Regensburg 2022 – wissenschaftliches Programm

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HL: Fachverband Halbleiterphysik

HL 21: Optical Properties 1

HL 21.2: Vortrag

Mittwoch, 7. September 2022, 15:15–15:30, H32

Coherent phonon oscillations and transient critical-point parameters in femtosecond pump-probe ellipsometry spectra — •S Zollner¹, C Emminger^1,2,3,4, S Espinoza⁵, S Richter^3,5,6, M Rebarz⁵, O Herrfurth^3,7, M Zahradnik⁵, R Schmidt-Grund⁸, and J Andreasson⁵ — ¹NMSU, Las Cruces, NM, USA — ²Masaryk University, Brno — ³Uni Leipzig — ⁴Humboldt Universität, Berlin — ⁵ELI Beamlines, Dolni Brezany, Czech Republic — ⁶Lund University — ⁷Active Fiber Systems GmbH, Jena — ⁸TU Ilmenau

Spectroscopic ellipsometry measures the dielectric function of materials, which is related to the electronic band structure of semiconductors, and allows the study of critical points and their parameters (energy, broadening, amplitude, phase angle). At ELI Beamlines, femtosecond pump probe ellipsometry measurements can be performed with a white-light continuum probe beam, resulting in a time resolution of 120 fs. We report the transient dielectric function of Ge as a function of delay time between the pump and probe beams near the E₁ and E₁+Δ₁ critical points. The changes of the critical-point amplitudes are attributed to band filling at the L-point, intervalley scattering, carrier diffusion, lattice heating, and recombination. We also calculate the derivatives of these spectra using a novel Fourier analysis technique and determine the critical point parameters. We find oscillations of the critical-point energies with a period of 11 ps, related to the propagation of coherent acoustic phonons (strain waves) generated by the pump pulse. The amplitude of these oscillations shows that the strain is hydrostatic (isotropic) and has a magnitude of 0.1%.