The present status and development of strong-field nano-optics, an emerging field of nonlinear optics, is discussed. A nonperturbative regime of light-matter interactions is reached when the amplitude of the external electromagnetic fields that are driving a material approach or exceed the field strengths that bind the electrons inside the medium. In this strong-field regime, light-matter interactions depend on the amplitude and phase of the field, rather than its intensity, as in more conventional perturbative nonlinear optics. Traditionally such strong-field interactions have been intensely investigated in atomic and molecular systems, and this has resulted in the generation of high-harmonic radiation and laid the foundations for contemporary attosecond science. Over the past decade, however, a new field of research has emerged, the study of strong-field interactions in solid-state nanostructures. By using nanostructures, specifically those made out of metals, external electromagnetic fields can be localized on length scales of just a few nanometers, resulting in signficantly enhanced field amplitudes that can exceed those of the external field by orders of magnitude in the vicinity of the nanostructures. This leads not only to dramatic enhancements of perturbative nonlinear optical effects but also to significantly increased photoelectron yields. It resulted in a wealth of new phenomena in laser-solid interactions that have been discovered in recent years. These include the observation of above-threshold photoemission from single nanostructures, effects of the carrier-envelope phase on the photoelectron emission yield from metallic nanostructures, and strong-field acceleration of electrons in optical near fields on subcycle timescales. The current state of the art of this field is reviewed, and several scientific applications that have already emerged from the fundamental discoveries are discussed. These include, among others, the coherent control of localized electromagnetic fields at the surface of solid-state nanostructures and of free-electron wave packets by such optical near fields, resulting in the creation of attosecond electron bunches, the coherent control of photocurrents on nanometer length and femtosecond timescales by the electric field of a laser pulse, and the development of new types of ultrafast electron microscopes with unprecedented spatial, temporal, and energy resolution. The review concludes by highlighting possible future developments, discussing emerging topics in photoemission and potential strong-field nanophotonic devices, and giving perspectives for coherent ultrafast microscopy techniques. More generally, it is shown that the synergy between ultrafast science, plasmonics, and strong-field physics holds promise for pioneering scientific discoveries in the upcoming years.

Future PHz electronic devices may be able to perform operations on few-femtosecond time-scales. Such devices are based on the ability to control currents induced by intense few-cycle laser pulses. Investigations of this control scheme have been based on complex, amplified laser systems, typically delivering mJ or sub-mJ-level laser pulses, limiting the achievable clock rate to the kHz regime. Here, we demonstrate transient metallization and lightwave-driven current control with 300-pJ laser pulses at 80 MHz repetition rate in dielectric media (HfO₂ and fused silica), and the wide-bandgap semiconductor GaN. We determine the field strength dependence of optically induced currents in these media. Supported by a theoretical model, we show scaling behaviors that will be instrumental in the construction of PHz electronic devices.

Coupling of propagating and localized plasmons on nanostructured surfaces is evidenced by photoelectron spectroscopy, which provides high-sensitivity probing of near-fields.

Non-thermal and thermal carrier populations in plasmonic systems raised significant interest in contemporary fundamental and applied physics. Although the theoretical description predicts not only the energies but also the location of the generated carriers, the experimental justification of these theories is still lacking. Here, we demonstrate experimentally that upon the optical excitation of surface plasmon polaritons, a non-thermal electron population appears in the topmost domain of the plasmonic film directly coupled to the local fields. The applied all-optical method is based on spectroscopic ellipsometric determination of the dielectric function, allowing us to obtain in-depth information on surface plasmon induced changes of the directly related electron occupancies. The ultrahigh sensitivity of our method allows us to capture the signatures of changes induced by electron-electron scattering processes with ultrafast decay times. These experiments shed light on the build-up of plasmonic hot electron population in nanoscale media.

The carrier-envelope phase (CEP) is an important property of few-cycle laser pulses, allowing for light field control of electronic processes during laser-matter interactions. Thus, the measurement and control of CEP is essential for applications of few-cycle lasers. Currently, there is no robust method for measuring the non-trivial spatial CEP distribution of few-cycle laser pulses. Here, we demonstrate a compact on-chip, ambient-air, CEP scanning probe with 0.1 µm³ resolution based on optical driving of CEP-sensitive ultrafast currents in a metal−dielectric heterostructure. We successfully apply the probe to obtain a 3D map of spatial changes of CEP in the vicinity of an oscillator beam focus with pulses as weak as 1 nJ. We also demonstrate CEP control in the focal volume with a spatial light modulator so that arbitrary spatial CEP sculpting could be realized.