Santamaría Botello, Gabriel ArturoSedlmeir, FlorianRueda, AlfredoAbdalmalak Dawoud, Kerlos AtiaBrown, Elliott R.Leuchs, GerdPreu, SaschaSegovia Vargas, DanielStrekalov, Dmitry V.García Muñoz, Luis EnriqueSchwefel, Harald G. L.2023-02-032023-02-032018-10-20Santamaría Botello, G., Sedlmeir, F., Rueda, A., Abdalmalak, K. A., Brown, E. R., Leuchs, G., Preu, S., Segovia-Vargas, D., Strekalov, D. V., García Muñoz, L. E. & Schwefel, H. G. L. (2018). Sensitivity limits of millimeter-wave photonic radiometers based on efficient electro-optic upconverters. Optica, 5(10), 1210.2334-2536https://hdl.handle.net/10016/36465Conventional ultra-high sensitivity detectors in the millimeter-wave range are usually cooled as their own thermal noise at room temperature would mask the weak received radiation. The need for cryogenic systems increases the cost and complexity of the instruments, hindering the development of, among others, airborne and space applications. In this work, the nonlinear parametric upconversion of millimeter-wave radiation to the optical domain inside high-quality (Q) lithium niobate whispering-gallery mode (WGM) resonators is proposed for ultra-low noise detection. We experimentally demonstrate coherent upconversion of millimeter-wave signals to a 1550 nm telecom carrier, with a photon conversion efficiency surpassing the state-of-the-art by 2 orders of magnitude. Moreover, a theoretical model shows that the thermal equilibrium of counterpropagating WGMs is broken by overcoupling the millimeter-wave WGM, effectively cooling the upconverted mode and allowing ultra-low noise detection. By theoretically estimating the sensitivity of a correlation radiometer based on the presented scheme, it is found that room-temperature radiometers with better sensitivity than state-of-the-art high-electron-mobility transistor (HEMT)-based radiometers can be designed. This detection paradigm can be used to develop room-temperature instrumentation for radio astronomy, earth observation, planetary missions, and imaging systems.10eng© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreementresearch articleTelecomunicacioneshttps://doi.org/10.1364/OPTICA.5.001210open access1210101219Optica5AR/0000022290