Time-Resolved Spectroscopy Peers in on Irreversible Phenomena

A team of researchers at the University of Ottawa developed a terahertz (THz) spectroscopy technique for recording movies in real time at 50,000 fps.

High-speed video captures and slow-motion movies allow scientists to observe the mechanical dynamics of complex phenomena in detail. When the images in each frame are replaced by THz waves, the movies make it possible to monitor low-energy resonances and fast structural and chemical transitions in sample materials. As a result, the THz spectroscopy system, developed in collaboration with researchers from the Max Planck Institute for the Science of Light, could become a powerful tool for observing phenomena that are currently impossible to investigate because they are too fast, nonreproducible, or both.

Scientists from the University of Ottawa and the Max Planck Institute for the Science of Light developed an approach that could facilitate discoveries in materials physics by combining THz spectroscopy and real-time monitoring via detector electronics. Courtesy of the University of Ottawa.

The system combines two spectroscopy techniques — chirped-pulse spectral encoding and a photonic time-stretch technique — with fast detection electronics. The first technique imprints the information carried by a THz pulse onto a chirped supercontinuum in the optical region. The second technique stretches the pulse in time, inside a long fiber, slowing down the rate of information so that it can be recorded in real time. These steps are repeated using a train of pulses at 20-µs intervals. The pulses can be combined to make a movie of the low-energy dynamics taking place inside the sample material.

The system relies on a single ultrafast source that enables the detection of every single generated THz pulse that is emitted every 20 µs. With the single-pulse detection technique, the researchers can probe the sample at the repetition rate of the laser (i.e., 50 kHz) to obtain a series of measurements that allow them to trace microscopic dynamics that may change on a pulse-to-pulse basis.

Single-pulse THz detection scheme. Courtesy of Nature Communications (2023). DOI: 10.1038/s41467-023-38354-3.

To demonstrate the spectroscopy system, the team monitored pulse-to-pulse, submillisecond dynamics of hot carriers injected in a silicon wafer, using successive pairs of near-infrared (NIR) pump and THz probe pulses in a transient regime. Each THz wave transmitted through the sample was time-resolved every 20 µs, providing phase and amplitude information, to achieve single-pulse THz spectroscopy of the pump-induced change in the complex dielectric function.

Using a theory based on the Drude model, the researchers were able to extract the density and relaxation time of the injected carriers by analyzing the complex transmission spectrum of the THz pulse. The experimental model included dynamic effects, such as inhomogeneous carrier distribution in the sample along the THz propagation direction, spatial diffusion, and carrier density-dependent scattering time.

Although the THz spectroscopy experiments were performed at a repetition rate of 50 kHz, revealing submillisecond dynamics in silicon, the team said that the acquisition rate was limited only by the signal-to-noise ratio at higher repetition rates. Further development of the system will enable the researchers to reach acquisition rates in the megahertz range, to uncover submicrosecond processes in systems resonant to THz frequencies.

Professor Jean-Michel Ménard, who led the research, said that the compact, tabletop system could replace a technology that was previously only accessible in large synchrotron facilities.

The single-pulse, time-resolved THz spectroscopy system could enable scientists to investigate irreversible physical, chemical, and biological phenomena, such as electronic transport in semiconductors, chemical exothermic reactions, and protein folding in biological systems, for the first time, according to the researchers. Ménard said that the system could be used in experiments that trace vibrational resonances of molecules to study the role of enzymes in chemical reactions and to observe invisible changes in living organisms when they are exposed to a sudden rise in temperature.

“In condensed matter experiments, our rapid THz photonic system will be used to observe a range of nonreversible electronic or lattice reconfigurations, notably occurring during phase transitions,” Ménard said. The development is also poised to render THz spectroscopy an even more efficient characterization tool in support of discoveries in materials physics, he said.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-023-38354-3).