Compared with bulky gas lasers and fiber lasers, semiconductor lasers have the advantages of small size, high energy efficiency, high coherence, and high controllability. However, the use of semiconductor materials as the working material to produce excited emission of the laser, but also has its own inherent defects: poor temperature characteristics, easy to generate noise, the output light dispersion serious. One of the consequences of these defects is that - it is difficult to achieve the brightness level for industrial-grade cutting of thick steel and so on.
Photonic crystals are composed of regular, nanoscale inflatable holes perforated in a semiconductor sheet. Photonic crystal lasers are one of the "potential players" in the field of high-brightness lasers, but until now, engineers have not been able to bring them up to provide beams bright enough to be used for actual metal cutting and processing. Researchers have been working to optimize the performance of semiconductor lasers, including power conversion efficiency, output power, beam quality, laser energy level, spectral characteristics, size, robustness to undesirable noise and thermal management, reliability, etc. (Note: Brightness is a measure of laser output power and beam quality, which encompasses the degree of focus and divergence of a beam of light. (The threshold value for metal processing is about 1 gigawatt/cm2.)
The above-mentioned research team led by Academician Susumu Noda has accumulated more than 20 years of research experience in PCSEL development. In terms of concrete results: they were able to develop a laser with a diameter of 3 mm, which is a 10-fold increase in area over the previous PCSEL devices with a diameter of 1 mm. The output power of this innovative laser is 50W, which is a significant increase compared to the 5-10W output power of 1mm PCSELs. The brightness of this new laser is approximately 1GW/cm2/str, which is sufficient for a range of applications currently dominated by gas and fiber lasers, such as precision smart manufacturing in the electronics and automotive industries. This high brightness level is also sufficient for more specific applications such as satellite communications and satellite propulsion.
In increasing the size and brightness of photonic crystal lasers, a number of challenges are encountered. Specifically, semiconductor lasers encounter bottlenecks when their emission area is expanded: a wider laser area means that there is room for continuous oscillations of light in the emission direction and laterally, and these lateral oscillations (known as Higher-order Modes/Higher-order Modes) destroy precisely the quality of the beam. In addition, if the laser undergoes continuous operation, the heat inside the laser changes the refractive index of the device, leading to further deterioration of the beam quality.
The key breakthrough point brought by Susumu Noda's research team is that they have embedded photonic crystals in the laser and modified the internal reflection layer to enable single-mode oscillation over a larger area and to compensate for thermal damage. These two changes allow the PCSEL to maintain high beam quality even during continuous operation.
In a typical photonic crystal laser, these cavities, which have a different refractive index than the surrounding semiconductor, deflect the light inside the laser in a precise manner. And Susumu Noda's research team designed the pattern of holes in the crystal so that the light would be deflected by a set of circular and elliptical holes that remain one-quarter of a laser wavelength away from each other. Eventually, these deflections cause losses in the higher-order patterns, resulting in a high-quality beam with almost no divergence.
This concept is good enough for a 1 mm laser, but extending it to a 3 mm area requires further innovation. To achieve single-mode oscillation over a larger area, the researchers adjusted the position of the reflector at the bottom of the laser, which caused more unwanted mode loss in the vertical direction.
His team has established the Center of Excellence for Photonic Crystal Surface Emitting Lasers at Kyoto University, which covers an area of 1,000m2, and more than 85 companies and research institutes are involved in the development of PCSEL technology. The team is industrializing their PCSEL design for mass production.
As part of this process, they have completed the conversion from electron beam lithography for photonic crystals to nanoimprint lithography for photonic crystals. Electron-beam lithography is very precise, but usually too slow for mass production. Nanoimprint lithography, which basically stamps a pattern onto a semiconductor, is valuable for creating very regular patterns quickly.
Noda explained that in the future the team will further expand the diameter of the laser from 3 mm to 10 mm, a size that could produce 1 kW of output power, although this goal could also be achieved by using an array of 3 mm PCSELs. He anticipates that the same technology as the 3 mm device could be used to scale up to 10 mm (which is expected to produce a 1 kW beam), and that using the same design would be sufficient.
