Junji Hirohashi

Control of specific domain structure in KNbO3 single crystals by differential vector poling method

The controllability of specific domain structures in KNbO3 single crystals was investigated by electric poling to several different orientations at room temperature. By applying electric field to ...

Polaron dynamics in lithium niobate upon femtosecond pulse irradiation: Influence of magnesium doping and stoichiometry control

The formation and relaxation dynamics of electron polarons in lithium niobate crystals were investigated by measuring transient absorption induced by blue femtosecond pulses. Anisotropy in the absorption change distinguished between small free polarons and small bound polarons, revealing that the dynamics were influenced by MgO doping and stoichiometry control. In crystals doped with MgO at concentrations above threshold, small free polarons were generated within 100 fs and decayed after tens of nanoseconds. In the presence of antisite defects, sequential formation of polaronic states occurred: electrons initially trapped as small free polarons became trapped as small bound polarons on picosecond time scale. The results are relevant for nonlinear optical applications of pulsed or high-power lasers.

Line-focus-beam acoustic microscopy characterization of optical-grade LiTaO3 single crystals

Basic data for evaluating homogeneities of LiTaO3 single crystals for optical use are investigated by line-focus-beam acoustic microscopy. First, the relationships among leaky surface acoustic-wave (LSAW) velocities, chemical compositions, Curie temperatures, densities, and lattice constants are experimentally investigated as the calibration lines for crystal evaluation using X-, Y-, and Z-cut substrates prepared from three LiTaO3 single crystals grown along the crystallographic Y axis with different Li2O contents ranging from 48 to 49 mol %. It is shown that as the Li2O content increases around the congruent composition, the LSAW velocities linearly increase for all specimen surfaces and all propagation directions, and the increase rate is maximum for Z-cut, Y-axis propagating (ZY) LiTaO3. Next, homogeneities of the above three crystals and a commercially available optical-grade crystal are evaluated using ZY-LiTaO3 specimens and the obtained calibration lines. The chemical composition variations along b...

Ultrasonic microspectroscopy of congruent LiNbO3 crystals

Experimental procedures for evaluating the compositional homogeneity of LiNbO3 single crystals were developed using the line-focus-beam ultrasonic material characterization system, and the true congruent composition was determined. First, the relationships among leaky surface acoustic wave (LSAW) velocities (VLSAW), chemical compositions, Curie temperatures, densities, and lattice constants were investigated for crystal evaluation, using X-, Y-, and Z-cut substrates prepared from three LiNbO3 crystals grown along the crystallographic Z axis with different Li2O contents of 48.0, 48.5, and 49.0mol%. We selected VLSAW for Z-cut Y-axis propagating (ZY) LiNbO3 with the highest sensitivity to chemical composition changes [0.0253Li2Omol%∕(m∕s)], and also VLSAW for Y-cut X-axis propagating (YX) LiNbO3 [0.0464Li2Omol%∕(m∕s)] that was advantageous for detailed evaluation of distributions in the chemical composition along the pulling direction as well as the diameter direction.The congruent composition was estimated...

Fabrication of 90° Domain Structures in KNbO 3 Single Crystals

New domain (90° domain) structure in the KNbO 3 single crystal could be fabricated by applying electric field at room temperature. The coercive electric field of 90° domain is very low (140 V/mm) compared with that of 180° domain (250 V/mm). A very sharp boundary was observed across the whole sample thickness (2 mm). Fabricated 90° domain structures were also confirmed by the (101) surface profile measurement using the optical interferrometer and two peaks of the 404 X-ray diffraction analysis. We also demonstrated the fabrication of periodically 90° domain structures with 200 w m period without generating unwanted domain structures. As 90° domain structure feels the difference of the refractive index, new applications other than 180° domain will be possible.

Homogeneity Evaluation of LiNbO3 and LiTaO3 Single Crystals for Determining Acoustical Physical Constants.

The homogeneity of surface acoustic wave (SAW) grade LiNbO3 and LiTaO3 single crystals, which are used to accurately measure acoustical physical constants, is evaluated using several specimens, by measuring the leaky surface acoustic wave (LSAW) velocity, longitudinal velocity, density, lattice constant, and Curie temperature. The specimens are taken perpendicular to the pulling-axis direction of each crystal. The variations of LSAW and longitudinal velocities along the pulling-axis direction are at most 1 m/s, and the corresponding changes of chemical composition ratio are estimated to be about 0.02 Li2O-mol% for both LiNbO3 and LiTaO3 crystals. The evaluation proves that the crystals employed can be regarded as homogeneous within the measurement accuracy, when the effect of the variations in the crystals on the constant determination is investigated.

