Tracing attosecond electron emission from a nanometric metal tip.

Solids exposed to intense electric fields release electrons through tunnelling. This fundamental quantum process lies at the heart of various applications, ranging from high brightness electron sources in d.c. operation1,2 to petahertz vacuum electronics in laser-driven operation3-8. In the latter process, the electron wavepacket undergoes semiclassical dynamics9,10 in the strong oscillating laser field, similar to strong-field and attosecond physics in the gas phase11,12. There, the subcycle electron dynamics has been determined with a stunning precision of tens of attoseconds13-15, but at solids the quantum dynamics including the emission time window has so far not been measured. Here we show that two-colour modulation spectroscopy of backscattering electrons16 uncovers the suboptical-cycle strong-field emission dynamics from nanostructures, with attosecond precision. In our experiment, photoelectron spectra of electrons emitted from a sharp metallic tip are measured as function of the relative phase between the two colours. Projecting the solution of the time-dependent Schrödinger equation onto classical trajectories relates phase-dependent signatures in the spectra to the emission dynamics and yields an emission duration of 710 ± 30 attoseconds by matching the quantum model to the experiment. Our results open the door to the quantitative timing and precise active control of strong-field photoemission from solid state and other systems and have direct ramifications for diverse fields such as ultrafast electron sources17, quantum degeneracy studies and sub-Poissonian electron beams18-21, nanoplasmonics22 and petahertz electronics23.

Ultrabroadband suppression of mid-infrared reflection losses of a layered semiconductor by nanopatterning with a focused ion beam.

Moth-eye structures are patterned onto gallium selenide surfaces with sub-micrometer precision. In this way, Fresnel reflection losses are suppressed to below one percent within an ultrabroad optical bandwidth from 15 to 65 THz. We tune the geometry by rigorous coupled-wave analysis. Subsequently, ablation with a Ga+ ion beam serves to write optimized structures in areas covering 30 by 30 µm. The benefits are demonstrated via optical rectification of femtosecond laser pulses under tight focusing, resulting in emission of phase-stable transients in the mid-infrared. We analyze the performance of antireflection coating directly in the time domain by ultrabroadband electro-optic sampling.

Passive elimination of correlated amplitude fluctuations in ultrabroadband supercontinua from highly nonlinear fibers by three-wave mixing.

The nonlinear transformation of fluctuations by frequency broadening is found to produce strong anti-correlations in the spectral output. This effect is investigated by dispersive Fourier transform measurements. We exploit the anti-correlations in order to cancel the intensity noise in a subsequent sum-frequency mixing step. This principle allows for the generation of tunable visible pulses by cascaded nonlinear mixing whilst maintaining the same intensity noise performance as the input pulses. In addition, we demonstrate that the power fluctuations occurring in the process of passive stabilization of the carrier-envelope phase locking via difference frequency generation may be cancelled by an analogous strategy.

Ultrabroadband out-of-loop characterization of the carrier-envelope phase noise of an offset-free Er:fiber frequency comb.

Recent demonstrations of passively phase-locked fiber-based combs motivate broadband characterization of the noise associated with the stabilized carrier-envelope offset frequency. In our study, we analyze the phase noise of a 100 MHz Er:fiber system in a wide range spanning from microhertz to the Nyquist frequency. An interferometric detection method enables analysis of the high-frequency output of an f-to-2f interferometer. The dominant contribution of a broadband white noise floor at high frequencies attests quantum-limited performance. An out-of-loop measurement of the carrier-envelope phase reveals its jitter to be as low as 250 mrad when integrated over 12 orders of magnitude of the radio-frequency spectrum.