Photonic molecules for improving the optical response of macroporous silicon photonic crystals for gas sensing purposes.

D Cardador,D Segura,A Rodríguez

doi:10.1364/oe.26.004621

Abstract

In this paper, we report the benefits of working with photonic molecules in macroporous silicon photonic crystals. In particular, we theoretically and experimentally demonstrate that the optical properties of a resonant peak produced by a single photonic atom of 2.6 µm wide can be sequentially improved if a second and a third cavity of the same length are introduced in the structure. As a consequence of that, the base of the peak is reduced from 500 nm to 100 nm, while its amplitude remains constant, increasing its Q-factor from its initial value of 25 up to 175. In addition, the bandgap is enlarged almost twice and the noise within it is mostly eliminated. In this study we also provide a way of reducing the amplitude of one or two peaks, depending whether we are in the two- or three-cavity case, by modifying the length of the involved photonic molecules so that the remainder can be used to measure gas by spectroscopic methods.

Highlights

Photonic Molecules (PMs), understood as two or more coupled micro or nanocavities, called photonic atoms, have been extensively studied during the recent years thanks to their unique optical characteristics [1,2]
This principal characteristic makes the photonic crystal not suitable for conventional spectroscopic gas sensing since the fingerprint of the main gases are comprised in few hundreds of nanometers—i.e. the CO2 has its spectrum lines within 4.20 μm and 4.35 μm
To conclude, we have reported a novel way of enhancing the features of macroporous silicon optical filters by means of photonic molecules

Summary

Introduction

Photonic Molecules (PMs), understood as two or more coupled micro or nanocavities, called photonic atoms, have been extensively studied during the recent years thanks to their unique optical characteristics [1,2]. The bandgap is enlarged and the transmission baseline— called offset along this study—is reduced because more periods have been added in order to place the new cavity We corroborate this experimentally, showing narrower peaks and better figures for the bandgap. In order to filter one of the two peaks, we shorten the deeper cavity for varying the phase between the two resonant frequencies and we use a chirp modulation, consisting of a linear decreasing of the length of the periods involved in the PC, to optimize the filtering of the resonant states given in the second cavity This procedure can be extrapolated to the case of three defects, showing a substantial improving of the optical figures respect to the previous cases

Experimental and simulations

Results and discussion

Conclusions