| 1. |
- Canalias, Carlota, et al.
(author)
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Polarization switching characteristics of flux grown KTiOPO4 and RbTiOPO4 at room temperature
- 2005
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 97:12, s. 124105-
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Journal article (peer-reviewed)abstract
- A study of polarization-switching characteristics under an applied electrical field at room temperature is presented for flux-grown KTiOPO4 and RbTiOPO4. By optimizing the experimental conditions, we determined the coercive field and the domain-switching time quantitatively by direct observation of the switching current. For both isomorphs, the inverse of the polarization-switching time, 1/t(s), follows an exponential dependence on the applied field E in low-field regime, and a linear dependence on E in the high-field regime. Domain morphology of KTiOPO4 based on selective etching reveals laminar structures elongated in the b crystallographic direction. An estimation of the domain-wall velocity shows that the domain speed in the polar direction is, at least, two orders of magnitude larger than in the a-b plane. The velocity along the b direction is similar to 30 times larger than along the a axis.
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| 2. |
- Hirohashi, Junji, et al.
(author)
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Blue light-induced infrared absorption in bulk and periodically poled ferroelectrics
- 2006
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In: Conf. Lasers Electro-Opt. Quantum Electron. Laser Sci. Conf., CLEO/QELS. - 1557528136 - 9781557528131
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Conference paper (peer-reviewed)abstract
- Blue light-induced infrared absorption is investigated in bulk and periodically poled KTiOPO4, MgO:stoichiometric-LiNbO3, MgO:stoichiometric-LiTaO3 KNbO3. KNbO3 and MgO:stoichiometric-LiNbO3 show the lowest susceptibility to the induced absorption. Periodic poling increases the susceptibility in KTiOPO 4 and MgO:stoichiometric-LiTaO3.
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| 3. |
- Hirohashi, Junji, 1975-
(author)
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Characterization of domain switching and optical damage properties in ferroelectrics
- 2006
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Doctoral thesis (other academic/artistic)abstract
- 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.
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| 4. |
- Hirohashi, Junji, et al.
(author)
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Controllability of specific domain structures in KNbO3 single crystals by electric poling at room temperature
- 2005
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 98:034107, s. 034107-
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Journal article (peer-reviewed)abstract
- 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 the direction corresponding to the differetial direction between the original and intended spontaneous polarization directions (differential vector poling method), 60°-, 90°-, and 180°-domain pairs were successfully fabricated under control in KNbO3. The measured threshold electric field for 90°-domain pairs was the lowest (140 V/mm), which could be related to the difference between the piezoelectric coefficients of the poling direction. For 60°- and 90°-domain pairs, theoretical uncharged domain walls fabricated under control without the generation of other domain structures. For 120°-domain pairs, (011)pc uncharged domain wall was not observed, which suggests the difficulty in generating uncharged 120°-domain walls without passing through the phase transition.
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| 5. |
- Hirohashi, Junji, et al.
(author)
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Embryonic nucleation method for fabrication of uniform periodically poled structures in potassium niobate for wavelength conversion devices
- 2004
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In: Japanese journal of applied physics. - 0021-4922. ; 43:2, s. 559-566
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Journal article (peer-reviewed)abstract
- A new electric poling concept called the 'embryonic nucleation method' for the fabrication of periodically poled structures has been proposed. By applying this method to KNbO3, 1-mm-thick uniform periodically poled KNbO3 (PPKN) with domain inverted period of 35.5 mum and an interaction length of 10 mm has been successfully fabricated by applying only 300 V/mm without the generation of unwanted domains. Furthermore, the reproducibility of the periodic poling process has been greatly improved. SHG using PPKN fabricated by this method was demonstrated and 100 mW second-harmonic laser generation at a wavelength of 532 nm was obtained from 1 W pumping without photorefractive damage at approximately 40 degrees C.
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| 6. |
- Hirohashi, Junji
(author)
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Fabrication of 90-degree domain structures in KNbO3 single crystals
- 2003
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In: Ferroelectrics (Print). - 0015-0193 .- 1563-5112. ; 282, s. 29-36
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Journal article (peer-reviewed)abstract
- 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 μ 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.
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| 7. |
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| 8. |
- Hirohashi, Junji, et al.
(author)
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Picosecond blue light-induced infrared absorption in single-domain and periodically poled ferroelectrics
- 2007
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101:033105
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Journal article (peer-reviewed)abstract
- Picosecond blue-light-induced infrared absorption is investigated in bulk and periodically poled ferroelectrics known to have high photorefractive resistance (KTiOPO4, MgO-doped congruent LiNbO3, MgO-doped stoichiometric LiNbO3, MgO-doped stoichiometric LiTaO3, and KNbO3). KNbO3 and MgO-doped congruent and stoichiometric LiNbO3 show the lowest susceptibility to induced absorption. Periodic poling slightly increases the susceptibility to the induced absorption in all materials but most noticeably in KTiOPO4, MgO-doped stoichiometric LiTaO3, and KNbO3. Different dynamics of induced absorption are investigated. Relatively high thresholds for induced absorption were observed in MgO-doped stoichiometric LiTaO3 and KNbO3. By increasing the peak power intensity of blue light, the induced absorption for LiNbO3, KTiOPO4, and KNbO3 are saturated, while in MgO-doped stoichiometric LiTaO3, the induced absorption increases almost linearly with the blue peak intensity. The low susceptibility to induced absorption observed in KNbO3 corroborates well with the earlier proposed existence of very shallow traps close to the valence band.
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| 9. |
- Hirohashi, Junji, et al.
(author)
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Quasi-phase-matched frequency conversion in KNbO3 structures consisting of 90-degree ferroelectric domains
- 2005
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In: Applied physics. B, Lasers and optics (Print). - : Springer Science and Business Media LLC. - 0946-2171 .- 1432-0649. ; 81, s. 761-763
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Journal article (peer-reviewed)abstract
- Periodic 90° domain structure with 18.6 μm periodicity was fabricated by applying a 200 V/mm electric field over (1̄01)-cut KNbO3 plate. The blue light generation by quasi-phase-matched frequency doubling in this periodic 90° domain structure is demonstrated. The 90° domain structure completely compensates for a Poynting vector walk-off of the second-harmonic beam. Moreover, these structures exhibit amphoteric Poynting vector refraction properties, i.e., the refraction can be negative or positive depending on the angle of incidence at the 90° domain boundary.
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| 10. |
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