High Excitation Photoluminescence Research Articles

Triggered high-purity telecom-wavelength single-photon generation from p-shell-driven InGaAs/GaAs quantum dot.

We report on the experimental demonstration of triggered single-photon emission at the telecom O-band from In(Ga)As/GaAs quantum dots (QDs) grown by metal-organic vapor-phase epitaxy. Micro-photoluminescence excitation experiments allowed us to identify the p-shell excitonic states in agreement with high excitation photoluminescence on the ensemble of QDs. Hereby we drive an O-band-emitting GaAs-based QD into the p-shell states to get a triggered single photon source of high purity. Applying pulsed p-shell resonant excitation results in strong suppression of multiphoton events evidenced by the as measured value of the second-order correlation function at zero delay of 0.03 (and ~0.005 after background correction).

Radiative efficiency of inelastic exciton-exciton scattering in ZnO nanocrystalline films

By using high-excitation photoluminescence spectroscopy, we explored the emissions due to inelastic exciton-exciton scattering in ZnO nanocrystalline films at low temperature. It was found that the threshold excitation intensity for occurrence of inelastic exciton-exciton scattering dramatically increases as the crystalline size increases. The radiative efficiency of the inelastic exciton-exciton scattering also decreases rapidly as the crystalline size increases from 120 nm to 170 nm and eventually, no emission due to inelastic exciton-exciton scattering can be detected for crystalline size of 220 nm even at low-temperature. We believe that the spatial confinement effect is the most determinative factor influencing the efficiency of inelastic exciton-exciton scattering.

MBE growth of P-doped 1.3 μm InAs quantum dot lasers on silicon

The authors report on the growth of InAs/GaAs quantum dots lasers on silicon emitting in the 1.3 μm wavelength regime with p-doped active regions. A growth optimization procedure guided by a combination of high and low excitation photoluminescence is presented for the InAs quantum dot growth. Growth conditions derived from this procedure are used to produce high optical quality quantum dots embedded in a GaAs/AlxGa1−xAs graded index separate confinement heterostructure waveguide. Ridge waveguide lasers fabricated from the as-grown material achieve room temperature continuous wave lasing at low thresholds, with high output power and elevated T0.

Many-particle effects in self-organized quantum dots

The influence of spectator particles on the electronic and optical properties is investigated for MOCVD grown In(Ga)As/GaAs quantum dots (QDs). The correlation between optical and structural properties allows for an analysis of the structural-dependent few-particle effects in single quantum dots, demonstrating the importance of correlation in self-organized quantum dots. For the investigation of the ground and excited QD states in view of Coulomb interaction, photoluminescence excitation and high-excitation photoluminescence experiments are used in presence of the maximum possible number of spectator electrons, holes and excitons. The results allow on the one hand to identify the electron and hole levels participating at the observed QD transitions and on the other hand to quantify clearly the differences between single-exciton and many-particle transitions.

Electronic structure of InAs/GaAs self-assembled quantum dots studied by high-excitation luminescence in magnetic fields up to [formula omitted

We report on high-excitation photoluminescence (PL) measurements of an ensemble of InAs/GaAs self-assembled quantum dots with large inter-shell spacing (75 meV) in magnetic fields up to 73 T . The PL spectra show a complex picture of levels splitting and crossings. A simple two-band single-particle model provides a good approximation to explain the observed magneto-PL spectra.

Stimulated emission due to the inelastic scattering from the heavy-hole exciton to the light-hole exciton in CuI thin films

We have investigated high-excitation photoluminescence properties of CuI thin films with a thickness of 100 nm grown on a NaCl substrate by vacuum deposition. The energies of the heavy-hole and light-hole excitons in the CuI thin film are split at low temperatures by the thermal strain resulting from the difference in the thermal expansion coefficients of CuI and NaCl: the heavy-hole exciton is lower in energy than the light-hole exciton. Under resonant excitation of the heavy-hole exciton, a new photoluminescence band appears on the low-energy side of the heavy-hole exciton. The energy spacing between the new photoluminescence band and the heavy-hole exciton is almost equal to the splitting energy of the heavy-hole and light-hole excitons. The photoluminescence intensity exhibits an almost quadratic dependence on the excitation power. These results indicate that the new photoluminescence band originates from an inelastic scattering process from the heavy-hole-exciton state to the light-hole-exciton state. We have confirmed the existence of optical gain in the energy region of the new photoluminescence band by using a variable-stripe-length method. This fact demonstrates that the inelastic scattering process from the heavy-hole-exciton state to the light-hole-exciton state may lead to stimulated emission.

Photoreflectance spectroscopy of vertically coupled InGaAs/GaAs double quantum dots

Photoreflectance and high excitation photoluminescence spectra have been measured at 10 K for In 0.6Ga 0.4As/GaAs double quantum dot structures with various thicknesses of the GaAs separating layer. Several transitions between split states, due to the dot–dot and wetting layer well–well interaction, have been observed. The transitions have been identified using the results of the effective mass approximation calculations for double quantum wells and lens-shaped double quantum dots. The splitting energy of the quantum dot transitions has been obtained as function of the GaAs barrier width.

High-excitation photoluminescence in GaN: Hot-carrier effects and the Mott transition

Photoluminescence under intense excitation is studied in GaN. As the excitation density increases, we show the Mott transition between an excitonic recombination to a plasma-type recombination. The carrier density at the Mott transition is given. At and above the Mott density, we show that the carrier temperature is higher than the lattice temperature. The energy relaxation of the hot plasma is shown to be dominated by LO-phonon emission. Coulomb screening and band-gap renormalization are observed from the photoluminescence peak position and the measured renormalization factor is in good agreement with elementary many-body theory. Finally the dependence of the Mott density on carrier temperature is shown to follow a Debye-H\"uckel model.

