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  • 1.
    Cordero-Edwards, Kumara
    et al.
    CSIC, Catalan Inst Nanosci & Nanotechnol, Campus UAB, Barcelona 08193, Spain.;BIST, Campus UAB, Barcelona 08193, Spain.;Univ Geneva, DPMC MaNEP, Quai Ernest Ansermet 24, CH-1211 Geneva, Switzerland..
    Kianirad, Hoda
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Sort, Jordi
    UAB, Dept Fis, Edifici Cc, E-08193 Bellaterra, Spain.;ICREA, Pg Lluis Co 23, E-08010 Barcelona, Catalonia, Spain..
    Catalan, Gustau
    CSIC, Catalan Inst Nanosci & Nanotechnol, Campus UAB, Barcelona 08193, Spain.;BIST, Campus UAB, Barcelona 08193, Spain.;ICREA, Pg Lluis Co 23, E-08010 Barcelona, Catalonia, Spain..
    Flexoelectric Fracture-Ratchet Effect in Ferroelectrics2019In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 122, no 13, article id 135502Article in journal (Refereed)
    Abstract [en]

    The propagation front of a crack generates large strain gradients and it is therefore a strong source of gradient-induced polarization (flexoelectricity). Herein, we demonstrate that, in piezoelectric materials, a consequence of flexoelectricity is that crack propagation is helped or hindered depending on whether it is parallel or antiparallel to the piezoelectric polar axis. The discovery of crack propagation asymmetry proves that fracture physics cannot be assumed to be symmetric in polar materials, and indicates that flexoelectricity should be incorporated in any realistic model.

  • 2.
    Kianirad, Hoda
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Studies on Domain Dynamics in Nonlinear Optical Ferroelectric Oxide Crystals2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Nonlinear optical frequency conversion is the key technology for modifying laser output radiation, in order to target specific applications. The most powerful technique to obtain tailored second-order nonlinear interactions is the quasi-phasematching (QPM) approach. QPM is based on periodic modulation of the medium’s nonlinearity and allows versatile and efficient frequency conversion in the whole transparency region of the material. QPM is commonly implemented in ferroelectric oxide crystals by periodically inverting the spontaneous polarization, so-called, periodic poling. However, in order to achieve QPM structures of practical relevance, both the optical properties of the material and the domain engineering techniques have to be suitable for the targeted nonlinear interaction.

    Rb-doped KTiOPO4 (RKTP) and vapor-transport-equilibrated stoichiometric LiTaO3 (VTE-SLT) are two of the most promising ferroelectric oxides used for nonlinear optics. The former is suitable for high peak-power applications and for engineering of QPM devices with sub-µm periodicity. The latter shows a short cut-off wavelength with low linear absorption, which makes it very attractive for UV-light generation. However, in order to fully exploit the potential of these two materials, it is of utmost importance to understand the domain dynamics and stability from a fundamental point of view, as well as to investigate ways to overcome their limitations.

    This thesis presents studies on domain dynamics and stability in these two materials. A novel method for periodic poling of RKTP has been investigated. The method, based on using a micro-structured silicon chip as the contact electrode, has been used to fabricate periodically poled RKTP crystals with 9.01 µm period. The samples became well-poled and showed high conversion efficiency for second harmonic generation. The domain dynamics, when the silicon stamp was used as an electrode were studied, showing potential for short-pitch poling and complex patterning.

    Furthermore, the domain stability in RKTP during thermal annealing at high temperatures was investigated. The results show anisotropic domain wall motion, with severe domain contraction along the crystallographic b-axis when the periodicity was reduced. A technique to suppress this domain contraction was developed based on dicing away the edges of the QPM grating so that the domain b-faces terminate in air. This gave excellent results for a broad range of periodicities.

    Studies of the domain stability of periodically poled VTE-SLT during chemical etching were performed by on-line second harmonic generation, and optical microscopy. The results show that wet etching directly after poling leads to domain-wall motion, resulting in back-switching or domain merging. 

    Furthermore, the domain wall motion induced by electron beam irradiation was investigated using a scanning electron microscope. It was found that domain switching strongly depends on the ratio of secondary electrons to incident electrons. These results are discussed in terms of electron beam and screening charges interaction.

