the experimental setup was based on a broadband 
 oscillator ( venteon pulse one ) delivering pulses of 5.5 fs duration , 
 800 nm central wavelength , and 250 pj pulse energy at a repetition 
 rate of 80 mhz .
double - chirped mirrors and a pair of glass wedges 
 were used for dispersion compensation to achieve the optimal pulse 
 duration on the sample .
the pulse was sent through a properly calibrated , 
 compact michelson interferometer with a piezo mirror enabling a range 
 of delays between 250 fs and + 250 fs in steps of 67 as .
the 
 beam was then focused by a 20 cm focal length achromat lens onto the 
 sample surface at an angle of 65 to the surface normal .
the 
 resulting focal spot dimensions were approximately 50  100 m on the sample .
after the interferometer , a flip mirror could 
 send the beam to a separate arm for pulse characterization . in this 
 separate pulse characterization arm ,
the beam passed through a window 
 identical to the entrance window of the peem chamber and was focused 
 by an identical lens into a thin bbo crystal for second harmonic generation . 

by measuring the second harmonic spectrum as a function of glass thickness 
 in the beam , the pulse could be completely characterized using the 
 d - scan technique . to obtain the 
 experimental near - field autocorrelation trace ,
the images were first 
 drift compensated using an automatic hot - spot tracking routine .
then , 
 the intensity in the area of the spot was measured from each image 
 and a background intensity measured from a featureless area was subtracted . 

the resulting near - field autocorrelation trace was subjected to a 
 narrow gaussian smoothing filter (  = 60 as ) .
the samples 
 consisted of polyol - synthesized rice - shaped ag nanoparticles that were drop - cast from ethanol solution onto 
 substrates of ito on glass .
the samples were directly transferred 
 into a ultrahigh vacuum chamber with a base pressure of 10 mbar .
after the peem measurements , the samples were studied in a 
 scanning electron microscope ( hitachi su8010 ) .
finite - difference 
 time - domain modeling was performed using the commercial software fdtd 
 solutions from lumerical .
the ag nanoparticles were modeled as ellipsoidal 
 particles with a dielectric function taken from literature , lying on a fused silica substrate . a staircase - type 
 mesh with a minimum element size of 2.5
the calculations 
 were performed with a broadband modulated gaussian total field scattered 
 field plane wave source .
the excitation spectral range was set to 
 match the actual laser spectrum covered by the experiment .
frequency - dependent 
 quantities , such as absorption cross - section and field patterns , were 
 obtained through fourier transform and normalization by the excitation .
 the local enhancement of few - cycle 
 laser pulses by plasmonic nanostructures opens up for spatiotemporal 
 control of optical interactions on a nanometer and few - femtosecond 
 scale . 
 however , spatially resolved characterization of few - cycle plasmon 
 dynamics poses a major challenge due to the extreme length and time 
 scales involved . in this letter 
 , we experimentally demonstrate local 
 variations in the dynamics during the few strongest cycles of plasmon - enhanced 
 fields within individual rice - shaped silver nanoparticles . 
 this was 
 done using 5.5 fs laser pulses in an interferometric time - resolved 
 photoemission electron microscopy setup . 
 the experiments are supported 
 by finite - difference time - domain simulations of similar silver structures . 
 
 the observed differences in the field dynamics across a single particle 
 do not reflect differences in plasmon resonance frequency or dephasing 
 time . 
 they instead arise from a combination of retardation effects 
 and the coherent superposition between multiple plasmon modes of the 
 particle , inherent to a few - cycle pulse excitation . 
 the ability to 
 detect and predict local variations in the few - femtosecond time evolution 
 of multimode coherent plasmon excitations in rationally synthesized 
 nanoparticles can be used in the tailoring of nanostructures for ultrafast 
 and nonlinear plasmonics . 
Methods