The growth of high In content InGaN material is notorious for being challenging because of high mechanical strain and thermodynamic instability of the system. It has been shown that one can improve the growth quality by using variable surface miscut. In this study, we demonstrate the use of micropatterning of bulk GaN substrates in order to improve the quality of high In content layers. During MOVPE growth the quality of the InGaN layers and the In content depends on the local shape of the substrate surface, reaching the highest emission intensity at the top of every pattern. We study patterns with characteristic sizes ranging from 1 to 6 µm created using two methods: binary photolithography combined with a thermal reflow process as well as multilevel photolithography. The latter allows for achieving lower inclination of the sidewall of the pattern which in turn supports a more stable growth process. The properties of the samples are studied using fluorescence microscopy, microphotoluminescence mapping and carrier diffusion measurements.
The goal of this work lies in expanding the integrated circuit technology to short wavelengths with the use of nitride emitters. We propose an approach that allows monolithic fabrication of lasers and waveguides using the same epitaxial structure. This is achieved by increasing the misorientation of the substrate locally, prior to the epitaxy, which allows local modification of the indium incorporation into the InGaN layers. Such areas are then used for etching down waveguides with low absorption. Within this work, we develop our technology for the fabrication of waveguide combiners, which involves creating waveguides with bends that bring two or more optical modes into close proximity. We compare systems consisting of 1 mm long laser diodes coupled to 1 mm bent waveguides with bend angles from 2.5° to 45° and different bend radiuses. We estimate the losses based on the optoelectrical parameters of the working system, treating it as a laser diode with a passive region introducing optical losses.
Nitride semiconductor-based light emitters (LEDs and laser diodes) are influenced by magnesium (Mg) acceptors, limiting conductivity and operational temperature due to high ionization energy. Mg also causes strong optical absorption, reducing laser diode efficiency. Dielectric polarization engineering using wurtzite nitride lattice symmetry (polarization doping) has been proposed to manipulate electrical properties. Our study demonstrates low threshold current density (2.5 kA/cm2), low internal losses (around 5 cm-1), and good thermal stability in fabricated laser diodes, enabling operation at cryogenic temperatures. Notably, polarization-doped p-layers yield lower voltage than Mg-doped ones. Understanding hole injection from polarization-doped layers remains a challenge.
We investigate the role of dislocations in nitride light emitters by comparing a set of laser diodes grown on GaN substrates with their counterparts grown on sapphire. Studied structures emit light in the range 383-477nm. We observe decrease in intensity of electroluminescence for samples with low indium composition and high dislocation density. To understand this effect we measured cathodoluminescence and investigated thermal stability of chosen structures. Results show that significance of dislocation related nonradiative recombination increases for low indium content structures due to shorter diffusion path of carriers. Results of TEM and SEM indicate that edge dislocations are the main source of nonradiative recombination in our structures.
Recently emerged “quantum technologies” motivate researchers and engineers to develop a specific class of light sources. The expected devices should emit spectrally narrow and tunable light with excellent beam properties. Semiconductor laser diodes are the light source of choice for these applications. InGaN laser diodes, emitting in the visible part of the spectrum, should play very important role in these applications. Within this presentation we will describe InGaN external cavity laser diodes, semiconductor optical amplifiers and distributed feedback lasers. We will discuss the progress in development of these devices as well as main physical and technological challenges.
In this paper, we try to resolve problems related to decreasing the size of an LED, and find a solution that would let us preserve optoelectronics parameters. The main idea is to use tunnel junctions to define the current path and, therefore, define the size of µLED. This way, during fabrication, there is no need to etch the active region. That way, it does not introduce any degradation nor problems related to surface states or differences in electrical fields inside the device.
We have fabricated such devices with sizes ranging from 100 µm-5 µm. In the characterization of these devices, it became apparent that, both electrical and optical parameters, are fully scalable with size. Most importantly, we do not observe an increase in the non-radiative recombination coefficient even for the smallest device. In addition, we observe excellent thermal stability of their light emission characteristics.
consumption and financial burden of the multiple light sources required for such systems. The AlGaInN material system allows for single transverse mode laser diodes to be fabricated with optical powers up to 100 mW over a wide range from ~380 nm up to ~530 nm. By tuning the indium content and thickness of the GaInN quantum well, we have developed a range of AlGaInN diode-lasers targeted to meet the wavelength and power requirements suitable for optical clocks and atom interferometry systems.
