The use of index-patterned Fabry-Perot lasers, where a small number of slot-like features are introduced along the laser cavity, is well established as a route to low cost, reliable single-frequency devices. We use a Fourier-transform based inverse scattering method to show how a modified choice of inverse function can deliver a significant improvement in modal threshold gain selectivity and SMSR compared to the originally proposed use of a constant inverse function for slot selection. We propose that the approach used can deliver a high yield of devices, with emission wavelength at or close to a target wavelength.
Electro-absorption modulators operate based on the quantum-confined Stark effect (QCSE). In quantum well and existing quantum dot (QD) structures, the use of top and bottom contacts allows application of an electric field along the growth direction. In this work, we theoretically analyse the QCSE in QD structures, investigating whether a lateral field orientation provides an appreciable QCSE sufficient to implement lateral QD-EAMs suitable for integration in photonic integrated circuits. We focus on InAs/GaAs QD structures close to 1300 nm, showing how the dot dimensions and built-in piezoelectric potential impact the calculated absorption spectrum as a function of applied lateral field.
To harness the advanced fabrication capabilities and high yields of the electronics industry for photonics, monolithic growth and CMOS compatibility are required. One promising candidate which fulfils these conditions is GeSn. Introducing Sn lowers the energy of the direct Γ valley relative to the indirect L valley. The movement of the conduction band valleys with Sn concentration is critical for the design of efficient devices; however, a large discrepancy exists in the literature for the Sn concentration at which GeSn becomes a direct band gap. We investigate the bandgap character of GeSn using hydrostatic pressure which reversibility modifies the bandstructure. In this work we determine the movement of the band-edge under pressure using photocurrent measurements. For a pure Ge sample, the movement of the band-edge is dominated by the indirect L valley with a measured pressure coefficient of 4.26±0.05 meV/kbar. With increasing Sn concentration there is evidence of band mixing effects, with values of 9.4±0.3 meV/kbar and 11.1±0.2 meV/kbar measured for 6% and 8% Sn samples. For a 10% Sn sample the pressure coefficient of 13±0.5 meV/kbar is close to the movement of the direct bandgap of Ge, indicating predominately direct Γ-like character for this GeSn alloy. This further suggests a gradual transition from indirect to direct like behaviour in the alloy as also evidenced from theoretical calculations. The implications of this in terms of optimising device performance will be discussed in further detail at the conference.
GaAsBi offers the possibility to develop near-IR semiconductor lasers such that the spin-orbit-split-off energy (ΔSO) is greater than the bandgap (Eg) in the active region with lasing wavelengths in the datacom/telecom range of 1.3-1.6 μm. This promises to suppress the dominant efficiency-limiting loss processes as Auger recombination, involving the generation of “hot” holes in the spin-orbit split-off band (the so-called “CHSH” process), and inter-valence band absorption (IVBA), where emitted photons are re-absorbed in the active region, thereby increasing the internal optical losses and negatively impacting upon the laser characteristics being responsible for the main energy consumption. In addition to growth and fabrication processes refinement, a key aspect of efforts to continue the advancement of the GaAsBi material system for laser applications is to develop a quantitative understanding of the impact of Bi on key device parameters. In this work, we present the first experimental measurements of the absorption, spontaneous emission, and optical gain spectra of GaAsBi/AlGaAs QW lasers using a segmented contact method and a theoretical analysis of these devices, which shows good quantitative agreement with the experiment. Internal optical losses of 10-15 cm-1 and peak modal gain of 24 cm-1 are measured at threshold and a peak material gain is estimated to be 1500 cm-1 at current density of 2 kA/cm-2, which agrees well with the calculated value of 1560 cm-1. The theoretical calculations also enabled us to identify and quantify Bi composition variations across the wafer and Bi-induced inhomogeneous broadening of the optical spectra.
We present a detailed analysis of wave function localization effects in InxGa1−xN alloys and quantum wells. Our work is based on density functional theory to analyze the impact of isolated and clustered In atoms on the wave function localization characteristics in InxGa1−xN alloys. We address the electronic structure of In0.25Ga0.25N/GaN quantum wells by means of an atomistic tight-binding model. Random alloy fluctuations in the quantum well region and well-width fluctuations are explicitly taken into account. The tight-binding model includes strain and built-in field fluctuations arising from the random In distribution. Our density functional theory study reveals increasing hole wave function localization effects when an increasing number of In atoms share the same N atom. We find that these effects are less pronounced for the electrons. Our tight-binding analysis of In0.25Ga0.27N/GaN quantum wells also reflects this behavior, revealing strong hole localization effects arising from the random In atom distribution. We also show that the excited hole states are strongly localized over an energy range of approximately 50 meV from the top of the valence band. For the quantum wells considered here we observe that well-width fluctuations lead to electron wave function localization effects.
