Optical Modal Gain Research Articles

Effect of Composition Disorders on Band Structure and Optical Gain Spectra of GaInNAs/GaAs Quantum Wells

The effects of non-uniform compositions (the composition disorders) of indium and nitrogen, in a Ga0.59In0.41N0.038As0.962/GaAs single-quantum well (QW) along the growth direction, on the QW's band structure and further on its optical gain spectrum have been studied theoretically. The 10-band k·p model and the many-body optical gain model have been employed, respectively, to calculate the band structure and the optical gain of the QW. The subband energy dispersions, the optical gains of the transverse electric (TE) and transverse magnetic (TM) modes, and the radiative current densities have been investigated. The composition disorders lead to blueshift in carrier's transition energy, which is mainly due to indium composition disorder while nitrogen composition disorder only plays minor role. The TM mode optical gain is significantly enhanced and the threshold current density is increased in the composition disordered QW structure. These results may provide important supports in the design and fabrications of GaInNAs/GaAs QW based optoelectronic devices.

Modal optical gain and cavity mode analysis of unstructured and optically structured ZnO nanocrystalline thin films

Deposition of high-crystal-quality ZnO nanocrystalline thin films (NCTFs) on a lattice-mismatched sapphire substrate was achieved using a pulsed laser deposition technique, and surface channel waveguides were fabricated by using a laser irradiation process. The optical structure defined on the film was characterized using a scanning electron microscope while the optical gain of unstructured and optically structured NCTFs was measured using the variable stripe length method (VSLM). We present a new analysis of a VSLM to study gain saturation for various stripe lengths, and its validity was confirmed by the consistent result obtained by using a widely used method for short stripe lengths. In the case of unstructured films, the inelastic exciton–exciton scattering dominates the stimulated emission (P-band) below Mott density while not only N-band due to the electron–hole plasma but also the exciton–exciton scattering contribute to the lasing beyond Mott density. These results suggest that the exciton–exciton scattering occurs in a localized space of a coherence scale, so the coexistence of the N-band and the P-band stimulated emission beyond the Mott density originates from the non-uniform spatial distribution. An induced cavity defined on a ZnO NCTF sample, showing constant periodic mode features, is confirmed which is attributed to local densification produced due to a refractive index change in a structured arc region.

Optical properties and modal gain of InGaN quantum dot stacks

AbstractWe present investigations of the optical properties of stacked InGaN quantum dot layers and demonstrate their advantage over single quantum dot layer structures. Measurements were performed on structures containing a single layer with quantum dots or threefold stacked quantum dot layers, respectively. A superlinear increase of the quantum dot related photoluminescence is detected with increasing number of quantum dot layers while other relevant GaN related spectral features are much less intensive when compared to the photoluminescence of a single quantum dot layer. The quantum dot character of the active material is verified by microphotoluminescence experiments at different temperatures. For the possible integration within optical devices in the future the threshold power density was investigated as well as the modal gain by using the variable stripe length method. As the threshold is 670 kW/cm2 at 13 K, the modal gain maximum is at 50 cm–1. In contrast to these limited total values, the modal gain per quantum dot is as high as 10–9cm–1, being comparable to the IIVI and III‐As compounds. These results are a promising first step towards bright low threshold InGaN quantum dot based light emitting devices in the near future (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modal gain enhancement by cylindrical waveguide and gain saturation in CdSe nanocrystal quantum dots

Optical modal gain enhancement of CdSe nanocrystal quantum dots in a cylindrical glass-waveguide has been observed by using a variable stripe length method. While poor modal gain was observed near the threshold excitation energy (3.5 mJ) for a thin film structure, significant modal gain (∼ 500 cm−1) was seen in the cylindrical structure due to the waveguide effect. Modal gain saturation for a stripe length was also investigated with a new gain analysis (Kyhm et al 2001 Appl. Phys. Lett. 79 3434). The stripe length dependence of gain saturation is also suppressed in the waveguide structure above the threshold excitation energy (5.5 mJ). We suggest the modal gain enhancement is involved with the intrinsic two electron–hole pair excited states (anti-bonding biexcitons) and the extrinsic waveguide effect.

Pulse generation at 346 GHz using a passively mode locked quantum-dash-based laser at 1.55 μm

We report on subpicosecond pulse generation at 346 GHz repetition rate based on InAs/InP quantum dash passively mode locked lasers emitting at 1.55 μm. This is achieved owing to the high optical modal gain of the multilayer InAs/InP quantum dash active region.

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.

