Adaptive photodetectors based on charge gratings in the problems of frequency-modulated optical signal detection

Nonstationary photovoltage excitation by frequency-modulated light in an adaptive photodetector based on GaAs is studied. To observe the effect, the crystal is exposed to light beams with the relative frequency shift Δf(t). Linear frequency modulation Δf(t) = At is used in the experiments. As a result of such illumination, a pulsed electrical signal is induced in the crystal. The pulse appears at the deceleration of the interference pattern motion, and its duration is controlled by the frequency variation rate A and the time of the charge grating formation. The possibility of using the effect in the systems for measuring velocities and accelerations of moving objects is shown.     

Change in the shape of a symmetric light pulse passing through a quantum well

A theory for the response of a 2D two-level system to irradiation by a symmetric light pulse is developed. Under certain conditions, such an electron system approximates an ideal solitary quantum well in a zero field or a strong magnetic field H perpendicular to the plane of the well. One of the energy levels is the ground state of the system, while the other is a discrete excited state with energy ?ω₀, which may be an exciton level for H=0 or any level in a strong magnetic field. It is assumed that the effect of other energy levels and the interaction of light with the lattice can be ignored. General formulas are derived for the time dependence of the dimensionless “coefficients” of the reflection ?(t), absorption A(t), and transmission ?(t) for a symmetric light pulse. It is shown that the ?(t), A(t), and ?(t) time dependences have singular points of three types. At points t ₀ of the first type, A(t ₀)=T(t ₀)=0 and total reflection takes place. It is shown that for γ_r?γ, where γ_r and γ are the radiative and nonradiative reciprocal lifetimes, respectively, for the upper energy level of the two-level system, the amplitude and shape of the transmitted pulse can change significantly under the resonance ω_l=ω₀. In the case of a long pulse, when γ_l<γ_r, the pulse is reflected almost completely. (The quantity γ_l characterizes the duration of the exciting pulse.) In the case of an intermediate pulse duration γ_l?γ_r, the reflection, absorption, and transmission are comparable in value and the shape of the transmitted pulse differs considerably from the shape of the exciting pulse: the transmitted pulse has two peaks due to the existence of the point t ₀ of total reflection, at which the transmission is zero. If the carrier frequency ω_l of light differs from the resonance frequency ω₀, the oscillating ?(t), A(t), and ?(t) time dependences are observed at the frequency Δω=ωl?ω₀. Oscillations can be observed most conveniently for Δω?γ_l. The position of the singular points of total absorption, reflection, and transparency is studied for the case when ω_l differs from the resonance frequency.     

Studying the effect of biaxial anisotropy on nonlinear magnetization oscillations accompanying the 90° pulsed magnetization process in ferrite–garnet films with easy-plane anisotropy

Time dependences of the azimuthal component of the torque T _φ(t) acting on magnetization are calculated to understand the nature of the delayed magnetization acceleration effect observed during the 90° pulsed magnetization of real ferrite–garnet films, in which biaxial anisotropy exists alongside with in-plane anisotropy. A calculation technique based on analyzing an operating point trajectory is used. Calculations show that if the effective anisotropy field H _K2 is comparable to the magnetizing pulse amplitude H _ma, abruptly ascending regions at characteristic times t* in curves T _φ(t) arise, in the limit of which nonlinear magnetization oscillations formed. The shape of these regions depends weakly on the magnetizing pulse front duration τ_f. This explains the reason of the weak dependence of the nonlinear magnetization oscillations on duration of the magnetizing pulse front. Calculations also show that the main features of the delayed acceleration effect are less clear upon an increase of the pulse amplitude: the behavior of curves T _φ(t) becomes smoother near times t*, and an increase in the pulse front duration is accompanied by a stronger drop in the intensity of magnetization oscillations.     

