Phonons In Bismuth Research Articles

Hot carrier transport limits the displacive excitation of coherent phonons in bismuth

We performed femtosecond transient reflectivity measurements on epitaxially grown bismuth (Bi) films in the weak photoexcitation regime. Single crystalline ultrathin Bi films down to a thickness of 7 nm enabled us to determine a clear correspondence between the amplitude of the coherent A1g phonon and the photoexcitation level. We were able to empirically measure the effective hot carrier penetration length that determines the excited carrier density governing the magnitude of the coherent A1g phonon in Bi. Our findings suggest that the transport behavior of hot carriers is to be taken into consideration in order to provide insights into the mechanism for the displacive excitation of coherent phonons.

Femtosecond reflectivity study of coherent phonons in doubly photoexcited bismuth

Ultrafast optical excitation generates coherent A1g 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.

Fano interference at the excitation of coherent phonons: Relation between the asymmetry parameter and the initial phase of coherent oscillations

The theoretical assertion that the Fano asymmetry parameter and the asymptotic initial phase of a harmonic oscillator interacting with a continuum are interrelated is experimentally verified. By an example of coherent fully symmetric A1g phonons in bismuth that are excited by ultrashort laser pulses at liquid helium temperature, it is demonstrated that, for negative values of the asymmetry parameter, the asymptotic phase increases as the modulus of the parameter decreases.

Inhomogeneity as a source of collapse and revival for large-amplitude chirped coherentA1gphonons in bismuth

We report a spatially resolved femtosecond pump-probe measurement of the lattice dynamics in bismuth made at helium temperature over a wide range of excitation levels. We demonstrate that the collapse and revival of chirped coherent ${A}_{1g}$ phonons arises due to laterally inhomogeneous excitation conditions, whereas their enhanced decay can be due to potential damping.

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 E g and A 1 g 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 A 1 g phonons, cannot be reduced to the Raman scattering responsible for the generation of lower symmetry coherent lattice modes.

Absence of phase-dependent noise in time-domain reflectivity studies of impulsively excited phonons

There have been several reports of phase-dependent noise in time-domain reflectivity studies of optical phonons excited by femtosecond laser pulses in semiconductors, semimetals, and superconductors. It was suggested that such behavior is associated with the creation of squeezed phonon states although there is no theoretical model that directly supports such a proposal. We have experimentally re-examined the studies of phonons in bismuth and gallium arsenide, and find no evidence of any phase-dependent noise signature associated with the phonons. We place an upper limit on any such noise at least 40--50 dB lower than previously reported.

Nonequilibrium phase transition in v-group semimetals, induced by ultrashort laser pulses

Coherent, totally symmetric large-amplitude phonons in bismuth and antimony were investigated by the pump—probe method using femtosecond laser pulses. The obtained results are compared to time-resolved Raman data. It is shown that intense photoexcitation by laser pulses with a duration shorter than the lifetime and reverse phonon frequency in Bi and Sb can lead to a nonequilibrium semimetal—metal phase transition, most likely caused by the instability of the crystal’s electron subsystem.

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.

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.

Quantum dynamical study of the amplitude collapse and revival of coherent A 1g phonons in bismuth: a classical phenomenon?

We parameterize the potential energy surface of bismuth after intense laser excitation using accurate full-potential linearized augmented plane wave calculations. Anharmonic contributions up to the fifth power in the A 1g phonon coordinate are given as a function of the absorbed laser energy. Using a previously described model including effects of electron–phonon coupling and carrier diffusion due to Johnson et al., we obtain the time-dependent potential energy surface for any given laser pulse shape and duration. On the basis of this parameterization we perform quantum dynamical simulations to study the experimentally observed amplitude collapse and revival of coherent A 1g phonons in bismuth considering work of Misochko et al. Our results strongly indicate that the observed beatings are not related to quantum effects and are most probably of classical origin.

Coupling of ultrafast laser energy to coherent phonons in bismuth

Energy coupling to coherent phonons in Bi during femtosecond laser–bismuth interaction is investigated using a double-pulse femtosecond pulse train generated from a temporal pulse shaper. It is found that the increase of bismuth temperature is dependent on the separation time between the two laser pulses. Using a numerical fitting, which considers the effect of convolution between the incident pulses and the material response, the measured temperature increases using different pulse-to-pulse separations allow quantitative determination of the amount of laser energy coupled from excited electrons to coherent phonon vibration.

