Is it possible to control light across four dimensions?
In a long-running international collaboration, an international group of researchers have shown that it is not only possible to control how light moves, but also how it behaves in time. We talk to Dr Victor Pacheco-Pena, Senior Lecturer in Physics at Newcastle University, to find out what this means for the future of relativity research and what practical applications this discovery could have.
Controlling light in space and time is a complex subject. Could you explain your recent research findings for us?
Absolutely.
Controlling how light interacts with materials is important for many everyday technologies, like lenses, computers, antennas, and phones. This is usually done by carefully manipulating the way light moves through three-dimensional space, also known as how light propagates.
By placing structures made from certain materials and shapes along the propagation path, we can control the light in three spatial dimensions (cartesian coordinates, for instance, x, y, z).
The research talks about controlling light in four dimensions. What does that mean practically?
We asked the question, what happens if we go further and introduce new dimensions into the mix? What about time, for instance? Could we manipulate light in both space and time?
We introduced time (t) as a new degree of freedom to enable a four-dimensional manipulation of light. Now we can demonstrate that by controlling light in four dimensions across space and time, we can completely stop light, amplify it and then release it. In doing so, light can continue its propagation at the same or different frequency.
Controlling light in four dimensions means shaping it not just in space (like a traditional lens), but also in time. Normally, we use materials to bend and focus light in three-dimensional space (x, y, z), such as shaping the geometry of glass to enable light to focus on a spot in space to create lenses. We know that we need to do this due to Snell’s law of refraction.
This is what we do in our work. We study the fundamental physical phenomena that occurs when the permittivity of a material changes from positive to a negative value in time. During our research, we discovered that light can be stopped and amplified without the use of ‘repeaters’. Combining both spatial and temporal control creates fascinating effects, like temporal lensing (focusing light in time) and temporal chirp (changing the frequency of light dynamically).
Why is the ability to stop and amplify light such a significant breakthrough?
To truly understand the importance of this, let us revisit two important facts of how light propagates in space.
The first consideration is speed. Light usually travels incredibly fast – about 300,000 km/s in empty space. When it moves through materials like glass, it slows down due to the material's refractive index (n). For example, in glass, red light slows to about 200,000 km/s.
The second consideration is losses. Yes, any material causes losses when light propagates through it. This means that as light ‘moves’ through a material, it will reduce its amplitude. This has prompted many researchers and industries to implement lower loss materials for certain frequency ranges, such as silicon, which has a low loss spectral window, in the designs for fibre optic-based telecommunications.
What makes our recent findings significant is that we have found a way to fully stop light from moving while actually making it stronger at the same time.
This can be done by changing the properties of the materials in time, not only in space. We also show that this method can change the frequency of light (or its colour). Being able to change it opens up new potential uses to control light.
What are the practical applications of being able to stop and restart light in space and time?
When sending information through, for instance, optical fibres over long distances, we need ’repeaters’. Repeaters are devices that amplify or regenerate signals – effectively boosting the light – ensuring efficient transmission over extended distances.
Being able to stop and recharge waves without the need for repeaters may be an alternative to this. We can think of this as a sort of ‘service station’ for light. A usual repeater receives an electromagnetic signal, transforms it into an electrical signal, passes it through an analogue-digital converter, then through an amplifier, and then converts it back to analogue via a digital-analogue converter. Only after all this is it re-transmitted. Our research cuts most of this out, as we show that one could simply stop the wave, amplify it, and then allow it to propagate again.
Of course, this is a single scenario, but this study has the potential for a wide range of applications in optical communications, memory and data storage, computing, and photonics.
Earlier, you mentioned Snell’s Law. Could you explain why this law is so important when studying light?
Snell’s law explains how light refracts and changes direction when travelling from one medium to another medium by relating their refractive index and angles of the propagating waves. This law was recently generalised in space by introducing subwavelength geometries at the interface between two materials, opening new avenues in the field of photonics, metasurfaces and nanotechnology.
In the spatial generalisation of the Snell’s law, it is mainly the ’direction of propagation‘ of light in the second medium that we try to control (that is, the refracted signal) while the frequency of the wave does not change through the process.
How does controlling the passage of light through time allow for a new kind of Snell’s Law?
This is a wonderful question. A tricky one to explain, but I will do my best:
Think about, for instance, a glass of water. If we put a straw within the glass, we will notice how the straw seems to bend within the water. This is the effect of water having a larger refractive index than air, so light travelling from air will refract and change direction within water.
An artistic representation of controlling light through space and time
In our work, however, we introduce time as a new degree of freedom for controlling light. The direction of propagation remains unchanged; instead, it is the frequency of light that is altered. We then make use of this intriguing feature by stacking together different layers of materials in space and modulating each of them in time, using different values but operating simultaneously. This means that the frequency will then change across space, as we have multiple layers stacked together.
