Ultraviolet (UV) lasers and light emitting diodes (LEDs) have potential applications in high-density optical storage, high-resolution laser printing, solid-state lighting and display. We are employing the photonic band structures to reduce the threshold of UV lasers and to increase the efficiency of UV LEDs. In collaboration with Professor R.P.H. Chang's group in the Materials Science and Engineering Department of Northwestern University, we have developed various techniques to fabricate two-dimensional and three-dimensional photonic crystals with a wide-band-gap semiconductor ZnO.

Recently we have realized the first UV photonic crystal lasers in photonic crystal slabs under optical pumping. Due to the short wavelength, structural disorder that is introduced unintentionally during the fabrication process has a significant effect on the fundamental band gap. We propose a new approach, i.e. to utilize high-order band structures. This approach increases the feature size and reduces the amount of lattice defects. Our latest experiments demonstrate effective and robust lasing associated with high-order flat bands of ZnO inverse opals as compared to lasing in the fundamental gap.

Owing to the large exciton binding energy, ZnO exhibits strong exciton emission even at room temperature. However, the non-directionality of exciton emission limits the efficiency of UV LEDs. We are using the photonic band structures to control the directionality of exciton emission in ZnO. An incomplete photonic band gap can inhibit exciton emission in undesirable directions so that the energy is redistributed to the desired directions.

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