Hung-Ta Chien

Improved output power of GaN-based light-emitting diodes grown on a nanopatterned sapphire substrate

This letter describes the improved output power of GaN-based light-emitting diodes (LEDs) formed on a nanopatterned sapphire substrate (NPSS) prepared through etching with a self-assembled monolayer of 750-nm-diameter SiO2 nanospheres used as the mask. The output power of NPSS LEDs was 76% greater than that of LEDs on a flat sapphire substrate. Three-dimensional finite-difference time-domain calculation predicted a 40% enhancement in light extraction efficiency of NPSS LEDs. In addition, the reduction of full widths at half maximum in the ω-scan rocking curves for the (0 0 2) and (1 0 2) planes of GaN on NPSS suggested improved crystal quality.

Focusing of electromagnetic waves by periodic arrays of air holes with gradually varying radii

We study theoretically the focusing effects when a Gaussian beam passing through a graded photonic crystal consisting of air holes with gradually varying radii. The device is used to couple the light between the conventional and photonic crystal waveguides. The coupling efficient is about 4 times higher than that by the linear taper structure.

Output power enhancement of light-emitting diodes via two-dimensional hole arrays generated by a monolayer of microspheres

The output power enhancement of the GaN-based light-emitting diodes (LEDs) featuring two-dimensional (2D) hole arrays is demonstrated. The 2D air hole arrays were first generated in the photoresist by utilizing the focusing nature of microspheres, and then transferred onto the GaN surface through dry etching. The maximum output power of the surface-textured LEDs was enhanced by 45% compared with the LEDs without surface texturing. The finite-difference time-domain calculation was performed and revealed that the light extraction efficiency of the textured LEDs increased with increasing etching depth.

The Comparison Between the Graded Photonic Crystal Coupler and Various Couplers

The coupling between a dielectric waveguide and a photonic crystal waveguide is studied theoretically and experimentally. The coupling methods studied in this work include graded photonic crystal coupler, photonic crystal taper, waveguide taper, and parabolic mirror coupler. The properties of each coupler are discussed and compared.

Patterning Periodical Motif on Substrates Using Monolayer of Microspheres: Application in GaN Light-Emitting Diodes

In this article, we present a method of patterning a GaN substrate using polystyrene spheres and its inversed structure as the mask. By dry etching, different surface morphologies and depths of the GaN substrate can be obtained by changing the etching time. We used such a sphere-patterned GaN substrate to fabricate GaN-based light-emitting diodes (LED). It was found that the total lighting output of the sphere-patterned GaN LED was increased by 37%.

Directional Coupler Formed by Photonic Crystal InAlGaAs Nanorods

In this paper, we demonstrate the use of photonic crystal (PC) directional couplers to separate light of wavelengths at 1.31 and 1.55 m. The PC structure consists of InAlGaAs nanorods arranged in hexagonal lattice. The simulation of our device is implemented by the finite-difference time-domain method. The devices are fabricated by e-beam lithography and conventional photolithography. We use the strong inverse method (SIM) of e-beam lithography to make the pattern smoother. The measurement results confirm that 1.31/1.55-m wavelength splitter can be realized in PC structures formed by nanorods.

Two-dimensional photonic crystal beamsplitters

We demonstrate a beamsplitter in 2-dimensional square-lattice photonic crystals with two orthogonal line defects, and a point defect placed in the intersection of the two line defects. This structure divides the input power into each output ports. The size and position of the point defect has been varied to observe the related output power. The 2/spl times/2 beamsplitter can divide input power into each output port identically. This beamsplitter may be used in photonic crystal Mach-Zehnder interferometers or switches.