Retrofittable T-ray antireflection coatings

Abstract

Terahertz time-domain spectroscopy (THz-TDS) is able to extract optical or dielectric properties of materials, whether in the solid, liquid, or gas phase, in the T-ray frequency region. Spectroscopy of a liquid or gas often requires a receptacle to confine the sample. In order to allow T-rays to probe the sample effectively, the receptacle must have T-ray transparent windows. However, even though windows are transparent to T-rays, attenuation exists, because of multiple reflections at air-window and window-air interfaces, which accounts for a major energy loss. Due to the recent emergence of T-ray technology, there has been very little work carried out to-date on the reduction of reflection losses. This paper analyses the reduction of T-ray reflection loss by means of an antireflection coating. Because T-ray wavelengths are much larger than visible wavelengths, the antireflection layer thickness for T-rays is much larger than the usual optical case. This creates an interesting opportunity for retrofittable antireflection layers in T-ray systems. In the experiment, a coating material made from polyethylene sheets is applied onto the surfaces of a silicon window. The coated window shows enhancement of the transmittance within a range of frequencies.