Combining spintronics and nanophotonics in 2D material


Research

Spintronics in materials of just a few atoms thick is an emerging field in which the ‘spin’ of electrons is used to process data, rather than the charge. Unfortunately, the spin only lasts for a very short time, making it (as yet) difficult to exploit in electronics. Researchers from the Kavli Institute of Nanoscience at TU Delft, working with the Netherlands Organisation for Scientific Research’s AMOLF institute, have now found a way to convert the spin information into a predictable light signal at room temperature. The discovery brings the worlds of spintronics and nanophotonics closer together and might lead to the development of an energy-efficient way of processing data, in data centres, for example. The researchers have given an account of their results in Science.

The research revolved around a nano-construction consisting of two components: an extremely thin silver thread, and a 2D material called tungsten disulfide. The researchers attached the silver thread to a slice of tungsten disulfide measuring just four atoms in thickness. Using circularly polarised light, they created what are known as ‘excitons’ with a specific rotational direction. The direction of that spin could be intitialized using the rotational direction of the laser light.

Original state

Excitons are actually electrons that have bounced out of their orbit. With this technique, the laser beam ensures that the electrons are launched into a wider orbit around a positively charged ‘hole’, in much the same way as a hydrogen atom. The excitons thus created want to return to their original state. On their return to the smaller orbit, they emit an energy package in the form of light. This light contains the spin information, but it emitted in all directions.

To enable the spin information to be put to use, the Delft researchers returned to an earlier discovery. They had shown that when light moves along a nanowire, it is accompanied by a rotating electromagnetic field very close to the wire: it spins clockwise on one side of the wire, and anti-clockwise on the other side. When the light moves in the opposite direction, the spin directions change too. So the local rotational direction of the electromagnetic field is locked one-to-one to the direction with which the light travels along the wire. ‘We use this phenomenon as a type of lock combination,’ explains Kuipers. ‘An exciton with a particular rotational direction can only emit light along the thread if the two rotational directions correspond.’

Opto-electronic switches

And so a direct link is created between the spin information and the propagation direction of the light along the nanowire. It works almost perfectly: the spin information is ‘launched’ in the right direction along the thread in 90% of cases. In this way, fragile spin information can be carefully converted into a light signal and transported over far greater distances. Thanks to this technique, which works at room temperature, you can easily make new optoelectronic circuitry. Kuipers: ‘You don’t need a stream of electrons, and no heat is released. This makes it a very low-energy way of transferring information.’

The discovery clears the way for combining the worlds of spintronics and nanophotonics. Kuipers: ‘This combination may well result in green information processing strategies at the nanoscale.’

More information: S.-H. Gong, F. Alpeggiani, B. Sciacca, E. C. Garnett and L. Kuipers, Nanoscale chiral valley-photon interface through optical spin-orbit coupling, Science 359, 6374: 443-447 (2018)

 

Lorenzo De Angelis wins Emil Wolf award


Awards

We congratulate with Lorenzo De Angelis for his recent achievement at the 2017 Frontiers in Optics conference held in Washington DC (USA). On this occasion, Lorenzo was selected as a finalist for the Emil Wolf Award and awarded the final prize for the best presentation in the category “Optical Interactions”. Good job!


 

Kuiperslab goeSUP!


Fun

Just before the “long” and “hotDutch summer was over, group members of Kuipers Lab paddled their way through the historical city center of Delft while standing surf boards (SUP)! A fun experience as well as a very good workout to stretch a bit the muscles after long days of intense and successful research. Ad maiora!

New group member Irina Komen


People

Irina Komen recently started a PhD position in the Kuipers Lab here in the Department of Quantum Nanoscience at the TU Delft. She did her Master projects at the Leiden University. In a first project, she was working in the Quantum Optics group on Superconducting Nanowire Single Photon Detectors, varying either the magnetic field or the polarization of the incoming light. In a second project in the Single Photon Detection group, she combined both photothermal imaging of gold nanorods and molecule fluorescence enhancement with optical tweezers. She is looking forward to explore the world of near-field microscopy and nano optics and develop herself scientifically in the exciting research environment of TU Delft.

