Light-hole Resonances Research Articles

Quantum Confinement-Controlled Exchange Coupling in Manganese(II)-Doped CdSe Two-Dimensional Quantum Well Nanoribbons

The impact of quantum confinement on the exchange interaction between charge carriers and magnetic dopants in semiconductor nanomaterials has been controversially discussed for more than a decade. We developed manganese-doped CdSe quantum well nanoribbons with a strong quantum confinement perpendicular to the c-axis, showing distinct heavy hole and light hole resonances up to 300 K. This allows a separate study of the s-d and the p-d exchange interactions all the way up to room temperature. Taking into account the optical selection rules and the statistical distribution of the nanoribbons orientation on the substrate, a remarkable change in particular of the s-d exchange constant with respect to bulk is indicated. Room-temperature studies revealed an unusually high effective g-factor up to ~13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications.

Spectral dependence of the optical Stark effect in ZnSe-based quantum wells

An experimental study of the optical Stark effect in ZnSe-based quantum wells using pump-probe spectroscopy is presented. The Stark shift at both the heavy-hole and the light-hole resonances is measured as a function of pump detuning for samples with different heavy-hole\char21{}light-hole separations. The coupling between the heavy hole and the light hole has to be taken into account to explain the observed dependence on the pump wavelength detuning and polarization of pump and probe pulses. For a range of samples the Stark shift behavior is characterized by the ratio of the detuning to the heavy-hole to light-hole separation; higher order Coulomb correlations play a prominent role when the ratio is less than 2.

Light-hole Stark-ladder photoluminescence induced by heavy-hole–light-hole resonance in a GaAs/InAlAs superlattice

We have studied the photoluminescence properties of a GaAs/InAlAs strained superlattice as a function of an applied electric field. We have detected the appearance of light-hole Stark-ladder photoluminescence resulting from heavy-hole–light-hole resonance induced by the applied electric field. From calculated overlap integrals between electron and hole wave functions, it is demonstrated that the light-hole Stark-ladder photoluminescence is observable in a GaAs/InAlAs superlattice.

Light-hole resonant tunnelling through tensile-strained GaInAs quantum wells

We have experimentally investigated the light- and heavy-hole states in tensile-strained GaInAs quantum wells using photoluminescence and photocurrent spectroscopies. Under special conditions, the strain resulting from the lattice mismatch is large enough to reverse the order of the light- and heavy-hole levels (the first light-hole level becomes the ground state). We obtained an experimental light-to-heavy-hole splitting of 50 meV in agreement with calculations. To take advantage of this situation, we realized and characterized the first p-i-p resonant tunnelling diode on an InP substrate. Resonances are clearly visible in the conductance-voltage characteristics but the first resonance is not yet enhanced as expected in this structure. We discuss the reasons that prevent the observation of the first light-hole resonance.

First resonant tunneling via a light-hole ground state

We report the demonstration of resonant tunneling of light-holes through an AlAs/GaAs 0.7P 0.3 double-barrier heterostructure. The tensile strain in the quantum well reverses the order of the light- and heavy-hole levels, the first light-hole level becoming the ground state. The I( V) characteristics are measured at different temperatures and compared to those of a standard AlAs/GaAs unstrained structure. The peak current density of the first light-hole resonance and its peak-to-valley current ratio are enhanced. They reach 28 A/cm 2 and 3.4 : 1 at 15 K. A negative differential resistance is observed up to 250 K.

Observation of strain-relaxation-induced size effects inp-type Si/SiGe resonant-tunneling diodes

We have studied the effect of strain relaxation in small Si/SiGe resonant-tunneling diodes (RTD's) on the tunneling of holes through these structures. We have used RTD's mesa-etched into dots and wires, the lateral dimensions ranging from 10 \ensuremath{\mu}m down to 230 nm. In the dots we find a very strong shift of the light-hole (LH) resonance in the tunneling spectrum as the dot diameter decreases below 1 \ensuremath{\mu}m, while the position of the heavy-hole (HH) resonance is constant. In the wires, on the contrary, this size effect in the tunneling is completely absent: both peak positions are constant. This behavior, including the surprising insensitivity of the tunneling spectrum to the wire width, arises from a substantial degree of strain relaxation in the SiGe layers of the devices. This interpretation is supported by the strain dependencies we derive for the HH and LH barrier heights, and the HH-LH splitting in the quantum well. The combined effect of these quantities on the peak voltages agrees qualitatively with the experimental data, when we assume that in the dots the relaxation is biaxial, while in the wires it is predominantly uniaxial. The interpretation is also consistent with magnetotunneling-spectroscopy data, which reflect the in-plane anisotropy of the LH quantum-well subband. We find for all dot diameters a fourfold rotational symmetry of the shift of the LH resonance and for the wires a remarkable transition from a fourfold to a pronounced twofold rotational symmetry of this shift as the wire width decreases below 900 nm. This transition is interpreted as evidence for the strong influence of uniaxial relaxation on the in-plane dispersion.

