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Maxwell's demon (MD) has proven an instructive vehicle by which to explore the relationship between information theory and thermodynamics, fueling the possibility of information driven machines. A long standing debate has been the concern of entropy violation, now resolved by the introduction of a quantum MD, but this theoretical suggestion has proven experimentally challenging. Here, we use classical vectorially structured light that is non-separable in spin and orbital angular momentum to emulate a quantum MD experiment. Our classically entangled light fields have all the salient properties necessary of their quantum counterparts but without the experimental complexity of controlling quantum entangled states. We use our experiment to show that the demon's entropy increases during the process while the system's entropy decreases, so that the total entropy is conserved through an exchange of information, confirming the theoretical prediction. We show that our MD is able to extract useful work from the system in the form of orbital angular momentum, opening a path to information driven optical spanners for the mechanical rotation of objects with light. Our synthetic dimensions of angular momentum can easily be extrapolated to other degrees of freedom, for scalable and robust implementations of MDs at both the classical and quantum realms, enlightening the role of a structured light MD and its capability to control and measure information., Comment: 14 pages, 6 Figures

One dimensional confinement in waveguide Quantum Electrodynamics (QED) plays a crucial role to enhance light-matter interactions and to induce a strong quantum nonlinear optical response. In two or higher dimensional settings, this response is reduced since photons can be emitted within a larger phase space, opening the question whether strong photon-photon interaction can be still achieved. In this study, we positively answer this question for the case of a 2D square array of atoms coupled to the light confined into a two-dimensional waveguide. More specifically, we demonstrate the occurrence of long-lived two-photon repulsive and bound states with genuine 2D features. Furthermore, we observe signatures of these effects also in free-space atomic arrays in the form of weakly-subradiant in-band scattering resonances. Our findings provide a paradigmatic signature of the presence of strong photon-photon interactions in 2D waveguide QED., Comment: 18 pages, 11 figures

This work aims at investigating the optical transmission system needed for such lightweight sail, taking into account the physical constraints of such unprecedented link and focusing on the optimal scheme for the optical signal emission. In particular, the optical signal is distributed to several emitters on the sail. The light diffraction resulting from the pattern of the emitters acting coherently determines the characteristics of the whole beam transmitted by the sail and of the received signal on the Earth. The performance of the digital communication system using pulse position modulation (PPM) can be assessed and channel coding schemes are proposed. We are using the paradigm for which the entire sail communication system is described as a Tree-of-light: the detectors, CPU, memory and laser transmitter are the central unit, representing the trunk of the tree. The branches of the tree are the waveguides, directed to the sail surface. By means of multimode splitters, the signal is further distributed via the petioles to the emitters, the leaves, realized by grating couplers (GCs), on which this work is more focused.

The control of structured waves has recently opened innovative scenarios in the perspective of radiation propagation and light-matter interaction. In particular, the transmission of customized electromagnetic fields is investigated for telecommunications, with the aim of exploring new modulation formats besides the traditional, almost saturated, division multiplexing techniques. Beams carrying twisted wavefronts have long been recognized as the promising candidates, however their phase singularities and efficient multiplexing still raise open issues. In a more general insight into structured-phase beams, we introduce and develop here a new and unique paradigm based on the transmission of beams with harmonic phases having a multipole structure. The outlined framework encompasses multiplexing, transmission, and demultiplexing as a whole for the first time, describing wavefields evolution in terms of conformal mappings, and solving straightforwardly the critical issues of previous solutions. Because of its potentialities, versatility, and ease of implementation, we expect this completely new paradigm to find widespread applications for space division multiplexing especially in free space, from the optical to the microwave and radio regimes.

