O. V. Misochko

Temperature dependence of coherent A1g and Eg phonons of bismuth

Bismuth has been a model material in the study of femtosecond dynamics of coherent lattice oscillations. The generation mechanism was first proposed to be displacive for the symmetric A1g mode, which was the only mode observed as a coherent phonon. The absence of the other Raman active mode Eg has not been fully explained, but was phenomenologically attributed to the exclusive coupling of the hot electrons at k∼0 and high symmetry phonons. In the present study, we demonstrate that both A1g and Eg modes are excited as coherent phonons at low temperature and confirm that the coherent phonons are generated via a Raman process in bismuth. We found a puzzling π∕2 difference in the initial phases of the two coherent phonons, which suggests that the initial phase cannot be a clear-cut index for the generation mechanism in absorbing media.

On the nature of coherent artifact

The coherent interaction of femtosecond laser pulses in the pump-probe regime has been experimentally studied in the time domain by monitoring light reflection from a tellurium single crystal. The optical response of the probed medium exhibits periodic variations at a frequency equal to that of the exciting laser radiation. Experimental dependences of the observed “coherent artifact” on the pump/probe intensity ratio, the number of accumulated pulses, and the mutual orientation of the polarization vectors of electromagnetic fields and the crystallographic axes are well described by the proposed phenomenological model.

Coherent A1g and Eg phonons of antimony

We report the ultrafast dynamics of the coherent A1g and Eg phonons of antimony as a function of temperature and optical polarization. Like in bismuth, the two phonon modes exhibit nearly π/2 difference in their initial phase, suggesting their different coupling strengths with photoexcited electrons. The dependence of the phonon amplitude on the optical polarization and temperature indicates the generation of the coherent A1g phonons through both displacive and Raman processes, rather than a purely displacive one. In contrast, the generation of the coherent Eg phonons can be understood within Raman framework alone.

Coherent Lattice Oscillations in Solids and Their Optical Control

Coherent optical phonons are the lattice atoms vibrating in phase with each other over a macroscopic spatial region. With sub-10 fs laser pulses, one can impulsively excite the coherent phonons of a frequency up to 50 THz, and detect them optically as a periodic modulation of electric susceptibility. The generation and relaxation processes depend critically on the coupling of the phonon mode to photoexcited electrons. Real-time observation of coherent phonons can thus offer crucial insight into the dynamic nature of the coupling, especially in extremely nonequilibrium conditions under intense photoexcitation.

Fully symmetric and doubly degenerate coherent phonons in semimetals at low temperature and high excitation : similarities and differences

We report the ultrafast dynamics of simultaneously excited coherent A1g and Eg phonons in semimetals Bi and Sb. At a low level of excitation, the doubly degenerate Eg phonons in pump–probe data occur at low temperature only, and they nearly vanish at room temperature. Their initial phase is shifted by π/2 with respect to that of the fully symmetric A1g phonons indicating that the Eg and A1g phonons are excited predominantly impulsively and displacively, respectively. At a high level of excitation, both phonons display an asymmetric (varying in time) line shape with more spectral weight at low frequencies testifying to quantum interference between the one-phonon state and a continuum of states. Furthermore, above a threshold fluence, these phonons of different symmetry exhibit collapse and revival, albeit with different characteristic times.

Coherent phonons and their properties

The optical phonons in semimetals, semiconductors, and superconductors were studied by the light reflection techniques with femtosecond time resolution and by the method of spontaneous Raman scattering. During measurements in the time domain, the phonon system is converted into a coherent state by the first ultrashort laser pulse and then probed at a variable delay by the second pulse. In this case, the phonons are shown to occur in a nonclassical state in which their fluctuational properties, different in various quadratures, are described by periodic functions of time. A comparison of the results obtained in the time and frequency domains gives evidence that the energies of thermal and coherent phonons coincide, while their dephasing and energy relaxation times are different.

Coherent A1 phonons in Te studied with tailored femtosecond pulses

We tailor the shape and phase of the pump-pulse spectrum in order to study coherent lattice dynamics in tellurium. Employing coherent control by splitting the pump pulse into a two-pulse sequence, we show that the oscillations due to A(1) coherent phonons can be cancelled but not enhanced as compared to single-pulse excitation of the same energy. We further demonstrate that a crucial factor for coherent phonon generation seems to be the bandwidth of the pulse spectrum. We observe that the coherent amplitude for long pump pulses decreases exponentially with pulse duration for both Gaussian and rectangular pulses. Finally, by varying the pulse chirp, we show that the coherent amplitude is independent of the chirp sign while the oscillation lifetime is slightly dependent on the chirp sign.

Peculiar noise properties of phonons generated by femtosecond laser pulses in antimony

In femtosecond pump-probe experiments on antimony, we have detected coherent oscillations that exhibit phase-dependent noise. The Fourier transforms of the coherent amplitude and its variance show that the two fluctuate at different frequencies, suggesting that the phonons created in the pump-probe experiment are of a squeezed nature.

Light scattering spectroscopy of Tl-based superconductors: phonon and electronic excitations

Abstract Raman spectra of Tl-based superconducting crystals are reviewed. Temperature, pressure and resonance dependencies are presented. A factor group analysis of the zone center phonons is also given. The polarized spectra and the temperature dependencies of light scattering due to electronic excitations are presented. Anisotropy of a superconducting gap is discussed.

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.