In vitro and in vivo biolasing of fluorescent proteins suspended in liquid microdroplet cavities.

Fluorescent proteins are indispensable for selective, quantitative visualization of localization, dynamics, and interactions of key molecular constituents of live cells. Incorporation of fluorescent proteins into an optical cavity can lead to a significant increase in fluorescence signal levels due to stimulated emission and light amplification in the cavity, forming a laser with biological gain medium. Utilization of lasing emission from fluorescent biological molecules can then greatly enhance the performance of fluorescence-based biosensors benefiting from the high sensitivity of non-linear lasing processes to small perturbations in the cavity and the gain medium. Here we study optofluidic biolasers that exploit active liquid optical resonators formed by surface-supported aqueous microdroplets containing purified yellow fluorescent protein or a suspension of live E. coli bacterial cells expressing the fluorescent protein. We first demonstrate lasing in fluorescent protein solutions at concentrations as low as 49 µM. Subsequently, we show that a single fluorescent bacterial cell of micrometre size confined in a droplet-based cavity can serve as a laser gain medium. Aqueous droplet microcavities allow the maintenance of the bacterial cells under conditions compatible with unimpeded growth. Therefore, our results also suggest a direct route to microscopic sources of laser light with self-regenerating gain media.

Optical manipulation of airborne particles: techniques and applications.

In the following paper, we discuss new methods to trap and manipulate airborne liquid aerosol droplets. We discuss the single gradient force trapping of water aerosols in the 2-14 micron diameter range using both 532 nm and 1064 nm light, as well as the holographic optical trapping of arrays of aerosols. Using this holographic technique, we are able to show controlled aerosol coagulation. We also discuss two techniques based on the radiation pressure trapping of aerosols, namely the dual beam fibre trap and the controlled guiding of aerosols using Bessel beams. We conclude with a discussion of new topics for study based upon these techniques and some possible applications.

Experimental study of the phenomenon of 1 x N spectral switch due to diffraction of partially coherent light.

Spectral properties of a class of partially coherent light with spectral profiles of varying bandwidths are studied on diffraction by a circular aperture in the far zone for different diffractive angles, i.e., for on-axis and off-axis points on the observation plane. It is found that the spectrum of the light in the far zone is different from that at the aperture plane. This change in the spectrum is termed spectral shift, which is found to be different at different diffractive angles. The spectral shift for a fixed diffactive angle shows a gradual change. However, for a critical value of the coherence at the aperture plane, the spectral shift shows a rapid transition, termed spectral switch. For different diffractive angles the coherence that causes the spectral switch also differs. Therefore the phenomenon of 1 x N spectral switch (consisting of one input port and N output ports) is studied experimentally.