A novel optical device is designed and fabricated in order to overcome the limits of the traditional sorter based on log-pol optical transformation for the demultiplexing of optical beams carrying orbital angular momentum (OAM). The proposed configuration simplifies the alignment procedure and significantly improves the compactness and miniaturization level of the optical architecture. Since the device requires to operate beyond the paraxial approximation, a rigorous formulation of transformation optics in the non-paraxial regime has been developed and applied. The sample has been fabricated as 256-level phase-only diffractive optics with high-resolution electron-beam lithography, and tested for the demultiplexing of OAM beams at the telecom wavelength of 1310 nm. The designed sorter can find promising applications in next-generation optical platforms for mode-division multiplexing based on OAM modes both for free-space and multi-mode fiber transmission., Comment: 12 pages, 8 figures

We present a method to efficiently multiply or divide the orbital angular momentum (OAM) of light beams using a sequence of two optical elements. The key-element is represented by an optical transformation mapping the azimuthal phase gradient of the input OAM beam onto a circular sector. By combining multiple circular-sector transformations into a single optical element, it is possible to perform the multiplication of the value of the input OAM state by splitting and mapping the phase onto complementary circular sectors. Conversely, by combining multiple inverse transformations, the division of the initial OAM value is achievable, by mapping distinct complementary circular sectors of the input beam into an equal number of circular phase gradients. The optical elements have been fabricated in the form of phase-only diffractive optics with high-resolution electron-beam lithography. Optical tests confirm the capability of the multiplier optics to perform integer multiplication of the input OAM, while the designed dividers are demonstrated to correctly split up the input beam into a complementary set of OAM beams. These elements can find applications for the multiplicative generation of higher-order OAM modes, optical information processing based on OAM-beams transmission, and optical routing/switching in telecom., Comment: 28 pages, 10 figures

Parallel sorting of orbital angular momentum (OAM) and polarization has recently acquired paramount importance and interest in a wide range of fields ranging from telecommunications to high-dimensional quantum cryptography. Due to their inherently polarization-sensitive optical response, optical elements acting on the geometric phase prove to be useful for processing structured light beams with orthogonal polarization states by means of a single optical platform. In this work, we present the design, fabrication and test of a Pancharatnam-Berry optical element in silicon implementing a log-pol optical transformation at 1310 nm for the realization of an OAM sorter based on the conformal mapping between angular and linear momentum states. The metasurface is realized in the form of continuously-variant subwavelength gratings, providing high-resolution in the definition of the phase pattern. A hybrid device is fabricated assembling the metasurface for the geometric phase control with multi-level diffractive optics for the polarization-independent manipulation of the dynamic phase. The optical characterization confirms the capability to sort orbital angular momentum and circular polarization at the same time., Comment: 15 pages, 10 figures

In this paper, the implementation of optical elements in the form of Pancharatnam-Berry optics is considered. With respect to 3D bulk and diffractive optics, acting on the dynamic phase of light, Pancharatnam-Berry optical elements transfer a phase which is geometric in nature by locally manipulating the polarization state of the incident beam. They can be realized as space-variant sub-wavelengths gratings that behave like inhomogeneous form-birefringent materials. We present a comprehensive work of simulation, realization, and optical characterization at the telecom wavelength of 1310 nm of the constitutive linear grating cell, whose fabrication has been finely tuned in order to get a {\pi}-phase delay and obtain a maximum in the diffraction efficiency. The optical design in the infrared region allows the use of silicon as candidate material due to its transparency. In order to demonstrate the possibility to assemble the single grating cells for generating more complex phase patterns, the implementation of two Pancharatnam-Berry optical elements is considered: a blazed grating and an optical vortices demultiplexer.

In a Quantum Walk (QW) the "walker" follows all possible paths at once through the principle of quantum superposition, differentiating itself from classical random walks where one random path is taken at a time. This facilitates the searching of problem solution spaces faster than with classical random walks, and holds promise for advances in dynamical quantum simulation, biological process modelling and quantum computation. Current efforts to implement QWs have been hindered by the complexity of handling single photons and the inscalability of cascading approaches. Here we employ a versatile and scalable resonator configuration to realise quantum walks with bright classical light. We experimentally demonstrate the versatility of our approach by implementing a variety of QWs, all with the same experimental platform, while the use of a resonator allows for an arbitrary number of steps without scaling the number of optics. Our approach paves the way for practical QWs with bright classical light and explicitly makes clear that quantum walks with a single walker do not require quantum states of light.

