CC BY
12LMI-I-18
Frequency conversion in nanocomposites
O. Fedotova1, O. Khasanov1, R Rusetski1, T. Smirnova2
Scientific-Practical Materials Research Centre NAS Belarus, Laboratory of Theory of Solids, Minsk, Belarus
2International Sakharov Environmental Institute BSU, Department of Environmental Information Systems, Minsk, Belarus
We consider ultrashort laser pulse interaction with nanocomposite consisted of semiconductor quantum dots (QD) incorporated into nonlinear dielectric matrix. Value, angular distribution and spatial dispersion of permanent dipole moments (PDM) are taken into account. The three-level system resonant to pump pulse at the lowest transition between QD exciton states is analyzed. The Hamiltonian describing the light-matter interaction includes fast oscillating diagonal elements due to PDM that impedes to apply traditional approach to calculate medium responses on pulsed excitation. Transformed Hamiltonian is involving QD interaction with fields at multiple frequencies. In result, we get generalized two-level system in rotating reference frame without fast oscillating terms, suitable for one- and two-quantum transitions, and Bloch equations for the QD with PDM in SVEA. The macroscopic photoinduced polarization contains multiple harmonics, which can be a source of responses at corresponding frequencies for the nanocomposite.
THz generation is shown to increase under phase-matching conditions (PMC) due to Zakharov-Benney resonance between THz and pump waves. When PDM is weak and one-photon transitions predominate, the THz field dependence on the pump pulse field is quadratic, in this case THz efficiency does not exceed 0.1%. With increasing PDM magnitude the underlying two-photon transitions (2PT) result in stronger dependence of THz harmonic on the pump pulse providing more efficient frequency conversion. For prevailing 2PT, THz efficiency reaches 1617% . Second harmonic (SH) generation is a parametric process of the same order as a frequency down-conversion in noncentrosymmetric media. As a result, the solutions for pump pulse and its effect on generated harmonics in both cases are similar. However, the frequency up-conversion process provides substantially larger harmonic yield than low frequency generation, so SH attains 70 % under proper PMC.
We compute photon echo (PE) signals under two-pulsed excitation taking into account excitation-induced shift (EIS) of absorption frequency. In addition to signals at resonant frequency under PMC, we establish the responses on two-pulse excitation at multiple frequencies. The spatial synchronism conditions of new responses will differ from those for the primary PE signal. The signals at multiple frequencies and THz responses are originated from PDM interaction with excitation pulses and depend strongly on its angular distribution. The most intensive signals appear at doubled and THz frequencies. Local field effect such as EIS parametrically generates different PE signals at resonant frequency with corresponding PMC. The larger order of PE signal the less its intensity. Spatial dispersion of PDM influences only on even responses. For short time delay between excitation pulses the nonresonant nonlinear matrix will contribute to the time-resolved four wave mixing signals mainly to resonant, SH and THz range. Having optimized the response on the parameters of radiation and materials, highly efficient frequency conversion to THz and SH has been established.
