Electroless deposited broadband omnidirectional multilayer reflectors for mid-infrared lasers

摘要

Hollow waveguides are very sensitive to bending and coupling conditions, which cause large losses. In order to overcome these limitations we suggest to develop a multilayer hollow waveguide based on one-dimensional photonic crystal. Photonic crystals have been investigated for many years. The most common photonic crystal structure is the one-dimensional photonic crystal. For over a decade many companies and research groups have been manufacturing "perfect mirrors" made of alternating pairs of dielectric materials with different index of refraction. These mirrors are made of a large number (～10) of pairs. Applying the same type of coating techniques, with large number of pairs, to tubular shapes is very difficult and hollow waveguides based on this technology cannot be manufactured. We suggest an alternative method of coating flat surfaces with pairs of layers of high ratio of index of refraction that can be applied later with minimal difficulties for tubes (hollow waveguides). We used a thin transparent metal layer (silver) as one of the dielectric materials of the pair. The thin metal layers have a large index of refraction in the MIR and the dielectric layer (silver iodine) has lower refractive index. Using these materials enables us to achieve a large ration of index of refraction, which is required for creating photonic crystal properties with a low number of pairs. We developed a mirror from alternating pairs of silver and silver iodine using an electroless chemical method. A mirror made of 4 pairs has reflectance close to 100% and omnidirectional behavior over a wide spectral region (6-10 μm). This experimental result is in agreement with our theoretical model as well as other approaches. Using a ray model we have shown that a hollow waveguide based on the same structure of layers will have negligible attenuation when bent and will not be sensitive to the focal length of the coupling lens (omnidirectional).