Starch-based second-harmonic-generated collinear frequency-resolved optical gating pulse characterization at the focal plane of a high-numerical-aperture lens

Abstract

We report the use of starch as an ideal nonlinear medium with which to perform collinear frequency-resolved optical gating measurements of ultrashort pulses at the focal plane of a high-numerical-aperture (NA) lens. We achieved these measurements by simply sandwiching starch granules (suspended in water) between two coverslips and placing them within the focal plane of a high-NA lens. The natural nonlinear characteristics of starch allow the correct phase matching of pulses at the focal plane of a high-NA lens at different wavelengths. This elegant arrangement overcomes all the complexity and problems that were previously associated with pulse characterization within a multiphoton microscope.

Invited Review Article: Imaging techniques for harmonic and multiphoton absorption fluorescence microscopy

“…Some examples of such structures include the aforementioned starch granules and other polysaccharides, 4,167 collagen, 168,169 striated muscle, 19,168 and chloroplasts. 4 As previously mentioned, SHG from some biological samples, for example, starch granules, is strong enough to allow pulse characterization measurements not only in the forward 37 but also in the backward direction. 170 Backward propagated SHG comes from backscattered forward propagated SHG and from direct emission, possible from surfaces 3 as well as scatterers smaller than the wavelength of light.…”

“…The fluorophore can be prepared in a simple cell mounted between coverslips-particularly important when using objectives that are designed to image through a standard coverslip glass type and thickness-in this way important aberrations, such as spherical aberration, are properly compensated and the measurement is performed in a manner commensurate with the actual imaging conditions. Amat-Roldán et al 37 pointed out a particularly useful specimen for performing pulse characterization measurements in microscopes-corn starch. The specimen is entirely innocuous and readily available from your local grocer!…”

“…By using a SHG signal as opposed to TPEF, more sophisticated pulse characterization is possible. For example, AmatRoldán et al 37 demonstrated that by spectrally resolving the autocorrelation signal, the phase of the excitation pulse can be extracted. This can be extremely useful in those instances where pulse shape impacts the imaging.…”

Harmonic nanoparticles: noncentrosymmetric metal oxides for nonlinear optics

“…By varying the time delay a collinear frequency resolved optical gating (FROG) trace was acquired. The high frequency components appearing due to the parallel beam geometry can be eliminated by Fourier filtering [86] and the resulting non-interferometric FROG spectrogram can be inverted by established algorithms to retrieve information about the spectral phase of the pulse at the focus [87]. A similar approach was successively applied in a series of experiments by the Liu group, attaching a HNP on a fiber taper via a carbon nanotube [88,89] making it possible to manipulate the probe with high precision via a piezoelectric stage for acquiring autocorrelation traces at the laser focus, using the HNP as nonlinear medium.…”

Spectral phase shaping for non-linear spectroscopy and imaging

“…Optimization of the SHG at the sample location ensures a transform limited pulse at the sample [91,72]. A phase profile applied to the incoming pulse with that maximizes the second harmonic intensity, has the opposite sign of the original phase profile [92].…”