On-going Projects

Color-tunable LEDs- microLED approach

Development of an LED with color-tuning capabilities based on a group-addressable micro-LED architecture, with individual micrometer-scale RGB pixels coated by jet-printing.

US patent application 11/780,082; International PCT application PCT/CN2007/002208

Color-tunable LEDs- stacked appraoch

LED chips of the primary colors are physically stacked on top of each other.  Light emitted from each layer of the stack passes through each other, and thus is mixed naturally without additional optics.

US patent application 12/572,345; International PCT application PCT/CN2009/001113

OSA Optics Express 17, 9873 (2009)

Photonic Crystals by Nanosphere Lithography

Two-dimensional photonic crystal consisting of a hexagonal array of triangular air-holes created on the surface of a GaN LED substrate using microsphere lithography.  A twofold increase in electroluminescence was observed from the PhC LED compared to an as-grown LED was identical geometry.

Applied Physics Letter 95, 133125 (2009)

Nanotechnology 19, 255302 (2008)

Photonic Crystal Bandedge Lasing

An ordered GaN nanopillar array fabricated by nanosphere lithography exhibited room temperature photopumped lasing via the photonic crystal band-edge effect.

Journal of Applied Physics 107, 063104 (2010)

350 nm Ultraviolet Nanopillar Cavity Lasing

A vertical cavity structure composing of an in situ grown bottom AlxGa1-xN/ AlyGa1-yN distributed Bragg reflector and a top SiO2/ HfO2 dielectric mirror for ultraviolet (UV) emission has been fabricated.  Optically-pumped UV lasing at a wavelength of 343.7 nm (3.608 eV) was observed at room temperature, with a threshold laser pulse energy of 100 μJ.

Applied Physics Letters 96, 241101 (2010)

Geometrical Shaping of LEDs by Laser Micromachining

Enabled by laser micromachining, LED chips of various 2-D and 3-D geometries have been demonstrated.  Such geometrical structuring is beneficial to light extraction and control of emission pattern.

IEEE Photonics Technology Letters 21, 1078 (2009)

IEEE Photonics Technology Letters 22, 513 (2010)

Journal of Applied Physics 108, 023110 (2010)

Vertically Mounted LEDs

An InGaN/GaN light-emitting diode (LED) chip mounted in a vertical configuration (vmLED) is demonstrated exhibiting significant enhancement to light extraction compared to an LED mounted in a conventional planar geometry.  By flipping the chip orthogonally, two large illumination surfaces of the device are exposed for direct light extraction.  Comparisons, through ray-trace modeling and experiment data with conventional surface-mounted LEDs, indicate that the vmLEDs achieves superior light extraction efficiency.  A sapphire-prism mounted vmLED is further proposed to improve heat-sinking, well-suited for higher current operations.

IEEE Tran Elect Dev (2011)

White LEDs using Fluorescent Microspheres

Using fluorescent microspheres as color-conversion agents, uniform and high-brightness white light emission has been demonstrated.  This method of white light generation provides an optical spectrum with high spectral purity, luminous efficacy, high CRI, variable CCT and overcomes all the existing IP issues associated with phosphor conversion.

Optics Express 16, 13 (2008)

Free-standing GaN microdisks Pivoted on Si Substrates

Whispering-gallery mode lasing has been demonstrated in pivoted microdisk structures fabricated on GaN grown on Si substrates, representing a practical solution towards GaN microcavity devices.

Applied Physics Letters 89, 211101 (2006)

Nanostructuring by Microsphere Lithography

We demonstrate the fabrication of ordered array of nano-pillars, based on the controllable and economical technique of microsphere lithography.  PL measurements revealed that the presence of nano-pillars enhanced the light emission intensity by about 230% due to increase in light extraction efficiency.

Journal of Vacuum Science and Technology B 26, 76 (2008)

Laser Micromachining

Laser micromachining, based on a pulsed UV laser at 349 nm, is being developed in our group for the direct patterning of microstructures onto optical materials such as GaN and sapphire.  Devices with laser-micromachined structures have been demonstrated.

Micro-structured Honeycomb LEDs

An optimized design for a micro-light-emitting diode array is presented.  Based on a honeycomb hexagonal closed-packed structure, the packing density of individual elements can be increased to over 80% to minimize wastage of junction area.  The honeycomb LED emits twice as much light compared to a conventional broad-area LED with equal junction area, and 50% more light than a microdisk array LED. 

Journal of Vacuum Science and Technology B 24, 800 (2006)