Polarization-Resolved Study of High Harmonics from Bulk Semiconductors

Area of Science:

• Solid-state physics
• Nonlinear optics
• Quantum mechanics

Background:

• High harmonic generation (HHG) is a key process in nonlinear optics.
• Understanding the polarization properties of HHG provides insights into material electronic structures.
• Previous models often rely on perturbative nonlinear optics, which may not explain all observed phenomena.

Purpose of the Study:

• To investigate the polarization properties of high harmonics generated from gallium selenide (GaSe).
• To explain the emergence of perpendicular polarization components in odd harmonics under high electric fields.
• To establish a link between the electronic band structure of solids and HHG polarization.

Main Methods:

• Experimental investigation using linearly polarized mid-infrared laser pulses on GaSe.
• Development of a two-dimensional single-band model to explain polarization phenomena.
• Validation through three-dimensional quantum mechanical simulations.

Main Results:

• Perpendicular polarization component of odd harmonics observed at high electric fields, absent at low fields.
• The anisotropic curvature of the energy band, particularly near the Brillouin zone edge, induces perpendicular odd harmonics.
• Quantum simulations confirm orientation dependence and reveal intraband current for odd harmonics and interband polarization for even harmonics.

Conclusions:

• The study demonstrates a strong correlation between a solid's band structure and the polarization of generated odd harmonics.
• A novel theoretical model successfully explains the emergence of perpendicular polarization in odd harmonics.
• This work advances the understanding of HHG mechanisms and their connection to material properties.