Type-II GaInAs/GaAsSb “W”-active regions offer the potential for greater control over the temperature sensitivity of semiconductor lasers operating in the near-IR. In this paper we explore the theoretical design space available using “W”-QWs and discuss the interplay between active region design choices and waveguide optimisation, highlighting the importance of simultaneous optimisation in these systems. We demonstrate the molecular beam epitaxy growth of GaAs-based “W”-lasers emitting around 1250 nm, achieving a room temperature threshold current density of 480±10 A/cm². These initial results demonstrate the promising potential of "W"-lasers for energy-efficient O-band applications in data communications networks.
Continued improvement of production-scalable characterisation methods is necessary to support the growth of semiconductor industries. In this work we present the application of compressed sensing for photoluminescence imaging in the temporal and spectral domains. The application can be enabled by using a digital micromirror device to programmatically control the spatial information of the excitation or detection source, allowing the use of single-point detectors for imaging applications, with benefits in terms of reduced measurement time and dark noise. We present the methodology for successful compressed sensing acquisition and reconstruction of spectral and temporal photoluminescence signals, developed through computational modelling work.
In this paper we discuss how a combination of band structure and band alignment engineering may be used to reduce the temperature sensitivity of semiconductor lasers operating in the near-IR. The use of back-to-back type-II band alignment “W”-structures, already successfully demonstrated in the mid-IR, provides a route to engineer the temperature sensitivity of the emission wavelength in near-IR devices through control of the band gap and band bending. Furthermore, utilising novel alloys such as the bismide-nitrides also provides a route to reduce non-radiative processes which underpin the temperature sensitivity of the threshold current across this wavelength range.
In this paper we discuss the emerging applications of photovoltaics for laser-based optical wireless power transfer. In particular, we focus on how key factors impact the design of the system, including wavelength, power range, operating temperature and environment and their impact on the choice of laser and photovoltaic cells. Two examples are considered: the first using bespoke InP-based photovoltaics targeting 1550 nm operation for eye-safe terrestrial applications, and the second concerning the use of visible and near-IR laser illumination of solar cells for satellites in low Earth orbit. The challenges and opportunities associated with each application will be discussed.
Type-II GaInAs/GaAsSb “W” quantum well heterostructures on GaAs show strong potential for temperature-stable data communications lasers. Devices emitting at 1255 nm show promising lasing characteristics including room-temperature threshold current densities, Jth < 300 A/cm^2, pulsed output powers >1 W, and a reduced wavelength temperature dependence of 0.31 nm/C. Temperature- and pressure-dependent characterisation techniques are used to determine the roles of radiative and non-radiative recombination. Analysis of these characteristics suggest a reduced influence of non-radiative recombination on the thermal stability of type-II “W”-lasers compared to type-I devices, as will be discussed along with recommendations for future device development.
We investigate the temperature and pressure dependence of a series of intrinsic and modulation p-doped InAs-based dot-in-well (DWELL) laser diodes grown on silicon substrates. Temperature dependence of the threshold current density (Jth) and pure spontaneous emission spectra provide an insight into inhomogeneity and non-radiative recombination mechanisms within the devices. Initial investigations showed that the intrinsic devices exhibited low temperature sensitivity in the range 170-200K. Above this, Jth increased more rapidly consistent with Auger recombination. P-doping increased the temperature at which Jth(T) started to increase up to 300K with a temperature insensitive region close to room temperature. P-doping delays the onset of carrier thermalization, leading to a high T0 but with an associated higher Jth. Temperature dependence of gain spectrum broadening was investigated by measuring the spontaneous emission spectral width parameter (1/e2) just below Jth (T). A strong direct correlation is found between the temperature dependence of peak width with the temperature dependence the radiative component of threshold, Jrad(T). At low temperature the correlation is consistent with strong inhomogeneous broadening of the carrier distribution. As temperature increases Jth reduces associated with carriers thermalizing to lower energy states. At higher temperatures homogeneous thermal broadening coupled with non-radiative recombination causes Jth to increase. Inhomogeneous broadening is more pronounced in the p-doped devices due to coulombic attraction between acceptor holes and injected electrons. A detailed analysis of recombination processes using high hydrostatic pressure and spontaneous emission in these lasers as a function of doping density will be presented and discussed at the conference.
