Change search
Refine search result
123 1 - 50 of 126
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1. Abbas, Syed Sohail
    et al.
    Popov, Sergei Yu
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Compact Er 3+-doped ZBLAN green upconversion fibre laser2011In: World Academy of Science, Engineering and Technology: An International Journal of Science, Engineering and Technology, ISSN 2010-376X, E-ISSN 2070-3740, Vol. 78, p. 756-758Article in journal (Refereed)
    Abstract [en]

    In this paper, a fibre laser at 546 nm has been studied for a signal power of -30 dB. Er 3+-doped ZBLAN fibre has been used by upconversion pumping of a 980 nm laser diode. Gain saturation effect has been investigated in detail. Laser performance has also been discussed. An efficiency of 35% has been calculated with a length of 5 mm fibre laser. Results show that Er 3+-doped ZBLAN is a promising candidate for optical amplification at 546 nm.

  • 2.
    Aggerstam, Thomas
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Pinos, Andrea
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Linnarsson, Margareta
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Lourdudoss, Sebastian
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Electron and hole capture cross-sections of Fe acceptors in GaN:Fe epitaxially grown on sapphire2007In: Journal of Electronic Materials, ISSN 0361-5235, E-ISSN 1543-186X, Vol. 36, no 12, p. 1621-1624Article in journal (Refereed)
    Abstract [en]

    Carrier trapping of Fe (3+)/Fe2+ deep acceptors in epitaxially grown GaN:Fe on sapphire was studied by time-resolved photoluminescence. For the investigated Fe doping levels on the order of 10(18) cm(-3), the luminescence decay times are strongly dependent on the Fe concentration, indicating that Fe centers act as predominant nonradiative recombination channels. Linear dependence of the decay time on the iron concentration allows estimation of the electron capture cross-section for the Fe3+ ions, which is equal to 1.9 x 10(-15) cm(2). The upper bound for the cross-section of the hole capture of Fe2+ was evaluated as 10 x 10(-15) cm

  • 3. Alonso, M. A.
    et al.
    Setala, T.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Optimal pulses for arbitrary dispersive media2011In: Journal of the European Optical Society - Rapid Publications, ISSN 1990-2573, E-ISSN 1990-2573, Vol. 6, p. 11035-Article in journal (Refereed)
    Abstract [en]

    A variational procedure is given for finding the pulses for which the initial temporal rms width and the rate of increase of this width are jointly minimized for propagation in non-absorbing media with arbitrary dispersive properties. We show that, while in linearly dispersive media the optimal pulses are Gaussian, in other situations such as a hollow metallic waveguide or for purely cubic dispersion departures from Gaussian behavior become evident. An interpretation of the results in terms of suitable phase-space representations is also given.

  • 4. Blomstedt, K.
    et al.
    Setala, T.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Effect of absorption on the spatial coherence in scalar fields generated by statistically homogeneous and isotropic sources2005In: Physical Review E, ISSN 1539-3755, Vol. 72, no 5Article in journal (Refereed)
    Abstract [en]

    We investigate the behavior of the spectral degree of coherence of fluctuating scalar wave fields generated by statistically homogeneous and isotropic sources within absorbing media. In the limit of negligible losses, the degree of coherence is shown to be of a known universal form (sinc function), whereas in the limit of infinite losses it is shown to asymptotically approach the degree of coherence of the source. The intermediate cases are studied for fields produced by sources with Gaussian and sinc-type coherence functions. These studies show that the concept of negligible losses is meaningful only when defined with respect to specific source distribution statistics, and that even for arbitrarily small nonzero losses, the coherence length of the field can be arbitrarily short.

  • 5. Blomstedt, K.
    et al.
    Setala, T.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Effective degree of coherence: general theory and application to electromagnetic fields2007In: Journal of Optics. A, Pure and applied optics, ISSN 1464-4258, E-ISSN 1741-3567, Vol. 9, no 10, p. 907-919Article in journal (Refereed)
    Abstract [en]

    The concept of overall or effective degree of coherence has proven useful and insightful in the scalar theory of light. In this paper we extend the concept to fields represented over arbitrary vector (Hilbert) spaces. We show that the effective degree of coherence remains unaltered by scaled unitary mappings, which describe for instance many physical interactions. In the context of random, vector-valued electromagnetic fields the invariance implies that the effective degree of coherence assumes the same value in commonly used representation spaces. Thereby, in contrast to the local degree of coherence, this quantity can be taken as an intrinsic property of the electromagnetic wave field. In particular, the effective degree of coherence does not change when the field is scattered by a lossless medium and it can be determined from the far-field pattern. Hence the effective degree of coherence of electromagnetic fields is readily measurable.

  • 6. Blomstedt, Kasimir
    et al.
    Setala, Tero
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Arbitrarily short coherence length in wave fields within finite lossless source regions2007In: Physical Review E, ISSN 1539-3755, Vol. 75, no 2Article in journal (Refereed)
    Abstract [en]

    We show that it is possible to construct stochastic source distributions that generate wave fields with any desired spatial coherence properties inside a prescribed finite region, regardless of the refractive-index distribution therein. In particular, we demonstrate that there is no universal sinc-function form for the field correlations produced by statistically homogeneous and isotropic source distributions within large lossless regions, contrary to what is suggested by previous work. The results also disprove the commonly held belief that in lossless source regions the coherence length of the light is bounded below by the blackbody coherence length-that is, by approximately half the wavelength of the field.

  • 7.
    Burvall, Anna
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Martinsson, Per
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Communication modes in large-aperture approximation2007In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 32, no 6, p. 611-613Article in journal (Refereed)
    Abstract [en]

    Simplified versions of the communication modes in the Fresnel domain are derived when the system apertures are large. The approximate modes, which are in the form of spherical waves and sinc functions with a spherical curvature, give physical insight into the communication modes approach and the basic limits of free-space optical communication systems. They also show that Gabor's information theory is readily derived from the communication modes.

  • 8. Chamaillard, K.
    et al.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Dainty, Chris
    Coherency matrix description for statistical linear regression of Stokes parameters in rough small-particle scattering2010In: Optics Communications, ISSN 0030-4018, E-ISSN 1873-0310, Vol. 283, no 22, p. 4519-4524Article in journal (Refereed)
    Abstract [en]

    A statistical analysis of the Stokes parameters in light scattering by randomly rough small particles shows a linear regression law between the squares of the first two components I-s and Q(s) of the Stokes vector. While the coefficients of this linear regression contain physical characteristics of the particles, they cannot be directly interpreted in terms of the degree of polarization of the scattered field. We propose an interpretation of this relationship between the Stokes parameters on the basis of the general coherence-density matrix formalism. The link between the statistical regression results and the polarization properties of the stochastic scattered components of the field is established through the coherency matrix elements.

