| I explore the possibility of detecting single nanoparticles below 100 nm in real time. The motivation behind this project is the development of an optical sensor capable of monitoring the environment for harmful viruses or other nanoscopic pathogens. The main difficulty in detecting small particles arises from a strong signal dependence on particle size. In a standard approach, it is customary to measure the intensity of light scattered by a particle. This measurement has R6 dependence on particle size, due to which the signal falls quickly below the noise level when attempting to detect smaller particles. The limit of currently available single particle detectors is approximately 200 nm for transparent glass-like particles. On the other hand, my approach investigates two different optical techniques, both of which employ a weaker particle size dependence. The weaker size dependence in our schemes leads to a better signal-to-noise ratio for small particles.; The first method is based on measuring the optical gradient force, which acts on the particle inside a strongly focused laser beam. The optical force is proportional to the third power of particle size, R 3. Similar to mass spectrometry, the laser focus influences the trajectory of a particle as it passes through a tightly-focused laser beam. Particle trajectory is monitored using quadrant detector placed in transmission. I successfully detect and recognize R = 50 nm and R = 100 nm particles in a mix. I show that such force measurement is not background-free due to the Brownian motion. I demonstrate the ways to minimize the contribution of the Brownian motion, and discuss the limitations of the scheme.; The other detection approach is based on the interferometric detection of the scattered light amplitude. The approach represents an interferometer, where the scattered light forms one arm. The scattered light and the reference beam are combined on a split photodetector, which renders a background free signal. I analyze signal-to-noise, and demonstrate real-time detection (with time resolution of 1ms) of particles down to R = 7 nm. In the final stage of this work, I apply this method to detecting live viruses. I successfully detect and distinguish Influenza and Sindbis viruses from a mix of polystyrene particles of similar size. |
