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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.

Metasurfaces are arrays of subwavelength spaced nanostructures that can manipulate the amplitude, phase, and polarization of light to achieve a variety of optical functions beyond the capabilities of 3D bulk optics. However, they suffer from limited performance and efficiency when multiple functions with large deflection angles are required because the non-local interactions due to optical coupling between nanostructures are not fully considered. Here we introduce a method based on supercell metasurfaces to demonstrate multiple independent optical functions at arbitrary large deflection angles with high efficiency. In one implementation the incident laser is simultaneously diffracted into Gaussian, helical and Bessel beams over a large angular range. We then demonstrate a compact wavelength-tunable external cavity laser with arbitrary beam control capabilities – including beam shaping operations and the generation of freeform holograms. Our approach paves the way to novel methods to engineer the emission of optical sources., The angular dependence is a well-known issue in metasurface engineering. Here the authors introduce a supercell metasurface able to implement multiple independent functions under large deflection angles with high efficiency, leading to a wavelength tunable laser with arbitrary wavefront control.

Polarization plays a key role in science; hence its versatile manipulation is crucial. Existing polarization optics, however, can only manipulate polarization in a single transverse plane. Here we demonstrate a new class of polarizers and wave plates—based on metasurfaces—that can impart an arbitrarily chosen polarization response along the propagation direction, regardless of the incident polarization. The underlying mechanism relies on transforming an incident waveform into an ensemble of pencil-like beams with different polarization states that beat along the optical axis thereby changing the resulting polarization at will, locally, as light propagates. Remarkably, using form-birefringent metasurfaces in combination with matrix-based holography enables the desired propagation-dependent polarization response to be enacted without a priori knowledge of the incident polarization—a behaviour that would require three polarization-sensitive holograms if implemented otherwise. Our work expands the use of polarization in the design of multifunctional metasurfaces and may find application in tunable structured light, optically switchable devices and versatile light–matter interactions. Using a metasurface that allows shaping of the polarization state of a light beam independently at each point of space along its propagation direction, longitudinally variable polarization optical components are demonstrated, inspiring new directions in structured light, polarization-switchable devices and light–matter interaction.

Direct visualization of nanometer-scale properties of moiré superlattices in van der Waals heterostructure devices is a critically needed diagnostic tool for study of the electronic and optical phenomena induced by the periodic variation of atomic structure in these complex systems. Conventional imaging methods are destructive and insensitive to the buried device geometries, preventing practical inspection. Here we report a versatile scanning probe microscopy employing infrared light for imaging moiré superlattices of twisted bilayers graphene encapsulated by hexagonal boron nitride. We map the pattern using the scattering dynamics of phonon polaritons launched in hexagonal boron nitride capping layers via its interaction with the buried moiré superlattices. We explore the origin of the double-line features imaged and show the mechanism of the underlying effective phase change of the phonon polariton reflectance at domain walls. The nano-imaging tool developed provides a non-destructive analytical approach to elucidate the complex physics of moiré engineered heterostructures., Direct visualization of moiré superlattices in van der Waals heterostructures is a needed diagnostic tool for the study of periodicity-induced electronic and optical phenomena. Here, the authors demonstrate that the moiré pattern in twisted bilayer graphene can be indirectly imaged by imaging the phonon polariton interference on the top hexagonal boron nitride encapsulation layer.

Metastructures hold promises for the realization of novel optical functions. Common topologies utilized in the form of metasurfaces featuring simple periodicities, however, are not exploiting the f...

Tailored nanostructures provide at-will control over the properties of light, with applications in imaging and spectroscopy. Active photonics can further open new avenues in remote monitoring, virtual or augmented reality and time-resolved sensing. Nanomaterials with χ(2) nonlinearities achieve highest switching speeds. Current demonstrations typically require a trade-off: they either rely on traditional χ(2) materials, which have low non-linearities, or on application-specific quantum well heterostructures that exhibit a high χ(2) in a narrow band. Here, we show that a thin film of organic electro-optic molecules JRD1 in polymethylmethacrylate combines desired merits for active free-space optics: broadband record-high nonlinearity (10-100 times higher than traditional materials at wavelengths 1100-1600 nm), a custom-tailored nonlinear tensor at the nanoscale, and engineered optical and electronic responses. We demonstrate a tuning of optical resonances by Δλ = 11 nm at DC voltages and a modulation of the transmitted intensity up to 40%, at speeds up to 50 MHz. We realize 2 × 2 single- and 1 × 5 multi-color spatial light modulators. We demonstrate their potential for imaging and remote sensing. The compatibility with compact laser diodes, the achieved millimeter size and the low power consumption are further key features for laser ranging or reconfigurable optics., Spatial light modulators (SLM) provide tailored light fields for many applications. Here, the authors present an SLM device based on an organic electro-optic material that manipulates the properties of individual pixels by electronic signals at speeds up to 50 MHz.

