Compared to the maturity of today’s blue laser diodes, which exhibit high efficiencies, low threshold currents, and long lifetimes, deep-ultraviolet (<280 nm) lasers have essentially just been born. We have only recently witnessed the first deep-UV, continuous-wave edge-emitting lasers operating at room temperature under electrical injection. And more complex laser structures in the deep-UV, such as vertical-cavity surface-emitting lasers and photonic crystal surface emitting lasers, are even further behind, having only been demonstrated under pulsed optical pumping. Among the many difficulties in transitioning from blue to deep-UV are the problems of efficient electrical injection, creation of optical waveguides and cavities in materials with low refractive index contrast, and high material defect densities. The question is, are these problems fundamental limitations to the technology, or just temporary growing pains to be overcome with hard work and persistence as we push lasers deeper into unseen wavelengths and frontiers?

In recent years, the need for secure data communication has increased. Here, quantum key distribution (QKD) offers fundamental advantages over classical key distribution. Despite its high level of security, QKD comes a long with numerous challenges regarding the application of single photons. In this view, the decoy state protocol offers the possibility to implement QKD using classical light sources such as semiconductor lasers. In this contribution we present the newest results of our approach for the realization of a monolithic 850 nm vertical-cavity surface-emitting laser (VCSEL) array capable for QKD via the BB84 and decoy state protocol.

We show that coherent supermode emission is observable in hexagonal ring shaped 6 and 12 element photonic crystal VCSEL arrays under relatively short (~100ns) pulse operation with all array elements connected in parallel.

There has been an increasing demand for Vertical Cavity Surface Emitting Laser (VCSEL) as a light source solution that offers higher power density, high conversion efficiency, and short pulse operation in these applications. In this report, we design and fabricate three-junction VCSELs with three oxide layers and a single oxide layer, aiming to explore the effects of oxide layers on device characteristics. The output characteristics indicate that the design with multiple oxide layers effectively confines carriers better, leading to higher conversion efficiency. However, in terms of thermal characteristics, the outcomes demonstrate that using only a single oxide layer effectively reduces resistance, resulting in better thermal performance in terms of heat dissipation characteristics. These results clearly demonstrate the pros and cons of multi-junction solutions and effectively provide information about multi-junction VCSELs for the next generation sensing applications.

We report on the design and performance of single-frequency VCSELs that are electro-optically tunable in the 852nm wavelength range. Electro-optic tuning of the index of refraction is achieved by changing the reverse-bias electric field in a secondary p-i-n junction that contains coupled quantum wells. The electro-optic tuning is enhanced by putting the index-tuning region in a secondary cavity of a dual-cavity VCSEL. Electro-optic tuning can achieve 1nm of wavelength tuning without changing laser power and can operate at modulation frequencies up to 1GHz.

VCSELs emitting in the red spectral range are auspicious candidates for various applications, for example in biomedical utilization. However, their implementation is occasionally hampered especially due to the limited fiber coupling efficiency resulting from the beam divergence further increasing for higher power emission, where higher order transverse modes are emitted. Hence, the emission of a highly collimated Gaussian beam at raised output power is pursued in this work. The suppression of higher order modes through etched surface reliefs spatially modulating the reflectivity is investigated. To reduce the angle of divergence, polymer microlenses are integrated directly on the VCSEL surface.

The common understanding of laser operation is based on the non-equilibrium balance between the optical gain and the losses of the device, resulting in the stimulated emission of light above critical pumping threshold. In this contribution, we demonstrate that a broad-area VCSEL can operate in a state close to thermal equilibrium, enabling the Bose-Einstein condensation of photons. We observe condensation to the fundamental optical mode of the 23 µm-diameter device, followed by a thermalised distribution of photons in higher-order modes. Moreover, we extracted experimentally the thermodynamic properties of the photon gas and found that it closely follows the equation of state of a 2D boson gas in thermal equilibrium. This work offers a novel avenue for collective quantum phenomena in a well-established VCSEL platform.

Consumer applications of VCSEL arrays demand larger sizes and improved reliability. Ge-substrates are drop-in replacements for GaAs, whilst offering additional benefits. Thinner Ge-substrates are readily available due to photovoltaic supply chains, offering a cheaper cost per wafer. We report on device performance of identical 940-nm VCSELs, grown on 675µm, 500µm and 225µm thick Ge-substrates. Threshold current densities vary by less than 10μA/µm2 and 36μA/µm2 at the wafer centre between 675µm and, 500µm and 225µm respectively. A 3nm wavelength shift with decreasing substrate thickness is also observed. Results show a potential route to larger manufacturing volumes with lower cost per wafer.

In recent years, continuous wavelength-tunable light sources leading micro-electro-mechanical system-based vertical cavity surface emitting lasers (MEMS-VCSELs) have been used as a technology for swept-source optical coherence tomography (SS-OCT). MEMS tuned lasers can provide wide linear tunability through engineering of the interfaces of the two cavities, a semiconductor cavity that contains the active region and an air cavity. The two cavities are coupled strongly with each other, and the tuning ratio can be altered by controlling the coupling between the air and semiconductor regions. In this work, we compare the simulation results of the three configurations and show that the extended cavity (EC) structure with 140 nm linear tuning range represents a compromise between the semiconductor coupled (SCC) and air-coupled (ACC) designs. We fabricated the SCC structure using vacuum bonding to combine an InP wafer to an SOI wafer and achieved 57 nm continuous tuning range at a sweep frequency of 1.5 MHz.

Sensing in the critical 1100-1400nm spectral region is well served by VCSELs and photodetector utilizing dilute nitride (DN) materials grown on GaAs substrates. For CMOS compatibility the growth of DN materials on 200mm Ge wafers has also been demonstrated. The DN VCSEL structure can also be grown using MOCVD for the DBR which gives the ideal mix of performance and high-volume throughput. The successful growth of equivalent performance DN materials by MOCVD has been achieved for photodetector, work on VCSEL is ongoing.