Kuipers Lab

NanoOptics

Radiationless anapole states in on-chip photonics

E. Díaz-Escobar, T. Bauer, E. Pinilla-Cienfuegos, A. I. Barreda, A. Griol, L. Kuipers and A. Martínez
Light: Science & Applications, 10, 204 (2021). https://doi.org/10.1038/s41377-021-00647-x

Enhanced on-chip photonic sensing and routing is playing an increasingly significant role in modern society. Realized as interference between different modes in high-index nanoparticles, radiationless states called anapoles are here one promising concept that combines reduced scattering with enhanced concentration of energy. Together with researchers from the Universitat Politècnica de València, Spain and the Friedrich Schiller University Jena, Germany, we now showed that when driven via integrated waveguides, these two striking properties decouple spectrally. The findings provide a crucial step towards the use of anapole states in photonic integrated circuitry.

For an in-depth discussion of the results, feel free to read this pdf!

Decoupling of anapole condition and near-field energy maximum in on-chip excitation of high-index nanodisks, with an experimental visualisation of the double-vortex structure of the contributing toroidal moment.

S. Arora, T. Bauer, R. Barczyk, E. Verhagen and L. Kuipers
Light: Science & Applications, 10, 9 (2021). https://doi.org/10.1038/s41377-020-00458-6

Photonic topological insulators are currently at the forefront of on-chip photonic research due to their potential for loss-free information transport. Realized in photonic crystals, they enable robust propagation of optical states along domain walls. But how robust is robust? In order to answer this, together with a group of researchers from AMOLF we quantified photonic edge state transport using phase-resolved near-field optical microscopy. The findings provide a crucial step towards error-free integrated photonic quantum networks.

For a more in depth explanation of the paper, feel free to read this pdf!

An article presenting our latest publication “Circular Dichroism Measurement of Single Metal Nanoparticles Using Photothermal Imaging” was published at the TU Delft website and also at the Leiden University website. We are proud of our collaborative work and we love to share it with the community.

In the paper, we show that we can measure the chirality of nano-objects, for example, gold nanostructures, with a ten-times improved sensitivity. This is now possible through the combination of two techniques, a challenging task we manage in collaboration with people from the single-molecule optics group at Leiden university.

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 23^th.

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).

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