Péter Rácz

Ultrafast strong-field photoemission from plasmonic nanoparticles.

We demonstrate strong-field photoemission from plasmonic nanoparticles by ultrashort pulses. Significant (x110) field enhancement attributed to surface plasmons enable 25-eV electron generation in nano-localized fields around nanoparticles. Correlation between plasmonic resonance and electron spectra is shown.

Strong-field plasmonic electron acceleration with few-cycle, phase-stabilized laser pulses

We carried out experimental investigations on surface plasmon enhanced electron acceleration with few-cycle, carrier-envelope phase (CEP) stabilized laser pulses. We determined the spectrum of electrons accelerated in the plasmonic field and found that signatures of the phase stabilized optical waveform driving the individual electron trajectories are washed out in the electron spectra. We attribute this effect to nanoscale surface roughness of the metallic samples, as supported by extensive numerical simulations. This finding explains the previously observed, low CEP sensitivity of photoemission processes from metallic films and enables the development of femtosecond electron sources for ultrafast time-resolved applications.

Ultrafast monoenergetic electron source by optical waveform control of surface plasmons

We propose coherent control of photoelectron acceleration at metal surfaces mediated by surface plasmon polaritons. A high degree of spectral and spatial control of the emission process can be exercised by amplitude and phase controlling the optical waveform (including the carrier-envelope phase) of the plasmon generating few-cycle laser pulse. Numerical results show that the emitted electron beam is highly directional and monoenergetic suggesting applications in contemporary ultrafast methods where ultrashort, well-behaved electron pulses are required.

Observation of few-cycle, strong-field phenomena in surface plasmon fields

We present experimental evidence of the generation of few-cycle propagating surface plasmon polariton (SPP) wavepackets. Ultrashort plasmonic pulses were generated by few-cycle laser pulses of 5.5 fs to 6.5 fs duration on a thin silver film of 50 nm thickness coated on the face of a right-angle prism to enable Kretschmann-type SPP coupling. SPP pulses were characterized by an autocorrelation-type measurement based on fourth order, nonlinear electron photoemission induced by the SPP field. The evaluation of the measured ultrashort, fringe-resolved autocorrelation curve of the SPP wavepacket (Fig. 1a) resulted in a retrieved SPP pulse length of 6.5 fs, as evidenced by the reconstructed curve in Fig. 1b. This first demonstration of the generation of few-cycle propagating SPP wavepackets on a metal surface has important applications in ultrafast plasmonics.

Measurement of nanoplasmonic field enhancement with ultrafast photoemission

Plasmonic enhancement of optical near-fields at nanostructures provides for localization of the energy of light on the nanoscale. This phenomenon allowed pioneering applications in spectroscopy, photovoltaics and sensorics. It remains a challenge to measure the extent of the maximum achievable nanoplasmonic field enhancement for a particular sample. Even though near-field probing methods such as scanning near-field optical microscopy (SNOM) can reach a resolution down to 8–10 nm, probing near-fields on ∼1 nm scale needs a different approach. Here, we demonstrate a method for nanoplasmonic near-field measurement with the help of photoemitted electrons induced by femtosecond laser pulses.

Conversion of chirp in fiber compression

Focusing positively chirped femtosecond pulses into nonlinear fibers provides significant spectral broadening and compression at higher pulse energies than achievable conventionally because self-focusing and damage are avoided. Here, we investigate the transfer of input to output chirp in such an arrangement. Our measurements show that the group delay dispersion of the output pulse, originating from the nonlinearities, is considerably reduced as compared to the initial value, by about a factor of 10. The mechanism of chirp reduction is understood by an interplay of self-phase modulation with initial chirp within the fiber. A simple model calculation based on this picture yields satisfactory agreement with the observations and predicts significant chirp reduction for input pulses up to the μJ regime. In practice, the reduction of chirp observed here allows for compressing the spectrally broadened intense pulses by ultrabroadband dispersive multilayer mirrors of quite moderate dispersion.

Strong-field plasmonic photoemission in the mid-IR at <1 GW/cm² intensity.

Strong-field ultrafast photoemission was studied by propagating surface plasmons generated on gold metal layer in Kretschmann configuration at 3.1 microns wavelength. Tunneling photoemission and electron acceleration was demonstrated at an unprecedently low laser intensity (1-5 GW/cm2).

Pre-excitation studies for rubidium-plasma generation

The key element in the Proton-Driven-Plasma-Wake-Field-Accelerator (PWFA) project is the generation of highly uniform plasma from Rubidium vapor. A scientifically straightforward, yet highly challenging way to achieve full ionization is to use high power laser which can assure the barrier suppression ionization (BSI) along the 10 m long active region. The Wigner-team in Budapest is investigating an alternative way of uniform plasma generation. The proposed Resonance Enhanced Multi-Photon Ionization (REMPI) scheme can be probably realized by much less laser power. In the following we plan to investigate the resonant pre-excitations of the Rb atoms, both theoretically and experimentally. In the following our theoretical framework is presented together with the status report about the preparatory work of the planned experiment.

Surface-plasmon–assisted electron pair formation in strong electromagnetic field

In this work the strong electromagnetic field of femtosecond Ti:Sa lasers was used to excite surface plasmon oscillations (SPOs) in gold films at room temperature in the Kretschmann geometry. Experimental investigations were carried out using a surface plasmon near field scanning tunneling microscope, measuring its response to excitation at SPO hot spots on the gold surface. Furthermore, the spectra of photoelectrons, liberated by multiplasmon absorption, have also been measured by a time-of-flight spectrometer. In both cases new type of anomalies in both the STM and electron TOF signals have been measured in the same laser intensity range. The existence of these anomalies may be qualitatively understood, by using the intensity-dependent expression for the effective electron-electron scattering potential, derived earlier in a different context. In this theoretical work an effective attraction potential has been predicted in the presence of strong inhomogeneous radiation fields.

Noble gas clusters and nanoplasmas in high harmonic generation

Abstract We report a study of high harmonic generation from noble gas clusters of xenon atoms in a gas jet. Harmonic spectra were investigated as a function of backing pressure, showing spectral shifts due to the nanoplasma electrons in the clusters. At certain value of laser intensity this process may oppose the effect of the well-known ionization-induced blueshift. In addition, these cluster-induced harmonic redshifts may give the possibility to estimate cluster density and cluster size in the laser–gas jet interaction range.