Chi-O Cho

Enhanced light extraction from GaN-based light-emitting diodes with holographically generated two-dimensional photonic crystal patterns

We observed a significant enhancement in light output from GaN-based light-emitting diodes (LEDs) in which two-dimensional photonic crystal (PC) patterns were integrated. Two-dimensional square-lattice air-hole array patterns with a period that varied from 300 to 700 nm were generated by laser holography. Unlike the commonly utilized electron-beam lithographic technique, the holographic method can make patterns over a large area with high throughput. The resultant PC-LED devices with a pattern period of ∼500nm had more than double the output power, as measured from the top of the device. The experimental observations are qualitatively consistent with the results of three-dimensional finite-difference-time-domain simulation.

GaAs-based near-infrared omnidirectional reflector

We introduce a compound-semiconductor-based omnidirectional reflector. A four-layer-pair stack of GaAs/AlAs was grown epitaxially using molecular-beam epitaxy, and was then converted to a GaAs/Al2O3 multilayer stack by selective oxidation of the AlAs layers. The resultant one-dimensional photonic crystal exhibited omnidirectional reflection properties in near-IR wavelength range below 1 μm. Reflectance spectra measured at various incidence angles and polarizations were observed to be in good agreement with theoretically simulated results.

Photonic Crystal Slab Waveguides Fabricated by the Combination of Holography and Photolithography

We have fabricated air-bridge type two-dimensional photonic crystal waveguides (2D-PCWs) using two high-throughput processes only: holography and photolithography. Despite the existence of misalignments of a defect line with respect to the air-hole arrays, waveguiding in both straight and 90°-bent PCWs was clearly observed at the wavelength of 1.55 µm. We also estimated the upper bound in propagation loss of our straight PCWs, which was measured to be about 40 cm-1.

Photonic crystal band edge laser array with a holographically generated square-lattice pattern

We fabricated a photonic band edge laser array based on a two-dimensional square-lattice photonic crystal (PC) slab waveguide using a laser holography (LH) method, instead of the commonly used electron-beam lithography (EBL). The nature of the LH process enabled high-throughput large-area fabrication of band edge lasers. Moreover, the laser performance was comparable to that of reported EBL counterparts. Careful examination of the spectral positions of the observed modes with respect to the calculated photonic band structure identified three main band edge modes as the origins of lasing: M2, X2, and M1. Owing to the gradual change in the air-hole size of the PC, the lasing modes shifted monotonically along the laser array, resulting in an M1 mode span of ∼30nm (centered at 1550nm) over a distance of 5mm.

Extraction efficiency enhancement in GaN-based light emitters grown on a holographically nano-patterned sapphire substrate

We observed over two-fold enhancement in light extraction efficiency from GaN-based light emitters grown on a nano-patterned sapphire substrate. Nano-hole array patterns were generated by laser holography, which enabled a large area process with high throughput.