  • 3.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Domain wall motion in stoichiometric LiTaO3 induced by low-energy electron beam2019In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118Article in journal (Refereed)
    Abstract [en]

    Scanning electron microscopy was used to study the mobility of domain walls in vapor-transport equilibrated stoichiometric LiTaO3. By using low-acceleration voltage, switching occurs solely for polarization pointing-up domains, resulting in fast domain-wall motion. When the incoming electron beam was more energetic with larger penetration depth, the switching instead occurred for polarization pointing down domains. Our results are discussed in terms of the interaction of the scanning electron beam with the polarization-screening charges at the crystal surface.

  • 4.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Domain wall motion in stoichiometric LiTaO3 induced by low-energy electron beam2019In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 115, no 5, article id 052901Article in journal (Refereed)
    Abstract [en]

    Scanning electron microscopy was used to study the mobility of domain walls in vapor-transport equilibrated stoichiometric LiTaO3. By using low-acceleration voltage, switching occurs solely for polarization pointing up domains, resulting in fast domain-wall motion. When the incoming electron beam was more energetic with larger penetration depth, the switching instead occurred for polarization pointing down domains. Our results are discussed in terms of the interaction of the scanning electron beam with the polarization-screening charges at the crystal surface.

  • 5.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Lindgren, Gustav
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Peña, A.
    Zukauskas, Andrius
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Ménaert, B.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Boulanger, B.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Stabilization of domain structures in Rb-doped KTiOPO 4 for high-temperature processes2019In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 114, no 5, article id 052904Article in journal (Refereed)
    Abstract [en]

    A way to stabilize the domain structure in periodically poled Rb-doped KTiOPO 4 samples at high-temperatures is presented. The domain contraction along the b-crystallographic axis that is observed when crystals are annealed at high temperatures is suppressed when the ends of the domains along the b-axis are diced away. Additionally, the thermal stability of self-assembled domain gratings with a sub-μm average periodicity of 650 ± 200 nm and a domain-width of 225 ± 75 nm in mm-thick samples is investigated, and it is shown that the key factor for the domain stability is the domain width rather than the interdomain distance.

  • 6.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Zakauskas, Andrius
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Domain dynamics in Stoichiometric Lithium Tantalate revealed by etching and on-line Second Harmonic Generation2015Conference paper (Refereed)
    Abstract [en]

    Periodically poled LiTaO3 has a shorter cut-off wavelength than KTP and LiNbO3, which makes this material attractive for UV generation. However, the standard congruent material suffers strongly from photorefractive damage which has limited its use in the UV. It is expected that stoichiometric LiTaO3 (SLT) would make a superior material in this context, as the defect concentration, which directly relates to the damage, is strongly reduced. Close to perfectly stoichiometric crystals can be obtained by post growth diffusion of Li2O into the crystal at high temperatures to compensate the Li2O deficiency, so called vapour transport equilibrated (VTE) SLT [1]. The coercive field is drastically reduced for SLT which facilitates the way for fabricating QPM devices [2]. However lower defects and reduced coercive field might cause instability for charged domain walls (DW) or walls that are not in their lowest energy state. Since the stability mechanism for VTE SLT is not very well known, its investigation would lead to improvement in device fabrication with short period and also helps to understand the DW dynamics.

     

    In this work, we study the impact of chemical etching after poling on domain stability of SLT. We show that chemical etching right after poling induced DW movement without any external field. We demonstrate that when the original DW configuration is head-to-head (HH) or tail-to-tail (TT), the DW movement results in backswitching or merging with neighboring domians, respectively. Our finding are monitored by in situ SHG during etching followed by characterization of the domain structure with optical microscopy. We also show that this behavior is independent of the domain orientation with respect the crystallographic axes.

  • 7.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Zukauskas, Andrius
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Domain dynamics in stoichiometric lithium tantalate revealed by wet etching and on-line second harmonic generation2017In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 18, article id 184103Article in journal (Refereed)
    Abstract [en]

    The effect of chemical etching on the stability of domains in periodically poled stoichiometric lithium tantalate is studied by on-line second harmonic generation and microscopy. It is found that wet etching directly after poling leads to domain-wall movement, resulting in back-switching or domain merging. Head-to-head domains tend to backswitch, while tail-to-tail domains merge. For samples where the domain structure stabilized for longer time, no domain motion is observed when etched.