  • 9.
    Chen, Jiajia
    et al.
    Centre for Optical and Electromagnetic Research, Joint Laboratory of Optical Communications of Zhejiang University, Zhejiang University.
    Chen, Biao
    Centre for Optical and Electromagnetic Research, Joint Laboratory of Optical Communications of Zhejiang University, Zhejiang University.
    He, Sailing
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    A novel hierarchical algorithm for intra-ONU scheduling in an ethernet passive optical network2005In: Network Architectures, Management, and Applications III, 2005, Vol. 6022, p. W220-W220Conference paper (Refereed)
    Abstract [en]

    The quality of service (QoS) requirements of end users and services are totally different, and thus the multi-oriented (user-oriented and service-oriented) scheduling for access networks based on a remote scheduling system especially for EPONs is desired to replace the single-oriented (user-oriented or service-oriented) scheduling. In this paper we introduce a novel hierarchical algorithm of intra-ONU scheduling for multi-oriented QoS, which can guarantee both the priority of the differentiated services and the fairness of the different users. Numerical results have shown that our overall scheduling algorithm can fulfill various requirements of delay and throughput for the transmission of multimedia traffic for each end user.

  • 10.
    Dong, Lin
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Optical Properties of Nanoparticles in Composite Materials2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Nanoparticles are synthetic structures with dimension from 1 to 100 nanometers and are various in types. Some favorable properties peculiar to the nanoparticles (generally owing to size effects) make them prevailing and beneficial for applications in different scientific and engineering fields. A large portion of these properties find their connection to optics and photonics. In the context of optics, the thesis is devoted to study of two specific categories of nanoparticles, gold nanoparticles and CdSe-CdS core-shell quantum dots, aiming at investigating the influence and potential of the particles in applications of lasing and medical diagnosis/treatment.

    Gold nanoparticles have been widely exploited in radiative decay engineering to achieve fluorescence enhancement or quenching of fluorophores, with the help of a localized surface plasmon resonance band in visible range. As the technique is recently introduced to lasing applications, the influence of the gold nanoparticles on the photostability of the gain medium needs more attention. In this work, the effect of size and concentration of gold nanoparticles on altering the photostability of aqueous solution of Rhodamine 6G in lasing process is demonstrated and analyzed. Energy transfer and nanoparticle induced heat are found to be responsible for the acceleration of photobleaching. It is shown that coating the gold nanoparticles with a 15 nm thick silica layer can effectively diminish the photostability degradation of the gain medium.

    Gold nanorods are popular for in vivo diagnostic and therapeutic applications due to their strong absorption of near-infrared light. A novel type of multimodal nanoparticles based on gold nanorods is synthesized here and optically characterized. The coating of silica and gadolinium oxide carbonate hydrate renders the nanoparticles superior performance as MRI/CT contrast agents than commercially available products. Meanwhile, the precise temperature control of bio-tissues using the particles under laser irradiation makes them promising for photothermal treatment of cancer cells.

    The thesis also addresses several open questions with respect to CdSe-CdS core-shell quantum dots. A numerical model is built to study the spatial separation of electrons and holes in the dots with different core/shell sizes. QDs in different geometrical shapes are investigated. It is found that the spherical core-shell QDs can be flexibly tuned between the type-I and the type-II regime by varying the dimensions of the core and the shell. The feature is confirmed by time-resolved photoluminescence measurements, in which the carrier recombinations from different spatial paths can be distinguished. A sign of amplified spontaneous emission is observed with spherical dots of an appropriate combination of core radius and shell thickness, indicating the potential of the QDs for lasing applications.

  • 11.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Iyer, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Friberg, Ari
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    3D fabrication of waveguide and grating coupler in SU-8 by optimized gray scale electron beam lithography2010In: 2010 Asia Communications and Photonics Conference and Exhibition, ACP 2010, 2010, p. 542-543Conference paper (Refereed)
    Abstract [en]

    Gray scale electron beam lithography is optimized for simple and accurate prototyping of 3D waveguides and grating output couplers in SU-8. Gratings with complex profiles and free of lag effect can be realized with this technique.

  • 12.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    One-step fabrication of polymer components for microphotonics by gray scale electron beam lithography2011In: J EUR OPT SOC-RAPID PUBL, ISSN 1990-2573, Vol. 6, p. 11010-Article in journal (Refereed)
    Abstract [en]

    We demonstrate an application of gray scale electron beam lithography (EBL) for the fabrication of polymer waveguides and grating output couplers with depth variable features, using the SU-8 resist. The technique is mainly applicable for multi-level binary profile, where groove depths of the structure are controlled by choosing a proper exposure dose. Unlike reactive ion etching which is limited by the lag effect, the gray scale EBL allows free combination of groove widths and depths. Shrinking effect which is critical in polymer couplers' writing is taken into account and can be compensated. For better fabrication feasibility, the grating couplers can be simultaneously produced with waveguides with no inter-step alignment required. Therefore, this is a promising technique in manufacturing grating output couplers for polymer based waveguides with high performance in terms of mode matching/confinement and coupling efficiency.

  • 13.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics.
    Sergeyev, Sergey
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Spatial light modulator as a reconfigurable intracavity dispersive element for tunable lasers2010In: Central European Journal of Physics, ISSN 1895-1082, E-ISSN 1644-3608, ISSN 1895-1082 print, Vol. 8, no 2, p. 228-234Article in journal (Refereed)
    Abstract [en]

    An improved approach for narrow-band wavelength selection in tunable lasers is described. To provide the tunability, a reconfigurable diffractive optical element (DOE) based on a programmable spatial light modulator (PSLM) is applied. With a proper choice of the phase transfer function of the PSLM, the device can be used as a dispersive intra-cavity component for precise tuning within the lasing spectral band of a solid-state dye laser. The suggested design allows avoiding the mechanical movement of any cavity components. The tunability performance and simulation are demonstrated using the Fourier optics method.

