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     

Theoretical and experimental aspects of optical storage of information via refreshing by inverse seeding in a photorefractive Ba0.77Ca0.23TiO3 crystal (BCT)

We present a new experimental setup for the optical storage of information via refreshing by inverse seeding (OSIRIS), which allows a sixfold increase of the storage time of holograms in a Ba_0.77Ca_0.23TiO₃ crystal (BCT). The setup consists of two four-wave mixing processes with common amplified signal waves and phase-conjugated (pc) waves. Temporal behaviours of the amplified and pc signal waves for the OSIRIS experiment as well as for the common four-wave mixing experiment are compared and discussed. The solutions of coupled equations under the depleted-pump approximation are obtained in order to estimate the pc reflectivities and coupling gains of gratings inside the crystal. Received: 13 December 2000 / Revised version: 26 January 2001 / Published online: 27 April 2001     

measurement of Third Order Hyperpolarizability of Some Dyes

A degenerate four-wave mixing experiment with the solutions of some dyes has been performed using 10ns pulses at 530nm. Large resonant and off-resonant third order hyperpolarizabilities were obtained.     

Degenerate four-wave mixing experiments in Methyl green dye-doped gelatin film

Degenerate four-wave mixing (DFWM) was performed in Methyl green dye-doped gelatin films using continuous-wave laser radiation () generated by a He-Ne laser of total power 35 mW. Various parameters which influence the phase-conjugate (PC) signal during the DFWM process were studied. The PC signal contributions from induced holographic transmission and reflection gratings were measured. We observed a maximum PC beam reflectivity of 0.13% in these dye-doped gelatin films.     

Tailoring the nonlinear refractive index of fluoride-phosphate glasses for laser applications

In this paper we report on measurements of the nonlinear refractive index n₂ at 1047 nm of various fluoride-phosphate laser glasses relative to fused silica using degenerate four-wave mixing. We find good agreement with empirical estimates obtained from the d-line linear refractive index and the Abbe number for pure fluoride-phosphate glasses. Significant increase of n₂ is observed when adding sulfate and niobium oxide to the glass composition offering tailorable nonlinear properties for glasses employed in short pulse laser oscillators. Received: 27 December 1999 / Published online: 24 March 2000     

Mirrorless coherent oscillation due to vectorial four-wave mixing in LiNbO3

Mirrorless coherent oscillation in a four-wave mixing arrangement with two phase-conjugate counterpropagating pump waves is studied in a LiNbO₃:Fe crystal. Oscillation waves are observed within a wide, but limited angular window where the gain factor takes its maximum value. The spatial structure and the intensities of particular oscillating components vary continuously in time.     

汞蒸汽中双光子共振四波和频产生125nm的相干辐射

利用汞原子6~1S_0—7~1S_0双光子共振四波和频产生了125nm的相干辐射.对双光子共振增强效应、双光子共振克尔(Kerr)效应进行了研究.讨论了平面波和聚焦情况下最佳相位匹配的条件.实验中还观测到了四波参量振荡和四波差频的信号输出.     

Mirrorless parametric oscillation in BaTiO3

We report on the first observation of mirrorless oscillation in BaTiO₃ pumped by two nearly counterpropagating mutually incoherent light waves, both containing an ordinary and an extraordinary component. The oscillation waves appear at a certain angle defined by the phase matching condition for four-wave mixing of orthogonally polarized waves.     

Performance of combined 〈1 0 0〉-〈1 1 0〉 ZnTe crystals in an amplified THz time-domain spectrometer

In this work we report on the performance of combined index-matched ZnTe 〈1 1 0〉-〈1 0 0〉 THz emitters and detectors for reduction of etalon effects in a nonlinear crystal-based THz time-domain spectrometer, driven by an amplified femtosecond laser. We demonstrate that the application of a combined electrooptic detector crystal reduces the strength of the first etalon and the average spectral dynamic range oscillations by a factor of two. We also show that even in a simple emitter configuration the emitter-related etalon is significantly reduced due to the transient dielectric effects of carriers created by two-photon absorption in the emitter.     

Coexisting Raman- and Rayleigh-Enhanced Four-Wave Mixing in Femtosecond Polarization Beats

Based on the polarization interference of Raman- and Rayleigh-enhanced four-wave mixing processes, heterodyne detection of the Raman, Rayleigh and coexisting Raman and Rayleigh femtosecond difference-frequency polarization beats is investigated in the cw and the three Markovian stochastic models, respectively. These two processes exhibit asymmetric and symmetric spectra, respectively, and the thermal effect in them can be suppressed by field-correlation method. Such studies of coexisting Raman- and Rayleigh-enhanced four-wave mixing processes can have important applications in coherence quantum control, and quantum information processing.     

Phase-matched light-induced scattering,mirrorless self-oscillation,and generation of nearly retropropagating waves in LiNbO3:Fe in “forbidden” interaction geometry

Two mutually incoherent pump waves impinging symmetrically on a LiNbO₃:Fe crystal from different faces in a plane normal to the C-axis produce chartacteristic patterns of light-induced scattering (lines and rings) and a pair of collimated waves propagating roughly in backward directions with respect to the pump waves. We show that the bright line of scattered light is in fact the mirrorless oscillation due to backward-wave four-wave mixing in the crystal with local (photovoltaic) response, while the retropropagating waves appear only because of high-order diffraction processes.     

