Optical limiting of laser radiation in semiconductor monocrystals, quantum dots and multilayer microresonators in the visible and near infrared spectral range

摘要

Investigations of nonlinear optical properties and mechanisms of optical limiting in semiconductor materials are of persistent interest for the design of laser radiation protection elements. Studies into nonlinear optical properties of ZnS semiconductor monocrystals in visible range and InP in NIR range allowed to determine the main optical limiting mechanisms: two-photon absorption, free-carrier absorption and absorption in forming microplasma (at high fluencies). Theoretical (based on Z-scan measurements of nonlinear constants) and experimentally obtained values of the limiting threshold for monocrystals are: 2 J/cm2 and 0.5 J/cm2 correspondingly (ZnS, λ = 532nm), 0.2J/cm2 and 0.1J/cm2 correspondingly (InP, λ = 1540nm). Polyvinylpirrolidone-stabilized ZnS quantum dots (QD)-based composite media synthesis helped to enhance optical limiting characteristics, in particular, to shift the UV edge of transmittance due to an increased band gap from 3.6eV for ZnS monocrystal up to 3.9 ÷ 4.7eV for Zn QD owing to sizes of synthesized QD (1.4-2.0nm). Introduction of C60 fullerene, perylene, tetramethylbenzidine and their combinations into CdSe/ ZnS QD colloid solutions led to formation of hybrid photoactive structures (photoreactors) in the form of donor-acceptor complexes in the ground state, and thereby to widening of spectral rage into the IR region, and also to a decrease of the limiting threshold on the grounds of “light quenching” mechanism. To solve an issue of low-threshold limiting, we worked out multilayer microresonators with a nonlinear layer visible and NIR limiters. To set up a low-threshold NIR limiter we employed a technique of GaAs/AlAs layers molecular epitaxy on a GaAs substrate. It gave way to construction of a resonator with transmittance line width 2.5nm at λ = 1117nm. A shift of the transmittance line maximum induced by the laser radiation owed to changes of the refraction index, the latter phenomena being connected with two-photon absorption and free-carrier generation. The said shift helped to get optical limiting with a low threshold value at a level of 10-3J/cm2.