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Magnetic fields can increase the intensity of terahertz (THz) waves due to changing the dipole moment direction using the Lorentz force. This study reports the increase in the THz-wave intensity generated by differential frequency mixing using commercial permanent magnets under exciton-excitation. While a weak magnetic field applied to a multiple quantum well increases the THz-wave intensity due to excitons, a strong field causes its decrease. According to the calculations, the increase is caused by the electron-hole separation due to the Lorentz force. Furthermore, the calculations suggest the importance of carrier acceleration to enhance the intensity. Importantly, the increase in the THz-wave intensity due to differential frequency mixing does not require a strong magnetic field and can be achieved with inexpensive commercially available magnets.

A comparison is presented between the experimental dynamic behavior of detuned AlInGaAs ridge waveguide distributed feedback lasers operating at O band and C band at two different temperatures. The nominal center wavelengths at 25°C for the set of devices studied were 1270, 1310, and 1550 nm. Preliminary static characterization of the samples showed consistent trends between wavelength detuning, characteristic temperature, and power output. For all set of samples, the characteristic temperature was substantially higher for the samples detuned to longer wavelengths (red) at room temperature. At the temperature of 85°C, a higher power efficiency was observed for those samples that thermally shifted nearer the peak optical gain. Resonance frequencies were extracted at 25°C and 85°C by means of electro‐optical transmission measurements and relative intensity noise measurements. The extracted values confirmed that at room temperature the samples detuned to shorter wavelengths (blue) showed the highest modulation efficiencies. At the temperature of 85°C, however, the wavelength and optical gain thermal shift makes those same samples to have the lowest resonant frequencies. The dynamic trends over temperature were clearer for the O band set of samples, while the detuning had a less pronounced effect at C band. The range of maximum resonance frequencies studied was from 12 to 14 GHz, which makes the devices suitable for uncooled communication applications at multi‐gigabit data rates.

We report on the characterization and analysis of a GaN-based distributed feedback laser diode (DFB-LD) with 3 rd -order laterally etched sidewall gratings centered at a wavelength of 420 nm. We also compare the device parameters with two commonly used Fabry-Perot (FP) devices operating at 450 nm and 520 nm. Intrinsic properties of the devices were extracted, including damping factor, carrier and photon lifetimes, modulation efficiency, differential gain, and parasitic capacitance. These parameters showed that the DFB exhibits a lower damping rate and parasitic capacitance while demonstrating a higher modulation efficiency, indicating that the DFB shows good potential for communications applications. Additionally, spectral linewidth of a GaN DFB is reported. To the authors’ knowledge, this is the first demonstration of parameter extraction and spectral linewidth measurement for GaN-based DFB-LDs.

Photonic crystal surface emitting lasers (PCSELs) are a new class of laser diode, offering control over emission (wavelength, polarisation, beam shape) through photonic crystal design, as well as power scalability and low beam divergence. We present developments in 2D arrays of large scale (~150x150um) PCSELs, coherently coupled by 1mm long, 100 μm wide waveguides that can be electrically driven into loss or gain. By studying the spectral and current-power characteristics, we show coherent power scaling between multiple devices. We discuss injection locking between devices achieved through controllable 2D in-plane feedback and its effect on the near and far field emissions.

In this paper, we show the effect of lateral external\ud optical feedback on an all semiconductor photonic crystal surface\ud emitting laser (PCSEL). Initially, a PCSEL is grown and fabricated\ud with a square lattice of triangles, the device is shown to\ud operate electrically driven at room temperature under continuous\ud wave condition. We investigate, theoretically and experimentally,\ud the effect of lateral feedback on the performance of photonic crystal\ud lasers. Demonstrating a reduction in mode competition and\ud a modification to spatial mode distribution, opening routes to all\ud electronic beam shaping and divergence control.

A broadly tunable THz source is realized via difference frequency generation, in which an enhancement to χ(3) that is obtained via resonant excitation of III–V semiconductor quantum well excitons is utilized. The symmetry of the quantum wells (QWs) is broken by utilizing the built-in electric-field across a p–i–n junction to produce effective χ(2) processes, which are derived from the high χ(3). This χ(2) media exhibits an onset of nonlinear processes at ~4 W cm−2, thereby enabling area (and, hence, power) scaling of the THz emitter. Phase matching is realized laterally through normal incidence excitation. Using two collimated 130 mW continuous wave (CW) semiconductor lasers with ~1-mm beam diameters, we realize monochromatic THz emission that is tunable from 0.75 to 3 THz and demonstrate the possibility that this may span 0.2–6 THz with linewidths of ~20 GHz and efficiencies of ~1 × 10–5, thereby realizing ~800 nW of THz power. Then, transmission spectroscopy of atmospheric features is demonstrated, thereby opening the way for compact, low-cost, swept-wavelength THz spectroscopy.

