Quantum Efficiency of Cold Electron Bolometer Optical Response

In this paper, we present the measurements of optical response dependence on power load of a Cold Electron Bolometer integrated in a twin slot antenna. These measurements are also compared to the models of the bolometer limit and the photon counter limit. The responsivity of 0.22*10⁹ V/W was measured at 0.22 pW radiation power from a black body at 3.5 K. According to our estimations, for optimized device the voltage responsivity at 100 mK electron temperature can approach S v = 10 ¹⁰ V/W for power load below 0.1 pW and decreases down to 10⁷ V/W at 300 mK for 5 pW signal power in a sample with absorber volume of 5*10^-20 m³. In the case of low bath temperatures and high applied RF power the changes of tunneling current, dynamic resistance and voltage response are explained by non-thermal energy distribution of excited electrons. Distribution of excited electrons in such system at lower temperatures can be of non-Fermi type, hot electrons with energies of the order of 1 K tunnel from normal metal absorber to superconductor instead of relaxing down to thermal energy kT_e in absorber before tunneling. This effect can reduce quantum efficiency of the bolometer at 350 GHz from η = hf/kT_ph> 100 in ideal case down to single electron per absorbed photon ( η = 1) in the high power case. Methods of preserving high quantum efficiency are discussed.