Contents I. Introduction
Broadband mirrors () with very high reflectivity () are essential for numerous applications, including telecommunications, surveillance, sensors, and imaging, ranging from 0.7–12- wavelength regimes. Metal mirrors have larger reflection Scheme of the subwavelength grating reflector. The low index material under the grating is essential for the broadband mirror effect. bandwidths but lower reflectivities (), limited by absorption loss. As a result, they are not suitable for fabricating transmission-type optical devices such as etalon filters. Dielectric mirrors have a low loss and, thus, can achieve a higher reflectivity. However, the deposition methods are often not precise enough to lead to very high reflectivities. Furthermore, the typical material combinations often have a rather small bandwidth, limited by the refractive index difference of the materials used. For tunable etalon type devices, such as microelectro-mechanical (MEM) vertical cavity surface emitting lasers (VCSEL), filters [1], and detectors [2], the tuning range is often limited by semiconductor-based distributed Bragg reflectors (DBRs) to to 9%. The challenge of designing a mirror with broadband reflection, low loss and compatibility with optoelectronic processing has not been overcome yet.
