Transformation of Sign of Nonlinear Refraction between Mo（W）/S/Cu Planar Metal Clusters

Optical nonlinear refractive properties of a series of Mo（W）/S/Cu planar square clusters are investigated using the Z-scan technique with the ns laser pulses at the wavelength of 532 nm. The result shows that the planar metal clusters containing the halogen ligands demonstrate the self-focusing effect, and the other planar metal dusters demonstrate the self-defocusing effect. These facts indicate that the halogen ligands can act as crucial factors in determining the sign of the nonlinear refraction of the Mo（W）/S/Cu planar metal clusters. The analysis of the experimental data shows that the planar clusters with halogen ligands possess greater refraction volume of the excited state than that of the ground state, while the other planar clusters possess the smaller refraction volume of the excited state than that of the ground state.     

Energy Squeeze of Ultrashort Light Pulse by Kerr Nonlinear Photonic Crystals

Self-phase modulation can efficiently shape the spectrum of an optical pulse propagating along an optical material with Kerr nonlinearity. In this work we show that a one-dimensional Kerr nonlinear photonic crystal can impose anomalous spectrum modulation to a high-power ultrashort light pulse. The spectrum component at the photonic band gap edge can be one order of magnitude enhanced in addition to the ordinary spectrum broadening due to self-phase modulation. The enhancement is strictly pinned at the band gap edge by changing the sample length, the intensity or central wavelength of the incident pulse. The phenomenon is attributed to band gap induced enhancement of light-matter interaction.     

Nonlinear optical properties of (B3O7)5- and (B3O6)3- groups

3 O₅ (LBO) crystal has been studied by using the bond valence theory of complex crystals. Chemical bond parameters and linear and nonlinear optical (NLO) properties of each type of constituent chemical bonds are quantitatively determined. Because of the different crystal structure characteristics of LBO from those of β-BaB₂O₄ (BBO), the two anionic groups, (B₃O₇)^5- in LBO and (B₃O₆)^3- in BBO, play different roles in contributions to their own total NLO tensor coefficients of LBO and BBO, respectively. By comparison, we find that planar (B₃O₆)^3- groups are the ideal structure model, leading to little cancellation of contributions of each kind of bond in these groups, and this gives us a useful guide to design new NLO materials in the future. Received: 24 January 1997/Accepted: 27 March 1997     

Non-steady-state photocurrents in SnS2 crystals

We report on non-steady-state photocurrent measurements of SnS₂ grown by vapor transport method. The main characteristics of photoinduced carriers, such as type of photoconductivity, average diffusion length, dielectric relaxation time, and the Debye screening length, were estimated for the excitation wavelengths in the red and blue region of the spectrum. Received: 12 November 1998 / Revised version: 22 December 1998 / Published online: 31 March 1999     

Nonlinear refraction of undoped and Fe-doped KTiOAsO4 crystals in the femtosecond regime

The third-order optical nonlinearities in undoped and Fe-doped KTA crystals have been measured using the Z-scan technique with femtosecond pulses at 780-nm wavelength. The nonlinear refractive index is determined to be 1.7×10⁻¹⁵ cm²/W and 0,9×10⁻¹⁵ cm²/W for undoped and Fe-doped KTA, respectively. No two-photon absorption occurs in these crystals. It is shown that doping with Fe₂O₃ could weaken refractive nonlinearity of KTA, suggesting that Fe:KTA will improve the performance of KTA in high-intensity femtosecond laser applications. In addition, the measured nonlinear index of refraction in KTA crystals is about five times lower than that predicted by the two-band theory. One of the reasons for the discrepancy is given as the applicable limit of the simple theory in which a two-parabolic band model has been assumed in the analysis. Received: 3 June 1999 / Published online: 20 October 1999     

Studies of impurity levels in Rh-doped and Ce-doped photorefractive BaTiO3

Impurity levels, light-induced and thermo-induced charge transfer processes in Rh-doped and Ce-doped photorefractive BaTiO₃ were studied. The thermal depths and optical transitions of these impurity levels in the crystals were determined by using such methods as light-induced absorption, thermo-induced absorption, and grating dark decay. The origins of these impurity levels were discussed. We demonstrated, for the first time to our knowledge, that thermo-induced absorption spectroscopy is a useful and complementary technique for investigating the impurity levels in photorefractive crystals. With this technique, the deep impurity level generated by Ce in BaTiO₃:Ce was revealed and identified. Received: 7 July 1999 / Revised version: 24 September 19999 / Published online: 2 February 2000     