We have explored the possibility to extend the data transmission rate for standard 850-nm GaAs-based VCSELs beyond
the 10 Gbit/s limit of today's commercially available directly-modulated devices. By sophisticated tailoring of the design
for high-speed performance we demonstrate that 10 Gb/s is far from the upper limit. For example, the thermal
conductivity of the bottom mirror is improved by the use of binary compounds, and the electrical parasitics are kept at a
minimum by incorporating a large diameter double layered oxide aperture in the design. We also show that the intrinsic
high speed performance is significantly improved by replacing the traditional GaAs QWs with strained InGaAs QWs in
the active region. The best overall performance is achieved for a device with a 9 μm diameter oxide aperture, having in
a threshold current of 0.6 mA, a maximum output power of 9 mW, a thermal resistance of 1.9 °C/mW, and a differential
resistance of 80 Ω. The measured 3dB bandwidth exceeds 20 GHz, and we experimentally demonstrate that the device is
capable of error-free transmission (BER<10-12) under direct modulation at a record-high bit-rate of 32 Gb/s over 50 m of
OM3 fiber at room temperature, and at 25 Gb/s over 100 m of OM3 fiber at 85 °C. We also demonstrate transmission at
40 Gb/s over 200 m of OM3+ fiber at room temperature using a subcarrier multiplexing scheme with a spectrally
efficient 16 QAM modulation format. All transmission results were obtained with the VCSEL biased at current densities
between 11-14 kA/cm2, which is close to the 10 kA/cm2 industry benchmark for reliability. Finally, we show that by a
further reduction of the oxide capacitance and by reducing the photon lifetime using a shallow surface etch, a record
bandwidth of 23 GHz for 850 nm VCSELs can be reached.
We have generalized the concept of nonlinear periodic structures to dielectric systems that show
arbitrary spatial and temporal variations of the refractive index. Nonlinear pulse propagation
through these spatiotemporal photonic crystals can be described, for shallow nonstationary
gratings, by coupled mode equations which are a generalization of the traditional equations
used for stationary Bragg gratings. Novel gap soliton solutions are found analytically. They
represent a generalization of the gap solitons in static photonic crystals and resonance solitons
found in dynamic gratings.
Theoretically predicted "dipole lasing" as spontaneous excitation of coherent oscillations of dipole momentum of metal nano-particles placed inside or near the surface of the medium with optical amplification. It has close analogy with ordinary lasing, but the polarization of nano-particles stands for the optical cavity mode. Oscillations of polarization cause coherent radiation from nano-particles acting as "nano-antennas". Optical cavity is not necessary, so that the minimum size of the dipole laser can be on the nano-scale. Dipole laser frequency corresponds to the localized plasmon resonance of nano-particles. The manifestation of the dipole lasing is in the divergence of the nano-particle polarisability, that is a second-order phase transition. Threshold conditions, enhancement of the spontaneous emission and optical bistability is dipole lasers are found.
We study the role of noise during the growth process of opal-based photonic crystals, and demonstrate
that noise significantly improves their structural properties. We observe a stochastic resonance-like
behaviour, where the ordering of the resulting structure improves up to a certain optimal noise level
and then deteriorates for larger noise volumes. This demonstrates that noise can have a nontrivial
effect when applied during a non-equilibrium pattern forming process.
In this paper we demonstrate the potential of a numerical algorithm that describes intersubband optical spectra by combining many body effects and nonparabolicity characterized by strong k-dependence of both dispersion relations and dipole moments. Applications of our numerical scheme are given for highly strained III-V quantum wells. Preliminary results results for dilute nitride systems are also discussed.
