Coherent crystallization of bismuth under strong excitation by ultrashort laser pulses

By means of coherent control, fully symmetric bismuth phonons A _1og are investigated at helium temperature depending on the degree of crystal excitation. It is shown that coherent phonons with large amplitude show phase rigidity that is absent at small amplitude. Such a change in the phase properties of crystal lattice excitations can be understood using the concept of a coherent crystal, the achievement of which results from condensation of phonon modes arising with increasing degree of excitation.     

On the nature of coherent phonons generated by ultrashort laser pulses in single-crystal antimony

Reflectivity oscillations generated by A_1g coherent phonons in an antimony single crystal have been studied by a method involving pumping and probing by femtosecond laser pulses, which was complemented by spectral filtration of the signal. An analysis of the spectrally resolved signal showed that not only the integrated intensity but also the spectrum of the probe pulse are functions of the delay time between the pumping and probing and oscillate between the Stokes and anti-Stokes components at the optical-phonon frequency. A comparison of the integrated lattice excitation relaxation dynamics with the spectrally resolved lattice excitation relaxation dynamics revealed new facets in the nature and generation mechanism of coherent phonons.     

Generation of coherent terahertz phonons by sharp focusing of a femtosecond laser beam in the bulk of crystalline insulators in a regime of plasma formation

The generation of coherent terahertz phonons in a regime of plasma formation by a femtosecond laser radiation with an intensity of 10¹³ W/cm² in the bulk of crystalline quartz has been detected by the method of probing by a probe pulse of the third harmonic. A smooth increase in the frequency of coherent terahertz phonons from 2.2 to 5.5 THz has been detected, along with its subsequent sharp decrease down to 2.2 THz due to an α-β phase transition in crystalline quartz. The generation of 1-THz coherent phonons has been detected in BaF₂ crystals. A smooth variation of the frequency of coherent phonons from 2 to 2.5 THz has been detected in leucosapphire. The generation of coherent phonons during local laser excitation in CaF₂ and LiF crystals develops at the frequencies of 2.3 and 0.1 THz, respectively.     

Investigation of coherent phonons in bismuth by femtosecond laser and X-ray pulse probing

Using femtosecond laser pulses, coordinated oscillations (coherent phonons) of Bi single-crystal atoms have been excited and recorded. The comparison with experimental results on the X-ray probing of coherent phonons at the same excitation energy density demonstrates that the observed lifetime and frequency shift of the oscillations are similar in both cases. Moreover, it has been revealed that the relaxation (incoherent) contributions are identical. This coincidence of the photoinduced response parameters indicates that probing in the visible spectrum correctly reflects the coherent dynamics of the Bi crystal lattice.     

Investigation of ultrafast processes in photoexcited bismuth by broadband probing in the wavelength range 0.4–0.9 μm

The decay of the photoexcited state of a bismuth single crystal is investigated in the wavelength range from 400 to 900 nm by means of femtosecond laser reflection spectroscopy. Oscillations produced by coherent fully symmetric A _1g phonons have been detected in the photoinduced response, along with a relaxation component. The dynamics of the electronic subsystem of the crystal is shown to be characterized by three values of the decay time: 1 ps, 7 ps, and ∼1 ns. The spectral dependence of the reflectivity oscillation amplitude has been measured; the possible cause of the shape of the derived curve is described.     

Period dependence of coherent oscillations in manganite superlattices

In three-component superlattices of [(NdMnO3)_n/(SrMnO3)_n/(LaMnO3)_n]_m consisting of nonferroelectric and antiferromagnetic layers, the generation of coherent phonons in the system is investigated by the relaxation process of the superlattice using a pump–probe technique. The coherent phonons are related to MnO₆ octahedra in the layers. While the frequency of the phonons is 49 GHz without regard to the period of each layer, the amplitude of the phonons is influenced by the period of the superlattices. The superlattices have the maximum amplitude of the acoustic phonons at a period of (5, 12) together with strong emergent interfacial effects such as ferromagnetism and ferroelectricity, which are not present in bulk materials. This implies a possible correlation of the lattice structure with interface-induced properties.     