Lock Acquisition Scheme For The Advanced LIGO Optical configuration

The lock acquisition scheme for the Advanced LIGO optical configuration, which makes use of ‘‘resonant sideband extraction’’, is under investigation in the 40 meter prototype interferometer at Caltech. The 40m has a similar optical configuration to the one planned for Advanced LIGO which has 5 degrees of freedom for length control. So far we have succeeded in locking the 5 degrees of freedom routinely. The differential mode of arm cavities was locked in the same state as the final setup, and the peak of optical resonance was verified to be around 4 kHz. Currently, since an offset remains in the common mode of the arm cavities, another optical resonance can be seen in common mode optical gain.

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

Growth temperature dependence of optical modal gain and loss in GaN:Eu active medium on Si

The dependence of optical modal gain and loss on GaN:Eu growth temperature is reported. GaN:Eu thin films were grown on Si substrates with AlGaN transition and cladding layers at temperatures ranging from 600 degrees C to 850 degrees C. The modal gain and loss in the GaN:Eu layer were a strong function of the optically active Eu atomic concentration and of the interface quality between the active layer and the top cladding layer, which in turn depended on the growth temperature. Optimum optical properties of maximum modal gain of ~ 100 cm(-1) and minimum loss of ~ 46 cm(-1) were obtained for growth at 800 degrees C.

Stimulated emission in the active planar optical waveguide made of silicon nanocrystals

Thin layers of silicon nanocrystals prepared by silicon-ion implantation into silica substrate form active planar optical waveguides. Testing experiments of optical gain have been performed on a sample implanted to a dose of 4 × 1017 cm–2 by using the variable stripe length (VSL) technique. The photoluminescence collected from the sample facet shows spectrally narrow, polarization-resolved transverse electric (TE) and transverse magnetic (TM) substrate modes. Continuous wave VSL revealed a reduction of optical losses in both modes only. On the contrary, a fast decay component due to amplified spontaneous emission is observed in time-resolved VSL experiments. This fast component shows positive net modal optical gain of ∼12 cm–1. The observed superlinear emission as a function of pump intensity, accompanied by shortening of the fast component lifetime, supports further the observation of stimulated emission. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Infrared Quantum Dots

AbstractColloidal nanocrystals are quantum‐size‐effect tunable; offer an abundance of available surface area for electronic and chemical interactions; and are processible from organic or aqueous solution onto substrates rigid or flexible, smooth or rough, flat or curved, inorganic or organic (including biological), crystalline or amorphous, conducting, semiconducting, or insulating. With the benefit of over a decade's progress in visible‐light‐emitting colloidal‐quantum‐dot synthesis, physical chemistry, and devices, significant progress has recently been made in infrared‐active colloidal quantum dots and devices. This progress report summarizes the state‐of‐the‐art in infrared colloidal quantum dots, with an emphasis on applications and devices. The applications of interest surveyed include monolithic integration of fiber‐optic and free‐space‐communications photonic components with electronic substrates such as silicon and glass; in‐vivo biological tagging in infrared spectral bands in which living tissue is optically penetrable to a depth of 5–10 cm; solar and thermal photovoltaics for energy conversion; and infrared sensing and imaging based on non‐visible, including thermal, signatures. The synthesis and properties of quantum dots are first reviewed: photoluminescence quantum efficiencies greater than 50 % are achievable in solution, and stable luminescent dots are available in organic and aqueous solvents. Electroluminescent devices based on solution processing have been reported with external quantum efficiencies approaching 1 %. Photoconductive devices have been realized with 3 % internal quantum efficiencies, and a photovoltaic effect was recently observed. Electro‐optic modulation achieved by either field‐ or charge‐induced modification of the rate of optical absorption has been demonstrated based both on interband and intersubband (intraband) transitions. Optical gain from these processible materials with a threshold of 1 mJ cm–2 and an optical net modal gain coefficient of 260 ± 20 cm–1 have been reported.

