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Laser Physics

Echo-spectroscopy of nanocomposites

Oleg Kh Khasanov, Olga M. Fedotova, Русецкий Григорий Александрович, Victor G. Nikiforov

Published Oct 25, 2019• 2 citations
Publisher

Abstract

Abstract Methods of characterization of nanocomposites consisting of semiconductor quantum dots incorporated into a transparent nonlinear dielectric matrix are elaborated. Spectral composition, observation time, and phase-matching conditions for echo-responses excited by two noncollinear pulses are analysed. More specifically, spectral and spatial features of echo signals caused by permanent dipole moments (PDMs) inherent to quantum dots and local field effects in them, as well as by the angular distribution function of the PDM orientations and spatial dispersion of transition dipole moments, are considered.

DOI
10.1088/1555-6611/ab4bd2

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Abstract Methods of characterization of nanocomposites consisting of semiconductor quantum dots incorporated into a transparent nonlinear dielectric matrix are elaborated. Spectral composition, observation time, and phase-matching conditions for echo-responses excited by two noncollinear pulses are analysed. More specifically, spectral and spatial features of echo signals caused by permanent dipole moments (PDMs) inherent to quantum dots and local field effects in them, as well as by the angular distribution function of the PDM orientations and spatial dispersion of transition dipole moments, are considered.
We elaborate methods of femtosecond time resolved spectroscopy of nanocomposites. Features of photon echo (PE) and time resolved four-wave mixing (TRFWM) signals in nanocomposites consisted of semiconductor quantum dots (QD) incorporated into nonlinear dielectric matrix are studied. In particular, impacts of value and angular distribution of permanent dipole moments (PDM) as well as their spatial dispersion on two pulse responses are under investigation. As is known, large values of permanent dipole moments are inherent to many semiconductor QDs, including ZnO QDs. Moreover, the presence of giant dipole moment in cellulose nanocrystals what reaches 4400+400 Debye has been demonstrated [1]. To the best of our knowledge, the PDM impact on the signals of time resolved four-wave mixing as a whole, and, in particular, photon echo has been yet unrevealed.
Experimental results of observations of a femtosecond photon echo excited in an ensemble of quantum nanometer-sized objects in thin semiconductor (ZnO, SiB, SiP) films at room temperature are presented. The observed properties of the photon echo are discussed. Recorded relaxation times of exciton quantum transitions and beats at the photon echo decay as a function of the increasing interval between the exciting pulses reveal the role played by the defects of the thin film in the formation of its exciton structure of quantum transitions investigated using angular optical echo spectroscopy.
In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.
An overview on photon echo spectroscopy under resonant excitation of the exciton complexes in semiconductor nanostructures is presented. The use of four-wave-mixing technique with the pulsed excitation and heterodyne detection allowed us to measure the coherent response of the system with the picosecond time resolution. It is shown that, for resonant selective pulsed excitation of the localized exciton complexes, the coherent signal is represented by the photon echoes due to the inhomogeneous broadening of the optical transitions. In case of resonant excitation of the trions or donor-bound excitons, the Zeeman splitting of the resident electron ground state levels under the applied transverse magnetic field results in quantum beats of photon echo amplitude at the Larmor precession frequency. Application of magnetic field makes it possible to transfer coherently the optical excitation into the spin ensemble of the resident electrons and to observe a long-lived photon echo signal. The described technique can be used as a high-resolution spectroscopy of the energy splittings in the ground state of the system. Next, we consider the Rabi oscillations and their damping under excitation with intensive optical pulses for the excitons complexes with a different degree of localization. It is shown that damping of the echo signal with increase of the excitation pulse intensity is strongly manifested for excitons, while on trions and donor-bound excitons this effect is substantially weaker.