What challenges did you face in developing a theoretical framework for space-time interfaces?
One of the main challenges was developing a theory that could strongly corroborate that having a change of permittivity from positive to a negative value was physically possible. This has taken a great deal of time to do. Being theoretical and numerical works, we had to make sure that all the theory was indeed correct, and we had to work hard to evaluate every single theory to make sure that all was consistent. The review process was tough but helped us to push boundaries and provide a full framework of the ideas we wanted to discuss in our papers. We are grateful to all the reviewers of both manuscripts.
The research published in Nature Communications started back in 2017, and we produced the first draft of the paper in 2018. But it took until this year to be published, after submitting it for consideration in 2023.
It has been a long but exciting journey that could not have happened without the commitment and insights from all the authors.
This scientific breakthrough was a result of an international collaboration. Tell us about the team and the skills they brought to the work.
This research would not have been possible without the incredible collaboration that brought together different areas of expertise from around the world.
I’ve been working closely with Professor Nader Engheta from the University of Pennsylvania (USA) for many years now. He’s a pioneer in the field and was instrumental in the success of this work. His insights, especially around electromagnetic theory and metamaterials, were fundamental in refining and pushing the boundaries of what we thought was possible.
In addition, the collaboration with colleagues from University of Extremadura (special note must go to Dr. Diego M. Solís, now at the University of Vigo) in Spain brought a strong theoretical foundation to our work. Their expertise in wave propagation and materials science really helped us build the core models we needed.
And working with colleagues from ESPCI Paris (École supérieure de physique et de chimie industrielles de la ville de Paris), Professor Mathias Fink in France opened our eyes to new ways of thinking. Their background in acoustic and water wave manipulation helped us see parallels across different physical systems and broadened the scope of what we were aiming to do.
Together, we published two new research studies. In one (published in Nature Communications), we demonstrated how to stop and amplify light using time-varying materials.
In a second research study (in Physical Review B), we propose a new version of Snell’s Law that applies to space-time boundaries, changing how light behaves not just directionally, but also in terms of frequency.
How might these findings influence future technologies or fields like telecommunications or quantum computing?
Controlling light in space and time is adding a new degree of freedom (time) to manipulate how light and matter interact. We have shown in other theoretical works, for instance, that we can control the direction of light in real time by using time interfaces too (something that we called temporal aiming).
Furthermore, time interfaces have also started to be studied in the quantum regime, too. While more needs to be done, our works represent key and fundamental studies to understand more about how we can manipulate at will the propagation of light in both space and time.
Practically, these findings could reshape industries and have a profound impact on future technologies, particularly in telecommunications and quantum computing. For instance, to enable faster and low-energy communication systems.
What drew you to this line of research originally?
I always like to ask the question: ‘what would happen if ... ?’ I believe that asking fundamental questions is key to enabling scientific progress. We may not know the answer the first time we ask it, but exploring the topic may lead to exciting and intriguing results, such as those we found in these works.
We asked, for instance, what would happen if the permittivity of the medium changes to a negative value? What would happen if we had certain regions in space that are filled with time-modulated media and others that are not? What interesting physical phenomena may occur?
The field of spacetime media is very new in comparison to what we know about controlling light in space. But excitingly, the results in the area of four-dimensional media are just the tip of the iceberg. There are much more exciting theories to discover and explore.
What’s next for your research? Are there follow-up experiments or theories you’re already pursuing?
Right now, we are looking into how we can create time interfaces by slowly changing a material over time. We are also exploring ways to mimic more complex, directional effects using simpler, uniform materials.
What’s something about light that still surprises you, even as an expert?
The simple answer to this is how little we know about it.
Many breakthroughs have been discovered, proposed, and demonstrated – such as lasers, topological photonics (where light in a photonics circuit can propagate without being scattered in multiple directions when encountering a sharp corner in the circuit, for instance), and twistronics (which explores how twisting layers of 2D materials alters their electrical properties). Yet there is still so much more to uncover, making this an exciting time to work in the field.
You might also like
- read the paper that proposes a new version of Snell’s Law, ‘Temporal chirp, temporal lensing, and temporal routing via space-time interfaces’, published in Physical Review B. DOI: 10.1103/PhysRevB.111.L100306
- read the paper that explains how to stop and amplify light, ‘Holding and amplifying electromagnetic waves with temporal non-foster metastructures’, published in Nature Communications. DOI: 10.1038/s41467-025-57739-0
- find out more about our interviewee, Dr. Victor Pacheco-Pena, Senior Lecturer in Physics at Newcastle University
- find out more about the co-author, Professor Nader Engheta, H. Nedwill Ramsey, Professor at the University of Pennsylvania
- find out more about the co-author, Dr Mathias Fink, Professor at ESPCI Paris and member of the French Academy of Sciences
