Effect of magnetic field on the stimulated photon echo in ytterbium vapors

Polarization of the stimulated photon echo (SPE) at the 0 ? 1 transition in ytterbium vapors in the presence of a longitudinal magnetic field of 0–40 G is experimentally and theoretically studied. The SPE is generated using three light pulses with identical linear polarizations, so that the SPE polarization is the same at zero magnetic field. In the presence of a weak magnetic field, the SPE polarization vector rotates around the magnetic field vector and the depolarization of the SPE signal takes place. Each of the SPE polarization components exhibits biharmonic oscillations depending on the magnetic field. In the presence of a strong magnetic field, these oscillations vanish and the SPE becomes depolarized. The experimental data are in qualitative agreement with the results of the numerical calculations performed with the method of the evolution operator for the finite-duration excitation pulses. The application of the results for the processing of optical data is discussed.     

Photon echo in the presence of a magnetic field: New prospects for data accumulation and processing

The possibility for echo signal switching-off and the switching of echo polarization between the ±45° positions is demonstrated for the photon echo (PE) and stimulated photon echo (SPE) generated in Yb vapor at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition by two pulses of the resonant linearly polarized radiation at appropriate experimental parameters in the weak magnetic field limit. The experimental data are in qualitative agreement with the theoretical analysis and the calculations made for the 1 ? 0 transition. The strong magnetic field limit leads to unpolarized PE and SPE signals generated by linearly polarized radiation pulses. The possibility of the generation of a long-lived echo in ytterbium vapor due to the magnetic field induced mixing of the upper working level with the metastable level is discussed. The results can be employed for the optical data storage and processing.     

Effect of locking of photon echo signals in multichannel data recording

The effect of locking of a long-lived photon echo is investigated in the case where nonresonant laser radiation (a standing or traveling wave with an artificially created spatial inhomogeneity) serves as an inhomogeneous external perturbation, which results in random shifts or splitting of the initial monochromatic components of the inhomogeneously broadened line. The use of the mutual spatial orientation of the gradients of electric fields as an associative key of access to recorded information allows one to produce a large number of independent channels of data recording. It is demonstrated that the proposed scheme for recording and associative sampling of information has the advantage that, in each channel, information is recorded simultaneously at all optical centers (the whole inhomogeneously broadened resonance line), which does not decrease the intensity of the response from each channel. Moreover, the echo holographic information coded in the wavefronts of the exciting pulses can also be recorded in each channel, which does not affect the independence of individual channels of data recording.     

用互补向量修正阈值的光电混合联想存储器模型

本文提出了一种在联想存储中评价向量间相似程度的新判据.并完成了一种采用互补向量修正阈值的光电混合联想存储器模型的实验研究,该模型可以控制读出向量的精度,能够消除向量的误读.     

Nutational stimulated photon echoes

Stimulated photon echoes (SPEs) with time duration comparable to the coherent lifetime and Rabi period have been investigated theoretically and experimentally with an angled beam configuration. The Rabi oscillation effects on both the transmitted field (optical nutation) and the SPE fields are explained by analytic solutions of Maxwell-Bloch equations. The theory also predicts that an echo can exist in the noncausal direction, and this was confirmed by experiments with Tm:YAG crystal.     

多状态,多阈值神经网络模型的光电混合实现

提出了一种采用高分辨率液晶电视（ＬＣＴＶ）实现Ｈｏｐｆｉｅｌｄ神经网络多值算法的光电系统，文章给出了平面多状态，多阈值的全互连Ｈｏｐｆｉｅｄ神经网络模型，并采用该系统对颜色进行了联想和记忆的实验，初步的实验结果可以证实，此种高分辨率液晶电视神经网络系统是可行的。     

光计算中关联存储器的一种新模式

本文提出了光计算中关联存储器的一种新的存储模式,称为自反关联存储器。在这种模式中,矢量是以矢量对的形式被贮存在存储矩阵中。用预先贮存的矢量对中的一个矢量的部分信息不但能取出这个矢量的完整矢量,而且能取出这个矢量对中的另一个矢量。文中并提出了自反关联存储器的光学实现方法。     

Echo responses of an ensemble of cold atoms in optical lattices

The dynamics of an ultracold dilute boson gas in an optical lattice is described in the framework of the Bose-Hubbard model in which the parameters of the system are controlled by light. Within this model, an ensemble of Bose-Einstein condensates trapped in cells of a double optical lattice generates an echo response in the quantum-mechanical probability current. The echo signal is excited by biharmonic radiation pulses under two-photon (Raman) resonance conditions. The time profile of the echo signal and the role played by the inhomogeneous broadening and the interaction of atoms in the formation of the boson echo are discussed.     

