We have investigated the lasing and output coupling characteristics of two-dimensional photonic crystals. The lasers/couplers are experimentally realized with organic semiconductors and involve photo-excitation of patterned combinations of organic semiconductor and dielectrics (e.g., SiO2). The patterning is accomplished by advanced optical lithography or electron-beam lithography followed by dry etching and the subsequent deposition of the organic gain medium as a thin coating. We show that by employing advanced optical lithography, which is driven by silicon electronics, feature sizes required for photonic crystals that operate in the visible can be attained. Organic media are also very convenient in investigating the basic physics of novel resonators and optical structures [1]. Most of the photonic crystals employed in this work do not possess a complete gap; however theoretical extensions of this work to systems with a complete bandgap will be discussed along with some experimental data.
The basic principle employed in our designs is based on our recent discovery that the output coupling characteristics of two-dimensional gratings (or PC’s) are significantly different from one-dimensional gratings. Since we do not employ the lowest energy bandstructure features to provide the feedback necessary for laser oscillation, phase matching conditions result in diffractive coupling of some of the laser emission out of the plane of the waveguide. In two-dimensional PC’s, phase matching conditions result in the coupling of light to one or a discrete number of directions instead of a cylindrical wave. We have designed numerous combinations of lasers and couplers which have the potential to couple to a single spot normal to the plane of the waveguide.
One design that has been successfully implemented is a square two-dimensional PC, which functions as a laser and output coupler. Detailed theoretical calculation show that a careful study of band-structure features is required to attain an accurate understanding of mechanism of laser action in two-dimensional photonic crystals. Laser action occurs at points in the Brillouin zone where the density of photon states is peaked and where the photon group velocity goes to zero. Laser action does not necessarily take place at band extrema; occurring at saddle points were the density of states is sometimes higher. We have also found a variety of ways to combine lasers and couplers resulting in composite devices with interesting and potentially useful optical properties.
[1.] M. Meier et al., Appl. Phys. Lett. 74, 7 (1999).