Attoworld physicists led by Ludwig Maximilian University and Max Planck Institute of Quantum Optics are currently working to develop laser-based instruments with unprecedented sensitivity, specificity, and measurement reproducibility for future healthcare applications.
One of their goals is to detect early hints of diseases, like cancer, from subtle changes in the molecular composition of human blood samples. To do this, they’re using controlled ultrashort mid-infrared (mid-IR) pulses to excite molecular vibrations within samples and recording the emitted electric-field response. Weak response signals provide a molecular fingerprint that depends on the complex composition of the blood samples, which can reveal information about a person’s health status.
Another goal is “to push the frontiers of electronic signal processing and use field- and time-resolved techniques to study the interaction of light and matter within solid-state systems,” explains Nathalie Nagl, a postdoc within the Attosecond Physics group at Max Planck Institute of Quantum Optics in Germany. “To do so, light sources with powerful and phase-stable pulse trains need to be developed to generate highly reproducible electric-field waveforms in a wide variety of spectral ranges.”
Launching and steering the few-femtosecond electron-hole wave packets within solids requires control over the electric field’s temporal evolution. “Moreover, to study low-bandgap highly doped semiconductors and increase the bandwidth in electric signal processing, such control must be extended to the longer (mid-infrared) wavelengths,” says Nagl.
To date, common approaches for mid-infrared pulse control tend to suffer from system complexity and limited reproducibility of output waveforms. So Nagl and colleagues, including Philipp Steinleitner and Maciej Kowalczyk, developed a carrier-envelope-phase-stabilized and Kerr-lens-modelocked Cr:ZnS laser system.
“With our low-noise laser systems based on diode-pumped chromium-doped II-VI gain media (Cr:ZnS), we demonstrated a compact and robust way to extend waveform control to the mid-IR spectral region and measured single-cycle waveforms with high fidelity,” Nagl says. “Our new approach yields the first multi-octave control of single-cycle mid-infrared waveforms, with a continuously adjustable, highly reproducible electric-field evolution.”
They did this by sending the 28-fs-long output of a diode-pumped oscillator directly into a thin bulk dielectric medium (rutile) to generate a super-octave-spanning spectrum. After compression with custom chirped mirrors, it achieved 7.7-fs-long pulses.
“At our spectral centroid position of 2.24 µm, this corresponds to a mere single cycle of the electric light field,” she says. “The broadened pulses are subsequently split with an uncoated wedge, and we use the two reflected beams for carrier-envelope phase (CEP) stabilization and for gating electric-field resolved detection with electro-optic sampling (EOS), respectively.”
The main part of the beam traversing the wedge is efficiently down-converted via cascaded intrapulse-difference-frequency generation (IPDFG), providing a coherent near-infrared to mid-infrared supercontinuum, spanning 3.7 octaves. So the physicists can tap into the CEP dependence of this process to continuously manipulate the mid-infrared waveform.
To explain this dependence, Nagl provides a closer look at the cascaded IPDFG process itself: “Due to sequential difference-frequency generation and remixing, the CEP of the newly generated wavelength components will either be invariant to (even cascading orders) or follow the CEP changes of the driving laser (odd cascading orders),” she says. “The spectral region where the even and odd orders overlap exhibits pronounced interference effects when tuning the CEP of the driving laser. Any change in the driver CEP modifies the electric-field waveform of the multi-octave spanning output, and this is what we’ve measured with EOS.”
This work is significant because the team’s work is a new and monolithic approach to mid-infrared waveform manipulation, in which the control parameter is the CEP of the driver laser. “It’s based on a robust, simple, and efficient conversion scheme that requires no spatial separation of spectral bands—eliminating instabilities arising from beam-pointing fluctuations and timing jitter typically present in multichannel synthesizer schemes,” Nagl says.
Nagl points out its ultrahigh phase and amplitude stabilities, as well as the reproducibility of the single-cycle waveforms. Their diode-pumped Cr:ZnS driver laser technology is the key enabler, and it recently evolved as a new source for generating powerful, ultrashort, and low-noise, few-cycle pulses with wavelengths between 2 to 3 µm.
“We believe our work will impact research on several fronts—including our own efforts to advance electronic-signal processing and electric-field-resolved molecular fingerprinting of biological systems,” she says.
The most surprising aspect of this work, from Nagl’s vantage, is that a comparably low pulse energy is sufficient to generate and record multi-octave waveforms with high fidelity. “No further amplification of the nanojoule-level laser output is needed to obtain multi-octave-spanning supercontinua with two-digit optical-to-optical conversion efficiencies for the IPDFG process,” she adds.
Next up, the team plans to boost the output power of their laser systems via multi-pass diode-pumped Cr:ZnS amplifiers. “The higher peak power and correspondingly higher peak intensities will allow us to use nonlinear crystals with lower nonlinear coefficients but broader transparency windows to extend the bandwidth of the supercontinua to shorter and longer wavelengths,” says Nagl. “Moreover, we’ll further explore the implications of our new technology in both basic research and toward applications in biology and medicine.”
FURTHER READING
P. Steinleitner et al., Nat. Photonics (2022); https://doi.org/10.1038/s41566-022-01001-2.
