Fabrication and characterization of fast TESs with small area for single photon counting

Transition edge sensors (TESs) with high energy resolution (Delta E) at telecommunication wavelengths have been realized in the last years for application to experiments of quantum optics and quantum information. In these fields, lasers with repetition rate of the order of 50 MHz are usually applied and hence TES with recovery times lower than 20 ns would be desirable. To this aim, Ti TESs with area ranging between 1 mu m x 1 mu m and 2 mu m x 2 mu m are fabricated and characterized, to study the effects of the different sizes of active areas and critical temperatures on the energy resolutions and on the response times. Titanium TESs have been fabricated by thermal evaporation and Electron Beam Lithography technique on Si/SiN substrates, and they have been electrically and optically characterized. The experimental data have been analyzed and correlated to the detector geometrical parameters. The superconducting transitions of TESs with areas down to 1 mu m x 1 mu m result to be affected by lateral proximity effects of Nb wirings. Ti TESs with critical temperature of 470 mK show response time of about 150 ns and energy resolution of 0.16 eV.