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Three-dimensional characterization of the effective second-order nonlinearity in periodically poled crystals
KTH, Superseded Departments, Physics.
KTH, Superseded Departments, Physics.ORCID iD: 0000-0002-2508-391X
KTH, Superseded Departments, Physics.
KTH, Superseded Departments, Physics.ORCID iD: 0000-0001-7688-1367
2003 (English)In: Optic letters, ISSN 0146-9592, Vol. 28, no 17, 1555-1557 p.Article in journal (Refereed) Published
Abstract [en]

A novel technique for characterization of the second-order nonlinearity in nonlinear crystals is presented. It utilizes group-velocity walk-off between femtosecond pulses in type II SHG to achieve three-dimensional resolution of the nonlinearity. The longitudinal and transversal spatial resolution can be set independently. The technique is especially useful for characterizing quasi-phase-matched nonlinear crystals, and it is demonstrated in potassium titanyl phosphate.

Place, publisher, year, edition, pages
2003. Vol. 28, no 17, 1555-1557 p.
National Category
Atom and Molecular Physics and Optics
Identifiers
URN: urn:nbn:se:kth:diva-8756DOI: 10.1364/OL.28.001555ISI: 000184836900019OAI: oai:DiVA.org:kth-8756DiVA: diva2:14166
Note
QC 20100830Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2015-03-30Bibliographically approved
In thesis
1. Fabrication and characterization of periodically poled KTB and RB-doped KTB for applications in the visible and UV
Open this publication in new window or tab >>Fabrication and characterization of periodically poled KTB and RB-doped KTB for applications in the visible and UV
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis deals with the fabrication and the characterization of periodically-poled crystals for use in lasers to generate visible and UV radiation by second-harmonic generation (SHG) through quasi-phasematching (QPM). Such lasers are of practical importance in many applications like high-density optical storage, biomedical instrumentation, colour printing, and for laser displays.

The main goals of this work were: (1) to develop effective monitoring methods for poling of crystals from the KTiOPO4 (KTP) family, (2) to develop useful non-destructive domain characterization techniques, (3) to try to find alternative crystals to KTP for easier, periodic poling, (4) to investigate the physical mechanisms responsible for optical damage in KTP. The work shows that the in-situ SHG technique used together with electro-optic monitoring, makes it possible to obtain reliable, real-time information regarding the poling quality over the whole crystal aperture during the electric-field poling process. Using this combined monitoring method, both KTP and Rb-doped KTP (RKTP) crystals were successfully poled. By comparing these two crystals, we found that a low-doped KTP has a substantially reduced ionic conductivity and, thus, a high-quality periodic poling can be obtained without otherwise affecting the properties of the crystal. RKTP is a good alternative candidate to KTP for poling purpose. We have also shown that Atomic Force Microscopy (AFM) is an informative tool for investigating domain nucleation, growth, and merging. Furthermore, we have demonstrated a simple technique for 3D characterization of QPM samples. It utilizes a group-velocity mismatched, type-II SHG of femtosecond pulses for layer-by-layer monitoring of the effective nonlinearity along the propagation direction of the beam. The quality of these crystals was finally reflected in a number of SHG experiments with a variety of laser sources. High energies and high efficiencies were thus demonstrated using CW, mode-locked and Q-switched lasers. Gratings with pitches smaller than 3 µm, were demonstrated for first-order UV generation. Type-II QPM SHG was demonstrated as a technique for reducing the fabrication constraints.

High intensity light in the visible and the UV leads to modification of the material properties and, eventually, to optical damage. In KTP and its isomorphs, the first sign of material change is an optically-induced absorption. We have used thermal-lens spectroscopy with a common-path interferometer for high-sensitivity measurements of green light-induced infrared absorption dynamics in single-domain and periodically-poled KTP (PPKTP). The saturated, green light-induced absorption has been shown to be consistently higher in periodically-poled crystals, and is attributed to the creation of stoichiometric and interstitial defects in the crystals during the poling process. Finally, irreversible bulk damage thresholds in PPKTP have been determined for pulsed frequency converters. As the characteristics of optical damage are closely related to the material quality, this investigation can provide useful information for crystal manufactures and will help to optimise the crystal growth conditions.

