Optical Pumping Conditions Research Articles

1.55-µm-Waveband Emissions from Sb-Based Quantum-Dot Vertical-Cavity Surface-Emitting Laser Structures Fabricated on GaAs Substrate

We have developed antimonide-based quantum-dot vertical-cavity surface-emitting laser (Sb-based QD-VCSEL) structures for operation in the 1.3 and 1.55 µm optical communication wavebands. They were fabricated on a (001)-oriented GaAs substrate and contain high-density InGaSb QDs as the active medium owing to the use of silicon atom irradiation. Testing demonstrated that they have optical-communication-waveband emission peaks at around 1.55 µm in cw wave operation at room temperature under conditions of optical pumping and current injection. Additionally, it was found that these Sb-based QD-VCSELs have threshold characteristics in the emission intensities vs both optical-pumping power and current.

Impact of Electronic Asymmetry on Photoexcited Triplet-State Spin Distributions in Conjugated Porphyrin Oligomers Probed via EPR Spectroscopy

The photophysics of triplet excitons in a series of electronically asymmetric “push−pull” π-conjugated meso-to-meso ethyne-bridged (porphinato)metal oligomers, along with electronically symmetric analogues, were studied by X-band electron paramagnetic resonance (EPR) spectroscopy under continuous-wave (CW) optical pumping conditions in the 4−100 K temperature range. In all of the systems studied, the spatial extent of the triplet wave function, as inferred from the |D| zero-field splitting (ZFS) parameter, never exceeds the dimensions of a single porphyryl moiety and its meso-pendant ethynyl groups. The |D| values determined for an oligomeric series of these electronically asymmetric species that span one through four porphyryl units are respectively 0.0301, 0.0303, 0.0300, and 0.0301 cm-1, indicating a common triplet wave function spatial delocalization of approximately 0.4−0.45 nm. Electron spin−spin and spin−lattice relaxation times were determined over the 4−30 K temperature range using progressive mi...

Slab-edge modes in two-dimensional photonic crystals

We demonstrate the existence of surface waves in two-dimensional photonic crystal slab edges. In finite-sized air-terminated photonic crystal boundaries, the slab-edge mode turns into a lasing mode under pulsed optical pumping conditions. Analyses of the modal behaviors of the slab-edge modes by means of calculations based on the plane wave expansion method and the finite-difference time-domain method show that resonant frequencies and quality factors are strongly dependent on the termination parameter and the shape of the slab corner.

Light emitters fabricated on bulk GaN substrates. Challenges and achievements.

AbstractWe used high-pressure grown GaN single crystal substrates to fabricate dislocation free optoelectronic devices like light emitting diodes and laser diodes structures. The latter ones demonstrated laser action under optical pumping condition with the threshold of about 200 kW/cm2 at room temperature. In the present paper we would focus on the specific aspects of the homoepitaxial growth by MOVPE method including epi-ready substrate preparation and surface polarity choice. We believe that our results demonstrate clearly the feasibility of device fabrication based on high-pressure grown GaN bulk crystals.

Optico-magnetic effects in diode spectroscopy of Rb vapors

In the conditions of velocity selective optical pumping we have experimentally observed interference oscillations of D 1-line absorption coefficient of Rb vapor in weak magnetic field and alignment induced crossover resonance in the saturated absorption spectrum of D 2 line of 87Rb. The first effect was caused by precession of macroscopic magnetic momentum induced by strong pulsed circularly polarized resonant laser radiation. The second effect was caused by alignment in a weak external magnetic field oriented along the electric field vector E → of the linearly polarized laser field.

High-Temperature W Diode Lasers Emitting at 3.3µm

AbstractW lasers based on type-II antimonides were recently operated nearly to room temperature under the conditions of cw optical pumping. However, the development of electrically pumped mid-infrared lasers has not yet reached the same level of performance. This is largely related to the more challenging task of simultaneously optimizing the doping/transport and gain/optical properties of the devices. Here we report a demonstration of type-II mid-IR diode lasers employing W active quantum wells. Laser structures with 5 or 10 active periods sandwiched between broadened-waveguide separate confinement regions and quaternary optical cladding layers were processed into 100-µm-wide stripes, cleaved into 1-mm-long cavities, and mounted junction side down. For 0.5-1 µs pulses at a repetition rate of 200 Hz, lasing was obtained up to a maximum operating temperature of 310 K, where the emission wavelength was 3.27 µm. The threshold current densities were 110 A/cm2and 25 kA/cm2 at 78 and 310 K, respectively. The characteristic temperature, To, was 48 K for temperatures between 100 and 280 K. Operation in cw mode was obtained to 195 K, with threshold current densities of 63 A/cm2and 1.4 kA/cm2at 78 and 195 K, respectively, with To = 38 K between 78 and 195 K. Significant further improvements in the operating characteristics are expected once the optimization of the designs and fabrication procedures is complete.

