Femtosecond photon echo in dye-doped polymer films at room temperature

The signals of primary and stimulated femtosecond photon echo in polymer films doped with dye (phthalocyanine) molecules have been experimentally investigated at room temperature. A femtosecond echo spectrometer for these echo experiments is described. The decay curves of echo signals with increasing time intervals between excitation femtosecond pulses are obtained and blue shifts of the spectra of femtosecond echo signals with respect to the spectrum of excitation pulses are revealed. The possibilities of using the studied doped polymer films as recording media for high-temperature echo processors and coolants in optical refrigerators are analyzed.     

Femtosecond primary and stimulated photon echoes in a dye-doped polymer film at room temperature

The signals of both primary and stimulated femtosecond photon echoes are observed and investigated in a dye-doped polymer film at room temperature using a modernized femtosecond echo-spectrometer. It should be noted that stimulated photon echo in the solid-state sample is observed for the first time at such a high temperature. Experimentally obtained decay curves of these signals have a nonexponential character. The spectra of these echo signals are also measured. It is found that the spectrum of the primary photon echo is short-wave shifted with respect to the spectrum of excitation. This can be used for the coherent laser cooling of a sample. The spectrum of the stimulated photon echo is also shifted to the short-wave range relative to the spectrum of excitation, but its shift is much less than that of the primary photon echo. The experiment shows that the femtosecond echo signals at room temperature are excited via the phonon-side band of the absorption line.     

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).     

Investigation of relaxation in quantum dot ensembles in thin semiconductor films by photon-echo technique

Experimental results of observations of a femtosecond photon echo excited in an ensemble of quantum nanometer-sized objects in thin semiconductor (ZnO, SiB, SiP) films at room temperature are presented. The observed properties of the photon echo are discussed. Recorded relaxation times of exciton quantum transitions and beats at the photon echo decay as a function of the increasing interval between the exciting pulses reveal the role played by the defects of the thin film in the formation of its exciton structure of quantum transitions investigated using angular optical echo spectroscopy.     

Signals of the femtosecond photon echo in inorganic films and their recording

We report the results of the first experiment on recording of photon echo and self-diffraction signals in a zinc oxide semiconductor film of nanoscale thickness, which are excited by two-photon absorption of femtosecond laser pulses with wavelengths of 800 to 840nm for the exciton transition at room temperature.     

Femtosecond photon echo in a dye-doped polymer film and the possibility of coherent optical cooling

The signals of primary and stimulated femtosecond photon echoes are investigated in a dye-doped polymer film at room temperature. The homogeneous S ₀ → S ₁ spectral line width, which is due to the interaction between the impurity molecules and the quasi-local low-frequency modes, is estimated (≈5 × 10¹² Hz). Special attention is paid to the study of spectra of femtosecond echo signals. The short-wave shifts of these spectra, with respect to the spectrum of femtosecond exciting pulses, are observed. These shifts indicate that the anti-Stokes regime of femtosecond pulse emission is realized. Therefore, the coherent regime of laser cooling of solids appears to be possible. The prospects of using of this new cooling regime in the function of a solid-state optical refrigerator are discussed.     

Anti-Stokes femtosecond photon echo in dye-doped polymer films and the possibility of coherent optical cooling

An anti-Stokes model is proposed to explain the known experimental data on the femtosecond photon echo in dye-doped polymer films at room temperature. The possibility of implementing the anti-Stokes mode of coherent laser cooling of such films is analyzed.     

Photon echo in oscillator-type multilevel systems

The effect of photon echo is studied in several multilevel systems with quasi-equidistant spectra. The analysis focuses on the echo schemes that are of interest for femtosecond photon echo experiments involving both fundamental and combination vibrational modes of molecules.     

Implementation of room temperature femtosecond echo-processing

Conditions for implementing room temperature femtosecond echo-processing in a phtalocyanine-doped polyvinylbutyral film were studied. A logical operation of bit-by-bit multiplication on the basis of the nonlinear third-order optical response (primary photon echo) of the medium was performed in a time of less than 1 ps.     

