Conference Agenda

Field-resolved detection at near-petahertz frequencies provides exceptional sensitivity, broad bandwidth, and high dynamic range, enabling attosecond temporal and sub-diffraction spatial resolution. In a technique known as Femtosecond Fieldoscopy, ultrashort laser pulses impulsively excite molecular vibrations in resonance with the near-petahertz carrier frequency. This initiates vibrational coherence at the trailing edge of the pulse, which decays exponentially based on the molecular dephasing time.

The transmitted electric field encodes information about the excitation pulse, the sample’s picosecond-scale response, and a prolonged signal from atmospheric gases lasting hundreds of nanoseconds. By detecting this response in the time domain and applying Fourier analysis, the technique yields spectroscopic data with outstanding sensitivity and dynamic range. This performance is achieved by temporally gating the molecular response away from the excitation pulse.

Enabled by recent advancements in ytterbium laser systems, Femtosecond Fieldoscopy has successfully resolved overtone, Raman, and combination bands in liquid samples. Moreover, the method has been advanced for real-time sampling and extended to non-perturbative, label-free imaging.

This talk presents an overview of these recent developments as demonstrated by my group, highlighting the broad potential of this emerging spectroscopic tool.

We combine super-resolution and label-free microscopy by using a donut-shaped pump beam to confine harmonic generation to a sub-diffraction region. This Harmonic Deactivation Microscopy (HADES) can enable (sub-)fs temporal and at least 100-nm spatial resolution.

We present the comprehensive characterization of a series of harmonic fields generated in a ZnO crystal by a few-cycle MIR driving pulse that spans the visible to mid-infrared (MIR) spectral region. The characterization is conducted using the recently developed Plasma-Induced Frequency Resolved Optical Switching (PI-FROSt) technique. We demonstrate the ability of this method to accurately characterize the MIR driving field (λ = 3.2 μm), as well as both odd and even harmonics up to the fifth order. The total spectral bandwidth extends over an exceptionally wide range of 2.6 octaves. All assessments validate the high precision of the field reconstructions and confirm the suitability of the PI-FROSt method for the metrology of over-octave-spanning waveforms. The results offer valuable insights into the fundamental mechanisms governing harmonic generation and emphasize the crucial influence of propagation and cascading.

A plethora of recent studies have shown optical modulation of high-harmonics generation in solids, in particular, suppression of high-harmonics generation has been observed by synchronized or delayed multi-pulse sequences. These works illustrate that high-harmonic generation can effectively be used to study the microscopic electron dynamics in solids on the femtosecond timescale. Moreover, the all-optical switching demonstrated has numerous potential applications: These range from super-resolution microscopy to nanoscale-controlled chemistry, and highly tunable nonlinear light sources. Here we demonstrate the use of elliptically polarized light to spatially shape and confine high-harmonic generation from solids. This technique of spatial polarization gating provides a step towards a universal high-harmonic-based all-optical super-resolution imaging technique in solids. We demonstrate the common ellipticity response of high-harmonic generation in solids and show that we can reproduce these results with simulations based on the semiconductor Bloch equations. Proof-of-principle measurements show that with spatial polarization gating we can obtain sharper and smaller emission features than with conventional high-harmonic imaging, enabling higher-resolution imaging. This opens the door to resolving ultra-fast phenomena such as the insulator-to-metal phase transition in strongly correlated materials not only in time but also in space.