Han Zhang

Integrated biosensor platforms have become subjects of high interest for consolidated assay preparation and analysis to reduce sample-to-answer response times. By compactly combining as many biosensor processes and functions as possible into a single lab-on-chip device, all-in-one point-of-care devices can aid in the accessibility and speed of deployment due to their compact size and portability. Biomarker assay preparation and sensing are functionalities that are often carried out on separate devices, thus increasing opportunity of contamination, loss of sample volume, and other forms of error. Here, we demonstrate a complete lab-on-chip system combining sample preparation, on-chip optofluidic dye laser, and optical detection. We first show the integration of an on-chip distributed feedback dye laser for alignment-free optical excitation of particles moving through a fluidic channel. This capability is demonstrated by using Rhodamine 6G as the gain medium to excite single fluorescent microspheres at 575 nm. Next, we present an optofluidic PDMS platform combining a microvalve network (automaton) for sample preparation of nanoliter volumes, on-chip distributed feedback dye laser for target excitation, and optical detection. We conduct concurrent capture and fluorescence tagging of Zika virus nucleic acid on magnetic beads in 30 min. Target-carrying beads are then optically excited using the on-chip laser as they flow through an analysis channel, followed by highly specific fluorescence detection. This demonstration of a complete all-in-one biosensor is a tangible step in the development of a rapid, point-of-care device that can assist in limiting the severity of future outbreaks.

We present a chip-scale integrated pH sensor with high sensitivity by using an optofluidic ring resonator (OFRR) laser. An optical fiber with a high refractive index (RI) is employed both as an optical cavity and the sensing reactor along a microchannel, while disodium fluorescein (DSF) aqueous solution with a low RI is served as the cladding gain medium and fluorescent probes. The pump light is introduced along the fiber axis and guided by the total internal reflection at the fiber/cladding interface. The evanescent field of the pump light extends out of the fiber surface and efficiently excites the dye molecules residing in the evanescent field region of the Whispering Gallery Modes (WGMs) of the OFRRs to produce lasing emission. This pumping scheme provides a uniform excitation to the gain medium and significantly increases the signal-to-noise ratio, ensuring a low lasing threshold and highly sensitive sensing. The lasing threshold property under different pH conditions is experimentally and theoretically conducted to evaluate the sensing performance, which shows that the lasing threshold highly depends on the pH value of the cladding solution due to the increasing deprotonation process. We further verify that the intensity of the lasing emission and the pH value shows good linearity in the pH range 6.51-8.13, with a 2-order-of-magnitude sensitivity enhancement compared to fluorescence measurement. The proposed OFRR lasing platform shows excellent robustness and low sample consumption, providing a powerful sensing strategy in medicine, and hazardous/toxic/volatile sensing, which require label-free, real-time, and in situ detection.

In this paper, we present the design and operation of a solid-core/liquid-cladding (SL) waveguide excited by an evanescent wave. To do this, an optical fiber is integrated into a microfluidic channel and pumped along the fiber axis, ensuring the cladding solution is excited by the evanescent field of the guided mode at the core/cladding interface. The pump beam is guided by the total internal reflection in the fiber, providing a uniform excitation along the microfluidic channel. The evanescent wave provides precise excitation to the dye molecules in close proximity to the core/cladding interface, which significantly reduces the background fluorescence and increases the signal-to-noise ratio. Fluorescence intensity measurements of different dye concentrations and refractive indices of the cladding solution are conducted to evaluate their influences on the propagation loss, which shows that the peak intensity propagation loss can be as low as about 0.1 dB/cm. We further exemplify that the intensity of the fluorescence emission and the dye concentration show good linearity when the dye is in the low concentration region (<250 mu M). A broad-band and simultaneous light source with a single pump light is also demonstrated by employing cascade SL waveguide segments through fluorescence resonance energy transfer. The proposed SL waveguide demonstrates excellent robustness and is easy to fabricate and use, providing a versatile platform for a variety of applications, such as high-sensitivity detection of low-concentration samples, multiband on-chip light sources, and simultaneous measurement of multiplexed parameters.