A robust and reproducible preparation of self-standing nanoporous gold leaves (NPGL) is presented, with optical characterization and plasmonic behaviour analysis. Nanoporous gold (NPG) layers are tipically prepared as thin films on a bulk substrate. Here we present an alternative approach consisting in the preparation of NPGL in the form of a self-standing film. This solution leads to a perfectly symmetric configuration where the metal is immersed in a homogeneous medium and in addition can support the propagation of symmetric and antisymmetric plasmonic modes. With respect to bulk gold, NPG shows metallic behaviour at higher wavelengths, suggesting possible plasmonic applications in the near / medium infrared range. In this work the plasmonic properties in the wide wavelength range from the ultraviolet up to the mid-infrared range have been investigated.

We present and test the integration of a static orbital angular momentum mode multiplexer with a dynamical geometric-phase optical element enabling on-demand spin-controlled angular momentum multiplexing. A diffractive optics multiplexer fabricated with 3D high-resolution electron beam lithography performs a conformal mapping for the conversion from linear to azimuthal phase gradients. The latter is functionalized by a dynamic spin-orbit add-on that consists of a self-engineered electrically-activated liquid crystal optical vortex generator having large clear-aperture and high-resolution. By combining several functionalities based on the optical angular momentum of light in a compact manner, the proposed hybrid device could find applications in next-generation high-dimensional mode switchers and routers based on orbital angular momentum.

Label-Free Multiphoton Microscopy is a very powerful optical microscopy that can be applied to study samples with no need for exogenous fluorescent probes, keeping the main benefits of a Multiphoton approach, like longer penetration depths and intrinsic optical sectioning, while opening the possibility of serial examinations with different kinds of techniques. Among the many variations of Label-Free MPM, Higher Harmonic Generation (HHG) is one of the most intriguing due to its generally low photo-toxicity, which enables the examination of specimens particularly susceptible to photo-damages. HHG and common Two-Photon Microscopy (TPM) are well-established techniques, routinely used in several research fields. However, they require a significant amount of fine-tuning in order to be fully exploited and, usually, the optimized conditions greatly differ, making them quite difficult to perform in parallel without any compromise on the extractable information. Here we present our custom-built Multiphoton microscope capable of performing simultaneously TPM and HHG without any kind of compromise on the results thanks to two, separate, individually optimized laser sources with full chromatic aberration compensation. We also apply our setup to the examination of a plethora of ex vivo samples in order to prove the significant advantages of our approach.

In this paper, we present the design, fabrication and optical characterization of computer-generated holograms (CGH) encoding information for light beams carrying orbital angular momentum (OAM). Through the use of a numerical code, based on an iterative Fourier transform algorithm, a phase-only diffractive optical element (PH-DOE) specifically designed for OAM illumination has been computed, fabricated and tested. In order to shape the incident beam into a helicoidal phase profile and generate light carrying phase singularities, a method based on transmission through high-order spiral phase plates (SPPs) has been used. The phase pattern of the designed holographic DOEs has been fabricated using high-resolution Electron-Beam Lithography (EBL) over glass substrates coated with a positive photoresist layer (polymethylmethacrylate). To the best of our knowledge, the present study is the first attempt, in a comprehensive work, to design, fabricate and characterize computer-generated holograms encoding information for structured light carrying OAM and phase singularities. These optical devices appear promising as high-security optical elements for anti-counterfeiting applications., Comment: 24 pages, 9 figures. Supplementary info: 8 pages, 3 figures

In recent years, mode-division multiplexing (MDM) has been proposed as a promising solution in order to increase the information capacity of optical networks both in free-space and in optical fiber transmission. Here we present the design, fabrication and test of diffractive optical elements for mode-division multiplexing based on optical transformations in the visible range. Diffractive optics have been fabricated by means of 3D high-resolution electron beam lithography on polymethylmethacrylate resist layer spun over a glass substrate. The same optical sequence was exploited both for input-mode multiplexing and for mode sorting after free-space propagation. Their high miniaturization level and efficiency make these optical devices ideal for integration into next-generation platforms for mode-division (de)multiplexing in telecom applications., Comment: 4 pages, 1 extended references page, 6 figures. arXiv admin note: substantial text overlap with arXiv:1610.07443