Resonance Diffraction Radiation from Ultrarelativistic Particles

A Smith–Purcell radiation from ultrarelativistic electrons on a flat grating made up of a set of conducting strips is considered. The proposed model treating the Smith–Purcell effect (SPE) as the resonance diffraction radiation makes it possible to estimate the SPE characteristics for small outgoing photon angles 1, where conventional approaches cannot be used due to computational difficulties. A new type of an optical grating, simple and technologically inexpensive, is proposed. It consists of parallel aluminum strips deposited onto a quartz substrate whose thickness is determined using the criterion of the minimum reflected intensity of optical radiation with a wavelength corresponding to SPE.     

On the feedback holographic associative memory

Theoretical expressions and convergence conditions are developed here for the output of a feedback holographic associative memory. It is shown that a feedback holographic associative memory becomes a pinhole holographic associative memory under certain conditions, and thus is a generalization of the pinhole holographic associative memory. The crosstalk in the output of a feedback holographic associative memory can also be removed by a threshold device or controlled by the overall optical efficiency of the memory. A simulation is provided to verify the theoretical results shown here. Using the theoretical results and simulation verification, a feedback holographic associative memory can be designed efficiently. In addition, if a threshold device is used, optical elements with high optical efficiency can be used in a feedback holographic associative memory.     

Spatial regularities in the effect of locking of photon echo signals and associative optical memory

The efficiency of suppression of the generation of a photon echo response depending on the mutual spatial orientation of gradients of external nonumform electric fields acting on a resonant medium is studied. The possibility of creating an associative memory where the mutual orientation of the gradients of external electric fields is an associative key is discussed.     

Coherent optical spectroscopy of promising materials for solid-state optical processors

State of the art of optical coherent spectroscopy of doped solids that are promising as information carriers for optical processors is reviewed. Special attention is paid to optical echo spectroscopy of doped crystals classified as the Van-Vleck paramagnets where the long-lived stimulated echo is observed with the optical-memory time reaching several hours at low temperatures. Modern elaborations of optical echo processors based on this echo phenomenon are discussed. Physical principles of femtosecond echo spectroscopy and coherent four-wave mixing spectroscopy are formulated. The abilities of these methods in the diagnostics of fast processes at room temperature are illustrated using examples of a doped polymer films. The results of elaborations of a new branch of optical spectroscopy (biphoton spectroscopy) are also presented. The advantages and possible applications of this method are demonstrated using an example of two crystals (Er³⁺:YAG and Cr³⁺:Al₂O₃).     

Raman echo under medium excitation by extremely short pulses

A generalization of the Bloembergen-Shen model to Raman active molecules with an arbitrary number of normal modes was suggested. The generalized model was used to study Raman echo signal characteristics when a system of molecules was excited by pulses of widths up to one period of optical oscillations. It was shown that a large number of echo responses on Stokes and anti-Stokes components could arise even under a two-pulse action in a continuous monochromatic pumping field. The number of echo responses depended on the number of molecular normal modes and the geometry of measurements. At small exciting pulse “areas,” the echo responses whose Stokes and anti-Stokes components corresponded to normal vibrational modes of a molecule had the highest intensity, whereas the components formed by normal mode combinations were strongly suppressed.     

Multipulse excitation of photon echo and its application for developing the principle of design of an optical echo processor

A method for optical data processing, based on the mode of multipulse excitation of photon echo signals, has been proposed and applied to solution of the knapsack problem. In this case, along with the recognition of solvability, the solution itself is found in the optical processor. A scheme of a possible optical echo processor on molecular iodine vapor is presented.     