  • 14.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Sugunan, Abhilash
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Hu, Jun
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Zhou, Sicheng
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Li, Shanghua
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Toprak, Muhammet S.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Muhammed, Mamoun
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Photoluminescence from quasi-type-II spherical CdSe-CdS core-shell quantum dots2013In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 52, no 1, p. 105-109Article in journal (Refereed)
    Abstract [en]

    Spherical CdSe-CdS core-shell quantum dots (QDs) are found to be flexible in the transition between the type-I regime and the type-II regime with different core/shell dimensions. The quasi-type-II feature of the colloidal dots is confirmed with time-resolved photoluminescence (PL) measurements. Two recombination paths of the excitons with significantly different decay rates are observed and analyzed. The spherical CdSe-CdS core-shell QDs are numerically simulated to investigate the carrier separation. A relatively long radiative lifetime and high degree of spatial carrier separation provide good potential to achieve lasing under continuous-wave excitation. Amplified spontaneous emission at room temperature is detected from the QDs embedded in the polymer matrix. It is shown that a larger shell thickness results in a lower pumping threshold, while a smaller shell thickness leads to higher PL efficiency.

  • 15.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Ye, Fei
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Chughtai, Adnan
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Liuolia, Vytautas
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Muhammed, Mamoun
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Lasing From Water Solution of Rhodamine 6G/Gold Nanoparticles: Impact of SiO2-Coating on Metal Surface2012In: IEEE Journal of Quantum Electronics, ISSN 0018-9197, E-ISSN 1558-1713, Vol. 48, no 9, p. 1220-1226Article in journal (Refereed)
    Abstract [en]

    Gold nanoparticles embedded in an optical gain material, particularly in a water solution of Rhodamine 6G, used in dye lasers can both increase and damp dye flourescence, thus changing the laser output intensity. Simultaneously, such nanoparticles influence the gain material's resistance against photobleaching. In this paper, we report our study on the impact of the SiO2 coating of nanoparticles on the enhancement or quenching and photobleaching of the fluorescence. The investigation demonstrates a noticeable improvement of the gain material's photostability compared to uncoated gold nanoparticles when silicon dioxide coating is implemented.

  • 16.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Ye, Fei
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Chughtai, Adnan
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Muhammed, Mamoun
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Enhanced photostability of aqueous solution of Rhodamine 6G with gold nanoparticles in lasing process by silica coating2012In: 2012 Conference on Lasers and Electro-Optics, CLEO 2012, IEEE , 2012, p. 6325399-Conference paper (Refereed)
    Abstract [en]

    Gold nanoparticles are mixed in aqueous solution of Rhodamine 6G to modify the lasing output intensity. The photostability deterioration of the gain medium by gold nanoparticles is successfully compensated by silica coating on the nanoparticles.

  • 17.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Ye, Fei
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Chughtai, Adnan
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Muhammed, Mamoun
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Photostability of lasing process from water solution of Rhodamine 6G with gold nanoparticles2012In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 37, no 1, p. 34-36Article in journal (Refereed)
    Abstract [en]

    We report the lasing performance and photobleaching of gain material containing a water solution of Rhodamine 6G dye and gold nanoparticles (NPs). In comparison to a pure dye solution, the investigated material demonstrated both enhancement and quenching of the lasing output, depending on the relative concentration of the gold NPs. Although the presence of NPs with an optimized concentration looks preferable in terms of the lasing output enhancement, such additives deteriorate the operational resource of the gain material; i.e., the photobleaching rate speeds up.

  • 18.
    Dong, Lin
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Ye, Fei
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Hu, Jun
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Muhammed, Mamoun
    KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
    Fluorescence quenching and photobleaching in Au/Rh6G nanoassemblies: impact of competition between radiative and non-radiative decay2011In: JOURNAL OF THE EUROPEAN OPTICAL SOCIETY-RAPID PUBLICATIONS, ISSN 1990-2573, Vol. 6, p. 11019-Article in journal (Refereed)
    Abstract [en]

    We report the study of fluorescence quenching from nanoassemblies formed by Rhodamine 6G and gold nanoparticles (Au NPs) of 2.6 nm radius. The presence of Au NPs induces long-term degradation of the photostability (photobleaching) of Rhodamine 6G used as a gain medium in a Fabry-Perot laser cavity. We found that the degradation gets profound when the Au NPs concentration is significantly increased. Calculation of the radiative rate and direct time-resolved measurement of the fluorescence decay indicates that both the decrease of radiative decay rate and increase of non-radiative decay rate are responsible for the fluorescence quenching and photostability degradation. An energy transfer from the dye molecules to gold nanoparticles is dominating within small distance between them and suppresses the quantum efficiency of Rhodamine 6G drastically. In a long time scale, the photobleaching rate was slowing down, and the laser output intensity reached a stabilized level which depends on the gold nanoparticles concentration.

  • 19. Erkintalo, Miro
    et al.
    Surakka, Minna
    Turunen, Jari
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Genty, Goery
    Coherent-mode representation of supercontinuum2012In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 37, no 2, p. 169-171Article in journal (Refereed)
    Abstract [en]

    We apply the coherent-mode expansion to correlation functions used to describe the coherence properties of supercontinuum generated in nonlinear fibers. We show that the leading term of the expansion represents the quasi-coherent part of the field while the quasi-stationary part is embedded into the higher-order modes. The evolution of the modal expansion and the number of modes needed to describe the supercontinuum field are also discussed.

  • 20.
    Friberg, Ari T.
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Lajunen, Hanna
    Torres-Company, Victor
    Spectral elementary-coherence-function representation for partially coherent light pulses2007In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 15, no 8, p. 5160-5165Article in journal (Refereed)
    Abstract [en]

    A broad class of partially coherent non-stationary fields can be expressed in terms of the recently proposed independent-elementary-pulse model. In this work we first introduce a corresponding dual representation in the frequency domain and then extend this concept by considering shifted and weighted elementary spectral coherence functions. We prove that this method, which closely describes practical optical systems, leads to properly defined correlation functions. As an example, we demonstrate that our new model characterizes, in a natural way, trains of ultra-short pulses, affected by noise and timing jitter, emitted by usual modulators employed in telecom applications.

  • 21. Genty, Goery
    et al.
    Surakka, Minna
    Turunen, Jari
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Complete characterization of supercontinuum coherence2011In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 28, no 9, p. 2301-2309Article in journal (Refereed)
    Abstract [en]

    Using second-order coherence theory of nonstationary light we examine in detail the coherence properties of supercontinuum radiation generated in nonlinear fibers. We show that the supercontinuum can be divided into quasi-coherent and quasi-stationary parts and that the relative contributions depend on the dynamics involved in the spectral broadening process. We establish the correspondence between the quasi-coherent part of the two-frequency correlation function of the second-order theory and the usual Dudley-Coen degree of coherence used to characterize the shot-to-shot stability of supercontinuum sources. Experimental implementation for measuring separately the quasi-coherent and quasi-stationary contributions is further addressed. Our results open the route for complete characterization of supercontinuum coherence.

