Optical Pumping Conditions Research Articles

Time-resolved photoluminescence from self-assembled Ge(Si) islands in multilayer SiGe/Si and SiGe/SOI structures

The results of a study of the spectral and temporal characteristics of the photoluminescence (PL) from multilayer structures with self-assembled Ge(Si) islands grown on silicon and “silicon-on-insulator” substrates in relation to temperature and the excitation-light wavelength are presented. A substantial increase in island-related PL intensity is observed for structures with Ge(Si) islands grown on silicon substrates upon an increase in temperature from 4 to 70 K. This increase is due to the diffusion of nonequilibrium carriers from the silicon substrate into the active layer with the islands. In this case, a slow component with a characteristic time of ∼100 ns appears in the PL rise kinetics. At the same time, no slow component in the PL rise kinetics and no rise in the PL intensity with increasing temperature are observed for structures grown on “silicon-on-insulator” substrates, in which the active layer with the islands is insulated from the silicon substrate. It is found that absorption of the excitation light in the islands and SiGe wetting layers mainly contributes to the excitation of the PL signal from the islands under sub-bandgap optical pump conditions.

Voltage control of electron-nuclear spin correlation time in a single quantum dot

We demonstrate bias control of the hyperfine coupling between a single electron in an InAs quantum dot and the surrounding nuclear spins monitored through the positively charged exciton X+ emission. In applied longitudinal magnetic fields we vary simultaneously the correlation time of the hyperfine interaction and the nuclear spin relaxation time and thereby the amplitude of the achieved dynamic nuclear polarization under optical pumping conditions. In applied transverse magnetic fields a change in the applied bias allows to switch from the anomalous Hanle effect to the standard electron spin depolarization curves.

Gain-switched pulses from InGaAs ridge-quantum-well lasers limited by intrinsic dynamical gain suppression

Gain-switched pulses of InGaAs double-quantum-well lasers fabricated from identical epitaxial laser wafers were measured under both current injection and optical pumping conditions. The shortest output pulse widths were nearly identical (about 40 ps) both for current injection and optical pumping; this result attributed the dominant pulse-width limitation factor to the intrinsic gain properties of the lasers. We quantitatively compared the experimental results with theoretical calculations based on rate equations incorporating gain nonlinearities. Close consistency between the experimental data and the calculations was obtained only when we assumed a dynamically suppressed gain value deviated from the steady-state gain value supported by standard microscopic theories.

Large Area of Ultraviolet GaN-Based Photonic Quasicrystal Laser

In this study, large-area GaN-based photonic quasicrystal (PQC) nanopillars were fabricated on an n-GaN substrate using the nanoimprint lithography (NIL) technique. Under optical pumping condition, a high lasing action from the GaN photonic quasicrystals was observed. The lasing wavelength is at 366 nm with a low threshold power density of 0.009 kW/cm2. To confirm the band-edge lasing mode, the finite-element method (FEM) was used to perform the simulation for the 12-fold symmetry photonic quasicrystal lattices.

Correlation between emission intensity of self-assembled germanium islands and quality factor of silicon photonic crystal nanocavities

We present a comparative micro-photoluminescence study of the emission intensity of self-assembled germanium islands coupled to the resonator mode of two-dimensional silicon photonic crystal defect nanocavities. The emission intensity is investigated for cavity modes of L3 and Hexapole cavities with different cavity quality factors. For each of these cavities many nominally identical samples are probed to obtain reliable statistics. As the quality factor increases we observe a clear decrease in the average mode emission intensity recorded under comparable optical pumping conditions. This clear experimentally observed trend is compared with simulations based on a dissipative master equation approach that describes a cavity weakly coupled to an ensemble of emitters. We obtain evidence that reabsorption of photons emitted into the cavity mode is responsible for the observed trend. In combination with the observation of cavity linewidth broadening in power dependent measurements, we conclude that free carrier absorption is the limiting effect for the cavity mediated light enhancement under conditions of strong pumping.

Effects of nitrogen quenching gas on spin-exchange optical pumping ofHe3

We consider the degree of conservation of nuclear spin polarization in the process of optical pumping under typical spin-exchange optical pumping conditions. Previous analyses have assumed that negligible nuclear spin precession occurs in the brief periods of time the alkali-metal atoms are in the excited state after absorbing photons and before undergoing quenching collisions with nitrogen molecules. We include excited-state hyperfine interactions, electronic spin relaxation in collisions with He and N_2, spontaneous emission, quenching collisions, and a simplified treatment of radiation trapping.

