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Journal of Experimental and Theoretical Physics Letters

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

A. M. Smirnov, Anastasiya D. Golinskaya, Kseniia V. Ezhova, Vladimir N. Mantsevich, В. С. Днепровский

Published Nov 1, 2016• 11 citations

Abstract

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 (Δ n ≈ 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.

DOI: 10.1134/s0021364016220136

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The possibility and the conditions of the formation of correlated “gratings” of electrons and their spins in a semiconductor film containing CdSe/CdS nanoparticles (which are the quantum dots) have been studied. Notably, the induction of electron spin “gratings” on quantum dots has been considered for the first time. We have suggested to perform an excitation of the film in the two photon absorption mode using two crossed (at an angle of 60?) femtosecond laser beams. As a result of the excitation, two nonequilibrium “gratings” are induced simultaneously in the excitation area of the film. If the exciting pulses have the same polarizations, then induced “gratings” of electrons are formed. However, in the case when the exciting pulses have mutually orthogonal polarizations, the formation of induced “gratings” of electron spins takes place. The same exciting pulses excite the electrons of CdSe/CdS nanoparticles into a superposition state. Then the free induction decay signal is generated. This signal being diffracted by induced “gratings” plays the role of a probe pulse in the suggested experimental scheme. The diffracted signals propagating in two mutually opposite directions are correlated (i.e., they are identical). Their wavefronts can carry the correlated transient holograms. Other possible applications of correlated “gratings” (in spintronics and in coherent femtosecond laser spectroscopy) are also discussed.

We report the observation of an intensity-dependent refractive-index nonlinearity in CdS due to a resonance state where an excitonic molecule is created by induced absorption of light. The refractive indexnas a function of the incident laser photon energyEis measured directly by light refraction in thin crystal prisms. A renormalized dielectric function describes the measuredn(E)spectra well. This strong refractive-index nonlinearity is well suited to produce an optical bistability and to further strengthen the evidence of CdS to be an important material for laser-induced dynamic gratings.

Electromagnetically induced phase grating is theoretically investigated in the driven two-level quantum dot exciton system at the presence of the exciton-phonon interactions. Due to the phonon-induced coherent population oscillation, the dispersion and absorption spectra are sharply changed and the phase modulation is enhanced via the high refractive index with nearly-vanishing absorption, which could effectively diffract a weak probe light into the first-order direction with the help of a standing-wave control field. Moreover, the diffraction efficiency of the grating can be easily manipulated by controlling the Huang-Rhys factor representing the exciton-phonon coupling, the intensity and detuning of the control field, and the detuning of the probe field. The scheme we present has potential applications in the photon devices for optical-switching and optical-imaging in the micro-nano solid-state system.

The observation of quantum mechanical interference in the exciton photoluminescence excitation (PLE) process in a semiconductor quantum dot is reported. A simple PLE spectroscopy revealed in a single InGaAs quantum dot a dynamic and continuous evolution of a resonance profile from a peak into a dip and to near transparency caused by an increase of excitation power density of only a few 100 W/cm2. A one-photon process via exciton states and a two-photon process via biexciton states, both of which return to the radiative exciton state, interfere quantum mechanically, resulting in a progressive decrease in exciton absorption under increasing excitation.