An optical transition-edge sensor with high energy resolution

Optical transition-edge sensors (TESs) have shown an energy resolution for resolving the number of incident photons at the telecommunication wavelength. However, a higher energy resolution is required for biological imaging and microscopic spectroscopy. In this study, we tested an Au/Ti (10/20 nm) bilayer TES that showed a high energy resolution. The high energy resolution was achieved by lowering the critical temperature T-c to 115 mK, and the resultant energy resolution was 67 meV full width at half maximum (FWHM) at 0.8 eV. When T-c was reduced to 115 mK, the theoretical resolution would scale up to 30 meV FWHM, considering that the typical energy resolution of optical IESs was 150 meV and T(c )was 300 mK. To investigate the difference between the theoretical expectation (30 meV) and the measured value (67 meV), we measured the complex impedance and current noise of the TES. We found excess Johnson noise in the TES; the excess Johnson noise term M was 1.5 at a bias point where the resistance was 10% of the normal resistance. For reference, the above mentioned TES was compared with a TES showing typical energy resolution (156 meV FWHM). We also discussed factors that improved or inhibited the energy resolution.