Chung-Hao Tien

Microlens Arrays by Direct-Writing Inkjet Print for LCD Backlighting Applications

In this paper, we demonstrated the Inkjet print method to fabricate a ultraviolet (UV)-curable epoxy microlens deflecting array with controllable curvature and filling factor, by which a 7-in light guide plate with 82% uniformity and 70% light efficiency was successfully implemented. The microlenses can be directly formed by plastic discharged by inkjet head, eliminating the need for molds and slashing development time. Proposed method will certainly has a promising impact on rapid prototyping and other specialized microfeatures due to its simplicity and versatility.

Microlens array fabricated by excimer laser micromachining with gray-tone photolithography

We demonstrate the fabrication of a refractive microlens array by using 248 nm excimer laser micromachining with coded gray-tone mask photolithography. With pre-corrections to the nonlinear exposure process, the maximum deviation from the designed shape was below 5%. The fabricated hemispherical lens of 30 µm radius was used as a solid immersion lens (SIL) and combined with a 0.54 numerical aperture (NA) objective to achieve a 0.87 effective NA through the knife-edge scanning test. The experimental results agreed with those of the simulation. Unlike the methods such as the thermal melting process, this one-step optical exposure method with a coded mask provides a relatively fast and cost-effective way to realize a microlens array in optical data storage, information processing, and optical interconnection applications.

Optical cavity modes of a single crystalline zinc oxide microsphere

A detailed study on the optical cavity modes of zinc oxide microspheres under the optical excitation is presented. The zinc oxide microspheres with diameters ranging from 1.5 to 3.0 µm are prepared using hydrothermal growth technique. The photoluminescence measurement of a single microsphere shows prominent resonances of whispering gallery modes at room temperature. The experimentally observed whispering gallery modes in the photoluminescence spectrum are compared with theoretical calculations using analytical and finite element methods in order to clarify resonance properties of these modes. The comparison between theoretical analysis and experiment suggests that the dielectric constant of the ZnO microsphere is somewhat different from that for bulk ZnO. The sharp resonances of whispering gallery modes in zinc oxide microspheres cover the entire visible window. They may be utilized in realizations of optical resonators, light emitting devices, and lasers for future chip integrations with micro/nano optoelectronic circuits, and developments of optical biosensors.

Design and Evaluation of Light Spread Function for Area-Adaptive LCD System

A methodology, based on two-dimensional super-Gaussian light spread functions (LSFs), was studied for the light-emitting-diode (LED) backlighting system in the high dynamic range (HDR) liquid crystal displays (LCDs). We proposed a novel LED-based optomechanical configuration to implement the desirable luminous pattern. A dual-liquid-crystal-panel system was constructed as a pseudo-HDR LCD to evaluate the super-Gaussian illumination. The proposed HDR scheme was verified via the current platform: overall luminance uniformity, contrast ratio and processing speed could be improved by factors of 1.55, 1.95-4.15, and 4.82, respectively. The design flexibility made the methodology applicable to any panel dimension and backlight division of the HDR LCDs, and to the conventional full-on backlighting LCDs as well.

Multispectral mixing scheme for LED clusters with extended operational temperature window

LEDs have changed the concept of illumination not only in an expectation of the highest electroluminance efficiency but also in tremendous chances for smart lighting applications. With a cluster mixing, many studies were addressed to strategically manipulate the chromaticity point, system efficiency and color rendering performance according to different operational purposes. In this paper, we add an additional thermal function to extend the operational thermal window of a pentachromatic R/G/B/A/CW light engine over a chromaticity from 2800K to 8000K. The proposed model is experimentally validated to offer a full operable range in ambient temperature (Ta = 10° to 100°C) associated with high color quality scale (above 85 points) as well as high luminous efficiency (above 100 lm/watt).

Phosphor-converted LED modeling by bidirectional photometric data

For the phosphor-converted light-emitting diodes (pcLEDs), the interaction of the illuminating energy with the phosphor would not just behave as a simple wavelength-converting phenomenon, but also a function of various combinations of illumination and viewing geometries. This paper presents a methodology to characterize the converting and scattering mechanisms of the phosphor layer in the pcLEDs by the measured bidirectional scattering distribution functions (BSDFs). A commercially available pcLED with conformal phosphor coating was used to examine the validity of the proposed model. The close agreement with the measurement illustrates that the proposed characterization opens new perspectives for phosphor-based conversion and scattering feature for white lighting uses.

Cluster LEDs mixing optimization by lens design techniques

This paper presents a methodology analogous to a general lens design rule to optimize step-by-step the spectral power distribution of a white-light LED cluster with the highest possible color rendering and efficiency in a defined range of color temperatures. By examining a platform composed of four single-color LEDs and a phosphor-converted cool-white (CW) LED, we successfully validate the proposed algorithm and suggest the optimal operation range (correlated color temperature = 2600-8500 K) accompanied by a high color quality scale (CQS > 80 points) as well as high luminous efficiency (97% of clusters theoretical maximum value).

High-transmission hybrid-effect-assisted nanoaperture

We present a C-aperture encircled by a groove that makes possible the hybrid effect of coupling surface plasmon resonance to propagating waves. Compared to a single C aperture, the groove-encircled aperture can increase transmission by a factor of 2.45 in the near field and by a factor of 1.88 in the far field, showing good agreement with our theoretical calculation.

Extending the Capture Volume of an Iris Recognition System Using Wavefront Coding and Super-Resolution

Iris recognition has gained increasing popularity over the last few decades; however, the stand-off distance in a conventional iris recognition system is too short, which limits its application. In this paper, we propose a novel hardware-software hybrid method to increase the stand-off distance in an iris recognition system. When designing the system hardware, we use an optimized wavefront coding technique to extend the depth of field. To compensate for the blurring of the image caused by wavefront coding, on the software side, the proposed system uses a local patch-based super-resolution method to restore the blurred image to its clear version. The collaborative effect of the new hardware design and software post-processing showed great potential in our experiment. The experimental results showed that such improvement cannot be achieved by using a hardware-or software-only design. The proposed system can increase the capture volume of a conventional iris recognition system by three times and maintain the systems high recognition rate.

Plasmonic gap-mode nanocavities with metallic mirrors in high-index cladding.

We theoretically analyze plasmonic gap-mode nanocavities covered by a thick cladding layer at telecommunication wavelengths. In the presence of high-index cladding materials such as semiconductors, the first-order hybrid gap mode becomes more promising for lasing than the fundamental one. Still, the significant mirror loss remains the main challenge to lasing. Using silver coatings within a decent thickness range at two end facets, we show that the reflectivity is substantially enhanced above 95 %. At a coating thickness of 50 nm and cavity length of 1.51 μm, the quality factor is about 150, and the threshold gain is lower than 1500 cm(-1).