Terahertz Generation via Excitation of Surface States Formed from Spatially Separated Electrons and Holes in Nanocomposites

Terahertz (THz) generation in nanocomposites consisted of metal-oxide semiconductor nanocrystals incorporated into a dielectric matrix is studied. We consider the THz emission from excitons formed by spatially separated... [ view full abstract ]

Terahertz (THz) generation in nanocomposites consisted of metal-oxide semiconductor nanocrystals incorporated into a dielectric matrix is studied. We consider the THz emission from excitons formed by spatially separated electron and hole in above nanostructures when the quantum dot (QD) dielectric permittivity is substantially higher than the dielectric matrix one and the QD sizes are large enough. According to our calculations starting from the critical QD radius the electron-hole binding energy asymptotically approach the binding energy of corresponding 2D exciton. Another reason of the large binding energy of the excitonic states is the dielectric enhancement effect originated from the field produced by the nanoparticles in matrix. We study frequency down-conversion processes in nanocomposites with the large permanent dipole moment (PDM). Dielectric matrix is supposed to be non-centrosymmetric and transparent in THz spectral range. We consider THz generation by a femtosecond laser pulse being in resonance with the QDs and out of resonance with dielectric matrix. As a result, the nonlinear polarization response of the medium consists of resonant and non-resonant components. One- and two-photon resonances are under study. The local field effects are also accounted for. The resonant mechanism of down-frequency conversion is shown to be more effective than non-resonant one. As a first step, solitonic regimes of the input pulse propagation and harmonic formation in its field have been studied. The local field and the PDM impact is revealed to modify sufficiently the pump pulse envelope in comparison with hyperbolic secant form. When such influence is weak, the dependence of the THz field on the pump pulse is quadratic. In this case the maximal THz field is observed for pump pulse area θ=3π but its generation efficiency does not exceed 10^-3. With increasing the PDM magnitude the underlying two-photon processes modify the pump pulse form to a great extent and lead to stronger dependence providing more efficient frequency conversion. According to our numerical results, for nanocomposites with optimal parameters the THz generation can reach 2% in magnitude under phase capture conditions.