Mehdi Aas

Thermal tuning of spectral emission from optically trapped liquid-crystal droplet resonators

Surfactant-stabilized emulsion droplets of liquid crystals (LCs) suspended in water and labeled with a fluorescent dye form active, anisotropic optofluidic microresonators. These microresonators can host whispering gallery modes (WGMs), high-quality morphology-dependent optical resonances that are supported due to the contrast of refractive index between the LC droplets and the surrounding aqueous medium. In addition, owing to the refractive index contrast, such LC emulsion droplets can be stably trapped in three dimensions using optical tweezers, enabling long-term investigation of their spectral characteristics. We explore various combinations of fluorescently dyed LC droplets and host liquid-surfactant systems and show that the WGM emission spectra of optical resonators based on optically trapped LC emulsion droplets can be largely and (almost) reversibly tuned by controlled changes of the ambient temperature. Depending on the actual range of temperature modulation and LC phase of the studied droplet, thermally induced effects can either lead to phase transitions in the LC droplets or cause modifications of their refractive index profile without changing their LC phase. Our results indicate feasibility of this approach for creating miniature thermally tunable sources of coherent light that can be manipulated and stabilized by optical forces.

Optofluidic microlasers based on liquid droplet resonators for biophotonics

Optofluidic sources of laser light that can be integrated into lab-on-a-chip systems allow for dynamic control of laser resonator geometry and gain media, and open up new paradigms in biosensing.1 With their unique features, liquid droplets stand out among different optical resonator alternatives for developing optofluidic lasers. Thanks to their spherical geometry and smooth surface, droplet-based cavities host high-quality optical resonances called whispering gallery modes (WGMs).2 These low-loss resonant modes then allow droplet lasers to operate at low-threshold pump powers. Liquid droplets are easy to produce using aerosol generators in air or flow focusing or T-junction geometries in a microfluidic chip. Upon generation, droplets can be captured and manipulated using optical micromanipulation techniques such as optical tweezing or stretching. Their easily deformable nature makes them attractive for developing tunable optical components, e.g., tunable light sources. In addition, water-based droplet cavities are biologically compatible, allowing for the use of aqueous solutions of biologically relevant molecules as laser gain media. Optical tweezing enables noninvasive confinement and manipulation of liquid droplets in air or another host fluid. Such optically trapped droplets containing a suitable gain medium can subsequently serve as movable active laser cavities. Using an experimental setup that combined an independent continuous trapping laser beam at 1064nm and a pulsed pump laser beam at 532nm at the focus of a high-numerical-aperture microscope objective, we were able to optically pump droplets that were simultaneously manipulated in three dimensions (see Figure 1). Thanks to the dye gain medium placed in the droplets, we observed dye lasing for two experimental configurations: Figure 1. Dye lasing in liquid microdroplets that are trapped and manipulated in 3D using optical tweezers. a.u.: Arbitrary units.