Subwavelength core/shell cylindrical nanostructures for novel plasmonic and metamaterial devices
© The Author(s) 2017
Received: 25 September 2017
Accepted: 9 November 2017
Published: 11 December 2017
In this review, we introduce novel plasmonic and metamaterial devices based on one-dimensional subwavelength nanostructures with cylindrical symmetry. Individual single devices with semiconductor/metal core/shell or dielectric/metal core/multi-shell structures experience strong light–matter interaction and yield unique optical properties with a variety of functions, e.g., invisibility cloaking, super-scattering/super-absorption, enhanced luminescence and nonlinear optical activities, and deep subwavelength-scale optical waveguiding. We describe the rational design of core/shell cylindrical nanostructures and the proper choice of appropriate constituent materials, which allow the efficient manipulation of electromagnetic waves and help to overcome the limitations of conventional homogeneous nanostructures. The recent developments of bottom-up synthesis combined with the top-down fabrication technologies for the practical applications and the experimental realizations of 1D subwavelength core/shell nanostructure devices are briefly discussed.
One-dimensional (1D) dielectric nanostructures with high refractive indices offer unique opportunities for exploring light-sensitive responses of materials, affording a series of optical resonances that further boost light–matter interaction compared to their bulk counterparts [1–8]. For example, high-refractive-index semiconductor nanowires (NWs) and NW array devices have become attractive platforms for next-generation solar cells since they support strong optical resonances that facilitate the increase of energy conversion efficiency [9–17]. The high quality resonant modes oscillating along the 1D structure provide strongly directional optical properties that are essential in some key applications such as coherent/incoherent optical light sources, requiring a highly directional emission with minimum spatial divergence [1, 2]. Individual subwavelength Si nanostructures (e.g., NWs and nanoparticles) exhibit strong morphology-dependent resonant scattering, allowing their arrays to be used for controllable structural coloration [5, 18]. The excellent optical responses of 1D nanostructure have attracted the increased interest of researchers working in the fields of plasmonics and metamaterials [19–39]. Importantly, plasmonic materials and metamaterials/metasurfaces allow one to overcome the limitations of conventional uniform and homogeneous dielectric materials. One can utilize deep subwavelength-scale optical constituents to expand the functionality of such 1D nanostructure and realize a variety of nanophotonic devices requiring both efficient light manipulation and strong light–matter interaction. In particular, structures with cylindrical symmetry can be readily combined with properly designed metallic components and serve as key building blocks for unique photonic and optoelectronic nanodevices with unprecedented optical functions. For example, the geometrical advantage allows for the 1D core/shell structures to be not only configured into single devices but also easily integrated with other optical and electrical components without breaking the symmetry and distorting the original optical properties, which are hard to obtain from the spherical core/shell nanostructures. In addition, the ability of plasmonic material NWs to support strongly localized surface plasmon modes has led to the development of deep subwavelength plasmonic waveguides [19, 40, 41], optical antennas , and surface-enhanced Raman scattering sensors [22, 42, 43]. Rationally designed complex dielectric/metal core/shell or core/multi-shell structures can significantly enhance or suppress the optical responses of nanostructures and allow conventionally inaccessible functionalities such as invisibility cloaking [24–32], super-scattering/super-absorption [33, 34], enhanced luminescence and nonlinear optical activities [35–37], and deep subwavelength optical waveguiding [38, 39]. In this review, we introduce various novel plasmonic and metamaterial devices based on 1D subwavelength core/shell nanostructures. We discuss the rational design of core/shell NW structures composed of materials appropriately chosen for targeted functionalities and successful experimental applications using core/shell NW structures. Moreover, we briefly describe the recently developed synthesis/fabrication technologies for realizing 1D subwavelength core/shell nanowires and their practical applications.
2.1 Invisible core/shell plasmonic NWs
2.1.1 Plasmonic cloaking based on core/shell NW structures
2.1.2 Invisibility cloaking of a hybrid metal–semiconductor NW structure
The demonstrated invisibility of the gold-covered Si NW was explained by scattering cancellation : the Si NW and the judiciously designed gold cover were oppositely polarized with an equal magnitude, affording a net-zero local polarization vector and thus resulting in scattering cancellation. The near-field distribution around the Si NW obtained by full-wave numerical simulations further supported the experimentally observed plasmonic cloaking (Fig. 2d). Without the gold cover, the subwavelength-scale bare Si NW distorted the regular interference pattern (left, Fig. 2d), whereas the presence of the gold cover restored the planar wavefronts and made the NW invisible (right, Fig. 2d). Moreover, the authors have demonstrated that the gold covered Si NW can be an invisible photodetector. They measured spectral photocurrents for gold-coated and uncoated regions revealed that the photocurrent at the invisible wavelength of ~ 650 nm was only decreased by a factor of four, whereas plasmonic cloaking suppressed scattering by over two orders of magnitude. These results indicate that the described plasmonic NW device can be utilized as an invisible sensor for the detection of optical signals without near- and far-field distortion [49, 50].