Ultraviolet GaN-based microdisk laser with AlN/AlGaN distributed Bragg reflector

We demonstrated a 4.7 μm GaN-based microdisk laser with 25-pair AlN/AlGaN distributed Bragg reflector in ultraviolet range without undercut or deeply-etching procedures. The distributed Bragg reflector provides a high reflectivity of 85%, and selects lasing mode around 375 nm wavelength. Under optical pumping conditions, the lasing action was observed with a low threshold power density of 0.03 kW/cm2. We also characterized the whispering gallery mode profiles of the microdisk with finite-different time-domain simulation.

Optical Superradiance from Nuclear Spin Environment of Single-Photon Emitters

We show that superradiant optical emission can be observed from the polarized nuclear spin ensemble surrounding a single-photon emitter such as a single quantum dot or nitrogen-vacancy center. The superradiant light is emitted under optical pumping conditions and would be observable with realistic experimental parameters.

Possibilities of using modern infrared imaging devices for measuring the temperature of the lasing regions of solid-state lasers pumped with high-power laser diodes

The modern infrared imaging technique is applied to studies of the temperature fields and heat loads of Er:YAG laser crystals under conditions of optical pumping of these crystals with high-power (20 W) radiation from diode arrays with fiber outputs. It is shown that state-of-the-art infrared imagers operating in the range 8–14 μm can be efficiently used to monitor the near-surface temperatures in the lasing regions of solid-state lasers under high-power diode pumping. The possibilities of modern infrared imagers for evaluating the degree of heating of the lasing region in a laser crystal, the temperature distribution in this region, and the efficiency of operation of cooling systems and for preliminarily rejecting active laser elements have been demonstrated.

Hyperfine interaction in InAs/GaAs self-assembled quantum dots: dynamical nuclear polarization versus spin relaxation

We report on the influence of the hyperfine interaction on the optical orientation of singly charged excitons X ± in self-assembled InAs/GaAs quantum dots. All measurements were carried out on individual quantum dots studied by micro-photoluminescence at low temperature. We show that the hyperfine interaction leads to an effective partial spin relaxation, under 50 kHz modulated excitation polarization, which becomes, however, strongly inhibited under steady optical pumping conditions because of dynamical nuclear polarization. This optically created magnetic-like nuclear field can become very strong (up to ∼ 4 T ) when it is generated in the direction opposite to a longitudinally applied field, and exhibits then a bistability regime. This effect is very well described by a theoretical model derived in a perturbative approach, which reveals the key role played by the energy cost of an electron spin flip in the total magnetic field. Finally, we emphasize the similarities and differences between X + and X − trions with respect to the hyperfine interaction, which turn out to be in perfect agreement with the theoretical description. To cite this article: O. Krebs et al., C. R. Physique 9 (2008).

Optimization of the Q factor of the magnetic M x resonance under optical pump conditions

Optimization of parameters of the M x resonance, which excited between Zeeman sublevels of a sin- gle HFS level of the ground state of an alkali metal in a vacuum cell in a circuit with a single beam for pumping and detection ( M x magnetometer circuit), has been carried out. A simple model taking into account all main factors controlling the resolving power of the M x resonance (including spin-exchange broadening and absorp- tion in a thick layer of the cell has been constructed. It is shown that the spin-resonance broadening of the res- onance line is mainly determined by the requirements imposed on the optical thickness of the cell, which con- siderably restricts the realization of advantages of ultranarrow (<1 Hz) lines in magnetometry. The experiment confirming the efficiency of the model has been carried out.

Exciton, biexciton and trion recombination dynamics in a single quantum dot under selective optical pumping

Continuous wave- and time-resolved micro-photoluminescence spectroscopy has been performed on single InAs self-assembled quantum dots grown on GaAs. The presence of residual impurities (donors and acceptors) in samples with low dot density opens the possibility to switch from trion to neutral exciton states inside quantum dots by selective optical pumping. We propose a microstate model to describe the recombination dynamics of all the excitonic especies (neutral exciton, positive/negative trion and biexciton) under the considered optical pumping conditions when increasing the excitation power.

