# Evidence for terahertz acoustic phonon parametric oscillator based on acousto-optic degenerate four-wave mixing in silicon doping superlattice

## TL;DR

This paper presents the first experimental evidence of a terahertz acoustic phonon parametric oscillator in silicon superlattice, utilizing acousto-optic four-wave mixing to generate coherent THz phonons, opening new avenues for phonon field studies.

## Contribution

It demonstrates the realization of a terahertz acoustic phonon parametric oscillator using acousto-optic four-wave mixing in a silicon doping superlattice, a novel approach in phononics.

## Key findings

- Detection of coherent THz compressional and shear waves
- Verification of Bragg scattering and energy conservation in D4WM
- Observation of self-starting mirrorless phonon oscillation

## Abstract

We report first evidence for a 1.0 terahertz (THz) self-starting mirrorless acoustic phonon parametric oscillator (MAPPO) produced from acousto-optic phase-conjugate degenerate four-wave (D4WM) mixing in a THz laser-pumped silicon doping superlattice (DSL). The DSL was grown by molecular beam epitaxy on a (100) boron-doped silicon substrate. A superconducting NbTiN subwavelength grating was used to couple the THz laser radiation into the DSL. Superconducting granular aluminum bolometric detection, coupled with Si:B piezophonon spectroscopy, revealed excitation of THz coherent compressional and shear waves, along the <111> direction only. The Bragg scattering condition for distributed feedback, and the energy conservation requirement for the D4WM process, were both verified. A THz MAPPO could provide a testbed for studies of non-classical acoustic phonon fields.