Synchronous pulsed infrared detection in AgGaSe2 crystal using 1.318 μm pump

2 laser by nonlinear upconversion, is reported for the first time in AgGaSe₂ crystal under near noncritical phase-matching, pumped by an electro-optically Q-switched Nd:YAG laser operated at 1.318 μm. The capability for low-level infrared signal detection at room temperature with a fast response was also studied. Received: 15 December 1996/Revised version: 20 February 1997     

Reduced space-charge fields in near-stoichiometric LiTaO3 for blue, violet, and near-ultraviolet light beams

The wavelength dependence of the space-charge field as induced in near-stoichiometric LiTaO₃ crystals is investigated by measuring the bulk photogalvanic effect and the photoconductivity. LiTaO₃ crystals of composition nearer to stoichiometry exhibit a pronounced reduction of the saturated space-charge field, which results from a larger increase in the photoconductivity relative to the photogalvanic effect. Especially, at near-UV wavelengths, the saturated space-charge field of the order of 0.1 kV/cm reaches a minimum that is two orders of magnitude lower than typical values observed in undoped near-stoichiometric LiNbO₃. Received: 11 July 2002 / Revised version: 2 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +41-1/633-1056, E-mail: mojca@fiz.uni-lj.si     

Analysis of THz-wave surface-emitted difference-frequency generation in periodically poled lithium niobate waveguide

It was shown that the periodically poled LiNbO₃-waveguide with period of poling λ≈λ/n_g (λ is the wavelength of emitted THz-wave, n_g is a refractive index corresponding to optical group velocity) emits THz-wave difference-frequency generation (DFG) in the direction normal to the surface of the planar waveguide. The 5% distinction between the manufactured and required periods of gratings results only in a small deflection (∼6°) of the output THz-beam from the normal direction. The dependence of DFG efficiency on mode size is analyzed. The output THz power at λ=150 μm is estimated as 2 mW, taking into account imperfections in coupling incident beams with guided modes. It was shown that the efficiency of THz-wave DFG in surface-emitting geometry is more than for collinear geometry in bulk crystal, especially in the high-absorption wavelength region. Received: 16 May 2001 / Revised version: 13 August 2001 / Published online: 2 November 2001     

X-ray-induced photochromic effects in copper-doped lithium niobate crystals

X-Ray-induced absorption changes in Cu-doped lithium niobate crystals (LiNbO₃) are investigated. All induced photochromic effects are fully reversible, i.e., illumination with white light annihilates the absorption changes. With increasing doping level, a decrease in the saturation value of the absorption changes is found. Changes of the Li concentration of the crystal to a more stoichiometric composition via Li indiffusion further decreases the photochromic effect. For sufficient X-ray doses, the saturation value is independent of the radiation energy in the measured range from 50–150 kV. The spectral distribution of the X-ray-induced absorption changes and the slow dark decay indicate that the additional absorption results from an increase of the Cu⁺ concentration. Received: 20 December 2000 / Revised version: 24 January 2001 / Published online: 27 April 2001     

Dark conductivity in copper-doped lithium niobate crystals

The impact of protons and copper on the dark conductivity of copper-doped photorefractive lithium niobate crystals is investigated by observation of the dark decay of holograms. The dark conductivity is significantly smaller than that of iron-doped lithium niobate. A non-mono-exponential hologram decay is observed, which needs to be considered for multiplexing of holograms. Received: 25 March 2002 / Published online: 2 May 2002     

Electrooptic properties of lithium niobate crystals for extremely high external electric fields

The electrooptic effect in lithium niobate crystals (LiNbO₃) for extremely high externally applied electric fields of up to 65 kV/mm is investigated. Homogeneous electrooptic refractive-index changes of up to 4.8×10^-3 are found for ordinarily polarized light. No quadratic electrooptic effect is observed. An upper bound for the quadratic electrooptic coefficient of |s₁₃|≤2.3×10^-21 m²/V² is determined. Electrooptic, angular, and wavelength tuning of the Bragg condition of a thermally fixed hologram are demonstrated. Received: 29 October 2002 / Revised version: 14 January 2003 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. E-mail: ml@uni-bonn.de     

Poling dynamics of lithium niobate crystals

Ferroelectric domain reversal via electric field poling of congruently melting lithium niobate (LiNbO₃) crystals is investigated. An electro-optic interferometric observation technique reveals spatial and temporal dynamics of the poling process. Starting from seeds, the domains grow until the entire crystal has a switched polarization. During the switching process the boundaries are preferentially aligned along the crystallographic axes. The coercive field between two sequenced domain inversions is transiently reduced after a poling event, and recovers exponentially with a time constant of about half a minute. No light-induced change of the recovery time constant, neither with green nor with ultraviolet light, is observed. The results are of relevance for domain engineering of LiNbO₃ crystals. Received: 6 February 2003 / Published online: 9 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-228/734038, E-mail: wengler@physik.uni-bonn.de     