Room temperature InGaSb quantum well microcylinder lasers at 2μm grown monolithically on a silicon substrate

Fabrication of microcylinder laser cavities in III-Sb material on a silicon substrate is reported. Room temperature lasing near 2μm wavelength under optical pumping conditions is demonstrated in the microcylinders with InGaSb quantum wells. The III-Sb material is grown monolithically on a silicon substrate. High quality epitaxy is enabled by an interfacial misfit array.

Interband light absorption and Pauli blocking in InAs/GaAs quantum dots covered by InGaAs quantum wells

Spectra of interband light absorption in InAs/GaAs quantum dots covered by InGaAs quantum wells were investigated experimentally and theoretically. The main peaks in the spectra are connected with electron transitions between ground and excited quantum dot states. The value of an interband absorption cross section was determined. Interband light absorption was also studied under the conditions of interband optical pumping. The photoinduced change of interband absorption for polarized light was studied as a function of interband optical pump intensity and the light amplification was found. The change of absorption, including light amplification, is caused by the occupation of quantum dot states with non-equilibrium electrons and holes.

LIGHT EMISSION, ABSORPTION AND AMPLIFICATION IN InAs/GaAs QUANTUM DOTS AND GaAs/AlGaAs QUANTUM WELLS RESULTING FROM OPTICAL PUMPING

Optical phenomena in quantum dot and quantum well nanostructures aimed at the development of mid-infrared lasers based on intraband electron transitions are investigated in the conditions of interband optical pumping. Evolution of photoluminescence spectra and interband light absorption with intensity of interband optical excitation is investigated in InAs / GaAs quantum dot structures. Optically induced intersubband mid-infrared light absorption is studied in undoped tunnel-coupled GaAs / AlGaAs quantum wells. The stabilization of the electron concentration at the ground level of tunnel-coupled quantum wells under increasing pumping level is established.

Transient photoreflectance in Al 0.18Ga 0.82N/GaN thin film

The analysis of transient photoinduced subgap photoreflectance under optical pumping conditions in double heterostructure Al 0.18Ga 0.82N/GaN layers led to the observation of a positive and a negative reflectance change in the millisecond time range. It is shown that its dependence on the optical pumping, and the respective shift of the zero-reflectance coordinates, cannot be explained as a thermal effect, but as an optoelectronic one, reflecting the excess carrier distribution and the dynamic competition between the relaxation steps of trapping, thermalization, and recombination.

Lasing characteristics of InAs quantum dot microcavity lasers as a function of temperature and wavelength

A Strong temperature dependence of microdisk lasers and photonic crystal nanocavity lasers with InAs quantum dot active regions is reported. These lasers operate at 1.3 microm at room temperature under optical pumping conditions. T(0, microdisk) = 31 K. T(0, photonic crystal nanocavity) = 14 K. The lasing threshold dependence on the lasing wavelength is also reported. We observe a minimum absorbed threshold pump power of 9 microW. This temperature and wavelength dependent lasing behavior is explained qualitatively by a simple model which attributes the experimental observations predominantly to surface recombination at threshold and the high quality factors of these cavities.

Development of a dynamic nuclear polarization system based on the optical pumping method

We have developed a dynamic nuclear polarization system based on the optical pumping NMR method. It consists of an NMR spectrometer, laser and optical system, optical-pumping double-resonance probe, cryostat and superconducting magnet. The double-resonance probe newly developed for this system is of a top-loading type, characterized by a single-coil double-tuning tank circuit with traps. The system allows simultaneous control of two radio-frequency fields and a light beam irradiated to samples, enabling us to perform cross-polarization experiments under the optical pumping condition. As a test of the system, a cross-polarization experiment has been performed with indium phosphide doped with iron.

Chemiluminescence of bromine vapor under pulsed lamp pumping

The chemiluminescence kinetics and spectrum of gaseous Br2 under the conditions of broadband optical pumping have been investigated. It has been found that the transition A 3Π1u → X 1Σ g + is the major contributor to the luminescence spectrum.

Optical-pumping double-resonance NMR system for semiconductors

We describe an optical-pumping double-resonance NMR system equipped with a single-coil double-tuning (XY) probe with an optical fiber attachment working in a helium-gas flow cryostat. The system enables simultaneous control of an infrared light and two radio-frequency fields irradiated to a sample, so that solid-state NMR techniques such as spin locking, decoupling, and cross polarization can be invoked under the optical-pumping condition. A nuclear hyperpolarization transfer experiment in optically pumped InP:Fe is presented.