Seeing small

This paper reviews a number of used and/or proposed ideas for optical detection of small particles including single-molecules. Different techniques (direct absorption and scattering, interferometry, use of sub Poissonian statistics, cavity enhancement, and thermal lens detection) are compared in terms of signal-to-noise (SNR) ratio. It is shown that scattering (resonance and non-resonance) remains the method of choice for most practical applications. Thermal lens detection can do potentially better, but its performance critically depends on the thermal properties of the particle surroundings.