Scanning interferometric microscopy for the detection of ultrasmall phase shifts in condensed matter

We describe a scanning optical microscope based on polarization Sagnac interferometry for measuring ultrasmall phase shifts, and use this method to detect small numbers of absorbing molecules in a solid without the use of fluorescence. The absorption and concomitant optical phase shift of terrylene dopant molecules in a p-terphenyl host crystal are made time dependent by periodic optical saturation of the sample. A detection sensitivity of 8.75 10−8 rad is achieved with a 0.078 Hz bandwidth, and detection of signals from as few as 19±3 terrylene molecules is demonstrated at room temperature.