Light-hole Transitions Research Articles

Localized and delocalized states in GaNAs studied by microphotoluminescence and photoreflectance

Optical transitions in GaNAs bulk layer containing 2.2% N have been studied with microphotoluminescence (μ-PL) and photoreflectance. At low temperatures and low excitation conditions, the μ-PL spectra showed sharp PL lines of 100–300μeV widths about 10–20meV below the energy gap. Those lines were attributed to the recombination of localized excitons trapped at local potential minima. When the excitation power was increased, an additional smooth PL band appeared at the higher-energy side. This band corresponds to the light-hole transition in photoreflectance spectrum, i.e., transition between the delocalized states.

Room temperature nano- and microstructure photon detectors

The development of room temperature infrared (IR) detectors for wavelengths beyond NIR will open up many applications that are currently limited due to cooling requirements. Three approaches are discussed, which show promise for room temperature IR detection. Tunneling quantum dot (QD) detector, utilizes a tunneling barrier in order to block the dark current while permitting the photocurrent to pass through due to resonance effects, has shown room temperature response for a detector operating at 6 and 17μm. The PbS QDs in a dielectric medium utilizes electronic polarizability of QDs, sensing only the variations of the radiation intensity, operating at ambient temperature. This method allows narrow multiple response bands. A GaAs/AlGaAs heterojunction detector, utilizing light, heavy and split-off hole transition in a p-doped semiconductor, shows a threshold of 3.4μm operating up to 330K.

Spin entanglement using coherent light and cavity-QED

A scheme for probabilistic entanglement generation between two distant single electron doped quantum dots, each placed in a high-$Q$ microcavity, by detecting strong coherent light which has interacted dispersively with both subsystems and experienced Faraday rotation due to the spin selective trion transitions is discussed. In order to assess the applicability of the scheme for distant entanglement generation between atomic qubits proposed by T. D. Ladd et al. [New J. Phys. 8, 184 (2006)] to two distant quantum dots, one needs to understand the limitations imposed by hyperfine interactions of the quantum dot spin with the nuclear spins of the material and by nonidentical quantum dots. Feasibility is displayed by calculating the fidelity for Bell state generation analytically within an approximate framework. The fidelity is evaluated for a wide range of parameters and different pulse lengths, yielding a trade-off between signal and decoherence, as well as a set of optimal parameters. Strategies to overcome the effect of nonidentical quantum dots on the fidelity are examined and the time scales imposed by the nuclear spins are discussed, showing that efficient entanglement generation is possible with distant quantum dots. In this context, effects due to light hole transitions become important and have to be included. The scheme is discussed for one- as well as for two-sided cavities, where one must be careful with reflected light which carries spin information. The validity of the approximate method is checked by a more elaborate semiclassical simulation which includes trion formation.

Optical properties of InAs/In 0.15Ga 0.85As quantum dots-in-a-well studied by piezomodulated reflectance spectroscopy

Piezomodulated reflectance (PzR) spectroscopy has been used to study the InAs/In 0.15Ga 0.85As quantum dots-in-a-well (DWELL) samples in the temperature range of 70–300 K. The optical transitions from ground state to excited states in InAs DWELL were observed. The heavy- and light-hole transitions from the hybrid quantum well (HQW) were identified by combining the one-dimensional effective mass approximation (EMA) with the selected modulation effect in PzR spectrum. A remarkable red shift of the ground state has been observed in the InAs DWELL with optimized growth of In 0.15Ga 0.85As quantum well layers. Cooling both DWELL samples produces an abnormal broadening of the QD transitions in PzR spectra.

Shift currents from symmetry reduction and Coulomb effects in (110)-orientatedGaAs∕Al0.3Ga0.7Asquantum wells

We report on the observation of an additional shift current tensor element by interband excitation of undoped (110)-oriented $\mathrm{GaAs}∕{\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum wells with a $150\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ laser pulse. The additional tensor element results from the symmetry reduction of the quantum well structure and shows a strong enhancement and a phase shift at the light-hole exciton transition revealing that shift currents are a sensitive probe of Coulomb effects in semiconductors. Moreover, we show that the overall shift current does not directly follow the envelope of the optical excitation pulse but contains a noninstantaneous contribution.