Reflection holograms in iron-doped lithium niobate

Received: 7 November 1996/Revised version: 16 December 1996     

Role of stoichiometric point defect in electric-field-poling lithium niobate

Non-congruent LiNbO₃ crystals has been used to determine the influence of the intrinsic defect density in the ferroelectric domain inversion mechanism. The poling processes have been carried out either at high temperature or at RT throughout the electric-field-poling technique. It is shown that the coercive field of LiNbO₃ crystals is strongly influenced by the intrinsic defect density in the crystals. Moreover it is shown that it is possible to realign the ferroelectric domain structure of near-stoichiometric LiNbO₃ crystals to prepare periodic poled structures which allows second harmonic generation at shorter wavelengths than with congruent crystals at a fixed wavelength. Received: 6 July 1999 / Accepted: 24 November 1999 / Published online: 24 March 2000     

Influence of intrinsic defects on light-induced changes in the refractive index of lithium niobate crystals

3 crystals at continuous-wave illumination are reported. The investigation clearly indicates an increase in the photorefraction amplitude Δn_s (at saturation) with light intensity J for all crystals studied. The main result is a correlation between ∂(Δn_s)/∂J and the density N of intrinsic defects such as Nb_Li. This effect is independent of the accidental iron contamination. Therefore at low J, Δn_s decreases with an increase in N, whereas at high J the opposite dependence is observed. The threshold intensity value of such a transformation correlates with the optical quality of the crystal. It has been established that these photorefraction changes are connected with the light induced absorption of the secondary centers, caused by the existence of the metastable small polarons at room temperature during illumination. Received: 7 January 1997/Revised version: 7 February 1997     

Photorefractive properties of highly-doped lithium niobate crystals in the visible and near-infrared

Light-induced refractive-index changes, bulk-photovoltaic current densities, and photoconductivities of photorefractive iron-doped lithium niobate crystals (iron concentrations between 0.02 and 0.17 wt. %) are investigated in detail using visible and near-infrared light. It turns out that the one-center model predicts the material performance correctly for small iron concentrations (c_Fe<0.06 wt. % Fe₂O₃), only. A strong increase of the photoconductivity for higher doping levels limits the space-charge fields. Refractive-index changes up to 7×10^-4 for green and 2.8×10^-4 for near-infrared ordinarily polarized light are obtained. The corresponding hologram multiplexing numbers are 11 for green and 5 for near-infrared light. Received: 18 November 1998 / Revised version: 8 January 1999 / Published online: 7 April 1999     

The dielectric behaviour of doped near-stoichiometric lithium niobate in the terahertz range

The dielectric properties of near-stoichiometric LiNbO₃:Fe and LiNbO₃:Ce single crystals have been investigated using terahertz time domain spectroscopy in a frequency range of 0.7-1.6 THz at room temperature. When coupled with an applied external optical field, obvious photorefractive effects were observed, resulting in a modulation of the complex dielectric constant for the crystals. The variation in refractive index, |Δn|, had a linear relationship with the applied light intensity, accompanied by a step-like decrease at high intensity. The findings were attributed to the internal space charge field of the photorefraction and the light-induced domain reversal in the crystals.     

Compositional characterisation of Zn-diffused lithium niobate waveguides

Rutherford backscattering (RBS) and secondary-ion mass spectrometry techniques have been used to investigate the two-step process involved during waveguide fabrication in LiNbO₃ using Zn-vapour diffusion. Compositional analysis (O, Nb, Li and Zn) in the two steps has been characterised. RBS analysis reveals that the first step, involving a heating of the substrate under a metallic Zn atmosphere, gives rise to a partial exchange between the Nb and Li ions from the crystals and the Zn from the vapour source. The second treatment at higher temperature in an open atmosphere diffuses the Zn deeper into the substrate, thus forming an optical waveguide, while the Nb and Li ions recover their bulk values. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

High optical gain using counterpropagating beams in iron and terbium-doped photorefractive lithium niobate

