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  • Open access
  • 57 Reads
Transition between coherent perfect absorption and lasing in a bilayer cylinder structure by changing the excitation channels

We investigate a unique kind of bilayer cylinder structure with loss in core and gain in shell, which exhibits the fascinating properties of switching between coherent perfect absorption and lasing by changing the excitation channels with different angular momentums. In this paper, such cylinder can act as coherent perfect absorber (CPA) under the excitation of monopolar waves, while it becomes a laser under the excitation of dipolar waves. We also show that a CPA can be switched to a laser by covering a metasurface which has ability to change the monopolar wave into dipolar wave. Our work demonstrates a different approach for implementation of transition between CPA and laser.

  • Open access
  • 73 Reads
An Optofluidic Microlens for Continuous Light Tuning from Focus to Divergence via Collimation

In this research, an in-plane hydrodynamically reconfigurable optofluidic microlens is porposed [1]. The microlens is formed by the laminar flow of a high-refractive-index stream and two low-refractive-index streams as shown in Figure 1. A mathematical model based on the quadrupolar flow theory is established for prediction of the stream dispersion and focal length [2-3]. The calculated streamlines are shown in Figure 2. These streamlines indicate the potential interface position of the cladding and core streams. By properly controlling the flow rate ratio of the streams, the shape of the high-refractive-index stream can be adjusted into biconvex, flat and biconcave, and the curvature of the stream interfaces can be precisely manipulated.

The fabricated microfluidic chip is shown in Figure 3. In the experiment, silicone oil (refractive index = 1.579) is selected as the high-refractive-index stream and 28.9% calcium chloride solution is used as the low-refractive-index stream. The refractive index of the calcium chloride solution is matched to that of PDMS (refractive index = 1.403) to avoid light scattering. Figure 4 shows the variation of the mircolens versus the flow rate ratio between calcium chloride solution and silicone oil. The interfaces between the fluids change with the adjustment of the flow rate of calcium chloride solution. When the flow rate ratio is low, the stream of silicone oil squeezes the streams of calcium chloride solution and forms a biconvex microlens. The curvatures of the interfaces between the fluids are symmetrical and positive and become small with the increase of the flow rate ratio. The interfaces turn to flat when the flow rate ratio is 0.45. If the flow rate ratio is kept increasing, the streams of calcium chloride solution expands and the stream of silicone oil shrinks. The interfaces become concave, resulting in negative curvatures. A biconcave microlens is formed. The output light beam is observed in the beam-tracing chamber, as shown in Figure 5. When the flow rate ratio is small, the shape of the microlens is biconvex and a focused beam can be observed in the beam-tracing chamber. With the increase of the flow rate ratio, the convergent angle shrinks. Afterward, the shape of the microlens gradually turns into biconcave and the output beam becomes divergent. Furthermore, the divergent angle expands with the flow rate ratio. The light beam is stable and no fluctuation is observed.

The proposed optofluidic microlens is a promising candidate for various microfluidic or lab-on-a-chip applications including biomedical sensing, cellular imaging and on-chip photonic signal processing.

  • Open access
  • 87 Reads

The mapping of cellular traction forces is crucial to understanding the means by which cells regulate their physiological function. Polymeric micropillar arrays have been used for measuring cellular traction force. However, the field-of-view of existing measurement methods is limited and the biocompatibility of micropillar arrays needs to be further improved. Since the submicron scale deflection of pillars is typically measured with a 60x objective lens, the field-of-view is limited, ensuring that only a few cells can be imaged at a time, thereby reducing the measurement efficiency.

To realize high-throughput cellular force mapping of multiple cells in a large field-of-view, a cell force measurement method was studied based on MEMS-based micropillar arrays. The force-displacement relationship of micropillar was achieved from analytic model and numerical simulation. The micropillar arrays were designed, fabricated and activated to yield force measurement resolution on the scale of nano newton and more in-vivo like microenvironment. The resulting PDMS micropillar was 2 μm in diameter and 6 μm in height.

For high throughput cell force mapping, double-sided micropillar arrays and the measuring system based on moiré effect were designed and validated. The double-sided micropillar arrays (DMA) integrate two independent micropillar arrays into a single sensor. Cells were seeded on the top side of the DMA, where they spread, contract and distort pillars under the cell body. The micropillars on the opposite side serve as a reference grating which enables the generation of a two-dimensional moiré pattern with the deflected pillars on the top side upon laser illumination through the DMA. When micropillars on the cell culture side are deflected due to cell contraction, the distorted moiré fringes are readily seen. The cells cultured on DMA may be analyzed using both conventional microscopy and moiré-based force mapping. Compared to the moiré-based method using two independent substrates, the use of DMA precludes the need for precise alignment of two independent samples, simplifying the system and improving moiré pattern contrast.