Extraction of rate‐equation parameters from optical modal gain measurements for multiple‐quantum‐well semiconductor lasers

AbstractRate equations for multiple‐quantum‐well (MQW) laser structure with carrier transport effects and advanced models of optical modal gain were used to extract relevant laser's parameters used in simulations. The many‐body effects of bandgap renormalization, Coulombic scattering interactions, and a non‐Markovian distribution were included in gain calculations. The importance of carrier leakage for that particular structure is indicated. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 55–56, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10514

Modeling the electrostatic and band‐mixing effects on gain for double‐quantum‐well lasers

AbstractThe simultaneous role of electrostatic effects and band‐mixing effects on optical modal gain in two coupled wells is analyzed within the self‐consistent solution of the Poisson and Luttinger–Kohn effective‐mass equations. The analysis is performed for a 1.3‐μm InGaAsP/InP lattice‐matched system. It shows that the electrostatic effects play a significant role in selecting the optimum barrier width and composition of the wells and barriers. The gain is actually larger when electrostatic effects are included because of better charge localization in the wells. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 35–37, 2002; DOI 10.1002/mop.10223

Ground-state emission and gain in ultralow-threshold InAs-InGaAs quantum-dot lasers

Emission spectra and modal optical gain are investigated in ultralow-threshold MBE-grown InAs-InGaAs quantum dot (QD) structures. The record lowest room-temperature inversion current is found to be /spl sim/13 A cm/sup -2/. The rate-equation model is proposed describing the optical gain related to the ground-state (GS) transitions in QDs. The ground-state gain goes to the maximum value that corresponds to the total inversion of available levels. The gain cross section for the GS emission is estimated as /spl sim/7/spl times/10/sup -15/ cm/sup 2/.

High-temperature optical gain of 980 nm InGaAs/AlGaAs quantum-well lasers

The optical modal gain of ridge-waveguide single-quantum-well InGaAs/AlGaAs diode lasers with an emission wavelength of 980 nm were measured up to a temperature of 250 °C. Comparisons of the obtained gain curves with a simple semiclassical model based on the single-band envelope function theory allows straightforward curve fitting of the gain as it is used, e.g., in numeric models. As a result a simple temperature dependence of the line shape function has been deduced.

Optical gain in InAs/InGaAs quantum-dot structures: Experiments and theoretical model

The dependence of the mode optical gain on current in InAs/InGaAs quantum-dot structures grown by the method of molecular-beam epitaxy is obtained from the experimental study of ultra-low-threshold laser diodes. The record lowest inversion threshold at room temperature was about 13 A cm-2. A theoretical model is proposed that relates the optical gain to the ground-state transitions in quantum dots. The effective gain cross section is estimated to be ~7 × 10-15 cm-2.

The effect of substrate orientation on the modal gain of quantum-well semiconductor lasers

The properties of In1−xGaxAsyP1−y–In1−xGaxAsyP1−y were studied as a function of substrate orientation. The Luttinger–Kohn formalism was used for holes, and a simple effective mass equation was used for conduction electrons. A novel method to calculate the TE optical modal gain was used. It was found that the modal gain is strongly dependent on the substrate orientation. At carrier concentrations less than 3.5×10cm−3, it has its largest value for the (111) growth direction. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 407–410, 2000.

On the optical modal gain of coupled quantum wells

Optical modal gain has been investigated for coupled quantum wells. Subband energies and envelope functions were determined by solving the Luttinger–Kohn effective mass equation including band mixing. The analysis was performed for GaAs–Al0.3Ga0.7As system, and shows that well coupling substantially shifts the spectral gain peak and band mixing reduces the gain peak as compared to the parabolic model. The implications for the design of semiconductor lasers based on coupled quantum wells are briefly discussed. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 301–304, 1999.

Polarization switching in a tensile-strained InGaAs/InGaAsP multiple quantum well distributed feedback laser diode

An inhomogeneously biased distributed feedback (DFB) laser diode (LD) with two electrodes switched its polarization mode by 3 mA change of the bias current, maintaining single longitudinal mode oscillation. In the active layer of the LD, 13 nm thick and 0.6% tensile-strained InGaAs multiple quantum well (MQW) equalized the transverse electric and the transverse magnetic modal optical gain at 1.55 μm. With various grating pitches on the same MQW active layer, polarization switching DFB LDs were realized in the wavelength range as wide as 18 nm. The linewidth characteristics during the polarization switching were confirmed to be narrow due to the small switching current.

High-brightness tapered semiconductor laser oscillators and amplifiers with low-modal gain epilayer-structures

The dependence of the beam quality of tapered laser oscillators and amplifiers on the modal optical gain is demonstrated experimentally and theoretically for the first time. Tapered devices with high- (HMG) and low-modal gain (LMG) structures are compared in terms of output power and beam quality. At high-output powers the beam quality of LMG devices is by a factor of ten better than the beam quality of high-modal gain devices. The beam quality remains nearly unchanged up to power levels of more than 2-W continuous-wave (CW) where a beam quality factor of M/sup 2/<3 is achieved for both, tapered laser oscillators and tapered amplifiers.