Fiber-based angular filtering for high-resolution Brillouin spectroscopy in the 20-300 GHz frequency range.

A Rodriguez,O Ortiz,P Senellart,N D Lanzillotti-Kimura,A Lemaître,P Priya,M Esmann,C Gomez-Carbonell

doi:10.1364/oe.415228

Abstract

Brillouin spectroscopy emerges as a promising non-invasive tool for nanoscale imaging and sensing. One-dimensional semiconductor superlattice structures are eminently used for selectively enhancing the generation or detection of phonons at few GHz. While commercially available Brillouin spectrometers provide high-resolution spectra, they consist of complex experimental techniques and are not suitable for semiconductor cavities operating at a wide range of optical wavelengths. We develop a pragmatic experimental approach for conventional Brillouin spectroscopy that can integrate a widely tunable excitation-source. Our setup combines a fibered-based angular filtering and a spectral filtering based on a rotating single etalon and a double grating spectrometer for sequential reconstruction of Brillouin spectra. This configuration allows probing confined acoustic phonon modes in the 20-300 GHz frequency range with excellent laser rejection and high spectral resolution. Remarkably, our scheme based on the excitation and collection of the enhanced Brillouin scattering signals through the optical cavity allows for better angular filtering with decreasing phonon frequency. It can be implemented for the study of cavity optomechanics and stimulated Brillouin scattering over broadband optical and acoustic frequency ranges.

Highlights

Conventional Raman spectroscopy techniques are used to study optical phonons in the THz range, which are spectrally far from the laser line
We have applied a combination of optical filtering techniques that allow us to access the spontaneous Brillouin scattering signal originated by the confined acoustic mode of a semiconductor microcavity
The technique relies on the angular filtering with a single mode fiber permitted by the angular offset between the incoming laser and scattered signal

Summary

Introduction

Conventional Raman spectroscopy techniques are used to study optical phonons in the THz range, which are spectrally far from the laser line While these techniques are usually compatible with excitation sources over a wide range in optical wavelengths, they provide an insufficient straylight rejection in order to observe Brillouin modes as low as a few tens of GHz =~ 1 cm-1. The additional spectral filtering is implemented through tandem of an etalon and double Raman spectrometer This combination has enabled us to observe the low frequency acoustic modes of a cavity at 20 GHz with a simple etalon, which are otherwise concealed in the excitation laser background. Under this condition, the excitation laser and the Brillouin scattered signal are coupled to the optical cavity mode. We benefit from the DOR to spatially filter the Brillouin signal by selecting the scattered signal for a given k// with a fiber coupler [25,26]

Methods

Conclusion