Self-diffraction at a dynamic photonic crystal formed in a colloidal solution of quantum dots

Self-diffraction at a one-dimensional dynamic photonic crystal formed in the colloidal solution of CdSe/ZnS quantum dots has been discovered. This self-diffraction appears simultaneously with self-diffraction at induced transparency channels at the resonant excitation of the main electron–hole (excitonic) transition of quantum dots by two laser beams with a Gaussian intensity distribution over the cross section. It is shown that a nonlinear change in the absorption of colloidal quantum dots results in the formation of a transparency channel and an induced amplitude diffraction grating, and a significant nonlinear change in the refractive index (Δn ≈ 10^?3) in the absorbing medium is responsible for the formation of the dynamic photonic crystal. Self-diffracted laser beams are revealed propagating not only in directions corresponding to self-diffraction at the induced diffraction grating but also in directions satisfying the Laue condition.     

The nuclear magnetization as the origin of transient changes in the magnetic field in pulsed NMR experiments

The very measurement of the NMR signal in a pulsed NMR experiment disturbs the nuclear equilibrium magnetization. A small change in the magnetic field results, which can be of the order of −0.0025 ppm in protonated solvents. It is shown that the resonances in pulsed high-field NMR consequently are non-Lorentzian. The FID is properly described by S(t) = S(0)exp(−iω_0tt)exp(−iΔωf{t}) is the small resonance shift which is experienced immediately following the pulse, and f {t} characterizes the subsequent return toward zero by the response of the lock system and by spin-lattice relaxation. In particular in samples containing large concentrations of protons, shifted and distorted resonance lines can be observed.     

Change in the resonance frequency of surface plasmons in copper nanoparticles excited by femtosecond laser pulses

Changes in the optical density ΔD(t), halfwidth ΔH/2(t), and spectral position of the maximum Δλ_SP(t) of the surface plasmon band in Cu nanoparticles after their excitation by femtosecond laser pulses have been investigated. The Δλ_SP(t) dependence appears to be alternating and is accompanied by a nonmonotonic variation in ΔH/2(t) in the time interval 0–5 ps. The results are explained in a model based on the evolution of the dielectric response of such a composite medium excited by intense laser pulses.     

Dynamical theory for photon and photoelectric pulse distributions in single molecule fluorescence

A single two-level molecule driven by CW-laser field and a photomultiplier tube (PMT) are considered as two parts of the united dynamical system connected with each other by photons of molecular fluorescence. Each PMT is characterized by a rate α of photo-effect and by a rate β of PMT recovery. A theory for the photon distribution function w_N(t) and for the photoelectric pulse distribution function f_n(t) for such a system is built up. If times 1/ α and 1/ β characterizing PMT are much shorter as compared to the average time interval 1/ k between two successively emitted photons of fluorescence, the photon and the photoelectron distribution functions coincide with each other, i.e. f_n(t) ≅ w_N(t). A relation between w_N(t) and f_n(t) is studied in detail for the case in which PMT works slower as compared to the rate k of photon emission, i.e. at 1/ α, 1/ β ≥ 1/ k.     

Optimal coherence via adiabatic following

We propose a new method for creating an optimal coherence in a two-level system (TLS) based on adiabatic following. Optimal coherence is achieved by choosing the time-dependent phase of the pulse that provides vanishing field-TLS detuning δ(t) at central time when the pulse amplitude reaches maximum. For example, δ(t)=δ₀(1-exp((t-t₀)/τ₀))⁴ for t?t₀ and δ=0 for t>t₀. The pulse envelope used in calculations has Gaussian form such that the Rabi frequency is , where τ₀ is the pulse duration.     

The exact solution of an elimination problem in kinetic theory: I. The one-dimensional hard sphere gas

A class of initial value problems for a one-dimensional hard sphere gas is considered where a specified particle has a given distribution f⁽¹⁾(z₁; 0) and the rest are in equilibrium at t=0. An exact expansion is obtained for a certain n-particle reduced distribution function f⁽ⁿ⁾(z₁;…;z_n; t) in terms of the 1-particle reduced distribution function f⁽¹⁾(z₁; t) for the specified particle by starting with separate expressions for these functions in terms of f⁽¹⁾(z₁; 0). Expansions for the corresponding cluster functions are first obtained and then graph theoretic methods applied to obtain a solution.     

