Edge-Emitting Photonic Crystal Nanolaser

Breck Hitz

Tiny resonators and waveguides based on photonic crystals provide a promising approach to fabricating high-density photonic integrated circuits. Recently, collaborating scientists at the University of Southern California, Los Angeles, at the University of Texas at Austin, and at the University of Central Florida, Orlando, have demonstrated what they believe is the first edge-emitting photonic crystal nanocavity laser.

Figure 1. The nanocavity laser and output waveguide were fabricated in the same monolithic structure. Images reprinted with permission of Optics Letters.

Achieving edge emission is important because all previously reported photonic crystal nanocavity lasers have been surface emitters, but surface emission is not readily compatible with planar photonic circuits. An edge emitter, on the other hand, can be coupled easily to other components in the same planar circuit. As an initial demonstration of this capability, the scientists coupled the output of their laser into a photonic crystal waveguide that they monolithically fabricated in the same semiconductor crystal as the laser.

The device consisted of four photonic crystal sections, each with a slightly different lattice constant (Figure 1). The scientists embedded five layers of InAs quantum dots, each with a quantum dot density of about 200/μm², in the 220-nm-thick GaAs membrane. The lattice constant in the double-heterostructure cavity section of the device was 343 nm, whereas in the mirrors the lattice constant was 335 nm. In the butt-coupled output waveguide, the lattice constant was 351 nm. Throughout the structure, the hole-size: lattice-constant ratio was 0.3.

Figure 2. The laser spectrum was a narrow peak at ~1331 nm. Data shown in this article were taken with 16-ns pump pulses at a 1 percent duty cycle.

The mirror on the right had four fewer periods than the one on the left, essentially making it the output coupler. The scientists calculated that the resonator Q could be as high as ~10⁵ when 80 percent of the laser output emerged through the mirror on the right if the mirrors were nonabsorbing. The semicircular structure at the waveguide’s output facet helped collect the output light.

The scientists focused the 850-nm pump radiation from a diode laser onto a ~1.25-μm spot on the photonic crystal with a microscope objective. The same objective collected luminescence from the device and revealed the laser spectrum as a narrow spike at ~1331 nm (Figure 2).

In a practical device, it could be difficult to maintain precise alignment between the pump and the nanocavity, so the scientists investigated the effect of misaligning the pump beam. They observed that, even with significant misalignment, nonzero output power could be obtained from the laser (Figure 3).

Figure 3. Even when the pump beam was misaligned from the cavity section of the laser (as indicated by the green, red and black circles in the photograph at left), significant output could be obtained from the laser (right). The colors of the data points in (b) correspond to the pump spot locations in (a). Inset: The spectrum was essentially unchanged when the pump spot moved from the black position to the green position, although the pump power was increased from 0.6 mW to 6.25 mW to obtain the same power.

By observing the intensity profile of radiation emerging from the output facet of the waveguide, the scientists established that at least part of the laser radiation from the nanocavity’s edge emission was coupled into the waveguide. In the future, they want to refine the structure’s design to ensure that coupling into the waveguide is the laser’s dominant loss mechanism.

Subsequent to the publication of their Optics Letters paper, the scientists fabricated similar devices with InGaAsP quantum wells and observed as much as 100 μW of direct edge emission. They believe that this power level, significantly higher than has ever been generated from a photonic crystal nanocavity laser, will enable transmission of high-speed data at low error rates.

Optics Letters, May 1, 2007, pp. 1153-1155.