Elettra Mari

Encoding many channels on the same frequency through radio vorticity: first experimental test

We have shown experimentally, in a real-world setting, that it is possible to use two beams of incoherent radio waves, transmitted on the same frequency but encoded in two different orbital angular momentum states, to simultaneously transmit two independent radio channels. This novel radio technique allows the implementation of, in principle, an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. This paves the way for innovative techniques in radio science and entirely new paradigms in radio communication protocols that might offer a solution to the problem of radio-band congestion.

Experimental verification of photon angular momentum and vorticity with radio techniques

The experimental evidence that radio techniques can be used for synthesizing and analyzing non-integer electromagnetic (EM) orbital angular momentum (OAM) of radiation is presented. The technique used amounts to sample, in space and time, the EM field vectors and digitally processing the data to calculate the vortex structure, the spatial phase distribution, and the OAM spectrum of the radiation. The experimental verification that OAM-carrying beams can be readily generated and exploited by using radio techniques paves the way to an entirely new paradigm of radar and radio communication protocols.

Reply to Comment on ‘Encoding many channels on the same frequency through radio vorticity: first experimental test’

Our recent paper (Tamburini et al 2012 New J. Phys. 14 033001), which presented results from outdoor experiments that demonstrate that it is physically feasible to simultaneously transmit different states of the newly recognized electromagnetic (EM) quantity orbital angular momentum (OAM) at radio frequencies into the far zone and to identify these states there, has led to a comment (Tamagnone et al 2012 New J. Phys. 14 118001). These authors discuss whether our investigations can be regarded as a particular implementation of the multiple-input–multiple-output (MIMO) technique. Clearly, our experimental confirmation of a theoretical prediction, first made almost a century ago (Abraham 1914 Phys. Z. XV 914–8), that the total EM angular momentum (a pseudovector of dimension length × mass × velocity) can propagate over huge distances, is essentially different from—and conceptually incompatible with—the fact that there exist engineering techniques that can enhance the spectral capacity of EM linear momentum (an ordinary vector of dimension mass × velocity). Our OAM experiments (Tamburini et al 2012 New J. Phys. 14 033001; Tamburini et al 2011 Appl. Phys. Lett. 99 204102–3) confirm the availability of a new physical layer for real-world radio communications based on EM rotational degrees of freedom. The next step is to develop new protocols and techniques for high spectral density on this new physical layer. This includes MIMO-like and other, more efficient, techniques.

Near-Field Experimental Verification of Separation of OAM Channels

The experimental proof that near-field radio communication channels based on orbital angular momentum (OAM) are naturally isolated is presented. In near-field zone, we show that two antennas, built for producing a beam with the same value of OAM, have a good throughput. On the other hand, antennas of different types, i.e., built for generating beams with different OAM values, including a standard l = 0 antenna, exhibit a good modal isolation .

Space-Division Demultiplexing in Orbital-Angular-Momentum-Based MIMO Radio Systems

Radio beams that carry nonzero orbital angular momentum (OAM) are analyzed from the viewpoint of a multiple-input-multiple-output (MIMO) communication system. Often, the natural OAM-beam orthogonality cannot be fully exploited because of spatial constraints on the receiving antenna size. Therefore, we investigate how far OAM-induced phase variations can be exploited in spatial demultiplexing based on conventional (linear momentum) receivers. Performances are investigated versus position and size of the transmitting and receiving devices. The use of OAM-mode coherent superpositions is also considered, in view of recent work by Edfors et al. Our final goal is to assess the merits of an OAM-based MIMO system, in comparison with a conventional one.

Angular momentum properties of electromagnetic field transmitted through holey plasmonic vortex lenses

We performed three-dimensional finite elements simulations of the optical response of holey plasmonic vortex lenses, i.e., spiral grooves milled on a thin gold film with a hole at the center. We focus in particular on the properties of the wave transmitted in the underlying half-space, which is shown to be a relevant part of the transmitted field. We find out that the angular momentum selection rule for this part of the field is different from the one for the transmitted plasmonic vortex, although closely related to the plasmonic interaction of the impinging wave with the chiral geometry.

Sub-Rayleigh optical vortex coronagraphy

We introduce a new optical vortex coronagraph (OVC) method to determine the angular distance between two sources when the separation is sub-Rayleigh. We have found a direct relationship between the position of the minima and the source angular separation. A priori knowledge about the location of the two sources is not required. The superresolution capabilities of an OVC, equipped with an ℓ = 2 N-step spiral phase plate in its optical path, were investigated numerically. The results of these investigations show that a fraction of the light, increasing with N, from the secondary source can be detected with a sub-Rayleigh resolution of at least 0.1 λ/D.

Tripling the capacity of a point‐to‐point radio link by using electromagnetic vortices

In this paper we report the results from outdoor experiments showing that it is possible to increase the data transmission capacity using at least three coherent, orthogonal beams on the same frequency, 17.128 GHz, each in a unique orbital angular momentum state. Each beam was encoded with the digital modulations used in present-day telecommunications. We achieved an error-free throughput of 3 × 11 Mbit/s with four-Quadrature Amplitude Modulation over a 7 MHz bandwidth over 100 m and 150 m long links.

Focusing dynamics on circular distributed tapered metallic waveguides by means of plasmonic vortex lenses

We investigate the focusing effect on circularly distributed planar tapered plasmonic waveguides by means of three-dimensional (3D) finite elements simulations. The proposed configuration allows nanofocusing on four faced planar nanotips, showing efficient condensation of surface plasmons polaritons (SPPs) at the silver/air interface toward the endpoint of the tips. By means of a plasmonic vortex lens it is possible to illuminate the tips with SPP waves carrying orbital angular momentum (OAM), namely plasmonic vortices. Our 3D simulations show that by acting on the topological charge of the plasmonic vortex the electric field charge distribution at the tips apex can be controlled accordingly to the input electric field phase distribution. The results for three particular OAM values are shown, along with a generalization for arbitrary plasmonic vortex angular momentum values.

Fabrication and Testing of l = 2 Optical Vortex phase masks for Coronography

In this paper we present the fabrication process and tests of two different types of l = 2 spiral phase plates (SPPs), designed for an Optical Vortex Coronagraph (OVC) in the visible wavelength regime. Each phase mask is realized dividing the spirals area in sectors respectively of 8 and 512 of levels using lithographic nanofabrication approach. The SPPs produces different optical vortices (OVs) with topological charge l that depends on the number of steps and on the wavelength. We found that the residual light in the central dark region of the OV tends to zero as the number of steps increases.