Characterization of Optical Fast Transition-Edge Sensors With Optimized Fiber Coupling

Energy resolution (Δ
E
), recovery time (τ
_eff
), and quantum efficiency (
QE
) are some of the most important single photon-counting detector parameters. New applications in the quantum optics field place extreme demand on detector performances that go beyond the capabilities of established single-photon detectors. Unfortunately, it is hard to reach best results for all these parameters at the same time. Concerning transition-edge sensors (TESs) as single photon detectors, Δ
E
and τ
_eff
are directly and inversely proportional to the transition temperature
T
_c
, respectively. Moreover, the geometrical optical coupling between illuminating fiber and detector, the device material and the characteristics of the optical cavity or the antireflection coating, drastically influence the
QE
and in some cases also the Δ
E
. Previous TESs fabricated by Istituto Nazionale di Ricerca Metrologica were characterized by very high energy resolution (0.18 eV) but high τ
_eff
and low
QE
. In this work, we present a Ti/Au TES with higher
T
_c
(300 mK) that, using the optical alignment system implemented by AIST, shows an optimization of these important parameters, energy resolution of 0.26 eV, recovery time of 186 ns, and a quantum efficiency of 50% without using any optical structure deposited on the detector.