Magneto-photoluminescence of heavy- and light-hole excitons in site-controlled pyramidal quantum dots

Pyramidal, site-controlled InGaAs/AlGaAs quantum dots (QDs) are studied by micro-photoluminescence (PL) spectroscopy in a magnetic field. The light-hole (LH) excitonic transitions exhibit diamagnetic shifts similar to those of their heavy-hole (HH) counterparts. From the evolution of the excitonic lines, effective g-factors of g=1.7 and 0.4 for HH and LH neutral excitons are inferred.

Influence of SiO2 and TiO2 dielectric layers on the atomic intermixing of InxGa1−xAs/InP quantum well structures

We have studied the influence of SiO2 and TiO2 dielectric layers on the atomic intermixing of InxGa1−xAs/InP quantum well structures using the impurity-free vacancy disordering technique. Photoluminescence results revealed that an enhancement of interdiffusion was obtained when the samples were capped with SiO2. Although TiO2 layers were able to suppress the interdiffusion in the InGaAs/InP system, the suppression was not significant compared to the AlGaAs/GaAs system. Based on a fitting procedure that was deconvoluted from the photoluminescence spectra as well as a theoretical modeling, the electron–heavy hole and electron–light hole transitions were identified, and a ratio of the group V to the group III diffusion coefficients (k) was obtained. The k ratio of the InGaAs/InP samples capped with SiO2 is relatively larger than that of samples capped with TiO2 layers.

Strain-induced splitting of the valence band in epitaxially lifted-off GaAs films

We report a detailed study on the valence band splitting in epitaxial lift-off (ELO) GaAs film bonded to silicon. The GaAs film used in this study was grown by molecular beam epitaxy on epiready GaAs (100) substrate. Variable temperature photoluminescence and reflectivity spectra were obtained for the as-grown film, the freestanding ELO film, and the ELO GaAs film bonded to silicon. The PL spectra for the GaAs film on Si showed the removal of the valence band degeneracy with the light hole and heavy hole transitions separated by 4.2meV at 10K and decreased monotonously to 1.6meV at 230K. No similar splitting was observed for the as-grown and freestanding films. The strain and stress were calculated at ε=(1.2±0.04)×10−3 and X=0.8±0.05kbar, respectively, at 10K and ε=(2.3±0.04)×10−4 and X=0.3±0.05kbar at 230K. The temperature dependence of the heavy hole–light hole separation energy indicated a strain-induced effect caused by the difference in the coefficient of thermal expansion between GaAs and Si. This shows the efficiency of using ELO techniques on dissimilar materials for strain related spectroscopy.

MBE growth and optical properties of highly tensile-strained In 1−xGa xAs/In 0.52(Ga 0.4Al 0.6) 0.48As multi-quantum-wells using digital alloy

Highly tensile-strained (TS) InGaAs/lattice-matched (LM) InGaAlAs MQWs for 1.3 μm emission wavelength were grown by MBE and their properties were characterized by photoluminescence (PL) measurements and cross-sectional transmission electron microscopy (TEM). The energy band of the TS-InGaAs/LM-InGaAlAs MQW was theoretically calculated using a 6×6 Luttinger-Kohn Hamiltonian. The tensile strains for the wells were varied from 1.0% to 1.5%, while lattice match was used for barriers. The wells and barriers have well-defined interfaces for TS-InGaAs/LM-InGaAlAs MQWs with a tensile strain ( ε) of 1.0% and 1.25%, respectively. However, significant non-planarity between wells and barriers was observed for TS-InGaAs/LM-InGaAlAs MQWs with a tensile strain of 1.5%. For ε=1.0%, 1.25%, and 1.5%, two peaks were observed in each PL spectrum. The longer wavelength peak is attributed to an electron–light hole (E1–LH1) transition while the shorter wavelength peak to an electron–heavy hole (E1–HH1) transition. While the E1–HH1 transition is dominant at ε=1.0%, the E1–LH1 transition is dominant at ε=1.25% and 1.5%. The E1–LH1 transition was clearly observed with increasing well number. The total PL intensity increased as the well number increased from 1 to 4 QWs. However, the total PL intensity decreased with 5 QWs. Therefore, the maximum well number is limited to 4, constituting a compromise between well number and strain relaxation.

