CC BY
16LMI-I-1
Ultrashort-pulse-laser excited dielectric materials: Unexpected
transient optical properties
P. S. Sneftrup1, S. H. Mailer1-2, T. Winkler1, P. Balling1
1-Dept. of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark 2- Joint Attosecond Science Laboratory, University of Ottawa and National Research Council of Canada, 25
Templeton Street, Ottawa ON K1N 6N5, Canada
balling@phys.au. dk
It is well established that ultrashort laser pulses in the IR and visible parts of the electromagnetic spectrum can readily excite materials with a bandgap that vastly exceeds the photon energy. The excitation is initiated by strong-field excitation (e.g. multiphoton or tunnel excitation), and the carriers thus promoted from valence to conduction band give rise to major changes in the material. Many experiments, based on time-resolved (pump-probe) laser spectroscopy, have investigated the transient optical properties of different dielectric materials, and the behavior has often been attributed to the formation of an electron plasma described by a Drude model. For certain materials, predominately quartz, changes due to the formation of self-trapped excitons must also be included; see Ref. 1 for an overview.
In this presentation, we will focus on a few recent examples of experimental investigations of highly excited dielectric materials, which exhibit properties that are not explained by the current models. The first example reports on the ultrafast dynamics of fused silica, which has been investigated by measuring polarization-resolved reflectance of a wavelength-tunable probe pulse following a near-infrared pump pulse [2]. The data, which should, in principle, allow the determination of both real and imaginary parts of the time-dependent electric permittivity, turned out to become incompatible with an isotropic optical response ~200 fs after the excitation. This was reconciled with a uniaxial permittivity tensor, which predicts greatly enhanced absorption of p-polarized light, suggesting that an absorption channel is missing in the standard optical models for strongly excited dielectrics.
Another unexpected transient optical feature has been reported by investigating highly excited dielectric materials with slightly higher probe fluences than are normally used. By employing transmission pump-probe spectroscopy, it was demonstrated that both sapphire and quartz samples under appropriate conditions exhibit transient optical amplification of the probe pulses [3,4]. The gain is attributed to two-photon stimulated emission occurring due to an energetically localized population inversion between the bottom of the conduction band and the top of the valence band.
[1] P. Balling and J. Schou, Femtosecond-laser ablation dynamics of dielectrics: basics and applications for thin films, Rep. Prog. Phys. 76, 036502 (2013).
[2] S. H. M0ller, S. T. Andersen, and P. Balling, Transient optical properties of highly excited dielectric materials: Apparent birefringence and delayed reflectivity increase, Phys. Rev. Research 2, 043010 (2020).
[3] T. Winkler, L. Haahr-Lillevang, C. Sarpe, B. Zielinski, N. Gotte, A. Senftleben, P. Balling, and T. Baumert, Laser amplification in excited dielectrics, Nature Physics 14, 74-79 (2018).
[4] T. Winkler, P. Balling, B. Zielinski, C. Sarpe, N. Jelzow, R. Ciobotea, A. Senftleben, T. Baumert, Unveiling nonlinear light-amplification regimes in fused silica with femtosecond imaging spectroscopy, Physical Review Research 2, 023341 (2020).
