Optical Gain Measurements Research Articles

Photoluminescence and Gain of MBE Grown Cubic InxGa1—xN/GaN Heterostructures

We report on the molecular beam epitaxy of cubic InxGa1—xN/GaN heterostructures with different In contents. High resolution X-ray diffraction (HRXRD) measurements gave the In content in the layers. Reciprocal space mapping of the symmetrical (002) and asymmetrical (113) reflection reveal that cubic InxGa1—xN with low In content (x < 0.2) is single phase whereas layers with higher In content show evidence of phase separation. The photoluminescence (PL) of the InxGa1—xN layers was measured at 2 K. The full width at half maximum of the PL is as low as 200 meV for single phase layers whereas the emission is significantly broadened for phase separated InxGa1—xN. Optical gain measurements revealed a maximum gain of about 70 cm—1 from InxGa1—xN (x = 0.07) and a decrease of the gain with increasing In content.

Determination of single-pass optical gain and internal loss using a multisection device

We describe a technique for the measurement of optical gain and loss in semiconductor lasers using a single, multisection device. The method provides a complete description of the gain spectrum in absolute units and over a wide current range. Comparison of the transverse electric and transverse magnetic polarized spectra also provides the quasi-Fermi-level energy separation. Measurements on AlGaInP quantum well laser structures with emission wavelengths close to 670 nm show an internal loss of 10 cm−1 and peak gain values up to 4000 cm−1 for current densities up to 4 kA cm−2.

Optical gain measurements based on fundamental properties and comparison with many-body theory

We present high accuracy measurements of gain, loss, and transparency energy in long-wavelength semiconductors based on a hybrid approach using the fundamental relationship between the gain and the spontaneous emission spectra. Independent measurements of optical gain, transparency energy, and loss show the accuracy and validity of this technique. These results are compared with those obtained by the non-Markovian gain model with many-body effects under the spontaneous emission transformation method. It is found that the hybrid approach for the gain spectrum alleviates many of the problems related to the poor signal to noise ratio in the amplified-spontaneous emission near and below the band edge. The theoretical spectra compare well with the measured spectra for both the transverse electric and transverse magnetic polarizations.

Optical properties of α-ZnAl2S4:Cr single crystals

The optical properties of the chromium doped spinel type semiconductor α-ZnAl2S4 have been examined over the temperature range 2–540 K. The intrinsic photoluminescence (PL) showed an intense ultraviolet peak at 381 nm at 296 K. From extrinsic absorption and emission spectra the transitions between the ground state A2g4 and the excited levels Eg2, T1g2, T2g2, T2g4, and T1g4 of Cr3+ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the Eg2, T1g2, and T2g4 levels and the energy gaps between the Eg2–T1g2 and Eg2–T2g4 states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr3+ ions in a α-ZnAl2S4:Cr spinel host has been constructed. Optical gain measurements at 198, 298, and 380 K displayed two separate spectral regions associated to the Cr3+ ions and possibly Cr4+ impurity centers. Gain values of up to 25 cm−1 at 198 K were obtained.

Realization and optical characterization of etched mirror facets in GaN cavities

We report on the realization of etched mirror facets in GaN cavities by chemically assisted ion-beam etching. The etching conditions are adjusted to obtain a high degree of verticality and smoothness. Optical pumping experiments and gain measurements are performed in etched GaN cavities of various geometries. Stimulated emission and lasing are observed. The study of the value of the gain at threshold as a function of the cavity length allows a determination of the reflection coefficient of the etched mirror. The measured value of 15% is in good agreement with the one expected for a perfect air–GaN interface. This demonstrates the high quality of the etched mirror facets.

Room-temperature gain spectra and lasing in microcrystalline ZnO thin films

We report room-temperature measurements of optical gain and gain spectra of ultraviolet emission from ZnO thin film. An optical gain of 300 cm −1 is measured at a fluence of 3.8 μJ/cm 2. The large gain is attributed to the modification of the spontaneous emission rate by the dielectric photonic structure of these films. The presence of excitonic gain is also observed in the room-temperature gain spectra.

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.

