  Small scale renewable energy harvesting is an attractive solution to the
growing need for power in remote technological applications. For this purpose,
localized thermal gradients on-chip--created via radiative cooling--could be
exploited to create microscale renewable heat engines running on environmental
heat. This could allow self-powering in small scale portable applications, thus
reducing the need for non-renewable sources of electricity and hazardous
batteries. In this 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, 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.
