Measuring the spatiotemporal electric field of tightly focused ultrashort pulses with submicron spatial resolution

We demonstrate a powerful and practical spectral interferometer with near-field scanning microscopy (NSOM) probes for measuring the spatiotemporal electric field of tightly focused ultrashort pulses with sub-micron spatial resolution and high spectral resolution. To make these measurements we use SEA TADPOLE which is a high spectral resolution, experimentally simplified version of spectral interferometry that uses fiber optics to introduce the pulse into the device. To measure the spatiotemporal field of focusing pulses, the entrance fiber is scanned around the focus and a measurement at each fiber position is made, so that E(ω) is found at many positions along the beam's cross section so that we can reconstruct E(x,y,z,ω). The make these measurements we require that the fiber's mode size be smaller than the focused spot size. In the past using optical fibers we were limited to measuring foci with NA's less than 0.1, and here by replacing the fiber with an NSOM fiber, we measure the spatiotemporal field of focused pulses with NAs as high as 0.44. To demonstrate this technique we measured pulses that were focused with two different microscope objectives to verify that we achieved the expected results. We also measured foci that had severe distortions, such as the Bessel-like X-shaped pulse caused by spherical aberrations and the "fore-runner pulse" due to chromatic aberrations and we verified these results with non-paraxial simulations. In our measurements we observed spatial features smaller than 1μm.