We have fabricated photonic crystal nanocavity lasers, based on a high-quality factor design that incorporates fractional edge dislocations. Lasers with InGaAsP quantum well active material emitting at 1550 nm were optically pumped with 10 ns pulses, and lased at threshold pumping powers below 220 m W, the lowest reported for quantum-well based photonic crystal lasers, to our knowledge. Polarization characteristics and lithographic tuning properties were found to be in excellent agreement with theoretical predictions.
The quest for a compact nanocavity laser, with highquality factor (Q) and small mode volume (Vmode), has been a central part of research in the fifield of integrated optics. Photonic crystals,and planar photonic crystals in particular (PPC) , are promising manufacturable geometries for the realization of compact optical nanocavities and their integration with waveguides, modulators, and detectors. So far, there have been several reports on room-temperature lasing in PPC nanocavities, and more recently, new high-Q cavity designs based on modifification of two-dimensional (2D) photonic crystals have been proposed. In this letter, we report the experimental application of one of these designs. The cavities are based on fractional edge dislocations,and are used for the construction of a low-threshold laser in which the high fifield from the laser surrounds a void for chemical sensing or strong coupling to atomic light sources.
The structures were optically pumped using 10– 30 ns long pulses (periodicity 1 μm) from a semiconductor laser diode (l pump5 830 nm). The pump beam was focused through a 1003 objective lens onto the sample surface to obtain a spot size of about 3 μm. The emission from the cavities is collected through the same lens, and the spectrum of the emitted light signal is detected with an optical spectrum analyzer. An additional flflip-up mirror is used to obtain the optical images from the excitation pump spot and the cavity modes.
As the fifirst step, we have measured the emission from the unprocessed InGaAsP material. We have found that emission exists between 1300 and 1650 nm, with a maximum at around 1550 nm. This wavelength range corresponds to normalized frequencies of a/λP ∈0.264, 0.335! , which is within the band gap of the bulk photonic crystal mirrors surrounding the cavity. Next, we tested all six cavities (Fig. 2) in order to measure their resonant modes. We have found two prominent resonant peaks in the emission range of our InGaAsP material, and observed that these two modes are linearly polarized, but have orthogonal polarization (Fig. 3) . This is in excellent agreement with our 3D FDTD analysis that shows that two orthogonally polarized modes (LQ polarized along the y axis and HQ along the x axis) exist in this wavelength range (Fig. 3) . We have also found that the position of these resonances depends strongly on the value of the elongation parameter p (Fig. 4), as predicted in our earlier publication.8,9 Moreover, theory predicts that the mode at longer wavelengths (HQ) should have much higher Q values than the one at shorter wavelengths (LQ) . This was con- fifirmed in our experimental measurements, and Q values of approximately 2 000 were found in the case of HQ modes (p5 cavity! , while Qs of only several hundreds were measured in the case of LQ modes.
In conclusion, we have observed room-temperature lasing from high-Q cavities based on fractional edge dislocations in triangular lattice PPCs. Lasing is observed from the high-Q dipole mode of this nanocavity. In spite of the unusual design of our structures, which have a hole etched through the position of maximum fifield intensity and therefore reduced overlap with gain material, we observe lowthreshold powers in our devices. We have attributed this to the small mode volume and the high-Q factors inherent to our device design. Polarization and lithographic tuning properties of high- and low-Q modes are in an excellent agreement with theoretical FDTD predictions. The mode profifile taken by our IR camera shows that the lasing resonance is well localized to the center of our cavity. Based on these experimental results, we conclude that the observed lasing corresponds to the high-Q mode of our fractional edge dislocation cavity.
Fig1
The authors would like to thank William Green from Caltech for many valuable discussions. This work was supported by the NSF under Grants Nos. ECS-9912039 and DMR-0103134, and the AFOSR Contract No. F49620-01-1- 0497. Two of the authors （P.G. and Y.Q.） would like to acknowledge the partial support from the Cross Enterprise Technology Development Program at the Jet Propulsion Laboratory （under a contract with the National Aeronautics and Space Administration).
