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Photonic crystal microcavity lasers are potentially attractive optical sources for future communication systems. They operate at lithographically defined wavelengths and because of their small volumes they are expected to exhibit low operating powers. Much work remains to be done, however, in order for these sources to find mainstream applications. In this presentation we will report on our work on optically pumped photonic crystal lasers. Finite-difference time-domain and finite element simulations will be presented as part of a discussion of the resonant cavity design. The trade-offs in the design of photonic lattice hole radius and membrane thickness will also be included, and we will discuss strategies for minimizing the optical loss in these cavities. The photonic crystal laser cavities reported here are defined by electron beam lithography in pmma. The pmma is subsequently used as a mask to transfer the pattern into a Cr/Au layer in an ion beam milling step. This patterned metal layer is then used as a mask for a reactive ion etch that patterns a silicon nitride layer. Finally this layer is used as a mask to transfer the lattice into the InGaAsP semiconductor using an ECR etching step. Suspended membranes are formed by chemically undercutting the lattice. This provides strong optical confinement at the semiconductor/air interfaces at the top and bottom of the cavity.
We have demonstrated pulsed, optically pumped lasing at and above room temperature in these resonant cavities using a semiconductor diode laser as the pump. The resonant cavity in our demonstration is formed by removing 19 holes from a triangular lattice and is about 2.6 mm across. Incident threshold pump powers for this cavity size as low as 0.5 mW have been demonstrated at room temperature. The peak output power collected through an optical fiber is approximately 2 mW. Lasing is seen for pump pulses as long as 200 ns. We have also demonstrated lasing in these cavities at elevated substrate temperatures. This demonstration was done using an 860 nm top emitting VCSEL as the pumping source because we expect it to provide a direction towards monolithic, electrically addressable lasers. Input power versus output power lasing characteristics for substrate temperatures up to 50 °C have been obtained. We will also report on our work on lithographic fine-tuning of the lasing wavelength. This wavelength can be defined through the lattice constant or the hole radius. This feature of photonic crystal lasers allows the definition of multiwavelength arrays. We have built and characterized arrays in which the lattice constant varies 2 nm steps across the array. The lasing wavelength redshifts with increasing lattice constant with an average separation between adjacent lasing wavelengths of 4.6 nm. The lasing wavelength tunes through the gain spectrum before the laser mode hops. Finally, we will present data on the optical loss in these cavities obtained by varying the number of lattice periods. We observed a reduction in incident threshold pump powers with increasing number of lattice periods at least through 11 periods.
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