Analytical efficiency of the molecular rotational quantum echo

The sensitivity of the delay time of the molecular rotational quantum echo to change in the rotational constants of molecules was investigated with a model of the rotational quantum echo considered as an optically induced quantum synchronization of the Fourier rotational modes of an asymmetric molecular top. The J-echo and the K-echo were investigated for the A- and C-configurations of rotation of a top molecule. The contour maps of the dependence of the sensitivity of the delay time of these types of echo to changes in the rotational constants on the parameters of the molecular tops have been obtained for all the rotational constants. They show that the delay time of the rotational quantum echo is greatly dependent on the configuration of rotation of the top molecule and on the rotational constants. It is shown that the optical probing of the rotational quantum echo can also be performed by the optical rotation of the investigated molecule, and in this case the rotational force is dependent on the intramolecular orientation of the magnetic dipole moment. It was determined that a high sensitivity of the delay time of the echo to changes in the rotational constants can be responsible for the broadening of the echo pulse and even for its disappearance. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 F. Skorina Ave., Minsk, 220072, Belarus; e-mail: lssm@imaph.bas-net.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 758–764, November–December, 1999.     

Improving the intensity and efficiency of compressed echo in rare-earth-ion-doped crystal

We investigate the intensity and efficiency of a compressed echo, which is important in arbitrary waveform generation(AWG). A new model of compressed echo is proposed based on the optical Bloch equations, which exposes much more detailed parameters than the conventional model, such as the time delay of the chirp lasers, the nature of the rare-earth-iondoped crystal, etc. According to the novel model of compressed echo, we find that reducing the time delay of the chirp lasers and scanning the lasers around the center frequency of the inhomogeneously broadened spectrum, while utilizing a crystal with larger coherence time and excitation lifetime can improve the compressed echo's intensity and efficiency. The theoretical analysis is validated by numerical simulations.     

多值神经网络改进模型及其光学实现

本文提出一种改进的光学神经网络模型,并利用空间光调制器PROM构成的光学系统实现了这种模型的联想记忆运算.计算机模拟和实验结果表明,改进模型提高了光学神经网络的识别能力,并在—定程度上提高了存贮容量.     

Influence of wake bubbles on optical impulse scattering echo

The model of the optical impulse scattering echo based on Mie theory and backscattering properties of bubble populations is presented. The experimental results of optical impulse scattering echo for a ship in the ocean is in agreement with the simulated experiment. The wake bubbles significantly influence the laser scattering echo process in the ocean. When the testing system is outside the wake, the distinct scattering echoes of interface between wake bubbles and water are received. When the testing system is inside the wake, the intensity of the echo attenuates with the increased scattering of wake bubbles. These phenomena of laser echoes in different periods are analyzed.     

Implementation of optical associative memory by a computer-generated hologram with a novel thresholding scheme

An associative memory based on a model by Rizvi and Zubairy [Appl. Opt. 33, 3642 (1994)] is experimentally demonstrated. The parallel optical interconnection is realized compactly by computer-generated holograms. A novel scheme of real-time thresholding is proposed and is proved to be effective and simple to implement. Stored images are successfully retrieved with both complete and partial inputs.     

Soft-potential model and homogeneous width of spectral lines of impurity centers in molecular amorphous media

A study is made to analyze the possibility of using the soft-potential model in optical investigations of disordered molecular systems with impurities. A procedure is suggested for calculating the temperature dependence of the homogeneous width of a phononless line in amorphous media with impurities within the soft-potential model. A calculation is performed of the temperature dependence of the width of a phononless line (optical dephasing) in an amorphous system of polymethyl methacrylate (PMMA) with an addition of tetratert-butylterrylene (TBT) using the parameters of this system known from the literature. Calculations are performed of the contributions to the width of a phononless line due to the interaction of an impurity with tunneling two-level systems, with thermally activated barrier crossings in double-well potentials, and with quasilocal modes of the matrix. The model calculation results are compared with the experimental data on the photon echo for TBT/PMMA, measured by us in the temperature range from 0.3 to 20 K. It is found that the soft-potential model describes qualitatively correctly the temperature behavior of the homogeneous width of a phononless line. In the temperature range of T < 2 K, where the main contribution to optical dephasing is associated with tunneling two-level systems, the predicted values of phononless line width agree well with the experimental data. At higher temperatures, some difference is observed between the prediction and experimental data, which may be due to the effect of impurity on the formation of quasilocal oscillation of the matrix.