New group member Aron Opheij


People

Aron Opheij recently joined the Kuipers Lab in the Quantum Nanoscience department at TU Delft. He has previously worked in the group as PhD student before it moved from Amolf to Delft. Aron will support the group as a technician, applying the experience he acquired while working in the lab with lasers, optical setups and programming.

 

New group member Thomas Bauer


People

Thomas Bauer recently started a Postdoc position in the Kuipers Lab here in the Department of Quantum Nanoscience at the TU Delft. In before, he was working on his PhD at the Max Planck Institute for the Science of Light in Erlangen, Germany on the experimental reconstruction of tightly focused vectorial light fields, specifically concerning their polarization structure and containing sub-wavelength phenomena. He hopes to be able to take some of the background from there to the area of near-field microscopy in Delft, and is looking forward to the hopefully versatile interaction with all the exciting broad research topics dealt with here in the Department.


Successful defense Anouk de Hoogh


People

On December 12th Anouk de Hoogh successfully defended her PhD thesis Optical singularities and nonlinear effects on plasmonic nanostructures at the Delft University of Technology.

Congratulations Dr. de Hoogh!

The importance of being a vector: a story of darkness in light waves


Research

Darkness can be found in light. This typically happens at an optical vortex: a point in which the amplitude of light is zero and where it twists like a corkscrew. In fact, the projection of a vortex on a flat surface looks like a ring of light, with a dark spot in the center. Researchers in the group of Kobus Kuipers studied how a multitude of dark optical vortices are distributed in space with respect to each other. They demonstrated that the vectorial nature of light plays an important role: the chance of finding another vortex is different for directions along or perpendicular to the vector field direction. The researchers publish their findings in the journal Physical Review Letters on August 23th.

A liquid of vortices
When many waves with random phases come together from all directions a multitude of optical vortices appear in the resulting interference pattern. This holds for all waves. Researchers M.R. Dennis and M.V. Berry predicted that for scalar waves the vortices would be distributed in space like the ions in an ionic liquid: for any given vortex the chance of finding another at a certain distance is a damped oscillating function with a typical distance of half the wavelength. That means that the positions are correlated. The chance also depends on whether the vortices have the same charge, i.e., is their corkscrew left- or right-handed: unlike the ions in the liquid oppositely charged vortices can approach each other as close as they like, since they themselves are infinitely small. Like in a liquid it doesn’t depend on direction: there are no preferred directions. However, when considering light as a wave we have to remember that this is a vector wave. The electromagnetic field that constitutes light waves oscillates and vectors determine the direction in which this oscillation takes place.

Correlated vortices (or not)
In the paper, the researchers demonstrate that the distribution of the optical vortices in random light waves is strongly affected by the fact that light is a vector. By trapping light in a chaotic cavity a random light field was created. With a dedicated microscope the relative positions and charges of thousands of vortices were determined. In addition the local field vectors of the light were mapped. It was clear that the chance of finding another vortex relative to another depended on the direction of the field. First author Lorenzo De Angelis says, “It is intriguing to observe that depending on the direction along which you look for the next, vortices far away from each other can still be correlated, or not; it depends on whether you look along or perpendicular to the field direction”

The ideas and methods that the researchers present do not only apply to light waves, but they are ready to use for every physical quantity that is described by a vector wave.

Figure: Intensity maps of the electromagnetic field resulting from random interference of light. The two figures present the cases in which the electric field oscillates along the horizontal (left) or vertical (right) direction. For each “dark” spot in the maps, an optical vortex occurs.

Reference: L. De Angelis, F. Alpeggiani, A. Di Falco and L. Kuipers, Spatial distribution of phase singularities in optical random vector waves, Physical Review Letters 117, 093901 (2016).

Kobus Kuipers goes to TU Delft


People

Professor Kobus Kuipers will leave AMOLF to become head of the Quantum Nanoscience department at the TU Delft. He will move to Delft in the summer of 2016.