Metasurfaces are a promising platform to exceed their traditional counterparts not only in compactness but also for functionality. However, current designs are limited when trying to implement multiple, non-paraxial functions with a single metasurface as they are bound to either a small angular range or to low efficiencies. Here, we present a new non-local metasurface design that enables the implementation of multiple, independent functions with a large difference in deflection angle. We further demonstrate the capabilities of this approach for advanced control of light emission systems by creating a wavelength-tunable external cavity laser with holographic output based on such metasurface.

New opportunities for plasmonic applications at high temperatures have stimulated interest in refractory plasmonic materials that show greater stability at elevated temperatures than the more commonly used silver and gold (Au). Titanium nitride (TiN) has been identified as a promising refractory material, with deposition of TiN thin films through techniques ranging from plasma-enhanced atomic laser deposition to sputter deposition to pulsed laser deposition, on a variety of substrates, including MgO, polymer, SiO2, and sapphire. A variety of plasmonic devices have been evaluated, including gratings, nanorods, and nanodisks. An implicit metric for TiN behavior has been the comparison of its plasmonic performance to that of Au, in particular at various elevated temperatures. This paper carries out a one-to-one comparison of bowtie nanoantennas formed of TiN and Au (on both Si and MgO substrates), examining the far-field characteristics, related to the measured near-field resonances. In both cases, the optic...

Recent work on beams with light structured along the propagation direction will be presented, where the polarization and the OAM of the beam changed along the propagation direction.

We demonstrate an electrically tunable metasurface from an array of nanoresonators coated by a single layer of electro-optic molecules that are embedded in a polymer matrix. By periodic poling of the non-linear coating in plane, we tune the resonant frequency of the array under an applied bias at high speeds and over a broad range in the near-infrared.

It is a well-established truth that spatial hole burning (SHB) in a standing-wave cavity is an essential single-mode instability mechanism for multimode operation of quantum cascade lasers (QCLs). We discovered recently that another instability mechanism–phase turbulence–is capable of triggering an onset of previously unseen types of frequency combs in traveling-wave ring cavity QCLs in absence of SHB. This new regime of laser operation reveals a connection with Kerr combs and paves the way to manipulation and engineering of comb states in QCLs.

We explore transduction between the radio-frequency and near-infrared domain in nanoscale metasurfaces to demonstrate multi-pixel spatial light modulators. We custom-tailor their optical, radio-frequency and electro-optic response at the nanoscale. We investigate guided mode resonances and bound states in the continuum cross-functionalized with state-of the art organic nano-molecules JRD1 that have combined merits of record efficiency in a broadband range and low electronic consumption per pixel.

Vector beams can be structured to change their polarization state and topological charge locally with propagation. We report on the observation of Berry phase factor accompanying these transitions and provide a recipe for engineering it on demand.

The electromagnetic near field enables subwavelength applications such as near-field microscopy and nanoparticle manipulation. Present methods to structure the near field rely on optical antenna theory, involving nanostructures that locally convert propagating waves into confined near-field patterns. We developed a theory of remote rather than local near-field shaping, based on cascaded mode conversion and interference of counterpropagating guided waves with different propagation constants. We demonstrate how to structure at will the longitudinal and transverse variation of the near field, allowing for distributions beyond the conventional monotonic decay of the evanescent field. We provide an experimental realization that confirms our theory. Our method applies to fields with arbitrary polarization states and mode profiles, providing a path toward three-dimensional control of the near field.

Metasurfaces—subwavelength arrays of phase-shifting elements—present new possibilities for polarization optics and polarimetry. In particular, a periodic, polarization-sensitive metasurface diffraction grating can enable full-Stokes imaging polarimetry with a single polarization-sensitive component. In this work, we show that a suitably-designed metasurface grating can serve as a polarimetric “attachment” to an existing intensity-only imaging system, converting it into one capable of full-Stokes imaging polarimetry. Design rules and tradeoffs governing this adaptation are described and demonstrated using a machine vision imaging system as an example.