In this paper, the optoelectronic properties of InGaN-based blue (430 nm) superluminescent light-emitting diodes with a multi-section, three contact design are reported. The bias conditions of the rear absorber section and gain sections are explored in terms of enhancing and maximizing spectral bandwidth. We demonstrate that broader emission can be obtained with a short circuit, rather than an open circuit absorber section, and with two active regions at different current densities as opposed to a single active contact. Under optimal drive conditions, a −3 dB linewidth of 20 nm is obtained at 430 nm. Analysis of the device emission spectrum indicates that an axial resolution of ∼3.4 μm may be obtained in an optical coherence tomography system.

Continuous terahertz wave sources with narrow bandwidth and wide frequency tunability enable high-resolution terahertz spectroscopy and high-speed information communication. In this study, using the optical nonlinearity of excitons as the source of second-order nonlinear polarization, we realize a continuous terahertz electromagnetic wave demonstrating wide frequency tunability from 0.1 to 18 THz without a decrease in intensity due to phonon scattering. Because of excitation of two exciton states in a Ga As / Al As multiple quantum well using two continuous-wave lasers, terahertz waves are emitted as a result of difference-frequency mixing, where the intensity shows a square dependence on the excitation intensity. Using the inhomogeneous width of exciton lines, we achieve wide frequency tunability without phonon effects.

We present a method for the in-line non-destructive characterisation of photonic crystal surface emitting lasers. We show through photoluminescence measurement at the point of photonic crystal definition that the etch depth and fill factor can be determined through comparison to band structure modelling.

Photonic crystal surface emitting lasers (PCSEL) are of interest in order to achieve large area single mode operation, excellent beam quality, and high output powers. Coherent PCSEL arrays have also been realised and lasing mode control of PCSELs has been demonstrated by using external in-plane feedback. However, the design and analysis of PCSELs has been limited by current modelling techniques. The plane wave expansion method is only applicable to infinite structures whilst the finite difference time domain method is computationally intensive especially when modelling large area, three dimensional finite devices. Other analytical approach, such as coupled mode analysis, leads to significant mathematical complication. None of these are well suited to accurately describing complex PCSEL devices, nor the effect of in-plane feedback. We introduce a general quasi-analytical model based on the separation of variables and a quasi-scalar field distribution. The multilayer planar structure in the growth direction is accounted for by computing the effective indices corresponding to the in-plane bound guided modes. This operation results in a sculpted, periodic index distribution. Next, by first assuming index uniformity along the z-axis, a piecewise constant multilayer periodic media results. An effective wave impedance of the waves supported in this structure are then computed and used to construct the 3D device. In this way, the final device characteristics account for the 2D periodic photonic crystal media. We compare our modelling to other methods, and various experimental reports. In particular we focus upon the effect of external in-plane feedback on PCSELs.

The realization of a 1 × 2 coherently coupled photonic crystal surface emitting laser array is reported. New routes to power scaling are discussed and the electronic control of coherence is demonstrated.

Optical coherence tomography (OCT) exploits the coherent properties of light to permit noninvasive and in situ imaging of biological tissues. By expanding the range of OCT light sources from the traditional telecoms wavelengths to include ∼400 nm gallium nitride (GaN) based superluminescent light emitting diodes (SLEDs) subcellular axial and lateral resolution could be achieved, provided enhanced bandwidth is also achieved. Due to the focus on high-power applications for GaN SLEDs, there has been limited work on increasing the source bandwidth. In this paper, we demonstrate for the first time a ∼400 nm GaN SLED with >10 nm bandwidth employed within an OCT system, where an axial resolution of ∼7 μm is achieved. Bespoke GaN SLEDs suggest that