Femtosecond coherent spectroscopy of four-wave mixing and photon echoes in a GaAs/AlGaAs heterostructure at room temperature

The results from experiments employing coherent femtosecond spectroscopy in a layer of two-dimensional electron gas at the boundary of the GaAs/AlGaAs heterojunction at room temperature are presented. The decay curves of primary femtosecond photon echo are obtained. The decoherence time in two-dimensional electron gas depends strongly on the power of the exciting pulse and varies from 36 to 54 fs. The dephasing time is studied for the first time as a function of the power of exciting pulses at room temperature. It is established that this dependence obeys the law T ₂ ∼ N ^−0.22, which differs from the typical law T ₂ ∼ N ⁻¹ for unscreened electron-electron interaction in semiconductor crystals. Analysis shows that electron-phonon interaction plays an important part along with electron-electron interaction. The induced spin gratings in the GaAs/AlGaAs heterostructure are studied with an eye to their possible application in spintronics.     

Femtosecond quantum dynamics of diatomic molecules in optically dense gaseous media

A consistent quantum theory of a two-pulse pump-probe experiment in the femto-and picosecond spectroscopy of diatomic molecules is developed. The theory aims at the use of weak quantum femtosecond light pulses to excite molecular coherence. It is suggested that the experiment be realized using a double-frequency excitation scheme that is feasible for Na₂ molecules. On the basis of the theory developed, the luminescence signals are calculated at different temperatures and different optical densities of the medium for the time range from 100 fs to ~ 100 ps. The results obtained show the presence of a strong dependence of the time and frequency spectra of a luminescence signal on the optical density of the medium at temperatures from 50 to 300 K. The proposed approach makes it possible to use the quantum coherence properties of optically dense media for more detailed study of the vibronic dynamics of diatomic molecules, in particular, detection of weak optical transitions.     

Photon echo experiments on 3–3′ diethylthiacarbocyanine in glass-forming liquids with ultrafast laser pulses

Summary  Photon echo experiments on diethylthiacarbocyanine iodide (DTCI) in dibuthylphthalate (DBP) and glycerol in the glass phase with femtosecond pulses are presented. The PE signal shows nonexponential decays even at low temperatures. This behavior is attributed to the many vibronic resonances of the chromophore/ glass system which can be simultaneously excited by the spectral bandwith of the laser pulses. We are able to extract from the PE decays the contribution of the homogeneous dephasing rate of the 0-0 transition of DTCI up to 20 K in dibutylphthalate and 40 K in glycerol. A beating structure in the echo decay for the system DTCI/DBP is also observed from 8 K up to 180 K (glass transition temperature for DBP). This feature is attributed to an intereference process involving excitation of a vibronic transition of DTCI characterized by a vibrational frequency of 300 cm^-1.     

光学性质对超快激光辐照下铜膜热响应的影响

在超快激光照射过程中,金属靶材的光学性质是动态变化的。采用双温模型与分子动力学结合法,考虑动态和常数光学性质两种情况,对不同脉宽的超快激光照射下铜薄膜的热响应进行了模拟研究。其中,常数光学性质包括由激光沉积能量相等计算得到的等效平均反射率和室温下的吸收系数。结果表明:两种情况下的电子温度和晶格温度均差别较小,尤其是脉宽远小于电子-晶格弛豫时间的飞秒激光; 而当激光脉宽相当于或大于电子-晶格弛豫时间时,如皮秒激光,光学性质的动态变化对材料的熔化和重凝的影响则比较明显。     

光学性质对超快激光辐照下铜膜热响应的影响

在超快激光照射过程中,金属靶材的光学性质是动态变化的。采用双温模型与分子动力学结合法,考虑动态和常数光学性质两种情况,对不同脉宽的超快激光照射下铜薄膜的热响应进行了模拟研究。其中,常数光学性质包括由激光沉积能量相等计算得到的等效平均反射率和室温下的吸收系数。结果表明:两种情况下的电子温度和晶格温度均差别较小,尤其是脉宽远小于电子-晶格弛豫时间的飞秒激光; 而当激光脉宽相当于或大于电子-晶格弛豫时间时,如皮秒激光,光学性质的动态变化对材料的熔化和重凝的影响则比较明显。     

Simulation of coherent responses of resonant media excited by a series of ultrashort laser pulses

A technique for the numerical simulation of coherent optical responses of the photon echo type formed in resonance media with strong inhomogeneous broadening under the action of femtosecond laser pulses is developed. This approach is based on solving the Maxwell-Bloch equations using the finite-difference time-domain (FDTD) method without application of a fast rotating field and slowly varying envelope approximations. The method can be used to simulate coherent responses in different resonance media with a complex structure of energy levels. The technique was validated using the example of describing experiments on narrowing a photon echo pulse and femtosecond echo processing.     