Anomalous dephasing of bosonic excitons interacting with phonons in the vicinity of the Bose-Einstein condensation

The dephasing and relaxation kinetics of bosonic excitons interacting with a thermal bath of acoustic phonons is studied after coherent pulse excitation. The kinetics of the induced excitonic polarization is calculated within Markovian equations both for subcritical and supercritical excitation with respect to a Bose-Einstein condensation (BEC). For excited densities n below the critical density , an exponential polarization decay is obtained, which is characterized by a dephasing rate . This dephasing rate due to phonon scattering shows a pronounced exciton-density dependence in the vicinity of the phase transition. It is well described by the power law that can be understood by linearization of the equations around the equilibrium solution. Above the critical density we get a non-exponential relaxation to the final condensate value p⁰ with that holds for all densities. Furthermore we include the full self-consistent Hartree-Fock-Bogoliubov (HFB) terms due to the exciton-exciton interaction and the kinetics of the anomalous functions . The collision terms are analyzed and an approximation is used which is consistent with the existence of BEC. The inclusion of the coherent exciton-exciton interaction does not change the dephasing laws. The anomalous function F_k exhibits a clear threshold behaviour at the critical density. Received 13 December 1999     

Details of coherent phonon motion in tellurium

We study impulsively excited coherent phonons in tellurium at various lattice temperatures and excitation densities. From a comparison with temperature-dependent reflectivity and X-ray scattering data, we infer the direction and estimate the amplitude of the coherent ionic motion. In addition, we show quantitatively that electronic weakening of the crystal forms the dominant contribution to a density-dependent red-shift of the phonon frequency.     

Observation of coherent phonon states in porous silicon films

The dynamics of differential transmission and reflectance spectra of porous silicon films was studied using the femtosecond excitation technique (τ≈50 fs, ?ω_pump=2.34 eV) with supercontinuum probing (?ω_probe=1.6–3.2 eV) and controlled time delay with a step of Δt=7 fs between the pump and probe pulses. A short-lived region of photoinduced bleaching was observed in the differential transmission spectra at wavelengths shorter than the pump wavelength. The excitation of coherent phonon states with a spectrum corresponding to nanocrystalline silicon with an admixture of a disordered phase was observed. The relaxation of electronic excitation was found to slow down in the spectral region where the amplitude of excited coherent vibrations was maximal.     

Radiative recombination of excitons in semiconductors at high excitation intensities

The shape of the exciton luminescence band of the gaseous phase of free excitons in crystal Si is investigated under high level excitation. Only the exciton-exciton interaction is considered and the influence of collisions of excitons with phonons and electrons is not taken into account. The theoretical shape of the exciton luminescence band qualitatively agrees with experiment. The process of the radiative Auger-recombination of two indirect excitons without the participation of phonons is studied in Ge and Si. The expression for the frequency and temperature dependence of the probability of radiation is obtained. The band has the asymmetrical Gaussian form with the steep short-wave tail. The predicted luminescence band in Ge is shifted at the long-wavelength side from the well-known exciton luminescence bands with participation of the phonons. The selection rules for the probability amplitude of the process under consideration are obtained.     

Generation of coherent phonons in bismuth by ultrashort laser pulses in the visible and NIR: Displacive versus impulsive excitation mechanism

We have applied femtosecond pump-probe technique with variable pump wavelength to study coherent lattice dynamics in Bi single crystal. Comparison of the coherent amplitude as a function of pump photon energy for two different in symmetry Eg and A1g phonon modes with respective spontaneous resonance Raman profiles reveals that their generation mechanisms are quite distinct. We show that displacive excitation, which is the main mechanism for the generation of coherent A1g phonons, cannot be reduced to the Raman scattering responsible for the generation of lower symmetry coherent lattice modes.     

Nonlinear lattice dynamics of bismuth as a means of clarifying the nature of coherent phonons

The nonlinear regime of generation of coherent phonons in bismuth single crystals irradiated by ultrashort high-energy laser pulses has been investigated. In this regime, autoecho phenomenon is implemented at a low (helium) crystal temperature, manifesting itself as a collapse and revival of reflection oscillations generated by coherent A _1g phonons. Different mechanisms of the observed phenomena are considered.     

