A. M. Smirnov

Two-dimensional dynamic photonic crystal creation by means of three non-coplanar laser beams interference in colloidal CdSe/ZnS quantum dots solution

We demonstrated a simple way to create dynamic photonic crystals with different lattice symmetry by interference of three non-coplanar laser beams in colloidal solution of CdSe/ZnS quantum dots. Two-dimensional dynamic photonic crystal was formed due to the periodical changing of refraction and/or absorption of resonantly excited excitons in CdSe/ZnS quantum dots. The formation of dynamic photonic crystal was confirmed by the observed diffraction of the beams that have excited photonic crystal at the angles equal to that calculated for the corresponding two-dimensional lattice (self-diffraction regime).

Self-diffraction of laser beams in the case of resonant excitation of excitons in colloidal CdSe/ZnS quantum dots

Self-diffraction of two types has been discovered in the case of resonant excitation of excitons in CdSe/ZnS quantum dots (highly absorbing colloidal solution) by powerful beams of mode-locked laser with picosecond pulse duration. I. The bleaching of exciton transition provokes the creation of transparency channel and laser beam’s self-diffraction at the induced circular aperture. II. Self-diffraction arises for two laser beams intersecting in the cell with colloidal CdSe/ZnS quantum dots due to the induced transient diffraction grating. Nonlinear optical properties responsible for the observed self-action effects in CdSe/ZnS quantum dots are discussed and a method for estimating laser pulse duration is suggested.

The features of nonlinear absorption during resonance one- and two-photon excitation of the basic exciton transition in colloidal CdSe/ZnS quantum dots

The features of the nonlinear absorption of CdSe/ZnS quantum dots (colloidal solution) in the case of resonant one- and two-photon excitation of the basic exciton transition by powerful ultra-short laser pulses were determined. In one-photon excitation, with an increasing intensity of impulses, a decrease in absorption (bleaching) is relayed by an increase in absorption, which is associated with the process of the filling of the states (saturation) of a two-level system with the lifetime of the excited state depending on the light intensity. The arising Fresnel or Fraunhofer diffraction of the laser ray that pass through a colloidal solution with a high concentration of quantum dots is associated with the formation of the transparency channel and self-diffraction of laser ray on an induced diaphragm. In two-photon excitation, the features of the nonlinear absorption and luminescence tracks (the dependence of luminescence intensity on distance) were explained by the influence, in addition to the two-photon absorption, of the processes that are responsible for the slower growth of nonlinear absorption and luminescence quenching at high intensities of laser pulses.

Two-Photon Excited Diffraction Grating: Self-Diffraction and Nonlinear Optical Properties of Colloidal CdSe/ZnS QDs

Specific features have been revealed of nonlinear optical processes occurring when the total energy of two photons of a mode-locked Nd3+:YAG laser coincides with the energy of the main electron—hole (exciton) transition in colloidal CdSe/ZnS QDs and the effective self-diffraction of two laser beams arises on the induced diffraction grating.

Self-diffraction at a dynamic photonic crystal formed in a colloidal solution of quantum dots

Self-diffraction at a one-dimensional dynamic photonic crystal formed in the colloidal solution of CdSe/ZnS quantum dots has been discovered. This self-diffraction appears simultaneously with self-diffraction at induced transparency channels at the resonant excitation of the main electron–hole (excitonic) transition of quantum dots by two laser beams with a Gaussian intensity distribution over the cross section. It is shown that a nonlinear change in the absorption of colloidal quantum dots results in the formation of a transparency channel and an induced amplitude diffraction grating, and a significant nonlinear change in the refractive index (Δn ≈ 10−3) in the absorbing medium is responsible for the formation of the dynamic photonic crystal. Self-diffracted laser beams are revealed propagating not only in directions corresponding to self-diffraction at the induced diffraction grating but also in directions satisfying the Laue condition.

Peculiarities of the nonlinear absorption of colloidal solutions of CdSe/ZnS quantum dots under stationary single-photon excitation of excitons

Peculiarities of the nonlinear absorption of a colloidal solution of CdSe/ZnS quantum dots with various sizes under resonant stationary excitation of the ground electron–hole (exciton) transition have been revealed by the pump and probe technique. The detected peculiarities of the nonlinear change in absorption are explained by the coexistence and competition of the effects of state filling and charge-induced Stark and temperature long-wavelength shift of the absorption spectra.