Polydimethylsiloxane-based optofluidics provides a powerful platform for a complete analytical lab-on-chip. Here, we report on a novel on-chip laser source that can be integrated with sample preparation and analysis functions. A corrugated sidewall structure is integrated into a microfluidic channel to form a distributed feedback (DFB) laser using rhodamine 6G dissolved in an ethylene glycol and water solution. Lasing is demonstrated with a threshold pump power of 87.9 µW, corresponding to a pump intensity of 52.7 m W / c m 2 . Laser threshold and output power are optimized with respect to rhodamine 6G concentration and core index and found to be in good agreement with a rate equation model. Additionally, the laser can be switched on and off mechanically using a pneumatic cell inducing positive pressure on the grating.

Optofluidic lasers have been an emerging technology for the development of miniaturized light sources and biological and chemical sensors. However, most optofluidic lasers demonstrated to date are operated at the single optical cavity level, which limits their applications in high-throughput biochemical sensing, high speed wavelength switching, and on-chip spectroscopic analysis. Here we demonstrated an optofluidic droplet laser array on a silicon chip with integrated microfluidics, in which four individual droplet optical cavities are generated and controlled by a 2X2 nozzle array. Arrays of droplets with a diameter ranging from 115 μm to 475 μm can be generated, removed, and regenerated on demand. The lasing threshold of the droplet laser array is in the range of 0.63 - 2.02 μJ/mm². An image-based lasing threshold analysis method is developed, which enables simultaneous lasing threshold measurement for all laser units within the laser array using a low-cost camera. Compared to the conventional spectrum-based threshold analysis method, the lasing threshold obtained from the image-based method showed consistent results. Our droplet laser array is a promising technology in the development of cost effective and integrated coherent light source on chip for point-of-care applications.

An optical fiber with a diameter of 125 µm is integrated in a polydimethylsiloxane (PDMS) microfluidic channel. The lasing gain medium is consisted of a lower refractive index (RI) rhodamine 6G (Rh6G) dye solution. With this configuration, we have presented the study on the lasing coupling efficiency and lasing emission spectrum affected by the RI of the dye solution under different polarization states of the pump light. When the fiber is pumped by a linearly s-or p-polarized light, the intensity of the emitted whispering gallery modes (WGMs) lasing, i.e., transverse magnetic (TM) or transverse electric (TE) mode, decreases by approximately two-fold as the RI of the dye solution increases from 1.325 to 1.419, meaning a significant difference on the lasing coupling efficiency is achieved under different RI of the dye solution. When the fiber is excited by a s-and p-mixed polarized light, the mode separation between the adjacent TM and TE mode decreases with the increase of the RI of the dye solution, providing a simple and effective method for applications such as RI sensing.

Double emulsion droplets have found diverse applications in biosensing, chemical synthesis, drug delivery, and highthroughput screening and production. In this work, we demonstrate monodisperse double emulsion optofluidic microlasers with a dye-doped thin shell structure. In particular, robust generation of water-in-oil-in-water (W/O/W) double emulsion droplets in a hydrodynamic flow focusing microfluidic device is demonstrated. With careful arrangement of the three liquid components in the double emulsion, an optical microcavity is formed which supports the whispering gallery modes (WGMs) that locate near the surface of the shell layer. Low-threshold lasing has been achieved in W/O/W double emulsion droplets. Lasing emission from double emulsion droplets with different diameter and different shell layer thickness are characterized on the flow. Due to the high degree of control provided by the microfluidic device, both droplet size and shell layer thickness can be conveniently tuned, and thus the lasing properties. The unique two-phase core-shell structure of the double emulsion droplets provides great flexibility and potential in the development of biosensing platform based on the optofluidic laser system demonstrated in this work, especially in the high-throughput bioanalysis.

We present an optofluidic droplet dye laser that is generated by an array of microfluidic nozzles fabricated on a polycarbonate chip. A droplet resonator forms upon pressurizing the nozzle backside microfluidic channel. Multimode low-threshold lasing is observed from individual microdroplets doped with dye. Additionally, droplets can be conveniently released from the nozzle by water rinsing from the top microfluidic channel and subsequently regenerated, and thus achieve optofluidic lasers on-demand. Our work demonstrates a new approach to generating on-chip laser source and laser arrays in a simple, reproducible, reconfigurable, and low-cost fashion.