Ultrathin Plasmonic Metasurfaces Research Articles

Ultrathin Twisted Stacked Gap‐Plasmon Metasurface with Giant and Tunable Shortwave Infrared Chirality

AbstractUltrathin optical components with giant circular dichroism hold great promise for polarization‐sensitive nanophotonics. However, existing chiral metamaterials often rely on complex‐shaped nanoinclusions, and there remains a paucity of designs active in the shortwave infrared (SWIR) useful for telecommunication and night vision. Here a detailed numerical analysis of the chiroptical response of a simple twisted stacked variant of gap‐plasmon metasurface is presented based on periodic nanohole arrays. Impressively, the 100‐nm free‐standing plasmonic metasurface can attain a thickness‐normalized ellipticity of up to 74.2° µm−1 with tunable bands in the SWIR. Strong and robust optical activity is achieved through energy confinement by gap surface plasmons and effective inter‐nanohole coupling, resulting in an intense superchiral near field. The ultrathin plasmonic metasurface serves as an excellent platform for refractive index sensing with a sensitivity Sn of up to 648.1 nm RIU−1 and field‐enhanced enantiodiscrimination. The simple metasurface with strong SWIR chirality can enjoy a multitude of potential applications including polarization‐sensitive photodetection, optical telecommunication, machine vision, medical imaging, and spectroscopy. These findings also offer insights into some fundamental design principles for twisted stacked aperture‐based metasurfaces and should facilitate the development of new chiral metamaterials with exceptional performance.

Slow light enabled high-modulation-depth graphene modulator with plasmonic metasurfaces.

Graphene has attracted the interest of researchers seeking to develop compact optical modulators with the flexible tunability of graphene conductivity by tuning the Fermi level. Plasmonic structures have provided a robust way to enhance the modulation depth (MD) of graphene optical modulators, but the available schemes suffer from low MD, fabrication complexities, or both. Here, an ultra-thin plasmonic metasurface structure capable of guiding slow surface plasmons (SPs) is proposed to construct graphene-based optical modulators. The designs take advantage of the strong field enhancement of slow SP modes as well as the orientation match between the electric field and the graphene plane. A typical 0.96-μm-long metasurface-based graphene modulator presents a significantly improved MD of 4.66dB/μm and an acceptable insertion loss of 1.4dB/μm, while still having ease of fabrication.

Generation of three-dimensional optical cusp beams with ultrathin metasurfaces

Cusp beams are one type of complex structured beams with unique multiple self-accelerating channels and needle-like field structures owning great potentials to advance applications such as particle micromanipulation and super-resolution imaging. The traditional method to generate optical catastrophe is based on cumbrous reflective diffraction optical elements, which makes optical system complicated and hinders the nanophotonics integration. Here we design geometric phase based ultrathin plasmonic metasurfaces made of nanoslit antennas to produce three-dimensional (3D) optical cusp beams with variable numbers of self-accelerating channels in a broadband wavelength range. The entire beam propagation profiles of the cusp beams generated from the metasurfaces are mapped theoretically and experimentally. The special self-accelerating behavior and caustics concentration property of the cups beams are also demonstrated. Our results provide great potentials for promoting metasurface-enabled compact photonic devices used in wide applications of light-matter interactions.

Mid-infrared second-harmonic generation in ultra-thin plasmonic metasurfaces without a full-metal backplane

We report the design and operation of a nonlinear intersubband polaritonic metasurface for mid-infrared second harmonic generation. The metasurface is made of plasmonic nanoresonators filled with a multiple-quantum-well semiconductor heterostructure. Unlike the previously reported nonlinear intersubband polaritonic metasurfaces that employ full-metal backplanes below the etched metal–semiconductor nanoresonators, the metasurface reported here employs an incomplete backplane that is complementary to the pattern of the top metallization of the etched semiconductor heterostructure nanoresonators. The new approach produces high-electric-field localization and enhancement in the nanoresonators, while requiring simplified fabrication and allowing the metasurface to operate in both transmission and reflection regimes.

Plasmonic Metasurfaces for Simultaneous Thermal Infrared Invisibility and Holographic Illusion

AbstractIn 1860s, Gustav Kirchhoff proposed his famous law of thermal radiation, setting a fundamental contradiction between the infrared reflection and thermal radiation. Here, for the first time an ultrathin plasmonic metasurface is proposed to simultaneously produce ultralow specular reflection and infrared emission across a broad spectrum and wide incident angle range by combining the low emission nature of metal and the photonic spin–orbit interaction in spatially inhomogeneous structures. As a proof‐of‐concept, a phase gradient metasurface composed of sub‐wavelength metal gratings is designed and experimentally characterized in the infrared atmosphere window of 8–14 µm, demonstrating an ultralow specular reflectivity and infrared emissivity below 0.1. Furthermore, it is demonstrated that infrared illusion could be generated by the metasurface, enabling not only invisibility for thermal and laser detection, but also multifunctionalities for potential applications. This technology is also scalable across a wide range of electromagnetic spectrum and provides a feasible alternative for surface coating.