  • 22. Guina, Mircea
    et al.
    Suomalainen, Soile
    Hakulinen, Tommi
    Okhotnikov, Oleg
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Semiconductor saturable absorbers with recovery time controlled by lattice mismatch2007In: Optical Components and Materials IV / [ed] Shibin Jiang, Michel J. F. Digonnet, 2007, p. 64690P-1-64690P-7Conference paper (Refereed)
    Abstract [en]

    We propose and demonstrate a new method to reduce the absorption recovery time of semiconductor saturable absorber mirrors operating at the 1060-nm wavelength range. The method is based on controlling the amount of nonradiative recombination centers within the absorbing region by incorporating an InGaP epitaxial layer with a relative large lattice mismatch to GaAs (~2.2 %). The defect density within the absorbing region can be controlled by the thickness of a GaAs buffer layer grown between the InGaP lattice mismatched layer and the InGaAs/GaAs quantum-wells. For thickness of the GaAs buffer of ∼110 nm and∼570 nm the absorption recovery time was∼5 ps and ∼10 ps, respectively. It is important to note that the fast recovery time was achieved without degrading the nonlinear optical properties of the saturable absorber mirror. The practicality of the structures was proved by demonstrating a reliable self-starting operation of a mode-locked Yb-doped fiber laser.

  • 23. Guina, Mircea
    et al.
    Tuomisto, Pietari
    Okhotnikov, Oleg
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Mizohata, Kenichiro
    Keinonen, Juhani
    Semiconductor saturable absorbers with recovery time controlled through growth conditions2007In: Solid State Lasers XVI: Technology and Devices / [ed] Hanna J. Hoffman, Ramesh K. Shori, Norman Hodgson, 2007, p. 645113-1-645113-7Conference paper (Refereed)
    Abstract [en]

    We discuss a new method to shape the temporal response of saturable absorption in semiconductors. In particular, we investigate the possibility to control independently the absorption recovery time of each quantum-well forming the semiconductor absorber. The recovery time is tailored by irradiation with nitrogen ions produced by an RF-plasma source. The irradiation is performed in-situ as one step of the epitaxial growth process; the quantum-wells are individually exposed to a flux of N-ions after they are grown. The amount of non-radiative recombination centers within the quantum-wells is strongly related to the time interval during which the N-ions flux is active and to the thickness of the semiconductor layer grown on top of each quantum-well before the irradiation is performed. We apply this method to fabricate fast semiconductor saturable absorbers operating in the 1-μm wavelength range. The absorption recovery time could be changed from 300 ps to 10 ps without degradation of the nonlinear optical response. The practicality of the design is finally proved by using the semiconductor saturable absorbers for mode-locking Yb-doped fiber lasers.

  • 24. Hakkarainen, Timo
    et al.
    Setala, Tero
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Subwavelength electromagnetic near-field imaging of point dipole with metamaterial nanoslab2009Article in journal (Refereed)
    Abstract [en]

    We investigate near-field imaging of a point dipole by a lossy, nanoscale metamaterial slab. Making use of the electromagnetic angular-spectrum representation, we derive the Green tensor for the field transmission through the metamaterial slab, duly considering multiple reflections, polarizations, and wave-vector signs. With this general formalism, we calculate the point-spread function of the imaging system, which enables us to assess, for instance, resolution and image brightness. Our results demonstrate that with the metamaterial-slab lens one achieves resolution beyond the conventional diffraction limit of half the wavelength. In general, the resolution and image brightness are degraded when the slab thickness and absorption increase, but we show that in some cases the resolution is rather insensitive to the magnitude of the losses in the metamaterial.

  • 25. Hassinen, T.
    et al.
    Tervo, J.
    Setala, T.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Hanbury Brown-Twiss effect with electromagnetic waves2011In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 19, no 16, p. 15188-15195Article in journal (Refereed)
    Abstract [en]

    The classic Hanbury Brown-Twiss experiment is analyzed in the space-frequency domain by taking into account the vectorial nature of the radiation. We show that as in scalar theory, the degree of electromagnetic coherence fully characterizes the fluctuations of the photoelectron currents when a random vector field with Gaussian statistics is incident onto the detectors. Interpretation of this result in terms of the modulations of optical intensity and polarization state in two-beam interference is discussed. We demonstrate that the degree of cross-polarization may generally diverge. We also evaluate the effects of the state of polarization on the correlations of intensity fluctuations in various circumstances.

  • 26. Hassinen, Timo
    et al.
    Tervo, Jani
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Cross-spectral purity of electromagnetic fields2009In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 34, no 24, p. 3866-3868Article in journal (Refereed)
    Abstract [en]

    We extend Mandel's scalar-wave concept of cross-spectral purity to electromagnetic fields. We show that in the electromagnetic case, assumptions similar to the scalar cross-spectral purity lead to a reduction formula, analogous with the one introduced by Mandel. We also derive a condition that shows that the absolute value of the normalized zeroth two-point Stokes parameter of two cross-spectrally pure electromagnetic fields is the same for every frequency component of the field. In analogy with the scalar theory we further introduce a measure of the cross-spectral purity of two electromagnetic fields, namely, the degree of electromagnetic cross-spectral purity.

  • 27.
    Iyer, Srinivasan
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Effects of surface plasmons in subwavelength metallic structures2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The study of optical phenomena related to the strong electromagnetic response of noble metals (silver (Ag) and gold (Au) being most popular) over the last couple of decades has led to the emergence of a fast growing research area called plasmonics named after 'surface plasmons' which are electron density waves that propagate along the interface of a metal and a dielectric medium. Surface plasmons are formed by the coupling of light to the electrons on the metal surface subject to the fulfillment of certain physical conditions and they are bound to the metal surface. Depending on whether the metallic medium is a continuous film or a structure having dimensions less than or comparable to the wavelength of the exciting light, propagating or localized surface plasmons can be excited. The structure can be either a hole or an arbitrary pattern in a metal film, or a metallic particle. An array of subwavelength structures can behave as an effective homogeneous medium to incident light and this is the basis of a new class of media known as metamaterials. Metallic metamaterials enable one to engineer the electromagnetic response to  incident light and provide unconventional optical properties like negative refractive index as one prominent example. Metamaterials exhibiting negative index (also called negative index materials (NIMs)) open the door for super resolution imaging  and development of invisibility cloaks. However, the only problem affecting the utilization of plasmonic media to their fullest potential is the intrinsic loss of the metal, and it becomes a major issue especially at visible-near infrared (NIR) frequencies.

