Laser-induced plasma formation in solid dielectrics
When strong-field ionization (SFI) occurs in a dielectric solid, the plasma density increases in a nearly stepwise fashion with two steps per optical cycle of the driving field [Fig. 1 (a)]. The Fourier series of the plasma density evolution ρSFI(t) contains even harmonics of the incident laser field [Fig. 1 (b)], the so-called Brunel harmonics. Since only the rapid variations of ρSFI(t) contribute to the harmonic emission, time-resolved detection of Brunel harmonics allows to analyze sub-cycle ionization dynamics in the frequency domain. In a two-color pump-probe scheme Brunel harmonics occur at frequencies wn = wprobe + 2n x wpump where n is a natural number. An experimental spectrogram displaying the first three orders of Brunel harmonics obtained from a two-color pump-probe experiment (lpump = 2300 nm, tpump = 150 fs, lprobe = 790 nm, tprobe = 40 fs) in the bulk of fused silica is shown in Fig. 1(c). By performing an iterative phase-retrieval on the experimental spectrogram, it is possible to reconstruct the plasma formation dynamics due to SFI. A simultaneous measurement of the total plasma density generated in the material reveals the relative importance of the two competing ionization mechanisms (SFI and electron impact ionization), thus allowing unprecedented insights into ultrafast plasma formation dynamics in solid dielectrics.
Fig 1: (a) Temporal evolution of the plasma density in fused silica irradiated by an ultrashort laser pulse. (b) Fourier series expansion of (a) containing even harmonics of the driving laser field. (c) Time-resolved experimental spectrogram resulting from two-color irradiation of a 0.5 mm fused silica sample.
Ultrafast dynamics of plasmon-controlled nanoscale surface modification
The deeper understanding of laser-matter interaction processes at extreme time-scales is both of fundamental interest and enabling to create new technologies in materials processing, spectroscopy, plasmonics, or spintronics. In particular, the dynamics of femtosecond-laser induced surface modification and related enhancement mechanisms prove to be highly complex. Our studies are focused onto the following points:
- Exploration of the dynamics on fs-scale by pump-and-probe experiments
- Role of surface plasmon-polaritons in the initial phase of laser excitation
- Enhancement and feedback mechanisms in generating laser-induced periodic nanostructures
Recently we reported particular results on
- Description of LIPSS period in ZnO by modified Drude theory with multiphoton terms 
- Generation of far-sub-wavelength structures in metals and dielectrics 
- Application of nanostructures in high-field laser systems for particle acceleration 
- Identification of nonthermal-thermal mechanisms for LIPSS of Si by pump-probe scattering 
Fig. 2 shows the temporal dynamics of laser-induced periodic surface structure formation in Si surfaces. Results of pump-probe scattering experiments are compared to related steps of the mechanism. Time dependent scattering indicates a hybrid mechanism. A non-thermal phase with plasmons during the the laser pulse is followed by a thermal phase and material re-arrangement. The most probable model is that the plasmons write the spatial frequencies of ripples into deformable matter at femtosecond scale and the structural information is conserved and slightly modified by final solidification at thermal relaxation on picosecond to nanosecond scale.
Plasmon management on nanoscale is assumed to be the key for controlling spatial localization (enabling a further miniaturization of structures) and periodicity (reducing the degree of randomness). Currently, focused ion beam (FIB) fabricated nanostructures are tested for their suitability to define the boundary conditions for plasmon propagation and resonance.
 S. K. Das et al., ZnO nanorods for efficient third harmonic UV generation, Opt. Mat. Express 4, 701-709 (2014).
 S. K. Das et al., Highly periodic laser-induced nanostructures on thin Ti and Cu foils for potential application in laser ion acceleration, J. Appl. Phys. 119, 113101 (2016).
 A. Lübck et al., Prospects of target nanostructuring for laser proton acceleration, Sci. Rep. 7, 44030 (2017).
 A. Lübcke et al., Interaction of ultrafast laser pulses with nanostructure surfaces, in: K. Wandelt (Editor-in-Chief), Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Vol. 2, pp. 420-432, Elsevier, Oxford (2018).
Fig. 2: Dynamics of LIPSS formation and plasmon excitation in silicon. Right side: results of pump-probe diffraction/scattering experiments with a Ti:sapphire laser as pump and the SHG as probe, left side: corresponding physical regimes in a time scale and assumed steps of the mechanism (schematically). A combined mechanism is indicated where first plasmons appear during the duration of the femtosecond laser pulse (first 250 fs). The nonthermal phase is followed by thermal processes and material re-arrangement.