Dynamic properties of multiple grating formation in doped and thermally treated lead germanate

During holographic recording in lead germanate (Pb₅Ge₃O₁₁) crystals two types of refractive-index gratings are observed. One has a very fast response whereas the second builds up comparably slowly. Measurements of diffraction efficiency and two-beam coupling are carried out to study the formation of both gratings and to obtain the relative phase between them. Differently doped and thermally treated samples are divided into four classes due to their different time evolution of diffraction efficiency and of the energy transfer direction during two-beam coupling. The classification depends on doping and treatment. For Ni-doped and thermally treated samples dark and photo conductivities corresponding to the slow grating are determined, indicating that Ni-doping combined with oxidation enhances the properties of the slow grating. Received: 18 November 1998 / Published online: 7 April 1999     

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     

Third-Order Optical Nonlinearity in Novel Porphyrin Dimers

We investigate the third-order optical nonlinearities in four novel porphyrin dimers （dimers A to D） and a monomeric porphyrin H2 CPTPP measured by using the single-beam z-scan technique with a pulsed Q-switched Nd：YAG nanosecond laser at 532nm. All the samples show strong excited state absorption （ESA） and high value of Х^（3） in the ns domain at this wavelength. We perform a comparison between dimer A and its monomer H2 CPTPP in their third-order optical nonlinearity, and discuss the relationships between the values of Х^（3） and the different bridging groups for all the dimers.     

Holographic and photoelectric characterization of a novel photorefractive organic glass

We present a detailed study of the photoelectric as well as the holographic properties of a novel organic photorefractive glass based on triphenylamine. We studied the quantum efficiency Φ of the photogeneration of charges by means of photoinduced discharge measurements. The photoconductivity σ and the charge carrier mobility μ were obtained via dc photoconduction and pulsed time-of-flight experiments, respectively. The holographic characterization was performed by two-wave and degenerate four-wave mixing experiments allowing for the determination of properties such as diffraction efficiency η, modulation of the refractive index Δn, gain coefficient Γ, and phase-shift φ_p of the investigated system. The experimental data for Φ could be successfully described by the Onsager formalism with a thermalization radius of r₀=24 ? and a primary quantum yield of Φ₀=40%. We evaluated the E field and temperature-dependent measurements of μ using the B?ssler formalism yielding a width of the density of states of σ=0.13 eV and a disorder parameter Σ=3.6. On this basis the lifetime and the average drift length of the charge carriers could be estimated from the dc photoconduction experiments. From the photoelectric measurements we also calculated the holographic response time that matched very well to the measured response time and described the E-field dependence satisfactorily. The presented photorefractive system shows outstanding optical properties and stability with respect to degradation. We measured a gain coefficient of Γ=90 cm^-1, and a diffraction efficiency of η=27% at a response time of 30 ms for only 40-μm-thick samples. Orientational enhancement was observed and evaluated quantitatively. To our knowledge, this work presents the first determination of each of the above quantities all in one single organic photorefractive material. Received: 16 November 1998 / Revised version: 4 January 1999 / Published online: 31 March 1999     

Electric-field enhancement of beam coupling in Sn2P2S6

The influence of an external field on photorefractive recording in Sn₂P₂S₆ (SPS) crystals is studied. A large gain factor of more then 15 cm^-1 is achieved for a grating spacing of 12 μm at λ=0.9 μm. For an applied field exceeding ±200 V/cm a switching of the beam coupling direction is detected, exhibiting a pronounced hysteresis. Received: 25 October 2000 / Revised version: 18 January 2001 / Published online: 21 March 2001     

Simultaneous and sequential power transfer between coherent beams by photorefractive coupling

