Interferometric time- and energy-resolved photoemission electron microscopy for few-femtosecond nanoplasmonic dynamics

Abstract

We report a time-resolved normal-incidence photoemission electron microscope with an imaging time-of-flight detector using ∼7-fs near-infrared laser pulses and a phase-stabilized interferometer for studying ultrafast nanoplasmonic dynamics via nonlinear photoemission from metallic nanostructures. The interferometer’s stability (35 ± 6 as root-mean-square from 0.2 Hz to 40 kHz) as well as on-line characterization of the driving laser field, which is a requirement for nanoplasmonic near-field reconstruction, is discussed in detail. We observed strong field enhancement and few-femtosecond localized surface plasmon lifetimes at a monolayer of self-assembled gold nanospheres with ∼40 nm diameter and ∼2 nm interparticle distance. A wide range of plasmon resonance frequencies could be simultaneously detected in the time domain at different nanospheres, which are distinguishable already within the first optical cycle or as close as about ±1 fs around time-zero. Energy-resolved imaging (microspectroscopy) additionally revealed spectral broadening due to strong-field or space charge effects. These results provide a clear path toward visualizing optically excited nanoplasmonic near-fields at ultimate spatiotemporal resolution.

Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

“…In its simplest embodiment, a PEEM instrument uses electrostatic electron optics to generate images of the energy integrated photoelectron spatial distributions, but additional information and contrast can be gained from energy resolved photoelectron detection. This can be accomplished using either pulsed excitation with a time-of-flight (TOF) photoelectron energy analysis ,− or with more instrumentally demanding, electrostatic electron energy analysis. , Although photoelectron energy analysis relies on similar physical principles as photoelectron spectroscopy, one should be aware that the length and momentum are Fourier conjugate parameters, so high spatial resolution implies a low photoelectron momentum resolution, and vice versa, when performing a photoelectron energy and momentum analysis. In principle, it is possible to incorporate the high spatial and momentum resolution in different modes of operation of the same instrument, to realize a “momentum microscope”.…”

Complete characterization of ultrafast optical fields by phase-preserving nonlinear autocorrelation

“…The laser pulses were dispersion-minimized via chirped mirrors and a wedge pair, leaving only uncompensated nonlinear chirp. IAC scans were obtained with an actively phase-stabilized Mach–Zehnder interferometer capable of ~35-as root-mean-square stability of the optical delay over a long scan range 47 ; the balance factor was set by adjusting a variable ND filter in one of the arms. The two delayed pulses were then collinearly focused into the BBO crystal, and the SHG radiation was separated from the fundamental via a short-pass filter.…”

“…Field retrieval from the IFROG measurement ( s = 1) was performed with a ptychographic retrieval algorithm originally developed for conventional FROG 53 , which we adapted for IFROG 47 . It provides fast convergence and accurate results while being suitable for an unmodified IFROG spectrogram without the need to separate or filter its harmonic components 36 , 37 , 39 .…”

“…Almost any existing nonlinear IAC setup is easy to modify for PENGUIN measurements, either by inserting a ND filter in one of the arms or by using beam splitters with different splitting ratios, effectively converting it into a complete optical field characterization device for most practical ultrashort laser pulses similar to FROG or SPIDER but with significantly less complexity and cost. The rapid field retrieval algorithm allows live pulse shape monitoring when a fast optical delay scan is employed, and the delay accuracy can be maintained with a co-propagating continuous-wave reference laser 47 if necessary. Furthermore, single-shot IAC acquisition with no moving parts is readily available for Fourier-transform spectroscopy applications using a Wollaston prism to map the autocorrelation delay onto the transverse position on a linear photodetector array 54 .…”

“…Non-radiative nonlinear interactions of ultrafast electric fields are therefore inaccessible by spectroscopic characterization techniques. For example, optically induced ultrafast plasmonic near-fields at metallic nanostructures produce low-energy photoelectrons via nonlinear photoemission 45 – 47 , which is an incoherent process and therefore does not preserve sufficient spectral information about the plasmonic near-fields in the kinetic energy spectrum of the photoelectrons for field retrieval with FROG or other spectroscopic methods. A spectrally integrated nonlinear IAC signal of the plasmonic near-fields, on the other hand, is easily obtained by recording the nonlinear photoemission rate as a function of the autocorrelation delay; 47 – 49 however, the extraction of the underlying plasmonic electric fields from such a measurement has been impossible because of inaccessible spectral information or required sub-cycle sampling pulses 50 , 51 .…”