Small lasers a big topic in nano-photonics
In recent years there have been significant advances in the size and characteristics of small lasers, i.e. lasers with dimensions or modes sizes close to, or smaller than, the wavelength of emitted light. This work has primarily been led by innovative use of new materials and cavity designs.
In a recently published review article in the journal Nature Photonics  Martin Hill (University of Western Australia) and Malte Gather (University of St Andrews) analysed the progress that has been made over the last few decades in the development of small lasers. Both the development time scales and size scales for the various laser types are shown, put in context and compared, in order to clarify how the magnitude and speed of miniaturisation in lasers is occurring.
The most dramatic progress has been in the emergence of lasers made from small metallic structures, as well as refinements in dielectric cavity lasers and the beginning of their penetration into new areas such as biology. Via the use of a simple laser model, it can be seen that small lasers based on dielectric and metallic cavities use different strategies to reduce the size of lasers.
In general, small dielectric lasers employ cavities with long photon lifetimes to reduce demands on the laser gain medium. In contrast, metallic cavities typically have much shorter photon lifetimes, often due to absorption in the metal. Increased confinement of the optical mode to the gain medium does however provide a design window in which lasing can occur in the metallic structures, though the gain medium is still often pushed to its limit.
By analysing results from many publications on small lasers, Hill and Gather showed that, typically, dielectric small lasers have cavities with dimensions and volumes greater than the wavelength of light, and quality factors greater than 1,000 whereas the cavities of metal based small lasers can be smaller than the wavelength of light, and have quality factors less than 1,000.
Besides providing tight confinement, the metal structures in metallic and plasmonic lasers also provide good heat-sinking and a pathway for electrical pumping, which has contributed to the rapid development of electrically pumped devices operating at room temperature. Additionally, the ever smaller lasers have stimulated useful debate in the following areas:
- Propagation of light and gain in small, lossy dispersive structures;
- Spontaneous vs stimulated emission and the characteristics of lasers with cavity sizes well below the diffraction limit;
- Increasing the maximum optical gain available from various gain media.
A key issue to be solved for very small dielectric and metallic lasers will be device lifetime. The small gain regions in these devices have large surface to volume ratios and their fabrication often involves etching which can introduce surface defects and surface recombination that accelerate device aging. Small metallic and plasmonic lasers also require substantial improvement in room temperature threshold current and efficient out-coupling of light or plasmons into either free space or a (plasmonic) waveguide.
“We are employing the microelectronic fabrication facilities at the West Australian node of the ANFF, in particular Reactive Ion Etching, Plasma Enhanced Chemical Vapour Deposition, and metal deposition equipment, to produce new plasmonic laser structures which, in theory, address the above mentioned issues.” Hill said.
Interest in making lasers smaller does not seem to have been discouraged when established laser concepts approached the conventional diffraction limit; on the contrary, recent years have clearly seen a very significant interest in making ever smaller and lower power lasers. Concrete, high-impact applications where small size and low power are of key importance are starting to emerge, particularly in short distance communications.
To lay the groundwork for additional applications, continued research will be necessary in small lasers, in their fundamental properties and constituent materials, and in related research areas that depend on small lasers and optical amplifiers such as plasmonics and nano-photonics. Having access to facilities provided by the ANFF will play a vital role in the development of this work.
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