Tapered THz Waveguides


Due to its unique properties such as dispersionless propagation of the TEM mode with no frequency cut-off , the parallel plate waveguide (PPWG) has attracted significant attention. Recently a tapered PPWG (TPPWG) has been proposed. THz waves can be confined inside a TPPWG on subwavelength scale in both transverse dimensions.

Figure 1: Schematic of the tapered parallel plate waveguide, consisting of two aluminum plates of a varying width (input 3 mm, output 49 mm) and adjustable plate separation (1 mm at the input and between 0 mm and 500 mm at the output).

In our laboratory we investigate the field enhancement properties of a TPPWG. We have developed a simple analytical approach to the estimation of the THz electric field enhancement inside a TPPWG and also performed a full-wave numerical time-domain simulation of THz wave propagation. Moreover, two independent experimental methods, free-space electro-optic sampling (FSEOS) and air biased coherent detection (ABCD), are then used to measure THz electric field at the output of the waveguide.

Figure 2: Output intensity distributions measured using a pyroelectric detector with a 0.25 mm-diameter aperture placed 0.8 mm away from the waveguide tip.

For imaging the terahertz field inside parallel plate waveguides we have developed a new non-invasive method, about which you can read more here.

Figure 3: Schematic of the air photonic setup for non-invasive field imaging inside a tapered parallel plate waveguide. THz radiation is coupled into the TPPWG, propagates along it and then interacts with NIR probe inducing second harmonic generation in the presence of oscillating local electric bias field. L1, L2 – lenses, F – 400 nm filter, HVM – high voltage modulator, PMT – photomultiplying tube.

At DTU Fotonik we have shown that a field enhancement of over 20 is possible, resulting in a THz peak electric field exceeding 1.4 MV/cm for output gaps of 0.02 mm. Even higher values of the peak THz field are expected for smaller output gaps. The tight concentration of high intensity THz radiation can be potentially applied in exploration of nonlinear THz phenomena, deep subwavelength imaging, and allow for development of compact optoelectronic devices.

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© 2012 Terahertz.dk Our research is carried out at DTU Fotonik - Department of Photonics Engineering, Technical University of Denmark. Suffusion theme by Sayontan Sinha