Monolithic growth of III-V semiconductors on silicon is a promising path for the development of silicon-based lasers. The GaP binary has a lattice constant very close to that of silicon and can be grown defect free without anti-phase domains (APDs) or stacking faults on (001) exact orientated silicon substrates. These GaP on Si templates provide the base for growth and investigation of III-V lasers. The addition of boron can be used to partially replace Ga and further reduce the lattice constant. This can be balanced to match the lattice constant of silicon by adding As to partially replace P. The alloying also provides control of band gaps and band offsets as well as refractive index. The BxGa(1-x)P and BxGa(1-x)AsyP(1-y) alloys are being explored to provide lattice matching/ strain compensation, cladding and the Separate Confined Heterostructure (SCH). The effects of the inclusion of boron on device related alloy properties have not been studied extensively and are not well understood. We investigate the refractive index and extinction coefficient dispersion relation and the electronic band structure properties of these boron containing alloys using spectroscopic ellipsometry to provide inputs for device modelling and optimisation. Results from the spectroscopic ellipsometry are presented for a series of BGaP and BGaAsP alloy samples with boron fractions in the range 0-6.6% and arsenic fractions from 0-17% on GaP substrates and GaP/ Si templates. These results provide important information for the design of lasers with strong optical and electronic confinement, as shall be discussed.
Interband cascade lasers (ICLs) are a promising light source for the mid-infrared (mid-IR) spectral range. However, for certain applications such as spectroscopic techniques for chemical sensing and non-invasive disease diagnostics, a broadband incoherent radiation source such as an LED may be more desirable. Here we investigate both ICLs and interband cascade light emitting devices (ICLEDs). The ICLEDs follow the example of ICLs by cascading multiple active stages in series to improve efficiency and increase output power, but without an optical cavity to provide feedback.
In this work we will present studies of these devices using high hydrostatic pressure techniques to determine the key efficiency limiting processes so that they might be mitigated. The application of hydrostatic pressure causes reversible changes to the band structure, increasing the energy of the conduction band gamma point and moving other key points in the band structure. This makes it a useful technique to probe recombination processes that depend on band gap and offsets, independently of temperature. For a laser dominated by CHCC Auger recombination, as is typical in narrow band gap devices for the mid-IR, one would expect a decrease in threshold current with increasing pressure, as the Auger process decreases with increasing band gap. However, the lasers studied here exhibit an increase in threshold current with pressure, indicating that other processes also play a significant role. We will discuss the relative contributions from Auger recombination and other processes such as defect-related recombination and carrier leakage in these devices, with respect to relevant modelling.
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 investigate the possibility to selectively reflect certain wavelengths while maintaining the optical properties on other spectral ranges. This is of particular interest for transparent materials, which for specific applications may require high reflectivity at pre-determined frequencies. Although there exist currently techniques such as coatings to produce selective reflection, this work focuses on new approaches for mass production of polyethylene sheets which incorporate either additives or surface patterning for selective reflection between 8 to 13 μ m. Typical additives used to produce a greenhouse effect in plastics include particles such as clays, silica or hydroxide materials. However, the absorption of thermal radiation is less efficient than the decrease of emissivity as it can be compared with the inclusion of Lambertian materials. Photonic band gap engineering by the periodic structuring of metamaterials is known in nature for producing the vivid bright colors in certain organisms via strong wavelength-selective reflection. Research to artificially engineer such structures has mainly focused on wavelengths in the visible and near infrared. However few studies to date have been carried out to investigate the properties of metastructures in the mid infrared range even though the patterning of microstructure is easier to achieve. We present preliminary results on the diffuse reflectivity using FDTD simulations and analyze the technical feasibility of these approaches.
