[Invited] Localized phoxonic modes and application to sensor devices
Résumé
Photonic crystals and their acoustic counterpart, the so-called phononic crystals are now well-known for their ability to guide, control, and manipulate the propagation of the optic and acoustic waves. These properties are mainly related to the possibility of band gaps in their band structure that allow the existence of localized modes and confined optic/acoustic waves. Moreover, during the past few years, there has been an increasing interest towards structures exhibiting simultaneous phononic and photonic band gaps, the so-called phoxonic crystals, thus allowing dual confinement of phonons and photons. From the point of view of sensing applications, several papers have already shown the capability of photonic crystals for detecting small variations in the refractive index of gases and liquids and have opened the way to a platform for a new class of sensors. In contrast, phononic crystals have only been recently proposed as a possible platform for the investigation of the acoustic velocity of a liquid filling the hollow parts of the structure. Thus, the potentiality of different geometries of phononic crystals for sensing applications still needs several further investigations. Moreover, some of the structures may be suitable for a dual measurement of both acoustic and optical velocities of the analyte. The objective of this presentation is to partly fill the lack of knowledge in these topics. To make a phononic/photonic sensor, one needs to design a structure in which the transmission coefficient displays well-defined features that are very sensitive to the acoustic/optic velocity of the infiltrating liquid. These features should be relatively isolated in frequency in order to allow the sensing of the probed parameter on a sufficiently broad range. In this paper, we investigate theoretically the transmission spectra in several geometries of phononic crystals and discuss the physical origin of the peaks and dips in the spectra and their usefulness for the sensing applications. The emphasis will first be put on the phononic behaviors and then possibly completed with the trends of the photonic transmissions. For the sake of simplicity, the solid parts of the structures are assumed to be made of silicon. We will show that several situations are suitable for independent determinations of the acoustic velocity and the index of refraction of the liquid while some constraints can appear on the geometrical parameters to make a dual phononic/photonic sensor. The calculations are performed by using either a home-made Finite Difference Time Domain (FDTD) method or the Comsol Multiphysics Finite Element Method (FEM).