Research

Waveforms with durations short enough to probe the electronic timescale can be generated both in the extreme UV and in the visible-infrared (Vis-IR) region of the electromagnetic spectrum. High-order harmonic generation can be used to generate isolated attosecond pulses and pulse trains in the extreme UV, and since very recently the coherent synthesis of optical pulses is used to generate subcycle optical waveforms in the Vis-IR region. In the deep UV (DUV), where many basic aromatic molecules absorb radiation and undergo photochemical reactions, pulses with such extremely short durations have not yet been demonstrated.

Harmonic concatenation is our new method for the synthesis of ultrashort pulses. The principle is the concatenation of spatial and temporal harmonics that are created by two noncollinear Vis-IR pulses. Cascaded processes of frequency conversion and self-diffraction in a thin dielectric plate yield a multifaceted emission pattern of temporal and spatial harmonics. The emission angle of the harmonic orders depends on the frequency. This spatiotemporal coupling is exploited to concatenate two neighboring spatial harmonics by adjusting the crossing angle of the fundamental pulses. Broadband waveforms arise, of which the lower frequency bands belong to one spatial harmonic, and the higher frequency bands belong to the other spatial harmonic. In order to synthesize ultrashort waveforms from these frequency bands, their phases must be adjusted. This is accomplished by controlling the groove-envelope phase (GEP) of the fundamental pulses, which is determined by their delay on the subcycle timescale. Using temporal harmonics of third order, the concatenation of two spatial harmonics synthesizes record-short deep UV waveforms of only 1.5 fs! 

_{J. Reislöhner, C. Leithold, and A. N. Pfeiffer, "Harmonic concatenation of 1.5-femtosecond-pulses in the deep ultraviolet", ACS Photonics, 10.1021/acsphotonics.9b00219 (2019).}

The wavelength range with photon energies of 5-15 eV is very important for spectroscopic methods, since binding energies of valence electrons are usually located within this range. For example, in order to observe the transient change of band structures in dielectrics and the related screenings in the presence of a strong field pulse, probe pulses are required that i) have a duration shorter than the optical period of the strong field pulse, ii) encompass the wavelength of the dielectric bandgap, and iii) are available at the sample position.

We develope a miniature beamline for transient absorption, a novel method in attosecond spectroscopy. Harmonic concatenation is an ideal source, because a generic feature is that these waveforms are spatially separated from the fundamental pulses and are therefore available for immediate spectroscopic usage without any subsequent optical elements.

We explore the timing of the nonlinear response of thin glasses, crystals and liquid films. These materials have a band gap in the range of 5-10 eV and therefore a much shorter response time than semiconductors. The implications concern the limits of light-induced signal processing. 

Through noncollinear interaction of fundamental light pulses inside the samples, additional light pulses are generated whose time course reflects the nonlinear dynamics in the medium. This dynamic is measured by recombining the generated pulses with the fundamental pulses. A suitable optical system must guarantee equal path lengths for all pulses after the interaction in the sample, which is normally not possible due to surface irregularities of the optics. In our novel approach, surface irregularities are compensated through a dynamic optical system.