We report the generation of tuneable THz radiation (0.2–6 THz) and demonstrate a spectroscopic system operating at 750GHz. THz radiation is generated through second order nonlinear effects in the excitation of excitons in GaAs/AlAs multi-quantum wells (MQWs), using readily available continuous wave (CW) laser diodes at room temperature. An MBE grown 30×MQW GaAs(l 1.9nm)/AlAs(7.1nm) sample was designed to have excitonic resonances at a wavelength accessible by commercially available lasers (∼850nm), and have E1HH1-E1LH1 splitting of 9.1meV. Two collimated, normally incident lasers were used to excite the excitonic resonances. Figure 1 (a) shows THz power obtained in collinear and crossed polarisations of the lasers with one laser resonant with the HH exciton, and the other laser tuned across the excitonic bands with each laser operating at 130mW output power. A clear signal is observed in the case of collinear excitation which scales with the density of states of the excitons. Power dependence measurements confirm this is a second order non-linear effect. Using a simple interferometer, and fitting the measured power with the expected transmission of a Fabry-Perot etalon, frequency measurements confirm the ability to tune the THz radiation from 0.75–3THz. See Figs 1(b-d). Tuning over the full range of Fig 1(a) indicates that THz generation os possible from −0.2 to 6THz. For excitation at the peaks of HH and LH, a conversion efficiency of 1.2×10 was obtained. This was achieved without the use of plasmonic effects, nor any kind of an antenna, nor an applied E-field to the structure. Spectroscopy using this new tuneable/sweepable light source will be presented at the conference. Future prospects for this technology will be discussed.

We report the generation of tuneable THz radiation (0.75–3 THz) through second order nonlinear effects in the excitation of excitons in GaAs/AlAs multi-quantum wells (MQWs), using readily available continuous wave (CW) laser diodes at room temperature. A MQW GaAs/AlAs sample was designed to have excitonic resonances at a wavelength accessible by commercially available lasers (850nm, 260mW), and have ElHHl-ElLHl splitting of 9.1meV. The sample was grown by MBE with the MQW region containing 30 repeats of 11.9nm GaAs separated by 7.1nm AlAs barriers. These preliminary measurements removed the substrate and cap layer. The sample was then capillary bonded to a diamond heat spreader [1]. Two collimated lasers were used to excite the excitonic resonances. Both lasers were normally incident to the sample surface. Figure 1(a) shows THz power obtained in collinear and crossed polarisations of the lasers with one laser resonant with the HH exciton, and the other laser tuned across the excitonic bands with both lasers operating at 260mW. A clear signal is observed in the case of collinear excitation which scales with the density of states of the excitons. Power dependence measurements confirm this is a second order non-linear effect. Using a simple interferometer, and fitting the measured power with the expected transmission of a Fabry-Perot etalon, frequency measurements indicate the ability to tune the THz radiation from 0.75–3THz. See Figs 1(b-d). For excitation at the peaks of HH and LH, a conversion efficiency of 1.2×10−5 was obtained. This was achieved without the use of plasmonic effects, nor any kind of an antenna, nor an applied E-field to the structure. This offers the opportunity for the creation of compact, low cost, tuneable room temperature THz source.

All-semiconductor photonic crystal surface-emitting lasers (PCSELs) operating in CW mode at room temperature and coherently coupled arrays of these lasers are reviewed. These PCSELs are grown via MOVPE on GaAs substrates and include QW active elements and GaAs/InGaP photonic crystal (PC) layer situated above this active zone. Atoms of triangular shapes have been shown to increase optical power from the PCSEL but are also shown to result in a competition between lasing modes. Simulation shows that the energy splitting of lasing modes is smaller for triangular atoms, than for circles making high power single-mode devices difficult to achieve. In this work we experimentally investigate the effect of lateral optical feedback introduced by a facet cleave along one or two perpendicular PCSEL edges. This cleavage plane is misaligned to the PC resulting in a periodic variation of facet phase along the side of the device. Results confirm that a single cleave selects the lowest threshold 2D lasing mode, resulting in a ~20% reduction in threshold current and favours single-mode emission. The addition of a second cleave at right-angles to the first has no significant effect upon threshold current. The virgin device is shown to have a symmetric far-field (1 degree) whilst a single cleave produces a 1 degree divergence perpendicular to cleave and 5 degree parallel to cleave. The second orthogonal cleave results in the far field becoming symmetric again but with a divergence angle of 1 degree indicating that single-mode lasing is supported over a wider area.

