Coherent light in ultraviolet (UV) spectral region has numerous high-impact applications in material science, medicine, biological and chemical sensing, data storage, aerospace navigation, and for water, food & air disinfection. Currently, the only commercial UV laser sources are excimer lasers and frequency-harmonic generators operating with visible light input. However, both types have considerable drawbacks. The former suffers from very large size, limited wavelength flexibility, low efficiency and very high cost. The latter provides a solution to the wavelength flexibility issue, however at the cost of much lower efficiency, which is typical for non-linear optical processes such as second harmonic generation and frequency mixing. Thus, alternative smaller UV laser sources based on nanostructured semiconductor materials with outstanding performance, as is widely available in the near-infrared and visible wavelength region, are today in very high demand.
In this project, we bring together the platforms of AlGaN semiconductor nanowire-based light emitters, the new material graphene and optical cavities based on high-reflective Distributed Bragg Reflectors in order to develop the world’s first electrically injected semiconductor laser operating in the UV-C spectral range, that is, with an emission wavelength below 280 nm.
In UV-Nanolaser, we bring together the platforms of AlGaN semiconductor nanowire (NW)-based light emitters, the exciting new material graphene, utilised as both transparent contact electrode (TCE) material and epitaxial growth substrate, and optical cavities based on high-reflective Distributed Bragg Reflectors (DBR) in order to develop the world’s first electrically injected semiconductor laser operating in the ultraviolet (UV)-C spectral range, that is, with emission wavelength below 280 nm at a threshold current density below 10 kA/cm^2.
By utilising some of graphene’s critical advantages such as easy transferability due to the weak van der Waals binding to other substrates, its excellent heat and electricity conduction properties and high transparency in the UV-C spectral band, we have developed strategies to resolve several issues that have long hampered research on coherent light emitters in the deep UV. NWs grown by molecular beam epitaxy (MBE) on graphene are nearly dislocation-free, thus eliminating one of the major reasons for the low internal quantum efficiency of planar Al-rich AlGaN. Furthermore, by graphene transfer and hole-mask patterning these NWs can be combined with amorphous substrates, such as e.g. oxide-based DBRs, which are the ultimate solution for an end-cavity reflector, but generally unsuitable for AlGaN epitaxial growth, and fabricated in a precisely ordered 2D array, enabling the creation of a photonic crystal effect for suppressing in-plane light leakage.
Coupling such novel ideas with the project group’s vast experience and expertise on III-V NW growth on graphene and device processing, which has already resulted in the successful demonstration of optically pumped NW-based lasers in the near infrared and GaN/AlGaN UV-A LEDs, we expect to unlock numerous high-impact applications for coherent UV-C light sources in socially significant areas such as material science, biological and chemical sensing, medicine and water purification.