Study On The Propertyand Generation Of Optofluidic Laser Based On High-Q Fabry–Pérot Microcavities

Optofluidic laser is emerging as a new technology platform for novel on-chip photonic devices and sensitive biochemical sensors.Currently, various optical microcavities have been explored for microfluidic lasers, including Fabry–Pérot（FP） cavities, distributed feedback gratings, photonic crystals and optical ring resonators. Among them, the FP cavity has advantages in ease of implementation, good compatibility with microfluidics and bulk interaction between the electromagnetic field and the gain medium. In addition, the FP cavity is able to accommodate a solution of any refractive index, making it more flexible than other types of cavities.However, the current FP cavities suffer from two major drawbacks.First, most of the FP microcavities utilize metal-coated mirrors as thereflector, which have a maximum reflectivity of approximately 95%,corresponding to a theoretically low finesse and Q-factor. Second, the FP cavities are usually constructed in a plano-plano format, which is highly susceptible to optical misalignment during cavity assembly and integration with microfluidics and results in further degradation of the Q-factor. Thus, the FP cavities currently used in optofludic laser normally show high lasing thresholds.To solve these problems, we studied the property and generation of optofluidic laser based on stable and high Q-factor Fabry–Pérot microcavities. The program of our work can be divided into several parts:1) We created arrays of microwells on fused silica chips using a CO₂ laser, then the chips were coated with DBR layers to form the concave and planar mirrors（a mirror reflectivity of ≥ 99.9% at the center wavelength）. After assembling, the plano-concave FP microcavity had a Q-factor of 5.6 × 105 and finesse of 4 ×103, over 100 times higher than those for the FP microcavities in existing optofluidic lasers.2) The optofludic laser chips were assembled using a microfludic packaging technique and 1 mmol L-1R6 G dye in ethanol was used to test the plano-concave FP microcavities, showing an ultralow lasing threshold of only 90 nJ mm-2, over 10 times lower than that in the corresponding unstable plano-plano FP microcavities formed by thesame DBR coatings on the same chip. Meanwhile, by expanding the pump laser and shorting the cavity length respectly, simultaneous laser emission from the optofluidic laser array on the chip and single-mode lasing operation were also demonstrated.3) By changing the concentration of R6 G dye in ethanol, the lowest dye concentration which can achieve laser emission in p-c FP cavities was also detected.4) The dye molecules are optically pumped directly in the methods above, to realize the indirect optical pumping, the generation of optofluidic FRET（fluorescence resonant energy transfer） laser in FP resonator was also studied. Experimental results show that the acceptor exhibits a lasing threshold as low as 0.48 mJ/mm2 in the FP optofluidic cavity. Meanwhile, the detection of low concentration substance can be realized though FRET laser in terms of laser detection.Our work will lead to the development of optofluidic laser-based biochemical sensors and novel on-chip photonic devices with extremely low lasing thresholds（nJ mm-2） and mode volumes（fL）.