Stimulated Emission Peak Research Articles

Effect of thermal annealing on high indium content InGaN/GaN single quantum well structures

Photoluminescence measurement in surface-emitting geometry, optical pumping in edge-emitting geometry and spatially resolved spectra measurement were performed to study high-indium-content InxGa1−xN/GaN single quantum well (SQW) structures (x⩾32%) grown by low-pressure metalorganic chemical vapor deposition. Stimulated-emission (SE) was observed in the optical pumping spectra. Thermal annealing was performed to investigate the influence of high temperature treatment to the optical properties of the samples. Redshift of the SE peaks took place after the samples were annealed at 700 °C for 30 and 60 min. Blue shift of these SE peaks was observed after the samples were annealed for 120 min. Red shift of the SE peaks may be attributed to the reduction of the compositional fluctuation in quantum well (QW) leading to the decrease of the quantized energies. The interdiffusion of In and Ga atoms across the interface of barrier and QW can result in the increase or decrease of the quantized energies leading to the redshift or blueshift of the SE peaks. The relaxation of the strain in the QW by annealing can induce the blueshift of the SE peaks.

Metal-organic vapor phase epitaxial growth of high-quality ZnO films on Al2O3(00·1)

High-quality ZnO thin films were grown epitaxially at 250–550 °C Al2O3(00·1) substrates using low-pressure metalorganic vapor phase epitaxy. The reactants for the growth were diethylzinc and oxygen. Growth temperature, one of the important experimental parameters for epitaxial layers, was optimized. The films grown at 500 °C exhibited good crystallinity and strong ultraviolet absorption and emission. Photoluminescence spectra of the films showed a dominant excitonic emission with a weak deep level emission. More importantly, a strong stimulated emission peak was observed even at room temperature.

Study of gain mechanisms in AlGaN in the temperature range of 30–300 K

We report the results of an experimental study of the stimulated emission (SE) properties of AlGaN epilayers grown by metalorganic chemical vapor deposition under high optical excitation conditions in the temperature range of 30–300 K. The band gap and energy position of spontaneous and SE peaks were measured over the entire temperature range studied. Through an analysis of the temperature dependence of the relative energy positions and the SE threshold, combined with absorption and time-resolved photoluminescence measurements, we estimated the carrier density at threshold to be ∼1019 cm−3 throughout the temperature range studied. Such a high carrier density indicates that an electron-hole plasma is responsible for the generation of gain in this material system from 30 to 300 K. Issues related to the development of short-wavelength AlGaN-based light emitting devices are discussed.

Effects of Carrier Localization on the Optical Characteristics of MOCVD-Grown InGaN/GaN Heterostructures

We have studied both the spontaneous and stimulated emission (SE) properties as a function of excitation photon energy for InGaN/GaN multiple quantum wells (MQWs). A significant redshift of the SE peak with decreasing excitation photon energy was observed as the excitation photon energy was tuned below a certain photon energy (“mobility edge”) for the InGaN/GaN MQWs, with similar behavior observed for the spontaneous emission. The relative position of the mobility edge with respect to the absorption edge and the spontaneous and stimulated emission peak positions indicates the emission originates from carriers localized by extremely large potential fluctuations in the InGaN active layers of the MQWs. Therefore, carrier localization in the InGaN active regions explains the observed spontaneous and stimulated emission behaviors of these materials.

Stimulated emission in GaN thin films in the temperature range of 300–700 K

We report the results of an experimental study on stimulated and spontaneous emission from high-quality single-crystal GaN films grown on 6H-SiC and (0001) sapphire substrates in the temperature range of 300–700 K. We observed edge-emitted stimulated emission (SE) at temperatures as high as 700 K for samples grown on both SiC and sapphire substrates. The energy position of the SE and spontaneous emission peaks were shown to shift linearly to longer wavelengths with temperature and empirical expressions for the energy positions are given. We demonstrate that the energy separation between the spontaneous and SE peaks gradually increases from 90 meV at 300 K to 200 meV at 700 K indicating that an electron-hole plasma is responsible for the SE mechanism in this temperature range. The temperature sensitivity of the SE threshold for different samples was studied and the characteristic temperature was found to be 173 K in the temperature range of 300–700 K for one of the samples studied. We suggest that the unique properties of SE in GaN thin films at high temperatures could potentially be utilized in optoelectronic devices.

High temperature excitonic stimulated emission from ZnO epitaxial layers

The emission spectrum of high quality ZnO epilayers is studied from room temperature up to 550 K. At room temperature and low excitation power a single emission peak is observed which may be identified with the free exciton from its peak energy and dependence on temperature. However, when excitation intensities exceed 400 kW cm−2 a sharp peak emerges at lower energy which we attribute to exciton-exciton scattering. At higher excitation intensities (>800 kW cm−2) a second stimulated emission peak emerges at even lower energies: we attribute this peak to be stimulated emission of an electron hole plasma. Similar features are observed for all temperatures up to 550 K.

