Intrinsic Nonlinear Microscopy: From Neuronal Firing to Historical Artwork

Imaging based on nonlinear processes takes advantage of the localized excitation to achieve high spatial resolution, optical sectioning, and deeper penetration in highly scattering media. However, the use of nonlinear contrast for imaging has conventionally been limited to processes that create light of wavelengths that are different from the wavelengths used for excitation. Intrinsic nonlinear contrasts that do not generate light at distinct wavelengths are generally difficult to measure because of the overwhelming background from the excitation light. This dissertation focuses on extension of nonlinear microscopy to these new intrinsic processes by using femtosecond pulse shaping to encode the nonlinear information as new frequency components in the spectrum. We will present a pump-probe microscopy technique based on pulse train shaping technology to sensitively access nonlinear transient absorption or gain processes. This technique has recently been used to uniquely identify a variety of biological pigments with high spatial resolution. Here, we extend this technique to image and characterize several inorganic and organic pigments used in historical artwork. We also present a spectral reshaping technique based on individual femtosecond pulse shaping to sensitively access nonlinear refractive contrasts in scattering media. We will describe an extension of this technique to utilize two distinct wavelengths and discuss its application in biological imaging. This two-color implementation would allow the extension of widely employed phase contrast to the nonlinear regime.