Photonic Crystal Laser Features Low-Threshold Pumping
Paula M. Powell
Working with a cavity originally designed for cavity quantum electrodynamic experiments and nanospectroscopy, scientists at California Institute of Technology and the Jet Propulsion Laboratory, both in Pasadena, have observed lasing from photonic crystal nanocavity lasers at threshold pump powers below 220 µW. This may be the lowest threshold reported so far for quantum-well-based photonic crystal lasers, according to Marko Loncar of Axel Scherer's nanofabrication group at the institute.
The central row of this photonic crystal laser nanocavity is dislocated by 25 percent. The defect hole is significantly smaller than surrounding photonic crystal holes.
The laser design used in the project has a simple triangular lattice single-defect cavity with fractional edge dislocation, as proposed by Jelena Vuckovic, now a professor at Stanford University in California. Scientists lengthen the row containing the cavity defect hole by moving the two crystal half planes a fraction of a lattice constant apart. This introduces a dislocation of a specific width that can be tuned to boost cavity quality (Q) factors -- in one case as high as 30,000 -- while maintaining a very small mode volume.
The researchers fabricated laser structures in InGaAsP quantum-well active material that emits at 1550 nm. Optical pumping with 10-ns pulses from a semiconductor laser diode induced lasing. Based on experimental results from several structures, Loncar and colleagues conclude that lasing corresponds to the high-Q mode of the cavity.
One would think, though, that a hole at the point of maximum field intensity would not lead to optimum performance in low-threshold laser designs, because this reduces the overlap with the gain region (quantum wells). The researchers thus attribute the low-threshold powers observed with the cavity design to the small mode volume and high Q factor. They expect tweaking of the cavity design to yield further improvements in laser performance and to expand potential applications.
"An interesting feature of our laser is the presence of the airhole at the position of maximum optical field intensity," Loncar said. "Therefore, the lasers are suitable for nanospectroscopy applications, where interaction between light and material placed in the hole is investigated. For example, we plan to use our laser as a gas sensor in order to detect small quantities of gas. By changing the geometry of our structures, we can design the lasers to operate at different wavelengths, each of which could be used to detect [the] presence of a different gas. Due to the compact size of our lasers, we can integrate many devices on a small area, and in that way form compact gas sensors."
The nanolasers also may find use as compact light sources in telecommunications. Because laser resonant modes are dipolelike and the in-plane radiation is highly directional, the scientists have integrated several lasers spaced less than 5 µm apart and operating at different wavelengths. The emission wavelength is defined lithographically by changing the size of the central defect hole, a useful property for designing compact multiple-wavelength sources for wavelength division multiplexing applications. Low power consumption and compact size also make the devices good candidates for applications like low-power, on-chip, optical clock distribution for semiconductor microprocessor chips.
Poor heat dissipation still limits device operation to a pulsed regime with small duty cycles. One possible solution, Loncar said, is to make the lasers on low-dielectric-constant material with good thermal conductivity, such as sapphire.
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