Optical Gain Spectra Research Articles

Numerical studies of optical gain of coupled quantum wells based on InGaAsN material system

AbstractThe optical gain of two coupled quantum wells fabricated from a new material system of InGaAsN/GaAs is studied by applying 10‐band k·p Hamiltonian matrix. A self‐consistent scheme which involves simultaneous solution of matrix Schrödinger and Poisson equations was applied. The optical gain spectra were obtained and analyzed for various values of nitrogen composition and barrier widths. They show optimum values for particular barrier width, although the variations are not significant. However, the magnitude of gain peak undergoes significant changes when varying nitrogen composition. The determined parameters and their variations can play significant role in designing practical structures based on InGaAsN/GaAs material systems. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical Modal Gain Saturation of Exciton-Exciton Scattering and Electron-Hole Plasma in ZnO

The optical modal gain spectrum for a ZnO thin film was measured by using a variable stripe length method. Using the dierential equation for amplified spontaneous emission in a stripe geometry, we obtained stripe-length-dependent gain spectra, which enabled us to investigate the gain saturation in long stripe lengths. When the stripe was shorter than the saturation length, our analysis was in a good agreement with the gain obtained by commonly-used analysis [1]. For intermediate density ( nMott). Both gains saturate with increasing stripe length, but the electron-hole plasma becomes relatively dominant for long stripe lengths ( 1000 µm) while the exciton-exciton scattering is relatively suppressed. This result suggests that exciton-exciton scattering gain saturation is more vulnerable to increase in the stripe length increase since the excitonic stimulated emission occurs on a coherence scale of the localized area.

Band Structure and Optical Gain of 1.3 um GaAsSbN/GaAs Compressively Strained Quantum Well Laser

The band structures and optical gain spectra of GaAsSbN/GaAs compressively strained quantum well (QW) were studied using 10-band k.p approach. We found that a higher Sb and N composition in the quantum well and a thicker well give longer emitting wavelength. The result also shows a suitable combination of Sb and N composition, and QW thickness can achieve 1.3 μm lasing. And, the optical gain spectra with different carrier concentrations will be obtained.

Cross-phase modulation induced modulation instability in single-mode optical fibers with saturable nonlinearity

Taking into account the saturable nonlinearity of the single-mode optical fibers, condition and gain spectra of scalar modulation instability induced by cross-phase modulation of two optical waves of different frequencies with the same polarizations are investigated. In both of the normal and anomalous dispersion regions, the contour plots of gain spectra of modulation instability with gray levels, the critical perturbation frequency (or instability domain), and the peak perturbation gain versus the input powers of the two waves are analyzed and discussed. The results show that, the critical perturbation frequency (or instability domain) and the peak perturbation gain increase with the input power, reaching a maximum value, and then decrease. This leads to a unique value of peak perturbation gain and critical perturbation frequency for two different input powers.

Comparison between optical gain spectra of InxGa1-xN/In0.02Ga0.98N laser diodes emitting at 404 nm and 470 nm

We investigate the gain formation properties of Inx Ga1–x N laser diodes emitting at 404 nm and 470 nm (sample A and B, respectively). Employing the Hakki–Paoli method, their optical gain spectra were measured at 298 K and 333 K. Since spatial fluctuations of the well width and In composition are considerable for long-wavelength laser diodes, broadening of the optical gain and luminescence spectra are clearly enhanced in sample B when compared with sample A. Moreover, population inversion occurs in at least two discrete energy levels for the gain spectra of sample B. It was found that only the modal gain of the higher gain peak can overcome the mirror loss, therefore the lower one does not contribute to lasing action. We also estimate internal loss of A and B to be 25 cm–1 and 30 cm–1, respectively. The maximum gain value of A increases linearly with increasing injected current even at 333 K, while that of B grows sub-linearly. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theory of Optical Gain of ${\hbox {Ge--}}{\hbox {Si}}_{x}{\hbox {Ge}}_{y}{\hbox {Sn}}_{1-x-y}$ Quantum-Well Lasers

We develop a theoretical model for optical gain of a strained Ge--SixGeySn1-x-y quantum-well (QW) structure. By using a ternary SixGeySn1-x-y material system as the barriers, a tensile strained germanium QW with a direct band gap for the electron and hole confinements can be realized. We show our theoretical model for the strained band structure and the polarization dependent optical gain spectrum of the tensile strained germanium QW laser taking into account the carrier occupations in both the Gamma- and L-valleys of the conduction band. Reasonable material parameters are used to estimate the transition energy, optical gain spectrum, and effects of the carrier leakage in presence of the quantized subbands

