Förster Resonance Energy Transfer (FRET) is a radiationless distance-dependent transfer of energy from an excited donor fluorophore to an acceptor fluorophore. This radiationless interaction of a donor-acceptor pair through resonance is observed by an increase/decrease in the acceptor/donor fluorescence intensity, respectively. Here we present preliminary results on the fluorescence spectra of optically levitated micro-droplets doped with two different dyes that works as FRET pair. The laser light used for levitation (λ=660 nm) passes through a telecentric system of lenses to form a controllable double optical trap system. Micrometer sized droplets are produced using two on-demand piezo-driven dispensers. This allows independent trapping of differently dyed droplets in two traps where a collision between the droplets can be induced by moving the trap positions. The dye molecules mix when two droplets collide and coalesce. The emission spectrum obtained when the droplets are illuminated with laser having a wavelength of 532 nm is observed with a spectrometer which can record up to 26,000 spectra per second. We compare the results with the spectra taken from the same solutions in a cuvette. The results indicate that we are able to observe the FRET effect in single droplets with an exposure time as short as 100 µs. This spectroscopic investigation is an ongoing research project with the long-term goal to investigate environmental effects of aerosols in the atmosphere.
Ti:Sapphire lasers are powerful tools in the field of scientific research and industry for a wide range of applications such as spectroscopic studies and microscopic imaging where tunable near-infrared light is required. To push the limits of the applicability of Ti:Sapphire lasers, fundamental understanding of the construction and operation is required. This paper presents two projects, (i) dealing with the building and characterization of custom built tunable narrow linewidth Ti:Sapphire laser for fundamental spectroscopy studies; and the second project (ii) the implementation of a fs-pulsed commercial Ti:Sapphire laser in an experimental multiphoton microscopy platform.
For the narrow linewidth laser, a gold-plated diffraction grating with a Littrow geometry was implemented for highresolution wavelength selection. We demonstrate that the laser is tunable between 700 to 950 nm, operating in a pulsed mode with a repetition rate of 1 kHz and maximum average output power around 350 mW. The output linewidth was reduced from 6 GHz to 1.5 GHz by inserting an additional 6 mm thick etalon. The bandwidth was measured by means of a scanning Fabry Perot interferometer.
Future work will focus on using a fs-pulsed commercial Ti:Sapphire laser (Tsunami, Spectra physics), operating at 80 MHz and maximum average output power around 1 W, for implementation in an experimental multiphoton microscopy set up dedicated for biomedical applications. Special focus will be on controlling pulse duration and dispersion in the optical components and biological tissue using pulse compression. Furthermore, time correlated analysis of the biological samples will be performed with the help of time correlated single photon counting module (SPCM, Becker&Hickl) which will give a novel dimension in quantitative biomedical imaging.
In this paper a versatile experimental system for optical levitation is presented. Microscopic liquid droplets are produced on demand from piezo-electrically driven dispensers. The charge of the droplets is controlled by applying an electric field on the piezo-dispenser head. The dispenser releases droplets into a vertically focused laser beam. The size and position in 3 dimensions of trapped droplets are measured using two orthogonally placed high speed cameras. Alternatively, the vertical position is determined by imaging scattered light onto a position sensitive detector. The charge of a trapped droplets is determined by recording its motion when an electric field is applied, and the charge can be altered by exposing the droplet to a radioactive source or UV light. Further, spectroscopic information of the trapped droplet is obtained by imaging the droplet on the entrance slit of a spectrometer. Finally, the trapping cell can be evacuated, allowing investigations of droplet dynamics in vacuum. The system is utilized to study a variety of physical phenomena, and three pilot experiments are given in this paper. First, a system used to control and measure the charge of the droplet is presented. Second, it is demonstrated how particles can be made to rotate and spin by trapping them using optical vortices. Finally, the Raman spectra of trapped glycerol droplets are obtained and analyzed. The long term goal of this work is to create a system where interactions of droplets with the surrounding medium or with other droplets can be studied with full control of all physical variables.