Yanying Feng

Passive valves based on hydrophobic microfluidics

Fluid-surface interactions can become dominant in microfluidics, which is a central technology in a number of miniaturized systems for chemical, biological and medical applications. In this paper, two kinds of hydrophobic valves in microfluidic applications were presented. One is based on special geometrical designs and chemical modification for silicon dioxide and glass microchannels. Silicon dioxide and Pyrex glass surfaces, which are hydrophilic originally, are modified with octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) to be hydrophobic, with the contact angles up to ∼102 and 103°, respectively, for water. The formation of OTS SAMs takes <5 min. The OTS SAMs based stop valve can function well to enable stopping the flow of a liquid inside a microchannel in both directions. Tested for deionized water, the hydrophobic valve blocked successfully the liquid for many consecutive times and yielded pressure barrier up to ∼490 Pa, which is near to the theoretical prediction. The OTS SAMs, therefore the hydrophobic valve function, can be retained in a suitable environment for a long time and can rebirth conveniently when destroyed. The other kind of hydrophobic valve is based on hydrophobic pattern, which is formed by plasma depositing CHF 3 patterns on the surfaces of silicon dioxide. The hydrophobic CHF 3 patterns (measured for contact angle for water to be 102°) can block the liquid to flow forward. The theoretical analysis and the process design were presented.

Analysis of interfacial thermal stresses of chip-substrate structure

The interfacial thermal stresses of the chip-substrate structure near free edges play an important role in determining the reliability of electronic packaging structures. According to the heat conduction mechanism of integrated circuits, the temperature field of the chip and the substrate is derived when the chip works in a steady state. A simple method is developed to determine the stress field of the chip and the substrate, which can exactly satisfy the traction-free boundary conditions and continuity conditions on the interface. The corresponding stress field is solved in terms of the variational principle. Finally, the effect of geometrical parameters of the chip and the substrate on stress concentration is analyzed in detail.

An ELISA Chip Based on an EWOD Microfluidic Platform

Abstract We have developed an enzyme-linked immunosorbent assay (ELISA) chip based on a digital microfluidic platform, in which droplet generation and manipulation are implemented using the electrowetting on dielectrics (EWOD) effect. First, droplet transportability was experimentally tested on silicon test chips with different control electrodes for optimizing the EWOD-based droplet manipulation, and then tested on transparent indium-tin-oxide (ITO) chips. The latter were then used for the development of an ELISA chip, on which we carried out immunoassay with rat immunoglobulin G (IgG) and goat anti-rat IgG marked with horseradish peroxidase as an example. In driving the reagent solutions, the average velocity of droplets is up to 5–7 mm/s on the ELISA chip, and a chip-scaled immunoassay can be finished within 20 min by using colorimetric detection, with the volume of the sample and reagents of only 0.5–1 μl.

Note: Generation of Raman laser beams based on a sideband injection-locking technique using a fiber electro-optical modulator

Two phase-coherent Raman laser beams with a frequency offset of 6.835 GHz were generated by sideband injection-locking technique. A master diode laser was phase-modulated at 6.835 GHz by a fiber electro-optic modulator. A slave diode was injection-locked to the -1 sideband of the phase-modulated beam, and another diode was locked to the master laser carrier. This method produced stable and spatially separated Raman lasers with a large frequency shift range (>180 MHz). The relative linewidth of these two beams was ∼1 Hz, and the unwanted carrier mode was suppressed down to -24 dB. Stimulated Raman transitions and Ramsey fringes were driven by Raman lasers in a cold atomic beam.

A continuous cold atomic beam interferometer

We demonstrate an atom interferometer that uses a laser-cooled continuous beam of 87Rb atoms having velocities of 10–20 m/s. With spatially separated Raman beams to coherently manipulate the atomic wave packets, Mach–Zehnder interference fringes are observed at an interference distance of 2L = 19 mm. The apparatus operates within a small enclosed area of 0.07 mm2 at a bandwidth of 190 Hz with a deduced sensitivity of 7.8×10−5 rad/s/Hz for rotations. Using a low-velocity continuous atomic source in an atom interferometer enables high sampling rates and bandwidths without sacrificing sensitivity and compactness, which are important for applications in real dynamic environments.

