Yu-Hwa Lo

Fluidic adaptive lens with high focal length tunability

Fluidic adaptive lenses with an adjustable focal length over a wide range were demonstrated in this letter. The focal length adjustment was achieved by changing the shape of the fluidic lens without any mechanical moving parts. The shortest focal length demonstrated in such devices is 41 mm, which corresponds to a large numerical aperture of 0.24 and a small F number of 2.05. The highest resolution measured using a positive standard is 25.39 lp/mm in this fluidic adaptive lens.

Silicon nanowire detectors showing phototransistive gain

Nanowire photodetectors are shown to function as phototransistors with high sensitivity. Due to small lateral dimensions, a nanowire detector can have low dark current while showing large phototransistive gain. Planar and vertical silicon nanowire photodetectors fabricated in a top-down approach using an etching process show a phototransistive gain above 35u2009000 at low light intensities. Simulations show that incident light can be waveguided into vertical nanowires resulting in up to 40 times greater external quantum efficiency above their physical fill factor. Vertical silicon nanowire phototransistors formed by etching are attractive for low light level detection and for integration with silicon electronics.

Integrated fluidic adaptive zoom lens

An integrated fluidic adaptive zoom lens is demonstrated for what is believed to be the first time. A zoom lens was fabricated using an UV lithographic-galvanic-like process involving soft lithography and wafer bonding. The zooming capability of such a lens was achieved by varying the focal length instead of the distance between the lenses. A zoom ratio of greater than 2 was obtained for devices that are 8 mm thick and have a 20-mm lens diameter. Including the 30-mm image distance, the total physical length of the fluidic zoom lens was less than 43 mm. More-compact systems with a higher zoom ratio can be obtained by reduction of the aperture size.

Fluidic adaptive lens of transformable lens type

Fluidic adaptive lenses with a transformable lens type were demonstrated. By adjusting the fluidic pressure, not only can the lens properties, such as the focal distance and numerical aperture, be tuned dynamically but also different lens types, such as planoconvex, planoconcave, biconvex, biconcave, positive meniscus, and negative meniscus lenses, can be formed. The shortest focal length for a 20 mm aperture adaptive lens is 14.3 mm when the device is transformed into a positive lens, and −6.3 mm when transformed into a negative lens. The maximum resolution of the fluidic lens is better than 40 line pairs/mm.

A prealigned process of integrating optical waveguides with microfluidic devices

We present a novel technology to monolithically integrate buried heterostructure waveguides with microfluidic channels in a prealigned manner. The fabrication process produced sealed microfluidic channels, low-loss waveguides, and excellent waveguide-channel alignment. The resultant device allows efficient optical coupling between the channels and the waveguides. Thus, the waveguides can both deliver the optical power and collect the emitted light.

High-performance fluidic adaptive lenses

High-performance fluidic lenses with an adjustable focal length spanning a very wide range (30 mm to infinite) are demonstrated. We show that the focal length, F-number, and numerical aperture can be dynamically controlled by changing the shape of the fluidic adaptive lens without moving the lens position mechanically. The shortest focal length demonstrated is less than 30 mm for a 20-mm lens aperture. The fluidic adaptive lens has a nearly perfect spherical profile and shows a resolution better than 40 line pairs/mm in a plano-convex structure and 57 line pairs/mm in a biconvex structure.

High-sensitivity cytometric detection using fluidic-photonic integrated circuits with array waveguides

We demonstrate a new detection scheme for a microfabricated flow cytometer. The fluidic-photonic integrated circuits (FPICs) that perform flow cytometric detection possess new functionality, such as on-chip excitation, time-of-flight measurement, and above all, greatly enhanced fluorescence detection sensitivity. Using the architecture of space-division waveguide demultiplexer and the technique of cross-correlation analysis, we obtained high detection sensitivity with a simple light source and a detector, without high-power laser excitation and lock-in amplifier (or photomultiplier tube) detection. Besides improving cytometric detection, the technology of integrating microfluidic circuits with photonic circuits into the FPIC presents a new platform for sophisticated biomedical-sensing devices with significant cost, size, and performance advantages.

Low-threshold 1.57-μm VC-SEL's using strain-compensated quantum wells and oxide/metal backmirror

We present an electrically-pumped long wavelength vertical cavity surface-emitting laser (VC-SEL) using strain-compensated multiple quantum well gain medium and an oxide/metal backmirror. Design flexibilities such as using oxide/metal mirrors are possible because of the exceedingly high optical gain provided by strain-compensated multiple quantum wells. The gain medium is bonded to various substrates, including InP and Si, prior to device processing. This substrate transfer process facilitates heat sinking, and can be useful in the integration of VC-SELs with other devices. The device operates at a single wavelength of 1.57 /spl mu/m, and has a minimum threshold of 12 mA at room temperature under pulse pumping.<<ETX>>

Fluidic photonic integrated circuit for in-line detection

We present a microfabricated fluidic photonic integrated circuit (FPIC) performing the detection function for flow cytometry. This device was entirely made of polymer using micromolding and capillary filling techniques. An array waveguide design was chosen to achieve superb sensitivity and the time-of-flight measurement for each particle flowing by. With multichannel sampling and cross-correlation analysis, the results show significant enhancement of detection sensitivity.

Dislocation microstructures on flat and stepped Si surfaces: Guidance for growing high‐quality GaAs on (100) Si substrates

Type‐I dislocations at the GaAs/Si interface are beneficial because they effectively relax the mismatched stress, but do not propagate into the GaAs film. Accordingly, the best way to grow a low defect density GaAs film on a Si substrate is to form as many as possible type‐I dislocations or, equivalently, to suppress other kinds of defects. The high‐resolution transmission electron microscopy study shows that most of the type‐I dislocations are formed at the double step on a Si surface. It is further determined that the silicon surface steps are mainly due to the substrate tilting instead of the heating before growth. Based on our study, the (100) Si substrate with double steps along both [110] and [110] axes provides the best condition for growing low defect density GaAs on Si substrates.