Kuipers began his career in 1988 as a Masters student in the group of Ad Lagendijk at AMOLF. From 1990 to 1994 he did his PhD research in the group of Joost Frenken. From 1994 to 1997 he was a post-doc, and later lecturer in Cambridge and Birmingham, and in 1997 he moved to Twente University to become assistant professor and, in 2000, associate professor and program director at the MESA+ instititute. In 2003 he returned to AMOLF to start the NanoOptics research group. In 2006 he became head of the Nanophotonics department and member of the management team of AMOLF. In 2003 he received an NWO VICI grant and from 2005 to 2010 he was a member of ‘The Young Academy’ of the KNAW. In 2013 Kuipers received an ERC Advanced Grant, and in 2015 he chaired the Dutch organizing committee of the International Year of Light 2015. Kuipers holds part-time chairs at Twente University and at Utrecht University.

Kobus Kuipers is one of the pioneers in the field of nanophotonics. He is internationally recognized for developing techniques that probe the electric and magnetic field of light on the nanometer length scale and the femtosecond time scale. With these techniques he obtained novel insights in the fundamental properties of light in nanostructures.

Kuipers has strongly contributed to the development of the nanophotonics research field in the Netherlands. Together with Albert Polman, he founded the Center for Nanophotonics at AMOLF, and made it a leading center for nanophotonics research.

Vortices in hall of mirrors show spinning light the way out


Research

Researchers from FOM institute AMOLF have used the classical toolbox of physics to make predictions about the quantum world. Using a classical experiment, they showed how the direction in which a quantum light source emits light reveals the quantum state (spin) of that source. On 2 April the AMOLF researchers published the results in Nature Communications.

Classical experiment in the hall of mirrors
The physicists injected spinning light into a mini ‘hall of mirrors’, a photonic crystal, from which the light could only escape in two directions. The researchers discovered that they could influence which exit the light escapes from. This was achieved by very precisely choosing the location at which they injected the light.
In the hall of mirrors there are locations where light naturally starts to spin: the electrical field of the light rotates there. Light that travels in a single direction through the hall of mirrors would, for example, always rotate left at such a vortex location. At such a vortex location in the empty, lightless hall of mirrors, the researchers introduced some spinning light. If the spin direction of this light from the needle matched the natural spin direction at the vortex location, light left the hall of mirrors on the one side. If the spin direction of the light from the needle was the opposite of the standard vortex direction, the light went the other way.

Quantum world
This trick, which falls within the boundaries of classical physics, reveals how light will behave in the quantum world. That is because quantum light sources often emit light that spins (is circularly polarised), just like the light the researchers introduced to the hall of mirrors. The spin direction of the spinning light from quantum sources is directly dependent on the quantum state (spin) of the source. Therefore, if the experiment were repeated with a quantum source, the quantum state would determine the spinning direction of the light and with that the direction of escape.  In other words, the direction in which the emitted light escapes reveals the state of the light source: the direction of the light emitted has therefore become a source of quantum information.

Hall of mirrors
The aforementioned hall of mirrors that the researchers used for their experiment is in fact a photonic crystal that consists of an ultra-thin wafer of silicon that is just 220 nanometres thick (a nanometre is one millionth of a millimetre). In the silicon a pattern of holes has been etched, which ensures that just like in a real hall of mirrors, light is reflected in all directions and cannot simply escape. The properties of the photonic crystal determine where the vortex locations are and in which direction light at these locations normally rotates.

Figure: Relationship between the spin direction of the light and the direction in which it escapes: the grey wafer (bottom) is a sketch of the photonic crystal in which the light is captured. The blue-purple relief (top) indicates the measurement of the researchers. At a peak, the light chooses which side of the crystal it will escape from, based on the local spin direction. In a trough, the spin direction of the light has no influence on the escape direction. At a blue peak right-spinning light goes to the right and left-spinning light to the left. At a purple peak the opposite applies: left-spinning light goes to the right and right-spinning light goes to the left.

Reference
B. le Feber, N. Rotenberg & L. Kuipers
Nanophotonic control of circular dipole emission
Nature Communications 6, 6695 (2015) | DOI: 10.1038/ncomms7695