Photo- and radio-excited luminescence properties of Eu-doped La2O3–Al2O3 based eutectics

Eutectic crystal of 0.5% Eu-doped 30LaAlO₃–70Al₂O₃ (vol %) was prepared by micro-pulling down (μ-PD) technique under nitrogen atmosphere. Being excited at a wavelength of 320 nm, the crystal exhibited intense emission band with a maximum at 450 nm which is corresponding to 4f⁶5d-4f⁷(⁸S_7/2) transitions of Eu²⁺. The decay time and fluorescence quantum efficiency (QE) were determined to be about 475 ns and 60%, respectively. When alpha-ray excited the crystal, both Eu²⁺ 4f⁶5d-4f⁷(⁸S_7/2) and Eu³⁺ 4f⁶-4f⁶ (⁵D₀-⁷F_1,2) emission peaks were observed at 435 nm and 600 nm. By the pulse height spectra, the relative scintillation light yield of the crystal was about 4% compared with that of BGO commercial scintillator.     

Laser ranging based on electro-optic switch

Based on electro-optic switch effect in crystal, a novel laser ranging method is proposed. CW-laser emitted by laser transmitter propagates forward to the measured target, after being reflected by the target, and then goes back to the transmitter. Close to the transmitter, a special mono-block LiNbO₃ crystal is added into the round-trip light beams. High-voltage pulses with the sharp enough changes in rising edges are loaded on the crystal. Based on electro-optic effect, double refraction and internal double reflection effect in crystal, the crystal cuts off the round-trip light beams, and reflects a light pulse cut out by the crystal to a detector aside from the original beam path. The pulse width T is the period that laser propagates forward and back between the crystal and the target. The feasibility of the new idea is proved by our experiments and a brand-new way for the laser ranging is provided.     

Light-field-induced spatially periodic freedericksz transition in a planar nematic cell

It is shown that a threshold spatially periodic reorientation of the director by a light field is possible in a planar nematic liquid crystal cell if the ratio of the Frank elastic constants, K ₂/K ₁, exceeds a critical value. The periodic director structure arising in the cell leads to a self-diffraction of the incident light wave. The dependences obtained for this phenomenon make it possible to determine the values of the elastic constants K ₁ and K ₂, the director reorientation threshold, and the period of the director structure from the experimental values of the self-diffraction angle.     

Influence of the optical activity on hologram formation in photorefractive crystals

It is shown that dynamical hologram formation in photorefractive, optically active crystals is accompanied by polarization and energy exchange. These effects may be used like new effective holographic methods for determinations of crystal parameters and optical signal processing in real time. In self-diffraction a polarization rotation the order of degrees/cm for Bi₁₂TiO₂₀ and Bi₁₂SiO₂₀ is predicted.     

Speed and diffraction efficiency in feedback-controlled running holograms for photorefractive crystal characterization

We report the measurement of the diffusion length, the Debye screening length and the quantum efficiency of photoelectron generation in strongly light absorbing photorefractive Bi₁₂TiO₂₀ crystals, using fringe-locked running hologram experiments. The effective applied electric field inside the sample is also computed and self-diffraction is considered. The novelty here, as compared to formerly reported experiments, is that the diffraction efficiency is now measured simultaneously with the hologram speed v. From these data the above referred to photorefractive and experimental parameters are obtained without the need for additional experiments. The method is used to analyze two photorefractive Bi₁₂TiO₂₀ crystal samples, in different experimental conditions, using the 514.5 nm wavelength. The computed parameters are in good agreement with the available information about these samples. Received: 23 November 2000 / Published online: 21 March 2001     

Spatial frequency dependence of the energy transfer in two-wave mixing experiments with BSO crystals

Energy transfer between two beams interfering in an electrooptic Bi₁₂SiO₂₀ crystal is analyzed as a function of the spatial frequency of the recorded hologram. We report high coefficients in a wide range of low spatial frequencies $\text{(Λ}{}^{-1}\text{?44 mm}{}^{-1}\text{)}$, when the drift length of the photocarriers becomes comparable to the fringe spacing. Exponential gain coefficients of Γ = 7 cm^-1 have been reached for high intensity beams ratio (β > 10⁴) and an optimized fringe displacement speed (moving grating recording). Application to coherent image amplification (×20) is shown.     

