Synthesis of space-time terahertz waves in random systems

A research team has developed a new technique that allows precise spatio-temporal control of terahertz waves as they pass through disordered materials.

The method, published in ACS Photonics, could lead to advances in medical imaging, communications and other applications relying on broadband terahertz pulses. The research was carried out as part of the European Union ERC TIMING project, and the team included members of the Loughborough Emerging Photonics Research Center in collaboration with Professor Jacopo Bertolotti from the University of Exeter.

In optics, the traditional view has long viewed disordered systems – like looking through ground glass – as a limit to clarity. Just as fog, a disordered distribution of water particles, scatters light and blurs our vision, these materials scatter light in unpredictable ways. But this new study shows that we can exploit this dispersion to our advantage.

A more modern approach that describes these objects as “complex media” reveals a strikingly different story. The key lies in understanding that although information is indeed scrambled within these systems, it is not irretrievably lost and that this scramble can be used to manipulate light itself.

Terahertz waves are a form of electromagnetic radiation with a wavelength between microwaves and infrared light. They represent the bridge between electronics and photonics, making them remarkably difficult to generate, detect and manipulate. Yet they are highly sought after and unique because terahertz waves can penetrate materials such as clothing, paper and plastic, providing clear images without the ionizing damage of x-rays, and can carry exceptionally high performance communications links. .

However, terahertz waves become distorted as they propagate through complex structures such as certain biological tissues or technological structures. In fact, imaging via complex media is a challenge, but also an opportunity.

In this study, the researchers used a special type of ultrafast laser, known for its extremely short pulses, to create patterns of terahertz pulses (a few picoseconds long).

As these patterns interacted with a complex scattering material, the researchers manipulated the laser’s illumination by employing a specially designed genetic algorithm that mimics the process of natural evolution to solve complex problems.

As a result, they took control of how terahertz waves distribute themselves in space and evolve over time after matter. In a way, this level of control recomposes pieces of the wave scrambled by diffusion, into a new form with the desired patterns and colors.

“It is remarkable that complex media function as sophisticated devices that manipulate terahertz waves in ways beyond the reach of art, and yet they actually constitute a very accessible random assemblage of particles,” said Dr. Vittorio Cecconi, principal investigator of the study. He continues: “This opens up new possibilities for exploiting terahertz waves in imaging and sensing applications where scattering is an issue.”

Although this approach has multidisciplinary ramifications, in terahertz, it is made possible by the availability of methods to measure the evolution of the terahertz electric field over time, in a way that resembles the operation of an oscilloscope. Yet in photonics this is very rare because the electric field (the quantity that oscillates in electromagnetic waves) is generally not measurable for light, whereas the quantity commonly accessible via a photodetector is intensity.

This specific difference allows a methodology known as nonlinear ghost imaging design to obtain spatiotemporal information about waves and how they interact with optical materials.

“The synergy between nonlinear ghost imaging and complex media enabled this research and paves the way for several potential advanced applications, such as terahertz computing,” said Dr. Cecconi.

Professor Peccianti, Center Director and Principal Investigator of the ERC TIMING project, highlighted the Center’s mission, saying: “At the Emerging Photonics Research Center, our main philosophy is to explore the intersection between ultrafast photonics and complexity. . Here, light transcends its traditional role as simple illumination, evolving into a powerful tool capable of instantly capturing and processing a large amount of information, thus marking the path to new technological innovation.