Electromagnetically induced transparency and slow light with n-doped GaAs

The suitability of bound exciton system in semiconductors is studied for use in nonlinear optical schemes based on EIT, such as “slow” or “stored” photons. We match the desired properties of such a system exhibiting EIT with the known physical realities of a semiconductor system, and suggest, in particular, two suitable schemes using donor impurities in GaAs. In addition to generic properties, we also focus on the influence of many neighboring levels and continuum levels, and on the effect of strong hole-mixing.     

Biexponential intersubband relaxation in n-modulation-doped quantum-well structures

Intersubband scattering in an n-modulation-doped GaAs/Al_0.31Ga_0.69As quantum-well structure is systematically investigated as a function of temperature and pump intensity. For the first time biexponential relaxation is observed during infrared bleaching experiments. The fast component with a time constant of ∼1 ps, which represents the depopulation of the first excited well subband, is found to dominate the signal more and more with decreasing temperature and pump intensity. The decay time of the slower component rises with decreasing temperature from τ₂=8 ps atT=300 K to τ₂=23 ps atT=10 K. This component is believed to be connected with a carrier transfer to the potential minima in the barrier layers. The intensity dependent excitation mechanism and the relaxation processes are discussed with the help of detailed numerical simulations.     

Giant Kerr nonlinearity in an n-doped semiconductor quantum well

A scheme for enhancement of Kerr nonlinearity with vanishing linear and nonlinear absorption in a three-level ladder-configuration n-doped semiconductor quantum well is proposed. It is shown that the Kerr nonlinearity can be controlled and even enhance by the intensity of coupling fields. Also, phase control of Kerr nonlinearity is then discussed.     

Giant Kerr nonlinearities and slow optical solitons in coupled double quantum-well nanostructure

We investigate the enhancement of Kerr nonlinearities in an asymmetric coupled double quantum-well (CQW) by analyzing the nonlinear optical response. With the consideration of real parameters in AlGaAs-based CQWs, we indicate the possibility to obtain the giant Kerr nonlinearities with the cancellation of linear absorption simultaneously. We also reveal both analytically and numerically that under appropriate conditions a probe field with very low intensity can evolve into a stable shape-preserving waveform that propagates with a slow group velocity. The stability of such slow optical solitons is also demonstrated numerically.     

Two forms of polarization relaxation in polydomain ferroelectrics in an electric field

The characteristic features of the polarization and depolarization kinetics of polydomain ferroelectrics with square and narrow “extended” dielectric hysteresis loops are investigated for the model of TGS and Rb₂ZnCl₄ crystals. It is shown that for the second crystal, in contrast to the first crystal, the local free energy is asymmetric relative to the direction of polarization, the coercive field does not have a definite value, and only part of the crystal volume participates in the slow thermoactivational relaxation. The slow relaxation follows a universal empirical power law in all cases. The distribution functions of the relaxation times in crystals are constructed on the basis of experimental data, and comparative estimates are made of the relaxation parameters and the parameters of the energy barriers for domain walls. Fiz. Tverd. Tela (St. Petersburg) 41, 499–504 (March 1999)     

Nuclear relaxation in dynamic nuclear polarization

The behavior of nuclear polarization in a substance, i.e., a solution of the complex HMBA(Cr^V)Na⁺ in 1,2-propylene glycol used in polarized nuclear targets is experimentally investigated by magnetic spectroscopic methods under conditions of dynamic nuclear polarization at hv_S/kT=≈1.5−3.2. Nuclear polarization is measured and analyzed as a function of time at different values of the saturating microwave signal and temperature. It is shown that the process of decreasing the nuclear polarization involving free nuclear relaxation is described by a nonmonoexponential law with two damping decrements, which determine the time of reaching equilibrium between the Zeeman nuclear subsystem, the dipole-dipole pool, and the lattice. Specific features of dynamic processes proceeding in the electronic-nuclear system of the substance investigated are discussed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 363–366, May–June, 1997.     

Optical orientation and spin relaxation of resident electrons in n-doped InAs/GaAs self-assembled quantum dots

We report on optical orientation of electrons in n-doped InAs/GaAs quantum dots. Under non-resonant cw optical pumping, we measure a negative circular polarization of the luminescence of charged excitons (or trions) at low temperature (T=10 K). The dynamics of the recombination and of the circular polarization is studied by time-resolved spectroscopy. We discuss a simple theoretical model for the trion relaxation, that accounts for this remarkable polarization reversal. The interpretation relies on the bypass of Pauli blocking allowed by the anisotropic electron–hole exchange. Eventually, the spin relaxation time of doping electrons trapped in quantum dots is measured by a non-resonant pump–probe experiment.     