  • 8.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Zukauskas, Andrius
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Domain engineering using Si-electrode stamp2016Conference paper (Refereed)
  • 9.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Zukauskas, Andrius
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    On the stability of periodically poled Lithium Tantalate2014Conference paper (Refereed)
  • 10.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Zukauskas, Andrius
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Frisk, Thomas
    KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Contact poling of Rb:KTiOPO4 using a micro-structured silicon electrode2015In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 23, no 2, p. 636-641Article in journal (Refereed)
    Abstract [en]

    A contact poling technique for domain engineering of ferroelectrics using a micro-structured silicon electrode is demonstrated on Rb:KTiOPO4. High quality QPM gratings were reproducibly fabricated. The silicon electrode is reusable and the technique potentially suitable when complex structures with sub-mu m features are to be domain engineered, which otherwise is incompatible with conventional photolithography. A non-negligible domain broadening was seen and attributed to a low nucleation rate using this type of electrode. However, under the appropriate poling conditions, this could be exploited to obtain a QPM grating with a short pitch (2 mu m), equal to half of the electrode period.

  • 11.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Zukauskas, Andrius
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Frisk, Thomas
    KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Contact poling of RKTP using a micro-structured silicon electrode2014Conference paper (Other academic)
  • 12.
    Kianirad, Hoda
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Zukauskas, Andrius
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Frisk, Thomas
    KTH, School of Engineering Sciences (SCI), Applied Physics, Cell Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Contact poling of RKTP with silicon pillars2013In: 2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013, IEEE Computer Society, 2013, p. 6800873-Conference paper (Refereed)
    Abstract [en]

    Quasi-phase-matching (QPM) is a method to get tailored efficient second order nonlinear interactions [1]. Several techniques exist for fabrication of periodic domain structures in ferroelectric crystals for QPM frequency conversion. By far, electric field poling using lithographically patterned electrodes on the z-face of the crystal is the most common one [2]. High-quality periodically inverted ferroelectric domain structures in flux grown KTiOP 4 (KTP) crystals were fabricated already in the late 90's using this technique [3], and recently periodic domain sizes of few hundred nanometers were fabricated in 1 mm thick samples thanks to the quasi-one dimensional structure of KTP. It has recently also been shown that a slight Rb doping of the KTP crystal (RKTP) facilitates the periodic poling [4]. However, fabrication of two-dimensional (2D) domain structures in RKTP has not yet been investigated. A disadvantage with the lithographic patterning is that each sample needs to be patterned individually, which is tedious and time consuming. Moreover, when it comes to the small domain features, which are required by the next generation of nonlinear optical devices, a more versatile poling technique has to be developed due to the limitations of conventional photolithography. Structured silicon has been investigated as an alternative electrode for formation of 1D domains by contact poling in LiNb3 [5]. However, these electrodes were fabricated by wet etching and the sample thickness was limited to ∼200 μm.

  • 13.
    Kores, Cristine C.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Mutter, Patrick
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Kianirad, Hoda
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Canalias, Carlota
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Quasi-phase matched second harmonic generation in periodically poled Rb-doped KTiOPO4 ridge waveguide2018In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 26, no 25, p. 33142-33147Article in journal (Refereed)
    Abstract [en]

    A 10.8 mu m wide ridge waveguide was fabricated by diamond-blade dicing in an ion-exchanged periodically poled Rb-doped KTiOPO4 sample. The waveguide was used to generate blue second harmonic light at 468.8 nm in the TM(00 )mode through first order Type I quasi-phase matching, exploiting the large d(33) coefficient of the crystal. It was evaluated using a cw Ti:Sapphire laser, and 6.7 mu W of blue light was generated with 5.8 mW of fundamental radiation at 933.8 nm coupled through the waveguide. The results presented here pave the way for efficient nonlinear processes in a waveguide format. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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