Biomedical subjects
Publications and source records attributed to B Sturman.
It is shown experimentally and theoretically that a partial domain reversal in ferroelectric lithium niobate crystals containing a previously imprinted photorefractive space-charge grating results in a highly unusual change of the angular Bragg selectivity. This carries important in situ information about domain structures, including the quality of periodically poled samples.
The introduction of certain electronic feedback loops into photorefractive wave-coupling schemes makes them noise free and leads to dramatic changes of the whole nonlinear behavior. The familiar steady states can be transformed into periodic states that are characterized by ultimately high and low values of the diffraction efficiency eta of the recorded index grating or into quasisteady states with small values of eta. These transformations possess thresholds with respect to controllable experimental parameters like the coupling strength and the input intensity ratio. We present a general analysis of the threshold behavior for different modes of the feedback operation and different types of the nonlinear photorefractive response. The results obtained (analytical and numerical) allow one to predict the regions of stability for feedback-controlled steady states and the observable characteristics of the system, including the output amplitudes and diffraction efficiency of the spatial grating, beyond these regions. They extend strongly the potentialities of the feedback-controlled wave coupling.
Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta(p) ranges from approximately 3.5 cm/GW for lambda(p) = 388 nm to approximately 0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2lambda(p) = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (beta (r) = 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of < or approximately equal to 0.1 ps.
The propagation of high-power femtosecond light pulses in lithium niobate crystals (LiNb O3 ) is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength 388 nm fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is beta(p) approximately = 3.5 cm/GW. Furthermore, strong pump pulses induce a considerable absorption for the probe at 776 nm. The dependence of the probe transmission coefficient on the time delay Delta t between probe and pump pulses is characterized by a narrow dip (at Delta t approximately = 0) and a long (on the picosecond time scale) lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is beta(r) approximately = 0.9 cm/GW. The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at 776 nm is sigma(r) approximately = 8 x 10(-18) cm(2). The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of approximately = 0.24 ps.
A considerable deceleration of light pulses in crystals with nearly compensated space-charge field proves that the strong dispersion of dynamic photorefractive gratings in the vicinity of Bragg resonance is of primary importance for light slowing down.
We show, using the symmetry properties of the coupled-wave equations for the transmission and reflection geometries, that any readout characteristic of dynamic spatially nonuniform index gratings in photorefractive crystals can be explicitly expressed through the characteristics of the recording light beams. This approach is applied to describe the impact of beam coupling on the diffraction efficiency of dynamic gratings and on the output intensities of the light beams at instantaneous input phase changes (light grating translation). Further implications of this general approach are discussed.
We propose a novel geometric approach to the description of the relaxation phenomena in complex condensed-matter systems. It is shown within a fairly general random site hopping model that the stretched exponential decay law, exp([-(t/tau)(beta)], originates from the simple and general geometric features of a random distribution of transport and trapping sites in the 3D space. The value of the variable stretching index beta is determined by the localization radius of hopping electrons. The possibilities for generalization of the obtained results and interpretation of the relevant experimental data are discussed.
It is shown experimentally and theoretically that photorefractive wave coupling can be used for dramatic (< or approximately 0.025 cm/s) deceleration of light pulses whose width is larger than (or comparable with) the nonlinear response time. This classical nonlinear scheme exhibits similarities with the technique based on the quantum effect of electromagnetically induced transparency. The main distinctive feature of our scheme is amplification of the delayed output pulse. Advantages of the novel technique and its prospects for manipulation with light photons are discussed.
We investigate light beam propagation in a fast photorefractive medium placed in an alternating electric ac field to enhance the nonlinear response. It is shown that the joint action of the optical and material nonlinearities leads to formation of a narrow singularity of the light-induced space charge at the intensity maximum and to self-trapping of the light energy near the corresponding discontinuity of the index profile. Owing to the strong saturation of the material nonlinearity, the trapped beam propagates over long distances with only a weak loss of its power.
The first observation of parametric light scattering patterns (rings, lines, and dots) in bulk periodically poled nonlinear media is reported. Development of the scattering patterns proves efficient photorefractive grating recording and considerable parametric gain for seed radiation in this new nonlinear material. Several novel phase-matched parametric processes, caused by the periodicity of the domain structure, are revealed.
We show that a photoconductive crystal, placed in a rapidly alternating ac field and exposed to nonuniform light, exhibits singularities of the induced space charge and discontinuities of the corresponding space-charge field. The singularities appear at the local intensity maxima when the curvature of the intensity profile exceeds a certain (often very low) threshold value. We analyze the characteristic features of the singular ac response and consider its possible optical manifestations.