Changes in the spectral degree of polarization of nonparaxial stochastic electromagnetic pulsed beams

By using the generalized vectorial Rayleigh-Sommerfeld diffraction integral, the analytical expression for the spectral degree of polarization (SDP) of stochastic spatially and spectrally partially coherent electromagnetic pulsed beams (SSSPCEPB) is derived, and used to study the changes in the SDP of SSSPCEPB in the nonparaxial regime. Similar to that of continuous beams, the f parameter and f_αα parameter of pulsed beams also play an important role in determining the nonparaxiality of SSSPCEPB. However, the pulse duration and temporal coherence length of the pulse can change the nonparaxiality of SSSPCEPB. The dependence of SDP on the f parameter, pulse duration and temporal coherence length is emphasized and illustrated numerically.     

Auditory detection of paired pulses by a dolphin in the presence of a pulse jam

From behavioral studies of a bottlenose dolphin (Tursiops truncatus), the audibility thresholds were measured for a single pair of equal-amplitude pulses, i.e., clicks, presented to the dolphin in combination with a pulse jam. The pulse jam consisted of pairs of identical pulses with a pulse spacing τ_j within the pairs and a pair repetition rate f _j. Series of pulses were interrupted by a pause R>1/f _j, within which the pulse jam was absent while a pair of test pulses was supplied to one of the two channels at random. Each series had a duration T, and the total stimulation cycle was J=T+R. The dependence of the test pair detection threshold on the pulse spacing τ_j was studied at different fixed values of the pulse spacing in the test pair: τ_t=50, 100, 200, and 500 µs. Preliminary measurements performed with τ_j=τ_t=100 µs were used to adjust the parameters of the pulse jam. The threshold shift at τ_j=τ_t=100 µs reached 35 dB above the audibility threshold of the test pair in the absence of the pulse jam. On both sides of the point τ_j=τ_t=100 µs the thresholds decreased with varying τ_j to approximately 20 dB above the detection threshold of the test pair in the absence of the jam. However, in the course of training, the threshold curves gradually shifted downwards approaching the detection level of the test pair in the absence of the jam and becoming progressively flatter (the selectivity with respect to the pulse jam vanished). A decrease in the pause duration R restored the dependence of the test pair detection threshold on τ_j. In this case, a statistically significant maximum was obtained at τ_j=τ_t for τ_j within the critical interval (for τ_t<500 µs). Beyond the critical interval (for τ_t>500 µs), even with the smallest pause duration (R=15 ms), no dependence of the test pair detection thresholds on τ_j could be observed.     

Heats of solution/substitution in TlNO3 and CsNO3 crystals and in RbNO3 and CsNO3 crystals from heats of transition: the complete phase diagrams of TlNO3-CsNO3 and RbNO3-CsNO3 systems

From measurements of the decrease in the heat (enthalpy) of transition in the solid phase using differential scanning calorimetry, the apparent molar heats of solution, slope ΔH_t/x, the partial molar heats of solution at infinite dilution, χ, and the heats of solution, ΔH_s^°, of Tl⁺ in CsNO₃ crystal and Cs⁺ in TlNO₃ crystal and Rb⁺ in CsNO₃ crystal and Cs⁺ in RbNO₃ crystal along with their recovered lattice energies, ΔH_L^°, are reported. ΔH_s^° of Tl⁺ and Rb⁺ in CsNO₃ crystal are each found to be negligible or zero representing an ideal solid solution, i.e. ΔH_mix=0. The complete phase diagrams of the TlNO₃-CsNO₃ and RbNO₃-CsNO₃ systems with details of the sub-solidus regions are included. The properties of Tl_(1−x)Cs_xNO₃ and Rb_(1−x)Cs_xNO₃ compositions are discussed in terms of a ‘mixed crystal’ or ‘crystalline solid solution’ in relation to parallel compositions of Tl_(1−x)Rb_xNO₃.