research in field-resolved infrared metrology

— Ioachim Pupeza —

Novel tools and techniques for electric-field-resolved investigations of ultrafast light-matter interactions

The optical electric fields associated with light-matter interactions carry in-depth information on the underlying physical mechanisms. The outstanding coherence of laser light enables direct measurements of these fields on their natural (sub-) femtosecond time scales, providing unique spectroscopy tools for a wide range of applications.

Our research primarily addresses the development of novel tools and techniques for field-resolved spectroscopy (FRS) of molecular vibrations in the IR spectral region. However, the technologies and expertise developed in our group also impact on other fields, such as time-resolved photoelectron emission microscopy (PEEM) employing attosecond XUV pulses.

At heart of our experimental setups lie the generation of broadband, high-power, phase-stable infrared pulses and techniques for sampling their electric fields in the time domain. In particular, we target high pulse repetition rates (> 1 MHz) affording short measurement times and, thus, improved statistics.

Our radiation sources employ state-of-the-art Yb-thin-disk oscillators and fiber lasers. Furthermore, the high repetition rates enable the use of femtosecond enhancement cavities, both for the generation of broadband coherent radiation and for increasing the sensitivity in spectroscopy.

Field-resolved spectroscopy of molecular vibrations

With field-resolved spectroscopy (FRS) we record the electric fields emitted by impulsively excited molecular vibrations as the most fundamental ensemble-averaged physical measurable of coherently oscillating microscopic electric dipoles.

research in field-resolved metrology

— Ioachim Pupeza —

Biomolecular assemblies exhibit strong fundamental rotational and vibrational modes in theinfrared (IR) spectral range (wavelengths >2µm). IR vibrational spectroscopy yields insights into molecular composition, structure and conformation.

Among its numerous applications, further enhancement of the sensitivity of state-of-the-art vibrational spectroscopy promises to lead to breakthroughs in the early detection of diseases.

Our research focuses on harnessing the advantages of employing phase-stable, broadband, ultrashort IR pulses and electric-field-resolved detection for vibrational spectroscopy.

Most importantly, the phase-coherent superposition of all frequency components of the IR source into an ultrashort pulse enables confinement of the impulsive excitation event to a sub-picosecond time window. Sampling of the resulting electric field, as the most fundamental ensemble-averaged physical measurable of the vibrationally excited sample, allows one to distinguish between the time-domain fingerprints of these vibrations and the fast decaying instantaneous response of the sample (see illustration above).

Thus, FRS permits IR-background-free detection of molecular fingerprints and, therefore, has the potential to provide unparalleled sensitivity and specificity for vibrational spectroscopy.

Technologies and collaborations

We employ high-average-power, high-repetition-rate radiation sources, which enable, on the one hand, measurements with high signal-to-noise ratios and, on the other, the use of broadband enhancement cavities.

Our first-generation FRS device uses a state-of-the-art, non-linearly post-compressed Yb-based high-power thin-disk oscillator developed by the HFS, and subsequent frequency down-conversion via difference-frequency generation.

Few-cycle IR pulses emerge at a repetition frequency of 100 MHz and cover the range between 6 and 18 µm with an average power of a few tens of mW. Coherent detection is based on electro-optical sampling employing a copy of the short pulses from the Yb-based frontend.