Gain Spectroscopy of HVPE-Grown GaN

We report on photoluminescence and optical gain measurements of highly excited GaN crystals grown by hydride vapor physe epitaxy (HVPE). Inelastic scattering processes of excitons dominate the spontaneous emission spectrum under high excitation up to temperatures of 180 K. Towards room temperature phonon-assisted recombination of excitons and free carriers begins to dominate the spectrum. Similar characteristics are observed in temperature-dependent gain measurements.

Measurement of optical gain at 670 nm in an oxazine-doped polyimide planar waveguide

Optical gain measurements have been performed on several of the lower order modes of a multimode polyimide planar waveguide doped with the laser dye cresyl violet 670. A transverse pumping geometry was employed, resulting in larger gain for higher order modes. In a 21-mm-long device, an amplification of 14.4 dB was achieved for the sixth order guided mode at a wavelength of 670 nm. The photostability performance of the waveguide is comparable to the results reported for rhodamine-doped organically modified silica gels.

Optical gain and lasing in self-assembled InP/GaInP quantum dots

Optical gain measurements have been performed on self-assembled InP quantum dots embedded in GaInP. The dots are formed during the growth of InP on GaInP by molecular beam epitaxy due to the strong lattice mismatch and are overgrown with GaInP afterwards. The optical gain spectra show two peaks which could be clearly identified by their polarization properties as being due to the quantum dots and the wetting layer, respectively. As expected from the results of the gain measurements, optical pumping of cleaved samples at room temperature leads to lasing of either the quantum dots or the wetting layer, depending on the experimental conditions.

Design and performance of a high-temperature fluorescence cell for discharge and electron-beam excited gas mixtures

The design and performance is described for a high-temperature cell appropriate for kinetic and spectroscopic studies on electron-beam or electrical-discharge excited species emitting in the vacuum ultraviolet. The experimental setup operates at temperatures up to 750 K for total gas mixture pressures of up to 5 bar. Particularly, the apparatus permits investigations on the emission spectra and their temporal behavior of ionic excimer molecules after excitation with these two most-common gas laser-excitation techniques. The comparison of the quantum efficiency for the production of the vacuum ultraviolet emissions with both excitation techniques, together with the possibility to perform optical gain measurements, will allow us to evaluate whether lasing for this new class of molecules can be achieved. The performance of the experimental device is presented by investigations on the second continua of the rare gas.

Optical amplification in CsI:In(Tl) ionic crystal under N 2-laser excitation

Optical gain measurements on CsI:In(Tl) ionic crystal have been done using ASE technique under N 2-laser excitation, in order to investigate the possibility of getting optical amplification in the UV/visible region by doped alkali halide crystals. Also time resolved spectra were recorded in order to measure the lifetime of the fast component present in the colour centres developed. The studies have proved that CsI:In(Tl) crystal is a good material for tunable laser in the indigo blue region (∼430 nm).

Optical gain anisotropy in serpentine superlattice nanowire-array lasers

We report optical gain measurement from a serpentine superlattice nanowire-array laser sample, grown by molecular beam epitaxy on a 2°-off (100)GaAs vicinal substrate. Gain spectra, obtained from in-plane ridge-waveguide lasers with stripes either parallel or perpendicular to the nanowire arrays at 1.4 K, showed that the optical gain for the TM mode became greater than that of the TE mode when the optical cavity was placed along the nanowire direction. This provides strong evidence that the lateral quantum confinement in the serpentine superlattice is stronger than the vertical quantum confinement.

Optical gain measurements in multiple quantum wells at 2 K

The optical gain spectra of strongly excited multiple GaAs-(Ga,Al)As quantum wells have been determined using the variable stripe length method. The gain spectra are obtained by analysing the dependence of the amplified luminescence intensities on stripe length. In the novel fitting procedure we applied it is assumed that the optically generated carriers of the electron-hole plasma are in quasi equilibrium. It is found that this assumption is justified in all the cases considered here where saturation effects are absent.