Nonlinear Polariton Propagation

The nonlinear optical response of excitonic polaritons to the presence of a background of incoherent polaritons in quasi-equilibrium is investigated for thin ZnSe layers in the temporal and spectral domain. At low densities the transmission spectra are dominated by strain-split heavy-hole and light-hole resonances the former being further split into several Fabry-Pérot modes caused by polariton interference inside the layers. In the temporal domain this corresponds to complex structured transients of transmitted pulses exhibiting pronounced propagation beats. Under resonant pumping of the heavy-hole polariton the nonlinear spectra exhibit a strongly asymmetric shape of the quantized polariton modes. The transients show induced transmission around t = 50 fs which is followed by a complex-structured induced absorption at t > 200 fs. Almost quantitative agreement is found between the experimental data and calculations based on polariton theory clearly demonstrating the strong influence of spatial dispersion also on nonlinear polariton propagation. This allows to determine the density dependence of the oscillator strength, damping, and energy shift of the heavy-hole polariton.

Light-hole resonant tunneling through a tensile-strained GaAsP quantum well

We report the demonstration of resonant tunneling of holes through an AlAs/GaAs0.7P0.3 double-barrier heterostructure. The tensile strain in the quantum well is large enough to reverse the order of the light- and heavy-hole levels (the first light-hole level is the ground state). The I(V) characteristic of this structure is measured and compared to a standard AlAs/GaAs unstrained one. As expected, the peak current density of the first light-hole resonance and its peak-to-valley current ratio are enhanced (they reach 28 A/cm2 and 3.4:1 at 15 K). Negative differential resistance is observed up to 250 K.

Heavy-hole-light-hole quantum beats in nonlinear transmission spectroscopy.

The complex polarization behavior of quantum beating between heavy- and light-hole excitons in nonlinear pump-and-probe spectroscopy is experimentally investigated. The strain-split free excitons in thin high-quality ZnSe layers embedded in ternary ${\mathrm{ZnS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ barriers served as the three-level system involved. Spectrally integrated pump-and-probe experiments confirm the theoretically predicted dependence of the quantum-beat phase on the relative polarization of the exciting fields. These observations are discussed in comparison with results from four-wave mixing obtained when using the same experimental setup. Beyond that, spectrally resolved transients measured on the light-hole resonance exhibit pronounced, weakly damped beating phenomena even at negative delay, i.e., with the probe pulses preceding the pump pulses. Additionally, for cross-polarized beams the quantum-beat phase is strongly dependent on the energy of detection. It turns out that the phase observed in the spectrally integrated mode is determined by the off-resonant contributions. \textcopyright{} 1996 The American Physical Society.

Observation of the Stark Effect in GaAs/AlGaAs Coupled Quantum Wells by Electroluminescence and Circularly Polarized Photoluminescence Excitation

The quantum-confined Stark effect (QCSE) has been studied in electroluminescence (EL) spectra by using GaAs/AlQ 3GaQ 7As symmetric coupled double-quantum-well (CDQW) structures under a "forward" electric field. The Stark shift for the transition between the ground electron state and heavy-hole state was successfully observed, and was found to be symmetrical with regard to the flat-band bias for EL and photoluminescence (PL), in which the electric field was applied in the forward and reverse directions, respectively. Circularly polarized photoluminescence excitation (CPPLE) spectroscopy was also employed to clarify the origins of optical transitions related to either heavy-hole or light-hole resonance. Each transition between electrons and light- and heavy-holes in the CDQWs was clearly identified by PL and by polarization of CPPLE spectra.

Influence of the exciton lifetime on resonant Raman scattering in quantum wells.

We report huge enhancements in the intensity of LO Raman scattering for photon energies resonant with the band-edge exciton of GaAs/AlAs multiple quantum wells, due to a sharpening of the resonance caused by the increased exciton phase-coherence lifetime. This explains the redshift of the resonance Raman profile with respect to the absorption, the much stronger heavy-hole resonance than that of the light-hole resonance, and the rapid decline of Raman intensity with temperature.