The peculiar screening of darkness in light

Points of darkness in light are singularities of wave optics, precisely they are singularities of light’s phase, or phase singularities. As light twists around these zeros of intensity, phase singularities are given a topological charge – positive or negative – defined by the direction of this twisting. Interestingly, this topological quantity seems by all means to be an actual charge, since phase singularities “feel” the charge in their surroundings. The way they feel it, is that one singularity of charge plus is always surrounded by a cloud of minuses, and vice-versa, so to try to maintain the total charge in a big box equal to zero. In atomic and molecular physics this process is called charge screening, and in fact we observe this screening to take place for singularities in random light as well.

However, in our recent publication we found something peculiar about this topological screening. To explain it we can play a little game. Let’s take a square box of side L, in which we observe an ensemble of charged entities:

In absence of screening the, total charge in the box would be zero, because every charge can randomly be a plus or a minus. But there will be an uncertainty on the total charge, which intuitiveley scales with total number of charges, i.e., with the area L*L of the box. Naive: the more particles I look at, the more likely it is to have more pluses than minuses.

In presence of perfect screening, the story changes. The total charge in the screen would still be zero, but with a smaller uncertainty! In fact we know that every charge is being compensated by a “cloud” of opposite ones, so to maintain the total charge zero. Nevertheless, this trick will not work at the edges of the screen… So there will still be an uncertainty on the total charge, given by the number of particles near the edges, i.e., proportional to the perimeter L.

Finally, we get to singularities in light. Weirdly enough singularities don’t show neither of the previous behaviours, but they do something in between. In our work we prove that the uncertainty on the topological charge for the singularities in a box of side L scales as ~ L log L, which is bigger than L but still smaller than L^2, proving that singularities do indeed undergo screening, although in a sort of imperfect way.



L. De Angelis, L. Kuipers. Screening and fluctuation of the topological charge in random wave fields, Optics Letters 43, 12: 2740-2743 (2018)


Spintronics & Nanophotonics

Check out the beautiful work of my colleague dr. Su-Hyun Gong on how to couple spin excitations to nanophotonic modes. Electrons spinning clockwise or anticklockwise in a 2D material can here be translated into photons running right or left on a plasmonic nanowire!

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.

Feel free to read more information about this on &, or read the original article on Science!

A Game of Pairs: Spotlight on Light’s Darkness

Nothing lasts forever, and neither does darkness in light. Tangled traces of darkness are left behind by many infinitely small dark points when they move around in random light fields. However, such dark points can be destroyed along their evolution, but always in pairs. New pairs can be  created as well.


In our recent paper, we shine light on the evolution of optical random fields by drawing the attention to the finite lifespan between creation and destruction of their dark points. For the first time, we also take a close look at the lifelong fidelity: the case in which a point of darkness is destroyed with its birth partner. Unexpectedly, we find that the behavior for dark points that remain faithful to their creation partner is distinct from that of “promiscuous” ones. These findings contribute to reveal the complex evolution of the wide class of random systems that the studied case embodies.

Check the original publication on Physical Review Letters and read the news item on PhysicsBuzz!

Emil Wolf Award 2017

I am very happy to share with you the news item that recently appeared on our group’s website! Being selected for, and eventually receiving the Emil Wolf Award has been a huge honor for me. Thanks to anybody who contributed in achieving this.

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!

check it out at

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

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)

Source: AMOLF News

New Notes!

The notes for the course “Metodi Spettroscopici per la Materia Condensata” are finally available in the OLD NOTES page. It’s a LaTeX file, so it looks fancy, but the quality of the content is just the same as the other files (maybe worse). So don’t expect a book-like product, but enjoy authentic and coarse notes stile!

Blog at

Up ↑