Three-section semiconductor optical amplifier for monitoring of optical gain

A 1.5- mu m multiquantum well amplifier divided into three sections with short contacts at the input and output facets is described. A simple derivation shows that the amplifier optical gain is proportional to the ratio of voltage changes at the facet contacts induced by the optical signal. Measurements of optical gain and contact voltage as a function of amplifier bias current are in good agreement with the theory. >

Free-carrier plasma and optical amplification in undoped and modulation-doped Ga0.47In0.53As/Al0.48In0.52As multiple-quantum-well structures

We report the results of a systematic investigation of radiative recombination processes of the free-carrier plasma confined in Ga0.47In0.53As/Al0.48In0.52As multiple-quantum-well structures on InP substrate, either undoped or modulation doped. Photoluminescence under low- and high-excitation intensity, luminescence excitation, and optical gain measurements have been used to study the electronic transitions and the optical amplification in the temperature range 10–300 K. Space-resolved luminescence has also been adopted to distinguish between the spontaneous and the stimulated spectral contributions to the observed luminescence collected along different directions with respect to the [001] growth axis of the heterostructures. Optical gain up to 300 K has been observed in the undoped samples under photoexcitation quasiresonant with the confined states in the quantum wells. The theoretical analysis of the optical gain spectra furnishes quantitative data on the electron-hole plasma ground level. In the n-type modulation-doped samples the application of intense optical pumping allows us to observe the transition from the one-component electron plasma to a two-component electron-hole plasma through the rising of a sharp stimulated emission in the optical spectra. Also in this case we observe optical gain up to room temperature.

Linewidth, tunability, and VHF-millimeter wave frequency synthesis of vertical-cavity GaAs quantum-well surface-emitting laser diode arrays

The authors report the measurement of the laser linewidth, wavelength tunability, and generation of microwave frequencies between individually addressable elements of a vertical-cavity GaAs quantum-well surface-emitting laser diode array (lasing in the wavelength range 850-865 nm). Using heterodyne techniques, the authors obtain a deconvolved 65 MHz laser linewidth from the 109 MHz beat signal. The laser linewidth corresponds to a semiconductor laser linewidth enhancement factor alpha =5.7, which is in excellent agreement with that obtained independently from optical gain measurements and corresponding calculated refractive index changes. The authors measured heterodyne beat frequencies of 2-20 GHz. The bandwidth was limited by the microwave amplifiers. A simple calculation shows that a tuning range of 65 MHz to 3 THz can be achieved. >

Measurement and calculation of spontaneous recombination current and optical gain in GaAs-AlGaAs quantum-well structures

Experimental determinations have been made of the peak optical gain as a function of spontaneous recombination current density for GaAs quantum wells of width 25 and 58 Å bounded by AlGaAs barriers. These data were obtained from measurements of spontaneous emission spectra, observed through narrow windows in the 50-μm-wide contact stripes of oxide isolated lasers, using only a single reference value of the optical absorption coefficient above the band edge to calibrate the measurements in absolute units. These results are in good agreement with gain-current curves calculated using a model which includes unintentional monolayer well width fluctuations, band-gap narrowing and intraband carrier-carrier scattering. The characteristic intraband scattering time is calculated from first principles as a function of electron energy and carrier density on the basis of a 2-dimensional Auger-type process. This lifetime gives a much better representation of our observed spontaneous spectra than a lifetime which is simply dependent upon carrier density. The comparison between experiment and model calculation involves no adjustable parameters. For the 58-Å-wide wells there is a difference between the experimental and calculated gain-current curves at low values of gain. We show that this is a consequence of applying the Einstein relations to a broadened spectrum in the process of deriving the gain from the observed spontaneous emission spectrum. A direct comparison of the shapes of experimental and calculated spontaneous emission spectra at several injection levels provides a more rigorous, yet equally valid, verification of the computer model.

Monitoring the performance of a semiconductor optical amplifier

Measurements of optical gain and spontaneous emission of optical amplifiers show that the latter is a good measure of the former. A device structure is fabricated where a photodetector is integrated next to the amplifier. A fraction of the spontaneous emission from the amplifier impinges on the photodetector in this device producing a photocurrent. The photodetector current is found to be a monotonic function of the amplifier gain. Hence, the photodetector output can be used to monitor the performance of the amplifier.

Gain excitation spectrum and acceptor density in CdIn2S4

Optical gain measurements on the donor-acceptor (D-A) photoluminescence band of CdIn2S4 revealed maximum gain coefficients varying from 1.5 to 3.5 cm−1 at 300 K, in an optical excitation energy range of 2.016–2.101 eV. The threshold pumping intensity required for the onset of gain was found to be dependent on the excitation energy. Such characteristics were used to calculate the energy distribution of the acceptor levels in that energy range, which turned out to be exponential.