Sensors based on silicon photonic crystal mirrors with engineered phase

The miniaturization of optical systems introduces benefits similar to those for electronics, i.e., making fabrication efficient, simplifying packaging, and reducing cost. Scaling optics to smaller sizes presents a number of challenges, however. This is particularly true for optical sensors that are exposed to the environment. In such systems, the components must scale well, lend themselves to efficient parallel manufacturing, and be mechanically and chemically robust enough to perform reliably and with good long-term stability. Mirrors are indispensable components in many optics applications. However, traditional mirror technologies do not perform well in miniaturized optical sensors. Metal mirrors are not sufficiently mechanically or chemically robust. This shortcoming complicates fabrication and packaging, and makes operation of the sensors in challenging environments impossible. Bragg mirrors consisting of multiple dielectric layers are sufficiently hardy for such applications, but do not scale well. The mirror thickness is determined by the desired wavelength and the required reflectivity, and thus cannot be reduced to fit the requirements of miniaturized systems. Photonic crystal (PC) mirrors are simple devices that lend themselves readily to straightforward fabrication by standard integrated-circuit manufacturing technologies. In their simplest form, PCs consist of a plate of semiconducting material with a periodic array of holes: see Figure 1(a). The principle of PC operation is different from other mirror technologies because PCs depend on interference between different pathways. As illustrated in Figure 1(b), a plane wave incident on a PC has two available pathways for transmission: a direct path, as through a homogeneous plate; and an indirect path that involves Figure 1. (a) In its simplest form, a photonic crystal (PC) mirror is a high-index plate with a periodic array of holes. The array can be 2D, as shown here, or 1D, as in a high-index grating. (b) The high reflectivity of PC mirrors is caused by interference. Incident light is transmitted through the PC as a plane wave as well as through the excitation of guided resonances. These two pathways through the PC interfere and determine the reflection and transmission spectra.