Low noise CW 355 nm generation by simple single pass configuration with PPMgSLTs from fiber based laser

Low noise CW 355nm with more than 50 mW was generated by frequency conversion of 1064 nm fiber based laser with simple cascaded single pass configuration using two PPMgSLTs.

Wide Acceptance Angle of Second-Harmonic Green Generation by Periodically Poled Potassium Niobate

Periodically poled KNbO3 (PPKN) with a period of 40 µm were prepared to demonstrate the second-harmonic generation of green (532 nm). The propagation direction was set to be the b axis in order to use large nonlinear coefficient d31, and the embryonic nucleation method was applied to form a spatially uniform domain inversion. The conversion efficiency and angle tolerance of PPKN were compared with those of conventional critical phase-matched KN (CPMKN). The angle θ on the b–c plane) acceptance of PPKN was 0.96 degcm, which was 38 times higher than that of CPMKN (0.025 degcm), and wide-angle tolerance was demonstrated. Its conversion efficiency was approximately 2 times higher than that of CPMKN.

Total Refraction of P-Polarized Light at the Boundary of 90°-Domains in the Ferroelectric Crystal

Nonlinear optical frequency conversion is one of the most important key techniques in order to obtain lasers with wavelengths targeted for specific applications. In order to realize efficient and tailored lasers, the quasi-phase-matching (QPM) approach using periodically-poled ferroelectric crystals is getting increasingly important. Also understanding of damage mechanisms in nonlinear materials is necessary to be able to design reliable and well working lasers. This is especially true for high power application lasers, which is a rapidly growing field, where the damage problem normally is the ultimate limiting factor. In this thesis work, several promising novel ferroelectric materials have been investigated for nonlinear optical applications and the emphasis has been put on QPM devices consisting of periodically-poled structures. The materials were selected from three different types of ferroelectric materials: 1) MgO-doped stoichiometric LiNbO3 (MgO:SLN) and LiTaO3 (MgO:SLT), and non-doped stoichiometric LiTaO3 (SLT), 2) KTiOPO4 (KTP) and its isomorphs RbTiOPO4 (RTP), and 3) KNbO3 (KN). The focus in our investigations have been put on the spontaneous polarization switching phenomena, optimization of the periodic poling conditions, and the photochromic optical damage properties which were characterized by the help of blue light-induced infrared absorption (BLIIRA) measurements. With electrical studies of the spontaneous polarization switching, we were able to determine quantitatively, and compare, the coercive field values of different materials by applying triangularly shaped electric fields. We found that the values of the coercive fields depended on the increase rate of the applied electric field. The coercive field of KN was the lowest (less than 0.5 kV/mm) followed by the ones of KTP, SLT, and MgO:SLT (1.5 to 2.5 kV/mm). MgO:SLN, and RTP had relatively high coercive fields, approximately 5.0 to 6.0 kV/mm, respectively. Based on the domain switching characteristics we found, we successfully fabricated periodically-poled devices in all of the investigated materials with 30 μm periodicities and sample thickness of 1 mm. Blue light-induced infrared absorption (BLIIRA) has been characterized for unpoled bulk and periodically-poled samples using a high-sensitivity, thermal-lens spectroscopy technique. SLT showed a large photorefraction effect and the BLIIRA signal could not be properly measured because of the large distortion of the probe beam. The rise and relaxation time of BLIIRA, after switching the blue light on and off was in a time span of 10 to 30 sec except for KTP and its isomorphs, which needed minutes to hours in order to saturate at a fixed value. KN and MgO:SLN showed the lowest susceptibility to the induced absorption. Periodic poling slightly increased the susceptibility of KTP, MgO:SLT, and KN. Relatively high thresholds were observed in MgO:SLT and KN. By increasing the peak-power intensity of the blue light, the induced absorption for MgO:SLN, KTP and KN saturated at a constant value while that of MgO:SLT increase in a constant fashion. This trend is critical issue for the device reliability at high-power applications.

Compact Integrated Folded Šolc Filter using Periodic 90°-Domain Structures in KNbO3 Single Crystals

A compact integrated folded Solc filter using periodic 90°-domain structures in KNbO3 single crystals is proposed. The periodic 90°-domain structures with a period of 200 µm are fabricated on 0.7-mm-thick (101)-cut KNbO3 single-crystal plates by applying an electric field of 200 V/mm. Using the refractive index change between 90°-domains, we can construct the same arrangement for a folded Solc filter. The measured filtering properties agree well with the calculated ones. This is the first demonstration of an optical filter using periodic domain structures other than 180°-domain structures. It is now possible to fabricate a compact folded Solc filter without inconvenient alignment.