    The frequency of the surface plasmon is the same as that of the exciting light but its wavelength could be as short as that of X-rays. This property allows light of a given optical frequency to be conned into very small volumes via subwave lengthmetallic structures, that can be used to develop ecient sensors, solar cells, antennas and ultrasensitive molecular detectors to name a few applications. Also, interaction of surface plasmons excited in two or more metallic subwavelength structures in close proximity inuences the far-eld optical properties of the overall coupled system. Some eects of plasmonic interaction in certain coupled particles include polarization conversion, optical activity and transmission spectra mimicking electromagnetically-induced transparency (EIT) as observed in gas based atomicsy stems.

    In this thesis, we mainly focus on the optical properties of square arrays of certain plasmonic structures popularly researched in the last decade. The structures considered are as follows: (1) subwavelength holes of a composite hole-shape providing superior near-eld enhancement such as two intersecting circles (called' double hole') in an optically thick Au/Ag lm, (2) double layer shnets, (3) subwavelength U-shaped particles and (4) rectangular bars. The entire work is based on electromagnetic simulations using time and frequency domain methods.

    Au/Ag lms with periodic subwavelength holes provide extraordinarily high transmission of light at certain wavelengths much larger than the dimension of the perforations or holes. The spectral positions of the maxima depend on the shape of the hole and the intra-hole medium, thereby making such lms function as a refractive index sensor in the transmission mode. The sensing performance of the double-hole geometry is analyzed in detail and compared to rectangular holes.

    Fishnet metamaterials are highly preferred when it comes to constructing a NIM at optical frequencies. A shnet design that theoretically oers a negative refractive index with least losses at telecommunication wavelengths (1.4 1.5 microns) is presented.

    U-shaped subwavelength metallic particles, in particular single-slit split-ring resonators (SSRRs), provide a large negative response to the magnetic eld of light at a specic resonance frequency. The spectral positions of the structural resonances of the U-shaped particle can be found from its array far field transmission spectrum at normal incidence. An effort is made to clarify our understanding of these resonances with the help of localized surface plasmon modes excited in the overall particle. From an application point of view, it is found that a planar square array of SSRRs eectively functions as an optical half-wave waveplate at the main resonance frequency by creating a polarization in transmission that is orthogonal to that of incident light. A similar waveplate eect can be obtained purely by exploiting the near-eld interaction of dierently oriented neighbouring SSRRs. The physical reasons behind polarization conversion in dierent SSRR-array systems are discussed.

    A rectangular metallic bar having its dipolar resonance in the visible-NIR is called a nanoantenna, owing to its physical length in the order of nanometers. The excitation of localized surface plasmons, metal dispersion and the geometry of the rectangular nanoantenna make an analytical estimation of the physical length of the antenna from the desired dipolar resonance dicult. A practical map of simulated resonance values corresponding to a variation in geometrical parameters of Au bar is presented. A square array of a coupled plasmonic system comprising of three nanoantennas provides a net transmission response that mimicks the EIT effect. The high transmission spectral window possesses a peculiar dispersion profile that enables light with frequencies in that region to be slowed down. Two popular designs of such plasmonic EIT systems are numerically characterized and compared.

  • 28.
    Iyer, Srinivasan
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Impact of apexes on the resonance shift in double hole nanocavities2010In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 18, no 1, p. 193-203Article in journal (Refereed)
    Abstract [en]

    The transmission of light through metallic films with periodic double nanoholes is studied using vectorial three-dimensional finite element method. Special emphasis is given on understanding different transmission resonances arising in gold and silver films with periodic sub-wavelength holes of different shapes. The spectral shift of the hole-shape resonance resulting from a variation of the hole refractive index is analyzed for a double nanohole geometry in the transmission mode using numerical simulations. Specifically, the role of field enhancement at the apexes of the double nanohole in the sensing of medium within the hole cavity is pointed out and discussed. The presence of sharp apexes within the double nanoholes significantly improves the resonance sensitivity as compared to rectangular holes of comparable area. Impact of possible manufacturing errors on the overall sensitivity is also characterized. Robustness and a relatively simple fabrication procedure make these kinds of refractive index sensors practically attractive.

  • 29.
    Iyer, Srinivasan
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Friberg, Ari. T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Interpreting the dipolar plasmon resonance of rectangular metallic nanoantennasIn: Plasmonics, ISSN 1557-1955, E-ISSN 1557-1963Article in journal (Other academic)
  • 30.
    Iyer, Srinivasan
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari. T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Linear birefringence in split-ring resonators2012In: Optics Letters, ISSN 0146-9592, E-ISSN 1539-4794, Vol. 37, no 11, p. 2043-2045Article in journal (Refereed)
    Abstract [en]

    We study polarization-dependent transmission of light through arrays of single-slit split-ring resonator (SSRR) based systems at normal incidence using finite integration time domain (FITD) and finite element methods (FEM). It is found that a conventional planar array of SSRRs acts as an effective optical wave plate at certain polarizations of incident light. The effect is attributed to the intrinsic linear birefringence of individual SSRRs. A comparison is made with other split-ring resonator-based systems exhibiting wave-plate-like properties due to inter-SSRR coupling.

  • 31.
    Iyer, Srinivasan Shankararaman
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Loss optimization in double fishnet metamaterials at telecommunication wavelengths2011In: JOURNAL OF THE EUROPEAN OPTICAL SOCIETY-RAPID PUBLICATIONS, ISSN 1990-2573, Vol. 6, p. 11008-Article in journal (Refereed)
    Abstract [en]

    A hole shape optimization study was made for a double fishnet metamaterial producing a negative index of refraction within 1.4 - 1.5 mu m. It is found that within these wavelengths, elliptical holes offer lower losses as compared to rectangular ones and theoretically produce the best figure of merit (FOM) of approximately 6.