One of the major limiting factors in nitride laser diode development has been the lack of a suitable low defectivity and uniform GaN substrate. Recently, single crystal growth of large area, very low dislocation-density and uniform GaN substrates are grown using a combination of high temperature and high pressure enabling a range of AlGaInN laser technology to be developed. This direct light generation at the required wavelength is crucial to reduce complexity and size of the overall system, and to ensure a high wall-plug efficiency that is critical for space and mobile applications.
We will present our development of GaN based, low SWaP, frequency-stabilised external-cavity seed and tapered amplifiers to operate at 461nm for first stage strontium cooling. This includes growth of custom optimised GaN epitaxy for operation at 461 nm, a robust ECDL geometry, a novel tapered amplifier design and important work in characterising the optical performance and minimising surface reflectivity to identify suitable working parameters.
KEYWORDS: Semiconductor lasers, Gallium nitride, Near field optics, High power lasers, Waveguides, Waveguide lasers, Laser applications, Integrated optics, Diodes
GaN laser diodes have the potential to be a key enabling technology since the AlGaInN material system allows for laser diodes to be fabricated over a wide range of wavelengths from the u.v. to the visible. Novel applications include high power laser bars for optical pumping, to laser sources for quantum technologies based on atom interferometry, such as next generation optical clocks and gravity sensors.
We report our latest results on a range of AlGaInN diode-lasers targeted to meet the linewidth, wavelength and power requirements suitable for optical clocks and atom interferometry systems. This includes the [5s2S1/2-5p2P1/2] cooling transition in strontium+ ion optical clocks at 422 nm, the [5s21S0-5p1P1] cooling transition in neutral strontium clocks at 461 nm and the [5s2s1/2 – 6p2P3/2] transition in rubidium at 420 nm.
In addition, we report our latest results on tapered high power GaN laser diodes, for i) optical amplifiers, and ii) optimising the optical power of the device by reducing filamentation and hence avoiding catastrophic optical mirror damage (COMD).
GaN laser diodes has the potential to be a key enabling technology for a range of quantum technologies, including next generation optical atomic clocks and gravity sensors, based on cold-atom interferometry and also quantum communications, that have important applications for security and defence. Presently, such systems require a number of expensive, sophisticated and complex laser sources that limit quantum technologies to the laboratory. In contrast, GaN laser diode technology has the potential to provide a compact, rugged and reliable solutions, suitable for commercialisation. We report our latest results of GaN laser diodes suitable for both cold-atom interferometry and quantum communications.
GaN laser diodes fabricated from the AlGaInN material system is an emerging technology for
high power, optical integration and quantum applications. The AlGaInN material system allows
for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the
visible ~530nm, by tuning the indium content of the laser GaInN quantum well, giving rise to
new and novel applications including displays and imaging systems, free-space and underwater
telecommunications and the latest quantum technologies such as optical atomic clocks and atom
interferometry.
There are two physical phenomena governing the light emission in InGaN quantum structures: the internal electric fields and the In composition fluctuations. Both these effects manifest through the blue shift of the wavelength emission with the excitation intensity and both of them have the pronounced influence on the light emitting properties of these structures.
In order to discriminate between these two effects, we fabricated two identical structures: one with the quantum barriers doped with silicon (method for internal electric field screening) and the other with an undoped active region. Under the optical excitation the emission peak shifts by almost 35 nm (Si doped) and 50nm (without Si). Additionally, we studied temperature dependence of the emission peak position. In case of low temperatures and at RT and high pumping energy, emission energy position is almost the same for both samples. Our observations lead us to the conclusion that at low temperatures and at high pumping regime the Quantum Confined Stark Effect (QCSE) is totally suppressed. While this is understandable that at high carrier injection QCSE is screened, the origin of the low temperature effect is much less clear. We can speculate that at the lowest temperature the carriers are localized eliminating the spatial separation of holes and electrons wavefunctions.
Measured cathodoluminescence (CL) maps show the same level of the indium fluctuations for both samples. At higher excitation the fluctuations starts to be less visible suggesting band filling of states.
Finally we compare recombination times by means of time resolved photoluminescence.
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