Cross-phase modulation imaging
Spotlight summary: The nonlinearity of the index of refraction is responsible for various effects in optical beam propagation, such as self-phase modulation (spatial and temporal), self-focusing, self-dispersion and self-steepening. These optical effects have been used in a variety of applications, ranging from the design of very high energy lasers to long distance fiber optic communication systems, and continue to be incorporated in novel optical designs and imaging modalities.
A high intensity light pulse propagating through a medium can change the index of refraction, and that in turn affects the propagation of the pulse itself. The time dependent refractive index changes the phase of the light pulse that alters its temporal and spectral properties leading to spectral broadening that results in self-phase modulation. The index of refraction can change due to a number of physical mechanisms that depend on vibrational motion of the molecules and direct electronic cloud distortion, reorientation and redistribution.
The authors of this paper have proposed a technique for nonlinear phase contrast imaging that uses two beams of different wavelengths in what is called cross-phase modulation (XPM). XPM is a two-color analogue of self-phase modulation (SPM), which is defined as a self-induced modulation of refractive index by an amount proportional to the instantaneous intensity of the propagating light pulse. A cross-phase modulation measurement method is developed in a pump-probe scheme by modulating the amplitude of the pump beam and subsequently analyzing the probe beam in the spectral domain, thereby achieving a background-free nonlinear phase contrast in imaging biological samples.
A method known as Z-scan, in which a sample is translated through the focus of a laser beam (in the direction of propagation) is generally used to measure two-photon absorption or the self-phase modulation coefficients. The Z-scan method is used to measure SPM-induced phase shifts via its effect on the far-field diffraction pattern. However, this method is not efficient in strongly scattering samples like biological material.
In contrast to the Z-scan method, the authors propose a spectral reshaping technique to measure the nonlinear refractive index and nonlinear absorption that encodes the nonlinear response in the spectral rather than the spatial domain, whereby a self-induced phase shift results in a small change in the frequency spectrum of an ultrashort light pulse. A spectral hole is created in the center of the ultrashort pulse; two-photon absorption and self-phase modulation together change the spectrum inside the sample in such a way that refills the spectral hole. The phase of the spectral hole is shifted by 180 degrees with the rest of the pulse due to two-photon absorption process and by 90 degrees due to SPM. By adding a reference field, also known as a local oscillator in a homodyne detection scheme, that interferes with and amplifies the nonlinear signal, the nonlinear absorption and SPM change the spectrum of the local oscillator with different phases and can be separated by lock-in detection. The background and noise in the SPM measurements cannot be avoided in highly scattering samples. A sensitive nonlinear phase contrast imaging method is developed by combining the spectral reshaping with a two-color XPM technique in a laser scanning microscope.
Earlier, the authors had reported obtaining a functional self-phase modulation contrast in hippocampal brain slices. Here, they have applied the so-called cross-phase modulation technique to obtain nonlinear phase contrast images from breast cancer cells with substantial dynamic range and subcellular resolution.
The authors speculate that, due to its ability to achieve nonlinear phase contrast and to remove any background associated with self-phase modulation, this dual-color cross-phase modulation technique will become an extension to the existing linear phase contrast imaging methods, thereby “providing an intrinsic and ubiquitous nonlinear contrast for biological imaging.”
Technical Division: Light–Matter Interactions
ToC Category: Ultrafast Optics
|OCIS Codes:||(190.7110) Nonlinear optics : Ultrafast nonlinear optics|
|(320.5540) Ultrafast optics : Pulse shaping|
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