In this paper, we report dispersion-engineered metasurfaces with distinct functionalities controlled by wavelength. Unlike previous approaches based on spatial multiplexing or vertical stacking of metasurfaces, we utilize a single phase profile with wavelength dependence encoded in the phase shifters' dispersion. We designed and fabricated a multiwavelength achromatic metalens (MAM) with achromatic focusing for blue (B), green (G), yellow (Y), and red (R) light and two wavelength-controlled beam generators (WCBG): one focuses light with orbital angular momentum (OAM) states ( l = 0,1,2) corresponding to three primary colors; the other produces ordinary focal spots ( l = 0) for red and green light, while generating a vortex beam ( l = 1) in the blue. A full color (RGB) hologram is also demonstrated in simulation. Our approach opens a path to applications ranging from near-eye displays and holography to compact multiwavelength beam generation.

Mid-infrared nanoimaging and spectroscopy of two-dimensional (2D) materials have been limited so far to scattering-type scanning near-field optical microscopy (s-SNOM) experiments, where light from the sample is scattered by a metallic-coated atomic force microscope (AFM) tip interacting with the material at the nanoscale. These experiments have recently allowed imaging of plasmon polaritons in graphene as well as hyperbolic phonon polaritons in hexagonal boron nitride (hBN). Here we show that the high mechanical sensitivity of an AFM cantilever can be exploited for imaging hyperbolic phonon polaritons in hBN. In our imaging process, the lattice vibrations of hBN micrometer-sized flakes are locally enhanced by the launched phonon polaritons. These enhanced vibrations are coupled to the AFM tip in contact with the sample surface and recorded during scanning. Imaging resolution of Δ/20 is shown (Δ being the polaritonic fringes' separation distance), comparable to the best resolution in s-SNOM. Importantly, this detection mechanism is free from light background, and it is in fact the first photonless detection of phonon polaritons.

We introduce a new class of polarizing elements (analyzers and retarders) that virtually rotate their orientation as a function of the propagation distance, thus expanding the scope of conventional polarization devices into 3D space.

We demonstrate that ring lasers can attain multimode operation at a very low pumping level, contrary to common belief. The instability is explained via the Ginzburg-Landau theory, establishing a connection between laser and microresonator combs.

We present compact planar devices that enable light’s spin and orbital angular momenta to evolve, simultaneously, from one state to another along the propagation direction, and report on arbitrary spin-orbit coupling in three dimensional vortices.

Semiconductor ring lasers are miniaturized devices that operate on clockwise and counterclockwise modes. These modes are not coupled in the absence of intracavity reflectors, which prevents the formation of a standing wave in the cavity and, consequently, of a population inversion grating. This should inhibit the onset of multimode emission driven by spatial hole burning. Here we show that, despite this notion, ring quantum cascade lasers inherently operate in phase-locked multimode states, that switch on and off as the pumping level is progressively increased. By rewriting the master equation of lasers with fast gain media in the form of the complex Ginzburg-Landau equation, we show that ring frequency combs stem from a phase instability---a phenomenon also known in superconductors and Bose-Einstein condensates. The instability is due to coupling of the amplitude and phase modulation of the optical field in a semiconductor laser, which plays the role of a Kerr nonlinearity, highlighting a connection between laser and microresonator frequency combs.

α-MoO3 is demonstrated as a mid-infrared biaxial hyperbolic crystal supporting in-plane phonon polaritons with concave wavefronts., Hyperbolic media have attracted much attention in the photonics community due to their ability to confine light to arbitrarily small volumes and their potential applications to super-resolution technologies. The two-dimensional counterparts of these media can be achieved with hyperbolic metasurfaces that support in-plane hyperbolic guided modes upon nanopatterning, which, however, poses notable fabrication challenges and limits the achievable confinement. We show that thin flakes of a van der Waals crystal, α-MoO3, can support naturally in-plane hyperbolic polariton guided modes at mid-infrared frequencies without the need for patterning. This is possible because α-MoO3 is a biaxial hyperbolic crystal with three different Reststrahlen bands, each corresponding to a different crystalline axis. These findings can pave the way toward a new paradigm to manipulate and confine light in planar photonic devices.

Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase, there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here, we give a proof of concept of a compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive a dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region, and injection lock the laser. These results pave the way for applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source.

