Researchers generate and shape multi-octave mid-IR pulses with single-cycle durations
A so-called adiabatic frequency converter helps to create pulses with only 1.2 optical cycles, extendable to subcycle durations.
Creating shorter femtosecond pulses in the mid-IR would aid single-shot remote sensing, as well as single-frame captures of light/matter interactions -- for example, the effect of light-induced vibration on a molecule (rhodopsin) in the retina.
A group that includes researchers at MIT (Cambridge, MA), Tel Aviv University (Tel Aviv, Israel), Cornell University (Ithaca, NY), and the University of Hamburg (Hamburg, Germany) has developed a process for generating and shaping intense mid-infrared (mid-IR) pulses of light for these purposes.
Mid-IR wavelengths are of particular importance to materials scientists, chemists, biologists, and condensed-matter physicists. Recently, the advent of high-pulse-energy and ultrashort-duration mid-IR sources has enable the exploration of new nonlinear light–matter interactions, and establishing mid-IR sources that feature not only an extreme bandwidth, but also an arbitrary control of the pulse shape, is of great interest.
One method for analyzing short-duration phenomena is pump-probe spectroscopy. The first beam of laser light acts as the pump to generate a wanted reaction in a material, and the second is the probe, used to analyze the reaction.
To create pulses of light short enough to capture these events, the light must contain a wide range of frequencies within the IR spectrum. The problem, however, is that in shaping the light for a specific purpose, bandwidth is lost. To overcome that problem, the researchers developed a way to create and shape a broadband, near-IR light pulse and change its spectral region to mid-IR while retaining its bandwidth and shape. In fact, the relative bandwidth of the near-IR wave is effectively increased when converted to a mid-IR wave.
The result: pulses lasting only 1.2 optical cycles.
"When we go through this process, we have bandwidth in the near-IR that’s less than an octave," says Jeffrey Moses, assistant professor of applied and engineering physics at Cornell, "and we end up with bandwidth in the mid-IR that’s more than an octave."
Characterizing the retina of the human eye
One particular application of interest to the group is tracking the way electron energy flows in a system, such as human vision. Rhodopsin molecules in the retina absorb light and then change shape; it's this straightening that serves to send a signal through the optic nerve to the brain.
"The change in the electronic configuration of these molecules happens over tens of femtoseconds," Moses says. "We think we have the right source of light here to gain a lot more information about what’s going on during that ultrashort time period."
And what can that information tell a scientist? For one thing, that process is very efficient in humans, but there are other similar biological processes that are highly inefficient.
"Using this tool, we're trying to develop a method for studying this sort of class of structures that is responsible for the way molecules respond to light," Moses says. "This could help us understand something that we're fabricating and help us make it do whatever it does more efficiently."
This work was supported by grants from the Air Force Office of Scientific Research.
1. Peter Krogen et al., Nature Photonics (2017); doi:10.1038/nphoton.2017.34