Robert Taylor, James Robinson, James Rice, Kwan Lee, Anas Jarjour, Jong Na, Shazia Yasin, Rachel Oliver, Menno Kappers, Colin Humphreys, G. Andrew Briggs, David Williams, Eoin O'Reilly, Aleksey Andreev, Yasuhiko Arakawa
We present measurements of microphotoluminescence decay dynamics for single InGaN quantum dots. The recombination is shown to be characterized by a single exponential decay, in contrast to the non-exponential recombination dynamics seen in the two-dimensional wetting layer. The lifetimes of single dots in the temperature range 4 K to 60 K decrease with increasing temperature. Microphotoluminescence measurements of exciton complexes in single MOVPE-grown InGaN quantum dots are also reported. We find the exciton-biexciton and exciton-charged exciton splitting energies to be 25 meV and 10 meV to the higher-energy side of the exciton ground state, respectively. Assignments of the ground state exciton, biexciton and charged exciton are supported by theoretical calculations. These measurements have been extended to investigate the time-resolved dynamics of biexciton transitions in the quantum dots. The measurements yield a radiative recombination lifetime of 1.0 ns for the exciton and 1.4 ns for the biexciton. The data can be fitted to a coupled differential equation rate equation model, confirming that the exciton state is refilled as biexcitons undergo radiative decay.
Ultrafast intraband carrier dynamics strongly influence many important characteristics in bulk and quantum well lasers and amplifiers through Spectral-Hole Burning (SHB) leading to nonlinear gain effects. In Quantum Dot (QD) devices, where the inter-level relaxation times can be even longer than the intraband relaxation times in conventional devices, SHB effects should also be substantial. A number of promising applications of QD amplifiers in high-speed optical processing (Cross-Gain Modulation, for instance) are based on features of the carrier dynamics in QD structures. In the present paper, based on a density matrix approach, we develop a theory of SHB-based nonlinear gain in QD lasers and amplifiers, which can affect such important characteristics as the modulation bandwidth in QD lasers and the saturation power and pulse energy in QD amplifiers. We give an expression for the nonlinear gain in QD devices, and show how it depends, particularly, on the capture/escape and relaxation/excitation rates.
We have developed a theory of the nonlinear refractive index in Quantum Dot (QD) Semiconductor Optical Amplifiers (SOAs) due to Spectral Hole Burning (SHB). Estimates show that this SHB nonlinear refractive index can be of order of 4x10-16 m2/W that is by four orders higher than the nonlinear refractive index in silica, and offers the possibility of an efficient ultrafast Cross-Phase-Modulation (XPM) in QD SOAs. The opportunity of XPM without patterning effects via this refractive index nonlinearity is discussed. The Pattern-Effect-Free (PEF) XPM is possible in QD SOAs at high pumps, when maximal (constant) gain is achieved in SOAs, and the linear and nonlinear refractive indices also become independent of the total carrier density in the QD structure. In whole, use of the ultrafast refractive index nonlinearity in the regime of maximum gain in QD SOAs can lead to the development of a new generation of nonlinear interferometers for ultrafast optical switching.
We report a systematic investigation of the band structure of
pentacene crystals, using both density functional theory (DFT) and a simpler tight-binding (TB) approach. We use the DFT method to determine the equilibrium crystal structure and the band structure along the crystallographic axes. We then introduce an analytic π bonding TB model which fully accounts for the calculated DFT band dispersion. The calculated band widths, effective masses,
and their anisotropy are in very good agreement with previous
experimental estimates. The band anisotropy in the ab plane and the
close similarity between the conduction and valence band dispersion are fully accounted for within the TB model.
We investigate the prospects for difference-harmonic genration due to intersubband transitions in n-doped quantum dots. The third order susceptibility tensor is calculated for flat conic quantum dots, where the lateral size is significantly greater than the height. The energy levels and the dipole matrix elements associated with the intersubband transitions are obtained in the framework of the effective mass approximation, including a simple model for self-consistent screenign in the dots. The spectral dependence of the susceptibility is calculated and the double-resonant enhancement of the response is considered. Numerical estimates of the efficiency of transformation of a two-color near-IR pump indicate that it may be possible to observe a mid-IR output of the order of a millimwatt for the case of a multi-QD-sheet structure placed in the center of a resonator.
By measuring the spontaneous emission from normally operating ~1.3um GaInNAs/GaAs-based lasers grown by MBE and by MOVPE we have quantitatively determined the variation of monomolecular (defect-related ~An), radiative (~Bn2) and Auger recombination (~Cn3) as a function of temperature from 130K to 370K. We find that A, B and C are remarkably independent of the growth method. Theoretical calculations of the threshold carrier density as a function of temperature were also performed using a 10 band k·p Hamiltonian from which we could determine the temperature variation of A, B and C. At 300K, A=11x10-8 sec-1, B=8x10-11 cm3 sec-1 and C= 6x10-29 cm6 sec-1. These are compared with theoretical calculations of the coefficients and good agreement is obtained. Our results suggest that by eliminating defect-related currents and reducing optical losses, the threshold current density of these GaInNAs/GaAs-based edge-emitting devices would be more than halved at room temperature. The results from studies of temperature and pressure variation of ~1.3um VCSELs produced by similar MBE growth could also be explained using the same recombination coefficients. They showed a broad gain spectrum and were able to operate over a wide temperature range.
