Invisibility cloaks and the world’s most accurate clock share the spotlight at the 25th International “Nicolás Cabrera” Summer School, devoted to nanophotonics

Few physicists would have imagined two decades ago that a macroscopic object could be turned wholly invisible or that we would one day be able to observe viruses or atoms with a simple smartphone mounted lens or get light to pass through holes narrower than itself. The 25th edition of the Nicolás Cabrera School, held in the Madrid municipality of Miraflores de la Sierra on 10-14 September, will welcome some of the top international names in the nanophotonics area.

7 September, 2018

This year’s edition of the school organized annually by the Instituto Universitario de Ciencias de Materiales Nicolás Cabrera (UAM) with the collaboration of the BBVA Foundation, takes as its title “Manipulating Light and Matter at the Nanoscale.” Taking part will be 25 world-class experts and over 264 pupils –mostly students but also researchers – from as many as twenty-four countries.

Nanophotonics has emerged in these past decades as an unexpected fruit of humanity’s newly acquired ability to control matter on the nanometric scale; the scale of molecules and atoms. When researchers began studying how light interacted with matter at these levels, they encountered some unexpected phenomena apparently at odds with the theoretical predictions. And the still reeling physicist community is just now coming to grips with how these insights can be harnessed to control light at the nanometric scale.

A few nanophotonic applications have already reached the market, among them optical tweezers to trap individual molecules and ultra precise sensors. And a number of existing patents use nanophotonics for the development of aircraft antenna or to exploit the optical properties of nanoparticles in creams, cosmetic products and even security features for stamps and banknotes based on extremely bright nanophotonic pixels.

But the list of potential applications is s good deal longer, with major implications for sectors as diverse as “energy, chemistry, biology and also telecommunications,” affirms Antonio I. Fernández Domínguez, a researcher at the UAM’s Instituto de Física de la Materia Condensada (IFIMAC) and one of the school’s organizers together with his IFMAC colleagues Iohannes Feist and Francisco J. García Vidal.

“Photons are the probe, the messenger, responsible for delivering most of the visual information we receive in our day-to-day lives,” Fernández Domínguez continues. “Nanophotonics seeks to harness this mode of operation, of seeing, in space and time regimes that were previously unachievable, not through lack of technology but due to light’s very nature. We could say that nanophotonics is trying to make the invisible visible, and also, at times, to turn the visible invisible. With nanophotonics, we can control light at spatial and temporal scales we believed were inherently impossible.”

Among the participants in this 25th edition are the UK’s John Pendry and Swiss scientist Ursula Keller, two world authorities on the spatial and temporal control of light respectively. Pendry is famous for proposing the development of invisibility cloaks and a “perfect” lens, while Keller has created the attoclock, measuring intervals as short as the time it takes light to travel the width of an atom.

Beyond the wavelength barrier

Since the late 19th century and the formulation of the theory of microscope optical resolution, we have known that light only strikes and rebounds off obstacles of a given size relative to its wavelength – to see something is to receive the particles of light rebounding from that object. Light’s wavelength is accordingly its size; the space that its particles, the photons, occupy as they advance. The wavelength of visible light runs from 0.4 to 0.8 thousandths of a millimeter approximately; photons of this wavelength do not collide with anything smaller, meaning such objects are invisible to the human eye.

This is an undisputed fact. Yet studies of how light interacts with matter at the nanometric scale have uncovered ways of getting round this principle, making it possible to operate with light in spaces smaller than its wavelength. It is this discovery that is driving the boom in nanophotonics. “We have achieved quite unprecedented control over light at scales below its wavelength,” explain the school’s organizers. “And this has marked a leap in knowledge about the interaction of light and matter, opening new development avenues in nanoscale materials science and photonic technologies.”

Invisibility and the “perfect” lens

One of the phenomena arising when light meets matter at the nanoscale is the formation of plasmons. When light strikes a metal, the interaction between the latter’s electrons and light’s electromagnetic field is a bit like throwing a stone into water: it produces waves. In these waves, called surface plasmons, the metal electrons couple with the photons. By controlling the behavior of plasmons, researchers can bring to life the dreams of science fiction.