Study of surface-emitted stimulated emission in GaN

We report the results of a study of spatially resolved surface-emitted stimulated emission in GaN epilayer samples under conditions of strong optical pumping. We observe that even at excitation powers near the damage threshold, no surface-emitted stimulated emission occurs from samples with a high quality GaN epilayer. In parts of the samples with inferior surface quality, we show that stimulated emission comes from cracks, burned spots, and other imperfections, and is due to the scattering of a photon flux propagating parallel to the surface. Our results suggest that these defects are effective scattering centers and can severely affect the accuracy of optical gain measurements.

Electron and hole excitonic spin population inversions in strained semimagnetic semiconductor quantum wells (abstract)

The large magnetic field-induced band-edge spin splitting that characterizes semimagnetic semiconductors has been used in the past to create tunable quantum confining potentials resulting in spin dependent localization of carriers. Previous work has demonstrated a field-dependent type I/type II transition for the lower energy spin component of the heavy-hole exciton. We report here a magneto-optic study of Zn1−x−yCdxMnySe/Zn1−zMnzSe multiple quantum wells in which judicious selection of composition results in a structure that exhibits a hole and/or electron spin population inversion under conditions of optical pumping. In these structures, the higher energy component of the spin split heavy-hole exciton exhibits a type I to type II transition with applied field (in contrast to previous work in which the lower energy component exhibited such a transition), preventing fast recombination via the normal interband radiative transition. Consequently, the spin-up carriers forming this exciton, at higher energy than their spin-down counterparts, may relax instead via intersubband transitions. In the conduction band, the short spin flip relaxation times lead to rapid decay of the spin-up electron population. The heavy holes, however, have much longer spin relaxation times in these strained structures, resulting in an optically pumped spin population inversion in the Zn1−zMnzSe barriers. Polarization dependent magnetoreflectivity is used to follow the type conversion of the higher energy component and the formation of the hole spin population inversion.

Optically enhanced NMR of plastically deformed GaAs

$^{69}\mathrm{Ga}$ and $^{75}\mathrm{As}$ NMR of plastically deformed GaAs under optical pumping conditions at low temperatures is shown to exhibit substantial differences compared to results obtained from as-grown material. NMR signal enhancement has been observed in a wide energy range of the irradiation light above and below the band gap. The phase of the NMR signals is independent of the helicity of the pumping light. A continuum of intragap states introduced in the GaAs upon plastic deformation is involved in the polarization of the nuclei.

Optically enhanced high-field NMR of GaAs.

The $^{69}\mathrm{Ga}$ NMR signals of differently doped GaAs samples were directly observed under optical pumping conditions in high magnetic field (4.2 T). This technique greatly enhances the sensitivity of the NMR signals and allowed us to study the effect of optical pumping on the nuclear polarization under conditions very different from optically detected NMR. The strongest NMR signals were observed when pumping with a light energy below the band gap, suggesting the involvement of impurities and defects for the most efficient polarization transfer. Inversion of the helicity of the pumping light at the lowest temperatures results in a phase inversion of the NMR signal of a bulk-GaAs and a Si-doped sample, but no phase inversion was observed for a Be-doped sample. \textcopyright{} 1996 The American Physical Society.

New physical phenomenon of dynamical self-organization in molecular electron transfer systems

A bifurcation of the optical transmission coefficient for the monostable-bistable transition was found for the first time in experiments with reaction centers of the purple bacteria Rb. Sphaeroides under conditions of optical pumping. A theoretical interpretation of the effect in terms of dynamical self-organization of the structure with a photo-induced electron flux is given.

Exciton-induced enhancement of optical waveguide confinement in (Zn,Cd)(Se,S) quantum well laser heterostructures

Room-temperature waveguide properties have been studied experimentally in ZnSe-based multiple-quantum-well laser structures. The spectra of waveguide transmission show distinct spectral features associated with heavy- and light-hole quantum well excitons. Refractive index dispersion in the multiple-quantum-well region is derived from interference spectra measured in short waveguide samples. The refractive index increases with approaching the exciton resonances, resulting in perfect confinement and low losses of the fundamental TE waveguide mode in a narrow spectral region below the heavy-hole exciton resonance energy. The spectra of laser generation have been measured under the conditions of optical pumping, showing the position of the lasing line to correlate with the spectral region of the waveguide transparency.

Quantitative investigation of the resonant nonlinear Faraday effect under conditions of optical hyperfine pumping.

We extend our previous discussion of the resonant nonlinear Faraday effect in terms of the precession of an induced quadrupole moment to the realistic experimental situations where depopulation, repopulation, and hyperfine optical pumping contribute. We derive, to lowest order in the light intensity, an analytical expression for the magnetorotation angle near an electric-dipole resonance in an atomic medium of arbitrary spins. We find excellent quantitative agreement with absolute measurements performed on the Cs ${\mathit{D}}_{2}$ line.

Optical detection of impurity NMR in the magnetic circular dichroism of F centres in alkali halide crystals

Nuclear magnetic resonance of thallium impurities has been detected by monitoring the magnetic circular dichroism of the absorption of F centres produced by room temperature X-irradiation under optical pumping conditions in Tl-doped KCl and RbCl crystals. In addition, electron nuclear double resonance signals of the F centre ligands were detected without microwaves. The observed effects are caused by hyperfine coupling of the F centres to the surrounding nuclei. F centres are probably produced with a spatial correlation to the thallium impurities.

