The rapid economic development and increasing consumption of electrical energy requires people to generate more energy. The new energy sources developed are required to be clean and environmentally friendly. Microfluidic energy harvesting device is relatively less known compared with other popular renewable energy sources, such as solar cell and bio-fuels, but can provide such a clean and environmentally-friendly source.

The classical electrokinetic energy conversion mechanism relies on a single stage conversion by forcing liquid through a channel with charged walls. When the net charges inside the electrical double layer (EDL) are transported by water flow, the produced electrical energy can be harvested via connection of electrodes at two ends of a channel.

Different from traditional single phase flow energy conversion, the liquid microjet was used for energy conversion. Applied pressure forces water flow through a micropore, forming a liquid jet. Then the jet broke into droplets, which are absolutely isolated by air. Droplet kinetic energy is converted to electrical energy when the charged droplets decelerate in the electrical field that forms between membrane and target. It operates entirely different with traditional energy conversion from streaming potential, hence we term it “ballistic energy conversion”. Conversion occurs in two stages: first pressure is converted to kinetic energy and subsequently kinetic energy is converted to electrical energy. In the first stage a stream of high-velocity high-charge droplets is produced by forcing water through a micropore. In the second stage the charged droplets travel through an electrical field towards a high-potential target, decelerating them to zero speed. The target acquires its potential by droplet impact. Current is drawn from the target to do useful work. We strongly reduced loss factors by optimizing the setup from the physical model. At present this resulted in an energy conversion efficiency of almost 50% with as main loss factor hydrodynamic friction losses in the micropore.

Now, inspired by Kelvin’s water dropper, we apply the electrostatic charge self-induction mechanism to an inertia-driven (ballistic) energy conversion system, and show the disadvantages of Kelvin’s water dropper do not any more apply. The droplet charge thereby is derived from the same inductive charging mechanism as in Kelvin’s droplet generator, employing two separate systems and cross-connecting targets and induction rings. To prevent overcharging of the droplets by the inductive mechanism and consequent droplet loss by repulsion from the target, we use voltage dividers, trying both a resistor divider and a diode divider, to enable stable energy conversion with a self-induction system. The current rapidly increased once the electrical circuit was connected. With reversely connected diodes, the induction voltage could be properly controlled to a saturated value (about 500V), whereby experimentally maximal 17.9% energy conversion efficiency was obtained with the diode-induced system.

This research demonstrates an optical force enabled NEMS actuator, whose travel range can be extended by as much as 20%. By taking advantage of the high quality factor of the cavity, the cavity optomechanics can not only change the travel range of the electrostatic capacitive actuator, but also provide an ultra-sensitive approach to detect the mechanical motion. This method gives a new approach to extend the actuation range of NEMS actuator and an ultra-sensitive way to detect the small actuator motion. There are several problems for traditional electrostatic mechanical actuators, for example, the breakdown of the electrostatic force, in which the actuator can only move small ratio of the designed gap. Even through several proposals are used to solve this problems, such as adding another capacitor or feedback control. The unwanted capacitance becomes a much bigger problem, because the capacitance is much larger than the natural capacitance. In this way, this method is quite ineffective. The optical gradient force which arises from the optical energy, on the other hand, plays an important role in the actuator at the nano scale. Therefore, the optical force can play an important role in the NEMS systems and have more good ways to manipulate. In this paper, we worked out a NEMS actuator enhanced by gradient optical force. More importantly, this system can provide a good method to detect small mechanical motion.

Integrating nanoscale electromechanical transducers and nanophotonic devices potentially can enable new acousto-optic devices to reach unprecedented high frequencies and modulation efficiency. We demonstrate acousto-optic modulation of a photonic crystal nanocavity using acoustic waves with frequency up to 19 GHz, reaching the microwave K band. Both the acoustic and photonic devices are fabricated in piezoelectric aluminum nitride thin films. Excitation of acoustic waves is achieved with interdigital transducers with periods as small as 300 nm. Confining both acoustic wave and optical wave within the thickness of the membrane leads to improved acousto-optic modulation efficiency in the new devices than that obtained in previous surface acoustic wave devices. Our system demonstrates a novel scalable optomechanical platform where strong acousto-optic coupling between cavity-confined photons and high frequency traveling phonons can be explored.

Exploitation of light-sound interactions in various types of media has led to a plethora of important optical technologies ranging from acousto-optic devices for optical communication to photo-acoustic imaging in biomedicine. With advances of integrated photonics and nanofabrication technology, it is now more feasible to miniaturize and integrate acousto-optic devices to augment their speed and performance so they can assume indispensable roles in integrated photonic systems for chip-scale optical communication. Moreover, in addition to electromechanical excitation, acoustic waves or localized mechanical vibrations can also be optically stimulated through optomechanical forces including radiation pressure, gradient force and electrostriction. Such optomechanical effects recently has been extensively investigated in various optomechanical systems with dimensional scales ranging from meters to nanometers. Therefore, with these recent developments, acousto-optics is entering a new era with plenty of research opportunities.

