Locking of a stimulated photon echo

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 1, pp. 84–88, July, 1991.     

Heating effects in stimulated photon echo spectroscopy of low-temperature glasses

Photon echo spectroscopy has been extensively used to characterize the low-temperature relaxations in amorphous solids. However, the relatively high excitation energies used in the actual experiments (20–200μJ/g) lead to serious problems with sample heating since the specific heat of amorphous solids is on the order of 300μJ/gK at 2 Kelvin. We present data for the amorphous system polymethylmethacrylate and discuss the different contributions to the heating process. Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997.     

Modified stimulated photon echo with hyperfine structure of resonant levels

The intensity and polarization of a modified stimulated photon echo generated on resonant levels with hyperfine structure are obtained. It is proposed that the polarization properties of this kind of a photon echo can be used for the identification of the structure of the resonant transitions on which the echo is formed as well as for the separation of the modified stimulated photon echo pulse from the third exciting pulse.     

Long-lived stimulated photon echo in optically thick van vleck paramagnets

Long-lived stimulated photon echo (LSPE) excitation by the sequence ofN pulse pairs in optically thick LaF₃:Pr³⁺ crystals is investigated numerically. Hyperfine structure of the ground and excited states of medium atoms is taken into account. Stable LSPE amplification along the total sample length is realized in the case when the total initial area of the pulse sequence is larger than the threshold value π. Cumulative effect depends on the threshold conditions and the crystal length. At measuring medium spectroscopic parameters the large evaluation errors can be introduced due to neglect of light propagation effects.     

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.     

Formation of stimulated photon echo in ytterbium vapours in a longitudinal magnetic field

Intensity and polarization are derived for stimulated photon echo produced in a longitudinal magnetic field by transitions between resonance levels with the angular momenta 0 and 1. It is shown that stimulated photon echo generated in a longitudinal magnetic field can be used to obtain spectroscopic data on the times of relaxation of population, orientation, and alignment of the level ³P₁ of a ¹⁷⁴Yb atom.     

Photon echo under superradiance

The dynamics of optical coherence and phase memory in three-level medium in conditions of superradiance pulse emission is considered. In the case of coherent excitation of the upper level in three-level system of lambda configuration it is shown that the superradiance eliminates optical coherence on the adjacent transitions of excitation and superradiance emission and induces optical coherence on the remain transition. This, in turn, makes it possible to observe new effects of photon-echo, simple example of which is described in the present Letter.     

Photon echo in superradiant media

It was shown that if an additional emission of a superradiance pulse by a medium occurs after an initial exciting pulse, two-pulse photon echoes are formed at a different frequencies and at a new time as compared to an ordinary medium. Conditions for the spatial synchronism of these photon echo were found.     

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.     

Low-temperature photon echo study of 2-MTHF glass under high pressure

A high-pressure photon echo study of the low-temperature dynamics of rhodamine 640 in a 2-methyltetrahydrofuran (2-MTHF) molecular glass is reported. High pressure is found to decrease the homogeneous dephasing rate comparably to the volume compression ratio. The pressured-induced line narrowing is also qualitatively similar to high-pressure hole burning results reported for a 2-MTHF glass doped with bacteriochlorophyll a. The results are consistent with a pressure-induced decrease in the number of tunneling two-level systems (TLSs) that may be associated with collapsible void space in the molecular glass. Comparable high-pressure photon echo studies of organic polymer glasses over the same low-temperature range do not show line narrowing at high pressure, suggesting an intrinsic difference in the nature of TLSs for molecular versus polymer glasses.     

Photon distribution functions of the fluorescence photons from a single nanoparticle in various photon counting methods

Simple expressions have been derived for three photon distribution functions w _N ^M (T), w _N ^Z (T), and w _N ^O (T) corresponding to three different methods for counting fluorescence photons from a single nanoparticle excited by continuous laser radiation. In contrast to the previously derived expressions represented in the form of N multiple integrals, the new expressions contain only single or double integrals of Poisson functions, which makes it possible to easily perform the numerical calculation of the photon distribution. The simplest photon counting method corresponds to the lengthiest function w _N ^M (T); on the contrary, the simplest function w _N ^O (T) corresponds to the most complex photon counting method. The functions w _N ^M (T), w _N ^Z (T), and w _N ^O (T) are noticeably different in short time intervals T; however, the distributions calculated using these functions are almost indistinguishable from each other in long T intervals. This circumstance makes it possible to use the simplest function w _N ^O (T) to consider the photon statistics measured by the simplest method. This possibility is particularly important for investigating the fluorescence photon statistics, where the intensity fluctuates.     

Photon echo in gases: From the non-Faraday rotation to unpolarized echo

A retrospective review of both theoretical and experimental works on photon echo in gases in the presence of longitudinal magnetic fields is presented from the viewpoint of new possibilities opened by this research for polarization echo spectroscopy of gases. The main attention is given to the physical scenario of the magnetic field’s effect on the properties of the photon echo. New results on the photon echo and stimulated photon echo in ytterbium vapor at the 1 ? 0 transition in the presence of the longitudinal magnetic field whose strength ranges from weak to strong are presented. Possible applications of the magnetic field effects for optical data storage and processing are analyzed.     

A classical model of the photon echo

An observation and the theory of a nonlinear response in the ensemble of nonlinear classical oscillators excited by two field pulses — similar to the phenomenon of a photon echo in optics — are reported. It has been shown that an echo arises in any ensemble with an inhomogeneously broadened line regardless of the type of interaction with a field (classical or quantum). New peculiarities appear during the determinate distribution of frequencies in the ensemble. On the one hand, an echo pulse is observed with a relatively small (about 100) number of oscillatorsN, on the other, new pulses spaced at a distance that is proportional toN arise in the nonlinear response.     

Excitation of photon echo by noise pulses

The excitation of photon echo by noise pulses that are formed by modulation of the carrying frequency with Gauss noise is modeled. The modeling is based on optical Bloch equations the solution of which for noise pulse realizations is constructed by their stepwise approximation. In terms of the formalism of state transfer matrices, the two- and three-pulse excitation modes are analyzed. The complex envelopes of the primary and stimulated echo responses are determined. In the linear (low-level-signal) mode, the shape of the two-pulse echo corresponds to that of the time delayed and inverted noise pulse. The boundary of the linear mode, upon exceeding of which distortions of the shape of the noise pulse become noticeable, is determined. The shape of the stimulated (three-pulse) echo in the linear mode corresponds to that of the autocorrelation function of the noise pulse realization. Upon passage beyond the boundary of the linear mode, the shape of the three-pulse echo corresponds either to the cross-correlation function of distorted noise pulses (with different intensities) or to the autocorrelation function of distorted pulses (with the same intensities). The modeled photon echo excitation modes can be used in photon echo processors to process signals in the light range.