Macroscopic attosecond currents driven in dielectrics by strong light fields after carrier injection by extreme-ultraviolet (EUV) light reveal nonequilibrium conduction band carrier dynamics. To introduce spatial resolution to attosecond physics, we demonstrate novel EUV metasurfaces.

Ultrashort pulses at infrared wavelengths are advantageous when studying light–matter interaction. For the spectral region around 2 µm, multi-stage parametric amplification is the most common method to reach higher pulse energies. Yet it has been a key challenge for such systems to deliver waveform-stable pulses without active stabilization and synchronization systems. Here, we present a different approach for the generation of infrared pulses centered at 1.8 µm with watt-level average power utilizing only a single nonlinear crystal. Our laser system relies on a well-established Yb:YAG thin-disk technology at 1.03 µm wavelength combined with a hybrid two-stage broadening scheme. We show the high-power downconversion process via intra-pulse difference frequency generation, which leads to excellent passive stability of the carrier envelope phase below 20 mrad—comparable to modern oscillators. It also provides simple control over the central wavelength within a broad spectral range. The developed infrared source is employed to generate a multi-octave continuum from 500 nm to 2.5 µm opening the path toward sub-cycle pulse synthesis with extreme waveform stability.

Direct measurements of the electric field of light enable new observations of light–matter interactions. In the near‐infrared and visible spectral ranges, this typically relies on techniques that exploit nonlinearities in gases or solids, which limits their sensitivity. Here, a method for the detection of broadband near‐infrared fields spanning more than one octave from 110 to 220 THz based on linear absorption in a semiconductor is demonstrated. This technique, which avoids complex vacuum setups and works under ambient conditions, employs linear photoconductive sampling (LPS) in gallium phosphide. Simulations reveal that the response function of LPS is concerned with the intensity envelope of the gate field, in contrast to electro‐optic sampling, relaxing the stringent temporal requirements on the gate pulse.

Photoconductive sampling of optical light fields in gallium phosphide is performed by Nicholas Karpowicz, Matthias F. Kling, and co-workers (article number 2202994). Ultrashort ultraviolet pulses (blue line) generate electrons (blue cloud) and holes (red cloud) that are accelerated towards electrodes with the optical field (red line). The measured current in the material enables sampling of the waveform of the optical field.

The ability to directly measure the electric field of light enables novel observations of light-matter interactions, proving a powerful tool in modern ultrafast science [1]. Techniques such as attosecond streaking and nonlinear photoconductive sampling (NPS) [1], [2] exploit nonlinearities in gases or solids in order to achieve short gating events that enable field sampling in the near-infrared (NIR) and visible spectral ranges. The dependency of these techniques on strong-field ionisation constrains their sensitivity. Presented here is a method for the detection of broadband near-infrared fields spanning more than one octave from 110 to 220 THz, based on linear absorption in gallium phosphide (GaP). In contrast to NPS, this Auston-type [3] linear photoconductive sampling (LPS) approach circumvents complex vacuum setups, avoids phase-matching and does not require high power or high pulse energy to sample the electric field of light. By applying a short visible-UV (VIS-UV) pulse to GaP, an appreciable change in the carrier density (within a few femtoseconds) occurs. This change in carrier density acts as a gating event, permitting the detection of electric fields across the near-infrared.

Our real-time study of nonlinearities in solids demonstrates that ultrafast light fields can transform nickel from a conducting to a semiconducting material, while silicon can be turned from an indirect to a direct band-gap absorber.

If the energy of a photon exceeds the band-gap of a material, an electron can be promoted from the valence into the conduction band with a probability described by the linear absorption coefficient. For photons with energies below the bandgap, the medium is transparent. However, even large band-gap materials that are transparent for visible light in the linear regime can become absorptive in the presence of strong electric fields either through multi-photon, or via tunneling transitions. To study the intricate dynamics of these excitation pathways we developed Attosecond Polarization Sampling (APS). This method resolves the light-matter energy transfer dynamics with attosecond temporal resolution and allows real-time recording of the evolution of both the linear and nonlinear polarization wave driven by the external electric field inside a material. In APS, the laser electric field E(t) is recorded after passage through the sample in two different settings: First, we attenuate the laser pulses before sending them through the material (here: 10 μm fused silica (SiO2)), ensuring only linear effects occur. In the second step, we use intensities close to the damage threshold of SiO2 so that both linear and non-linear interaction occurs. Comparison of the two electric fields yields slight differences in the instantaneous temporal phase of the pulse carrying information about the time evolution of the induced nonlinear polarization wave. The observed phase shifts record the field amplitude dependency of the nonlinear refractive index due to the Kerr-effect including its response time and can be directly related to the degree of light-field induced conduction band population. Consequently, these measurements, as shown in Fig. 1, allow to determine the amount of energy exchanged between light-field and sample with sub-optical-cycle time resolution. In the case of strong-field illumination of SiO2 we find that the nonlinear energy transfer has a strong reversible component occurring only during short time intervals around the field crests of the strong laser field and corresponding to the non-permanent transfer of electrons into conduction band-states. Build-up and immediate decay of this transient conduction band population are the result of a bi-directional energy transfer between field and matter and conform to the assumption of energy absorption from the field during the first half of the laser pulse and re-emission of energy into the same field within the duration of the few-cycle laser pulses. Most interestingly, we find settings where the transient energy exchange yields a significant modification of the optical and electronic material properties for femtosecond time intervals but no lasting energy transfer (i.e. residual conduction band population) is observable. This ultrafast, dissipation free switching of material properties might turn useful for a future all-optical, ultrafast and loss-free signal metrology.