Time Integrated Detection and Applications of Fs- Laserpulses Scattered by Small Droplets

A characteristic of femtosecond pulse scattering on small particles is the temporal separation of the pulses, which is due to the different optical path lengths through the particle. Therefore the particle converts the incident pulse in a train of pulses, with the pulse heights and temporal separation depending on particle size, refractive index and observation angle. Because of the temporal pulse separation, signal parts from different scattering orders are no longer coherent. Therefore interference structures, which are in the case of continuous illumination the cause of scattering lobes, disappear. The interference inside a single scattering order, as for the rainbow, exists as long as the pulses from different paths overlap. The detection does not require high temporal resolution but is time integrated. This offers the possibility to improve or extend techniques which are disturbed by additional scattering orders, as the rainbow technique for small particles. A second characteristic of femtosecond pulses is the broad electromagnetic spectrum of the illumination. For continuous laser illumination with one spectral line, a spherical particle generates optical resonances for various wavelength-diameter ratios (morphology dependent resonances, MDRs). These optical resonances increase the optical cross-section of the particle and generate strong oscillations in the diameter-intensity relation. Because the power of a pulse is distributed over several spectral lines, which generally do not fulfill the resonant conditions at the same time, the influence of optical resonances is suppressed strongly. We demonstrate an electrodynamic trap with a novel geometry of electrodes making it possible to follow the evaporation of water droplets over seconds and prove the monotonic nature of the recorded intensity as a function of diameter. The illumination with femtosecond laserpulses forces a monotonous relation between intensity and diameter without any ambiguity. This opens the way for particle sizing using the scattered intensity. 1. Laser pulse and continuous wave scattering on single droplets Illuminating single droplets with electromagnetic waves yields an angular intensity distribution depending on frequency, refractive index and coherence length. Investigating this interdependence for microscopic scale droplets leads to new possibilities in particle characterization. A spatially extended droplet with corresponding size and refractive index will generate multiple scattering orders when interacting with electromagnetic waves. Incident waves are partly reflected by, and partly refracted into the droplet. Internal waves split further because of ongoing reflection and refraction. If the droplet is imaged, different scattering orders are observed on the sphere’s surface as a juxtaposition of point sources. In the special case of an incoming plane wave the Lorenz-Mie Theory yields the total scattered field, which can be expanded into Debye-series. Individual complex terms of the series are interpreted as individual scattering orders (Albrecht et al. 2003). In analogy to summing up the contributions of individual complex terms, the angular intensity distribution in the far field of the droplet is a result of interference: Electromagnetic field vectors of individual scattering orders add up (Fig. 1-a). The assumption of a monochromatic, 14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 7-10 July, 2008 2 incident plane wave is valid for continuous wave (CW) lasers or for pulsed lasers with pulse lengths far larger then the diameter of a droplet. Fourier decomposition can be used to expand the scope of the Lorenz-Mie Theory to inhomogeneous waves scattered from a spherical, homogeneous, isotropic droplet. In the special case of ultrashort laser pulses temporal effects become important, because the laser pulse length is in the order of microns (a 10fsec pulse corresponds to a 3μm pulse length). For laser pulses far shorter than the diameter of the scattering object this results in temporally separated scattering orders described by the Debye-expansion (Bech and Leder 2004; Damaschke et al. 2002; Mees et al. 2001). Because the coherence length corresponds to the pulse length, no far field interference takes place between the individual scattering orders. In this case the angular intensity distribution of the droplet is not a sum of electromagnetic field vectors, but of the intensities of individual scattering orders detected at specific scattering angles (Fig. 1-b).