Effect of partial coherence on four-wave mixing in photorefractive materials via reflection grating approximation

We investigate the effect of beam coherence on four-wave mixing via reflection gratings in photorefractive media. For the case of phase conjugation, the results of our theoretical analysis indicate that partial coherence always leads to a drop of signal gain and phase conjugate reflectivity in non-depleted cases. In general, the mutual coherence of the signal beam and the pump beam can be enhanced due to the process of wave mixing. The mutual coherence of the phase conjugate beam and one of the pump beams depends on the beam intensity ratio as well as the optical path difference. This is distinctly different from the four-wave mixing case with a transmission grating. Received: 15 October 1999 / Revised version: 26 June 2000 / Published online: 7 February 2001     

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     

Thermally tunable double-peak phase-conjugate reflectivity in InP:Fe at 1.32 μm

We report on double-peak thermally tuned reflectivity of optical phase conjugation in InP:Fe under dc fields. A single-pump four-wave mixing geometry generates a phase-conjugate reflectivity up to 6.7% for a pump intensity of 28 mW/cm² at 1.32 μm wavelength. The phase-conjugate reflectivity exhibits strong temperature dependence that is attributed to the phase-shift variations of the index grating due to optical power density of the intensity distribution. Two strong peaks in phase-conjugate reflectivity are observed near room temperature and are separated by 3 °C. Our theoretical predictions, which are based on the modified single-defect model, are in good agreement with the experimental findings. Received: 6 December 2000 / Revised version: 13 February 2001 / Published online: 27 April 2001     

Competition between Dispersion and Absorption of Doubly-Dressed Four-Wave Mixing and Dressed Six-Wave Mixing

We study the competition between dispersion and absorption of doubly-dressed four-wave mixing （DDFWM） and dressed six-wave mixing. In the case of weak coupling fields limit, we find DDFWM signal is affected by destructive interference between four-wave mixing（FWM） and six-wave mixing as wen as constructive interference between FWM and eight-wave mixing. By analysing the difference between two kinds of doubly dressing mechanisms （parallel cascade and nested cascade） in this opening five-level system, we can further understand the generated high-order nonlinear optical signal dressed by multi-fields.     

Temperature-tuned difference-frequency mixing in periodically poled KTiOPO4

4 (PPKTP). We generated 12 μW of radiation tunable around 1.6 μm by difference-frequency mixing of the outputs of a frequency-doubled Nd:YLF laser at 523 nm (240 mW) and a tunable Ti:sapphire laser near 760 nm (340 mW). A temperature tuning rate of 0.73 nm/°C for the generated wavelength and a FWHM temperature acceptance bandwidth of 6.9 °C cm was observed. The effective d₃₃ coefficient was estimated to be ∼5 pm/V. Received: 02 September 1998     

Nonlinear optical studies in tetraphenyl-porphyrin-doped boric acid glass using picosecond pulses

We report the results of our studies on the nonlinear optical response of thin films of tetraphenyl porphyrin doped in boric acid glass for picosecond-duration pulses. The fluence-dependent transmission measurements show that the sample exhibits reverse saturation behavior at 532 nm and saturable absorber action at 683 nm. By fitting the nonlinear transmission data at these wavelengths to a rate equation model, values for σ^S _e and σ_L the excited singlet and ground state cross sections were obtained. The large value (≈8) for σ^S _e/σ_L makes it a potential material for optical limiting at 532 nm, whereas at 683 nm the ratio σ_L/σ^S _e≈4.3. Intensity dependence of degenerate forward four-wave mixing at 683 nm was also investigated to establish the order and magnitude of the nonlinearity. The forward four-wave mixing signal at 683 nm showed a cubic dependence on the input intensity, establishing the third-order nonlinearity. A value of 4×10^-12 esu for the third-order susceptibility |χ⁽³⁾| was estimated from the measured efficiency. The time response of the nonlinearity was measured by the pump-probe transmission experiment, resulting in a decay constant of about 1.8 ns for the repopulation of the ground singlet state at 683 nm. Received: 13 April 2000 / Published online: 20 September 2000     

Interactions of Three Dual-Dressing Effects of Four-Wave Mixing in a Five-Level Atomic System

We study the four-wave mixing （FWM） in an opening five-level system with two dressing fields. There axe three kinds of doubly dressing mechanisms （parallel cascade, sequential cascade, and nested cascade） in the system for doubly dressed four-wave mixing. These mechanisms reflect different correlations between two dressing fields and different effects of two dressing fields to the FWM. Investigation of these mechanisms is helpful to understand the generated high-order nonlinear optical signal dressed by multi-fields.     

Exciton-related optical nonlinearities in semiconductors and semiconductor microcrystallites

We report on large optical nonlinearities due to specific many-body effects in high-density electron-hole pair systems. On bulk CdS we study the effect of free-carrier screening on the absorption and refraction in the vicinity of the band-gap at room temperature. Steady-state saturable absorption at mW-power levels and four-wave mixing with first-order efficiencies as large as 2% of the incoming light are demonstrated on a 1m slab. On CuBr microcrystallites embedded in glass we investigate the changes of the exciton absorption caused by many-exciton effects both at cryo and room temperature. In contrast to bulk semiconductors, we observe a blue shift of the exciton peak at resonant optical excitation. In addition, strong saturation of absorption with very large contrast shows up. We find Lorentzian saturation intensities in the 100 kW/cm² range.     

Coherent Phase Control of Ultrafast Polarization Beats in Reverse V-Type Three-Level System

We investigate the third-order nonlinear absorption and dispersion of femto- and atto-second polarization beats between the one-photon degenerate four-wave mixing process and the two-photon nondegenerate four-wave mixing process in the pure homogeneously-broadened reverse V-three-level using twin noisy fields. The third-order nonlinear response can be controlled and modified by the colour-locked correlation of twin noisy fields. Most importantly, the coherent phase control in optical heterodyne detection for studying the phase dispersion of the third-order susceptibility is demonstrated. The radiation-matter detuning oscillation is also considered in the freauencv domain.