Filippo Romanato

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.

High-efficiency multilevel zone plates for keV X-rays

The development of high brilliance X-ray sources coupled with advances in manufacturing technologies has led to significant improvements in submicrometre probes for spectroscopy, diffraction and imaging applications. The generation of a small beam spot size is commonly based on three principles: total reflection (as used in optical elements involving mirrors or capillaries), refraction (such as in refractive lenses) and diffraction. The latter effect is employed in Bragg–Fresnel or Soret lenses, commonly known as Fresnel zone plate lenses. These lenses currently give the best spatial resolution, but are traditionally limited to rather soft X-rays—at high energies, their use is still limited by their efficiency. Here we report the fabrication of high-efficiency, high-contrast gold and nickel multistep (quaternary) Fresnel zone plates using electron beam lithography. We achieve a maximum efficiency of 55% for the nickel plate at 7 keV. In addition to their high efficiency, the lenses offer the advantages of low background signal and effective reduction of unwanted diffraction orders. We anticipate that these lenses should have a significant impact on techniques such as microscopy, micro-fluorescence and micro-diffraction, which require medium resolution (500–100 nm) and high flux at fixed energies.

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.

Strain relaxation in graded composition InxGa1-xAs/GaAs buffer layers

A model to compute the strain relaxation rate in InxGa1−xAs/GaAs single layers has been tested on several compositionally graded buffer layers. The existence of a critical elastic energy has been assumed as a criterion for the generation of new misfit dislocations. The surface strain accuracy results are within 2.5×10−4. The influence of different grading laws and growth conditions on residual strain, threading dislocation density, misfit dislocation confinement, and surface morphology has been studied. The probability of dislocation interaction and work hardening has been shown to strongly influence the mobility and the generation rate of the dislocations. Optimization of the growth conditions removes residual strain asymmetries and smoothes the surface roughness.

Lattice parameter in Si1-yCy epilayers: deviation from Vegard's rule

The precise C content of a series of Si1−yCy epilayer samples (0<y<0.012) was determined by resonant backscattering experiments using a 4He+ ion beam at 5.72 MeV. This beam energy is more suitable for the determination of the C content than the previously used 4.265 MeV. From the correlation of these investigations with x-ray diffraction experiments, a significant deviation of the lattice parameter variation in Si1−yCy from Vegard’s rule between Si and diamond or β-SiC was observed, which amounts up to 30% or 13%, respectively, for y<0.012. This negative deviation is in agreement with recent theoretical predictions by Kelires.

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.

Nanoporous gold plasmonic structures for sensing applications

The fabrication, characterization and functionalization of periodically patterned nanoporous gold layers is presented. The material shows plasmonic properties in the near infrared range, with excitation and propagation of surface plasmon polaritons. Functionalization shows a marked enhancement in the optical response in comparison with evaporated gold gratings, due to a great increase of the active surface. Due to its superior response, nanoporous gold patterns appear promising for the realization of compact plasmonic platforms for sensing purposes.

Fabrication of 3D metallic photonic crystals by X-ray lithography

Photonic crystals (3D) represent one of the most important building blocks towards the achievement of a full optics communication technology. So far the largest interest has been attracted by two-dimensional photonic crystals because they are potentially more amenable to fabrication and much closer to application. Straightforward application of the photonic band gap concept is generally thought to require three-dimensional (3D) photonic crystals that, however, represent a challenge from a fabrication point of view. Recent works have shown that 3D metallic PC can be fabricated and that they can be advantageous in the low frequency region where the metals become almost completely reflectors. In this work we show the possibility to fabricate 3D PC structures by X-ray lithography. Gold and nickel 3D photonic crystals with threefold (Yablonovite) and fourfold rotation symmetry have been fabricated with a lattice parameter ranging from 1 µm down to 300 nm. The total thickness of the 3D PC is of the order of 10 µm, a value which should allow to achieve a complete bulk behavior. This is supported by variable angle reflectance measurements, which have shown clear indications for true 3D dimensionality of our samples.

Generation of high-order Laguerre–Gaussian modes by means of spiral phase plates

Spiral phase plates for the generation of Laguerre-Gaussian (LG) beam with non-null radial index were designed and fabricated by electron beam lithography on polymethylmethacrylate over glass substrates. The optical response of these phase optical elements was theoretically considered and experimentally measured, and the purity of the experimental beams was investigated in terms of LG modes contributions. The far-field intensity pattern was compared with theoretical models and numerical simulations, whereas interferometric analyses confirmed the expected phase features of the generated beams. The high quality of the output beams confirms the applicability of these phase plates for the generation of high-order LG beams.

The role of polarization on surface plasmon polariton excitation on metallic gratings in the conical mounting

The polarization effects of surface plasmon polaritons (SPP) in an azimuthally rotated gratings have been investigated. Mixed s and p modes can be excited using the polarization angle of the incident light to optimize the SPP coupling. Experimental results and simulations show which component of polarization is effective for the SPP excitation. The optimum angle direction has been calculated analytically. The SPR tunability by polarization may uncover important features in SPP applications.