Wai Y. Leung

Fabrication of photonic band gap materials using microtransfer molded templates

A method of manufacturing photonic band gap structures operable in the optical spectrum has been presented. The method comprises the steps of creating a patterned template for an elastomeric mold, fabricating an elastomeric mold from poly-dimethylsiloxane (PDMS) or other suitable polymer, filling the elastomeric mold with a second polymer such as epoxy or other suitable polymer, stamping the second polymer by making contact with a substrate or multilayer structure, removing the elastomeric mold, infiltrating the multilayer structure with ceramic or metal, and heating the multilayer structure to remove the second polymer to form a photonic band gap structure.

Soft holographic interference lithography microlens for enhanced organic light emitting diode light extraction

Very uniform 2 μm-pitch square microlens arrays (μLAs), embossed on the blank glass side of an indium-tin-oxide (ITO)-coated 1.1 mm-thick glass, are used to enhance light extraction from organic light-emitting diodes (OLEDs) by ~100%, significantly higher than enhancements reported previously. The array design and size relative to the OLED pixel size appear to be responsible for this enhancement. The arrays are fabricated by very economical soft lithography imprinting of a polydimethylsiloxane (PDMS) mold (itself obtained from a Ni master stamp that is generated from holographic interference lithography of a photoresist) on a UV-curable polyurethane drop placed on the glass. Green and blue OLEDs are then fabricated on the ITO to complete the device. When the μLA is ~15 × 15 mm(2), i.e., much larger than the ~3 × 3 mm(2) OLED pixel, the electroluminescence (EL) in the forward direction is enhanced by ~100%. Similarly, a 19 × 25 mm(2) μLA enhances the EL extracted from a 3 × 3 array of 2 × 2 mm(2) OLED pixels by 96%. Simulations that include the effects of absorption in the organic and ITO layers are in accordance with the experimental results and indicate that a thinner 0.7 mm thick glass would yield a ~140% enhancement.

Slot antennas on photonic band gap crystals

The radiation patterns of a slot antenna placed on a photonic band gap crystal have been measured. We used a layer-by-layer photonic band gap crystal having a three-dimensional stop band between 12 and 15 GHz. The slot antenna radiation depends sensitively on the relative position and orientation of the slot in the surface unit cell of the photonic crystal. We have found configurations of the slot antenna with an increase of radiated power by 2-3 dB. The photonic band gap crystal can considerably improve the performance of a simple slot antenna.

Design of midinfrared photodetectors enhanced by resonant cavities with subwavelength metallic gratings

We propose a metallic Fabry–Perot cavity with a Au grating and a Au film acting as two reflectors to enhance the field and absorption in the active detector region, leading to better performance of quantum-dot-based photodetectors at a wavelength of 10 μm. One- and two-dimensional Au gratings are applied to achieve enhancement for polarized and unpolarized light, respectively. With optimizing grating parameters, the absorption can be enhanced by about 20 times in the active detector region compared to conventional photodetectors without the Au reflectors.

Diffracted moiré fringes as analysis and alignment tools for multilayer fabrication in soft lithography

We studied the first-order diffracted moire fringes of transparent multilayered structures comprised of irregularly deformed periodic patterns. By a comparison study of the diffracted moire fringe pattern and detailed microscopy of the structure, we show that the diffracted moire fringe can be used as a nondestructive tool to analyze the alignment of multilayered structures. We demonstrate the alignment method for the case of layer-by-layer microstructures using soft lithography. The alignment method yields high contrast of fringes even when the materials being aligned have very weak contrasts. The imaging method of diffracted moire fringes is a versatile visual tool for the microfabrication of transparent deformable microstructures in layer-by-layer fashion.

Layer-by-layer photonic crystal fabricated by low-temperature atomic layer deposition

Layer-by-layer three-dimensional photonic crystals are fabricated by low-temperature atomic layer deposition of titanium dioxide on a polymer template created by soft lithography. With a highly conformal layer of titanium dioxide, a significantly enhanced photonic band gap effect appears at 3.1μm in transmittance and reflectance. From optical investigations of systematically shifted structures, the robust nature of the photonic band gap with respect to structural fluctuations is confirmed experimentally. With angle-resolved Fourier-transform spectroscopy, the authors also demonstrate that the fabricated photonic crystal can be a diffraction-free device as the photonic band gap exists over the diffracting regime.

Polarized thermal radiation by layer-by-layer metallic emitters with sub-wavelength grating

Metallic thermal emitters consisting of two layers of differently structured nickel gratings on a homogeneous nickel layer are fabricated by soft lithography and studied for polarized thermal radiation. A thermal emitter in combination with a sub-wavelength grating shows a high extinction ratio, with a maximum value close to 5, in a wide mid-infrared range from 3.2 to 7.8 mum, as well as high emissivity up to 0.65 at a wavelength of 3.7 microm. All measurements show good agreement with theoretical predictions. Numerical simulations reveal that a high electric field exists within the localized air space surrounded by the gratings and the intensified electric-field is only observed for the polarizations perpendicular to the top sub-wavelength grating. This result suggests how the emissivity of a metal can be selectively enhanced at a certain range of wavelengths for a given polarization.

The Effect of Photonic Crystals on Dipole Antennas

ABSTRACT We study the radiation patterns of antennas placed on top and inside three dimensional photonic band-gap (PBG) materials. The Finite Difference Time Domain method has been used for the calculation of the radiation patterns. Measurements are in good agreement with theoretical results. The different factors influencing the patterns (position, height, and frequency) have been studied.

Optimizing the Q value in three-dimensional metallic photonic band gap crystals

A metallic photonic band gap crystal with different defect structures is fabricated. The structure is designed and built to operate in the 8–26 GHz frequency range. Defects with sharp peaks in the transmission are created by removing portions of the metallic rods in a single defect layer. A high quality factor (Q) for the defect state is obtained by larger filling ratios and spatial separations between the unit cells. An optimized value of Q⩾300 is found for three unit cell metallic photonic band gap structure. The experimental observations agree very well with theoretical calculations using the transfer matrix method.

Laser-machined layer-by-layer metallic photonic band-gap structures

Metallic photonic band-gap crystals operating in the microwave frequency range were fabricated by laser precision machining. They consist of stainless steel plates with a tetragonal lattice of holes and a lattice constant of 15 mm. Transmission measurements show that periodic crystals exhibit a cutoff frequency in the 8–18 GHz range, below which no propagation is allowed. The cutoff frequency can be easily tuned by varying the interlayer distance or the filling fraction of the metal. Combinations of plates with different hole diameters create defect modes with relatively sharp peaks, which are tunable. The experimental measurements are in good agreement with theoretical calculations.