The advent of optical coherence tomography (OCT) has permitted high-resolution, non-invasive, in vivo imaging of the eye, skin and other biological tissue. The axial resolution is limited by source bandwidth and central wavelength. With the growing demand for short wavelength imaging, super-continuum sources and non-linear fibre-based light sources have been demonstrated in tissue imaging applications exploiting the near-UV and visible spectrum. Whilst the potential has been identified of using gallium nitride devices due to relative maturity of laser technology, there have been limited reports on using such low cost, robust devices in imaging systems. A GaN super-luminescent light emitting diode (SLED) was first reported in 2009, using tilted facets to suppress lasing, with the focus since on high power, low speckle and relatively low bandwidth applications. In this paper we discuss a method of producing a GaN based broadband source, including a passive absorber to suppress lasing. The merits of this passive absorber are then discussed with regards to broad-bandwidth applications, rather than power applications. For the first time in GaN devices, the performance of the light sources developed are assessed though the point spread function (PSF) (which describes an imaging systems response to a point source), calculated from the emission spectra. We show a sub-7μm resolution is possible without the use of special epitaxial techniques, ultimately outlining the suitability of these short wavelength, broadband, GaN devices for use in OCT applications.

This paper considers multiple structural designs for photonic crystal surface emitting lasers operating at key wavelengths. Initially a structure from Williams et al. is modelled, the structure is modified to include an additional GaAs waveguide layer (termed ballast layer) and to include an additional PC layer (termed double decker). These structures are modelled by a combination of coupling calculation and waveguide modelling and are compared to the original structure. We show that both of these schemes give an increase in coupling, but present fabrication challenges. Next, we model standard laser structures operating at key wavelengths (400 nm, 1.3 and 10 µm) where a photonic crystal is located above the active region and explore the effect of increasing thickness of photonic crystal. We find that increasing the thickness increases the coupling coefficient but not true for the full range of thicknesses considered. This study allows a more universal comparison of the use of all-semiconductor, or void containing PCSELs to be conducted and we find that the realisation of all semiconductor PCSELs covering a wide range of material and wavelengths are possible.

Electronic control of coherence in 2D arrays of photonic crystal surface emitting lasers is discussed.

We investigate the beam divergence in far-field region, diffraction loss and optical confinement factors of all-semiconductor and void-semiconductor photonic-crystal surface-emitting lasers (PCSELs), containing either InGaP/GaAs or InGaP/air photonic crystals using a three-dimensional FDTD model. We explore the impact of changing the PC hole shape, size, and lattice structure in addition to the choice of all-semiconductor or void-semiconductor designs. We discuss the determination of the threshold gain from the diffraction losses, and explore limitations to direct modulation of the PCSEL. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The multiple quantum well (MQW) structure designed to utilize light hole (lh) and heavy hole (hh) energy states for producing coherent excitation of lh and hh excitons and resulting in radiation in the THz range of frequencies. Energies of states can be tunable with applied electric bias, resulting in a tunability of the radiation frequency. In our work, we aim to produce room temperature tunable THz radiation by optically exciting the QB sample using continues wave (CW) laser sources. In order to maximize the incident power upon the sample, and simplify the collection of the THz radiation, a structure, which has had the substrate, removed by using different method as thinning and wet etching, and capillary bonded to diamond is required.

We demonstrate a semiconductor PCSEL array that uniquely combines an in-plane waveguide structure with nano-scale patterned PCSEL elements. This novel geometry allows two-dimensional electronically controllable coherent coupling of remote vertically emitting lasers. Mutual coherence of the PCSEL elements is verified through the demonstration of a two-dimensional Young’s Slits experiment. In addition to allowing the all-electronic control of the interference pattern, this type of device offers new routes to power and brightness scaling in semiconductor lasers and opportunities for all-electronic beam steering.

980 nm GaAs-based photonic crystal surface emitting lasers containing all semiconductor GaAs/InGaP and GaAs/air photonic crystals (PC) inside their cavity are theoretically investigated. We use a combination of an average index approach and optical coupled mode theory to optimize the PC interaction with optical modes of the laser waveguide and draw guidelines for design of PCSELs based on a range of material systems and operating wavelengths. Results show that the all-semiconductor PC provides a higher coupling with the optical mode in most cases.

In this work we report on the simulation of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs). We simulate an etched mesa structure (substrate emission) with the substrate acting as the current spreading layer. The effect of contact misalignment on the carrier distribution within the active element is explored and confirms the validity of the model in describing the carrier distribution. We go on to discuss the effects of the substrate thickness and trench depth on the intensity profile. Simulation results show that a thicker substrate and a trench partially etched into the substrate may improve the intensity profile in future devices.

We investigated the beam divergence in far-field region, diffraction loss and optical confinement factors of semiconductor photonic crystal surface emitting lasers containing either InGaP/GaAs or InGaP/air photonic crystals. The maximum in-plane feedback in PCSELs with InGaP/GaAs PCs has been observed at the ratio between PC atom radius and the lattice constant of 0.2 and 0.38. The surface emission grows as the size of atom increases.

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