LiTaO3 Crystal Research Articles

Optical waveguides in LiTaO3 crystals fabricated by swift C5+ ion irradiation

We report on the optical waveguides, in both planar and ridge configurations, fabricated in LiTaO3 crystal by using carbon (C5+) ions irradiation at energy of 15MeV. The planar waveguide was produced by direct irradiation of swift C5+ ions, whilst the ridge waveguides were manufactured by using femtosecond laser ablation of the planar layer. The reconstructed refractive index profile of the planar waveguide has showed a barrier-shaped distribution, and the near-field waveguide mode intensity distribution was in good agreement with the calculated modal profile. After thermal annealing at 260°C in air, the propagation losses of both the planar and ridge waveguides were reduced to 10dB/cm.

Effect of electron focusing in x-ray sources using LiTaO3 crystals excited by neodymium-doped yttrium lithium fluoride laser light

The intensity of x-rays from a 15 mm-diameter metal target bombarded by electrons emitted from a LiTaO3 crystal is found to depend upon the distance, h, between the crystal and the target. The electron emission of the LiTaO3 crystal is generated by being excited with light from a neodymium-doped yttrium lithium fluoride laser. The x-ray intensity is highest for h = 6 mm, coinciding also with a minimum electron spot size. The coincidence of the minimum electron spot size and maximum x-ray intensity with the 15 mm-diameter target is a result of self-focusing of the electrons by the higher electric field at the edge of the cylindrical crystal. The maximum electron energy at h = 6 mm with 120 s of laser irradiation is 47 keV, which is estimated from the maximum energy of bremsstrahlung x-rays.

Growth of Near-Stoichiometric Lithium Tantalite Crystal and its Optical Characterization

Near-stoichiometric lithium tantalite (LiTaO3) crystals were grown by the top-seed solution pulling method in congruent melt with the addition of K2O. The crystals were characterized by measurement of the ultraviolet absorption edge, ability of photo-damage resistance, and the domain structure analysis. Compared with the congruent crystal shows that, the ultraviolet absorption edge of the near-stoichiometric crystals has a blue shift towards a shorter wavelength and curie temperature is also improved; the domain structure of near-stoichiometric LiTaO3 crystals is hexagonal; and the photo-damage resistance of the crystals is greatly improved.

Crystal growth, spectroscopic characterization, and laser operation of Tm/Mg: LiTaO_3 crystal

Tm, Mg co-doped LiTaO3 crystal is grown by the traditional Czochralski method. By analyzing the absorption and emission measurements of the Tm/Mg: LiTaO3 single crystal with the Judd-Ofelt analysis, the intensity parameters Ω2,4,6, transition probabilities, exited state lifetimes, branching ratios, and emission cross-sections were calculated. Non-photorefractive continuous wave laser operation with a Tm/Mg: LiTaO3 single crystal is demonstrated for the first time. We obtained 1.51 W output power at 1.92 μm with a slope efficiency of near 38.5%, which, to the best of our knowledge, are the largest output power and highest slope efficiency obtained for this crystal thus far. The long-term photorefractive effect was also quantitatively analyzed.

Continuous tuning of a narrow-band terahertz wave in periodically poled stoichiometric LiTaO3 crystal with a fan-out grating structure

Continuous tuning of terahertz waves was demonstrated with seamless change in grating period in a periodically poled stoichiometric lithium tantalite (SLT) crystal. The periodically poled fan-out structure allows wide tunability such as 610 GHz with the bandwidth as narrow as 17 GHz at a carrier frequency of 1.00 THz. Temperature-dependent measurements show a gradual intensity increase of the THz pulses as the temperature decreases. Furthermore, absorption and refractive index of SLT in the THz range were estimated and compared with those of LiNbO3 (LN). The absorption coefficient of the LT crystal at ordinary wave was almost half of that in the LN crystal. SLT could be one of the powerful crystals for high-power THz generation with large optical aperture because of the fewer defects inside the crystal.

Theoretical explanation of g factors for Ti3+ ions in LiNbO3 and LiTaO3 crystals

The EPR g factors g// and g⊥ for Ti3+ ions at the trigonal octahedral Li+ sites of LiNbO3 and LiTaO3 crystals are calculated from the third-order perturbation formulas of g factors for 3d1 ion in trigonal symmetry. In the calculations, the crystal-field parameters are obtained from the structural data by using the superposition model. The calculated values are in reasonable agreement with the observed values. The results are discussed.

