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

We show that the reply by Tamburini et al (2012 New J. Phys. 14 118002) to our previous comment (2012 New J. Phys. 14 118001) on the experiment reported in (2012 New J. Phys. 14 033001) actually does not invalidate any of the issues raised in our initial comment.Comment: 3 pages, 1 figur

Introducing Berry phase gradients along the optical path via propagation-dependent polarization transformations

Abstract As a classical or quantum system undergoes a cyclic evolution governed by slow change in its parameter space, it acquires a topological phase factor known as the geometric or Berry phase. One popular manifestation of this phenomenon is the Gouy phase which arises when the radius of curvature of the wavefront changes adiabatically in a cyclic manner, for e.g., when focused by a lens. Here, we report on a new manifestation of the Berry phase in 3D structured light which arises when its polarization state adiabatically evolves along the optical path. We show that such a peculiar evolution of angular momentum, which occurs under free space propagation, is accompanied by an accumulated phase shift that elegantly coincides with Berry's prediction. Unlike the conventional dynamic phase, which accumulates monotonically with propagation, the Berry phase observed here can be engineered on demand, thereby enabling new possibilities; such as spin-dependent spatial frequency shifts, and modified phase matching in resonators and nonlinear interactions. Our findings expand the laws of wave propagation and can be applied in optics and beyond

Theory, design and measurement of near-optimal graphene reconfigurable and non-reciprocal devices at terahertz frequencies

This thesis explores the applications of graphene for terahertz and far infrared optical components and antennas, with particular emphasis on tunable and non-reciprocal devices. Both terahertz technologies and graphene are emerging fields which hold many promises for a number of future applications, including ultra-broadband communications, sensing and security. A very important amount of research has been devoted to explore the potential applications of graphene and its advantages over existing technologies. Conversely, there is a clear set of applications that could benefit from the development of terahertz technologies, but there are several technical challenges in terms of very limited availability of materials and components to generate, manipulate and detect terahertz waves. The main idea of this work is to bring these two topics together to demonstrate that terahertz and far infrared technologies can greatly benefit from the unique optical properties of graphene. The first original contribution of this thesis is an important theoretical upper bound for the performance of non-reciprocal and tunable devices, demonstrating that both these components can achieve a target performances at the expense of an unavoidable optical loss, which depends uniquely on the properties of graphene. If graphene with higher mobility is used, this unavoidable loss can be reduced; however, independently of the design geometry (waveguide devices, free space planar devices, ...), the loss will always appear. This theoretical limit is an important guideline for the design of graphene optical devices, as it can predict the best possible performances prior to any design effort or numerical simulation. It is also demonstrated that devices able to reach the upper bound are actually possible, and hence these devices (modulators, isolators among others) are optimal. The thesis explores then a number of designs of graphene antennas for terahertz and mid infrared frequencies, where it is shown that gated graphene can be used to achieve frequency reconfiguration in resonant plasmonic antennas and beam steering in graphene based reflectarrays. Circuit models are provided as a simple way to understand the behavior of the device in a simple way. Furthermore, an experimental technique able to measure the complex conductivity of graphene at infrared frequency is demonstrates, providing a very useful evaluation of graphene quality at those frequencies. The potential of graphene for non-reciprocal applications is then demonstrated experimentally, with the design, fabrication and measurement of the first terahertz isolator (operation frequency between 1 THz and 10 THz). The isolator is a device which allows the unilateral propagation of light, and for that reason is often called âoptical diodeâ. The isolator uses graphene immersed in a magnetostatic field, and exhibits approximately 7 dB of loss in one direction and more than 25 dB in the other. The device is shown to be quasi-optimum according to the theoretical bound and greatly improved performances are predicted for devices with next generation CVD graphene. Finally, the first tunable graphene reflectarray is presented, which is a metasurface able to steer in a desired direction an incoming beam of terahertz radiation. The device acts as a mirror, but, upon graphene gating, the direction of the reflected beam can be controlled and the beam itself can be modulated with complex modulation schemes

Engineering Phonon Polaritons in van der Waals Heterostructures to Enhance In-Plane Optical Anisotropy

Van der Waals heterostructures assembled from layers of 2D materials have attracted considerable interest due to their novel optical and electrical properties. Here we report a scattering-type scanning near field optical microscopy study of hexagonal boron nitride on black phosphorous (h-BN/BP) heterostructures, demonstrating the first direct observation of in-plane anisotropic phonon polariton modes in vdW heterostructures. Strikingly, the measured in-plane optical anisotropy along armchair and zigzag crystal axes exceeds the ratio of refractive indices of BP in the x-y plane. We explain that this enhancement is due to the high confinement of the phonon polaritons in h-BN. We observe a maximum in-plane optical anisotropy of {\alpha}_max=1.25 in the 1405-1440 cm-1 frequency spectrum. These results provide new insights on the behavior of polaritons in vdW heterostructures, and the observed anisotropy enhancement paves the way to novel nanophotonic devices and to a new way to characterize optical anisotropy in thin films

Nonlocal Electromagnetic Response of Graphene Nanostructures

Nonlocal electromagnetic effects of graphene arise from its naturally dispersive dielectric response. We present semi-analytical solutions of nonlocal Maxwell's equations for graphene nano-ribbons array with features around 100 nm, where we found prominent departures from its local response. Interestingly, the nonlocal corrections are stronger for light polarization parallel to the ribbons, which manifests as additional broadening of the Drude peak. For the perpendicular polarization case, nonlocal effects lead to blue-shifts of the plasmon peaks. These manifestations provide a physical measure of nonlocal effects, and we quantify their dependence on ribbon width, doping and wavelength