Light-induced drift under conditions of ground state optical hyperfine pumping

Analytical expressions describing the light-induced drift (LID) of alkali metal vapours reasonably well both under conditions of ground state optical hyperfine pumping and in its absence are obtained. It is shown that under the conditions of optical pumping the LID effect of the particles arises under the action of its own fluorescent radiation. The drift velocity for sodium atoms in a argon atmosphere reaches a value of 1 cm s-1 for fluorescent radiation of intensity 20 mW cm-2.

Dynamics of atomic sublevel coherences during modulated optical pumping.

The dynamics of coherences between sublevels of an electronic ground state is investigated under conditions of modulated optical pumping. It is shown that the modulation corresponds to an effective reduction of the apparent sublevel splitting, thereby allowing the excitation of coherences between atomic sublevels whose energy splitting would require very high laser intensities for dc excitation. In addition, the frequency and phase of the modulation introduce additional degrees of freedom that can be utilized for the design of new experiments. The theoretical evaluations are compared with experiments on the ground state of atomic sodium.

Some features of ultrashort pulse generation in a distributed feedback dye laser with two-photon pumping

Tunable generation of ultrashort pulses was achieved using a dynamic distributed-feedback (DFB) rhodamine 6G dye laser under conditions of two-photon optical pumping by single fundamental-frequency pulses from an actively mode-locked YAG:Nd laser. At a dye concentration of 1.6 × 10 −2 mol/liter, the lasing threshold of the DFB laser was reached at a pump energy of ~ 6 mJ which corresponded to an intensity of ~ 7.7 GW/cm2 in the active zone. The laser operated stably in the 580–606 nm wavelength range. The width of the emission line was 0.1–0.5 nm. A positive chirp of the lasing frequency was observed due to nonlinear modulation of the active medium refractive index in the high-intensity two-photon pump field.

Xenon and hydrogen gas mixtures as laser active media

It is suggested that gaseous mixtures of xenon and molecular hydrogen may be used as active media of Xe2 (172 nm) and XeH ( ~ 250 nm) photochemical lasers. By adding more than 3 Torr of hydrogen to xenon, amplification can be achieved in the 172 nm range as a result of quenching of the 1u/0u− absorbing state under optical pumping conditions. The hydrogen atoms produced by the quenching process can be utilized to populate XeH* by three-body recombination with Xe* ( 3P1/2) atoms.

Kinetic, structural, and spectroscopic indentification of geminate states of myoglobin: a ligand binding site on the reaction pathway

Elementary steps or geminate states in the reaction of gaseous ligands with transport proteins delineate the trajectory of the ligand and its rebinding to the heme. By use of kinetic studies of the 765-nm optical "conformation" band, three geminate states were identified for temperatures less than approximately 100 K. MbCO, which is accumulated by photolysis between 1.2 and approximately 10 K, was characterized by our previous optical and X-ray absorption studies [Chance, B., Fischetti, R., & Powers, L. (1983) Biochemistry 22, 3820-3829]. Between 10 and approximately 100 K, geminate states that are also identified that have recombination rates of approximately 10(3) s-1 and approximately 10(-5) s-1 (40 K). Thus, it is possible to maintain a steady-state nearly homogeneous population of the slowest recombining geminate state, Mb, by regulated continuous illumination (optical pumping). Both X-ray absorption and resonance Raman studies under similar conditions of optical pumping show that the heme structure around the iron in Mb is similar to that of MbCO. In both geminate states, the iron-proximal histidine distance remains unchanged (+/- 0.02 A) from that of MbCO while the iron to pyrrole nitrogen average distance has not fully relaxed to that of the deoxy state. In MbCO the CO remains close to iron but not bound, and the Fe...CO angle, which is bent in MbCO (127 +/- 4 degrees C), is decreased by approximately 15 degrees [Powers, L., Sessler, J. L., Woolery, G. L., & Chance, B. (1984) Biochemistry 23, 5519-5523]. The CO molecule in Mb, however, has moved approximately 0.7 A further from iron. Computer graphics modeling of the crystal structure of MbCO places the CO in a crevice in the heme pocket that is just large enough for the CO molecule end-on. Above approximately 100 K resonance Raman studies show that this structure relaxes to the deoxy state.

Parameters of a dye laser with transverse pumping

The authors describe a new method for determining the excitation and output parameters of dye laser media under conditions of transverse optical pumping by laser radiation. The dye is modeled as a medium with two electron-vibrational levels. The method is based on the variation of the value of the distance on the mirror reflection coefficients of the resonator and on estimates of inactive or parasitic losses, the probability of exciting active dye molecules with the radiation incident on the front face of the cell, and the unsaturated gain coefficient of that output frequency. Experimental data are then derived for these conditions for a rhodamine 4C solution in ethanol. Pumping was done using a neodymium laser.