Optical gain spectra for near UV to aquamarine (Al,In)GaN laser diodes

Optical gain spectra presented for (Al,In)GaN laser diodes with lasing wavelength ranging from UV (375 nm) to aquamarine (470 nm) show a strong increase in inhomogeneous broadening, caused by Indium composition and quantum well width fluctuations which increase with Indium mole fraction. These gain spectra provides a standard data set for the calibration of microscopic many-body simulations. We demonstrate by comparison with basic simulations that the different assumptions of a global constant carrier density or of global constant quasi-Fermi levels for electrons and holes lead to a strikingly different dependency of optical gain on carrier density. For constant quasi-Fermi levels the threshold carrier density becomes insensitive to inhomogeneous broadening for realistic parameters. This is in agreement with the observation that the threshold current is nearly independent over the wavelength range from near UV to aquamarine.

Substrate Modes of (Al,In)GaN Semiconductor Laser Diodes on SiC and GaN Substrates

In semiconductor laser diodes layers with high refractive index can act as parasitic waveguides and cause severe losses to the optical mode propagating in the longitudinal direction. For (Al,In)GaN laser diodes, the parasitic modes are typically caused by the SiC or GaN substrate or buffer layers, hence the name substrate modes. A set of four different experiments shows the effect of substrate modes in the near-field (the most direct evidence of substrate modes), as side lobes in far-field, oscillations of the optical gain spectra, and as dependency of threshold current on n-cladding thickness. We derive several basic properties of the substrate modes by simple estimates. For a quantitative analysis we employ a 2-D finite element electromagnetic simulation tool. We simulate periodic variations in the cavity gain spectrum that explain the measurements in terms of absolute value and oscillation amplitude. We show that it is necessary to include the refractive index dispersion in order to get the correct period of the gain oscillations. Furthermore, we use the simulations to optimize the laser diode design with respect to substrate mode losses within the constraints given, e.g., by growth conditions

Gain suppression phenomena observed in InxGa1−xN quantum well laser diodes emitting at 470nm

Optical gain spectra were measured for InGaN-based laser diodes (LDs) emitting at 406nm (LD406) and 470nm (LD470) by employing the Hakki-Paoli method. The internal loss coefficient was as large as 35cm−1 for the LD470 compared to 25cm−1 for LD406. Moreover, gain saturation was observed at about 490nm just below the lasing threshold, and a gain band appears at higher photon energies for further carrier injection resulting in lasing at 470nm. Spontaneous emission peaks of electroluminescence were measured as a function of injection current density below threshold for both samples. The authors attribute the huge blueshift of the spontaneous emission of LD470 up to 450meV to a filling of the localized tail states in addition to that caused by the screening of the piezoelectric field. The blueshift for the LD406 was as small as about 30meV and can be interpreted as a result of the screening by both injected carriers and dopants.

Suppression mechanism of optical gain formation in In xGa 1− xN quantum well structures due to localized carriers

We have studied the optical gain and its suppression mechanism of In x Ga 1− x N multiple quantum well structures with different indium compositions. The variable stripe length method and the up-conversion method were employed in this research. It was found that the optical gain spectra became broad and the differential gain decreased with increasing the indium composition. These results indicated that inhomogeneous broadening affected not only spontaneous emission but also stimulated emission. The rise time of photoluminescence under a strong excitation condition was changed from 0.95 ps to 12.4 ps depending on the indium composition. Assuming the rise time reflects the relaxation lifetime from the absorption edge energy to population inversion levels, our calculation suggested that long rise times led to gain suppression in In x Ga 1− x N laser diodes with larger indium compositions.

The Effect of Barrier Composition on the Vertical Carrier Transport and Lasing Properties of 1.55-<tex>$mu hbox m$</tex>Multiple Quantum-Well Structures

