Ultrafast Microscopy: Imaging Light with Photoelectrons on the Nano–Femto Scale

Abstract

Experimental methods for ultrafast microscopy are advancing rapidly. Promising methods combine ultrafast laser excitation with electron-based imaging or rely on super-resolution optical techniques to enable probing of matter on the nano-femto scale. Among several actively developed methods, ultrafast time-resolved photoemission electron microscopy provides several advantages, among which the foremost are that time resolution is limited only by the laser source and it is immediately capable of probing of coherent phenomena in solid-state materials and surfaces. Here we present recent progress in interference imaging of plasmonic phenomena in metal nanostructures enabled by combining a broadly tunable femtosecond laser excitation source with a low-energy electron microscope.

Nucleon effective masses in neutron-rich matter

“…After averaging over all target nucleons, the last term in the above expression leads to the ±(N T − Z T )/A T term of the optical potential felt by a projectile neutron/proton. It has been used extensively in extracting information about the energy/momentum dependence of the Lane potential in many earlier [147,148,149,150,151] and recent [41,42,96,145,146,152,153,154] studies of nucleon-nucleon scattering, (p,n) and/or ( 3 He,t) charge exchange reactions. As new data are accumulated, depending on the data sets, number of model parameters as well as the form factors and/or density profiles used for the target, the quantitative results from model analyses sometimes have some discrepancies.…”

“…It is necessary to first briefly recall how one can get the U 0 (ρ 0 , k), U sym,1 (ρ 0 , k) and U sym,2 (ρ 0 , k) in nuclear matter at saturation density from the optical model potentials U 0 (E), U sym,1 (E) and U sym,2 (E) at a beam energy E. The question was discussed in refs. [41,151] and some useful relationship for conveniently transforming the potentials were given. Using the kinetic energy T J and δ as two independent variables necessary in expressing the three parts of the nucleon potential given in Eq.…”

“…(20) and expanding all terms up to δ 2 , the Eq. (34) gives the following transformation relations [41,151]…”

“…Since the Coulomb potential is normally explicitly considered in the optical model analyses of proton-nucleus scattering and one normally does the calculations in the theoretically uncharged isospin asymmetric nucleonic matter, the above transformations are thus valid for both neutrons and protons. However, it was known that to transform to the interior of nuclei in β equilibrium an extra relationship between the Coulomb potential and the symmetry potential is required [151].…”

Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate

“…17 A noncollinear optical parametric amplifier (NOPA) is capable of combining the design paradigms of broadband pulses with a wide-ranging flexibility of central wavelengths from the near-infrared (NIR) to the ultraviolet (UV) regime as well as high (MHz) repetition rates for time-resolved spectromicroscopy. [18][19][20] First laser-excited PEEM experiments using a MHz-NOPA were demonstrated by Höfer 20 and the technique was since applied for ultrafast photoemission imaging of plasmonic modes in nanostructured materials 22,23 and ferroelectric domains. 24 Moreover, high-repetition rate NOPA systems were also used in spatially averaged time-resolved photoemission experiments.…”