Strong-field photoemission of electron pulses from sharp metallic tips

Summary form only given. The investigation of strong-field light-matter interactions is increasingly being applied to surfaces and nanostructures. Local enhancement and sub-wavelength confinement facilitate highly nonlinear optics in tailored optical near-fields. Using ultrashort pulses at visible and near-infrared frequencies, metallic nanotips have been shown to allow for enhanced nonlinear photoelectron emission confined to nanometric volumes; numerous effects regarding strong-field emission and carrier-envelope sensitivity were recently identified [1-3]. Currently, there is increasing interest in the impact of spatial inhomogeneity on electron dynamics in nanoscale systems[4]. Here, we investigate localized photoelectron emission from single gold nanotips with ultrashort laser pulses at near- and mid-infrared frequencies. We observe localized optical field emission throughout a wavelength range from 800 nm to 8 μm. The emission is found to be polarization sensitive, and photoelectron energies of hundreds of electronvolts are recorded. Simulations within a two-step photoemission model reveal that the dynamics are governed by the spatial confinement of the driving field. The talk will disseminate these recent results [5,6] and will present novel applications in time-resolved electron imaging based on such localized electron sources. Figure 1a displays the general experimental scenario, in which electrons are field-emitted from a nanometric gold tip using femtosecond laser pulses at various wavelengths. In the low frequency limit, the physical process underlying strong-field infrared photoemission is transient tunnelling through a barrier generated by the driving field. It can be characterized using the intensity dependence of the emission yield, which agrees well with a simple Fowler-Nordheim type tunnelling model (Fig. 1c). Alongside the transition to the strong-field regime, electron acceleration in the locally enhanced optical fields becomes important.- Specifically, the kinetic energy spectra consist of a plateau region and an intensity-dependent cutoff. Because of the low absorption of gold at mid-infrared wavelengths and the scaling of the Keldysh-parameter with frequency [5], we can more deeply access the strong-field regime than in previous experiments on nanostructures. The intensities of the local driving fields are substantial enough to accelerate electrons to kinetic energies of hundreds of electronvolts (Fig. 1b). This causes a wavelength scaling of the observed cutoffs that directly reflects the strong spatial localization of the driving field at the nanotip apex [5]. Specifically, the strong-field acceleration at such high energies leads to conditions, in which electrons escape the enhanced near-field within much less than an optical cycle (cf. Fig. 1a). In conclusion, the combination of ultrashort mid-infrared pulses and optical field enhancement allows for unique access to a parameter regime in strong-field physics, in which the spatial and temporal scales of the driving field become coupled. This illustrates new prospects for ultrafast control of electron dynamics in ways that are exclusive to nanostructures.