Spectrally dispersed femtosecond CARS investigation of vibrational characteristics in ethanol

Spectrally dispersed femtosecond time‐resolved coherent anti‐Stokes Raman spectroscopy is applied to study the ultrafast vibrational dynamics in ethanol at room temperature. The anti‐Stokes intensities were monitored as a function of delay time and wavenumber. By simply changing the timing of the laser pulses, the vibrational dynamics between the excited Raman transitions in ethanol molecules can be tracked and detected. Copyright © 2014 John Wiley & Sons, Ltd.     

New mechanism for non-trivial intra-molecular vibrational dynamics

We investigate the time evolution process of one selected (initially prepared by optical pumping) vibrational molecular state S, coupled to all other intra-molecular vibrational states R of the same molecule, and also to its environment Q. Molecular states forming the first reservoir R are characterized by a discrete dense spectrum, whereas the environment reservoir Q states form a continuous spectrum. Assuming the equidistant reservoir R states we find the exact analytical solution of the quantum dynamic equations. S-Q and R-Q couplings yield to spontaneous decay of the S and R states, whereas S-R exchange leads to recurrence cycles and Loschmidt echo at frequencies of S-R transitions and double resonances at the interlevel reservoir R transitions. Due to these couplings the system S time evolution is not reduced to a simple exponential relaxation. We predict various regimes of the system S dynamics, ranging from exponential decay to irregular damped oscillations. Namely, we show that there are possible four dynamic regimes of the evolution: (i) independent of the environment Q exponential decay suppressing backward R - S transitions, (ii) Loschmidt echo regime, (iii) incoherent dynamics with multicomponent Loschmidt echo, when the system state is exchanged its energy with many states of the reservoir, (iv) cycle mixing regime, when long time system dynamics looks as a random-like. We suggest applications of our results for interpretation of femtosecond vibration spectra of large molecules and nano-systems.     

Femtosecond selective optical Kerr-effect spectroscopy: Molecular dynamics study of chloroform

The femtosecond selective spectroscopy of vibrational-rotational dynamics of molecules in a liquid was realized using optical Kerr effect registration under two-pulse nonresonant excitation. The object chosen for the study was the chloroform at room temperature. It was shown that control of the separate molecular motions by creating the constructive or destructive interference of corresponding wave packets allows one to determine directly from the experiment such constants of molecular dynamics as the relaxation times of the coherent vibrations (≈1.5 ps) and those of orientational anisotropy (≈1.2 ps).     

Ultrafast time-resolved coherent degenerate four-wave-mixing spectroscopy for investigating molecular dynamics in different states

In this paper, ultrafast time-resolved coherent degenerate four-wave-mixing (DFWM) spectroscopy is performed to investigate molecular dynamics in the gaseous phase. Laser pulses lasting for 40 fs are used to create and monitor different vibrational eigenstates of iodine at room temperature (corresponding to a low saturation pressure of about 35 Pa). Using an internal time delay in the DFWM process resonant with the transition between the ground X-state and the excited B-state, the vibrational states of both the electronically excited and the ground states are detected as oscillations in the DFWM transient signal. The dynamics of either the electronically excited or ground state of iodine molecules obtained are consistent with the previous high temperature studies on the femtosecond time-resolved DWFM spectroscopy.     

Simultaneous detection of Raman- and collision-induced molecular rotations of O2 and N2 via femtosecond multi-pulses in combination with quartz-enhanced photoacoustic spectroscopy

Molecular rotational states of nitrogen and oxygen molecules at room temperature and atmospheric pressure are excited by femtosecond double- and multi-pulses with variable temporal pulse distances, and quartz-enhanced photoacoustic spectroscopy is used for their detection. A simple extrapolation of measured double-pulse data is presented, which predicts form and position of Raman-excited spectral features and thus enables us to distinguish between spectral lines caused by Raman-scattering- and collision-induced absorption both appearing when excitation by pulse trains is used.