Microwave spectroscopy system for molecules trapped in low temperature condensed media using opto-thermal detection with superconducting thin-film sensor

A simple and widely applicable microwave spectroscopy system for low temperature matrices has been developed. We applied the technique of opto-thermal detection with a superconducting thin-film sensor (STS) for the detection of very weak absorption signals of molecules trapped in quantum solids such as solid para-hydrogen (p-H₂). For testing of the system, the microwave transition of an ortho-hydrogen (o-H₂) dimer in solid p-H₂ was observed. The S/N ratio of the observed spectrum of the G₁ transition at 1.4 K was approximately 8. The ultrahigh sensitivity of STS makes it possible not only to obtain sufficient sensitivity but also to make the apparatus very simple especially around the sample cell.