Publications

2019
Zheng, Z., et al. A mid-infrared biaxial hyperbolic van der Waals crystal. Science Advances 5, 5, (2019). Publisher's VersionAbstract
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.
Chaudhary, K., et al. Engineering phonon polaritons in van der Waals heterostructures to enhance in-plane optical anisotropy. Science Advances 5, 4, (2019). Publisher's VersionAbstract
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.
Toninelli, E., et al. Concepts in quantum state tomography and classical implementation with intense light: a tutorial. Adv. Opt. Photon. 11, 1, 67–134 (2019). Publisher's VersionAbstract
A tomographic measurement is a ubiquitous tool for estimating the properties of quantum states, and its application is known as quantum state tomography (QST). The process involves manipulating single photons in a sequence of projective measurements, often to construct a density matrix from which other information can be inferred, and is as laborious as it is complex. Here we unravel the steps of a QST and outline how it may be demonstrated in a fast and simple manner with intense (classical) light. We use scalar beams in a time reversal approach to simulate the outcome of a QST and exploit non-separability in classical vector beams as a means to treat the latter as a classically entangled'' state for illustrating QSTs directly. We provide a complete do-it-yourself resource for the practical implementation of this approach, complete with tutorial video, which we hope will facilitate the introduction of this core quantum tool into teaching and research laboratories alike. Our work highlights the value of using intense classical light as a means to study quantum systems and in the process provides a tutorial on the fundamentals of QSTs.
Huang, Y.-W., et al. Versatile total angular momentum generation using cascaded J-plates. Opt. Express 27, 5, 7469–7484 (2019). Publisher's VersionAbstract
Optical elements coupling the spin and orbital angular momentum (SAM/OAM) of light have found a range of applications in classical and quantum optics. The J-plate, with J referring to the photon&\#x02019;s total angular momentum (TAM), is a metasurface device that imparts two arbitrary OAM states on an arbitrary orthogonal basis of spin states. We demonstrate that when these J-plates are cascaded in series, they can generate several single quantum number beams and versatile superpositions thereof. Moreover, in contrast to previous spin-orbit-converters, the output polarization states of cascaded J-plates are not constrained to be the conjugate of the input states. Cascaded J-plates are also demonstrated to produce vector vortex beams and complex structured light, providing new ways to control TAM states of light.
2018
Zheng, Z., et al. A mid-infrared biaxial hyperbolic van der Waals crystal. arXiv:1809.03432 (2018). Publisher's VersionAbstract
Hyperbolic media have attracted much attention in the photonics community, thanks to their ability to confine light to arbitrarily small volumes and to their use for super-resolution applications. The 2D counterpart of these media can be achieved with hyperbolic metasurfaces, which support in-plane hyperbolic guided modes thanks to nanopatterns which, however, pose significant fabrication challenges and limit the achievable confinement. We show that thin flakes of the van der Waals material α-MoO3 can support naturally in-plane hyperbolic polariton guided modes at mid-infrared frequencies without any patterning. This is possible because α-MoO3 is a biaxial hyperbolic crystal, with three different Restrahlen bands, each for a different crystal axis. Our findings can pave the way towards new paradigm to manipulate and confine light in planar photonic devices.
Ambrosio, A. Structuring visible light with dielectric metasurfaces. Journal of Optics (Topical Review) 20, 113002 (2018). Publisher's Version
Oscurato, S.L., Salvatore, M., Maddalena, P. & Ambrosio, A. From nanoscopic to macroscopic photo-driven motion in azobenzene-containing materials. Nanophotonics 7, 1387-1422 (2018). Publisher's Version
Ambrosio, A., et al. Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals. Light: Science & Applications 7, 1, 27 (2018). Publisher's VersionAbstract
We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron nitride thin flakes on a gold substrate. Such modes show substantially improved confinement and a group velocity that is hundreds of times slower than the speed of light, thereby providing a new way to create slow light in the mid-infrared range with a simple structure that does not require nano-patterning. One mode is the fundamental mode in the first Restrahlen band of hexagonal boron nitride thin crystals on a gold substrate; the other mode is equivalent to the second mode of the second Restrahlen band of hexagonal boron nitride flakes that are suspended in vacuum.
Tamagnone, M., et al. Ultra-confined mid-infrared resonant phonon polaritons in van der Waals nanostructures. Science Advances 4, 6, (2018). Publisher's VersionAbstract
Hexagonal boron nitride has been proposed as an excellent candidate to achieve subwavelength infrared light manipulation owing to its polar lattice structure, enabling excitation of low-loss phonon polaritons with hyperbolic dispersion. We show that strongly subwavelength hexagonal boron nitride planar nanostructures can exhibit ultra-confined resonances and local field enhancement. We investigate strong light-matter interaction in these nanoscale structures via photo-induced force microscopy, scattering-type scanning near-field optical microscopy, and Fourier transform infrared spectroscopy, with excellent agreement with numerical simulations. We design optical nano-dipole antennas and directly image the fields when bright- or dark-mode resonances are excited. These modes are deep subwavelength, and strikingly, they can be supported by arbitrarily small structures. We believe that phonon polaritons in hexagonal boron nitride can play for infrared light a role similar to that of plasmons in noble metals at visible frequency, paving the way for a new class of efficient and highly miniaturized nanophotonic devices.
J., L.J., et al. Photothermal Effect: Large Photothermal Effect in Sub‐40 nm h‐BN Nanostructures Patterned Via High‐Resolution Ion Beam (Small 22/2018). Small 14, 22, 1870101 (2018). Publisher's VersionAbstract
In article number 1800072, Josué J. López and co‐workers use photothermal microscopy to measure the near‐field absorption of nanostructured hexagonal boron nitride (h‐BN) and reveal a large photothermal expansion in the nanostructures. This effect is attributed to the anisotropy of the thermal expansion coefficients of h‐BN and the nanostructuring implemented. The photothermal expansion should be present in other van der Waals materials and may lead to nanomechanical switches driven by light.
Wintz, D., et al. Guided Modes of Anisotropic van der Waals Materials Investigated by near-Field Scanning Optical Microscopy. ACS Photonics 5, 4, 1196-1201 (2018). Publisher's VersionAbstract
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.
2017
Devlin, R.C., Ambrosio, A., Rubin, N.A., Mueller, J.P.B. & Capasso, F. Arbitrary spin-to-orbital angular momentum conversion of light. Science 358, 896-901 (2017). Publisher's VersionAbstract