  • 32.
    Jacobsen, Gunnar
    et al.
    Acreo AB, Electrum 236, Kista, Sweden.
    Kazovsky, L. G.
    Stanford University, USA.
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Li, Jie
    Acreo AB, Electrum 236, Kista, Sweden.
    Zhang, Y.
    Tianjin University, China.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics (Closed 20120101), Optics (Closed 20120101).
    Phase noise influence in optical OFDM systems employing RF pilot tone for noise cancellation2011In: Journal of optical communications, ISSN 0173-4911, E-ISSN 2191-6322, Vol. 32, no 2Article in journal (Refereed)
    Abstract [en]

    For coherent and direct-detection Orthogonal Frequency Division Multiplexed (OFDM) systems employing radio frequency (RF) pilot tone phase noise cancellation the influence of laser phase noise is evaluated. Novel analytical results for the common phase error and for the (modulation dependent) inter carrier interference are evaluated based upon Gaussian statistics for the laser phase noise. In the evaluation it is accounted for that the laser phase noise is filtered in the correlation signal detection. Numerical results are presented for OFDM systems with 4 and 16 PSK modulation, 200 OFDM bins and baud rate of 1 GS/s. It is found that about 225 km transmission is feasible for the coherent 4PSKOFDM system over normal (G.652) fiber.

  • 33. Jacobsen, Gunnar
    et al.
    Lidon, Maria Sol
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Zhang, Yimo
    Influence of pre-and post-compensation of chromatic dispersion on equalization enhanced phase noise in coherent multilevel systems2011In: Journal of optical communications, ISSN 0173-4911, E-ISSN 2191-6322, Vol. 32, p. 257-261Article in journal (Refereed)
    Abstract [en]

    In this paper we present a comparative study of the equalization enhanced phase noise (EEPN) for preand post-compensation of chromatic dispersion in high capacity and high constellation systems. This is - to our knowledge - the first detailed study in this area for precompensation systems. Our main results show that the local oscillator phase noise determines the EEPN influence in post-compensation implementations whereas the transmitter laser determines the EEPN in pre-compensation implementations. As a result of significance for the implementation of practical longer-range systems it is to be emphasized that the use of chromatic dispersion equalization in the optical domain - e.g. by the use of dispersion compensation fibers - eliminates the EEPN entirely. Thus, this seems an efficient solution for such systems operating at high constellations in the future.

  • 34. Jacobsen, Gunnar
    et al.
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Li, Jie
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Zhang, Yimo
    EEPN and CD study for coherent optical nPSK and nQAM systems with RF pilot based phase noise compensation2012In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 20, no 8, p. 8862-8870Article in journal (Refereed)
    Abstract [en]

    A radio frequency (RF) carrier can be used to mitigate the phase noise impact in n-level PSK and QAM systems. The systems performance is influenced by the use of an RF pilot carrier to accomplish phase noise compensation through complex multiplication in combination with discrete filters to compensate for the chromatic dispersion (CD). We perform a detailed study comparing two filters for the CD compensation namely the fixed frequency domain equalizer (FDE) filter and the adaptive least-mean-square (LMS) filter. The study provides important novel physical insight into the equalization enhanced phase noise (EEPN) influence on the system bit-error-rate (BER) versus optical signal-to-noise-ratio (OSNR) performance. Important results of the analysis are that the FDE filter position relative to the RF carrier phase noise compensation module provides a possibility for choosing whether the EEPN from the Tx or the LO laser influences the system quality. The LMS filter works very inefficiently when placed prior to the RF phase noise compensation stage of the Rx whereas it works much more efficiently and gives almost the same performance as the FDE filter when placed after the RF phase noise compensation stage.

  • 35. Jacobsen, Gunnar
    et al.
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Li, Jie
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Zhang, Yimo
    Receiver implemented RF pilot tone phase noise mitigation in coherent optical nPSK and nQAM systems2011In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 19, no 15, p. 14487-14494Article in journal (Refereed)
    Abstract [en]

    In this paper, a novel method for extracting an RF pilot carrier signal in the coherent receiver is presented. The RF carrier is used to mitigate the phase noise influence in n-level PSK and QAM systems. The performance is compared to the use of an (ideal) optically transmitted RF pilot tone. As expected an electronically generated RF carrier provides less efficient phase noise mitigation than the optical RF. However, the electronically generated RF carrier still improves the phase noise tolerance by about one order of magnitude in bit error rate (BER) compared to using no RF pilot tone. It is also found, as a novel study result, that equalization enhanced phase noise - which appears as correlated pure phase noise, amplitude noise and time jitter - cannot be efficiently mitigated by the use of an (optically or electrically generated) RF pilot tone.

  • 36. Jacobsen, Gunnar
    et al.
    Xu, Tianhua
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Li, Jie
    Zhang, Yimo
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Error-rate floors in differential n-level phase-shift-keying coherent receivers employing electronic dispersion equalisation2011In: Journal of optical communications, ISSN 0173-4911, E-ISSN 2191-6322, Vol. 32, p. 191-193Article in journal (Refereed)
    Abstract [en]

    A model for the phase noise influence in differential n-level phase shift keying (nPSK) systems and 2nlevel quadrature amplitude modulated (2nQAM) systems employing electronic dispersion equalization and quadruple carrier phase extraction is presented. The model includes the dispersion equalization enhanced local oscillator phase noise influence. Numerical results for phase noise error-rate floors are given for dual polarization (DQPSK, Dl6PSK and D64PSK) system configurations with basic baud-rate of 25 GS/s. The transmission distance in excess of 1000 km requires local oscillator lasers with sub-MHz linewidth.

  • 37. Jaroszewicz, Zbigniew
    et al.
    Adamkiewicz, Grazyna
    Duran-Bosch, Vicente
    Friberg, Ari
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Kolodziejczyk, Andrzej
    Kowalik, Andrzej
    Lancis, Jesus
    Popov, Sergei
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Kinoform apodization by using of programmable diffractive optical elements2004In: Proceedings of SPIE - The International Society for Optical Engineering: Volume 5456, 2004 / [ed] Frank Wyrowski, SPIE - International Society for Optical Engineering, 2004, p. 153-156Conference paper (Refereed)
    Abstract [en]

    The apodizationof diffractiveopticalelementscan be realized by a local change of their diffraction efficiency. In the case of lithographic elementswith step-like structure of the period, the variable diffraction efficiency can be achieved by a gradual transformation of the 2m stepkinoform into its conjugate counterpart across theapodization region. In the present contribution we show experimental results confirming this idea, which until no was verified only by simulations. The apodized quaternary grating with locally varying diffraction efficiency was obtained on a SLM device as aprogrammablediffractiveopticalelement by changing gradually the period's profile. Knowledge of the phase heights of the SLM's pixels is required for successful implementation of theapodization function. It was determined from Fresnel images of the binary phase gratings with different phase step height programmed on the SLM. The Fresnel images become then binary and their visibility depends on the phase height of the grating in a known way, what makes possible to calibrate the SLM.

