Optically Transportable Optofluidic Microlasers with Liquid Crystal Cavities Tuned by the Electric Field

Abstract

Liquid crystal microdroplets with readily adjustable optical properties have attracted considerable attention for building reconfigurable optofluidic microsystems for sensing, imaging, and light routing applications. In this quest, development of active optical microcavities serving as versatile integrated sources of coherent light and ultra-sensitive environmental sensors has played a prominent role. Here, we study transportable optofluidic microlasers reversibly tunable by an external electric field, which are based on fluorophore-doped emulsion droplets of radial nematic liquid crystals manipulated by optical tweezers in microfluidic chips with embedded liquid electrodes. Full transparency of the electrodes formed by a concentrated electrolyte solution allows for applying an electric field to the optically trapped droplets without undesired heating caused by light absorption. Taking advantage of independent, precise control over the electric and thermal stimulation of the lasing liquid crystal droplets, we characterize their spectral tuning response at various optical trapping powers and study their relaxation upon a sudden decrease in the trapping power. Finally, we demonstrate that sufficiently strong applied electric fields can induce fully reversible phase transitions in the trapped droplets even below the bulk melting temperature of the used liquid crystal. Our observations indicate viability of creating electrically tunable, optically transported microlasers that can be prepared on-demand and operated within microfluidic chips to implement integrated microphotonic or sensing systems.

Optofluidic lasers and their applications in biochemical sensing

“…According to the different resonance principles, optical microcavities can be classified as whispering gallery mode (WGM) microcavities, [55][56][57][58][59][60][61][62][63][64][65][66][67][68][69] Fabry-Perot (FP) microcavities, [70][71][72][73][74][75][76][77][78][79][80][81] photonic crystal (PC) microcavities, 53,82,83 resonators of distributed feedback (DFB) lasers, 54,[84][85][86][87][88][89] random laser (RL) structures, [90][91][92][93] and other optical microcavities with special structures. 48,[94][95][96][97][98] Each type of optical microcavity listed above will be systematically described below.…”

“…There are various types of WGM microcavities. In terms of the geometric shapes, optical microcavities can be simply classified as annular, cylindrical, spherical cavities, etc., and more specifically, microdisk cavities, 30,99 microbottle cavities, 66 microdroplets, 59,61,62,64,68,100 microring cavities, [55][56][57][58]60,63,67,69 microbubble cavities, 65,101 etc. Compared with other types of optical microcavities, WGM microcavities usually have higher Q-factors (>10 6 ), which results in lower thresholds of laser output.…”

“…These features make WGM microcavities more valuable for practical applications and are often used in the design of OFLs by scholars. [54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69] In the design of OFLs, microdroplets and microring cavities are the most used microcavities. Microfluidic-based microdroplet generation techniques have been developed continuously in recent years.…”

“…Biological gain materials can be embedded or implanted into biological cells or tissues for the analysis of biological processes. Many types of biological gain materials have been used as gain media for OFLs, including fluorescein, 52 fluorescent proteins, 32,51,62,105 flavin mononucleotide, 125 chlorophyll, 33 riboflavin, 124 and indocyanine green. 135 Fluorescent proteins have a broad range of applications in many areas of research in biology.…”

Motor-like microlasers functioning in biological fluids

“…[19][20][21][22][23][24][25] Recent studies have also incorporated microlasers with cells or tissues, aiming to reveal biological behaviours at the cellular level. [26][27][28][29][30] In spite of the progress that has been developed, [31][32][33][34] manipulation of microlasers under large dimensions with controllable and versatile functions remains a key challenge. Exploring a versatile way for driving and controlling coherent light sources in a liquid environment holds great potential in photostimulation and photoactivable reactions.…”