Fano interference for large-amplitude coherent phonons in bismuth

We report femtosecond time-resolved measurements of lattice dynamics inbismuth made over a wide range of temperatures and excitation levels. Wedemonstrate that time-integrated Fourier transforms for both the fully symmetricA1g and doublydegenerate Eg coherent oscillations of large amplitude exhibit asymmetric line shapes described by theFano formula. Measuring the real and imaginary part of the phonon self-energy, we attemptto identify the nature of the continuum responsible for the configuration mixing. Based onthe measured pump and temperature dependences, we suggest that the continuumresponsible for the interference includes both the electronic and lattice degrees of freedom.

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.

Laser-induced phonon-phonon interactions in bismuth

We compute laser-induced interactions between coherent phonons in bismuth and demonstrate that they are key to understanding important experiments performed with intense femtosecond laser pulses. In particular, we find mixing signals and higher harmonics arising from the coupling between phonons of different and the same symmetries, respectively. We show that the phonon-phonon interaction is strongly dependent on the laser fluence and is for that reason only observable when sufficiently strong laser pulses are used. Our results offer a unified description of the different experimental observations performed so far on bismuth.

Ultrafast Dynamics of Laser-Excited Solids

AbstractWe discuss recent experimental and theoretical results on ultrafast materials dynamics. Intense, femtosecond lasers can deposit energy in a time that is short compared with relaxation processes and can generate extremely large carrier densities that drive bond softening, nonthermal melting, and ablation. In particular, we present optical experiments on electronic softening of coherent phonons in bismuth and x-ray experiments on ultrafast disordering in indium antimonide that probe the bonding of the lattice under successively higher carrier concentrations. We review a number of molecular dynamics simulations and their assumptions, which address nonthermal melting. Large-scale molecular dynamics simulations elucidate the role of void formation in laser ablation.

Observation of an amplitude collapse and revival of chirped coherent phonons in bismuth.

We have studied the A(1g) coherent phonons in bismuth generated by high fluence ultrashort laser pulses. We observed that the nonlinear regime, where the phonons' oscillation parameters depend on fluence, consists of subregimes with distinct dynamics. Just after entering the nonlinear regime, the phonons become chirped. Increasing the fluence further leads to the emergence of a collapse and revival, which next turns into multiple collapses and revivals. This is explained by the dynamics of a wave packet in an anharmonic potential, where the packet periodically breaks up and reconstitutes in its original form, giving convincing evidence that the phonons are in a quantum state, with no classical analog.

Observation of the magneto-phonon effect due to acoustic phonons in the planar Hall effect of bismuth

The magneto-phonon effect due to acoustic phonons in bismuth, first observed in the thermal variation of the longitudinal Shubnikov–de Haas oscillations, has been investigated by the measurement of the oscillatory planar Hall coefficient. The oscillatory planar Hall coefficient R was obtained from the measurements of magneto-resistivity component ρ yx ( y) of bismuth in magnetic fields up to 5 T at several fixed temperatures. The obtained R became largest in amplitude at temperatures around 6.5 or 5.4 K, and became less and less intense as the temperature decreased. This temperature dependence was understood by considering the magneto-phonon effect, in which the number of phonons responsible for the inter-Landau level transition of electrons decreased with lowering temperature.

Dephasing of coherent THz phonons in bismuth studied by femtosecond pump–probe technique

We have studied the effect of point defects on coherent optical phonons and photo-generated carriers in ion-implanted bismuth polycrystalline films and single crystals by means of pump–probe reflectivity technique. The dephasing time of the A 1g phonon decreases monotonically with increasing ion dose, which is explained by phonon–defect scattering. The relaxation time of photo-excited carriers decreases as the ion dose increases, being attributed to trapping of carriers by defect-induced deep levels. The behavior of the carrier and phonon dynamics for the polycrystalline film is compared with those for the single crystal.

Coherent phonons in bismuth under high-density excitation

We investigate the dynamics of coherent phonons in bismuth under high-density excitation with a pump–probe technique. As the pump power density is increased, the E g mode appears in addition to the A 1g mode, and the A 1g mode in the Fourier-transformed spectra is asymmetrically broadened. Analysis based on a time-partitioning Fourier transformation reveals that the frequency of the coherent A 1g phonon is instantaneously downshifted upon the excitation and that the line shape of the A 1g mode changes from an asymmetric to symmetric one as the coherent oscillation decays. These behaviors of the coherent phonons are attributed to anharmonicity of the lattice vibrations.

Dynamics and coherent control of high-amplitude optical phonons in bismuth

High-amplitude optical phonons in bismuth are generated through impulsive stimulated Raman scattering and detected through the induced change in the optical reflectivity. Reflectivity changes greater than 1% have been recorded. Phonon dynamics and coherent control in the high-amplitude regime are reported.