Long-range surface plasmon polariton detection with a graphene photodetector.

We present an integration of a single Ag nanowire (NW) with a graphene photodetector and demonstrate an efficient and compact detection of long-range surface plasmon polaritons (SPPs). Atomically thin graphene provides an ideal platform to detect the evanescent electric field of SPPs extremely bound at the interface of the Ag NW and glass substrate. Scanning photocurrent microscopy directly visualizes a polarization-dependent excitation and detects the SPPs. The SPP detection responsivity is readily controlled up to ∼17 mA/W by the drain-source voltage. We believe that the graphene SPP detector will be a promising building block for highly integrated photonic and optoelectronic circuits.

Strain Multiplexed Metasurface Holograms on a Stretchable Substrate.

We demonstrate reconfigurable phase-only computer-generated metasurface holograms with up to three image planes operating in the visible regime fabricated with gold nanorods on a stretchable polydimethylsiloxane substrate. Stretching the substrate enlarges the hologram image and changes the location of the image plane. Upon stretching, these devices can switch the displayed holographic image between multiple distinct images. This work opens up the possibilities for stretchable metasurface holograms as flat devices for dynamically reconfigurable optical communication and display. It also confirms that metasurfaces on stretchable substrates can serve as platform for a variety of reconfigurable optical devices.

Three-dimensional grating nanowires for enhanced light trapping.

We propose rationally designed 3D grating nanowires for boosting light-matter interactions. Full-vectorial simulations show that grating nanowires sustain high-amplitude waveguide modes and induce a strong optical antenna effect, which leads to an enhancement in nanowire absorption at specific or broadband wavelengths. Analyses of mode profiles and scattering spectra verify that periodic shells convert a normal plane wave into trapped waveguide modes, thus giving rise to scattering dips. A 200 nm diameter crystalline Si nanowire with designed periodic shells yields an enormously large current density of ∼28 mA/cm2 together with an absorption efficiency exceeding unity at infrared wavelengths. The grating nanowires studied herein will provide an extremely efficient absorption platform for photovoltaic devices and color-sensitive photodetectors.

Tunable Metasurface and Flat Optical Zoom Lens on a Stretchable Substrate.

A mechanically reconfigurable metasurface that can continuously tune the wavefront is demonstrated in the visible frequency range by changing the lattice constant of a complex Au nanorod array fabricated on a stretchable polydimethylsiloxane substrate. It is shown that the anomalous refraction angle of visible light at 632.8 nm interacting with the tunable metasurface can be adjusted from 11.4° to 14.9° by stretching the substrate by ∼30%. An ultrathin flat 1.7× zoom lens whose focal length can continuously be changed from 150 to 250 µm is realized, which also demonstrates the potential of utilizing metasurfaces for reconfigurable flat optics.

Design of high Q-factor metallic nanocavities using plasmonic bandgaps.

The surface plasmon polariton modes often excited in metallic nanocavities enable the miniaturization of photonic devices, even beyond the diffraction limit, yet their severe optical losses deteriorate device performance. This study proposes a design of metallic nanorod cavities coupled to plasmonic crystals with the aim of reducing the radiation loss of surface plasmon modes. Periodic Ag disks placed on an insulator-metal substrate open a substantial amount of plasmonic bandgaps (e.g., Δλ=290 nm at λ=1550 nm) by modifying their diameter and thickness. When an Ag nanorod with a length of ∼400 nm is surrounded by the periodic Ag disks, its Q-factor increases up to 127, yielding a 16-fold enhancement compared with a bare Ag nanorod, while its mode volume can be as small as 0.03(λ/2n)³. Ag nanorods with gradually increasing lengths exhibit high Q-factor plasmonic modes that are tunable within the plasmonic bandgap. These numerical studies on low-radiation-loss plasmonic modes excited in metallic nanocavities will promote the development of ultrasmall plasmonic devices.

Strong Exciton-Plasmon Coupling in MoS2 Coupled with Plasmonic Lattice.

We demonstrate strong exciton-plasmon coupling in silver nanodisk arrays integrated with monolayer MoS2 via angle-resolved reflectance microscopy spectra of the coupled system. Strong exciton-plasmon coupling is observed with the exciton-plasmon coupling strength up to 58 meV at 77 K, which also survives at room temperature. The strong coupling involves three types of resonances: MoS2 excitons, localized surface plasmon resonances (LSPRs) of individual silver nanodisks and plasmonic lattice resonances of the nanodisk array. We show that the exciton-plasmon coupling strength, polariton composition, and dispersion can be effectively engineered by tuning the geometry of the plasmonic lattice, which makes the system promising for realizing novel two-dimensional plasmonic polaritonic devices.

Fano Resonance and Spectrally Modified Photoluminescence Enhancement in Monolayer MoS2 Integrated with Plasmonic Nanoantenna Array.