Energy selective optically pumped stimulated emission from InGaN/GaN multiple quantum wells

Optically pumped stimulated emission (SE) from InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition has been systematically studied as a function of excitation photon energy (Eexc) to further understand the origin of SE in these structures. Optically pumped SE was observed for excitation photon energies well below that of the absorption edge of the MQWs, indicating the states responsible for the soft absorption edge in these structures can efficiently couple carriers with the gain region. “Mobility edge”-type behavior in the SE peak was observed as Eexc was varied. The effective mobility edge measured in these SE experiments lies ∼110 meV above the main spontaneous emission peak and ∼62 meV above the SE peak. Tuning the excitation energy below the mobility edge was found to be accompanied by a drastic increase in the SE threshold due to a decrease in the effective absorption cross section. The experimental results indicate that the SE peak observed here has the same microscopic origin as the spontaneous emission peak, i.e., radiative recombination of localized states.

Excited-state dynamics of poly(para-phenylene)-type ladder polymers at high photoexcitation density

Ultrafast dynamics in methyl-substituted poly(para-phenylene)-type ladder polymer films was studied by femtosecond pump-probe spectroscopy in a wide range of excitation densities, up to values typical for the regime of narrow band emission. The singlet excitons relaxation dynamics, monitored at the stimulated emission peak (2.53 eV), showed an intensity-dependent ultrafast decay component, which occurs on a subpicosecond time scale, ascribed to the onset of an amplified spontaneous emission process. In addition, we attributed the photoinduced absorption bands at 1.48 and 1.9 eV to singlet excitons and polarons, respectively. At high excitation density an additional absorption band becomes evident at 2.63 eV. Based on anisotropy decay kinetics we assigned it to short-lived interchain species.

Stimulated emission from optically pumped GaN quantum dots

Stimulated emission was observed from optically pumped GaN quantum dots in an AlxGa1−xN separate confinement heterostructure fabricated on 6H-SiC(0001) substrate by metal organic chemical vapor deposition. Nanostructural GaN quantum dots, with an average size of ∼10 nm width, ∼1–2 nm height, and density of ∼1011 cm−2, were self-assembled on the AlxGa1−xN cladding layer surface. The stimulated emission peak was observed at ∼3.48 eV, which is ∼50 meV lower than that of spontaneous emission. The excitation power dependence on the emission intensity clearly indicates threshold pump power density of 0.75 MW/cm2 for the onset of stimulated emission.

Ultraviolet spontaneous and stimulated emissions from ZnO microcrystallite thin films at room temperature

Room-temperature free excition absorption and luminescence are observed in ZnO thin films grown on sapphire substrates by the laser molecular beam epitaxy technique. At moderate optical pumping intensities, an excition-exciton collision induced stimulated emission peak is observed at 390 nm. The existence of this peak is related to the presence of closely packed hexagonally shaped microcrystallites in these films. Stimulated emission due to electron-hole plasma recombination process is also observed at higher pumping intensities.

Dynamical Properties of Optical Excitations in II-VI Structures

Ultrafast dynamical properties and lasing in two-dimensional II–VI heterostructures reveal a manifold of physical mechanisms which are partly unique and of great interest for the general understanding of the optical behavior of these structures and related devices. We review our investigations of polariton propagation effects strongly governing transmission of fs pulses perpendicular to II–VI waveguiding layers, similar to those in respective laser structures, for low to high excitation densities, being explained by the interference of simultaneously excited modes from four polariton branches involved. Second, exciton diffusion properties can be deduced from results of transient-grating experiments. Extremely large exciton diffusion constants could be measured in ultrapure ZnSe layers. For the first time, ultrafast time-resolved laser mode and gain dynamics is determined for II–VI laser diode structures. Coupling of longitudinal ground modes and higher-order transversal modes results in a density-dependent temporal beat pattern which is well described in a model of weakly phase-coupled modes in II–VI resonator cavities. Gain in laser diodes under optical pumping exhibits a complicated spectro-temporal shape with two subsequent stimulated-emission peaks and kinetic- as well as spectral-hole-burning features, being explained on base of carrier heating and cooling effects during lasing. Challenges for the development of a microscopic theory of lasing in II–VI diodes are discussed.

Quantum shift of band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes

We report on the band-edge stimulated emission in InGaN–GaN multiple quantum well light-emitting diodes with varying widths and barrier thicknesses of the quantum wells. In these devices, we observe that the stimulated emission peak wavelength shifts to shorter values with decreasing well thickness. From the comparison of the results of the quantum mechanical calculations of the subbands energies with the measured data, we estimate the effective conduction- and valence-band discontinuities at the GaN–In0.13Ga0.87N heterointerface to be approximately 130–155 and 245–220 meV, respectively. We also discuss the effect of stress on the estimated values of band discontinuities.

