Light under Control

As a leading physicist with an established reputation, Stefan Maier knows a great deal about the physics of light, and is only too happy to talk about it. Having moved to LMU from Imperial College London a year ago, he is busy putting together a team of young researchers at the new Nano-Institute. Indeed, one encounters many young scientists in the corridors. The cleanroom is receiving the final finishing touches and most of the laboratories on the third floor are fully equipped. Maier defines his mission as follows: “Our job is to find ways of exploiting physical processes that have to do with light. More specifically, we want to control how light interacts with matter, using a combination of optics and nanotechnology.” A more detailed understanding of these interactions promises to lead to innovations that could help solve urgent global problems. For instance, Maier’s work is directly relevant to sustainable energy generation – in particular, the conversion of solar into chemical energy. In this context, he points out, ‘light’ does not just mean visible light, but encompasses a large fraction of the electromagnetic spectrum – including infrared and ultraviolet light, microwaves and terahertz radiation.

To defeat the diffraction limit, Maier works with what are called plasmonic antennas – nanometer-scale metallic structures that are tuned to interact with light with a defined set of wavelengths, like old-fashioned TV aerials. Nano-antennas allow the incident light to be coupled to the oscillations of the electrons in the metal. Depending on the structure of the antenna, it is possible to set up a very strong absorption or emission interaction. “This is because the light is confined to a volume with a radius of a few dozen nanometers around the tip of the antenna,” Maier explains. “What one gets is a minute light spot, a hotspot.” So this gets around the diffraction limit. “A few decades ago, nobody would have believed that this could be done with the degree of control that has now been attained,” says Maier.

However, attempts to elucidate the realm of nanoparticles run into a fundamental problem. Light is electromagnetic radiation and, as a wave-like phenomenon, it is subject to diffraction: On passage through an aperture such as a lens, light originating from a point spreads out. This effect gives rise to the diffraction limit – a physically defined barrier that restricts the resolution of standard optical microscopes to around 200 nm. Thus, extremely small structures, such as virus particles or quantum dots, with dimensions of a few nanometers cannot be imaged clearly – which means that the nanoworld is off limits for conventional light microscopes. Similarly, typical optic fibers cannot be miniaturized sufficiently that they fit onto computer chips.

As a physicist, Stefan Maier looks for ways to overcome such limits. The diffraction limit is always equal to about half the wavelength of the incident light. “The longer the wavelength of the radiation, the worse the problem becomes, as the degree of coincidence between the light waves and the nanoparticle drops off,” he says – in other words, there is effectively no interaction.

“Metallic gold or silver nanoparticles are capable of interacting very strongly with light,” Maier explains. Light impinges on the metal structures and causes the surface electrons to oscillate as a cloud. If the wavelength of the light is just right, it amplifies the collective oscillation. This in turn affects both absorption and scattering at that resonant wavelength, which accounts for the color changes.

This is also the principle behind plasmonic antennas. After decades of basic research on the phenomenon, researchers are increasingly focusing on specific applications. The basic problem is to define conditions that enable optical light-matter interactions across a wide range of wavelengths, which requires even more precise control of the light. Scientists are experimenting with various metals and various shapes of antenna. The antennas themselves are usually made of gold, silver, copper and aluminum, but new hybrid nanomaterials are now coming to the fore. Fabrication of the miniature antennas can be very complex. Maier mentions antennas that look like cats’ heads, which are used to optimize directional emission. The response characteristics of different forms are modelled on the computer, and the optimal shapes are then fabricated on the nanoscale. “The design of such specialized antennas requires a great deal of developmental work,” says Maier, and leading nanophotonics groups around, including his own, are actively engaged in this task.

The mystery of the ancient cup

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The term ‘plasmonic antenna’ refers to the fact that the interaction excites surface plasmons – collective oscillations of surface electrons. “The name was invented at Caltech in Pasadena while I was there,” Maier says. “Shortly afterwards, I wrote what was probably the first modern introduction to the topic.” The term soon found its way into Russian and Chinese, and Maier’s text has served to acquaint young researchers around the world with the new field.

However, technological application of the phenomenon goes back a long way. “It has been exploited for more than 1500 years to impart color to glass objects,” Maier says, citing the example of the Lycurgus Cup, which is now the British Museum in London. This Late Roman object was made in the 4th century AD. It is decorated with a scene depicting the shimmering green figure of King Lycurgus of Sparta, entangled in ambrosia vines, being dragged into the Underworld. When illuminated from within, the vessel takes on a reddish hue. This striking dichroic effect is due to the presence of gold, silver and copper nanoparticles of varying sizes in the glass matrix.