The manipulation of light-matter interactions in two-dimensional atomically thin crystals is critical for obtaining new optoelectronic functionalities in these strongly confined materials. Here, by integrating chemically grown monolayers of MoS2 with a silver-bowtie nanoantenna array supporting narrow surface-lattice plasmonic resonances, a unique two-dimensional optical system has been achieved. The enhanced exciton-plasmon coupling enables profound changes in the emission and excitation processes leading to spectrally tunable, large photoluminescence enhancement as well as surface-enhanced Raman scattering at room temperature. Furthermore, due to the decreased damping of MoS2 excitons interacting with the plasmonic resonances of the bowtie array at low temperatures stronger exciton-plasmon coupling is achieved resulting in a Fano line shape in the reflection spectrum. The Fano line shape, which is due to the interference between the pathways involving the excitation of the exciton and plasmon, can be tuned by altering the coupling strengths between the two systems via changing the design of the bowties lattice. The ability to manipulate the optical properties of two-dimensional systems with tunable plasmonic resonators offers a new platform for the design of novel optical devices with precisely tailored responses.

Full three-dimensional power flow analysis of single-emitter-plasmonic-nanoantenna system.

We present a full three-dimensional (3D) power flow analysis of an emitter-nanoantenna system. A conventional analysis, based on the total Poynting vector, calculates only the coupling strength in terms of the Purcell enhancement. For a better understanding of the emitter-nanoantenna system, not only the Purcell enhancement but also complete information on the energy transfer channels is necessary. The separation of the pure scattering and emitter output Poynting vectors enables the quantification of the individual energy transfer channels. Employing the finite-difference time-domain method (FDTD), we examine a nanodisk antenna that supports the bright dipole and dark quadrupole resonance modes for which the power flow characteristics are completely distinct, and we analyze the power flow enhancements to the energy transfer channels with respect to the wavelength, polarization, and position of the emitter coupled to the antenna. The 3D power flow analysis reveals how the constructive or destructive interference between the emitter and the antenna resonance mode affects the power flow enhancements and the far-field radiation pattern. Our proposed power flow analysis should play a critical role in characterizing the emitter-antenna system and customizing its energy transfer properties for desired applications.

Shape-dependent light scattering properties of subwavelength silicon nanoblocks.

We explore the shape-dependent light scattering properties of silicon (Si) nanoblocks and their physical origin. These high-refractive-index nanostructures are easily fabricated using planar fabrication technologies and support strong, leaky-mode resonances that enable light manipulation beyond the optical diffraction limit. Dark-field microscopy and a numerical modal analysis show that the nanoblocks can be viewed as truncated Si waveguides, and the waveguide dispersion strongly controls the resonant properties. This explains why the lowest-order transverse magnetic (TM01) mode resonance can be widely tuned over the entire visible wavelength range depending on the nanoblock length, whereas the wavelength-scale TM11 mode resonance does not change greatly. For sufficiently short lengths, the TM01 and TM11 modes can be made to spectrally overlap, and a substantial scattering efficiency, which is defined as the ratio of the scattering cross section to the physical cross section of the nanoblock, of ∼9.95, approaching the theoretical lowest-order single-channel scattering limit, is achievable. Control over the subwavelength-scale leaky-mode resonance allows Si nanoblocks to generate vivid structural color, manipulate forward and backward scattering, and act as excellent photonic artificial atoms for metasurfaces.

A double-strip plasmonic waveguide coupled to an electrically driven nanowire LED.

We demonstrate the efficient integration of an electrically driven nanowire (NW) light source with a double-strip plasmonic waveguide. A top-down-fabricated GaAs NW light-emitting diode (LED) is placed between two straight gold strip waveguides with the gap distance decreasing to 30 nm at the end of the waveguide and operated by current injection through the p-contact electrode acting as a plasmonic waveguide. Measurements of polarization-resolved images and spectra show that the light emission from the NW LED was coupled to a plasmonic waveguide mode, propagated through the waveguide, and was focused onto a subwavelength-sized spot of surface plasmon polaritons at the tapered end of the waveguide. Numerical simulation agreed well with these experimental results, confirming that a symmetric plasmonic waveguide mode was excited on the top surface of the waveguide. Our demonstration of a plasmonic waveguide coupled to an electrically driven NW LED represents important progress toward further miniaturization and practical implementation of ultracompact photonic integrated circuits.

Low-power nano-optical vortex trapping via plasmonic diabolo nanoantennas.

Optical vortex trapping can allow the capture and manipulation of micro- and nanometre-sized objects such as damageable biological particles or particles with a refractive index lower than the surrounding material. However, the quest for nanometric optical vortex trapping that overcomes the diffraction limit remains. Here we demonstrate the first experimental implementation of low-power nano-optical vortex trapping using plasmonic resonance in gold diabolo nanoantennas. The vortex trapping potential was formed with a minimum at 170 nm from the central local maximum, and allowed polystyrene nanoparticles in water to be trapped strongly at the boundary of the nanoantenna. Furthermore, a large radial trapping stiffness, ~0.69 pN nm(-1) W(-1), was measured at the position of the minimum potential, showing good agreement with numerical simulations. This subwavelength-scale nanoantenna system capable of low-power trapping represents a significant step toward versatile, efficient nano-optical manipulations in lab-on-a-chip devices.

