The polarization property of high harmonics from gallium selenide is investigated using linearly polarized midinfrared laser pulses. With a high electric field, the perpendicular polarization component of the odd harmonics emerges, which is not present with a low electric field and cannot be explained by the perturbative nonlinear optics. A two-dimensional single-band model is developed to show that the anisotropic curvature of an energy band of solids, which is pronounced in an outer part of the Brillouin zone, induces the generation of the perpendicular odd harmonics. This model is validated by three-dimensional quantum mechanical simulations, which reproduce the orientation dependence of the odd-order harmonics. The quantum mechanical simulations also reveal that the odd- and even-order harmonics are produced predominantly by the intraband current and interband polarization, respectively. These experimental and theoretical demonstrations clearly show a strong link between the band structure of a solid and the polarization property of the odd-order harmonics.

We studied the propagation length of surface plasmon polaritons (SPPs) at the gold/air interface in the mid-infrared range. We showed that SPPs propagate for a distance of about or above 10 mm at a wavelength of 10.6 μm, in good agreement with the value predicted from the dielectric constant of polycrystalline gold. We also demonstrated that a simple treatment of thermal annealing led to noticeable elongation of SPP propagation length, accompanied by increased grain size and decreased surface roughness. Quantitative evaluation of SPP propagation length, in correlation with material's morphology, is important in designing plasmonic devices and beneficial for understanding the mechanisms of SPP's losses that underlie electric-field enhancement.

The excitation of four coherent phonon modes of different symmetries are realized in copper metaborate CuB2O4 via impulsive stimulated Raman scattering (ISRS). The phonons are detected by monitoring changes in the linear optical birefringence using the polarimetric detection (PD) technique. We compare the results of the ISRS-PD experiment to the polarized spontaneous Raman scattering spectra. We show that agreement between the two sets of data obtained by these allied techniques in a wide phonon frequency range of 4-14 THz can be achieved by taking into account the symmetry of the phonon modes and corresponding excitation and detection selection rules. It is also important to account for the difference between incoherent and coherent phonons in terms of their contributions to the Raman scattering process. This comparative analysis highlights the importance of the ratio between the frequency of a particular mode, and the pump and probe spectral widths. We analytically demonstrate that the pump and probe pulse durations of 90 and 50 fs, respectively, used in our experiments limit the highest frequency of the excited and detected coherent phonon modes to 12 THz, and define their relative amplitudes.

High harmonic generation (HHG) in solids has great potential for coherent extreme ultraviolet (EUV) sources, all-optical band-structure reconstruction, and electron dynamics metrology. Solid HHG driven by plasmonic near-fields will open a new paradigm, enabling high repetition-rate HHG with a compact laser, HHG manipulation with meta-surfaces, and precise control over carrier trajectory. In this paper, we demonstrate antenna-enhanced HHG in a wide-bandgap semiconductor ZnO. By exploiting gold nano-antennas resonating at the driver wavelength of 2 μm, we successfully trigger HHG at input intensity of ~0.02 TW/cm and observe harmonic radiations up to 9th-order. Orders-of-magnitude enhanced conversion efficiency at the hot-spots brings about ten-fold enhancement in the total yield. The spectral selection rule is found to reflect crystal symmetry, suggesting the possibility of nano-scaled EUV sources and band-structure reconstruction.

We demonstrate high harmonic generation from GaSe in the deep-UV range with its photon energy reaching three times the bandgap energy. The harmonic spectrum represents a clear dependence on the crystal orientation with respect to the incident field, reflecting the 6 ¯ 2 m point group. This reflection indicates the possibilities of high harmonic spectroscopy and high harmonic waveform control at frequencies well above the bandgap. The 3D time-dependent density matrix simulations reveal that the interband polarization mechanism dominates over the intraband current mechanism at frequencies above the bandgap and can generate attosecond bursts with characteristic sine-like waveforms.