Sy-Hann Chen

Using a slightly tapered optical fiber to attract and transport microparticles

We exploit a fiber puller to transform a telecom single-mode optical fiber with a 125 microm diameter into a symmetric and unbroken slightly tapered optical fiber with a 50 microm diameter at the minimum waist. When the laser light is launched into the optical fiber, we can observe that, due to the evanescent wave of the slightly tapered fiber, the nearby polystyrene microparticles with 10 microm diameters will be attracted onto the fiber surface and roll separately in the direction of light propagation. We have also simulated and compared the optical propulsion effects on the microparticles when the laser light is launched into a slightly tapered fiber and a heavily tapered (subwavelength) fiber, respectively.

InGaN-based light-emitting diodes with an embedded conical air-voids structure.

The conical air-void structure of an InGaN light-emitting diode (LEDs) was formed at the GaN/sapphire interface to increase the light extraction efficiency. The fabrication process of the conical air-void structure consisted of a dry process and a crystallographic wet etching process on an undoped GaN layer, followed by a re-growth process for the InGaN LED structure. A higher light output power (1.54 times) and a small divergent angle (120°) were observed, at a 20 mA operation current, on the treated LED structure when compared to a standard LED without the conical air-void structure. In this electroluminescence spectrum, the emission intensity and the peak wavelength varied periodically by corresponding to the conical air-void patterns that were measured through a 100 nm-optical-aperture fiber probe. The conical air-void structure reduced the compressed strain at the GaN/sapphire interface by inducing the wavelength blueshift phenomenon and the higher internal quantum efficiency of the photoluminescence spectra for the treated LED structure.

Enhanced luminescence efficiency by surface plasmon coupling of Ag nanoparticles in a polymer light-emitting diode

This study achieved a substantial enhancement in electroluminescence by coupling localized surface plasmons in a single layer of Ag nanoparticles. Thermal evaporation was used to fabricate 20-nm Ag particles sandwiched between a gallium-doped zinc oxide film and a glass substrate to form novel window materials for use in polymer light-emitting diodes (PLEDs). The PLEDs discussed herein are single-layer devices based on a poly(9,9-di-n-octyl-2,7-fluorene) (PFO) emissive layer. In addition to low cost, this novel fabrication method can effectively prevent interruption or degradation of the charge transport properties of the active layer to meet the high performance requirements of PLEDs. Due to the surface-plasmon-enhanced emission, the electroluminescence intensity was increased by nearly 1-fold, compared to that of the same PLED without the interlayer of Ag nanoparticles.

InGaN light emitting diodes with a laser-treated tapered GaN structure.

InGaN light-emitting diode (LED) structures get an air-void structure and a tapered GaN structure at the GaN/sapphire interface through a laser decomposition process and a lateral wet etching process. The light output power of the treated LED structure had a 70% enhancement compared to a conventional LED structure at 20 mA. The intensities and peak wavelengths of the micro-photoluminescence spectra were varied periodically by aligning to the air-void (461.8nm) and the tapered GaN (459.5nm) structures. The slightly peak wavelength blueshift phenomenon of the EL and the PL spectra were caused by a partial compressed strain release at the GaN/sapphire interface when forming the tapered GaN structure. The relative internal quantum efficiency of the treated LED structure (70.3%) was slightly increased compared with a conventional LED (67.8%) caused by the reduction of the piezoelectric field in the InGaN active layer.

High PLED Enhancement by Surface Plasmon Coupling of Au Nanoparticles

In this investigation, a simple, rapid, and low-cost sputtering system was employed to deposit a Au-nanoparticles (Au-NPs) layer in polymer light-emitting diode (PLED) at room temperature. The green-emitting PLEDs considered herein are single-layer devices based on a poly[9,9-dioctylfluorene-co-benzothiadiazole] emissive layer. This novel fabrication effectively avoids interruption or degradation of the charge transport properties of the active layer and therefore satisfies the high performance requirements for PLEDs. Because of the surface-plasmon-enhanced emission, the electroluminescence intensity of the green-emitting PLED based on the Au-NPs/ITO anode increased nearly 2.7-fold, compared to that of the standard green-emitting PLED with a bare ITO substrate.

InGaN light emitting diodes with a nanopipe layer formed from the GaN epitaxial layer

A Si-heavy doped GaN:Si epitaxial layer is transformed into a directional nanopipe GaN layer through a laser-scribing process and a selectively electrochemical (EC) etching process. InGaN light-emitting diodes (LEDs) with an EC-treated nanopipe GaN layer have a high light extraction efficiency. The direction of the nanopipe structure was directed perpendicular to the laser scribing line and was guided by an external bias electric field. An InGaN LED structure with an embedded nanopipe GaN layer can enhance external quantum efficiency through a one-step epitaxial growth process and a selective EC etching process. A birefringence optical property and a low effective refractive index were observed in the directional-nanopipe GaN layer.

Fabrication of efficient thermoacoustic device with an interdigitated-like electrode on indium tin oxide glass

A thermoacoustic device was fabricated on indium tin oxide (ITO) glass, exhibiting an interdigitated-like electrode pattern. Our fabrication method enhanced the sound performance by approximately 20 dB compared with that of plain ITO film. Two approaches were adopted in this study to enhance the sound pressure level (SPL). One was to decrease the heat capacity per unit area of the device by reducing the thickness of the conductor film, and the other was to increase the thermal diffusivity of the device by applying a thin Au film on the electrode. We observed that heat generated by electron accumulation on ITO protrusions resulted in a large temperature oscillation of the surroundings and induced an SPL increase. A 4 nm Au film coating on the fabricated thermoacoustic device assisted thermal energy exchange with close-proximity air, improving the efficiency by an SPL of 7 dB.

Differences between the luminescence efficiencies of PLEDs based on Ag-nanoparticles/GZO/PEN and GZO/Ag-nanoparticles/PEN anodes

This study presents a simple, rapid, and low-cost sputtering system for the deposition of Ag-nanoparticles (Ag-NPs) in polymer light-emitting diodes (PLEDs) at room temperature. Proposed PLED structures based on Ag-NPs/GZO/PEN (AGP) and GZO/Ag-NPs/PEN (GAP) anodes are discussed. Because of surface-plasmon-enhanced emission, the electroluminescence intensities of PLEDs based on AGP anodes increased nearly 3.4-fold compared to normal PLEDs without Ag-NPs.

Enhancement of Light Extraction Efficiency of InGaN Light-Emitting Diodes with an Air-Hole-Array Structure

The truncated-conical air-hole (TAH) array structure of an InGaN light-emitting diode (LED) was fabricated on the mesa-edge region to increase the light extraction efficiency. The fabrication consisted of a dry process and a crystallographic wet etching process on the AlN buffer layer to form a truncated-conical air-hole array pattern. The light output power of the TAH-LED structure has a 55% enhancement compared with the conventional LED structure at 20 mA operation current. At 20 mA operation current, the forward voltage and peak electroluminescence wavelength of the TAH-LED were measured to be 3.09 V and 455.6 nm, respectively, similar to those of the conventional LED structure because the truncated-conical air-hole array pattern was fabricated around the mesa-edge region without affecting the current injection area with a top transparent contact layer.