Nonvolatile photorefractive properties in triply doped stoichiometric Mg:Fe:Mn:LiTaO3 crystals

We have grown triply doped Mg:Fe:Mn:LiTaO3 crystals with near stoichiometry using the top seeded solution growth technique. The defect structure was investigated by infrared absorption spectra and Curie temperature. Using a blue laser as the source, excellent photorefractive properties were obtained. Nonvolatile holographic storage properties were investigated using the dual wavelength technique. We got a very high fixed diffraction efficiency and nonvolatile holographic storage sensitivity. The blue light has more than enough energy to excite holes of deep (Mn) and shallow (Fe) trap centers with the same phase, which enhance dramatically the blue photorefractive properties and the nonvolatile holographic storage. Mg2+ ion is no longer damage resistant at blue laser, but enhances photorefractive characteristics.

Laser-induced breakdown and damage generation by nonlinear frequency conversion in ferroelectric crystals: Experiment and theory

Using our experimental data for ns pulsed second harmonic generation (SHG) by periodically poled stoichiometric LiTaO3 (PPSLT) crystals, we consider in detail the mechanism underlying laser-induced damage in ferroelectric crystals. This mechanism involves generation and heating of free electrons, providing an effective kinetic pathway for electric breakdown and crystal damage in ns pulsed operation via combined two-photon absorption (TPA) and induced pyroelectric field. In particular, a temperature increase in the lattice of ≈1 K induced initially by ns SHG and TPA at the rear of operating PPSLT crystal is found to induce a gradient of spontaneous polarization generating a pyroelectric field of ≈10 kV/cm, accelerating free electrons generated by TPA to an energy of ≈10 eV, followed by impact ionization and crystal damage. Under the damage threshold for ns operation, the impact ionization does not lead to the avalanche-like increase of free electron density, in contrast to the case of shorter ps and fs pulses. However, the total number of collisions by free electrons, ≈1018 cm−3 (generated during the pulse and accelerated to the energy of ≈10 eV), can produce widespread structural defects, which by entrapping electrons dramatically increase linear absorption for both harmonics in subsequent pulses, creating a positive feedback for crystal lattice heating, pyroelectric field and crystal damage. Under pulse repetition, defect generation starting from the rear of the crystal can propagate towards its center and front side producing damage tracks along the laser beam and stopping SHG. Theoretical analysis leads to numerical estimates and analytical approximation for the threshold laser fluence for onset of this damage mechanism, which agree well with our (i) experiments for the input 1064 nm radiation in 6.8 kHz pulsed SHG by PPSLT crystal, (ii) pulsed low frequency 532 nm radiation transmission experiments, and also (iii) with the data published for other nonlinear crystals and operated wavelengths.

Assembly and evaluation of a pyroelectric detector bonded to vertically aligned multiwalled carbon nanotubes over thin silicon

A novel pyroelectric detector consisting of a vertically aligned nanotube array on thin silicon (VANTA/Si) bonded to a 60 μm thick crystal of LiTaO₃ has been fabricated. The performance of the VANTA/Si-coated pyroelectric detector was evaluated using National Physical Laboratory's (NPL's) detector-characterization facilities. The relative spectral responsivity of the detector was found to be spectrally flat in the 0.8-24 μm wavelength range, in agreement with directional-hemispherical reflectance measurements of witness samples of the VANTA. The spatial uniformity of response of the test detector exhibited good uniformity, although the nonuniformity increased with increasing modulation frequency. The nonuniformity may be assigned either to the dimensions of the VANTA or the continuity of the bond between the VANTA/Si coating and the pyroelectric crystal substrate. The test detector exhibited a small superlinear response, which is similar to that of pyroelectric detectors coated with good quality gold-black coatings.

Femtosecond laser micromachined optical waveguides in LiTaO3 crystal

AbstractBy using femtosecond laser micromachining, optical wave guides in both depressed cladding and dual‐line configurations have been produced in LiTaO3 crystal. The guiding properties and the thermal stability have been investigated for both geometries, which exhibit different performance. Depressed cladding waveguides support guidance along both extraordinary and ordinary index polarizations, while dual‐line waveguides support only extraordinary index polarization. Thermal annealing has been proved to be an effective method to reduce the propagation losses. For the cladding waveguide, the lowest propagation loss was as low as 0.38 dB/cm after the annealing treatment at 400 °C. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical breakdown threshold in nanosecond high repetition second harmonic generation by periodically poled Mg-doped LiTaO3 crystal

