Photonic Crystal Array Lases at Stanford

Breck Hitz

Lasers based on nanocavities in photonic crystals have been widely investigated because of the high density of states associated with these tiny devices, which leads to low lasing thresholds and to high direct-modulation rates. But their drawback to date has been a low output power, typically on the order of nanowatts, which is too low for nearly all applications.

Figure 1. Each nanocavity in the 280-nm-thick photonic crystal membrane produced a separate output beam, but the cavities were coupled so the beams combined coherently and output was not diminished by destructive interference among the individual beams.

Recently, researchers at Stanford University’s Ginzton Laboratory in Stanford, Calif., fabricated an array of multiple nanocavities in an InP material system. The individual nanocavities were phase-locked together to produce a coherent output beam containing two orders of magnitude more power than typically obtained from a single nanocavity. Moreover, theory predicts that such nanocavity arrays can be directly modulated at high speeds, leading the scientists to conclude that these new lasers have the potential to be high-speed, high-power single-mode sources for telecommunications and other applications.

Figure 2. Scanning-electron micrographs show the top surface of the membrane. The image at the top shows a single nanocavity, and the one on the bottom shows an array of nanocavities. One of the experimental lasers comprised a membrane with 81 coupled nanocavities in a 9 × 9 matrix. ©OSA.

They fabricated the array in a 280-nm-thick membrane containing the photonic-crystal structure and four InGaAsP quantum wells (Figures 1 and 2). The periodicity of the holes was 500 nm, and their diameters ranged from 160 to 230 nm in different membranes. Different hole sizes resulted in different resonant frequencies for the nanocavities in different membranes. They designed the cavities to have outputs in the communications band around 1540 nm.

A 9 × 9 array of nanocavities occupied an ~15-µm-square area in the membrane, and the scientists focused 808-nm pump light from a diode laser to the same size on the surface of the membrane.

Figure 3. The coherent beam from the array of nanocavities exhibited a single-mode spectrum. The dashed curve is a Lorentzian fit with a 0.23-nm linewidth. ©OSA.

The pump power was pulsed, with 20-ns-long pulses at a 1 percent duty cycle, to avoid overheating the membrane. They measured a single-mode peak output power of 12 µW with a peak pump power of 2.4 mW. The single-mode spectrum above threshold had a Lorentzian shape (Figure 3).

The researchers compared the performance of arrays of nanocavities with that of single nanocavities. They found that the laser threshold increased with the number of coupled nanocavities, but that the laser efficiency rose more rapidly than the threshold. That implies that more power can be extracted from each nanocavity in an array than from the same nanocavity alone. Indeed, they observed that the maximum power from the nanocavity array laser — more than 12 µW — was about 100 times what a single cavity produced, while the threshold increased only 10 times (Figure 4).

Figure 4. The pump power obtained from each nanocavity in an 81-cavity array was greater than that obtained from a single nanocavity. The scientists estimate that more than half of the 81 cavities were coupled and contributing to the output shown here. The total output from the array was ~100 times that of a single nanocavity, while the threshold power increased by only 10 times. ©OSA.

Comparing their experimental results with those from a computer simulation, they concluded that many of the available 81 cavities in the array were lasing together. They also showed that such a rapid increase in laser efficiency without sacrificing the low threshold is possible only if the coupled lasers have strong cavity effects, as in photonic crystal nanocavities.