From the force map, it is clear that the maximum force was generated in the center of the cell, near the nucleus, with a force value of approximately 40 nN. Compared to the pillars in the center of the cell, the pillars under lamellipodia were not deflected, though some pillars were entirely involved in the cytoskeleton. The resulting force maps were consistent with the force mapping results using conventional microscopy image analyses, enabling the real-time acquisition of cell force maps from a larger field-of-view.

Using optical microscopy and confocal microscopy, cell force measurements were conducted and the image analysis method was discussed. Cell force of human hepatic stellate cell line (LX-2) in their activated status was quantified, which will aid disease study and drug screen related to liver fibrosis. Also the 3 days cell culture on the micropillar arrays indicates that the micropillar arrays was biocompatible.

  • Open access
  • 75 Reads

In this work, we present the design and characterization of a 1xN beam splitter based on an MMI waveguide fabricated on MEMS technology. The proposed structure also has the advantage of allowing the splitter to be used at wavelengths below the 1.1 µm silicon cut off wavelength that is needed for many optofluidic applications. A compact splitter design is achieved by guiding the light in air in a parallel plate waveguide structure with the lowest achievable guiding refractive index, with paired excitation in the input and parabolic tapering in the waveguide width along its length. The structure is simulated at different wavelengths using a wide angle FD-BPM technique. Measurement results of the fabricated structure are also shown at wavelengths of 1310 nm and 980 nm.

  • Open access
  • 105 Reads
Fabrication of low-threshold ZnO nanorod array random laser using a laser-induced hydrothermal synthesis

Random lasers (RLs) have recently expected as unique speckle-free laser light sources for sensors and imaging. To improve the controllability of RLs, we had proposed a novel resonance-controlled RL, which were composed of agglomerated mono-dispersed spherical ZnO nanoparticles. Although the resonance-controlled RL showed unique features such as quasi-single-mode and low lasing threshold, it is difficult to realize efficient optical input-output and electrode formation using agglomerated nanoparticle film. In contrast, because a two-dimensional nanorod array can also induce light localization via in-plane multiple light scattering, the accessibility of excitation and lasing light in the vertical direction could be expected to be improved comparing with agglomerated nanoparticles. As the fabrication method of nanorod array structures, a laser-induced hydrothermal synthesis using a local heating by laser irradiation on a gold thin film has recently been proposed. Because, unlike the conventional hydrothermal synthesis method, this method can easily control the size of nanorods by the control of irradiated laser power and time, we attempted to realize the resonance-controlled two-dimensional RL in nanorod array structures. In the experiments, we succeed to induce random lasing in ZnO nanorod array structures fabricated by a laser-induced hydrothermal synthesis, and find that their thresholds and lasing probability strongly depend on the fabrication condition (irradiated laser intensity, growth time, precursor concentration). These results suggest the possibility to control the RL properties simply by tuning the irradiated laser conditions.

  • Open access
  • 60 Reads

This paper reports a new design of tunable thermal gradient index (GRIN) lens using the thermal diffusion between one liquid [1], and its application in dynamic trap of single living HEK 293 cell. This system consists of the GRIN lens part and the cell trapping part. The former includes a trapezoid and a rhombus region to form a GRIN lens. The optical properties can be modulated by flow rates and liquid temperature. The latter is isolated to the GRIN lens by a very thin PDMS wall for dynamic cell trap. The cells flowing in this region will be trapped and analyzed by the synergy of optical forces and drag forces. Unlike other counterparts using concentration diffusion [2] and thermal diffusion [3], this thermal GRIN lens is formed by thermal diffusion between only one liquid, and the RI along radial direction fits to a parabolic variation, and its enhanced performance shows more flexibility of controlling single living cell as compared to the solid optical tweezers.

  • Open access
  • 56 Reads
Integrated High-resolution Photonic Spectrometer

An integrated high-resolution on-chip spectrometer is designed, fabricated and experimentally tested. A thermally tunable microring resonator is adopted to function as a tunable filter so as to retrieve the input spectrum. The robust integrated spectrometer is quite compact, low cost and CMOS-compatible. It has high potential to be used in on-chip sensing and spectroscopy systems.

  • Open access
  • 51 Reads
NEMS Actuator Enhanced by Gradient Optical Force with A Large Range

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.

  • Open access
  • 115 Reads
On-chip Cavity-enhanced Fourier-transform Spectrometer

A microring resonator (MRR) cavity-enhanced on-chip Fourier-transform spectrometer (FTS) is demonstrated. High resolution (1.7 nm) and large bandwidth (at least 100 nm) are achieved with the help of high-Q cavity. The on-chip spectrometer is very robust, cheap and compatible with other integrated photonic devices. It is promising to be applied in integrated photonic sensing and on-chip spectroscopy system.