Determination of the Optical Index for Few-Layer Graphene by Reflectivity Spectroscopy

TL;DR

This study measures the wavelength-dependent optical refractive index of few-layer graphene using reflectivity spectroscopy, revealing that each layer number has a unique optical index that cannot be described by a common constant.

Contribution

It provides the first precise experimental determination of the complex refractive index for monolayer, bilayer, and trilayer graphene across visible wavelengths.

Findings

Each few-layer graphene has a unique optical index.

A constant optical index cannot describe all layers simultaneously.

Refractive indices were accurately extracted from experimental data.

Abstract

We have experimentally studied the optical refractive index of few-layer graphene through reflection spectroscopy at visible wavelengths. A laser scanning microscope (LSM) with a coherent supercontinuum laser source measured the reflectivity of an exfoliated graphene flake on a Si/SiO2 substrate, containing monolayer, bilayer and trilayer areas, as the wavelength of the laser was varied from 545nm to 710nm. The complex refractive index of few-layer graphene, n-ik, was extracted from the reflectivity contrast to the bare substrate and the Fresnel reflection theory. Since the SiO2 thickness enters to the modeling as a parameter, it was precisely measured at the location of the sample. It was found that a common constant optical index cannot explain the wavelength-dependent reflectivity data for single-, double- and three-layer graphene simultaneously, but rather each individual few-layer graphene possesses a unique optical index whose complex values were precisely and accurately determined from the experimental data.