Pump pulse duration dependence of coherent phonon amplitudes in antimony

Coherent optical phonons of A_1k and E_k symmetry in antimony have been studied using the femtosecond pump–probe technique. By varying the pump-pulse duration and keeping the probe duration constant, it was shown that the amplitude of coherent phonons of both symmetries exponentially decreases with increasing pulse width. It was found that the amplitude decay rate for the fully symmetric phonons with larger frequency is greater than that of the doubly degenerate phonons, whereas the frequency and lifetime for coherent phonons of both symmetries do not depend on the pump-pulse duration. Based on this data, the possibility of separation between dynamic and kinematic contributions to the generation mechanism of coherent phonons is discussed.     

Femtosecond reflectivity study of coherent phonons in doubly photoexcited bismuth

Ultrafast optical excitation generates coherent A_1g optical phonons in bismuth via the mechanism of displacive excitation of coherent phonons (DECP). Here, femtosecond pump-probe reflectivity spectroscopy examines the dynamics of coherent phonons in doubly photoexcited bismuth. Two successive pump pulses are employed to generate optic phonons and thus to investigate the phonon dynamics including bond softening and dephasing with variable pump fluence and inter-pump separation times. Combined with thermal analysis, it distinguishes thermal and nonthermal effects of photoexcitation on the phonon dynamics. It also reveals that photocarriers relax via electron-hole recombination and diffusion out of the optical penetration depth on ultrafast timescales of 10–20?ps.     

Coherent phonon excitation in bismuth

Ultrafast time-resolved reflectivity of a bismuth thin film evaporated on a silicon substrate is measured to investigate coherent phonons in bismuth. The reflectivity result is analyzed by a linear chirp approximation to obtain the time dependent frequencies of coherent phonons. Not only the optical modes are detected, which are generated by a combination of impulsive stimulated Raman scattering and displacive excitation of coherent phonon, acoustic phonon modes are also observed, which are emitted by the A_1g optical phonon.     

Influence of the optical pulse length on the amplitude and phase of photoinduced coherent phonons

The theoretical dependence of the amplitude and initial phase of photoinduced coherent vibrations of the crystal lattice on the length of a rectangular excitation optical pulse has been derived. Comparison with the experiment for tellurium has been carried out.     

Investigation of the dependence of the coherent dynamics of a bismuth lattice on the crystal excitation level

Fully symmetric coherent phonons in bismuth have been investigated in a wide range of pumppulse energies by the pump-probe method using femtosecond laser pulses. It has been shown that in the linear regime, which is implemented only for low pump intensities, the coherent amplitude is proportional to the pump intensity, whereas the relaxation rate of the coherent state and its frequency remain unchanged. In the nonlinear regime, which can be divided into superlinear and sublinear regions, the relaxation rate of a photoinduced lattice state can be approximated by a two-component response, where only one component depends strongly on the pump intensity. The nature of the coherence of the phonon subsystem, which is created by an ultrashort laser pulse, is discussed on the basis of the analysis of the data.     

Coherent molecular vibrational dynamics of CH2 stretching modes in polyethylene studied by femtosecond‐CARS

We have studied the coherent molecular vibrational dynamics of CH₂ stretching modes in polyethylene by time‐resolved femtosecond coherent anti‐Stokes Raman spectroscopy. We observed that the coherent vibrational relaxation of symmetric CH₂ stretching modes in polyethylene at room temperature is much faster than that previously measured in polyvinyl alcohol. In addition, it was detected that, at low temperature, the coherent vibrational relaxation of the symmetric stretching modes evidently becomes slower compared with that at room temperature. These temperature‐dependent measurements enable us to discriminate the contribution of pure dephasing mechanism, due to phonons and two‐level systems in polymer, from the contribution of lifetime of the vibrational excited state to the coherent vibrational relaxation of CH₂ stretching modes. We conclude that the coherent vibrational relaxation of symmetric CH₂ stretching modes at room temperature consists of the contribution of lifetime and approximately 1.5 times larger contribution of pure dephasing. Copyright © 2015 John Wiley & Sons, Ltd.     

Echo spectroscopy of two-level systems in a Y2SiO5: Pr3+ crystal

The results of an experimental investigation of low-temperature optical spectra and phase relaxation of electronic excitations of Pr³⁺ impurity ions in a Y₂SiO₅ crystal are reported. It is established that at low temperatures spectral lines are broadened by a mechanism that is uncharacteristic for crystals and is due to the interaction of impurity ions with two-level systems. The constants characterizing the interaction of Pr³⁺ impurity ions with phonons and two-level systems are found. Zh. éksp. Teor. Fiz. 115, 704–715 (February 1999)