Nonlinear Dark-Field Microscopy

Dark-field microscopy is a background-free imaging method that provides high sensitivity and a large signal-to-noise ratio. It finds application in nanoscale detection, biophysics and biosensing, particle tracking, single molecule spectroscopy, X-ray imaging, and failure analysis of materials In dark held microscopy, the unscattered light path is typically excluded from the angular range of signal detection. This restriction reduces the numerical aperture and affects the resolution. Here We introduce a nonlinear dark-field scheme that overcomes this restriction. Two laser beams of frequencies omega(1) and omega(2) are used to illuminate a sample surface and to generate a purely evanescent field at the four-wave mixing (4WM) frequency omega(4wm) = 2 omega(1) - omega(2). The evanescent 4WM field scatters at sample features and generates radiation that is detected by standard far-field optics. This nonlinear dark-field scheme works with samples of any material and is compatible with applications ranging from biological imaging to failure analysis.