| Abstract | Radiative coupling of objects with outer space, through the atmospheric spectral transparency window, allows passive cooling below ambient temperature. Such passively created cold sinks can be used for converting ambient heat into other useful forms of energy. Using this approach, conversion of ambient heat to electricity has been demonstrated in macroscopic settings, where the radiatively cooled heat sink is a macroscopic object coupled with ambient heat through a heterogeneously attached, standalone, thermoelectric generator. In contrast, miniaturization and on-chip integration of such heat engines has not been demonstrated, although it could be useful in portable remote technological applications. Such integration would first require that both the radiatively cooled heat sink and the hot (ambient temperature) side of the engine are monolithically integrated on a single chip. This first step is the focus of our work: we demonstrate the creation of a local thermal gradient on-chip by radiative cooling of a 90 nm thick freestanding silicon nitride nanomechanical resonator integrated on a silicon substrate that remains at ambient temperature. The reduction in temperature of the thin film is inferred by tracking its mechanical resonance frequency, under high vacuum, using an optical fiber interferometer. Experiments were conducted on 15 different days during fall and summer months, under various solar illumination, resulting in successful radiative cooling of the membrane in each case. Maximum temperature drops of 9.3 K and 7.1 K are demonstrated during the day and night, respectively, in close correspondence with our heat transfer model. Future improvements to the experimental setup could improve the temperature reduction to 48 K for the same membrane, while emissivity engineering potentially yields a maximum theoretical cooling of 67 K with an ideal emitter. |
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