High refractive index contrast systems enable very high-Q wavelength-scale microphotonic resonators that due to their high-Q and small size have the potential to dramatically alter the landscape for chemical, biological, thermal, electromagnetic, and mechanical sensor systems, enabling highly sensitive large-scale sensor arrays to be envisioned. An example sensor system is that enabled by microphotonic thermal detectors and imagers, a new class of thermal detectors and imagers that I proposed and have been developing at Sandia which offer a rich area of research spanning fundamental physics to high frequency electromagnetic design to the very large scale integration of microphotonic and nanophotonic elements.
A microphotonic thermal detector consists of a thermally isolated microphotonic resonator (i.e. suspended and tethered) coupled to an absorbing element and evanescently coupled to a bus waveguide. Incident radiation (infrared or otherwise) is absorbed in the absorbing element. The heat produced by the absorbed radiation raises the temperature of the thermally isolated resonator and via the thermo-optic effect, shifts its resonant frequency. The resonant frequency shift can then be readout by an RF or optically swept laser line. While the thermo-optic effect is by itself small, in a high-Q resonator, the slope of the resonance is steep, enabling a responsivity that is three orders of magnitude larger than that achieved by class-leading microbolometers (i.e. resistance-based thermal detectors). Moreover, since the detector is formed from a dielectric, microphotonic thermal detectors do not suffer from Johnson noise. Further, the metallic contacts of a microbolometer are replaced with evanescent optical coupling, enabling far greater thermal isolation of the detector. On account of the massive responsivity, lack of Johnson noise, and all-dielectric thermal isolation, microphotonic thermal detectors have the potential to reach the temperature fluctuation limit imposed by temperature exchange between the detector and the substrate to which it is attached. This fundamental limit of noise performance was recently achieved in an experiment I conducted at Sandia, demonstrating the best internal noise performance of any uncooled thermal detector. Given the size of the thermal detector industry, and relatively small-scale effort required to achieve this record result, the inherent advantages of microphotonic thermal detectors are clearly substantial. Microphotonic thermal detectors offer dramatic benefits over microbolometers and significant, but tractable research challenges, requiring rigorous electromagnetic design of the microphotonic resonator and nanophotonic antennas, testing fundamental noise limitations, and pushing nanofabrication to realize high-Q resonators suspended with nanoscale tethers. The benefits of microphotonic thermal imagers are sufficiently clear that we are already in discussions with a major microbolometer manufacturer. Still, imagers present significant challenges in the manufacturability of large arrays of microphotonic elements common to all microphotonic sensor systems that if surmounted, will enable chemical, biological, mechanical, and other sensing arrays, with sensitivity not achievable using traditional techniques.