Dynamic range of submicron/nanoparticle sizing with photon correlation LDA

Signal processing method was developed for simultaneous particle counting, sizing and flow velocity measurements with photon correlation laser Doppler anemometers. The high sensitivity of avalanche photodiodes in photon counting mode assures the ability to catch the individual particles in the submicron/nanometer size range. A detailed discussion is given about the optimal set up parameters (wavelength, detector position and refractive index) and calibration measurements are shown to determine system parameters in a particular arrangement and simulations to estimate the lower size limit of the particle characterization. Estimations are given for the required laser power to detect 20 photon counts in average for a single particle transit as a lower limit of the particle sizing procedure. In the calibration measurement the most sensitive size region was below 300nm down to the sizing limit (20 photons in average) at 514nm illumination wavelength. As a numerical example we conclude that the size estimation of a polystyrene sphere particle of 50nm diameter requires at least 123 kW/cm 2 laser power density at 350nm while 587 kW/cm 2 at 514nm in the studied system.