Photonic Crystal Laser Features Hexagonal Ring Resonator
Daniel S. Burgess
A team at Korea Advanced Institute of Science and Technology and at Electronics and Telecommunications Research Institute, both in Taejon, Korea, has demonstrated a photonic crystal laser with a hexagonal waveguide ring resonator. The photonic crystal structure improves the optical confinement of the device, potentially enabling higher output powers and minimizing loss.
The optically pumped photonic crystal laser with an 8-µm-diameter hexagonal waveguide ring resonator displays a qualilty factor of greater than 2000 and a lasing threshold of 3mW at room temperature. Courtesy of Se-Heon Kim.
The optically pumped InGaAsP/InP laser features 410-nm-diameter holes and a lattice constant of 570 nm, with the absence of holes forming the waveguide sides of the 8-µm-diameter hexagonal cavity. Se-Heon Kim, a graduate student at Korea Advanced Institute of Science and Technology and the lead author of a paper describing the device, noted that the triangular lattice not only lowers loss in the bending region, but also has a wider bandgap for TE polarization than square lattices, making it well-suited for the preferential confinement of the photons that the strained InGaAsP quantum wells in the device produce.
The researchers used electron-beam lithography to pattern the surface and ion-beam etching with a poly(methylmethacrylate) mask to write the features into the wafer. A 980-nm laser diode served as the pump source for the device, with the output focused through a microscope objective to cover the entire hexagon. The objective also collected the emitted radiation from the structure and directed it to a CCD camera for analysis.
Plotting the pump power vs. output at room temperature, the researchers estimated a lasing threshold of 3 mW at an absorbed pump-power density of approximately 1.2 kW/cm2. A spectral analysis of the output at near the transparency pumping condition suggested that the quality factor of the device is greater than 2000, leading to a total propagating and bending loss of <260 cm–1.
Although the laser was designed as an in-plane light source for a potential optical integrated circuit, the researchers suggest that it may play a role in the design and evaluation of such components. "Our ring cavity gives a simple, bending-effect estimation tool," Kim said. "Many people have tried to find an efficient waveguide-bending design. But our laser cavity consists of waveguide sections; thus, by investigating the spectral properties, one can find an efficient bending design."
The team, continuing its investigation of the design, is pursuing means of coupling the output of the laser to adjacent waveguide structures in the photonic crystal.
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