We report the growth by molecular beam epitaxy (MBE) of InAlSb/InSb superlattice structures to investigate their potential for reducing the Auger recombination and intervalence absorption effects, which currently limit the maximum operating temperature. The devices were all grown onto InSb(001) substrates and are not lattice matched. They are a psuedo double heterostructure, comprising an active region 3micrometers thick, consisting of 10 repeats of In0.904Al0.096Sb/InSb (10nm/6.5nm), surrounded by undoped layers of In0.944Al0.056Sb. Electrical confinement in the active region is by means of a 20nm thick wide gap In0.794Al0.206Sb barrier layer onto which a p type In0.944Al0.056Sb tunnel contact and highly doped n type In0.944Al0.056Sb region is grown, which together with a substrate highly doped n type In0.944Al0.056Sb region gives optical confinement, due to a Moss-Burstein shift of the refractive index. We have demonstrated laser operation up to 160K for devices ~1000micrometers long by 15micrometers wide. FTIR spectroscopy measurements revealed a lasing wavelength of 3.65micrometers at 80K. Under pulse bias conditions, the threshold current density was 320Acm-2 at 80K. The peak output power was in excess of 800mW. Detailed modeling of the structures shows that greater strain is required in the system in order to quench Auger losses at higher temperatures.
We calculate microscopically the gain and absorption, linewidth enhancement factor and carrier capture times for a GaInNAs/GaAs quantum-well laser operating in the 1.3 micrometers wavelength regime. The results are compared to those for an InGaAsP/InP and an InGaAlAs/InP structure with similar fundamental transition energies. The much higher confinement for carriers in the GaInNAs quantum well is shown to lead to larger gain bandwidths and, for low to moderate carrier densities, to lower linewidth enhancement factors than for the later two material systems. On the other hand, the high depth of the wells leads to longer carrier capture times in GaInNAs/GaAs.
The understanding and evaluation of the Auger coefficient, C, and its variation with band structure is essential for accurate device modeling of long wavelength quantum well devices. We have developed a calculation of the Auger coefficient C for both 'band-to-band' processes, which involve strict k-selection rules, and 'k-relaxed' processes, where the strict k-selection rule is relaxed by momentum from phonons. To identify which process is dominating in 1.5 micrometers QW devices we have compared hydrostatic pressure measurements of the lasing threshold current with theoretical predictions for each process. We find that the 'k-relaxing' models are in good agreement with experiment as a function of pressure while the 'band-to-band' processes overestimate the reduction of the non-radiative component of the threshold current with pressure. Based on these results, we predict the threshold current for a number of well characterized 1.5 micrometers QW devices in the literature with a variety of strains and well widths.
The temperature sensitivity of the threshold current of 1.3 micrometer semiconductor lasers, denoted by the characteristic temperature T0, has remained low, with values ranging from 40 K up to a maximum of order 100 K. We report here on a combined theoretical and experimental analysis to identify the dominant factors contributing to this poor temperature sensitivity. We have determined directly the temperature dependence of the radiative current density, Jrad, by measuring the integrated spontaneous emission, L, from bulk and strained quantum well buried heterostructure devices. We find an effective T0 for Jrad of around 200 K for the bulk device and around 300 K for the quantum well device, in good agrement with the theoretical prediction for ideal lasers. This radiative temperature dependence compares with the measured T0 of around 50 - 60 K for the total threshold current density in both devices, from which we conclude that radiative recombination is not the dominant mechanism of the temperature sensitivity of the laser. We also find from the spontaneous emission data that just below threshold L varies with current I as I varies direct as L3/2, which is expected in the Boltzmann approximation if auger recombination is the dominant current path. We have used these findings to estimate T0 from as simple analytic expression we have derived and find values at room temperature of 40 - 100 K, in agreement with experiment. This poor T0 results both from the temperature dependence of the differential gain and by the major contribution of auger recombination to the total threshold current.
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