Integrated high-Q lithium niobate resonator with broadband absorber for uncooled longwave infrared sensing application.

Optical resonators with high Q-factor are of interest in infrared thermal sensors for their high-temperature sensitivity. Thin-film lithium niobate (LN), with low propagation loss in near-infrared, is a competitive material for a comparative advantage in thermal-optic coefficient and wider absorption range from longwave infrared (LWIR) to terahertz frequencies if compared with SiN and SiO. Here, we present an LWIR sensor on the LN-photonic platform, where a high-Q microring resonator is monolithically integrated with a broadband LWIR radiation absorber, compatible with the multilayer-stacked LN photonic circuits. It is demonstrated that the radiation can induce a significant mode-frequency shift of near-infrared probe light on the thermal-optic effect, even if the device suffers from relatively high frequency noise. The noise equivalent temperature difference of the sensor is evaluated as 1.33 mK through a short-term frequency instability measurement. Under the radiation at a wavelength of = 9.1 µm, the device achieves a responsivity of 155.25 kHz/nW and a response time of 128 µs. The integrated device scheme on the LN-photonic platform holds great promise in developing a high-sensitivity infrared sensor, even to enable infrared imaging with a high signal-to-noise ratio.