To reach a greater concentration of light in space, using a higher numerical aperture (NA) objective lens is essential; however, for ultrashort pulses with a broadband spectrum, a higher NA objective lens will tremendously increase the pulse duration of light by introducing phase distortions originated from the dispersion of light in glass. Therefore, for a better correction of the phase distortions to obtain shorter pulses at a certain region, it is necessary to evaluate the phase distortions induced by the objective lens precisely. Currently, one of the most popular techniques to characterize the phase distortions is the multiphoton intrapulse interference phase scan (MIIPS), developed by the group of Dantus in 2002. The basic idea of MIIPS is to find a suitable phase mask that makes the group delay dispersion (GDD) zero, and to use this mask to retrieve the spectral phase distortions. Unfortunately, the GDD only covers the second order dispersion, and therefore the accuracy of this characterization is fundamentally limited to the second order correction.
In this paper, the authors demonstrate improved accuracy in spectral phase characterization without any increase in time for measurement by including the influences from higher order contributions in an ingenious way, in what they call Gated-MIIPS. This technique simply employs amplitude modulation to gate the spectrum around a specific frequency. With this implementation, the authors effectively improve the validity of second order and even higher order contributions of the phase, and further demonstrate that Gated-MIIPS successfully characterizes the phase distortions of femtosecond pulses focused by a very high NA objective lens. Therefore, Gated-MIIPS can be used to help concentrating light efficiently in 4 dimensions, potentially leading to improvements in the resolution and precision of time-resolved microscopy and micromachining.
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