Femtosecond Fieldoscopy
Field-resolved detection allows for the direct measurement of light-matter interactions with attosecond precision in a sub-cycle regime, capturing both amplitude and phase information. For decades, attosecond streaking was the sole method to probe the electric field of light with a bandwidth approaching petahertz (PHz), limited to vacuum operations. Over the past decade, various techniques have been developed that enable near-PHz field-resolved light detection in ambient air. Among these techniques, electro-optic sampling (EOS) stands out for its unparalleled detection sensitivity. In EOS a short probe pulse is employed to resolve the cycles of the electric field of light by up-converting its spectral bandwidth to higher frequencies, making it possible to apply silicon detectors for broadband near-infrared detection. Moreover, when combined with bright, ultra-short pulses, the heterodyne detection allows for higher detection signal-to-noise ratio and higher detection sensitivity, leaving the shot noise of the probe pulse the primary source of noise.
In femtosecond fieldoscopy, an ultrashort pulse impulsively excites molecules at their near-infrared resonances. In the picture, the molecules inside a cuvette represent the sample under scrutiny, while the surrounding molecules represent atmospheric water vapor molecules. The transmitted field contains the sample's global molecular response and the environment. A second short pulse at higher frequencies is used for up-conversion and generation of a delay-dependent signal in a nonlinear crystal, where the correlation signal is directly proportional to the electric field of the excitation pulse. The measured electric field contains the ultrashort excitation pulse, the liquid's delayed response spanning several picoseconds, and a long-lasting response of atmospheric gases lasting for hundreds of nanoseconds. Time filtering and subsequent data analysis can decompose the molecular response to the short-lived liquid and long-lived gas responses. Different compounds like proteins, carbohydrates, lipids, polyphenols, and alcohols have distinct vibrational modes at this range. (Nature Photonics 2024)
Time-domain Compressed Sensing
The molecular response, enriched with comprehensive spectroscopic information, lasts from tens of femtoseconds to nanoseconds and is spars in the frequency domain. The measurement speed in field-resolved detection is limited to i) the required number of sample points dictated by the Nyquist-Shannon criteria, ii) the speed of spatial-temporal scanning, and iii) the transportation and storage speed of the measured data. We have demonstrated that field-resolved spectroscopy can be accelerated by compressed sensing.
Nonlinear interaction of bright femtosecond pulses and soft tissue
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Over the last years, the team has investigated in vivo phototoxicity in soft tissue upon interacting with high repetition rate femtosecond pulses. Our study leveraged the vertebrate species zebrafish (Danio rerio) to delve into the mechanisms of photodamage in deep tissue at a cellular level triggered by femtosecond excitation pulses.
Our objectives revolved around elucidating several key questions: How do the dynamics of different photodamage mechanisms unfold across a spectrum of irradiation wavelengths? Is it feasible to reach plasma formation without inducing gradual damage to the sample due to thermal effects and chemical reactions? How does the repetition rate of laser pulses impact the photodamage threshold?
Atmospheric Gas Sensing
Frequency comb spectroscopy and field-resolved broadband absorption spectroscopy are promising techniques for rapid, precise, and sensitive detection of short-lived atmospheric pollutants on-site. Enhancing detection sensitivity in absorption spectroscopy hinges on bright sources that cover molecular resonances and fast signal modulation techniques to implement lock-in detection schemes efficiently. Yb:YAG thin-disk lasers, combined with optical parametric oscillators (OPOs), present a compelling solution to fulfill these requirements. We developed a bright OPO pumped by a Yb:YAG thin-disk Kerr-lens mode-locked oscillator delivering 2.8 W, 114 fs pulses at 2.06 μm with an averaged energy of 90 nJ. The OPO cavity operates at 30.9 MHz repetition rate—twice the repetition rate of the pump laser—allowing for a broadband, efficient, and dispersion-free modulation of the OPO output pulses at a 15.45 MHz rate. With 13% optical-to-optical conversion efficiency and a high-frequency intra-cavity modulation, this scalable scheme advances the detection sensitivity and frontiers of field-resolved spectroscopy. Moreover, the technology holds potential for other scientific fields, such as earth-orbit communication, where high bandwidth modulated lasers is required (APL Photonics 2024).
Solar-pumped Lasers
To reduce greenhouse gas emissions and establish an affordable, secure, and reliable energy supply, we need to generate most of our future energy requirements from renewable sources and use these sources efficiently. The sun is projected to last for another 4–5 billion years. Our planet receives more energy from the Sun in one hour than all humankind’s energy consumption in a year. This positions solar energy as arguably the most substantial and harnessable renewable energy resource, making it a recommended long-term energy alternative solution. Nonetheless, its contribution to our energy needs remains surprisingly minimal.
The Earth’s atmosphere reflects about 30% of this energy into the cosmos. Moreover, the diurnal cycle ensures that each night plunges us into total darkness. In regions like Europe, this dependence becomes even more pronounced during winter when only about 3% of sunlight penetrates the Earth. This variability, coupled with seasonal changes, introduces challenges in harnessing solar power, as these fluctuations can impact the efficiency of solar energy generation.
To overcome these limitations, harvesting energy in space and transporting it to the ground was suggested at the dawn of the space age. It is estimated that space-based solar power (SPSB) could generate 40 times as much energy as Earth-based solar power. However, advancements in solar-pumped lasers for space applications have faced obstacles. These challenges stem from the low conversion efficiency, complexities in focusing solar light, managing thermal load during the operation, and constraints in existing gain media.
Equipped with a decade of experience in laser development, the team is currently addressing these issues. The pathfinder program of the European Research Council in the APACE project funds part of these activities (APL Photonics 2024).