Anisotropic phonon polaritons in boron nitride on black phosphorus heterostructures are studied with near-field microscopy., Van der Waals (vdW) heterostructures assembled from layers of two-dimensional 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 phosphorus (h-BN/BP) heterostructures, demonstrating the first direct observation of in-plane anisotropic phonon polariton modes in vdW heterostructures. Notably, the measured in-plane optical anisotropy along the 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 αmax = 1.25 in the frequency spectrum at 1405 to 1440 cm−1. These results provide new insights into 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.

Graphene is expected to be an enabling technology for THz antennas and related devices. This chapter describes the foundations for the theoretical and numerical modeling of graphene devices in the framework of Maxwell's equations. Subsequently, several designs of graphene planar antennas for terahertz frequencies are proposed showing that high-quality-gated graphene can be used to achieve frequency reconfiguration in resonant plasmonic antennas and beam steering in graphene-based-plasmonic reflectarrays. Afterwards, the potential of graphene for non-reciprocal applications is demonstrated experimentally, with the design, fabrication, and measurement of the first terahertz graphene isolator (operating between 1 and 10 THz). Finally, preliminary results concerning the realization of graphene beam steering reflectarray antennas at terahertz frequencies are presented. All of the above takes advantage of a newly developed theoretical “upper bound”which allows one to evaluate the closeness of a given design to the theoretical optimum, and depends uniquely on graphene conductivity.

Highly efficient wave refraction at arbitrary angles has been a long-standing practical challenge and of great importance in scientific and engineering applications. In this paper, we reveal the possibility of exciting persistent radiating modes propagating at arbitrarily chosen negative angles with respect to the designed structure's surface normal. The interplay between the excited backward propagating guided modes of dielectric waveguides and the scattered field from the periodic dielectric meta-surfaces plays the crucial role in achieving such characteristics. Here, we study this in more details.

In this paper, two THz reflectarray surfaces have been designed and fabricated in order to deflect a plane wave with any polarization and with a specific incident angle to three different specific directions each at distinct three frequencies of 0.7, 1.0 and 1.5 THz. The surface is composed of an array of 100 $\times$ 100 cells, each comprised of gold crosses and parasitic dipoles printed on thin grounded high resistivity silicon. Finite-element method (FEM) simulations are in line with the measurement results obtained using THz time-domain spectroscopy (THz TDS) showing the intended deflections for the two fabricated samples each with an arbitrary frequency-vs-deflection angle relationship. In addition, the use of silicon as the substrate paves the way for the integration of reconfigurable technologies which enhances the reflectarray versatility.

We study ultra-confined polaritonic modes supported by thin micro- and nano-devices made of hexagonal boron nitride flakes. We image these modes using new optical scanning probes microscopy techniques. These results open new avenues for nano-photonics as a way to achieve extreme light confinement at the nanoscale.

We demonstrate single-layer metasurfaces with controllable multi-wavelength functions. A multiwavelength achromatic metalens for red, yellow, green and blue light, and metasurfaces generating focused beams with different orbital angular momentum states are designed and fabricated.

Faraday rotation is a fundamental property present in all nonreciprocal optical elements. In the THz range, graphene displays strong Faraday rotation; unfortunately, it is limited to frequencies below the cyclotron resonance. Here, we show experimentally that in specifically designed metasurfaces, magnetoplasmons can be used to circumvent this limitation. We find excellent agreement between theory and experiment and provide physical insights and predictions on these phenomena. Finally, we demonstrate strong tunability in these metasurfaces using electric and magnetic field biasing., This work has been financially supported by the Swiss National Science Foundation (SNSF) under GrantsNo. 133583 and No. 168545, the Hasler Foundation under Project No. 11149, and the European Commission under Graphene Flagship (Contract No. CNECT-ICT-604391).

Controlling the spacing of self-starting harmonic frequency combs in QCLs by design of fundamental laser parameters is arduous. New ways to shape such combs by means of original electrical, optical and radiofrequency techniques are presented.

This paper presents a wide-band RF shunt capacitive switch reconfigurable by means of electrically triggered Vanadium Oxide (VO2) phase transition to build a true-time delay (TTD) phase shifter. The concept of VO2-based reconfigurable shunt switch has been explained and experimentally demonstrated by designing, fabricating and characterizing an 819 μm long unit cell. The effect of bias voltage on losses and phase shift has been studied and explained. By triggering the VO2 switch insulator to metal transition (IMT) the total capacitance can be reconfigured from the series of two metal-insulator-metal (MIM) capacitors to a single MIM capacitor. Higher bias voltages are more effective in this reconfiguration and give a higher phase shift. The optimal achievable performance has been shown heating the devices above VO2 IMT temperature. A maximum of 16° per dB loss has been obtained near the design frequency (10 GHz).