Microwave signal generation with the frequency-selective sideband injection-locking of semiconductor lasers

Two longitudinally multimode Febry-Perot diode lasers have been sideband injection-locked to the +1 and -1 sidebands of a 3.4GHz electro-optical modulator (EOM). Optical heterodyne measurement showed that powers of 99.5% of the slave laser could be injection-locked to the +1 or -1 sidebands, and the unselected master laser carrier was suppressed down to -24dB. Generally, the long-term stability and efficiency of the injection-locking to the +1 sideband was worse than the -1 due to the asymmetry of the injection-locking bandwidth. The microwave signal at 6.8GHz had a measured 3dB linewidth of less than 200Hz, without considering the noise contribution by the driving signal of the additional acousto-optical modulator. The proposed method will be used for driving the stimulated Raman transitions in a Rubidium based atom gyroscope.

A self-sustaining atomic magnetometer with τ −1 averaging property

Quantum measurement using coherent superposition of intrinsic atomic states has the advantage of being absolute measurement and can form metrological standards. One example is the absolute measurement of magnetic field by monitoring the Larmor precession of atomic spins whilst another being the Ramsey type atomic clock. Yet, in almost all coherent quantum measurement, the precision is limited by the coherence time beyond which, the uncertainty decreases only as τ−1/2. Here we show that by non-destructively measuring the phase of the Larmor precession and regenerating the coherence via optical pumping, the self-sustaining Larmor precession signal can persist indefinitely. Consequently, the precision of the magnetometer increases with time following a much faster τ−1 rule. A mean sensitivity of 240  from 1 Hz to 10 Hz is realized, being close to the shot noise level. This method of coherence regeneration may also find important applications in improving the performance of atomic clocks.

Investigation of the nonlinear CPT spectrum of 87Rb and its application for large dynamic magnetic measurement

The coherent population trapping (CPT) phenomenon has found widespread application in quantum precision measurements. Various designs based on the narrow resonant spectrum corresponding to the linear Zeeman effect have been demonstrated to achieve high performance. In this article, the nonlinear Zeeman split of the CPT spectrum of 87Rb in the lin ∥ lin setup is investigated. We observe re-split phenomenon for both magnetic sensitive and magnetic insensitive CPT resonant lines at a large magnetic field. The re-split in the magnetic sensitive lines raises a practical problem to magnetometers worked in the lin ∥ lin setup while the other one shows a good potential for applications in large magnetic field. We propose a design based on the nonlinear split of the magnetic insensitive lines and test its performance. It provides a much larger measurement range compared to the linear one, offering an option for atomic magnetometers where a large dynamic range is preferred.

Loop-locked coherent population trapping magnetometer based on a fiber electro-optic modulator

We have set up a coherent population trapping (CPT)-based magnetometer prototype with the D1 line of ⁸⁷Rb atoms. The dichromatic light field is derived from a fiber electro-optic modulator (FEOM) connected to an external cavity laser diode. A CPT resonance signal with a 516 Hz linewidth is observed. By feeding back the derivative of the resonance curve to the FEOM with a proportional integral controller, of which the voltage output is directly converted to the measured magnetic field intensity, the resonance peak is locked to the environmental magnetic field. The measurement data we have achieved are well matched with the data measured by a commercial fluxgate magnetometer within 2 nT, and the sensitivity is better than 8 pT/√Hz in a parallel B field.

Studies on Microfluidic Systems and Related Topics

Microfluidic systems are getting attractive attentions for their promising applications in many field, especially in bio-chemical analysis. Researches on microfluidics and related topics have been carried out in our group since 1991. Recent progresses including characteristics of microflow in microchannels and micronozzles/diffusers, mechanical properties of thin silicon films, micro-electromagnetic pump, micro-thrusters, electrophoresis microchip and its analytical instruments are reviewed in the paper. Studies of microfluidics and its applications show that the miniaturization of devices could bring us more opportunities and the trends would be integrating micro-valves, pumps and react chambers with microdetectors and circuits to a complicated system.