Zhenhua Shen

Optically induced transparency in a micro-cavity

Electromagnetically induced transparency has the unique ability to optically control transparency windows with low light in atomic systems. However, its practical applications in quantum physics and information science are limited due to rigid experimental requirements. Here we demonstrate a new mechanism of optically induced transparency in a micro-cavity by introducing a four-wave mixing gain to nonlinearly couple two separated resonances of the micro-cavity in an ambient environment. A signature Fano-like resonance was observed owing to the nonlinear interference of the two coupled resonances. Moreover, we show that the unidirectional gain of the four-wave mixing can lead to the remarkable effect of non-reciprocal transmission at the transparency windows. Optically induced transparency may offer a unique platform for a compact, integrated solution to all-optical and quantum information.

Terahertz wave generation by plasmonic-enhanced difference-frequency generation

We propose an efficient and compact plasmonic surface-enhanced terahertz generation scheme based on nonlinear difference-frequency generation inside a metal–insulator–metal structure. Gold nanowire arrays are planted on top of the surface of a lithium niobate (LN) substrate with second-order nonlinearity to enhance both the nonlinear wavelength conversion and waveguide terahertz waves at the same time. Our numerical simulations show that our structures are capable of generating both tunable continuous and ultrafast-pulsed terahertz sources. We also discuss further improvements on the conversion efficiency by combining with Ti-diffusing LN waveguides.

Characterization of microdroplets using optofluidic signals

We develop a simple method to determine the microdroplet features in a microfluidic chip fabricated by conventional soft lithography. Different sizes of microdroplets are generated through a typical microfluidic T-junction by adjusting the flow rates of the two immiscible liquids. Droplet size and content can be determined by monitoring the optofluidic signals reflected at the fluid-polydimethylsiloxane (PDMS) interface. The demonstrated droplet characterization system can be readily integrated with other microfluidic networks, making it promising for biochemical and biosensing applications.

Random lasing action in a polydimethylsiloxane wrinkle induced disordered structure

This paper presents a chip-scale random lasing action utilizing polydimethylsiloxane (PDMS) wrinkles with random periods as disordered medium. Nanoscale wrinkles with long range disorder structures are formed on the oxidized surface of a PDMS slab and confirmed by atomic force microscopy. Light multiply scattered at each PDMS wrinkle-dye interfaces is optically amplified in the presence of pump gain. The shift of laser emission wavelength when pumping at different regions indicates the randomness of the winkle period. In addition, a relatively low threshold of about 27 μJ/mm2 is realized, which is comparable with traditional optofluidic dye laser. This is due to the unique sinusoidal Bragg-grating-like random structure. Contrast to conventional microfluidic dye laser that inevitably requires the accurate design and implementation of microcavity to provide optical feedback, the convenience in both fabrication and operation makes PDMS wrinkle based random laser a promising underlying element in lab-on-a-chi...

An integrated tunable interferometer controlled by liquid diffusion in polydimethylsiloxane

We demonstrated an integrated tunable interferometer in Polydimethylsiloxane (PDMS). In contrast to most on-chip interferometers which require complex fabrication, our design is realized by conventional soft lithography fabrication. The optical path difference occurs during propagation across a fluid-fluid interface. The diffusion level of the two miscible liquids which is controlled by liquid flow rates provides tunability. Different ratio of two liquid flow rates result in the interference spectral shift. Interference peak numbers are varied with flow rate ratio of two liquids. Mutual diffusion between two liquids changes the profile of the refractive index across the fluidic channel. The two arms structure of our design provides convenience for sensing and detection in biology system. This device not only offers the convenience for microfluidic networks but also paves the way for sensing in chemical microreactors.

On-chip tunable optofluidic dye laser

We demonstrate a chip-scale tunable optofluidic dye laser with Au-coated fibers as microcavity. The chip is fabricated by soft lithography. When the active region is pumped, a relatively low threshold of 6.7  μJ/mm2 is realized with multimode emission due to good confinement of the cavity mirrors, long active region, as well as total reflectivity. It is easy to tune the lasing emission wavelength by changing the solvent of laser dye. In addition, the various intensity ratios of multicolor lasing can be achieved by controlling flow rates of two fluid streams carried with different dye molecules. Furthermore, the convenience in fabrication and directional lasing emission outcoupled by the fiber make the tunable optofluidic dye laser a promising underlying coherent light source in the integrated optofluidic systems.

Random lasing at the edge of a TiO 2 nanotube thin film

In this paper, we introduce a random laser in which lasing action is observed at the edge of a dye-doped TiO2 thin film. A TiO2 nanotube membrane serves as a disordered structure that enhances the optical multiple scattering effect, while Rhodamine 6G dissolved in ethylene glycol is used as a gain medium. In the experiment, a random laser with a low threshold is observed when optically pumped at the fringe of a TiO2 nanotube membrane, which makes it practical for microfluidic integration. Simulation results show that multiple scattered light between the nanotubes and ethylene glycol solution is more likely to form a resonance loop with the help of a random edge structure. This well interrupted the appearance of coherent spikes in the emission laser spectrum in the experiment. The edge random laser offers simplicity and convenience in both fabrication and operation, which makes it a promising component for optofluidic laser integration with TiO2 functional material.

Optically Induced Transparency In a Micro-cavity

Optically induced transparency in an optical microresonator is observed by introducing four-wave mixing gain to couple nonlinearly two isolated resonances of the micro-cavity. Its optical-controlling capacity and non-reciprocity characteristics are also demonstrated.

Whispering gallery mode microlaser based on a fiber-stand polydimethylsiloxane microresonator

Abstract. The whispering gallery mode (WGM) lasing in a polydimethylsiloxane (PDMS)-based microresonator is demonstrated with a convenient and crafty approach. Fabricated by directly brushing dye-doped PDMS solution on an optical fiber, the microresonator is self-formed due to the high surface tension. The size of the resonator can be widely tuned by using different droplet volumes and brushing speeds across the optical fiber. Lasing with a threshold as low as 2.5  μJ/mm2 is observed in this kind of fiber-stand PDMS microresonator. We also investigate the dependence of the lasing threshold on the different polarizations of the pump laser and size of the microresonator. This kind of WGM microresonator will find applications in optical sensors and on-chip integrated systems.