Submillimeter Wave Spectroscopy

Our two quasi-optical spectrometers continuously cover the frequency range from 1 cm-1 to 35 cm-1 (= 35 GHz - 1100 GHz). This type of spectrometer ''Epsilon'' was developed by the group of G. Kozlov and A. Volkov at the Institute of General Physics (GPI), Russian Academy of Sciences, in Moscow. We can perform both polarized reflection and transmission measurements placing the sample either in He cryostats, which operate from room temperature down to 2 K, or in home-made ovens going up to 700 K. We are using a Janis continuous He-flow cryostat with a big custom-made sample chamber and a home-made bath crystat, with the sample placed in a static exchange gas. In addition low temperature experiments can be performed in a superconducting slit-ring magnet (Spectromag 4000) ranging up to 9 Tesla. For controlling the measurement temperatures different LakeShore T-controllers are used. The experiments are performed on superconducters, metals, dielectric solids, and on glass-forming liquids.

Ten different backward wave oscillators (BWOs) with overlapping frequency ranges are used as powerful and tunable coherent sources, connected to an extremely precise and stable high voltage power supply (dV/V » df/f » 5x10-5). Teflon and polyethelene lenses, Si mirrors, wire-grid polarizers, and wire-mesh beam splitters are used as optical components. For the signal detection (using lock-in amplification) either a golay detector or a He-Bolometer can be used. The experiments are computer controlled and operated.

For transmission measurements a Mach-Zehnder type interferometer is used as optical setup. This allows for simultaneous measurement of both the transmitted power (red arm) and the change in phase (both red and yellow arm) the beam suffers through the sample. The empty aperture is used as a reference for each measurement. This allows for straight foward calculation of the real and imaginary part of the dielectric constant or conductivity.


Measurements of thick transparent samples, of liquid samples in cells, or of thin films on transparent substrates, utilize the Fabry-Perot type multireflection in many-layer systems, in order to evaluate the complex dielectric constant, by fitting the interference pattern.

For reflection measurements, polished aluminum mirrors are used as reference. They are accurately placed at the sample position by linear translation at each temperature. For large samples (d > 5 l), no aperture is needed. Best results on small samples are obtained by keeping the aperture in the same position.

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