- VCSELs Pump Photonic Crystal Defect Lasers
Paula M. Powell
The potential for low-threshold pump power is one reason why photonic crystal de-fect lasers show promise for high-speed optical-interconnect applications. To validate just how low that threshold can go, scientists at the University of Southern California, Los Angeles, have demonstrated room-temperature 1609-nm lasing of two-dimensional crystal defect lasers, with optical pumping done by a top-emitting vertical-cavity surface-emitting laser (VCSEL). The pump power threshold obtained with the structure was 2.4 mW -- a factor some four times smaller than required for previous photonic crystals with similar-size resonant cavities.
VCSEL-pumped crystal defect lasers developed at the University of Southern California use 2-D photonic crystals for in-plane localization and a thin dielectric slab waveguide for vertical confinement. Crystals (left) reside on a suspended dielectric membrane (right).
How it was done
The work was the result of collaboration between the university research groups of John O'Brien, assistant professor of electrical engineering, and P. Daniel Dapkus, professor of engineering. The scientists formed the optical resonant microcavities by using 2-D photonic crystals for in-plane localization and a thin dielectric slab waveguide for vertical confinement. Each photonic crystal had a triangular lattice of airholes to ensure a complete bandgap for a range of TE mode frequencies. It also featured a lattice constant of 550 nm and hole radii of 215 nm. With 19 holes removed from the lattice, the crystal defect cavity was roughly 2.6 µm across the hexagon.
According to the researchers, the InGaAsP material system used to fabricate the typical laser structure presented a twofold benefit. Not only did the system achieve laser operation near 1.55 µm, but the scientists were able to take advantage of the material system's relatively low nonradiative surface recombination rate.
Using a metallorganic chemical vapor deposition process, they grew four 1.2 percent compressively strained InGaAsP quantum wells (10-nm quantum wells separated by 23-nm barriers) on an InP substrate with a total slab thickness of 224 nm. The compressively strained quantum wells promoted the emission of TE-polarized radiation.
During testing of the crystal defect laser at room temperature, the scientists used an 860-nm top-emitting VCSEL to optically pump the membrane defect cavity at normal incidence with a repetition rate of 0.5 to 1 MHz and a 5 percent duty cycle. They relied on a long-working-distance 100x objective lens to focus the pump beam to a spot size of approximately 4.5 µm. The researchers operated the laser in a single mode, although they could observe two side modes with an adjustment of the pump spot size and position.
O'Brien and his colleagues say that there is still plenty of work to be done, but they believe it may be possible to make photonic crystals that radiate like a VCSEL. This probably will require the scientists to combine the crystals in arrays to obtain both a well-behaved far-field pattern and enough output to make them practical.
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