II-VI Semiconductor Quantum Wells Research Articles

Universality of open microcavities for strong light-matter coupling

An optical resonator is utilized to enhance interactions between photons and solid-state emitters. In particular, when the coupling strength between the exciton within the material is faster than the dissipation rate, the eigenstates of the system are mixed light-matter quasiparticles referred to as exciton-polaritons. In this work, we demonstrate an open, planar cavity platform for investigating a strong coupling regime. The open cavity approach supports ease of integration of diverse material systems and in situ tunability of the photonic resonance. We characterize the strong coupling regime in systems ranging from thin 2D semiconductors, perovskites, and II-VI semiconductor quantum wells.

Universality of optical absorptance quantization in two-dimensional group-IV, III-V, II-VI, and IV-VI semiconductors

The optical absorptance of a single graphene layer over a wide range of wavelengths is known to be remarkably constant at the universal value $\ensuremath{\pi}\ensuremath{\alpha}$ where $\ensuremath{\alpha}$ is the fine structure constant. Using atomistic tight-binding calculations, we show that the absorptance spectra of nanometer-thin layers (quantum wells) of group-IV, III-V, II-VI, or IV-VI semiconductors are characterized by marked plateaus at integer values of $\ensuremath{\pi}\ensuremath{\alpha}$, in the absence of excitonic effects. In the case of InAs, the results obtained are in excellent agreement with the currently available experimental data. By revisiting experimental data on semiconductor superlattices, we show that $\ensuremath{\pi}\ensuremath{\alpha}$ is also a metric of their absorption when normalized to a single period. We conclude that the $\ensuremath{\pi}\ensuremath{\alpha}$ quantization is universal in semiconductor quantum wells provided that excitonic effects are weak and is therefore not specific to the zero-gap graphene case. The physical origin of this universality and its limits are discussed using analytical models that capture the main underlying physics of the lowest optical transitions in III-V and II-VI semiconductor quantum wells. These models show that the absorptance is ruled by $\ensuremath{\pi}\ensuremath{\alpha}$ independent of the material characteristics because of the presence of a dominant term in the Hamiltonian, linear in the wave vector $\mathbf{k}\phantom{\rule{4pt}{0ex}}(\ensuremath{\sim}\mathbf{V}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{k})$, which couples the conduction band to the valence bands. However, the prefactor in front of $\ensuremath{\pi}\ensuremath{\alpha}$ is not unity as in graphene due to the different nature of the electronic states. In particular, the spin-orbit coupling plays an important role in bringing the absorptance plateaus closer to integer values of $\ensuremath{\pi}\ensuremath{\alpha}$. The case of IV-VI semiconductor (PbSe) quantum wells characterized by a rocksalt lattice and multivalley physics is very similar to that of graphene, with the specification that a ``massful gap'' is formed around the Dirac points.

Magnetogyrotropic reflection from quantum wells induced by bulk inversion asymmetry

Bulk inversion asymmetry (BIA) of III-V and II-VI semiconductor quantum wells is demonstrated by reflection experiments in magnetic field oriented in the structure plane. The linear in the magnetic field contribution to the reflection coefficients is measured at oblique incidence of $s$ and $p$ polarized light in vicinity of exciton resonances. We demonstrate that this contribution to the reflection is caused by magnetogyrotropy of quantum wells, i.e. by the terms in the optical response which are linear in both the magnetic field strength and light wavevector. Theory of magnetogyrotropic effects in light reflection is developed with account for linear in momentum BIA induced terms in the electron and hole effective Hamiltonians. Theoretical estimates agree with the experimental findings. We have found the electron BIA splitting constant in both GaAs and CdTe based quantum wells is about three times smaller than that for heavy holes.

Optical Non-Linearities Related to Trions in Quantum Wells and Quantum Dots

We study the non-linear optical properties associated with negatively charged excitons (trions) in II-VI semiconductor quantum wells and single quantum dots. In n-type doped quantum well structures, we observe optical gain and lasing at the trion resonance. The trion momentum distribution spreads out deeper in k-space, while the electrons are closer to the zone center. This results in optical gain at moderate injection levels without inversion in the total number of trions and electrons. The non-linear response of single self-assembled CdSe/ZnSe quantum dots charged with an electron is indeed atom-like. We observe complete bleaching of linear trion absorption spectrum and appearance of induced absorption due to two-pair excitations. The data embody clear signatures for Pauli blocking, few-particle Coulomb correlation, as well as electron-hole exchange finestructure.

