Liquid crystal emulsion micro-droplet WGM resonators Jan Ježek 1* , Zdeněk Pilát 1 , Oto Brzobohatý 1 , Alexandr Jonáš 2,3 , Mehdi Aas 2 , Alper Kiraz 2 , Pavel Zemánek 1 1) ASCR, Institute of Scientific Instruments, Královopolská 147, 612 64 Brno, Czech Republic 2) Department of Physics, Koç University, Rumelifeneri Yolu, Sariyer, 34450 Istanbul, Turkey 3) Department of Physics, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey * jezek@isibrno.cz; phone +420 541 514 524; fax +420 541 514 402; www.isibrno.cz/omitec ABSTRACT We introduce tunable optofluidic microlasers based on optically stretched or thermally modified, dye-doped emulsion droplets of liquid crystals (LC) confined in a dual-beam optical trap. Droplets were created in microfluidic chips or by shaking. Optically trapped microdroplets emulsified in water and stained with fluorescent dye act as an active ultrahigh-Q optical resonant cavity hosting whispering gallery modes (WGMs). Tuning of the laser emission wavelength was achieved by a controlled deformation of the droplet shape using light-induced forces generated by dual-beam optical trap and by thermal changing of the order in the LC. Keywords: optical trapping, microfluidic chips, liquid crystals, whispering gallery modes, droplets 1. INTRODUCTION Various applications in physics, chemistry, biology, medicine, etc. are possible and convenient to carry out in small volumes, directly in the field of view of the optical microscope, and with high precision of the sample dosing. Downsizing brings many advantages such as reducing the consumption of reagents for chemical analysis. It is possible to observe a range of reactions in parallel, to shorten reaction times, maintain stable sample temperature, low reagent volumes imply increased safety, etc. These trends lead to complex microfluidic systems, such as “lab-on-a-chip” [1]. Microfluidic chips consist of channel systems, chambers and various control and management elements (pumps, mixers, valves, etc. [2, 3]) connected to generally complicated structures with dimensions in the order of units to hundreds of micrometers. The possibility of precise fluid flow control in such scale opens a wide field for possible inclusion of optical micromanipulation techniques, optical sorting [4], laser ablation, microspectroscopy, or microphotopolymerization [5, 6]. Excitation of whispering gallery modes (WGM) lasing in fluorescently labeled droplets of water emulsified in low refractive index oil in microfluidic channel was reported previously [7]. Droplet based microresonators from cholesteric (chiral) liquid crystals (LC) were fabricated in microfluidic channel and sorted according to their chirality by their interaction with laser beam [8]. WGM lasing of nematic LC droplets in microchannel was used for chemical sensing of surfactant concentration [9]. Spectral characteristics of WGM lasing in cholesteric LC droplets were shown as well [10]. Various principles of spectral tuning of WGM lasing were employed for microresonators based on LC, oil, or water droplets [11-13]. Optomechanical characteristics of solid chiral microresonators dependent on laser beam polarization were also studied [14]. 2. MATERIALS AND METHODS We devised tunable optofluidic microlasers based on optically stretched, dye-doped emulsion droplets of nematic liquid crystals (LC) confined in an optical trap. Optically trapped microdroplets of LC emulsified in water and stained with fluorescent dye act as active ultrahigh-Q optical resonant cavities hosting whispering gallery modes (WGM) which enable dye lasing with low threshold pump powers. In order to achieve tunable dye lasing, the droplets were trapped in a single beam optical tweezers, pumped with a pulsed green laser beam and simultaneously electrically heated or stretched by a dual-beam laser trap. Experimental setup for the observation of tunable lasing from optically trapped and stretched or heated emulsion microdroplets is shown in figure 1. 19th Polish-Slovak-Czech Optical Conference on Wave and Quantum Aspects of Contemporary Optics, edited by Agnieszka Popiołek-Masajada, Wacław Urbańczyk, Proc. of SPIE Vol. 9441, 944108 © 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2176025 Proc. of SPIE Vol. 9441 944108-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/07/2017 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx