Metasurfaces are known as a powerful tool for complex wavefront shaping. However, two-dimensional metasurface systems of nanoparticles exhibit only a weak spatial asymmetry perpendicular to the surface and therefore have mostly reciprocal optical transmission features. To influence this reciprocity, we present a metasurface design principle for nonreciprocal polarization encryption of holograms. Our approach is based on a two-layer plasmonic metasurface design that introduces a local asymmetry and allows full phase and amplitude control of the transmitted light. We experimentally show that our pixel-by-pixel encoded Fourier-hologram appears in a particular linear cross-polarization channel, while it is disappearing in the reverse propagation direction.
Free from phase-matching constraints, plasmonic metasurfaces have contributed significantly to the control of optical nonlinearity and enhancement of nonlinear generation efficiency by engineering subwavelength meta-atoms. However, high dissipative losses and inevitable thermal heating limit their applicability in nonlinear nanophotonics. All-dielectric metasurfaces, supporting both electric and magnetic Mie-type resonances in their nanostructures, have appeared as a promising alternative to nonlinear plasmonics. High-index dielectric nanostructures, allowing additional magnetic resonances, can induce magnetic nonlinear effects, which, along with electric nonlinearities, increase the nonlinear conversion efficiency. In addition, low dissipative losses and high damage thresholds provide an extra degree of freedom for operating at high pump intensities, resulting in a considerable enhancement of the nonlinear processes. We discuss the current state of the art in the intensely developing area of all-dielectric nonlinear nanostructures and metasurfaces, including the role of Mie modes, Fano resonances, and anapole moments for harmonic generation, wave mixing, and ultrafast optical switching. Furthermore, we review the recent progress in the nonlinear phase and wavefront control using all-dielectric metasurfaces. We discuss techniques to realize all-dielectric metasurfaces for multifunctional applications and generation of second-order nonlinear processes from complementary metal–oxide–semiconductor-compatible materials.
Optical holography became a powerful tool for arbitrarily manipulating the wavefronts of light. With the recent development of metasurface holography it became possible to tailor all the fundamental properties of light (amplitude, phase, polarization, wave vector and frequency) within a thin slab of material. However, for exploring the full capability of the information storage of metasurface holograms and enhance the encryption security, smart multiplexing techniques together with suitable metasurface designs are required.
Here, we demonstrate a novel method for achieving multichannel vectorial holography and show its potential for obtaining dynamic displays and high-security applications. We explore birefringent metasurfaces for the complete control of polarization channels with the freedom of designing both the polarization dependent phase shift and polarization rotation matrix. We show that although the target holographic phase profiles have quantified phase relations they can process very different information within different polarization manipulation channels. For our metasurface holograms, we demonstrate high fidelity, large efficiency, broadband operation, and a total of twelve polarization channels. Such multichannel polarization multiplexing can be used for dynamic vectorial holographic display and provide triple protection to the optical security devices. The concept is appealing for applications of arbitrary spin to angular momentum conversion and various phase modulation/beam shaping elements.
We study the polarisation and geometry dependence of four-wave mixing (FWM) on nanocross arrays. The arrays are composed of gold meta-atoms fabricated via EBL and lift-off on a glass substrate coated with a 15 nm ITO film. The individual nanocrosses are C4-symmetric, 360 nm by 360 nm, with 80 nm wide arms. The array period is 550 nm.
FWM is generated by two-colour illumination. The two input wavelengths are 1028 nm (wavelength 1) and 1310 nm (wavelength 2), and we look for the degenerate FWM signal at 846 nm (2*frequency 1 - frequency 2). Using all combinations of handedness for circularly polarised inputs, we verify the theoretical selection rules for FWM on systems of this type. They are LLL-L, RRR-R, LRR-L, and RLL-R, where the first letter is the handedness of beam 2, the following two are the handedness of beam 1, and the last letter is the handedness of the output FWM.
We measure several metasurfaces. In each, the two nanocrosses in a unit cell are rotated towards each other by an angle theta, which is varied 0 to 45 degrees in 7.5 degree increments. With co-polarised inputs (LLL and RRR) the FWM signal is the same from all metasurfaces. With cross-polarised inputs (LRR and RLL) it follows cos^2(4*theta). This behaviour, which is predicted theoretically, is due to the nonlinear Pancharatnam-Berry geometric phase of the FWM from the rotated nanocrosses.
We further support our results with numerical simulations, which match the experimental behaviour for all metasurfaces and show the angle-dependent phase of the nonlinear polarisations on the meta-atoms.
Metasurfaces provide great feasibilities for tailoring both propagation waves and surface plasmon polaritons (SPPs). Manipulation of SPPs with arbitrary complex field distribution is an important issue in integrated nanophotonics due to their capability of guiding waves with subwavelength footprint. Here, with metasurface composed of nano aperture arrays, a novel approach is proposed and experimentally demonstrated which can effectively manipulate complex amplitude of SPPs in the near-field regime. Positioning the azimuthal angles of nano aperture arrays and simultaneously tuning their geometric parameters, the phase and amplitude are controlled based on Pancharatnam-Berry phases and their individual transmission coefficients. For the verification of the proposed design, Airy plasmons and axisymmetric Airy beams are generated. The results of numerical simulations and near-field imaging are well consistent with each other. Besides, 2D dipole analysis is also applied for efficient simulations. This strategy of complex amplitude manipulation with metasurface can be used for potential applications in plasmonic beam shaping, integrated optoelectronic systems and surface wave holography.
The emerging field of metasurfaces has offered unprecedented functionalities for shaping wave fronts and optical responses. Here, we realize a new class of metasurfaces with nanorod array, which can generate abrupt interfacial phase changes to control local wave front at subwavelength scale. The physical mechanism under the phase modulation is geometry phase in essence, thus can achieve broadband operation, as well as helicity-dependent property. Multiple applications have been demonstrated, such as anomalous refraction, ultrathin dual-polarity metalenses, helicitydependent unidirectional surface plasmon polariton (SPP) excitation, and three-dimensional (3D) holography.
A new class of optical modes arising from the hybridization between one localized plasmon and two orthogonal
waveguide modes is described. Of particular interest is our observation that these hybrid modes simultaneously exhibit
extremely low-loss and highly dispersive characteristics, which translate into slow light propagation. We propose that
this is a new type of classical analogs of the electromagnetically induced transparency (EIT) in an atomic system. Based
on a fine balance of geometric and material dispersion in the system, destructive interference of the waveguide modes
cancels out the metal loss, resulting in a narrow transparent window within a broad absorption band. In accordance with
the developed phenomenological model, we show that the dispersion characteristics of the hybrid modes can be entirely
controlled by tuning the coupling strengths between the plasmon and waveguide modes while the mode losses remain the
Laser science has tackled physical limitations to achieve higher power, faster and smaller light
sources. The quest for ultra-compact laser that can directly generate coherent optical fields at
the nano-scale, far beyond the diffraction limit of light, remains a key fundamental challenge.
Microscopic lasers based on photonic crystals3, metal clad cavities4 and nanowires can now
reach the diffraction limit, which restricts both the optical mode size and physical device
dimension to be larger than half a wavelength. While surface plasmons are capable of tightly
localizing light, ohmic loss at optical frequencies has inhibited the realization of truly nano-scale
lasers. Recent theory has proposed a way to significantly reduce plasmonic loss while
maintaining ultra-small modes by using a hybrid plasmonic waveguide. Using this approach, we
report an experimental demonstration of nano-scale plasmonic lasers producing optical modes
100 times smaller than the diffraction limit, utilizing a high gain Cadmium Sulphide
semiconductor nanowire atop a Silver surface separated by a 5 nm thick insulating gap. Direct
measurements of emission lifetime reveal a broad-band enhancement of the nanowire's exciton
spontaneous emission rate up to 6 times due to the strong mode confinement and the signature
of apparently threshold-less lasing. Since plasmonic modes have no cut-off, we show downscaling
of the lateral dimensions of both device and optical mode. As these optical coherent
sources approach molecular and electronics length scales, plasmonic lasers offer the possibility to
explore extreme interactions between light and matter, opening new avenues in active photonic
circuits, bio-sensing and quantum information technology.
Inspired by the observation of Bloch oscillations of electrons in semiconductor supperlattices we recently predicted the existence of Wannier-Stark states as well as Wannier-Stark ladders and consequently the emergence of optical Bloch oscillations in evanescently coupled optical waveguide arrays. Here we show that the required linear variation of the propagation constant across the array can be realized by using the thermo-optic effect in polymers. Beyond the fundamental interest in waveguide arrays for the study of dynamical effects in discrete systems, they have a fair potential in all-optical signal processing. We demonstrate that waveguide arrays allow for temperature- controlled beam steering, while simultaneously minimizing the diffractive beam spreading. Homogeneous arrays of 75 waveguides are fabricated in an inorganic-organic polymer, with each waveguide guiding a single mode (<0.5 dB/cm) at a wavelength of 633 nm. By heating and cooling the opposite sides of the samples, a transverse linear temperature gradient is established. Exciting a few waveguides using a wide Gaussian beam we measure the oscillating transverse motion of the undiffracted output beam for an increasing temperature gradient.