Phys. Rev. Lett. 111, 033903 (2013)

Credit: © 2013 APS

The ability to detect features smaller than the diffraction limit of light would greatly enhance applications ranging from semiconductor device inspection to the detection of carcinogenesis in human cells. Several approaches for realizing optical detection on this scale have been demonstrated in recent years, but each has its limitations (for example, requiring labelling or fluorescence). Optical detection is especially challenging in systems with low refractive-index contrasts, such as biological cells. Now, Lusik Cherkezyan and colleagues from Northwestern University in the USA have shown that interferometric spectroscopy of scattered light can quantify the statistics of refractive index fluctuations on the subdiffraction length scale. In other words, it should be possible to detect very small changes in cells simply by analysing scattered light in the far field. The team developed the necessary theory for quantitatively determining the refractive-index fluctuations in weakly scattering systems from spectrally resolved far-field microscope images. The minimum size of detectable features is limited by the signal-to-noise ratio of the detector. Comparison with numerical solutions obtained using Maxwell's equations revealed very good agreement for a variety of sample and instrument parameters.