Nonlinear mixing in nanowire subwavelength waveguides.

The realization of nonlinear photonic circuits to achieve the control of light-by-light is contingent upon a strong nonlinear response that can be captured in a guided-wave geometry. There remains a need to further scale down waveguides while maintaining a strong nonlinear response. In this study, we report second-harmonic generation and optical parametric generation using the second-order nonlinear response in an 80 nm thick CdS nanowire subwavelength waveguide. Moreover, our three-dimensional finite-difference time-domain (FDTD) simulations demonstrate that it is possible to enhance the coherence length due to the very nature of the subwavelength geometry. Nonlinear mixing in a nanowire subwavelength waveguide represents an advance toward all-optical processing and all-optical switching in integrated photonic circuits.

Design of polarization-selective light emitters using one-dimensional metal grating mirror.

This paper proposes a polarization-selective light emitter that can enhance preferentially the spontaneous emission rate of one desired polarization state using a one-dimensional metal grating mirror. Systematic numerical simulations were performed to determine the optimized structural parameters of the metal grating mirror consisting of ITO/silver, in which the two orthogonally polarized lights reflected from the grating mirror undergo completely opposite phases. This metal grating mirror was incorporated into a GaN medium, and the spontaneous emission rate of one linearly polarized light was 1.3 times higher than that of the other at a specific distance between the light source and mirror. In addition, the polarization ratio can be increased to 15:1 by considering the extracted power in a practical vertical GaN slab light-emitting diode structure. This study will be useful for demonstrating high-efficiency polarization-selective light-emitting diodes without using additional optical components, such as a polarizer.

High-efficiency vertical GaN slab light-emitting diodes using self-coherent directional emitters.

We demonstrate a highly-efficient, large-area (1x1 mm2) GaN slab light-emitting diode using a vertically directional emitter produced from constructive interference. The vertical radiation can be coupled effectively into leaky modes from the beginning and thus a high-extraction efficiency can be expected with reduced material absorption. The far-field measurements show that the desired vertical emission profiles are obtained by varying the thickness of the dielectric layer between the emitter and bottom silver mirror. With the combination of a light extractor of a randomly textured surface, the output power was increased approximately 1.4 fold compared to a non-patterned device at a standard current of 350 mA without electrical degradation.

Full three-dimensional subwavelength high-Q surface-plasmon-polariton cavity.

We propose a full three-dimensional subwavelength surface-plasmon-polariton cavity based on a metal-coated dielectric nanowire with an axial heterostructure. Surface plasmon-polaritons are strongly confined at the nanowire-metal interface sandwiched by an effective plasmonic mirror that consists of lower-index nanowire core and metal shell. Numerical simulations show for a cavity <50 x 50 x 40 nm(3) (mode volume, V approximately 10(-5) microm(3)) that a quality factor, Q, >36000 is achieved at 20 K. This ultrasmall plasmonic cavity can be used as a plasmonic emitter or laser device coupled to a plasmonic waveguide with a high coupling efficiency in deep-subwavelength photonic systems.

Characteristics of dielectric-band modified single-cell photonic crystal lasers.

We demonstrate new types of dielectric-band photonic crystal lasers in a two-dimensional modified single-cell cavity with enlarged air holes. Finite-difference time-domain simulations performed in real and Fourier spaces show that the dielectric-band cavity modes originating from the first band edge point in the dielectric band have mode patterns that are distinguishable from conventional air-band cavity modes. In our experiment, the observed multimode lasing peaks are identified as the hexapole and the monopole dielectric-band cavity modes through the spectral positions and mode images. The thresholds of these lasers are measured as approximately 340 microW and approximately 450 microW, respectively, at room temperature. In addition, using the simulation based on the actual fabricated structures, quality factors and mode volumes are computed as 4900 and 1.09 (lambda/n)3 for the hexapole mode, and 4300 and 2.27 (lambda/n)3 for the monopole mode, respectively.

Modal characteristics in a single-nanowire cavity with a triangular cross section.

In this study, the modal characteristics of a single-GaN nanowire cavity with a triangular cross section surrounded by air or located on a silicon dioxide substrate have been analyzed. Two transverse resonant modes, transverse electric-like and transverse magnetic-like modes, are dominantly excited for nanowire cavities that have a small cross-sectional size of <300 nm and length of 10 microm. Using the three-dimensional finite-difference time-domain simulation method, quality factors, confinement factors, single-mode conditions, and far-field emission patterns are investigated for a nanowire cavity as a function of one length of the triangular cross section. The results of these simulations provide information that will be vital for the design and development of efficient nanowire lasers and light sources in ultracompact nanophotonic integrated circuits.