Gain spectra and stimulated emission in epitaxial (In,Al) GaN thin films

We measured the emission of (In,Al) GaN films under high intensity optical pumping both in the direction parallel and perpendicular to the film growth. In the edge emission geometry we determine the gain magnitude from the variable stripe length method. We use the spontaneous emission collected perpendicular to the layer plane to calculate the spectral dependence of the gain. Finally, we compare the emission spectra with those obtained for high quality GaAs thin films and find that the observation of a stimulated emission peak perpendicular to the nitride layer plane is predominantly due to scattering of the in-plane stimulated emission.

Observation of stimulated emission, line narrowing,and red shift ofemission peak from optically-pumped rectangular cross-sectional GaNoptical waveguides fabricated by selective growth

The authors examined low-loss III-V nitride optical waveguides for fabrication on sapphire substrates for active devices using a selective-area growth technique. A rectangular cross-section waveguide can be created by adjusting crystal growth conditions. A stimulated emission peak at 362 nm, line narrowing, and a red shift were observed from the selectively grown GaN waveguides by optical pumping.

Gain Spectra and Stimulated Emission in Epitaxial (In,Al) GaN Thin Films

ABSTRACTWe measured the emission of (In,Al) GaN films under high intensity optical pumping both in the direction parallel and perpendicular to the film growth. In the edge emission geometry we determine the gain magnitude from the variable stripe length (VSL) method. We use the spontaneous emission collected perpendicular to the layer plane to calculate the spectral dependence of the gain. Theoretical calculations are in good agreement with those experimentally determined gain spectra. We also show that the observalion of a stimulated emission peak perpendicular to the film is predominantly due to scattering of the in-plane stimulated emission but without ruling out contributions from microstructures in the films.

Spontaneous and stimulated emission from photopumped GaN grown on SiC

Photoluminescence of GaN layers grown on 6H–SiC substrates was studied in the temperature range 77–900 K. GaN layers were grown by metalorganic chemical vapor deposition. The temperature dependence of the band gap of GaN was measured throughout the entire temperature range. Edge cavity stimulated emission from photopumped GaN layers was observed in the temperature range 77–450 K. The full width at half-maximum (FWHM) of the stimulated emission peak was ∼3 nm at 300 K and ∼7 nm at 450 K. Multipass stimulated emission with Fabry–Pérot modes was detected from GaN. The FWHM of Fabry–Pérot modes was ∼0.2 nm (300 K).

Blue-light stimulated emission from a localized state formed by well-barrier fluctuation in a II-VI semiconductor superlattice

A well-defined exciton absorption peak from a localized state was observed for the first time. The localized state was formed by fluctuation of well-barrier interfaces in a ZnSe/ZnSSe superlattice structure. Stimulated emission was observed from the localized state up to 100 K, and the physical origin is discussed from the temperature dependence of the stimulated emission peak. This opens the possibility to form a natural quantum box structure by controlling the well-barrier fluctuation.

Effect of picosecond-laser-driven shock waves on spontaneous and stimulated emissions in GaSe.

Shock waves in GaSe semiconductors were produced by high-power picosecond laser pulses. Spontaneous and stimulated emissions in GaSe were used to probe the effect of shock waves. Under the laser-driven shock loading, a 24-nm spectral red-shift of the spontaneous-emission peak position, which corresponds to 14-kbar shock pressure, was detected. Significant line broadening of the spontaneous emission is attributed to the shock-wave-induced collision mechanism. The observed larger red-shift of 36 nm and the intensity decrease of the stimulated-emission peak were explained by the shock-wave-induced band-gap shrinkage through the gain-reduction mechanism based on the exciton-exciton scattering process in GaSe.

Stimulated raman spectra from H 2O, D 2O, HDO, and solutions of NaClO 4 in H 2O and D 2O

Stimulated Raman spectra were obtained from H 2O, D 2O, H 2OD 2O mixtures, and solutions of NaClO 4 in H 2O and D 2O using picosecond pulse excitation at 530.0 nm. The stimulated emission peaks could be predicted from the spontaneous Raman contour shapes for the OD stretching vibrations, but exclusive stimulation of either hydrogen-bonded or of nonhydrogen-bonded components was observed for the OH stretching vibrations. The large widths of the stimulated spectra are discussed in relation to self-focusing and refractive index modulation.

Thermally Stimulated Emission and Conductivity Peaks in the Case of Temperature Dependent Trapping Cross Sections

The original theory of thermally stimulated emission is extended to include the effect of a temperature dependent trapping cross section. A method is proposed whereby the ionization energy and temperature dependent capture cross section of a set of identical trapping states may be determined from the half-width and shape of a thermally stimulated emission peak obtained from a single experiment. The method is also applicable to thermally stimulated current peaks if the free electron lifetime varies only slowly with temperature.