Metallic nanostructures designed on the basis of sophisticated theoretical calculations now make it possible to cover almost any desired resonance frequencies in the region extending from UV to the near infrared. “These issues have now been largely solved. Only the mid-infrared continues to cause problems,” he says. But his team is making good progress, and the Nano-Institute possesses the necessary instrumentation and expertise.

Indeed, the making of nanostructures – nanofabrication – now plays a crucial role in nanoscience research. New techniques such as electron-beam lithography make it possible to produce metallic nanostructures that conform to the precise specifications worked out by theorists. Maier is interested in structures that can be fabricated on a chip and combined with waveguides and other semiconductor structures. In the Institute‘s cleanroom, novel structural configurations and hybrid materials can be assembled ready for testing in the laboratories.

These are the initial steps on the way to novel applications. The demand is great and the opportunities enormous. Some of these, such as light modulators for use in non-linear optics, lie in the field of nanophotonics itself. Energy conversion is another area in which expectations are high, as novel types of solar cells are also high on the wishlist. Energy conversion by purely chemical means also stands to benefit, and Tenure-Track Professor Emiliano Cortés is working on this topic at Maier’s chair. Indeed, an interdisciplinary team of physicists and chemists is working on the use of surface plasmons as catalysts.

In all these cases, the interaction between light and matter is the heart of the matter. Researchers at the Institute are all seeking ways to increase the efficiency of their favorite systems. In the area of nanophotonics, plasmonic antennas are in principle capable of enhancing light emission from quantum dots. This would provide a light source that can be modulated at rates in the region of a few hundred picoseconds. Developments like this are of great interest for quantum computing and efforts to achieve faster rates of information transmission. In this context, nano-antennas serve as accelerators.

Enhancing the level of light absorption

The whole complex of sustainable energy conversion technologies is another area where the potential for substantial advances is high. For example, one way to reduce the cost of solar cells is to make them thinner and lighter than today’s models. But as a consequence of the diffraction limit, cells need to have a minimal thickness in order to absorb any light at all. As explained above, plasmonic antennas could solve that problem by concentrating the light within a smaller region. “To do so, we have to couple light with oscillations that are intrinsic to the material it interacts with“, says Maier. Here, the goal is to increase the yield of electricity from solar cells by enhancing the level of light absorption, such that more light can interact productively with the absorbing material. If these efforts succeed, conversion efficiencies can be improved.

Similar reasoning holds for the field of chemical catalysis, where sunlight once again supplies the input energy. Here, researchers at the Nano-Institute are testing new materials, such as zircon nitride, for use as nano-antennas. The idea here is to equip the nanostructures with light-driven catalysts. The antennas serve to concentrate the incident light at the surface, which then interacts with a catalyst to drive a chemical reaction. “The development of hybrid nanosystems of this kind is one of the core tasks of my department,” says Maier. “The challenge is to combine structures that enhance light absorption with catalytically active structures without inhibiting the degree of light absorption or the functionality of the catalyst. Both goals must be realized at once."

This kind of combined approach involves lots of trial-and-error. “It’s like Lego“, says Maier. Researchers have to work out what sort of antenna works best with which type of catalyst, and developing the appropriate fabrication process presents both chemical and technological challenges. Some materials are easier to make using top-down procedures, such as electron-beam lithography. Others require the methodologies of colloidal chemistry, as practiced at the neighboring chair led by Jochen Feldmann. “We expect that the collaboration will give rise to useful synergetic effects,” says Maier.

In his own highly specialized field, progress depends on such interdisciplinary cooperation, he adds. That is why the Nano-Institute is assembling an international team of physicists, condensed-matter specialists, chemists and mathematicians, who will work together with experts in microscopy, laser technology, computer simulation and quantum theory. The proportion of early-career researchers is strikingly high, and they come from all over the world. “Training young scientists is very important to me,” says Maier, who was Director of Postgraduate Studies at Imperial College London for over three years. Interdisciplinarity and a strong emphasis on training are central elements of the Institute’s philosophy, and they were among the primary reasons for his decision to join the new Nano-Institute “in the heart of this invigorating city”.

Professor Stefan Maier holds the Chair of Experimental Physics with a Focus on Hybrid Nanosystems at LMU. Born in 1975, he studied Physics at the Technical University of Munich and at the California Institute of Technology (Caltech) in Pasadena, where he obtained his PhD. He then did a stint as a postdoc at Caltech, before moving to the University of Bath (UK) and then to Imperial College London, where he held the Lee-Lucas Chair in Experimental Physics. He was appointed to his present position at LMU in 2017.