Electromagnetically induced transparency from electron spin coherences in semiconductor quantum wells [Invited

Spectroscopy of intersubband transitions in Si–Si 1− xGe x quantum wells

Summary form only given. In this paper, we present coherent optical effects in a quantum well system with two electron subbands coupled by a pump laser. The Coulomb and light induced electronic renormalization is investigated with generalized density-functional theory. On the basis of the renormalized electron states, the absorption coefficient of a probe light beam is studied for intersubband transition by using a self-consistent field theory. Both the electromagnetically induced transparency (EMIT) and probe light amplification without population inversion are found.

The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers.

Excitonic trions in undoped GaAs quantum wells

Optical excitation and control of electron spins in semiconductor quantum wells

A new technique for the measurement of the intersubband lifetime in semiconductor quantum wells is demonstrated. Electrons are optically excited from the ground state to the first excited state of a doped quantum well. From measurements of the absorption cross section between excited states, we find a lifetime equal to τs=0.65±0.15 ps for a 85 Å GaAs quantum well and τs=0.8±0.2 ps for a 100 Å Ga0.47In0.53As quantum well, in good agreement with the theoretical predictions of Ferreira and Bastard [Phys. Rev. B 40, 1074 (1989)]. In addition, our experiments unambiguously show that the intersubband absorption line is homogeneously broadened at cryogenic temperatures (≤100 K).

In this paper, we present an analytical solution to the Maxwell–Bloch equations of the two-level semiconductor quantum well, G a A s / A l G a A s . In addition, we discuss the effects of coherent Rabi oscillations Ω ( t ) and the frequency of the semiconductor system ν ( t ) on atomic occupation probabilities, ρ 11 ( t ) and ρ 22 ( t ) , population inversion, ρ z ( t ) , and information entropies, H ( σ x ) , H ( σ y ) , and H ( σ z ) . We observe clearly the emergence of long-lived quantum coherence and the decay in curves for some special cases of Ω ( t ) and ν ( t ) . Also, we show that the dynamic nonlinear properties of the system can be controlled by changing the values of the coherent Rabi oscillations Ω ( t ) and the frequency of the semiconductor system ν ( t ) . Due to the lack of mathematical treatment of such systems, our study promises significant advantages for a large number of nonlinear dynamic systems, opening up a wide range of applications for semiconductor quantum wells.

The aim of the present chapter is to discuss excitonic processes in semiconductor bulk and Quantum Wells(QW)s. The concept of exciton is elaborated for bulk semiconductors and the expressions for dispersion relation, binding energy, and absorption coefficient of Wannier excitons are derived, illustrating the changes in the absorption spectra due to excitons. The recombination processes are then discussed. Theories for excitonic absorption in QWs are developed, first for the ideal two-dimensional system and then for QWs of finite widths and barriers by approximate treatments. The effects of electric field, the Franz–Keldysh effect in bulk and prominent quantum confined stark effects in QWs due to changes in subband structure and excitonic effects are then investigated. The chapter ends with a brief introduction to and development of Wannier equation in the fractional dimensional space and the methods of determination of non-integer dimension and excitonic binding energy for QWs.

We calculate the Hartree, exchange and correlation energies of a quasi-2D electron gas in doped semiconductor quantum wells, using an expansion in Coulomb interaction. This expansion, also valid for the Hartree term in usual experimental situations, allows one to obtain the analytical well width dependence of the energies. We find that the finite width corrections to the exact 2D exchange-correlation energy are quite often as large as the Hartree contributions.

Based on a microscopic many-particle theory, we predict large optical gain in the probe and background-free four-wave mixing directions caused by excitonic instabilities in semiconductor quantum wells. For a single quantum well with radiative-decay limited dephasing in a typical pump-probe setup we discuss the microscopic driving mechanisms and polarization and frequency dependence of these instabilities.

