Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures

Summary

Photonic band structure enables understanding how confined electromagnetic modes propagate within a photonic crystal. In photonic crystals that incorporate magnetic elements, such confined and resonant optical modes are accompanied by enhanced and modified magneto-optical activity. We describe a measurement procedure to extract the magneto-optical band structure by Fourier space microscopy.

Abstract

Photonic crystals are periodic nanostructures that can support a variety of confined electromagnetic modes. Such confined modes are usually accompanied by local enhancement of electric field intensity that strengthens light-matter interactions, enabling applications such as surface-enhanced Raman scattering (SERS) and surface plasmon enhanced sensing. In the presence of magneto-optically active materials, the local field enhancement gives rise to anomalous magneto-optical activity. Typically, the confined modes of a given photonic crystal depend strongly on the wavelength and incidence angle of the incident electromagnetic radiation. Thus, spectral and angular-resolved measurements are needed to fully identify them as well as to establish their relationship with the magneto-optical activity of the crystal. In this article, we describe how to use a Fourier-plane (back focal plane) microscope to characterize magneto-optically active samples. As a model system, here we use a plasmonic grating built out of magneto-optically active Au/Co/Au multilayer. In the experiments, we apply a magnetic field on the grating in situ and measure its reciprocal space response, obtaining the magneto-optical response of the grating over a range of wavelengths and incident angles. This information enables us to build a complete map of the plasmonic band structure of the grating and the angle and wavelength dependent magneto-optical activity. These two images allow us to pinpoint the effect that the plasmon resonances have on the magneto-optical response of the grating. The relatively small magnitude of magneto-optical effects requires a careful treatment of the acquired optical signals. To this end, an image processing protocol for obtaining magneto-optical response from the acquired raw data is laid out.

Introduction

Confined electromagnetic modes in photonic crystals can arise from a variety of different origins, such as plasmon resonances around metal/dielectric interfaces or Mie resonances in high refractive index dielectric nanostructures^1,2,3, and can be designed to appear at specifically defined frequencies^4,5. Their presence gives rise to many fascinating phenomena such as photonic band gaps^6,7,8, strong photon localization

Protocol

1. Mounting the setup

1. Optics
   NOTE: Build the setup as depicted in Figure 3A on an optical table with sufficient vibration isolation. To avoid spherical and other aberrations, center all the optical components (lenses, pinholes etc.) with respect to the beam. The optical arrangement is shown in Figure 2 with the distances between components indicated.
   1. Guide the light from the white light source to a monochromator to obtain a monochromatic light beam. See the Table of Materials for details of the setup used in this work. Set the mon....

Results

Figure 4A shows a scanning electron microscope (SEM) micrograph of a commercial DVD grating covered with Au/Co/Au multilayer that was used a demonstration sample in our experiments. Its optical and magneto-optical spectra are shown in Figure 4B,C respectively. Details on sample fabrication are presented elsewhere²³. Black lines in Figure 4A,B show the plasmon.......

Discussion

We have introduced a measurement setup and protocol to obtain angular resolved magneto-optical spectra of optical crystals. In particular, the case of ferromagnetic materials, that requires additional data analysis to account for the nonlinear permeability of the material, has been laid out. Angular resolved magneto-optical spectroscopy presents an additional advantage over non-angular resolved methods that the confined modes can be more readily identified as they appear as clearly defined bands in both optical and magne.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Beam splitter	Thorlabs	BSW27
Bertrand lens	Thorlabs	LA1608	f = 75 mm
CCD Camera	Thorlabs	1500M-GE-TE	Camera for real space imaging
Collecting lens	Thorlabs	ITL200	f = 200 mm
Collimating lens	Zeiss	420640-9800	Magnification 10x NA 0.3
Flip mirror	Thorlabs	CCM1-P01/M
Flip mirror mount	Thorlabs	FM90/M
L1-lens	Thorlabs	LA1986	f = 125 mm
L2-lens	Thorlabs	LA1461	f = 250 mm
Objective lens	Nikon	MUE10500	Magnification 50x NA 0.8
Pinhole	Thorlabs	ID8/M
Polarizer	Thorlabs	GTH10M	For LMOKE measurements, two polarizers are needed
sCMOS camera	Andor	ZYLA-4.2P-USB3