Light is at once the most sensitive and the most gentle probe of matter.  It is commonplace for light to measure a picometer displacement far below the nanometer scale of atoms, or to capture the emission of a single photon from a fluorescent dye molecule.  Light is easy to generate using light-emitting diodes or lasers, and to detect using ultrasensitive photodetectors as well as the now ubiquitous digital cameras.  Light also has the uncanny ability to penetrate living tissue harmlessly and deeply, while capturing valuable information on the health and function of cells.  For these reasons, light has become an indispensible tool for biology and medicine.  We all bear witness to the central role of light in microscopic imaging, in optical biosensors and in laser therapy and surgery. 

 

Interferometry, applied to biology and medicine, provides unique quantitative metrology capabilities.  The wavelength of light is, in effect, like a meterstick against which small changes in length (or phase) are measured.  This meterstick analogy is apt, because one micron is to one meter as one picometer is to one micron—at a dynamic range of a million to one.  Indeed, a picometer is detected routinely using interferometry at wavelengths around one micron.  This level of interferometric sensitivity has great utility in many biological applications, providing molecular sensitivity for biosensors as well as depth-gating capabilities to optically section living tissue.