Optical elements that convert the spin angular momentum (SAM) of light into vortex beams have found applications in classical and quantum optics. These elementsSAM-toorbital angular momentum (OAM) convertersare based on the geometric phase and only permit the conversion of left- and right-circular polarizations (spin states) into states with opposite OAM. We present a method for converting arbitrary SAM states into total angular momentum states characterized by a superposition of independent OAM. We designed a metasurface that converts left- and right-circular polarizations into states with independent values of OAM and designed another device that performs this operation
for elliptically polarized states. These results illustrate a general material-mediated connection between SAM and OAM of light and may find applications in producing complex structured light and in optical communication.

Ambrosio, A., et al. Mechanical Detection and Imaging of Hyperbolic Phonon Polaritons in Hexagonal Boron Nitride. ACS Nano 11, 9, 8741-8746 (2017). Publisher's Version
Oscurato, S.L., Borbone, F., Maddalena, P. & Ambrosio, A. Light-Driven Wettability Tailoring of Azopolymer Surfaces with Reconfigured Three-Dimensional Posts. ACS Applied Materials & Interfaces 9, 35, 30133-30142 (2017). Publisher's VersionAbstract
The directional light-induced mass migration phenomenon arising in the photoresponsive azobenzene-containing materials has become an increasingly used approach for the fabrication of controlled tridimensional superficial textures. In the present work we demonstrate the tailoring of the superficial wettability of an azopolymer by means of the light-driven reconfiguration of an array of imprinted micropillars. Few simple illumination parameters are controlled to induce nontrivial wetting effects. Wetting anisotropy with controlled directionality, unidirectional spreading, and even polarization-intensity driven two-dimensional paths for wetting anisotropy are obtained starting from a single pristine pillar geometry. The obtained results prove that the versatility of the light-reconfiguration process, together with the possibility of reversible reshaping at reduced costs, represents a valid approach for both applications and fundamental studies in the field of geometry-based wettability of solid surfaces.
Schöche, S., et al. Optical properties of graphene oxide and reduced graphene oxide determined by spectroscopic ellipsometry. Applied Surface Science 421, Part B, 778 - 782 (2017). Publisher's Version
Longo, A., et al. Graphene oxide prepared by graphene nanoplatelets and reduced by laser treatment. Nanotechnology 28, 22, 224002 (2017). Publisher's Version
Oscurato, S.L., et al. New microscopy technique based on position localization of scattering particles. Opt. Express 25, 10, 11530–11549 (2017). Publisher's VersionAbstract
We introduce the Holographic &\#x2013; Single Scatterer Localization Microscopy in which we combine dynamical laser speckle illumination with centroid localization of backscattered light spots in order to localize isolated scattering particles. The reconstructed centroid images show very accurate particle localization, with precision much better than the width of diffraction-limited image of the particles recorded by the CCD. Furthermore, the method provides an improved resolution in distinguishing two very close scattering objects compared to the standard laser scanning techniques and can be assimilated to a confocal technique in the ability of light background rejection in three-dimensional disposition of scattering objects. The illumination is controlled via a digital holography setup based on the use of a spatial light modulator. This allows not only a high level of versatility in the illumination patterns, but also the remarkable characteristics of absence of moving mechanical parts, typical of the laser scanning techniques, and the possibility of strongly miniaturizing the setup.
Ambrosio, A., Devlin, R.C., Capasso, F. & Wilson, W.L. Observation of Nanoscale Refractive Index Contrast via Photoinduced Force Microscopy. ACS Photonics 4, 846-851 (2017). Publisher's Version
Devlin, R.C., et al. Spin-to-orbital angular momentum conversion in dielectric metasurfaces. Optics Express 25, 1, 377-393 (2017). Publisher's Version
Wintz, D., Ambrosio, A., Zhu, A.Y., Genevet, P. & Capasso, F. Anisotropic Surface Plasmon Polariton Generation Using Bimodal V-Antenna Based Metastructures. ACS Photonics 4, 1, 22-27 (2017). Publisher's Version