Hole spin relaxation in Ge quantum dots

Hole spin relaxation in an isolated Ge quantum dot due to interaction with phonons is investigated. Spin relaxation in this case occurs through the mechanism of the modulation of the spin-orbit interaction by lattice vibrations. According to the calculations performed, the spin relaxation time due to direct single-phonon processes for the hole ground state equals 1.4 ms in the magnetic field H = 1 T at the temperature T = 4 K. The dependence of the relaxation time on the magnetic field is described by the power function H^?5. At higher temperatures, a substantial contribution to spin relaxation is made by two-phonon (Raman) processes. Because of this, the spin relaxation time decreases to nanoseconds as the temperature is raised to T = 20 K. Analysis of transition probabilities shows that the third and twelfth excited hole states, which are intermediate in two-step relaxation processes, play the main part in Raman processes.     

Mixing,ergodicity and slow relaxation phenomena

Investigations on diffusion in systems with memory [I.V.L. Costa, R. Morgado, M.V.B.T. Lima, F.A. Oliveira, Europhys. Lett. 63 (2003) 173] have established a hierarchical connection between mixing, ergodicity, and the fluctuation–dissipation theorem (FDT). This hierarchy means that ergodicity is a necessary condition for the validity of the FDT, and mixing is a necessary condition for ergodicity. In this work, we compare those results with recent investigations using the Lee recurrence relations method [M.H. Lee, Phys. Rev. B 26 (1982) 2547; M.H. Lee, Phys. Rev. Lett. 87 (2001) 250601; M.H. Lee, J. Phys. A: Math. Gen. 39 (2006) 4651]. Lee shows that ergodicity is violated in the dynamics of the electron gas [M.H. Lee, J. Phys. A: Math. Gen. 39 (2006) 4651]. This reinforces both works and implies that the results of [I.V.L. Costa, R. Morgado, M.V.B.T. Lima, F.A. Oliveira, Europhys. Lett. 63 (2003) 173] are more general than the framework in which they were obtained. Some applications to slow relaxation phenomena are discussed.     

Strong relaxation polarization in a double-layer capacitor

In dielectrics, strong relaxation polarizations develop that contribute much to the polarization of the substance. Such great contributions should be expected in heterogeneous dielectrics. Relaxation polarization is studied in the simplest of heterogeneous dielectrics: a double-layer capacitor. The areas of concentration of the development of strong relaxation polarizations, the locations of which depend on the ratios of permittivities and the through electrical conductivities of the layers, are determined. The transition to strong relaxation polarization in this case also occurs in accordance with criteria developed earlier.     

Long-time relaxation of an ordered magnetic structure

In a Mössbauer investigation of the parameters of the hyperfine interactions on Fe⁵⁷ nuclei in the ferromagnetic Tb_0.8Y_0.2Fe₂, we have made the first observation of long-time (days, months) relaxation of an ordered magnetic structure. In this effect the orientation of some moments in a sample changes under the action of a pulsed magnetic field (up to 250 kOe) and then slowly reverts to the initial state by means of gradual decomposition of clusters of the reoriented moments. A theoretical estimate of the relaxation time is made.     

Spin orbit interaction and polarization of slow neutrons

It is shown that the spin orbit (LS) interaction of an anomalous magnetic moment is twice that of a Dirac moment. In neutron scattering this interaction leads to an imaginary spindependent scattering amplitude. Possible experiments are discussed, where interference ofLS scattering and nuclear scattering in crystals without inversion symmetry leads to detectable neutron polarization.     

Strong and weak polarization relaxation processes in solid dielectrics

It is established that an increase in the reach-through electrical conductivity of a dielectric can affect the frequency characteristics of the quantity tanδ in different ways when relaxation polarization processes occur: the extrema of the frequency characteristics can be either suppressed or intensified. In the former case, relaxation processes are referred to as weak; in the latter case, they are referred to as strong. Strong processes lead to the emergence of extrema in the frequency dependence of the imaginary part of complex conductivity. The causes underlying the two polarization relaxation processes are identified.