Full-Color Plasmonic Metasurface Holograms.

Holography is one of the most attractive approaches for reconstructing optical images, due to its capability of recording both the amplitude and phase information on light scattered from objects. Recently, optical metasurfaces for manipulating the wavefront of light with well-controlled amplitude, phase, and polarization have been utilized to reproduce computer-generated holograms. However, the currently available metasurface holograms have only been designed to achieve limited colors and record either amplitude or phase information. This fact significantly limits the performance of metasurface holograms to reconstruct full-color images with low noise and high quality. Here, we report the design and realization of ultrathin plasmonic metasurface holograms made of subwavelength nanoslits for reconstructing both two- and three-dimensional full-color holographic images. The wavelength-multiplexed metasurface holograms with both amplitude and phase modulations at subwavelength scale can faithfully produce not only three primary colors but also their secondary colors. Our results will advance various holographic applications.

Antireflective Coatings: Conventional Stacking Layers and Ultrathin Plasmonic Metasurfaces, A Mini-Review

Reduction of unwanted light reflection from a surface of a substance is very essential for improvement of the performance of optical and photonic devices. Antireflective coatings (ARCs) made of single or stacking layers of dielectrics, nano/microstructures or a mixture of both are the conventional design geometry for suppression of reflection. Recent progress in theoretical nanophotonics and nanofabrication has enabled more flexibility in design and fabrication of miniaturized coatings which has in turn advanced the field of ARCs considerably. In particular, the emergence of plasmonic and metasurfaces allows for the realization of broadband and angular-insensitive ARC coatings at an order of magnitude thinner than the operational wavelengths. In this review, a short overview of the development of ARCs, with particular attention paid to the state-of-the-art plasmonic- and metasurface-based antireflective surfaces, is presented.

Giant Nonlinear Optical Activity of Achiral Origin in Planar Metasurfaces with Quadratic and Cubic Nonlinearities.

3D chirality is shown to be unnecessary for introducing strong circular dichroism for harmonic generations. Specifically, near-unity circular dichroism for both second-harmonic generation and third-harmonic generations is demonstrated on suitably designed ultrathin plasmonic metasurfaces with only 2D planar chirality. The study opens up new routes for designing chip-type biosensing platform, which may allow for highly sensitive detection of bio- and chemical molecules with weak chirality.

Wavelength-selective orbital angular momentum generation based on a plasmonic metasurface

Nanoapertures with space-variant geometries are designed in a gold thin film to construct an ultrathin plasmonic metasurface, which has been demonstrated both numerically and experimentally to selectively generate and focus orbital angular momentum (OAM) beams with different topological charges at the wavelengths of 930 nm and 766 nm, respectively. Moreover, the interference patterns between the different circularly polarized transmission light were used to confirm the topological charges unambiguously. The agreement between the simulated and measured results suggests that the metasurface of wavelength-selective OAM modes may have potential applications in future optical communication systems.

Femtosecond pulse shaping by ultrathin plasmonic metasurfaces

In this work, a compact and ultrathin plasmonic metasurface is proposed for an ultrashort pulse shaping method in the optical regime based on linear filtering principles. A planar array of nanoplasmonic particles is employed to manipulate the transmission spectrum of propagating ultrashort pulses to shape its temporal characteristics. We demonstrate that the method is able to broaden, compress, and reshape the polarization of ultrashort pulses with great control over its characteristics by engineering the resonances of the ultrathin plasmonic metasurface.

Modeling nanoscale V-shaped antennas for the design of optical phased arrays

We present a simplified numerical method to solve for the current distribution in a V-shaped antenna excited by an electric field with arbitrary polarization. The scattered far-field amplitude, phase, and polarization of the antennas are extracted. The calculation technique presented here is an efficient method for probing the large design parameter space of such antennas, which have been proposed as basic building blocks for the design of ultrathin plasmonic metasurfaces. Our calculation is based on the integral equation method of moments and is validated by comparison to the results of finite-difference time-domain (FDTD) simulations. The computation time is approximately five orders of magnitude less than for FDTD simulations. This speed-up relies mainly on the use of the thin-wire approximation, whose domain of validity is discussed. This method can be generalized to more complex geometries such as zigzag antennas.