We present the development of a novel semiconductor chip-based spectrometer for calibration of Earth observation instruments. The chip follows the Solo spectroscopy approach utilising an array of microdisk resonators evanescently coupled to a central waveguide. Each resonator is tuned to select out a specific wavelength from the incoming spectrum, and forms a p-i-n junction in which current is generated when light of the correct wavelength is present. In this paper we discuss important design aspects including the choice of semiconductor material, design of semiconductor quantum well structures for optical absorption, and design and optimisation of the waveguide and resonators.
In this work we present results from high performance silicon optical modulators produced within the two largest silicon
photonics projects in Europe; UK Silicon Photonics (UKSP) and HELIOS. Two conventional MZI based optical
modulators featuring novel self-aligned fabrication processes are presented. The first is based in 400nm overlayer SOI
and demonstrates 40Gbit/s modulation with the same extinction ratio for both TE and TM polarisations, which relaxes
coupling requirements to the device. The second design is based in 220nm SOI and demonstrates 40Gbits/s modulation
with a 10dB extinction ratio as well modulation at 50Gbit/s for the first time. A ring resonator based optical modulator,
featuring FIB error correction is presented. 40Gbit/s, 32fJ/bit operation is also shown from this device which has a 6um
radius. Further to this slow light enhancement of the modulation effect is demonstrated through the use of both
convention photonic crystal structures and corrugated waveguides. Fabricated conventional photonic crystal modulators
have shown an enhancement factor of 8 over the fast light case. The corrugated waveguide device shows modulation
efficiency down to 0.45V.cm compared to 2.2V.cm in the fast light case. 40Gbit/s modulation is demonstrated with a
3dB modulation depth from this device. Novel photonic crystal based cavity modulators are also demonstrated which
offer the potential for low fibre to fibre loss. In this case preliminary modulation results at 1Gbit/s are demonstrated.
Ge/SiGe Stark effect devices operating at 1300nm are presented. Finally an integrated transmitter featuring a III-V
source and MZI modulator operating at 10Gbit/s is presented.
A simple multiple-angle light scattering system was developed for the differential measurement of particle
concentrations in suspension even in high concentration where multiple scattering effects are significant based on size.
The system combines multiple-angle detection to collect scattered angle dependent light intensities, and Partial Least
Square Regression method (PLS-R) to compose the predictive models for analyzing scattered signal obtain
concentrations of samples under investigation. The system was designed to be simple, portable and inexpensive. It
employs a diode lasers (red AlGaInP-based) as a light source and a silicon photodiode as a detector and optical
components, all of which are readily available. The technique was validated using 1.1 μm and 3.0μm polystyrene latex
beads in both mono-dispersed and poly-dispersed suspensions. The measurement results showed good agreement
between the measured results and reference values. Their deviations from the reference values are 2.4% and 1.5%
relating to references' concentrations of 1.3×108 and 1.2times;107 particles/ml for 1.1 m and 3.0 μm in mono-dispersed
solutions and 2.3 % and 3.5% relating to references' concentrations of 1.1times;108 and 4.4 times;105 particles/ml for 1.1 μm
and 3.0 μm in mixed solutions, respectively. This system is a compact but high performance system allowing multiple
particle sizes in high concentration to be measured simultaneously.
A simple, dual wavelength, multiple-angle, light scattering system has been developed for detecting cryptosporidium
suspended in water. Cryptosporidium is a coccidial protozoan parasite causing cryptosporidiosis; a diarrheal disease of
varying severity. The parasite is transmitted by ingestion of contaminated water, particularly drinking-water, but also
accidental ingestion of bathing-water, including swimming pools. It is therefore important to be able to detect these
parasites quickly, so that remedial action can be taken to reduce the risk of infection. The proposed system combines
multiple-angle scattering detection of a single and two wavelengths, to collect relative wavelength angle-resolved
scattering phase functions from tested suspension, and multivariate data analysis techniques to obtain characterizing
information of samples under investigation. The system was designed to be simple, portable and inexpensive. It employs
two diode lasers (violet InGaN-based and red AlGaInP-based) as light sources and silicon photodiodes as detectors and
optical components, all of which are readily available. The measured scattering patterns using the dual wavelength
system showed that the relative wavelength angle-resolved scattering pattern of cryptosporidium oocysts was
significantly different from other particles (e.g. polystyrene latex sphere, E.coli). The single wavelength set up was
applied for cryptosporidium oocysts'size and relative refractive index measurement and differential measurement of the
concentration of cryptosporidium oocysts suspended in water and mixed polystyrene latex sphere suspension. The
measurement results showed good agreement with the control reference values. These results indicate that the proposed
method could potentially be applied to online detection in a water quality control system.
