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Radiometry in the sub-millimeter and THz region is required for applications as imaging, spectroscopy, earth observation, planetary missions and radio astronomy. The recent advances in the development of high electron mobility transistors (HEMT) low noise amplifiers (LNAs) have pushed their operation frequency to the sub millimeter range, showing noise figures below 4dB inside cryocoolers. In the THz range detection has been achieved with Schottky mixers working either at room temperature or inside cryostats. They have shown noise temperatures comparable with those of the most recent LNAs. Superconducting devices such as SIS mixers can achieve near quantum limited noise performance while operating at 4K. Sensitive detection is also possible by adapting the principles of photonic detectors to THz frequencies. In this case, low noise operation implies cryogenic cooling of the detectors. Bolometer-based THz receivers operating in cryostats have shown photon counting sensitivity. In this work we study another approach proposed in the last two decades for high sensitivity THz detection at room temperature. The detection principle consists on taking advantage of the nonlinearity of crystals such as lithium niobate to enable a sum frequency generation (SFG) process that boosts the THz photon energy to the optical domain. This occurs via electro-optic modulation of the laser pump by the THz radiation when the waves are phase-matched inside the crystal. To significantly increase the conversion efficiency, an ultra high-Q whispering-gallery resonator (WGR) is used to confine and enhance the optical field. The WGR is also resonant in the THz domain, enhancing further the photon conversion efficiency. In this work we present optimized geometries for the doubly resonant WGR structure and coupling mechanisms. Then, theoretical models are used to predict the photon conversion efficiencies of such electro-optic modulators along with their thermal occupation levels and overall noise performance as THz radiometers.
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