The power of several coherent laser beams can be transferred into a single diffraction-limited signal beam by photorefractive coupling. The efficiency of the power transfer is investigated experimentally in the case of two pump beams. Two different geometrical arrangements, one with overlapping (simultaneous) and the other with separate (sequential) pump beams are compared in a BaTiO₃:Ce crystal, with diffusion-driven photorefraction. We measured about the same power transfer efficiency in both arrangements for strong signal beams, but the efficiency was higher in the sequential arrangement for weak signals. A simple theoretical model of the power transfer process based on the standard linear two-wave-mixing theory is presented and the observed stationary beam-coupling behavior for different pump-to-signal intensity ratios is found to be in qualitative agreement with the theoretical predictions. Received: 10 November 1998 / Revised version: 22 December 1998 / Published online: 24 March 1999     

Reflection-type photorefractive gratings in tin hypotiodiphosphate

Reflection photorefractive gratings recorded by nearly counterpropagating light waves in the near infrared are studied in tin hypothiodiphosphate. The ratios are established for certain electrooptic tensor components responsible for reflection grating recording, and the Debye screening length is evaluated. Reflection holograms of binary objects are recorded. Received: 26 June 2001 / Revised version: 3 September 2001 / Published online: 29 November 2001     

Nonlinearity of a response in photorefractive crystals with a square-wave applied field

The space-charge-field formation in a photorefractive crystal illuminated by a one-dimensional light pattern is studied when an alternating square-wave electric field is applied to the crystal. We derive the general nonlinear equation for the time-averaged distribution of the space-charge field and employ the simplified versions of this equation to analyze the space-charge-field formation in the case of a Gaussian beam and interference light pattern. The borders of the applicability of simplified equations and analytic expressions to describe a self-action of light beams and the large modulation effects under two-beam coupling are estimated from the numerical calculations. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Different temperature dependences of photorefractive parameters of Ce-doped and Rh-doped BaTiO3

eff and the electro-optic coefficient r₃₃ in Ce-doped and Rh-doped BaTiO₃ were determined by two-beam coupling measurements. It was found that the effective trap density N_eff of BaTiO₃:Ce increases whereas that of BaTiO₃:Rh decreases with increasing temperature. The electro-optic coefficient r₃₃ of both crystals increases with temperature. The photorefractive response times were also measured and found to decrease with different rates as temperature increases in the two crystals. The results were discussed by using the two-centre model for BaTiO₃:Ce and three-charge-state model for BaTiO₃:Rh. We found that the different temperature dependence of N_eff in the two crystals was due to the fact that the deep- and shallow-trap levels in BaTiO₃:Ce are caused by different impurity centres whereas those in BaTiO₃:Rh are caused by different charge states of the same impurity centre. Received: 16 July 1998/Revised version: 26 October 1998     

Photorefractive properties and self-pumped phase conjugation in a series of BaTiO3 crystals

This paper presents the experimental results of two-wave mixing and self-pumped phase conjugation (SPPC) in barium titanate (BaTiO₃) crystals doped with Rh or Ce. It has been shown that the main parameters of SPPC depend very strongly on the concentrations of dopants, the incident angles and the crystal cut used. High gain in two-wave mixing corresponds to low phase-conjugate reflectivity in all samples. It is impossible to maintain the highest reflectivity and gain simultaneously, because the gain will definitely be limited when we try to get the highest reflectivity, and vice versa. Some qualitative explanation of the experimental results is also given. Received: 5 January 2001 / Final version: 25 May 2001 / Published online: 19 September 2001     

Optical beams in lossy non-local Kerr media

It is discussed that optical beams propagate in non-local Kerr medium waveguides with losses. A variational principle is carried out for the 1 + 1-D non-local non-linear Schrödinger equation in the presence of the losses. In the strongly non-local case, the approximate analytical solutions are obtained. The lossy soliton solution shows that, Unlike its local counterpart, such lossy strongly non-local soliton does not possess the adiabatic property anymore. In addition, the general approximate results for non-soliton cases are gained. The comparisons between our approximate analytic solutions and numerical simulations confirm our variational approximate solutions.     

Chirp control in third-harmonic generation due to cross-phase modulation

Received: 16 October 1997/Revised version: 16 January 1998     

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