Nonlinear properties in InxGa1 − xAs/GaAs multiple quantum well laser structures

In this paper we investigated nonlinear properties and lasing in InxGa1−xAs/GaAs multiple quantum wells grown by metal-organic chemical vapour deposition. A systematic study, performed by high excitation photoluminescence measurements as a function of excitation intensity, allowed us to identify the minimum well width for observing stimulated emission from well states. We also determined the threshold for stimulated emission for well and barrier lasing. Radiative recombination energies are identified by using theoretical data obtained in the effective mass approximation, including boundary conditions and strain.

Cavity size and temperature dependence of stimulated emission in single Zn1 − xCdxSe/ZnSe quantum-well lasers

We observed stimulated emission originating from light-hole transitions in single Zn1−xCdxSe quantum-well structures, using high excitation photoluminescence spectroscopy as a function of temperature and cavity length. We found that the lasing wavelength depends on the cavity length, and can be switched by varying the sample temperature. The occurrence of lasing from light-hole recombinations in short cavities is also accompanied by an increase in the threshold of stimulated emission, with respect to larger cavities.

Spectroscopic evidence for different laser gain mechanisms in optically pumped ZnCdS/ZnS quantum well structures

The role of biexcitons has been studied in two optically pumped quantum well (QW) structures of Zn1−xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x=18% QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x=3% QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton–exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron–hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations.

Optical-Gain Measurements on GaN and Alx Ga1-xN Heterostructures

ABSTRACTOptical gain processes in thin GaN and AlGaN are compared by means of gain spectroscopy using the stripe length method and high-excitation photoluminescence, both performed at various densities and temperatures. We find that inelastic excitonic scattering processes and biexciton decay are important at low temperatures and low excitation densities Both materials are similar in that increasing the excitation density results in gain spectra dominated by the electron-hole plasma and phonon-assisted band-to-band recombination. These also prevail at high temperatures.

Coupling of geometric confinement and magnetic confinement in In0.09Ga0.91As/GaAs quantum wells in magnetic fields with varying orientations.

High excitation photoluminescence investigations of a 30-nm-wide ${\mathrm{In}}_{0.09}$${\mathrm{Ga}}_{0.91}$As/GaAs quantum well have been performed in magnetic fields up to B=10 T with varying orientations and have been compared with the results of detailed numerical calculations. The coupling between the quantum-well confinement potential and the magnetic confinement potential is controlled by the tilt angle \ensuremath{\vartheta} of the magnetic field relative to the quantum-well growth direction. For magnetic fields normal (\ensuremath{\vartheta}=0) and parallel (\ensuremath{\vartheta}=90\ifmmode^\circ\else\textdegree\fi{}) to the quantum-well plane the two confinement potentials are decoupled. In parallel orientation the magnetic quantization enhances only the geometric one. With increasing magnetic field the length scale that determines the quantization changes from the quantum-well width to the magnetic length. In contrast, in a normal magnetic field, B induces, independently of the geometric quantization, the in-plane Landau quantization. Spectral lines arising from intersubband transitions between Landau levels of different quantum-well subbands cross each other when they come close. For all other field orientations the two confinement potentials are coupled. This coupling generates hybridized electronic levels, in which both geometric and magnetic quantization are mixed. Transitions involving levels of different symmetry cross, whereas transitions involving levels of the same symmetry anticross when they approach each other. In addition, the number of strongly allowed transitions is increased near the anticrossing regime due to the exchange of character between the levels.

Optical characterization of InGaAs/GaAs quantum dots defined by lateral top barrier modulation

We have produced effectively buried quantum well dots on the basis of InGaAs/GaAs single quantum wells using high resolution electron beam lithography and selective wet etching of the top barrier. The lateral barriers are realized by the difference in quantization energy in quantum wells with a semiconductor top barrier compared to quantum wells with a vacuum potential barrier. Dot structures with widths down to 20 nm have been generated. We observe that the structures maintain high luminescence efficiencies down to the smallest sizes. We also observe a blueshift of up to 9 meV for 23 nm quantum dots. High excitation photoluminescence shows a second lateral subband in small dot structures.

Band edge offset in strained InxGa1-xAs/GaAs quantum wells measured by high-excitation photoluminescence

A new method is presented for direct and reliable determination of the band edge offset in quantum well (QW) semiconductor heterostructures. It is based on luminescence measurements at various filling levels of the valence and conduction band quantum wells. Magnetoluminescence measurements were made to independently determine the effective masses. The conduction band offset to energy gap ratio for strained InxGa1-xAs QWS in an unstrained GaAs matrix was determined to be 0.79+or-0.05 for x=0.18 and 0.28.

Electron–Hole Plasma Expansion in Direct‐Gap GaAs1−xPx

AbstractThe e‐h plasma in the ternary alloy GaAs1−xP2 below direct‐indirect crossover is studied at 2 K by means of very high excitation photoluminescence measurements. The most interesting feature observed is an extremely efficient expansion of the excited region when the particle density reaches the value n* needed to minimize the free energy. This behaviour and the corresponding spectral evolution with power density are accounted for by a model where pressure‐induced flow of particles away from the pumped region becomes competitive with recombination mechanisms.

Luminescence of CdS generated by high intensity excitation below the bandgap

High excitation photoluminescence of CdS at low temperatures excited by photons below the bandgap show the same general features as those obtained by one-photon band to band absorption. The excitation spectrum of the M-line shows no structure corresponding to a giant two-photon absorption expected for the direct creation of excitonic molecules.