The orbital angular momentum (OAM) of light has recently attracted a growing interest as a new degree of freedom in order to increase the information capacity of today optical networks both for free-space and optical fiber transmission. Here we present our work of design, fabrication and optical characterization of diffractive optical elements for compact OAM-mode division demultiplexing based on optical transformations. Samples have been fabricated with 3D high-resolution electron beam lithography on polymethylmethacrylate (PMMA) resist layer spun over a glass substrate. Their high compactness and efficiency make these optical devices promising for integration into next-generation platforms for OAM-modes processing in telecom applications., Comment: 5 pages, 6 figures

A short range experimental communication system, based on Orbital Angular Momentum (OAM) multiplexing, is presented. We characterize circular arrays of patch antennas designed to transmit and receive OAM electromagnetic fields, reporting new results on communication links based on such antennas. We also experimentally study the antennas tolerance to misalignment errors (angular tilt and lateral shift) within which OAM multiplexing can be efficiently exploited. Starting from these results, we finally propose an application to short range communications of OAM-based systems that can lead to a high level of security in the information exchange.

The congestion of the radio frequency bands imposes serious limitations on the capacity and capability of modern wireless information infrastructures. One approach to enable frequency re-use is to exploit other physical conserved quantities of the electromagnetic fields, such as the orbital angular momentum (OAM) in addition to linear momentum, which is exploited in present-day telecommunications. Whereas in the optical regime the increase of channel capacity by using OAM states was demonstrated recently, the receiving antennas in commercial radio links have a much smaller extent than the transmitted beam, making the signal reception and characterization of the OAM state demanding. Moreover, radio data transmission with more than two channels per polarization state at the same frequency for radio links is known to be notoriously difficult to realize even with multiport techniques, long antenna baselines and digital post-processing. Here we report results from an outdoor, proof of concept experiment where the physical properties of OAM states were used to transfer information, using far-field multiplexing/demultiplexing of three coexisting collinear, vertically polarized and mutually independent OAM radio beams, opening new perspectives in wireless telecommunications., Comment: 14 pages, 6 figures, minor changes/typos corrected

We have shown experimentally that it is possible to propagate and use the properties of twisted non-monochromatic incoherent radio waves to simultaneously transmit to infinity more radio channels on the same frequency band by encoding them in different orbital angular momentum states. This novel radio technique allows the implementation of, at least in principle, an infinite number of channels on one and the same frequency, even without using polarization or dense coding techniques. An optimal combination of all these physical properties and techniques represents a solution for the problem of radio band congestion. Our experimental findings show that the vorticity of each twisted electromagnetic wave is preserved after the propagation, paving the way for entirely new paradigms in radio communication protocols., Comment: 17 pages, 6 figures, with a public experiment, three audio files in mp3 format

It has been known for a century that electromagnetic fields can transport not only energy and linear momentum but also angular momentum. However, it was not until twenty years ago, with the discovery in laser optics of experimental techniques for the generation, detection and manipulation of photons in well-defined, pure orbital angular momentum (OAM) states, that twisted light and its pertinent optical vorticity and phase singularities began to come into widespread use in science and technology. We have now shown experimentally how OAM and vorticity can be readily imparted onto radio beams. Our results extend those of earlier experiments on angular momentum and vorticity in radio in that we used a single antenna and reflector to directly generate twisted radio beams and verified that their topological properties agree with theoretical predictions. This opens the possibility to work with photon OAM at frequencies low enough to allow the use of antennas and digital signal processing, thus enabling software controlled experimentation also with first-order quantities, and not only second (and higher) order quantities as in optics-type experiments. Since the OAM state space is infinite, our findings provide new tools for achieving high efficiency in radio communications and radar technology., Comment: 4 pages, 3 figures