To combine acoustic and photonic devices, previously we and other groups have used piezoelectric aluminum nitride (AlN) film deposited on silicon wafers with a layer of silicon dioxide (SiO₂). Since AlN has a relatively high refractive index of about 2.1, photonic waveguides and cavities can be fabricated in AlN with the SiO₂ layer as the cladding. At the same time, acoustic waves can be exited in the AlN layer. On this platform, we have demonstrated SAW wave with frequency up to 12 GHz and its acousto-optic modulation of optical ring resonators and photonic crystal nanocavities. However, an important drawback of above devices based on SAW is that the SiO₂ layer has a lower sound velocity than both the top AlN layer and the bottom silicon substrate. As a result, the excited acoustic waves in the AlN layer tend to leak into and be guided in the SiO₂ layer whereas the optical modes are highly confined in the top AlN layer because of its high refractive index. Consequently, the modal overlap between the optical and acoustic modes is low, leading to relatively weak acousto-optic coupling efficiency.

To circumvent this problem, we further implemented integrated acousto-optic devices on a suspended AlN membrane. When the membrane thickness is less than or comparable to the acoustic wave, the generated acoustic wave will propagate in Lamb wave mode with very high acoustic velocity. The removal of the substrate enforces the optical mode and the acoustic wave to maximally overlap within the thickness of the membrane. With this approach, we demonstrate acousto-optic modulation of a photonic crystal nanocavity at frequency up to 19 GHz with significantly improved modulation efficiency. The system overcomes the limitation of the unsuspended AlN platform where the acoustic wave leaks into the substrate layer without contributing to the acousto-optic interaction. The platform is promising for studying interaction between cavity confined photons and propagating phonons of microwave frequency. In additional, the strong and high frequency acoustic waves realized in this platform can provide spatially-coherent time-domain modulation to induce non-reciprocity and break time-reversal symmetry in photonic systems. At this high acoustic frequency, this system can also be applied for microwave photonics technology where optical and microwave channels of communication can be linked and interchanged.

Cancer is a highly heterogeneous disease, thus a “one-size-fits-all” treatment approach has not been effective in cancer management. Interactions between tumor and their environment at the cellular and systemic levels have been shown to contribute to variability in therapy outcomes.  Tumor cells interact at the cellular level with immune cells and tumor associated fibroblasts to modulate tumor cell susceptibility to various treatment modalities. At the systemic level, anti-cancer drugs can cause differential toxic side effects to other tissues, which limits the maximum tolerable dose for patients. While patient derived xenografts (PDx) are being explored as mouse avatars for pre-clinical screening of an optimal personalized therapeutic regime, animal models are expansive and not scalable for screening applications. To fill this technological gap, we have developed Patient-Derived Micro-Avatar Chips (PD-MAC), which integrate the biological diverse characteristics of patient-derived tumor cells and high configurability of microfluidic systems, to study tumor-environment interactions at the cellular and systemic level. A 3D microenvironment can be engineered using micropillar structures to support the formation and remodeling of patient-derived parental and metastatic oral squamous cell carcinoma (OSCC) into 3D micro-tumors (PD-mTs). We have developed a modular approach to achieve system integration with various microfluidic components such as micro-pumps and valves, and a second tissue chip to facilitate the scaling of the PD-MAC to study systemic effects of chemotherapeutic drugs. Finally, we demonstrate the manipulation of parental and metastatic OSCC tumor and immune (NK) cells using hydrodynamic trapping arrays to investigate differential immune-mediated cytotoxicity.

This work reports one interesting optical microcavity: self-rolled up optical microcavity with tubular geometry. Several different fabrication processes for the microtubular optical cavities had been reported [1]. Different from transitional fiber-drawing technique [2], the nanomembrane with pre-defined geometries can be bend into a curved structure and forms a three dimensional (3D) tubular structure by predefined strain-engineering via lift-off technology [3, 4]. Our group focus on the research on the optical properties of these self-rolled up optical microcavities [5, 6].

A schematic view of the self-rolled up processes is illustrated in Figure 1 [1]. The nanomembranes released via the lift-off processes from the sacrificial layer on substrate. Then, the freestanding nanomembranes self-rolled up into microtubes and performed as optical microcavities. The typical micro-photoluminescence (μ-PL) spectra of these rolled-up microtubes in various color regions are shown in Figure 2 [5]. Various interesting materials (Figure 2) were used for the fabrication of self-rolled up tubular optical microcavities in difference spectral range [5]. As shown in Figure 3, the μ-PL spectra measured at different positions along the micro-tubes by rolling circular nanomembranes indicates an evident 3D optical confinement. The WGMs shift to lower wavelengths when moving from the middle to the end of the micro-tubular cavity along the z direction. We consider that this interesting phenomenon in WGMs (both emergence of sub-peaks and the shift of modes) should be intimately connected with the geometrical structure of the micro-tubular cavity. Effective surface modification and geometry design will introduce special optical properties in these self-rolled up microcavities [6].

The rolling process has been provides a convenient way to produce a stack of multilayers made from different materials with tunable geometry, demonstrating a new method to prepare optical microdevices. The future of rolled-up microcavities lies in many aspects from fundamental science to practical applications. The corresponding resonance can thus be tuned in terms of wavelength range and Q-factor. Functional materials may also be incorporated to achieve effective coupling between the light and other physical fields, giving birth to special micro devices with good tunability. Meanwhile, rolled-up thin-walled oxide tubular microcavity delivers a new optical component for light coupling and may imply interesting applications in the interaction between light and matter [6]. Considering the on-chip integratability of rolled-up structures, researchers may also be able to produce a sensing device to realize lab-in-a-tube [3].