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27 February 2006 Levitated droplet dye laser: a new approach for single molecule detection
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We present the first observation, to our knowledge, of lasing from a levitated, dye droplet. The levitated droplets are created by computer controlled pico-liter dispensing into one of the nodes of a standing ultrasonic wave (100 kHz), where the droplet is trapped. The free hanging droplet forms a high quality optical resonator, which shape can be externally controlled by the ultrasonic field, yielding wavelength tunability and directional control of the emission. Our 700 nL lasing droplets consist of Rhodamine 6G dissolved in ethylene glycol, at a concentration of 0.02 M. In our experiments the droplets are optically pumped at 532 nm light from a pulsed, frequency doubled Nd:YAG laser, and the dye laser emission is analyzed by a fixed grating spectrometer. With this setup we have achieved reproducible lasing spectra in the wavelength range 610 nm - 650 nm. The lasing spectra can controllably be modulated by shaping the droplet. Lasing micro-droplets have been demonstrated earlier, where the droplets in free fall passed the pumping laser beam. The levitated droplet technique has successfully been applied for a variety of bio-analytical applications at single cell level. In combination with the lasing droplets, the capability of this high precision setup can further be applied to create a highly sensitive intra cavity absorbance detection system.
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H. Azzouz, L. Alkhafadiji, S. Balslev, J. Johansson, N. A. Mortensen, S. Nilsson, and A. Kristensen "Levitated droplet dye laser: a new approach for single molecule detection", Proc. SPIE 6092, Ultrasensitive and Single-Molecule Detection Technologies, 60920D (27 February 2006);

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