Optofluidic Ring Resonator Laser Research Articles

Optofluidic ring resonator laser with an edible liquid laser gain medium.

We demonstrate a biocompatible optofluidic laser with an edible liquid laser gain medium, made of riboflavin dissolved in water. The proposed laser platform is based on a pulled-glass-capillary optofluidic ring resonator (OFRR) with a high Q-factor, resulting in a lasing threshold comparable to that of conventional organic dye lasers that are mostly harmful, despite the relatively low quantum yield of the riboflavin. The proposed biocompatible laser can be realized by not only a capillary OFRR, but also by an optical-fiber-based OFRR that offers improved mechanical stability, and is promising technology for application to in vivo bio-sensing.

Demonstration of polarization mode selection and coupling efficiency of optofluidic ring resonator lasers.

We demonstrate the polarization mode selection and the dependence of coupling efficiency on polarization state of pump light for an optofluidic ring resonator (OFRR) laser. An optical fiber is chosen to serve as the ring resonator and surrounded by rhodamine 6G dye solution of lower refractive index as the fluidic gain medium. When the ring resonator is pumped by a linearly s-polarized laser, the emitted whispering gallery mode (WGM) lasing is of parallel polarization (TM mode), while p-polarized laser excitation generates a vertically polarized lasing emission (TE mode), both TM and TE mode lasing emission coexist simultaneously if the ring resonator is pumped by the s- and p-mixed polarized light. Further investigation reveals that the lasing intensity of the TM mode is approximately twice that of the TE mode for the same pump energy density, meaning an obvious difference of coupling efficiency on the polarization state of pump light; the experimental results of coupling efficiency are well explained by an induced dipole model.

Monolithic optofluidic ring resonator lasers created by femtosecond laser nanofabrication.

We designed, fabricated, and characterized a monolithically integrated optofluidic ring resonator laser that is mechanically, thermally, and chemically robust. The entire device, including the ring resonator channel and sample delivery microfluidics, was created in a block of fused-silica glass using a 3-dimensional femtosecond laser writing process. The gain medium, composed of Rhodamine 6G (R6G) dissolved in quinoline, was flowed through the ring resonator. Lasing was achieved at a pump threshold of approximately 15 μJ mm(-2). Detailed analysis shows that the Q-factor of the optofluidic ring resonator is 3.3 × 10(4), which is limited by both solvent absorption and scattering loss. In particular, a Q-factor resulting from the scattering loss can be as high as 4.2 × 10(4), suggesting the feasibility of using a femtosecond laser to create high quality optical cavities.

Versatile optofluidic ring resonator lasers based on microdroplets

We develop a novel nL-sized microdroplet laser based on the capillary optofluidic ring resonator (OFRR). The microdroplet is generated in a microfluidic channel using two immiscible fluids and is subsequently delivered to the capillary OFRR downstream. Despite the presence of the high refractive index (RI) carrier fluid, the lasing emission can still be achieved for the droplet formed by low RI solution. The lasing threshold of 1.54 µJ/mm(2) is achieved, >6 times lower than the state-of-the-art, thanks to the high Q-factor of the OFRR. Furthermore, the lasing emission can be conveniently coupled into an optical fiber. Finally, tuning of the lasing wavelength is achieved via highly efficient fluorescence resonance energy transfer processes by merging two different dye droplets in the microfluidic channel. Versatility combined with improved lasing characteristics makes our OFRR droplet laser an attractive platform for high performance optofluidic lasers and bio/chemical sensing with small sample volumes.

A quasi-droplet optofluidic ring resonator laser using a micro-bubble

Optofluidic ring resonator lasers based on micro-bubbles filled with liquid gain medium are demonstrated. Due to the sub-micron wall thickness of the micro-bubble, significant amount of the electric field resides inside the liquid. Consequently, micro-bubbles mimic the droplets in air that have 3-dimensional optical confinement, extremely high Q-factors, and versatility in handling liquids of different refractive index. Furthermore, they enable repetitive interrogation and easy directional laser emission out-coupling without evaporation or size/shape variations. The laser using Rhodamine 6G in methanol is achieved with a threshold of 300 nJ/mm2 and 5.3 μJ/mm2 for 1 mM and 10 μM in concentration, respectively.

Tunable single mode lasing from an on-chip optofluidic ring resonator laser

Single mode lasing from the polydimethylsiloxane based on-chip coupled optofluidic ring resonator (OFRR) with the lasing threshold of a few μJ/mm2 is demonstrated using the Vernier effect. The single mode operation is highly stable even at high pump energy densities. The effect of the OFRR size and coupling strength on the single mode emission is investigated, showing that the excessive coupling results in incomplete side mode suppression. Tuning of the lasing wavelength is achieved by modifying the dye solution.

Demonstration of the coupling of optofluidic ring resonator lasers with liquid waveguides

Optofluidic lasers are of particular interest for lab-on-a-chip-type devices, with broad spectral tunability, convenient microfluidic integration, and a small footprint. Optofluidic ring resonator (OFRR) lasers are advantageous in terms of size but typically generate nondirectional emission that is of minimal practical use. We introduce two unique geometries for soft-lithography-based OFRR lasers--side-coupled rings and spiral rings--both of which can be produced in polydimethyl siloxane substrates with contact molding. These rings utilize evanescent and direct butt-coupling, respectively, to effectively couple the OFRR laser emission into microfluidic channels. A laser threshold of a few to tens of microJ/mm(2) is achieved.

Opto-fluidic ring resonator lasers based on highly efficient resonant energy transfer

We demonstrate an opto-fluidic ring resonator dye laser using highly efficient energy transfer. The active lasing material consists of a donor and acceptor mixture and flows in a fused silica capillary whose circular cross section forms a ring resonator and supports the whispering gallery modes (WGMs) of high Q-factors (>107). The excited states are created in the donor and transferred to the acceptor through the fluorescence resonant energy transfer (FRET), whose emission is coupled into the WGM. Due to the high energy transfer efficiency and high Q-factors, the acceptor exhibits a lasing threshold as low as 0.3 muJ/mm2. We further analyze the energy transfer mechanisms and find that non-radiative Förster transfer is the dominant effect to support the acceptor lasing. FRET lasers using cascade energy transfer and using quantum dots (QDs) as the donor are also presented. Our study will not only lead to development of novel microfluidic lasers with low lasing thresholds and excitation/emission flexibility, but also open an avenue for future laser intra-cavity bio/chemical sensing.

Versatile opto-fluidic ring resonator lasers with ultra-low threshold

We develop a versatile integrated opto-fluidic ring resonator (OFRR) dye laser that can be operated regardless of the refractive index (RI) of the liquid. The OFRR is a micro-sized glass capillary with a wall thickness of a few micrometers. When the liquid in the core has an RI lower than that of the capillary wall (n=1.45), the capillary circular cross-section forms the ring resonator and supports the whispering gallery modes (WGMs) that interact evanescently with the gain medium in the core. When the core RI is higher than that of the wall, the WGMs exist at the core/wall interface. In both cases, the WGMs can have extremely high Q-factor (>109), providing excellent optical feedback for low-threshold lasing. In this paper, we analyze the OFRR laser for various core RI's and then we demonstrate the R6G laser when the dye is in ethanol (n=1.36), chloroform (n=1.445), and quinoline (n=1.626). The lasing threshold of 25 nJ/mm(2) is achieved, two to three orders of magnitude lower than the previous work in microfluidic lasers. We further show that the laser emission can be efficiently out-coupled via an optical waveguide in touch with the OFRR for both high and low RI liquid core, allowing for easy guiding and delivery of the laser light.