  • 38.
    Jaroszewicz, Zbigniew
    et al.
    Institute of Applied Optics, Warsaw.
    Burvall, Anna
    National university of Ireland, Galway.
    Friberg, Ari
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Garcia-Sucerquia, Jorge Ivan
    Kolodziejkzyk, Andrzej
    Nowakowski, Arkadiusz
    Focal segments obtained by interference of multiple Bessel beams: experimental results2004In: PHOTON MANAGEMENT  : PROCEEDINGS OF THE SOCIETY OF PHOTO-OPTICAL INSTRUMENTATION ENGINEERS (SPIE), vol 5456 / [ed] Frank Wyrowsky, SPIE , 2004, p. 484-489Conference paper (Other academic)
    Abstract [en]

    In the present communicate the experimental results are shown, which deal with the interference pattern created by superposition of multiple Bessel beams. They confirm our earlier results obtained analytically as well as by simulations. The interfering Bessel beams were obtained in a standard way, i.e., a field produced by a set of concentric annular apertures of narrow width illuminated by a plane wave was transformed by a lens into a set of Bessel beams.

  • 39.
    Jaroszewicz, Zbigniew
    et al.
    Institute of Applied Optics, Warsaw.
    Burvall, Anna
    National university of Ireland, Galway.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Axicon:  the most important optical element2005In: Optics and photonics news (Print), ISSN 1047-6938, E-ISSN 1541-3721, Vol. 16, no 4, p. 34-39Article in journal (Refereed)
    Abstract [en]

    Last year, the optics community celebrated the 50th anniversary of the formal naming of the axicon. Long before that, however, axicons generated vivid discussions and disagreements, often of fundamental importance to our understanding of optics.

  • 40. Jouttenus, T.
    et al.
    Setala, T.
    Kaivola, M.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Connection between electric and magnetic coherence in free electromagnetic fields2005In: Physical Review E, ISSN 1539-3755, Vol. 72, no 4Article in journal (Refereed)
    Abstract [en]

    We introduce quantitative measures for the description of the electric and magnetic coherence in a stationary, random electromagnetic field at two points, in a volume, and in the Fourier space. These quantities are applied to free electromagnetic fields, and several theorems regarding the relationship between the two types of coherences in such fields are established. Fields which are statistically homogeneous, and those which, in addition, are statistically isotropic are considered separately. Furthermore, the connection between the electric and magnetic coherence is exemplified for some specific statistically homogeneous fields.

  • 41. Kivisto, Samuli
    et al.
    Puustinen, Janne
    Guina, Mircea
    Herda, Robert
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Dianov, Evgueny M.
    Okhotnikov, Oleg G.
    Pulse dynamics of a passively mode-locked Bi-doped fiber laser2010In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 18, no 2, p. 1041-1048Article in journal (Refereed)
    Abstract [en]

    The pulse evolution in Bi-doped soliton fiber laser with slow and fast saturable absorber has been studied both experimentally and numerically. Semiconductor saturable absorbers with balanced slow and fast absorption recovery mechanisms exhibit a bi-temporal recovery dynamics which permits both reliable start-up of passive mode-locking and short pulse generation and stabilization. The pulse dynamics within the Bi fiber laser cavity have been investigated.

  • 42. Lajunen, Hanna
    et al.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Ostlund, Petter
    Quasi-stationary plane-wave optical pulses and the van Cittert-Zernike theorem in time2006In: Journal of the Optical Society of America a-Optics Image Science and Vision, ISSN 1084-7529, Vol. 23, no 10, p. 2530-2537Article in journal (Refereed)
    Abstract [en]

    We study the properties of quasi-stationary, partially coherent, plane-wave optical pulses in the space-time and space-frequency domains. A generalized van Cittert-Zernike theorem in time is derived to describe the propagation of the coherence function of quasi-stationary pulses. The theory is applied to rectangular pulses chopped from a stationary light source, and the evolution characteristics of such pulse trains with different states of coherence are discussed and illustrated with numerical examples.

  • 43. Lajunen, Hanna
    et al.
    Torres-Company, Victor
    Lancis, Jesus
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Resolution-enhanced optical coherence tomography based on classical intensity interferometry2009Article in journal (Refereed)
    Abstract [en]

    We propose a fourth-order interference scheme for optical coherence tomography operating with broadband incoherent (or quasi-incoherent) light. It is shown that using this proposal, an axial resolution improvement by a factor of root 2 and a better sensitivity for weakly reflecting samples are obtained than with the standard second-order correlation scheme. From a practical perspective, we suggest the use of broadband Q-switched pulses and performing ultrafast intensity correlation with a nonlinear crystal. The global performance of our proposal is illustrated by means of numerical simulations.

  • 44. Lindberg, J.
    et al.
    Setala, T.
    Kaivola, M.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Coherence and polarization properties of a three-dimensional, primary, quasi-homogeneous, and isotropic source and its far field2010In: Optics Communications, ISSN 0030-4018, E-ISSN 1873-0310, Vol. 283, no 22, p. 4452-4456Article in journal (Refereed)
    Abstract [en]

    We study the spatial coherence properties of a three-dimensional, primary, quasi-homogeneous, and isotropic source and the far field that it generates. We assess the spectrum, degree of polarization, electromagnetic degree of coherence, and effective degree of coherence of the source and of the far field. We also demonstrate the formalism for a spherical Gaussian Schell-model current source. The polarization properties of light are fully accounted for in the analysis, and it is found that certain results pertaining to scalar coherence theory are reproduced also within the electromagnetic treatment.

  • 45. Lindberg, Jari
    et al.
    Setala, Tero
    Kaivola, Matti
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Spatial coherence effects in light scattering from metallic nanocylinders2006In: Journal of the Optical Society of America a-Optics Image Science and Vision, ISSN 1084-7529, Vol. 23, no 6, p. 1349-1358Article in journal (Refereed)
    Abstract [en]

    We study the scattering of a partially coherent electromagnetic beam from metallic nanocylinders and analyze the effects of plasmon resonances on the coherence and polarization properties of the optical near field. We employ the coherent-mode representation for the incident field and solve the scattering problem independently for each mode by using a boundary-integral method. Our results show that the plasmon resonances may significantly affect the coherence and polarization characteristics of the near field and that partial coherence influences the energy flow in nanocylinder arrays.

