The demonstration of a photonic-crystal surface-emitting laser (PC-SEL) that operates in the blue, at the shortest wavelength reported so far, could be good news for the development of large-area visible lasers with custom-designed beam shapes (Science 319, 445–447; 2008).

Credit: © 2008 AAAS

Photonic-crystal surface-emitting lasers combine a light-generating active semiconductor region with a two-dimensional photonic-crystal structure to create a laser cavity with unique properties. For example, the periodicity and size of the photonic crystal determines the laser's beam shape, enabling the creation of so-called doughnut beams and other patterns. Another virtue of the design is that the laser emits light from its top surface, so that large emitting areas are possible while retaining a single longitudinal and lateral mode. However, until now operation of PC-SELs has been limited to the infrared and wavelengths longer than 980 nm.

Now, Susumu Noda and co-workers from Kyoto in Japan have made a GaN PC-SEL and report lasing in the blue at a wavelength of 406 nm. The electrically driven laser operates at room temperature under pulsed conditions, and emits around 0.7 mW of blue light when a drive current of about 9 A is applied. The laser consists of a layer of InGaN multiple quantum wells (MQW), which are grown over the top of a GaN/air two-dimensional photonic-crystal structure and surrounded by cladding layers and electrodes.

Although the laser's threshold current of 6.7 A and its submilliwatt output powers are not ideal, Noda told Nature Photonics that the performance can be substantially improved by optimizing the MQW region and photonic-crystal-structure geometry, and by using a transparent or ring-shaped top electrode.

Noda envisages potential applications ranging from information storage and biology to materials processing. “These PC-SELs can be utilized as super-resolution light sources that could be focused to a spot much smaller than blue-violet wavelengths by using doughnut beams. This could be applied to next-generation DVDs,” commented Noda. “In addition, by using the merit of large-area coherent oscillation, very-high-power blue-violet laser sources can be achieved while keeping a single longitudinal and lateral mode.”