Figure of Merit for Detectors Based on Laser-Induced Thermoelectric Voltages in La1−xCaxMnO3and YBa2Cu3O7−σThin Films

A figure of merit (FOM) Z = U p /τ r , where U p is the peak voltage, and τ r is the rise time of the laser-induced thermoelectric voltage (LITV) signal, is defined for photodetector based on the LITV and the influence of the parameters on FOM is analysed based on the time dependence of LITVs in La 1− x Ca x MnO 3 (LCMO) and YBa 2 Cu 3 O 7−σ (YBCO) thin films grown on vicinal-cut substrates. We find that the FOM increases as the photon penetration depth decreases, and linearly increases with the thermal diffusion constant D. To achieve the highest FOM, the film thickness d has to be controlled to an optimum value. We also find that the FOM is directly proportional to the laser absorption coefficient α 0 , the laser energy density per pulse E, the illuminated length of film l, sin(2α) [αis the vicinal-cut angle], the Seebeck coefficient anisotropy ( S ab − S c ), and is inversely proportional to the mass density ρ and the specific heat c 0 .