Our study shows that a local temperature increase of ≈1 K in the crystal lattice caused by second harmonic generation (SHG) and two-photon absorption of 532 nm radiation at the rear of periodically poled stoichiometric LiTaO3 crystal by changing spontaneous polarization induces a pyroelectric field ≈10 kV/cm, accelerating free electrons to an energy of ≈10 eV, followed by optical breakdown and crystal damage. Theoretical analysis leads to an explicit expression for the threshold laser fluence of optical breakdown giving ≈1.2 J/cm2 for 1064 nm input radiation in 6.8 kHz pulsed SHG operation, agreeing well with the experimentally found value ≈1.32 J/cm2.

A high power, continuous-wave, single-frequency fiber amplifier at 1091 nm and frequency doubling to 545.5 nm

We present a high power single-frequency ytterbium fiber amplifier system with an output power of 30 W at 1091 nm. The amplifier system consists of two stages, a preamplifier stage in which amplified spontaneous emission is efficiently suppressed (>40 dB) and a high power amplifier with an efficiency of 52%. Two different approaches to frequency doubling are compared. We achieve 8.6 W at 545.5 nm by single-pass frequency doubling in a MgO-doped periodically poled stoichiometric LiTaO3 crystal and up to 19.3 W at 545.5 nm by frequency doubling with a lithium-triborate crystal in an external enhancement cavity.

Potentialities of an Electro-Optic Crystal Fed by Nuclear Magnetic Resonant Coil for Remote and Low-Invasive Magnetic Field Characterization

In this paper, we demonstrate the use of a LiTaO3 crystal associated with a typical nuclear magnetic resonant loop coil to perform an optically remote radio frequency magnetic-field characterization. The whole transduction scheme is theoretically and experimentally studied. The measurement dynamics reaches 60 dB. The minimum detectable magnetic field is lower than 1 nT, which corresponds to an induced inner crystal electric field as low as 30 mV/m. To evaluate the spatial potentialities of the sensor, a 1-D mapping of the field along an asymmetric butterfly-shaped loop coil is performed. The result is in good agreement with finite-difference time-domain simulations and demonstrates the vectorial behavior of the sensor device.

A Nd:MgO:LiTaO3 crystal as a gain medium for 1083 nm lasers

The growth, spectroscopic properties, laser parameters, and laser performance of a Nd3+/Mg2+ co-doped congruent LiTaO3 crystal were investigated. Continuous-wave (cw) laser operation at a wavelength of about 1083 nm with a slope efficiency of nearly 12% was achieved. The maximum cw output power obtained was 615 mW, which, to the best of our knowledge, is the highest output power obtained for this crystal thus far. We propose that Nd:MgO:LiTaO3 lasers could be an excellent source for the optical pumping of helium, and may achieve second harmonic generation and optical parametric oscillation in the same crystal.

High output power of differently cut Nd:MgO:LiTaO3 CW lasers

A high-quality Nd3+ and Mg2+ co-doped LiTaO3 (Nd:MgO:LT) crystal was grown by the Czochralski method. The polarized absorption spectra and fluorescence spectra were studied, and the absorption cross section was calculated by Judd–Ofelt (J–O) theory. The laser performance with different sample cuts of the crystal was investigated for the first time, and it was found that Nd:MgO:LT crystal with different cutting directions (a and c) exhibits different laser properties. By optimizing a partial reflectivity mirror in the laser experimental setting, a high continuous wave output power of 3.58 W was obtained at 1092 and 1076 nm with an optical-to-optical conversion efficiency of 22.78% and slope efficiency of 26.06%. The results indicate that Nd:MgO:LT crystal is a promising candidate for the manufacture of Nd3+ doped periodically poled MgO:LiTaO3 crystal (Nd:PPMgOLT), which should have considerable applications in self-frequency doubling and optical parametric oscillation laser devices.

Influence of Sc ions on optical properties of Ce:LiTaO3 crystals

Sc:Ce:LiTaO3 crystals were grown by using the Czochralski method. The two-wave coupling experiments and transmitted facula distortion method were used to measuring the photorefractive properties and the optical damage resistance, respectively. It was found that the photorefractive response speed of Sc:Ce:LiTaO3 was four times faster than that of Ce:LiTaO3 and the resistance ability to optical damage was two orders higher than that of Ce:LiTaO3. It could be attributed to the Ce ions losing their electron acceptor properties, which result in the increase of photoconductivity. In this paper, site occupation mechanism of impurities was also discussed to explain the increased of photoconductivity.

