Field-enhancing photonic devices utilizing waveguide coupling and plasmonics - a selection rule for optimization-based design.

Joakim Vester-Petersen,Brian Julsgaard,Peter Balling,Rasmus E Christiansen,Søren P Madsen,Ole Sigmund

doi:10.1364/oe.26.00a788

Abstract

This paper describes a systematic design study of periodic gold-nanostrip arrays placed on a thin film aimed at enhancing the electric field inside the film when irradiated by light. Based on the study, a "selection rule" is proposed, which provides optimization-based design methods with an a priori choice between field-enhancement dominated by coupling to guided modes, by plasmonic near-field enhancement or by a mix hereof. An appropriate choice of wavelength and grating period is shown to selectively suppress or include waveguiding effects for the optimized designs. The validity of the selection rule is demonstrated through a numerical topology optimization study in which gold nanostrips are optimized for electric-field enhancement in an erbium-doped TiO2 thin film, targeting increased spectral upconversion in the erbium ions. The obtained designs exhibit waveguide excitation within the predicted intervals and, for light polarized perpendicularly to the strips, plasmonic response outside.

Highlights

Localized enhancement of the electric field under external irradiation is desirable for a range of applications
This paper describes a systematic design study of periodic gold-nanostrip arrays placed on a thin film aimed at enhancing the electric field inside the film when irradiated by light
The validity of the selection rule is demonstrated through a numerical topology optimization study in which gold nanostrips are optimized for electric-field enhancement in an erbium-doped TiO2 thin film, targeting increased spectral upconversion in the erbium ions

Summary

Introduction

Localized enhancement of the electric field under external irradiation is desirable for a range of applications. The rear-placement ensures that the photonic devices only interact with the infrared light transmitted through the cell, not interfering with the intrinsic photon-absorption Such an embedded setup is one potential end goal for designing field-enhancing devices, considering instead a setup with photonic device placed on top of thin film is of great interest, e.g. for photonic crystal biosensors [12] and polarization-tuned color filters [13]. For such a configuration, one question is, how to efficiently couple incident light into the film and how to systematically target different field-enhancing mechanisms?. The topology optimization method is used as a numerical synthesis tool to design the 2D nanostrip cross section (assuming the strips are extruded infinitely in the out-of-plane direction)

The model problem

The selection rule

The optimization based design problem

Results and discussion

Conclusion