In this paper, the effect of barrier bandgap and composition on the optical performance of 1.55-mum InGaAsP/InGaAsP and InGaAsP/InGaAlAs multiple quantum-well structures and Fabry-Perot lasers is evaluated experimentally. Direct vertical carrier transport measurements were performed through strain-compensated multiple quantum-well (MQW) test structures using femto-second laser pulse excitation and time-resolved photoluminescence up-conversion method. MQW test structures were grown with different barrier composition (InGaAsP and InGaAlAs) and barrier bandgap (varied from lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> = 1440 to 1260 nm) having different conduction band DeltaE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and valence band discontinuity DeltaE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> , while keeping the same InGaAsP well composition for all the structures. The ambipolar carrier transport was found to be faster in the structures with lower valence band discontinuity DeltaE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> . Regrown semi-insulating buried heterostructure Fabry-Perot (SIBH-FP) lasers were fabricated from similar QWs and their static light-current-voltage characteristics (including optical gain and chirp spectra below threshold) and thermal characteristics were measured. Lasers with InGaAlAs barrier showed improved high-temperature operation, higher optical gain, higher differential gain, and lower chirp, making them suitable candidates for high-bandwidth directly modulated uncooled laser applications

Optical spectra of (1‐101) InGaN/GaN and GaN/AlGaN MQW structure grown on a 7 degree off axis (001) Si substrate

AbstractInGaN/GaN and GaN/AlGaN MQW waveguides were fabricated on a (1‐101) GaN facet grown on a 7‐degree off oriented (001) Si substrates by MOVPE. We achieved uniform layers with less dislocation density and superior flatness by virtue of the low growth rate on the facet. Using a nitrogen laser as the excitation source, the optical gain spectra as a waveguide was evaluated. In the InGaN/GaN MQW structure, we found that the optical gain was as high as +40 cm‐1 at near band edge emission peak. The GaN edge emission peak showed narrowing by high intensity excitation: the full width at half‐maximum 1.8nm at under 4.5 MW/cm2 at 77 K. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Optical gain and threshold properties of strained InGaAlAs/AlGaAs quantum wells for 850-nm vertical-cavity surface-emitting lasers

The valence subband structures, optical gain spectra, transparency carrier densities, and transparency radiative current densities of different compressively strained InGaAlAs quantum wells with Al 0.3Ga 0.7As barriers are systematically investigated using a 6 × 6 k · p Hamiltonian including the heavy hole, light hole, and spin–orbit splitting bands. The results of numerical calculations show that the maximum optical gain, transparency carrier densities, transparency radiative current densities, and differential gain of InGaAlAs quantum wells can be enhanced by introducing more compressive strain in quantum wells. However, further improvement of the optical properties of InGaAlAs quantum wells becomes minimal when the compressive strain is higher than approximately 1.5%. The simulation results suggest that the compressively strained InGaAlAs quantum wells are of advantages for application in high-speed 850-nm vertical-cavity surface-emitting lasers.

Optical modal gain in multiple quantum-well semiconductor lasers based on InP

An approach to determine the optical modal gain spectra in multiple quantum-well semiconductor lasers based on InP in terms of the current at electrodes is presented. The link between the current at the electrodes and the density of carriers inside each quantum well was provided by rate equations with the inclusion of carrier transport effects. Description of hole dispersion is based on a 4 × 4 Luttinger–Kohn Hamiltonian and was done with the electrostatic effects of the carrier charges. Electrostatic effects were included via a self-consistent solution of electron and hole wave equations and the Poisson equation. The optical gain for the TE (transverse electric) mode has been determined with the inclusion of non-Markovian effects, Coulombic enhancement, and band-gap renormalization. The theoretical approach was compared with experimental measurements of the optical gain on a laser structure consisting of four quantum wells using the Hakki–Paoli method. The important role played by the leakage current was revealed when the results were compared. PACS No.: 73.63.Hs, 78.20.Bh, 78.20.Ci

The role of high growth temperature GaAs spacer layers in 1.3-/spl mu/m In(Ga)As quantum-dot lasers

We investigate the mechanisms by which high growth temperature spacer layers (HGTSLs) reduce the threshold current of 1.3-/spl mu/m emitting multilayer quantum-dot lasers. Measured optical loss and gain spectra are used to characterize samples that are nominally identical except for the HGTSL. We find that the use of the HGTSL leads to the internal optical mode loss being reduced from 15 /spl plusmn/ 2 to 3.5 /spl plusmn/ 2 cm/sup -1/, better defined absorption features, and more absorption at the ground state resulting from reduced inhomogenous broadening and a greater dot density. These characteristics, together with a reduced defect density, lead to greater modal gain at a given current density.