The magnetotransport experiments based on the weak antilocalization (WAL) effect have confirmed the common belief that the Rashba spin-orbit coupling (SOC) of two-dimensional (2D) holes in semiconductor quantum wells (QWs) is a $k$-cubic term as the lowest order with negligible linear terms. However, an emerging finite linear Rashba SOC was recently found in 2D holes by semiempirical pseudopotential method (SEPM) due to the direct dipolar coupling of an external electric field to the valence subbands in the presence of heavy-hole--light-hole (HH-LH) mixing. Here, we resolve this discrepancy by illustrating that the hole densities in the experiments are so high that the emerging linear term becomes undetectable since its strength declines substantially as increasing the wavevector $k$. Taking the example of a strained $\mathrm{Ge}/{\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5}$ QW utilized in the experiment [Phys. Rev. Lett. 113, 086601 (2014)], we demonstrate that the hole density must be reduced by a factor of 5 to below $2.1\ifmmode\times\else\texttimes\fi{}{10}^{11}{\text{cm}}^{\ensuremath{-}2}$ in order to probe the $k$-linear term. We also evaluate the possibility to achieve WAL at low hole densities in order to measure SOC. These findings shed new light on the experimental measurement of Rashba SOC.

Four-wave mixing via electron spin coherence in a quantum well waveguide

The dynamics of the coherent superposition of exciton states in a semiconductor quantum well excited by the joint action of two successive light pulses with the time interval between them t12 has been theoretically studied. The dependences of the amplitude and initial phase of the quantum bits on t12 have been found. It has been shown that the time dependence of the Kerr rotation of the reflected probe pulse is determined not only by the dynamics of the excited state, but also by the optical properties of the heterostructure containing the quantum well. The dependence of the decay rate of the excited state on t12 has been predicted.

We provide a microscopic theory for the Doppler velocimetry of spin\npropagation in the presence of spatial inhomogeneity, driving electric field\nand the spin orbit coupling in semiconductor quantum wells in a wide range of\ntemperature regime based on the kinetic spin Bloch equation. It is analytically\nshown that under an applied electric field, the spin density wave gains a\ntime-dependent phase shift $\\phi(t)$. Without the spin-orbit coupling, the\nphase shift increases linearly with time and is equivalent to a normal Doppler\nshift in optical measurements. Due to the joint effect of spin-orbit coupling\nand the applied electric field, the phase shift behaviors differently at the\nearly and the later stages. At the early stage, the phase shifts are the same\nwith or without the spin-orbit coupling. While at the later stage, the phase\nshift deviates from the normal Doppler one when the spin-orbit coupling is\npresent. The crossover time from the early normal Doppler behavior to the\nanomalous one at the later stage is inversely proportional to the spin\ndiffusion coefficient, wave vector of the spin density wave and the spin-orbit\ncoupling strength. In the high temperature regime, the crossover time becomes\nlarge as a result of the decreased spin diffusion coefficient. The analytic\nresults capture all the quantitative features of the experimental results,\nwhile the full numerical calculations agree quantitatively well with the\nexperimental data obtained from the Doppler velocimetry of spin propagation\n[Yang {\\it et al.}, Nat. Phys. {\\bf 8}, 153 (2012)]. We further predict that\nthe coherent spin precession, originally thought to be broken down at high\ntemperature, is robust up to the room temperature for narrow quantum wells. We\npoint out that one has to carry out the experiments longer to see the effect of\nthe coherent spin precession at higher temperature due to the larger crossover\ntime.\n

A new form of the Hamiltonian and a boundary condition on the derivatives of the wave function are presented for use in the Schr\"odinger wave equation to calculate the energy eigenvalues in semiconductor quantum wells. This fourth-order Hamiltonian allows the use of a constant effective mass for the electron, and accounts for the effect of band nonparabolicity on the energy eigenvalues in a simple way. Calculations show that nonparabolicity contributes only a small effect for quantum wells grown in the material systems ${\mathrm{Al}}_{0.35}$${\mathrm{Ga}}_{0.65}$As/GaAs and InP/${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As. Larger effects are predicted in the [111] crystal-growth direction than the [100] direction.