Speaker
Description
Recent advances in high harmonic spectroscopy have enabled new methods to study ultrafast coherent dynamics of excited electron-hole wave packets in condensed matter. Electron-hole pairs are generated by the strong non-resonant light field via quantum tunneling and accelerated in the lattice to high energies. The electron and hole can eventually recombine leading to production of high-energy photons in the form of high harmonic frequencies. The three-step generation process is further supplemented by the presence of intraband anharmonic currents due to non-parabolic dispersion of electrons and holes in solid-state materials. The spectral and temporal properties of the output radiation and its dependence on the polarization state of the driving field reflects the crystal structure and ultrafast carrier dynamics. Besides the standard production of high harmonics using a single-frequency driving pulse [1], high-order frequency mixing can be induced by using two-color illumination. If a resonant pulse is used to coherently excite the electron-hole pairs, which are then driven by the non-resonant strong field, so-called high order sidebands are produced. The output photons have energies equal to $ω_{excitation}$+2 n*$ω_{non-resonant}$ where $ω_{excitation}$ and $ω_{non-resonant}$ are the frequencies of resonant and the non-resonant driving lasers respectively and n is an integer number. Odd-order sidebands are absent in a centrosymmetric crystal. The high-order sidebands were studied using terahertz driving pulses in bulk semiconductors, but mid-infrared pulses have not been used till now [2] [3].
In this contribution, we report on the observation of high-order frequency mixing and sideband generation in crystalline silicon using broadband near-infrared femtosecond pulses generated in a noncollinear optical parametric amplifier (650-950 nm) which serve for excitation of carriers and mid-infrared pulses (central wavelength of 2000 nm) for driving the excited carriers non-resonantly. The pulses are collinear, and the generated radiation is collected in the reflection geometry which is used to avoid propagation effects in the material. We study the generated high-energy photon spectra in the vacuum ultraviolet (VUV) spectral region. We focus on the dependence of the output radiation on the polarization of the driving laser pulses with respect to the crystal orientation of the sample. This may potentially allow to trace the electron dynamics as the electrons are accelerated by the non-resonant pulse after the initial excitation to the specific location in the Brillouin zone by the excitation pulse. We experimentally study the generation mechanisms and compare the efficiencies of high harmonic and high order sideband generation in the same material. The two-color illumination leads to a significant enhancement of the VUV emission compared to single-color high harmonic generation, which may bring interesting applications in high energy photon sources based on nonperturbative nonlinear optics in solids.
References
1. Ghimire, S., DiChiara, A., Sistrunk, E. et al. Observation of high-order harmonic generation in a bulk crystal. Nature Phys 7, 138–141 (2011). https://doi.org/10.1038/nphys1847
2. Zaks, B., Liu, R. & Sherwin, M. Experimental observation of electron–hole recollisions. Nature 483, 580–583 (2012). https://doi.org/10.1038/nature10864
3. Zaks B, Banks H, Sherwin MS. High-order sideband generation in bulk GaAs. Applied Physics Letters; 102. doi:10.1063/1.4773557