Modeling of self-diffraction from the induced aperture in colloidal quantum dots

Modeling of self-diffraction pattern formation from the induced diaphragm, arising in the case of the transparency channel saturation by one-photon resonant non-stationary excitation of the basic exciton transition in colloidal quantum dots (QDs) is realized. The simulation results allow us to obtain the reference image of self-diffraction pattern and dependence of the intensity transverse distribution of the output beam from the intensity of the excitation beam, forming a transparency channel. A powerful laser pulse creates a transparency channel, so that it self-diffracts on the induced diaphragm. The possibility to apply the obtained simulation results for intensity estimation of the laser radiation and for the possible application in the technique (nonlinear-optical limiters of intense laser radiation in the visible and nearinfrared region, optical switches) are discussed.

Self-action effects in semiconductor quantum dots

Two-dimensional (2D) dynamic photonic crystal regime has been utilized to investigate self-diffraction effect and nonlinear optical properties of excitons in CdSe/ZnS colloidal quantum dots (QDs). Self-diffraction at 2D photonic crystal arises for three intersecting beams of Nd+3:YAG laser second harmonic in the case of one-photon resonant excitation of the exciton (electron - hole) transition QDs. The relaxation time of excited excitons has been measured by pump and probe technique at induced one-dimensional transient diffraction grating. Two-exponential decay with initial fast and slow parts was discovered. Self-action effect has been discovered in the case of stationary resonant excitation of excitons in CdSe/ZnS QDs by the beam of second harmonic of powerful 12-nanosecond laser pulses. The bleaching of exciton absorption and the creation of transparency channel (this effect provokes self-diffraction of the second harmonic beam) was explained by the dominating coexisting and competing processes of state filling in stationary excited quantum dots and Stark-shift of exciton spectral band. The peculiarities of the influence of these processes at the change of exciton absorption in quantum dots in the case of different detuning from exciton resonance (quantum dots with different size have been used) was analyzed.

Nonlinear effects in colloidal nanoplatelets with two-dimensional electronic structure

We have investigated the nonlinear absorption of CdSe-based nanoplatelets (NPLs) with different thicknesses of shell in the case of resonant one-photon stationary excitation of exciton transitions by nanosecond pulses of mode-locked Nd:YAP laser. Decrease in absorption at the wavelength of whether light hole – electron and heavy hole – electron exciton transitions was revealed. Induced changing of both absorption doublet components was attributed to phase space filling effect and exciton energy conversion mechanism.We have investigated the nonlinear absorption of CdSe-based nanoplatelets (NPLs) with different thicknesses of shell in the case of resonant one-photon stationary excitation of exciton transitions by nanosecond pulses of mode-locked Nd:YAP laser. Decrease in absorption at the wavelength of whether light hole – electron and heavy hole – electron exciton transitions was revealed. Induced changing of both absorption doublet components was attributed to phase space filling effect and exciton energy conversion mechanism.

Tyndall scattering in colloidal solution of quantum dots under the action of laser pulses

The high-intensity nanosecond laser pulses scattering in strongly absorbing colloidal solutions of СdSe/ZnS quantum dots has been investigated. Different types of nonlinear Tyndall scattering mechanism was revealed as a function of excitation radiation intensity. At the low laser pulses intensity (up to 15 MW/cm2 ) saturation of the basic exciton transition in strongly absorbing colloidal solution of СdSe/ZnS quantum dots was observed. In this case of average laser pulses intensity (15-200 MW/cm2 ) the dominant scattering mechanism is scattering on dipoles induced by the electric field and scattering on density fluctuations of the dispersed medium around bleached quantum dots. At the higher intensity (200-4000 MW/cm2 ), the predominant scattering mechanism is the scattering on bubbles of gas formed around local heating centers – colloidal quantum dots.The high-intensity nanosecond laser pulses scattering in strongly absorbing colloidal solutions of СdSe/ZnS quantum dots has been investigated. Different types of nonlinear Tyndall scattering mechanism was revealed as a function of excitation radiation intensity. At the low laser pulses intensity (up to 15 MW/cm 2 ) saturation of the basic exciton transition in strongly absorbing colloidal solution of СdSe/ZnS quantum dots was observed. In this case of average laser pulses intensity (15-200 MW/cm 2 ) the dominant scattering mechanism is scattering on dipoles induced by the electric field and scattering on density fluctuations of the dispersed medium around bleached quantum dots. At the higher intensity (200-4000 MW/cm 2 ), the predominant scattering mechanism is the scattering on bubbles of gas formed around local heating centers – colloidal quantum dots.