One of the promises of photonic crystals is the dense integration of optical components (such as waveguides and laser sources) on a chip or wafer. However, mechanical fragility has proved to be a limiting factor, especially for fragile air-bridge photonic-crystal structures. Now, Takumi Watanabe and co-workers at Yokohama National University in Japan have demonstrated 100 × 100 μm2 samples that contain 1,089, 2,376 and 11,664 integrated photonic-crystal lasers (Appl. Phys. Lett. 104, 121108; 2014).

Although photonic crystals have been around for over a decade, many demonstrations still employ pulsed pumping. Additionally, very small mode volumes (less than the cube of the wavelength) together with room-temperature, continuous-wave operation were demonstrated only in 2007, according to Watanabe.

Talking to Nature Photonics, Watanabe explained that one of the main reasons for the success of their present approach is the simplicity of the fabrication process, making it suitable for large-scale array integration. Another factor is the use of the air-bridge structure, which provides strong optical confinement. However, such structures are mechanically fragile and are typically unsuitable for large-scale integration. To solve this problem, the team developed resin-mediated bonding to attach a slab to a glass substrate.

Credit: © 2014 AIP

In the present study, the spacing between the lasers was 5 μm, but the team had previously demonstrated independent lasing with a 2.5 μm spacing. Noticeable coupling between neighbouring nanolasers occurs if the spacing becomes much less than about 2 μm; this reduces the Q-factor of the cavities.

The cladding beneath the slab is the adhesive (polydimethylsiloxane), resulting in lower optical confinement than for a usual single photonic-crystal laser with an air-bridge structure. This slightly increases the threshold, according to Watanabe, who notes that in sensing applications the sensitivity would decrease by approximately half relative to that of a conventional air-bridge nanolaser.

Regarding the limitation on the total number of lasers that could be fabricated, Watanabe says that it is mainly a technical limitation. For example, producing the photonic-crystal patterns by electron-beam lithography took about 10 h using one machine for the 11,664-laser array and several days for many arrays. Of course, when lithography is conducted over a long time and over large areas, the pattern accuracy is degraded. Measurement of large numbers of lasers also takes considerable time.

“We use a spectrometer with a one-dimensional InGaAs image sensor and automatic stage control, and it takes 2 h to measure 1,089 lasers,” Watanabe explained. “These problems may be resolved if we use, for example, nano-imprint lithography for the patterning, and batch pumping and measurement using high-power lasers.”