2.1.3 Subwavelength metasurface and metamaterial shell structures
The increased attention enjoyed by invisibility cloaking of subwavelength-scale objects partly comes from the availability of suitable layered metamaterials . In particular, the concept of hyperbolicity in momentum space has been successfully utilized to explore the invisibility of single nanostructures in the visible frequency range. Kim et al. proposed a radial anisotropic hyperbolic metamaterial nanotube comprising an air core and a layered shell structure featuring multiple alternating thin layers of Ag and TiO2 (Fig. 3d) and numerically investigated its scattering properties under both TE- and TM-polarized incident light . Since the permittivities of Ag and TiO2 exhibit opposite signs in the visible frequency range, the optical response of the nanotube can be effectively tuned by controlling the thickness of constituent layers. Figure 3e shows the calculated scattering efficiency spectra of the metamaterial nanotube, revealing that scattering is dramatically reduced at a wavelength of ~ 450 nm under TE- and TM-polarized incident light, consequently leading to decreased visibility. To gain further insights, the authors modeled the layered nanotube as a radially anisotropic hyperbolic metamaterial nanotube [51, 52]. They showed that the decreased scattering occurs when the effective permittivity of the hyperbolic nanotube in the angular direction was close to zero, which is distinguishable from that of conventional plasmonic cloaking. Furthermore, the near-field intensity distribution at the invisible wavelength revealed the occurrence of unique field enhancement inside the nanotube (Fig. 3f), which was explained by the focusing of time-averaged Poynting vector power flow in the core region with a volume of less than ~ 0.16 (λ/2n)2, where λ is the wavelength of incident light and n is the refractive index of the core. Such strong light focusing is expected to be useful for various nanophotonic applications requiring strong light–matter interaction for invisible sensors as well as for spontaneous emission enhancement for fluorescence measurements without signal distortion by scatterers [53, 54].
2.2 Super-scattering and super-absorption
2.3 Plasmonic cavities for enhanced luminescence and second harmonic generation (SHG)
The NW/metal core/shell plasmonic nanocavity structural concept can also be used to enhance the emission quantum efficiency of conventional III–V compound semiconductor NWs, which has been challenging even for direct band gap semiconductor materials due to their large surface-area-to-volume ratio. This large ratio results in a high density of surface defects , which causes the undesirable surface non-radiative recombination and subsequently limits quantum efficiency and NW luminescence. For example, Mokkapati et al. demonstrated that the quantum efficiency of GaAs NWs can be increased by an order of magnitude when Ω-shaped Au nanocavities are used . Additionally, the authors revealed that the increased quantum efficiency originated from the enhanced radiative recombination (Fig. 5d). In this work, the GaAs NW core was passivated with an AlGaAs shell, and an additional GaAs layer was introduced to prevent the oxidation of Al in this shell (Fig. 5e). Next, the obtained core/shell/cap GaAs/AlGaAs/GaAs NWs were coupled to Ω-shaped Au nanocavities. From the measured photoluminescence spectra, they observed ~ 10 times stronger GaAs emission peak from the NW coupled with Ω-shaped Au nanocavity than that from the bare NW (Fig. 5f). Time-resolved photoluminescence measurements of the GaAs NWs at their emission peak also showed that the minority carrier lifetime was reduced from 1.5 ns (bare NW) to 0.5 ns (NW coupled to an Ω-shaped Au nanocavity) (Fig. 5g). The enhanced photoluminescence intensity and the reduced minority carrier lifetime were ascribed to the increased radiative recombination rate in the nanocavity-coupled NW. Based on experimental results, the authors estimated that nanocavity coupling enhanced the quantum efficiency by as much as 900%.
2.4 Graphene-coated NWs for deep-subwavelength-scale graphene plasmon guiding
2.5 Synthesis of conformal and uniform multilayer core/shell cylindrical micro/NWs for novel plasmonic and metamaterial devices
In this review, we described a variety of novel plasmonic and metamaterial devices based on 1D subwavelength core/shell nanostructures, focusing on the rational design of core/shell NW structures for efficient manipulation of light–matter interaction. Importantly, we highlighted that the core/shell or core/multi-shell NW structures comprising plasmonic, dielectric, metasurface/metamaterials or two-dimensional materials exhibit unique optical properties with a variety of functions such as invisibility cloaking, super-scattering/super-absorption, enhanced luminescence and nonlinear optical activities, and deep subwavelength-scale optical waveguiding, which are rarely achievable in homogeneous single-wire structures. Experimental demonstrations of core/shell NW photonic devices such as invisible gold-covered Si NW sensors and enhanced luminescence in Si, CdS, and III–V NWs coupled to plasmonic nanocavities envision the further development of core/shell photonic devices. Furthermore, the recent advances in bottom-up synthesis combined with the top-down fabrication technologies for practical applications and the experimental realizations of functional 1D subwavelength core/shell nanostructure devices were briefly discussed. Thus, rational design and realization/controlled fabrication of subwavelength core/shell NW structures paves the way to the development of novel photonic devices free from the limitations of a homogenous single-wire structures.
KHK and YSN wrote the manuscript. Both authors read and approved the final manuscript.
The authors declare that they have no competing interests.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03033668).
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