Exceptionally high values of photorefractive gain coefficient of up to 100 cm^-1 have been observed in Fe-doped and Fe/Tb-doped crystals of photorefractive lithium niobate. It is believed that these are the highest observed coefficients of any crystalline photorefractive medium. Accurate measurements of gain coefficient have been possible for the first time by using a specially cut triangular crystal which allows the use of short interaction lengths and the complete elimination of surface reflections by having the beams incident at Brewster’s angle. Experimental results are consistent with a simple model of photorefractive beam coupling up until the onset of noise which is observed to deplete the pump for interaction lengths longer than 1 mm. Received: 28 September 1998 / Revised version: 8 January 1999 / Published online: 31 March 1999     

Etch frustration in congruent lithium niobate single crystals induced by femtosecond ultraviolet laser irradiation

Modification of the chemical etching behaviour of undoped congruent z-cut lithium niobate single crystals is achieved by pre-illumination of the –z face of the crystal with sub-ps ultraviolet laser radiation at 248 nm, at energy fluences below the threshold for ablation. A systematic study of the effect of the energy fluence and total exposure on the etch frustration is presented. Received: 23 July 2001 / Accepted: 28 September 2001 / Published online: 20 December 2001     

Preparation and characterization of lithium niobate as a novel photocatalyst in hydrogen generation

Several chemical compounds based lithium niobate have been tested in the reaction for the photocatalytic hydrogen generation. The photocatalysts have been prepared by impregnation of Nb₂O₅ in the aqueous solution of lithium hydroxide and then the calcination at the temperature range of 400-650 °C. In this report, we present the interesting study showing that the most active catalyst for the photocatalytic generation of hydrogen is the one containing two lithium niobate phases such as LiNbO₃ and LiNb₃O₈. It means that the lithium niobates based catalyst without any further modification or doping can be applied as a novel material for this process.     

Detection of hydrogen isotope atoms by mass spectrometry combined with resonant ionization

We examined the application of mass spectrometric methods using resonant ionization by a tunable laser and proposed its use for analyzing hydrogen isotopes. We conducted resonance ionization mass spectrometry (RIMS) to detect gas-phase hydrogen isotope atoms. The ionization efficiency was increased by more than 1000 times that obtained with conventional methods using nonresonant ionization. Resonant laser ablation mass spectrometry (RLAMS) was applied for deuterium detection in solid samples. A graphite substrate implanted with deuterium was used for ordinary laser ablation mass spectrometry (LAMS) and RLAMS. The deuterium signal was observed very clearly by RLAMS, in contrast to LAMS. Mass spectrometry combined with resonance ionization was very useful for hydrogen isotope detection, because components with equal mass numbers were resolved and the method demonstrated higher ionization efficiency. Received: 4 November 1998 / Revised version: 12 January 1999 / Published online: 7 April 1999     

Collinear broadband optical parametric generation in periodically poled lithium niobate crystals by group velocity matching

Collinear broadband optical parametric generation (OPG) using periodically poled lithium niobate (PPLN) crystals were designed and experimentally demonstrated with the quasi-phase matching (QPM) periods of 21.5, 24.0, and 27.0 μm. The broad gain bandwidth was accomplished by choosing a specific set of the period and the pump wavelength that allows the group velocities of the signal and the idler to match close to the degeneracy point. OPG gain bandwidth and also the spectral region could be controlled by proper design of QPM period and pump wavelength. The total OPG gain bandwidth of 600, 900, and 1200 nm was observed for the PPLN devices with QPM periods of 21.5, 24.0, and 27.0 μm, respectively. We have also observed multiple color visible generation whenever the OPG spectrum was significantly broad. From the visible peaks of the three PPLN samples, it is found that broad gain bandwidth is crucial in the temperature-insensitive collinear simultaneous RGB generation from a single crystal.     

Photorefractive amplification and generation of light beams in BaTiO3 controlled by the intensity of the pump light

3 by photorefractive forward four-wave mixing. The intensity of a signal beam is amplified by a factor up to 3000. We show that heating of the crystal by absorption of the pump light influences the gain because of the thermo-optic effect. Thus both, amplification and generation can be controlled by the intensities of the pump beams. Received: 23 January 1998     

Polaron luminescence in iron-doped lithium niobate

Photoluminescence related to the bound polaron Nb_Li⁴⁺ is investigated as a function of temperature and incident light intensity in iron-doped lithium niobate crystals with various iron concentrations. Experiments are done under constant-wave (CW) and pulsed illumination. Its found that the decay time is always monoexponential. The radiative lifetime, the activation energy of the nonradiative lifetime and the quenching temperature are only weakly sensitive to iron concentration. On the other hand, the magnitude of the photoluminescence signal seems strongly correlated to the Fe²⁺ concentration, and the superlinear regime evidenced at low CW illumination definitely confirms that polaron excitation in lithium niobate is a two-step process.