Ultra-Thin Film Sensing With Terahertz Subwavelength Thin Disc Microresonators

Subwavelength thin disc terahertz microresonators have been proven to exhibit unprecedented quality factors, exceptionally low losses and extensive evanescent fields. We investigate the microresonators' potential for ultra-thin film sensing using poly(methyl methacrylate) films for a compelling proof-of-concept. Highly subwavelength thin films ($\lambda$/10 000) with thicknesses of tens of nanometers were deposited atop the thin disc microresonator, each causing a pronounced frequency shift and a marked reduction in the intrinsic quality factor; these differences were measurable using a standard continuous-wave terahertz spectrometer with an added terahertz microresonator frequency reference. This demonstration benchmarks the exceptional sensitivity of thin disc terahertz microresonators, and, to our knowledge, marks the first recorded use of a whispering-gallery mode resonator for ultra-thin film sensing in the THz domain. Our findings create exciting possibilities for future applications, such as detecting ultra-thin films of sophisticated analytes.