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.