Series
Trita-FYS, ISSN 0280-316X ; 2005:50
Keyword
Nonlinear optics, second-harmonic generation, quasi-phasematching, type-I QPM SHG, type-II QPM SHG, KTP, RKTP, periodic electric-field poling
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-483 (URN)91-7178-153-6 (ISBN)
Public defence
2005-11-25, Sal FB53, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:00
Opponent
Supervisors
Available from: 2005-11-09 Created: 2005-11-09 Last updated: 2012-03-22
2. Short Pulses in Engineered Nonlinear Media
Open this publication in new window or tab >>Short Pulses in Engineered Nonlinear Media
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Short optical pulses and engineered nonlinear media is a powerful combination. Mode locked pulses exhibit high peak powers and short pulse duration and the engineered ferro-electric KTiOPO4 facilitates several different nonlinear processes. In this work we investigate the use of structured, second-order materials for generation, characterization and frequency conversion of short optical pulses.

By cascading second harmonic generation and difference frequency generation the optical Kerr effect was emulated and two different Nd-based laser cavities were mode locked by the cascaded Kerr lensing effect. In one of the cavities 2.8 ps short pulses were generated and a strong pulse shortening took place through the interplay of the cavity design and the group velocity mismatch in the nonlinear crystal. The other laser had a hybrid mode locking scheme with active electro-optic modulation and passive cascaded Kerr lensing incorporated in a single partially poled KTP crystal. The long pulses from the active modulation were shortened when the passive mode locking started and 6.9 ps short pulses were generated.

High-efficiency frequency conversion is not a trivial task in periodically poled materials for short pulses due to the large group velocity mismatch. Optimization of parameters such as the focussing condition and the crystal temperature allowed us to demonstrate 64% conversion efficiency by frequency doubling the fs pulses from a Yb:KYW laser in a single pass configuration. Quasi phase matching also offers new possibilities for nonlinear interactions. We demonstrated that it is possible to simultaneously utilize several phase matched second harmonic interactions, resulting in a dual-polarization second harmonic beam.

Short pulse duration of the fundamental wave is a key parameter in the novel method that we demonstrated for characterization of the nonlinearity of periodically poled crystals. The method utilizes the group velocity mismatch between the two polarizations in a type II second harmonic generation configuration.

The domain walls of PPKTP exhibit second order nonlinearities that are forbidden in the bulk material. This we used in a single shot frequency resolved optical gating arrangement. The spectral resolution came from Čerenkov phase matching, a non-collinear phase matching scheme that exhibits a substantial angular dispersion. The second harmonic light was imaged upon a CCD camera and with the spectral distribution on one axis and the temporal autocorrelation on the other. From this image we retrieved the full temporal profile of the fundamental pulse, as well as the phase. The spectral dispersion provided by the Čerenkov phase matching was large enough to characterize optical pulses as long as ~200 fs in a compact setup. The Čerenkov frequency resolved optical gating method samples a thin stripe of the beam, i.e. the area close to the domain wall. This provides the means for high spatial resolution measurements of the spectral-temporal characteristics of ultrafast optical fields.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. x, 57 p.
Series
Trita-FYS, ISSN 0280-316X ; 2006:74
Keyword
nonlinear optics, KTiOPO4, frequency conversion, mode-locked lasers, ultra-fast lasers, visible lasers, short pulses, ultrafast nonlinear optics, diagnostic applications of nonlinear optics, nonlinear optical materials
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-4234 (URN)91-7178-540-X (ISBN)
Public defence
2006-12-18, FD5, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100831Available from: 2006-12-15 Created: 2006-12-15 Last updated: 2015-03-30Bibliographically approved

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Pasiskevicius, ValdasLaurell, Fredrik

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