Field-effect magnetization reversal in ferromagnetic semiconductor quantum wells

We predict that a bias-voltage-assisted magnetization reversal process will occur in Mn-doped II-VI semiconductor quantum wells or heterojunctions with carrier-induced ferromagnetism. The effect is due to strong exchange-coupling-induced subband mixing that leads to electrically tunable hysteresis loops. Our model calculations are based on the mean-field theory of carrier-induced ferromagnetism in Mn-doped quantum wells and on a semiphenomenological description of the host II-VI semiconductor valence bands.

Theory of optical spectra of polar quantum wells: Temperature effects

Theoretical and numerical calculations of the optical absorption spectra of excitons interacting with longitudinal-optical phonons in quasi-2D polar semiconductors are presented. In II-VI semiconductor quantum wells, exciton binding energy can be tuned on- and off-resonance with the longitudinal-optical phonon energy by varying the quantum well width. A comprehensive picture of this tunning effect on the temperature-dependent exciton absorption spectrum is derived, using the exciton Green's function formalism at finite temperature. The effective exciton-phonon interaction is included in the Bethe-Salpeter equation. Numerical results are illustrated for ZnSe-based quantum wells. At low temperatures, both a single exciton peak as well as a continuum resonance state are found in the optical absorption spectra. By contrast, at high enough temperatures, a splitting of the exciton line due to the real phonon absorption processes is predicted. Possible previous experimental observations of this splitting are discussed.

Mapping of electronic wave functions in II-VI semiconductor quantum wells viaMn2+electron paramagnetic resonance

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Accumulated photon echo in semiconductor microcrystalline quantum dots

The nonlinear coherent emission of excitons in CuBr quantum dots was investigated using a phase-stabilized photon-echo technique. The echo signal was observed to be quite strong, with the emission efficiency being three orders of magnitude larger than that of II-VI semiconductor quantum wells. Time resolving of the echo signal confirmed that the strong nonlinear emission originates from the accumulation effect of the population grating due to the presence of the bottleneck state. Formation and decay kinetics of the population grating was analyzed by taking into account the light-induced deexcitation effect.

Multi-Band Bloch Equations and Gain Spectra of Highly Excited II-VI Semiconductor Quantum Wells

Quasi-equilibrium excitation dependent optical probe spectra of II–VI semiconductor quantum wells at room temperature are investigated within the framework of multi-band semiconductor Bloch equations. The calculations include correlation effects beyond the Hartree-Fock level which describe dephasing, interband Coulomb correlations and band-gap renormalization in second Born approximation. In addition to the carrier–Coulomb interaction, the influence of carrier–phonon scattering and inhomogeneous broadening is considered. The explicit calculation of single particle properties like band structure and dipole matrix elements using k · p theory makes it possible to investigate various II–VI material combinations. Numerical results are presented for CdZnSe/ZnSe and CdZnSe/MgZnSSe semiconductor quantum-well systems.

Low-temperature anti-Stokes luminescence mediated by disorder in semiconductor quantum-well structures.

In low-temperature photoluminescence experiments on II-VI semiconductor quantum wells we find an energy transfer from confined quantum-well states to above-barrier states. The observed anti-Stokes barrier luminescence exhibits a characteristic intensity dependence showing that this transfer is caused by a two-step absorption process involving localized or impurity bound-exciton states in the quantum well. Time-resolved photoluminescence experiments show that the photon for the second intraband absorption step can be provided by the quantum-well luminescence, i.e., the anti-Stokes barrier luminescence is a direct consequence of photon recycling.

Simple formula for exciton binding energy in quantum wells with zero band offsets.

Band offsets in II-VI semiconductor quantum wells can be such as to show small or zero confinement in the conduction band while having considerable confinement in the valence band, or vice versa. By assuming purely two-dimensional motion and a simple variational exciton function, we calculate an explicit analytical expression for the 1s-exciton binding energy in this case and evaluate it for a range of electron- to hole-mass ratios.