T. Garl

Transient Dielectric Function of Fs-Laser Excited Bismuth

We present time-resolved dual-angle single-wavelength pump-probe study of the reflectivity of femtosecond laser excited bismuth crystal. The recovered real and imaginary parts of the dielectric function show complex a transition to a new quasi-stable excited state at about 20 ps. This state lasts for about 4 ns after the excitation and is not an intermediate state between the solid and the liquid phase. The results suggest that the photo-excited state does not correspond to neither a warm nor a liquid phase, and open general questions on photoexcited phase transition.

Ultrafast dielectric function dynamics in bismuth

The time-dependent reflectivity at two angles from a fs-laser-excited single crystal of Bi were simultaneously measured up to 25 ps after a 50-fs pump pulse excitation. We demonstrate that bismuth did not have transition to liquid state even at the absorbed energy twice the equilibrium enthalpy of melting. We attribute this to the fast electrons transport that carries the excess energy away from the heated surface before the thermalisation of electrons and phonons has occurred.

Probing coherently excited optical phonons by extreme ultraviolet radiation with femtosecond time resolution

We present a pump-probe experimental approach to study time-resolve coherent optical phonons using light pulses in the extreme ultraviolet wavelength range with femtosecond time resolution. Using this technique, a Bi (111)-oriented single crystal was excited by near-infrared (1.55eV) pulses and probed by a high-order harmonic generation source, whose intrinsic flux instability was reduced by making use of a normalization procedure. This unconventional approach allowed us to perform measurements in a previously inaccessible range of phonon wavevectors within the Brillouin zone.

Reflectivity oscillations of fs-laser excited Bismuth: excitation of coherent phonons

We present the experimental and theoretical studies of the optical response from the single-crystal of bismuth to the excitation by the femtosecond laser pulse. The experimental results revealed a complex, first - positive and a few picoseconds later - negative, change in time-dependent reflectivity, which could not be explained in the light of the existing theories. It is shown that reflectivity oscillations are related to the excitation of coherent phonons by the pulse with duration shorter of all relaxation times. We demonstrate that swiftly heated electrons are responsible for the phonon excitation due to the fast modification of the attractive (electronic) part of inter-atomic potential. The electronic perturbation of potential is also responsible for the red shift of phonon frequency and for the increase in the amplitude of phonons. The coherent phonon excitation process as well as the change in the reflectivity is related mainly to the modification of the electron-phonon momentum exchange frequency. The comparison between the theory and experiments shows an excellent agreement. Moreover, the reflectivity measurements allow direct recovery of the electron-phonon coupling rate in bismuth crystal, which has not been measured before.

Dynamics of the dielectric function in fs-laser excited bismuth

Time-resolved study of the dielectric function of femtosecond laser excited bismuth demonstrates that excitation of coherent phonons leads to a solid-plasma phase transition, and into a quasi-stable excited state lasting up to 4 ns.

Reflectivity oscillations of laser-excited Bi: imprint of atomic vibrations through electron-phonon coupling

Experimental and theoretical studies of probe beam reflectivity oscillations from single-crystal bismuth excited by fs-laser pulses are presented. Pump-probe technique with accuracy of 10-5 and time-resolution of 35 fs is used to measure the reflectivity. Results show that reflectivity oscillations are related to the atomic vibrations predominantly via electron-phonon coupling.

Coherent phonons imprinted into reflectivity oscillations of laser-excited Bi through electron-phonon coupling

We show that the reflectivity of laser-excited solid relates to phonons, driven by thermal forces, through the electron-phonon coupling rate. Controlled excitation of phonons is available by the optimum combination of laser and material parameters.