Using kinetic equation approach we study dynamics of electrons and phonons in current-carrying superconducting nanostrips after absorption of single photon of near-infrared or optical range. We find that the larger the ratio $C_e/C_{ph}|_{T_c}$ ($T_c$ is a critical temperature of superconductor, $C_e$ and $C_{ph}$ are specific heat capacities of electrons and phonons, respectively) the larger part of photons energy goes to electrons, they become stronger heated and, hence, could thermalize faster during initial stage of hot spot formation. Thermalization time $tau_{th}$ could be less than one picoseconds for superconductors with $C_e/C_{ph}|_{T_c}gg 1$ and small diffusion coefficient $Dsimeq 0.5 cm^2/s$ when thermalization occurs mainly due to electron-phonon and phonon-electron scattering in relatively small volume $sim xi^2d$ ($xi$ is a superconducting coherence length, $d<xi$ is a thickness of the strip). At larger times due to diffusion of hot electrons effective temperature inside the hot spot decreases, the size of hot spot increases, superconducting state becomes unstable and normal domain spreads in the strip at current larger than so-called detection current. We find dependence of detection current on the photons energy, place of its absorption in the strip, width of the strip, magnetic field and compare it with existing experiments. Our results demonstrate that materials with $C_e/C_{ph}|_{T_c} ll 1$ are bad candidates for single photon detectors due to small transfer of photons energy to electronic system and large $tau_{th}$. We also predict that even several microns wide dirty superconducting bridge is able to detect single near-infrared or optical photon if its critical current exceeds 70 $%$ of depairing current and $C_e/C_{ph}|_{T_c} gtrsim 1$.
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