  • 46. Lindfors, K.
    et al.
    Setala, T.
    Kaivola, M.
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Degree of polarization in tightly focused optical fields2005In: Journal of the Optical Society of America a-Optics Image Science and Vision, ISSN 1084-7529, Vol. 22, no 3, p. 561-568Article in journal (Refereed)
    Abstract [en]

    We analyze the degree of polarization of random, statistically stationary electromagnetic fields in the focal region of a high-numerical-aperture imaging system. The Richards-Wolf theory for focusing is employed to compute the full 3 X 3 electric coherence matrix, from which the degree of polarization is obtained by using a recent definition for general three-dimensional electromagnetic waves. Significant changes in the state of partial polarization, compared with that of the incident illumination, are observed. For example, a wave consisting of two orthogonal and uncorrelated incident-electric-field components produces rings of full polarization in the focal plane. These effects are explained by considering the distribution of the spectral densities of the three electric field components as well as the correlations between them.

  • 47. Lindfors, Klas
    et al.
    Priimagi, Arri
    Setala, Tero
    Shevchenko, Andriy
    Friberg, Ari T.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Kaivola, Matti
    Local polarization of tightly focused unpolarized light2007In: Nature Photonics, ISSN 1749-4885, Vol. 1, no 4, p. 228-231Article in journal (Refereed)
    Abstract [en]

    The polarization of light is important in a great variety of optical phenomena, ranging from transmission, reflection and scattering to polarimetric imaging of scenes and quantum-mechanical selection rules of atomic and molecular transitions. Among some less-well-known phenomena that illustrate the vectorial nature of light are the Pancharatnam(1) (or geometric(2)) phase, singularities in the polarization pattern of clear sky(3) and polarization of microwave background radiation(4). Here, we examine the partial polarization of focused light. We experimentally demonstrate a rather surprising phenomenon, where the focusing of unpolarized light results in rings of full polarization in the focal plane of the focusing optics. The polarization rings are imaged with a resolution of < 100 nm by probing the focal region using a gold nanoparticle.

  • 48.
    Liuolia, Vytautas
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Localization effects in ternary nitride semiconductors2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    InGaN based blue and near-ultraviolet light emitting diodes and laser diodes have been successfully commercialized for many applications such as general lighting, display backlighting and high density optical storage devices. Despite having a comparably high defect density, these devices are known for their efficient operation, which is attributed to localization in potential fluctuations preventing carriers from reaching the centers of nonradiative recombination. Nitride research is currently headed towards improving deep ultraviolet AlGaN and green InGaN emitters with higher Al and In molar fractions. The efficiency of these devices trails behind the blue counterparts as the carrier localization does not seem to aid in supressing nonradiative losses. In addition, the operation of ternary nitride heterostructure based devices is further complicated by the presence of large built-in electric fields. Although the problem can be ameliorated by growing structures in nonpolar or semipolar directions, the step from research to production still awaits.

    In this thesis, carrier dynamics and localization effects have been studied in three different nitride ternary compounds: AlGaN epitaxial layers and quantum wells with high Al content, nonpolar m-plane InGaN/GaN quantum wells and lattice matched AlInN/GaN heterostructures. The experimental methods of this work mainly consist of spectroscopy techniques such as time-resolved photoluminescence and differential transmission pump-probe measurements as well as spatial photoluminescence mapping by means of scanning near-field microscopy.

    The comparison of luminescence and differential transmission measurements has allowed estimating the localization depth in AlGaN quantum wells. Additionally, it has been demonstrated that the polarization degree of luminescence from m-InGaN quantum wells decreases as carriers diffuse to localization centers.What is more, dual-scale localization potential has been evidenced by near-field measurements in both AlGaN and m-InGaN. Larger scale potential fluctuation have been observed directly and the depth of nanoscopic localization has been estimated theoretically from the recorded linewidth of the near-field spectra. Lastly, efficient carrier transport has been observed through AlInN layer despite large alloy inhomogeneities evidenced by broad luminescence spectra and the huge Stokes shift. Inhomogeneous luminescence from the underlying GaN layer has been linked to the fluctuations of the built-in electric field at the AlInN/GaN interface.

  • 49.
    Liuolia, Vytautas
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Billingsley, D.
    Sensor Elect Technol Inc, Columbia, SC USA.
    Shatalov, M.
    Sensor Elect Technol Inc, Columbia, SC USA.
    Yang, J.
    Sensor Elect Technol Inc, Columbia, SC USA.
    Gaska, R.
    Sensor Elect Technol Inc, Columbia, SC USA.
    Shur, M. S.
    Rensselaer Polytech Inst, Troy, NY, USA .
    Photoexcited carrier dynamics in AlInN/GaN heterostructures2012In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 100, no 24, p. 242104-Article in journal (Refereed)
    Abstract [en]

    Photoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructures have been examined by time-resolved and scanning near-field photoluminescence (PL) spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited hole transfer from AlInN to GaN via sub-band-gap states. These states are attributed to extended defects and In clusters. Near-field PL scans show that diameter of the localization sites and the distance between them are below 100 nm. Spatial variations of the GaN PL wavelength have been assigned to the electric field inhomogeneities at the heterostructure interface.

  • 50.
    Liuolia, Vytautas
    et al.
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Marcinkevicius, Saulius
    KTH, School of Information and Communication Technology (ICT), Optics and Photonics, Optics.
    Lin, You-Da
    Ohta, Hiroaki
    DenBaars, Steven P.
    Nakamura, Shuji
    Dynamics of polarized photoluminescence in m-plane InGaN/GaN quantum wells2010In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 108, no 2, p. 023101-Article in journal (Refereed)
    Abstract [en]

    Spectrally-, polarization-, and time-resolved photoluminescence (PL) experiments have been performed on 2.5 nm thick m-plane single InGaN quantum wells. It has been found that PL decay is mainly determined by nonradiative recombination through several types of recombination centers, while PL rise is largely affected by exciton transfer into localization minima. Prolonged PL rise times and time-dependent spectral shift were used to study exciton transfer into the localization centers. CharacteristiC time of the exciton transfer is 80-100 ps at lower temperatures and about 50 ps at room temperature, which corresponds to the exciton diffusion length of 200-500 nm. Degree of PL linear polarization was found to decrease at a similar rate. Decreased PL polarization for the localized excitons suggests that the localization centers are related to areas with relaxed strain. (C) 2010 American Institute of Physics. [doi:10.1063/1.3460278]

123 1 - 50 of 126
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf