THz-micro-spectroscopy with backward-wave oscillators

We have developed a micro-spectrometer operating in the THz-range between 30 GHz and 1.4 THz, where we use backward-wave oscillators as continuous-wave sources, which supply highly monochromatic (Δν /ν ≈ 10−6) and coherent radiation with an output power of up to 300 mW. The microscopic unit consists of an aberration-free hyperbolic lens, a hemispherical solid immersion lens and a small pinhole. The hyperbolic lens gives a diffraction limited spot, the solid immersion lens (SIL) reduces this spot by an additional factor of n, where n is the refractive index of the lens material, and the small pinhole in close contact to the sample defines the resolution. In contrast to the visible, where the highest refractive indices are around 2, in the THz-range exist materials with very high n. In our setup, we use a silicon SIL with n = 3.5 which improves the resolution by this factor without a significant reduction in intensity compared to the far-field. The instrument allows imaging and spectroscopy of biological samples with high spatial and spectroscopic resolution.