Second harmonic generation in germanium quantum wells for nonlinear silicon photonics

TL;DR

This paper demonstrates strong second-harmonic generation in germanium quantum wells on silicon, achieved through symmetry breaking with asymmetric coupled quantum wells, enabling high nonlinear susceptibility in silicon-compatible materials.

Contribution

The study introduces a novel approach to induce second-order nonlinearity in centrosymmetric group-IV semiconductors using asymmetric coupled quantum wells.

Findings

Achieved giant second-order nonlinearity with susceptibility near 10^5 pm/V.

Demonstrated strong SHG at mid-infrared wavelengths between 9 and 12 microns.

Utilized symmetry breaking with ACQW to enable SHG in germanium quantum wells.

Abstract

Second-harmonic generation (SHG) is a direct measure of the strength of second-order nonlinear optical effects, which also include frequency mixing and parametric oscillations. Natural and artificial materials with broken center-of-inversion symmetry in their unit cell display high SHG efficiency, however the silicon-foundry compatible group-IV semiconductors (Si, Ge) are centrosymmetric, thereby preventing full integration of second-order nonlinearity in silicon photonics platforms. Here we demonstrate strong SHG in Ge-rich quantum wells grown on Si wafers. The symmetry breaking is artificially realized with a pair of asymmetric coupled quantum wells (ACQW), in which three of the quantum-confined states are equidistant in energy, resulting in a double resonance for SHG. Laser spectroscopy experiments demonstrate a giant second-order nonlinearity at mid-infrared pump wavelengths between 9 and 12 microns. Leveraging on the strong intersubband dipoles, the nonlinear susceptibility almost reaches 10^5 pm/V