Guided modes in nanometer thick anisotropic van der Waals materials are experimentally investigated and their refractive indices in visible wavelengths are extracted. Our method involves near-field scanning optical microscopy of waveguide (transverse electric) and surface plasmon polariton (transverse magnetic) modes in h-BN/SiO2/Si and Ag/h-BN stacks, respectively. We determine the dispersion of these modes and use this relationship to extract anisotropic refractive indices of h-BN flakes. In the wavelength interval 550–700 nm, the in-plane and out-of-plane refractive indices are in the range 1.98–2.12 and 1.45–2.12, respectively. Our approach of using near-field scanning optical microscopy allows for the direct study of the interaction between light and two-dimensional van der Waals materials and heterostructures.

The modulated scattering technique is based on the use of reconfigurable electromagnetic scatterers, structures able to scatter and modulate an impinging electromagnetic field in function of a control signal. The modulated scattering technique is used in a wide range of frequencies up to millimeter waves for various applications, such as field mapping of circuits or antennas, radio-frequency identification devices and imaging applications. However, its implementation in the terahertz domain remains challenging. Here, we describe the design and experimental demonstration of the modulated scattering technique at terahertz frequencies. We characterize a modulated scatterer consisting in a bowtie antenna loaded with a vanadium dioxide switch, actuated using a continuous current. The modulated scatterer behavior is demonstrated using a time domain terahertz spectroscopy setup and shows significant signal strength well above 0.5 THz, which makes this device a promising candidate for the development of fast and energy-efficient THz communication devices and imaging systems. Moreover, our experiments allowed us to verify the operation of a single micro-meter sized VO2 switch at terahertz frequencies, thanks to the coupling provided by the antenna.

This letter reports a detailed experimental investigation of the slope of the current switching between OFF and ON states exploiting the metal–insulator-transition (MIT) in vanadium dioxide devices. The reported devices are CMOS compatible two-terminal switches. We experimentally demonstrate for the first time the very little dependence on temperature of the steep slope of these switches, ranging from 0.24 mV/decade at room temperature, to 0.38 mV/decade at 50 °C. The fabricated devices show excellent ON-state conduction, with $I_{\mathrm{{\scriptscriptstyle ON}}}>1.8$ mA/ $\mu \text{m}$ or $R_{\mathrm{{\scriptscriptstyle ON}}} , for the whole range of investigated temperatures (from room temperature to the MIT transition temperature), which recommends them as future candidates for steep-slope, highly conductive, and temperature-stable switches.

The ITER project requires at least two Neutral Beam Injectors (NBIs), each accelerating to 1MV a 40A beam of negative deuterium ions, to deliver to the plasma a power of about 33 MW for one hour as additional heating.Since these requirements have never been experimentally met, it was recognized necessary to build-up a test facility, named PRIMA (Padova Research on ITER Megavolt Accelerator), in Italy, including both a 100 kV full-size negative ion source (SPIDER - Source for Production of Ion of Deuterium Extracted from Rf plasma) and a prototype of the whole 1 MV ITER injector (MITICA - Megavolt ITER Injector & Concept Advancement).The mission of SPIDER is to increase the understanding of the source operation and to optimize the source performance in terms of extracted current density, current uniformity and duration.The SPIDER experiment needs a dedicated Cooling Plant to remove up to 10 MW heat loads applied during the one hour pulses to the beam source, the beam dump/calorimeter and the power supplies. The total water flow rate necessary to remove the heat and guarantee the optimal control of temperatures is about 150 kg/s at full operational conditions.The plant is widespread over the whole PRIMA site area and is characterized by several physical interfaces with the SPIDER experiment, the related power supplies and control system and finally with PRIMA buildings and civil plants. A well-equipped system of sensors and feed-back control system is also present to guarantee the correct performances of the plant during different operational phases.The paper deals with the installation and testing phases of the plant under execution, with particular emphasis on integration problems, electrical insulation, tests of immunity to electromagnetic interference, vibration issues, monitoring and safety issues for possible activated water and finally the acceptance tests before integrated commissioning.