Effect of Induced Strain Due to Lattice Mismatch between MgyZn1-yS Cladding Layers and CdxZn1-xS/ZnS Quantum Wells on Effective Band Gap Energy in CdxZn1-xS/ZnS/MgyZn1-yS Separate-Confinement Heterostructures

We calculate the effect of the induced strain due to lattice mismatch between CdxZn1-xS/ZnS quantum wells (QWs) and MgyZn1-yS cladding layers on the effective band gap energy in CdxZn1-xS/ZnS/MgyZn1-yS separate-confinement heterostructures (SCHs). The effective band gap energy in the CdxZn1-xS/ZnS/MgyZn1-yS SCHs increases with increasing Mg content (y) for wider wells. The variation in effective band gap energy reflects the peak energy of optical gain spectra. The effect of the induced strain due to MgyZn1-yS cladding layers on the threshold carrier concentration for the production of a positive optical gain is very weak. The lattice-matching condition for the MgyZn1-yS cladding layers with respect to the CdxZn1-xS/ZnS MQWs is estimated. The induced strain due to the lattice mismatch between MgyZn1-yS cladding layers and CdxZn1-xS/ZnS MQWs is suppressed by adjusting y.

Spatially inhomogeneous optical gain in semiconductor photonic-crystal structures

The optical and electronic properties of semiconductor heterostructures in the vicinity of photonic crystals are discussed. The theoretical approach provides a self-consistent solution of the dynamics of the electromagnetic field and the material excitations. Due to the influence of the structured dielectric environment on the Coulomb interaction, the exciton resonances and the quasiequilibrium carrier densities in the spatially homogeneous semiconductor become space dependent. It is demonstrated that these inhomogeneities lead to distinct modifications of the optical absorption and gain spectra. As an application, numerically calculated density-dependent optical spectra are analyzed for an array of semiconductor quantum wires which are close to a two-dimensional photonic crystal. The spatial inhomogeneities result in novel excitonic absorption features and modification of the optical gain in these structures.

Theoretical and experimental study of optical gain and linewidth enhancement factor of type-I quantum-cascade lasers

A theoretical and experimental study of the optical gain and the linewidth enhancement factor (LEF) of a type-I quantum-cascade (QC) laser is reported. QC lasers have a symmetrical gain spectrum because the optical transition occurs between conduction subbands. According to the Kramers-Kronig relation, a zero LEF is predicted at the gain peak, but there has been no experimental observation of a zero LEF. There are other mechanisms that affect the LEF such as device self-heating, and the refractive index change due to other transition states not involved in lasing action. In this paper, the effects of these mechanisms on the LEF of a type-I QC laser are investigated theoretically and experimentally. The optical gain spectrum and the LEF are measured using the Hakki-Paoli method. Device self-heating on the wavelength shift in the Fabry-Perot modes is isolated by measuring the shift of the lasing wavelength above the threshold current. The band structure of a QC laser is calculated by solving the Schro/spl uml/dinger-Poisson equation self-consistently. We use the Gaussian lineshape function for gain change and the confluent hypergeometric function of the first kind for refractive index change, which satisfies the Kramers-Kronig relation. The refractive index change caused by various transition states is calculated by the theoretical model of a type-I QC laser. The calculated LEF shows good agreement with the experimental measurement.

Optically pumped lasing and gain formation properties in blue Inx Ga1–x N MQWs

AbstractLasing and gain formation properties have been studied in an InxGaN1–x multiple quantum wells lasing at around 460 nm by employing the modified variable stripe length method (VSLM) and pump and probe spectroscopy (P&amp;P). It was found that the spontaneous emission (Esp) appeared far below absorption edge (Ea) that is observed as a photo‐bleaching negative‐peak in P&amp;P, indicating the formation of localized tail states. Lasing peaks appeared in between Esp and Ea with wide spectral distribution. This is consistent with the results where VSLM revealed the broad feature of optical gain spectra associated with rapid peak saturation of lasing even just above the threshold photo‐pumping power density. Such mechanism observed in In‐rich InxGa1–xN MQWs is contributed not only from the broad distribution of localized density‐of‐states but also from hot carrier distribution (determined by Maxwell–Boltzmann statistics), temperature of which is raised up due to long energy relaxation time to localized tail states. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Linewidth enhancement factor of a type-II interband-cascade laser

Experimental results using the amplified spontaneous emission spectroscopy of a type-II interband-cascade laser are presented. Using the Hakki-Paoli method, the optical gain spectra of the laser are extracted for the wavelength of 3.13μm at various subthreshold current levels. A group index of around 3.33 is determined from Fabry-Pérot modal spacing. The change in refractive index with increased bias current is obtained from the peak wavelength shifts of the Fabry-Pérot spectrum. A low value of 0.71 for the linewidth enhancement factor at the lasing wavelength near threshold is found.