A monolithic, chip-based spectrometer based on the novel concept of a series of electrically addressable high-Q
resonators coupled to a ridge waveguide is presented. A discrete monolithic InP-based spectrometer chip has been
designed to detect wavelengths spanning a 10nm range in the near-infrared region functioning as a sensor. This
spectrometer approach has a number of advantages over traditional spectrometers. As well as being solid state and
having no moving parts, the co-location of wavelength dispersion and detection offers advantageous spectral
performance in a compact chip form. Optimization of resonator size and composition to detect wavelengths across the
spectral range of interest will be discussed together with preliminary experimental results.
InGaAsN is a promising material system to enable low-cost GaAs-based detectors to operate in the telecommunication
spectrum, despite the problems posed by the low growth temperature required for nitrogen incorporation. We
demonstrate that InGaAsN p+-i-n+ structures with nominal In and N fraction of 10% and 3.8%, grown by molecular
beam epitaxy (MBE) under non-optimal growth conditions, can be optimized by post growth thermal annealing to match
the performance of optimally grown structures. We report the findings of an annealing study by comparing the
photoluminescence spectra, dark current and background concentration of the as-grown and annealed samples. The dark
current of the optimally annealed sample is approximately 2 μA/cm2 at an electric field of 100 kV/cm, and is the lowest
reported to date for InGaAsN photodetectors with a cut-off wavelength of 1.3 μm. Evidence of lower unintentional
background concentration after annealing at a sufficiently high temperature, is also presented.
This paper reports the improvements and limitations of MBE grown 1.3μm GaAsSb/GaAs single QW lasers. At room
temperature, the devices show a low threshold current density (Jth) of 253 Acm-2, a transparent current density of 98
Acm-2, an internal quantum efficiency of 71%, an optical loss of 18 cm-1 and a characteristic temperature (T0) = 51K. The
defect related recombination in these devices is negligible and the primary non-radiative current path has a stronger
dependence on the carrier density than the radiative current contributing to ~84% of the threshold current at RT. From
high hydrostatic pressure dependent measurements, a slight decrease followed by the strong increase in threshold current
with pressure is observed, suggesting that the device performance is limited to both Auger recombination and carrier
leakage.
Quantum Cascade Lasers (QCLs) have been very successful at long wavelengths, >4μm, and there is now considerable
effort to develop QCLs for short wavelength (2-3μm) applications. To optimise both interband and QC lasers it is
important to understand the role of radiative and non-radiative processes and their variation with wavelength and
temperature. We use high hydrostatic pressure to manipulate the band structure of lasers to identify the dominant
efficiency limiting processes. We describe how hydrostatic pressure may also be used to vary the separation between the
Γ, Χ and L bands, allowing one to investigate the role of inter-valley carrier scattering on the properties of QCLs. We
will describe an example of how pressure can be used to investigate the properties of 2.9-3.3μm InAs/AlSb QCLs. We
find that while the threshold current of the 3.3μm devices shows little pressure variation even at room temperature, for
the 2.9μm devices the threshold current increases by ~20% over 4kbar at 190K consistent with carrier scattering into the
L-minima. Based on our high pressure studies, we conclude that the maximum operating temperature of InAs/AlSb
QCLs decreases with decreasing wavelength due to increased carrier scattering into the L-minima of InAs.
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.
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