Intervalence band solitary waves in semiconductor quantum wells

We show theoretically the possibility of excitation of a special class of two-frequency solitons, sustained by the nonlinearity provided by the nonradiative intervalence band coherence between heavy-hole and light-hole excitonic transitions in semiconductor quantum wells. The formation of such resonant solitons excited by femtosecond pulses in InxGa1‑xAsyP1‑y quantum wells is investigated numerically using a phenomenological model based on the local-field approximation, and a realistic configuration for their experimental observation is proposed. The pumping conditions for using complex resonant soliton dynamics as an efficient source of continuum generation are also discussed.

Evolution of the amount of InAs in wetting layers in an InAs/GaAs quantum-dot systemstudied by reflectance difference spectroscopy

The wetting layer (WL) in InAs/GaAs quantum-dot systems has been studiedby reflectance difference spectroscopy (RDS). Two structures related to theheavy-hole (HH) and light-hole (LH) related transitions in the WL have beenobserved. On the basis of a calculation model that takes into account thesegregation effect and exciton binding energies, the amount of InAs in the WL(tWL) and its segregationcoefficient (R) have been determined from the HH and LH transition energies. The evolutions oftWL and R exhibit a close relation to the growth modes. Before the formation of InAs dots,tWL increaseslinearly from ∼1 to∼1.6 monolayer (ML),while R increasesalmost linearly from ∼0.8 to ∼0.85. After the onsetof dot formation, tWL is saturated at ∼1.6 ML and R decreases slightly from 0.85 to 0.825. The variation oftWL can be interpreted by using an equilibrium model. Different variations of in-plane opticalanisotropy before and after dot formation have been observed.

Quantum-confined Stark shift in electroreflectance of a cylindrical GaN quantum dot

The electroreflectance (ER) spectra in the presence of the modulated electric field have been employed to study the fine structure of a cylindrical GaN quantum dot (QD), including the light hole and heavy hole interband transitions, and the ER spectra exhibit Franz–Keldysh oscillation characteristics with abscissa of energy (E−Eg). The quantum-confined Stark shift (QCSS) happened when the electric field intensity increased and the light and heavy holes dependent characteristics have been shown. The three-dimensional Schrödinger equation of QD has been calculated within the framework of effective-mass approximation, and the ER indices have been obtained from modulation absorption coefficients using the Seraphin coefficients and the Kramer–Kroning relation.

Inducing electron spin coherence in GaAs quantum well waveguides: Spin coherence without spin precession

Electron spin coherence is induced via light-hole transitions in a quantum well waveguide without either an external or internal DC magnetic field. In the absence of spin precession, the induced spin coherence is detected through effects of quantum interference in the spectral domain coherent nonlinear optical response. We interpret the experimental results qualitatively using a simple few-level model with only the optical transition selection rule as its basic ingredients.

Effective-mass theory for hierarchical self-assembly ofGaAs∕AlxGa1−xAsquantum dots

The electronic structures in the hierarchical self-assembly of $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum dots are investigated theoretically in the framework of effective-mass envelope function theory. The electron and hole energy levels and optical transition energies are calculated. In our calculation, the effect of finite offset, valence-band mixing, the effects due to the different effective masses of electrons and holes in different regions, and the real quantum dot structures are all taken into account. The results show that (1) electronic energy levels decrease monotonically, and the energy difference between the energy levels increases as the GaAs quantum dot (QD) height increases; (2) strong state mixing is found between the different energy levels as the GaAs QD width changes; (3) the hole energy levels decrease more quickly than those of the electrons as the GaAs QD size increases; (4) in excited states, the hole energy levels are closer to each other than the electron ones; (5) the first heavy- and light-hole transition energies are very close. Our theoretical results agree well with the available experimental data. Our calculated results are useful for the application of the hierarchical self-assembly of $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum dots to photoelectric devices.