Among the most striking examples are invisibility cloaks and “perfect” lenses, two directions being explored by Pendry at London’s Imperial College of Science and Technology. His field of expertise is metamaterials, whose optical properties depend on their nanoscale structure rather than their chemical composition. Metamaterials offer unprecedented control over light, and this led Pendry to theorize in 2001 that “by bending light in the right way, we could use it to see objects far smaller than itself (its wavelength), making visible objects so minute that they were regarded as invisible,” the organizers relate.

This “perfect” lens, which would defy the laws of physics in having no resolution limit, would, for instance, enable bacteria and viruses to be viewed via a smartphone. Although Pendry’s short paper setting out his proposal proved hugely controversial, numerous groups are now in the race to develop his lens.

In one congress Pendry jokingly predicted that sophisticated metamaterials could be used to fashion invisibility cloaks or shields. Enveloped by the cloak, the object is surrounded by light the way a stone is surrounded by the water of a lake, and continues on its path. Such an object would neither reflect light nor cast a shadow. The idea immediately garnered intense scientific and technological interest, and the first, tentative version of an invisibility shield was built in 2006 using microwave light. Pendry’s group and many others are working as we write to develop new shields for visible light and radiation across the whole of the electromagnetic spectrum.

Besides Pendry, the school will welcome another expert in metamaterials characterization and fabrication, Martin Wegener of Germany’s Karlsruhe Institute of Technology.

Billions of laser pulses per second

Nanophotonics also permits the deployment of photons on hitherto undreamt-of time scales. The expert here is Ursula Keller, a pioneer in the development of ultrafast lasers; pulses of light so brief that trillions can be squeezed into one second, making them the maximum expression of temporal light control.

Ultrafast lasers have multiple applications in biomedicine, consumer electronics – smartphones, touchscreens, etc. – and the motor industry. “The iPhone would not exist without ultrafast lasers,” affirms Keller, head of the National Center of Competence in Research for Molecular Ultrafast Science and Technology (MUST) in Zurich (Switzerland). The global market for ultrafast lasers is now worth over 2.1 billion euros and within five years should safely top the 8 billion mark.

Keller’s is also the mind behind the attoclock, capable of measuring the time it takes light to travel between two neighboring atoms – trillionths of a second. This allows the movement of electrons to be frozen in time so as to study subatomic reactions. Its development also provides a gateway to the quantum world – quantum time – and as such is now an essential tool in basic research.

Her work in ultrafast lasers earned Keller the 2018 European Inventor Award Lifetime Achievement Prize, bestowed by the European Patent Office on inventors whose work has provided answers to “some of the biggest challenges of our times.”

Photosynthesis and quantum computation

Another branch of nanophotonics is biophotonics, focusing on the photosynthesis of plants and bacteria. Photosynthesis is the process underpinning practically all life on Earth, but we still know little about the light-capturing antenna of photosynthesizing organisms or the elements that transfer solar energy to the reaction center where photosynthesis takes place. As the organizers explain: “There has been much debate in recent years about these light capture and transfer mechanisms, and our hope is to learn from nature how to improve photovoltaic technology.” Niek van Hulst, of the Instituto de Ciencias Fotónicas (ICFO), and Martin Plenio, from the University of Ulm (Germany) will address just this topic during the school.

Van Hulst will also join with Jeremy Baumberg of Cambridge University (United Kingdom) to consider the opportunities nanophotonics provides for the world of quantum computing. Controlling photons one by one is at the base of quantum computing and communication, and nanophotonics could deliver this goal by trapping light in cavities of nanometric dimensions.

Shanhui Fan of Stanford University (United States) and Luis Martín Moreno of the University of Zaragoza will discuss their work at the interface of nanoscale photonics and electronics, leading to the development of nanostructures for catalysis and photodetection, nanolasers, the control and engineering of the optical properties of two-dimensional materials like graphene, black phosphorus or boron nitride, and the control of radiative heat transfer at nanometric distances.