Resumen de Low noise thz detection via optical parametric upconversion
Gabriel Arturo Santamaría Botello
Recently, the interest in platforms that coherently convert photons from the RF to the optical domain has increased. This has been mainly motivated by the advent of quantum information technologies, because room-temperature photonic links could be used to transfer quantum states between cryogenic superconducting microwave circuits over long distances. While several approaches are under investigation to this end, there is a second potential application of efficient photonic upconverters that is sometimes overlooked. Indirect detection of weak signals at microwave, millimeter-wave, and THz frequencies could be carried out with optical detectors by using a room-temperature upconverter as an intermediary. The advantage is the high sensitivity provided by off-the-shelf photodetectors with no need for cryogenics. Thus, room-temperature microwave/THz receivers with unprecedented sensitivity could be designed. This fact provides upconverters a great potential for bridging the so-called THz gap, being millimeter and submillimeter-wave radio astronomy, earth observation, imaging, and spectroscopy among the direct beneficiaries. In this dissertation, we theoretically analyze RF-to-optical upconversion via electro-optic modulation (EOM) in nonlinear crystals. We emphasize resonant architectures, especially those using whispering-gallery-mode (WGM) resonators. Using a classical approach, we develop a theoretical framework to describe nonlinear interactions in generic waveguides. This framework is later used to study nonlinear mixing in WGM resonators, modeling them as loop-waveguides instead of harmonic oscillators as it is done in the literature. The reason is the harmonic oscillator model might result too simplistic because it presumes the energy is evenly distributed throughout the cavity. The loop-waveguide model allows us to account for the possible non-uniformities of the WGMs along the resonator’s perimeter, caused by exceptionally strong coupling or nonlinear interaction strength. Our results show that both models match when intrinsic and coupling quality (Q) factors are sufficiently large, but for highly overcoupled systems, however, the harmonic oscillator model underestimates the upconversion efficiency. We also found a similar behavior regarding the amount of thermal noise coupled to the upconverter due to its physical temperature: the harmonic oscillator model underestimates the thermal population in a strongly overcoupled WGM. Indeed, the loop-waveguide model imposes a theoretical minimum for the input-referred thermal noise temperature of an infinitely overcoupled resonator, in contrast to the harmonic oscillator model that allows for arbitrary thermal noise reduction. Furthermore, in the high-Q limit, both models agree except for exceptionally strong nonlinear interactions, requiring levels of pump power and modal overlap that are far from the experimental demonstrations reported so far. The waveguide model might be relevant, however, for recent proposals of ultra-efficient upconverter architectures. We propose a resonant upconverter design on a thin-film lithium niobate platform that is analyzed theoretically, predicting photon conversion efficiencies on the order of 1% per milliWatt of pump power. Finally, total noise calculations in a WGM upconverter followed by both incoherent and coherent optical detection stages are presented in a unified manner. It is found that homodyne, heterodyne, and direct optical detection schemes after upconversion lead to fundamentally different noise performance for radiometry. Contrary to a mixer (downconverter) or low-noise amplifier (LNA), there is no fundamental noise added by a 100% efficient sum-frequency-generation (SFG) upconverter except for the coupled thermal noise. Therefore, the upconverter can serve as an interface to indirectly count the incoming THz photons in the optical domain, which is not subject to the conventional quantum limit for coherent receivers. Hence, under certain realistic conditions, the quantum limit in a THz radiometer can be circumvented via upconversion followed by incoherent optical detection. The conventional quantum limit bounds the upconversion system sensitivity only when coherent optical detection is done afterward (homodyne or heterodyne). Predicted noise levels of upconversion-based THz receivers show that photon conversion efficiencies on the order of 1% or higher would significantly improve state-of-the-art room-temperature and cryogenic low noise amplifiers and mixers in the sub-millimeter-wave/THz bands. It is worth mentioning that such efficiency requirements are much less strict for ultra-low noise radiometers than for practical quantum-state links.
