Inverse photonic design of functional elements that focus Bloch surface waves

Yannick Augenstein,Andreas Vetter,Babak Vosoughi Lahijani,Hans Peter Herzig,Myun-Sik Kim,Carsten Rockstuhl

doi:10.1038/s41377-018-0106-x

Abstract

Bloch surface waves (BSWs) are sustained at the interface of a suitably designed one-dimensional (1D) dielectric photonic crystal and an ambient material. The elements that control the propagation of BSWs are defined by a spatially structured device layer on top of the 1D photonic crystal that locally changes the effective index of the BSW. An example of such an element is a focusing device that squeezes an incident BSW into a tiny space. However, the ability to focus BSWs is limited since the index contrast achievable with the device layer is usually only on the order of Δn≈0.1 for practical reasons. Conventional elements, e.g., discs or triangles, which rely on a photonic nanojet to focus BSWs, operate insufficiently at such a low index contrast. To solve this problem, we utilize an inverse photonic design strategy to attain functional elements that focus BSWs efficiently into spatial domains slightly smaller than half the wavelength. Selected examples of such functional elements are fabricated. Their ability to focus BSWs is experimentally verified by measuring the field distributions with a scanning near-field optical microscope. Our focusing elements are promising ingredients for a future generation of integrated photonic devices that rely on BSWs, e.g., to carry information, or lab-on-chip devices for specific sensing applications.

Highlights

The control of electromagnetic fields in integrated environments is of paramount importance for a large number of applications in the broader context of information transmission, acquisition, and processing[1,2,3,4,5]
We even obtain a focal width that is slightly smaller than half the wavelength when the focal point is placed directly behind the element. This is realized by exploiting near-field components directly behind the structured device. We experimentally demonstrate their anticipated functionality by means of measurements with a scanning near-field optical microscope (SNOM)[22]
We demonstrate that computational strategies for inverse photonic design are suitable to achieve functional elements that can control the propagation of Bloch surface waves (BSWs) to an extraordinary degree

Summary

Introduction

The control of electromagnetic fields in integrated environments is of paramount importance for a large number of applications in the broader context of information transmission, acquisition, and processing[1,2,3,4,5]. Even stronger integration with better accessibility is achieved by confining electromagnetic waves to surfaces[8]. This led to the notion of surface waves, i.e., self-consistent solutions to Maxwell’s equations localized at the interface between two media that exponentially decay away from the interface. While exploiting the coupling of light to an electronic excitation, the concentration of electromagnetic fields in a nanometric region close to the interface can be achieved. This has been instrumental for a large number of applications, e.g., to sense molecules or, more generally, to guide light at small length scales[10].

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