Army

Reflection-mode waveplate for operation in the terahertz (THz) region

Enables highly accurate remote sensing

Photonics

In recent years, it has been realized that the THz region of the electromagnetic spectrum is of considerable scientific and technological interest which has led to advances that include THz imaging, semiconductor material characterization, and chemical and biological sensing. However, the THz spectrum still suffers from the often cited THz gap – the lack of basic components available in this frequency range. One such component is the waveplate which alters the polarization of light traveling through it.

Many THz components created to date suffer high insertion loss (input power loss), which arises from Fresnel losses (signal loss from the difference in the refractive index between glass and air). The loss can be substantial as typical materials at THz frequencies have large refractive indices. This could be remedied through the use of anti-reflection (AR) coatings but the AR coatings would be thicker than the actual device, which would be problematic in fabrication or implementation.

To address the above, Army researchers have developed a THz waveplate with very high efficiency. This reflection mode device (45° angle of incidence), metamaterial (periodic gold/polyimide composite) waveplate can be static or dynamic. In the static case, the electromagnetic response of the metamaterial does not change. In the dynamic case, the electromagnetic response of the metamaterial can be altered by an electrical voltage or optical excitation beam.

The unique characteristics of THz radiation include transparency to materials such as cardboard, plastic, and styrofoam, which are opaque at other wavelengths, and the sensitivity to gas and solid phase molecular signatures including biological agents and chemical explosives makes THz sensing ideal for identification of embedded illicit or hazardous materials.

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