Spectroscopic characterization of 1.3μm GaInNAs quantum-well structures grown by metal-organic vapor phase epitaxy

We report optical studies of high-quality 1.3μm strain-compensated GaInNAs∕GaAs single-quantum-well structures grown by metalorganic vapor phase epitaxy. Photoluminescence excitation (PLE) spectroscopy shows clearly the electronic structure of the two-dimensional quantum well. The transition energies between quantized states of the electrons and holes are in agreement with theoretical calculations based on the band anti-crossing model in which the localized N states interact with the extended states in the conduction band. We also investigated the polarization properties of the luminescence by polarized edge-emission measurements. Luminescence bands with different polarization characters arising from the electron to heavy-hole and light-hole transitions, respectively, have been identified and verify the transition assignment observed in the PLE spectrum.

Optical two-dimensional Fourier transform spectroscopy with active interferometric stabilization

Optical two-dimensional Fourier transform spectroscopy is implemented near 800 nm with active stabilization. Excitation pulse delay is stabilized during data acquisition and stepped with interferometric accuracy. The reference used for heterodyne detecting the complete transient four-wave mixing signal is also phase-stabilized. The phase evolution of the four-wave mixing signal during the initial evolution period and the final detection period is then measured and correlated. Two-dimensional spectra with absorption and emission frequency axes are obtained by Fourier transforms with respect to the corresponding time variables. Measurement performed on a GaAs multiple quantum well sample shows light-hole and heavy-hole exciton transitions as the diagonal peaks and coupling between these two resonances as off-diagonal peaks.

MOVPE growth of AlGaInAs–InP highly tensile-strained MQWs for 1.3 μm low-threshold lasers

The low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) of tensile AlGaInAs multi-quantum wells (MQWs) for transverse magnetic (TM) 1.3 μm emitting lasers is presented. Al-containing wells have been mostly studied with compressive strain for transverse electric (TE) lasers. In this study, we report on highly tensile-strained AlGaInAs well layers (−0.72 to −1.65%) grown with compressive-strained AlGaInAs barrier layers (0.64%). The good agreement of high-resolution X-ray curves and simulated curves indicates that good crystalline quality and abrupt hetero-interfaces are obtained. An enhanced separation between light hole and heavy hole transitions is clearly observed by room-temperature photoluminescence as the strain increases. From broad-area laser results, it was observed that the strain had a low impact on the laser internal loss, the quantum efficiency and the transparency current density, which was as low as 0.32 A/cm 2 for a 6 QW structure. On the opposite, a doubling of the gain parameter g 0 when the strain increases from −0.72 to −1.65% was clearly observed. This result is associated with a 40% threshold density reduction on 300 μm long lasers. These investigations show that highly tensile-strained layers are very promising for the realisation of high-speed lasers.

Systematic study of polarized electron emission from strained GaAs∕GaAsP superlattice photocathodes

Spin-polarized electron photoemission has been studied for GaAs∕GaAs1−xPx strained superlattice cathodes grown by gas-source molecular beam epitaxy. The superlattice structural parameters are systematically varied to optimize the photoemission characteristics. The heavy-hole and light-hole transitions are reproducibly observed in quantum efficiency spectra, enabling direct measurement of the band energies and the energy splitting. Electron-spin polarization as high as 86% with over 1% quantum efficiency has been observed.

Laterally patterned band structure in micromachined semiconductors

We demonstrate that micromachining lattice-matched InGaAs quantum wells grown on (001) InP with strained barriers produces precise laterally patterned modifications to the semiconductor band structure. The light-hole and heavy-hole excitonic transitions are mixed and differentially shifted by the micromachining, inducing a surface-normal optical anisotropy characterized by a peak birefringence of Δn=0.028. The measured optical properties agree with calculations based on finite-element models of the strain combined with an eight-band k⋅p model that includes deformation potentials. This technique may find applications in fields such as surface-normal polarization modulators, quasi-phase matching, and optically-acitve piezoelectric materials.

Electromagnetically induced transparency viaelectron spin coherence in a quantum wellwaveguide

We propose and analyze a novel scheme to realize electromagnetically induced transparency (EIT) via robust electron spin coherence in semiconductor quantum wells. This scheme uses light hole transitions in a quantum well waveguide to induce electron spin coherence in the absence of an external magnetic field. For certain polarization configurations, the light hole transitions form a crossed double-V system. EIT in this system is strongly modified by a coherent wave mixing process induced by the electron spin coherence.