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Journal of Physics Conference Series

The phenomenon of nonstationary nutation in the system of two-level quantum wells of a periodic superlattice with strong magnetic quantization

H.S. Nikoghosyan, Vardan Manukyan, G. H. Nikoghosyan

Published Nov 1, 2019• 1 citation
Publisher

Abstract

Abstract We investigate the superposition of nonstationary nutation processes that arose during the interaction of two-level systems of quantum wells of a superlattice with two exciting coherent optical pulses with various resonance detunings. According to the model under consideration, dipole transitions in two-level systems of quantum wells under the action of exciting pulses occur between the Landau levels of the main mini-band of the superlattice conduction band. This gives a direct opportunity to study the controlled variation of the course of optical nutation processes, by changing the intensity of an external quantizing magnetic field.

DOI
10.1088/1742-6596/1400/5/055004

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Abstract We investigate the superposition of nonstationary nutation processes that arose during the interaction of two-level systems of quantum wells of a superlattice with two exciting coherent optical pulses with various resonance detunings. According to the model under consideration, dipole transitions in two-level systems of quantum wells under the action of exciting pulses occur between the Landau levels of the main mini-band of the superlattice conduction band. This gives a direct opportunity to study the controlled variation of the course of optical nutation processes, by changing the intensity of an external quantizing magnetic field.
The question is raised whether nonlinear coherence can be created in coupled systems whose subsystems interact with distinct radiation fields. The simplest coupled system consisting of two two-level subsystems is treated in interaction with two electromagnetic fields, each close to resonance of one of the subsystems. The problem is treated within the SU(4) symmetries. The generators of SU(4) are defined and their properties under transformations deduced. The Hamiltonian and the density operator of the coupled system are expressed in terms of the generators of SU(4) and the Liouville equation is then used to find the time evolution of the density operator. When the frequencies of the radiation fields are equal to the resonant frequencies of the uncoupled subsystems, one obtains two two-photon nutation processes, provided that the interactions of the subsystems with the radiation fields are weaker than the mutual interaction of the two subsystems. The nutation frequency is proportional to the product of the field strengths and inversely proportional to the coupling parameter which in this case represents the resonance offset of the intermediate levels. Alternatively, the frequencies of the radiation fields can be arranged so that only one two-photon transition is on resonance. The amplitude of the nutation then depends on the strengths of the radiation fields, and acquires maximum value for ${\ensuremath{\omega}}_{1}={\ensuremath{\omega}}_{2}$. The nutation frequency increases when one approaches the resonance with the intermediate level. With a two-photon $\ensuremath{\pi}$ pulse one transfers the population difference from the levels of one subsystem to those of the other. This procedure can be used to polarize rare spins in the case of strong spin-spin coupling with the abundant spins when the method of thermodynamic contact is inapplicable. Both magnetic and optical coupled systems can be described with the same equations.
The effect of an external magnetic field on optical coherent transient phenomena has been analyzed for semiconductor quantum well structures (QWS). The Lüttinger Hamiltonian is used to incorporate the complex valence band structure which is modified due to the application of the magnetic field. We have considered a GaAs/AlGaAs QWS with magnetic field applied in the direction perpendicular to the plane of the well. We get two quantum levels due to confinement, and doubly degenerate heavy-hole and light-hole bands corresponding to each quantum level, thus leading to eight hole subbands. This results in an 8×8 Hamiltonian in an appropriately chosen basis. Energy eigenvalues and eigenvectors have been calculated by numerically diagonalizing the above Hamiltonian. The absorption characteristics and the optical coherent transient phenomena have been studied by solving the effective semiconductor Bloch equations in the weak excitation regime. The polarization induced in conjunction with the results obtained by solving the modified Lüttinger Hamiltonian has been used to analyze the transient characteristics. We observe a blueshift in transition energy with an increase in magnetic field. The features are changed dramatically when one includes band mixing. The crossing of subbands in the absence of band-mixing effects changes over to anticrossing behavior when band mixing is included. Multiple oscillations are observed in the optical nutation signal that can be attributed to band-mixing effects. Absorption spectra also show a significant blueshift with an increase in magnetic field and additional peaks appear when band mixing is included.
The phenomenon of transfer of coherence and the effect of level shift caused by two-quantum (TQ) excitation were studied on 27Al^{27}\mathrm{Al}27Al nuclei in Al2{\mathrm{Al}}_{2}Al2​O3{\mathrm{O}}_{3}O3​. Transfer of coherence between two single-quantum (SQ) transitions and that between SQ and three-quantum transitions were examined. It was observed that the coherences transfer periodically with frequency equal to one-half the TQ transient nutation frequency accompanied by phase shifts caused by the level shift. Experimental results are in good agreement with theoretical predictions. The effect of magnetic dipole interaction was also studied. A precise measurement was made on the shift of TQ resonance frequency and it was confirmed that no shift exists on the TQ resonance frequency contrary to the prediction based on the three-level model.
A new type of nutation oscillations occurring only in the phase of an electromagnetic wave propagating in an ensemble of quantum dots is described. The interaction of the electromagnetic wave with quantum dots produces the transitions of electron between states of approximately equal energies without any changes in the intensity of the wave. The period of nutation oscillations is shown to be defined by the main parameters describing the electromagnetically induced electron transfer. Nutation effect is considered for the case of nonresonant electromagnetic pulse with a rectangular form of its amplitude.
In the past few years advance in nanofabrication techniques have made it possible to make well-like compound semiconductor structures of low nanometer size.In these structures,the state of the electron in the direction perpendicular to the length of the well is quantized with sufficiently large separation in energy levels and the motion of the electron parallel to the length is that of a quasi-free electron.One of the most remarkable properties of these one-dimensional electron system is that the optical transitions between the size-quantized subbands are feasible.The result shows there is a large optical nonlinearities in asymmetric quantum wells.The problem of optical saturation of intersubband absorption has recently attracted considerable attention.The problem is very important to the development of superlattice devices such as fast infrared detectors and(modulators) because one needs to know the range of input optical intensities for which linear operation is(possible.) The analytical expression of the linear and the third-order nonlinear optical intersubband absorption coefficients are obtained in Pschl-Teller wells using the density matrix formalism taking into account the(intrasubband) relaxation.Since there are two adjustable parameters κ and λ in Pschl-Teller wells,the shapes of the wells will change with the change of the parameters κ and λ,accordingly the linear and the third-order nonlinear optical absorption coefficients will change with the parameters κ and λ.It is shown that the optical absorption coefficient depends on the parameter,and the optical intensity strongly. The absorption saturation will occur with I increase.Numerical results are presented for a typical AlGaAs/GaAs Pschl-Teller well.