Probing optical resonances of silicon nanostructures using tunable-excitation Raman spectroscopy.

M Matthiae,A Larroche,A Kristensen,C Zhou,S Raza,K E S Nielsen

doi:10.1364/oe.385088

Abstract

Optical materials with a high refractive index enable effective manipulation of light at the nanoscale through strong light confinement. However, the optical near field, which is mainly confined inside such high-index nanostructures, is difficult to probe with existing measurement techniques. Here, we exploit the connection between Raman scattering and the stored electric energy to detect resonance-induced near-field enhancements in silicon nanostructures. We introduce a Raman setup with a wavelength-tunable laser, which allows us to tune the Raman excitation wavelength and thereby identify Fabry-Pérot and Mie type resonances in silicon thin films and nanodisk arrays, respectively. We measure the optical near-field enhancement by comparing the Raman response on and off resonance. Our results show that tunable-excitation Raman spectroscopy can be used as a complimentary far-field technique to reflection measurements for nanoscale characterization and quality control. As proof-of-principle for the latter, we demonstrate that Raman spectroscopy captures fabrication imperfections in the silicon nanodisk arrays, enabling an all-optical quality control of metasurfaces.

Highlights

Advancements in optical technology are fueled by our understanding and characterization of optical materials at the nanoscale
We introduce a Raman setup with a wavelength-tunable laser, which allows us to tune the Raman excitation wavelength and thereby identify Fabry-Pérot and Mie type resonances in silicon thin films and nanodisk arrays, respectively
To benchmark tunable-excitation Raman spectroscopy, we first demonstrate that Raman spectroscopy with a fixed excitation wavelength can successfully detect resonance-induced near-field enhancements

Summary

Introduction

Advancements in optical technology are fueled by our understanding and characterization of optical materials at the nanoscale. Raman spectroscopy measures inelastic scattering of photons from phonon and vibrational modes in materials and its signal strength depends dramatically on the electric near-field enhancement [6,7,8]. This dependency has driven research in surface-enhanced Raman spectroscopy to create surfaces that amplify the local electric field outside nanostructures for sensing of nearby molecules [9]. Instead of sensing external material, we exploit the phonons in the crystalline high-index material as a natural probe of the internal near field This enables us to use a far-field Raman measurement system to determine the internal near-field enhancement created by Mie resonances without the need of any external or invasive probes

Methods

Results

Conclusion