A lateral field excited (yxl)88° LiTaO3 bulk acoustic wave sensor with interdigital electrodes

In this work, to improve the sensitivity of lateral field excited (LFE) sensors to changes of liquid conductivity, LFE bulk acoustic wave sensors with interdigital electrodes are investigated. LFE bulk acoustic wave on thickness-shear mode is excited successfully by interdigital electrodes on (yxl)88° LiTaO3 and applied in liquid-phase sensor. The electric field direction of LFE (yxl)88° LiTaO3 plates is determined. Based on this, several LFE bulk acoustic wave sensors with interdigital electrodes are designed and fabricated to increase the sensitivity of LiTaO3 LFE sensors. The results show that the (yxl)88° LiTaO3 LFE sensor with interdigital electrodes is 10.9 times and 2.1 times more sensitive to changes in liquid conductivity compared to traditional LFE devices with single gap circular electrodes and Archimedes spiral electrodes, respectively. The results are important for investigating high-sensitivity LFE bulk acoustic wave sensors by using LiTaO3 crystal.

Nd:MgO:LiTaO3 crystal for self-doubling laser applications: growth, structure, thermal and laser properties

Nd3+ and Mg2+ co-doped LiTaO3 (Nd:MgO:LT) crystal has great potential for applications in self-frequency doubling (SFD) lasers, and self-optical parametric oscillation (SOPO) infrared lasers. However, a related high performance laser device has not yet been realized. No reliable data on the basic physical properties of Nd:MgO:LT have been obtained, because of the lack of high quality Nd:MgO:LT crystals available due to the great difficulty in growing this crystal. In this paper, by optimizing the growth parameters and the thermal field by means of simulation, a high quality Nd3+ and Mg2+ co-doped LiTaO3 crystal was grown by a modified Czochralski method. A comprehensive characterization of the crystal, including structure, composition, thermal properties, and CW laser performance, was performed. The experimental results indicate that Nd:MgO:LT is a monocrystal with strong anisotropic physical properties. The anisotropic thermal expansion was confirmed by observing the lattice parameter variation via temperature-dependent XRD analysis. Some important physical property data, such as thermal diffusivity, thermal conductivity and specific heat of the Nd:MgO:LT single crystal are reported for the first time, and should be of great utility for the application of this crystal. A CW laser output power of 3.58 W with an optical-to-optical conversion efficiency of 11.55, and a slope efficiency of 13.78%, was achieved with an incident pump power of 31 W, which is the highest laser output reported for this kind of crystal. It is demonstrated that single crystal Nd:MgO:LT will have important applications in SFD and SOPO lasers.

Growth, optical characterization and evaluation of laser properties of Yb3+, Mg2+:LiTaO3 crystal

High-quality Yb3+/Mg2+ co-doped LiTaO3 crystal was grown using the Czochralski technique for the first time. Absorption and fluorescence spectra, as well as fluorescence decay curve, were recorded at room temperature. The stimulated emission cross-sections were calculated using the reciprocity method and the Fuchtbauer–Ladenburg formula. The laser parameters and spectral features were investigated. Comparison with other Yb3+-doped laser crystals was made. The results show that the Yb3+/Mg2+ co-doped LiTaO3 crystal is a promising laser material for tunable and ultrashort laser generation.

The modification of LiTaO3 crystal by low-energy He-ion implantation

The effects of He-ion implantation on the surface morphology and transmittance of LiTaO3 single crystals are investigated. The samples were implanted with 250keV He-ion at different fluences at room temperature. The results show that the surface morphology and transmittance of implanted samples strongly depend on the ion fluence and the time when the samples expose to the air up to 60days. When the fluence is above 1.0×1016 He+/cm2, the transmission spectra indicate that a high concentration of defects is created. 3D-profile images show that at the higher fluence a great many triangular stripes appear on the surface of the samples. After 60days, the recovery of the transmittance occurs and varies with the fluence. For the sample at the fluence of 5.0×1016 He+/cm2, the raised stripes on the surface evolve into narrow cracks. Regional exfoliation, however, occurs on the surface of the sample with the fluence of 1.0×1017 He+/cm2. According to the experimental results and simulation of SRIM 2008 code, the evolvement of defects and the behavior of He are discussed.