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Abstract
With the ability to manipulate and precisely tailor the micro- and nanostructures with sizes that are comparable to or smaller than the wavelength of light, unique optical properties beyond those offered by natural materials can be achieved in various nanophotonic platforms including plasmonics, photonic crystals and metamaterials. In this feature issue, fifteen papers are included with broad coverage of the cutting-edge research in the fields ranging from optical sensing, structure-enhanced absorption, novel non-metallic plasmonic materials, structure-modified optical transmission to new types of metamaterials, topological and quantum photonics.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As the thriving advancement in nanotechnology and nanofabrication, micro/nano-structures with unit-cell sizes comparable to or smaller than the optical wavelength can now be achieved, enabling the realization of photonics crystals, metamaterials and plasmonic devices with exceptional optical properties.
Plasmonics studies phenomena induced by and associated with surface plasmons which are coherent collective oscillations of electrons with respect to the lattices, providing a means to enhance light-matter interactions, generating intense, highly localized electromagnetic fields [1]. Such prosperity provides plasmonics a plethora of applications in optical sensing [2], waveguiding [3], lasers [4], harmonic generation [5], photocatalysis [6] and photovoltaics [7].
Metamaterials are artificially engineered meso-structured multi-material systems which give an effective response to optical fields that is different from that of the component materials [8,9]. By carefully designing the subwavelength unit cell structures, unique effective properties that do not exist in nature can be attained. By virtue of such features of the metamaterials, novel types of optical devices beyond the traditional understanding of physics were realised, such as superlens with resolution beyond diffraction limit [10], invisible cloaking that otherwise only exists in the science fiction [11].
Photonic crystal is a periodic optical nanostructure that has attracted considerable interest for their ability to manipulate light [12], providing unique features for plenty of applications ranging from lasers [13,14], all-optical memories [15] to sensing [16] and emission control [17].
This special issue features 15 papers describing the state-of-art development that combines the fields of plasmonics, photonic crystals and metamaterials, including optical sensing [18–20], structure-enhanced absorption [21,22], novel non-metallic plasmonic materials [23,24], structure-modified optical transmission [25–27], new types of metamaterials [28–30] and beyond (quantum [31] and topological photonics [32]).
We hope with this feature issue to highlight some of the main trends in this rapidly evolving field and give a flavour for the current and exciting future research directions. In the following, we summarise the collection of papers in this feature issue in the context of the different emergent research directions.
2. Optical sensing improved by designed nanostructures
Compared to chemical sensing, the optical counterpart has the advantages of non-contacting, label-free, fast response and high sensitivity. To achieve the above-mentioned properties, it is always desired to localise light as intensely as possible to enhance the light-mater interactions, consequently driving the plasmonics/photonic crystals as an ideal platform to realise the function [16,33]. Analyte detection at single molecule level was achieved [34,35] and even at ultrafast preparation time [36].
In this feature issue, Champi et al. propose the separation of enantiomer from the optical force generated by asymmetric plasmonic nano-apertures [18]. They predict that the chiral spherical molecule pair at 10-nm can be separated by shining circular polarized light on the designed crescent moon aperture, shedding light on the all-optical enantioseperation of single chiral macromolecules such as proteins or carbohydrates. Baburin et al. report the sensing of fluorescent molecules based on plasmonic crystals composed of nanohole arrays on a thin Ag film [19]. Taking advantages of the Ebbesen effect [42], high sensitivity is achieved by suppressing of the parasitic luminescence. Li et al. propose to enhance the sensitivity of hollow-core Bragg fibres by geometric optimisation. A 32% sensitivity enhancement of refractive index is reported by using the optimised thickness of the bilayer of the Bragg fibre [20].
3. Structure-enhanced absorption
Broadband absorption is a desired feature in many fields ranging from energy harvesting related applications (such as photovoltaics, photothermal conversion and photocatalysis) to optical detectors, thermal emitters and optical modulators [37–39]. The intrinsic material properties always hinder the absorption from a broadband feature. To break this bottleneck, the geometric features comparable or smaller to the wavelength (i.e., inducing the strong matter interaction between the incident light and absorber) provides another degree of freedom to drastically enhance the absorption in both amplitude and bandwidth. Ultra-broadband absorber was achieved in different platforms, including plasmonic nanoparticles [40] and graphene metamaterials [41]. In this feature issue, Shen et al. report a 3D structure composed of bent wire arrays with gradually varied lengths that achieves a broadband absorption beyond 90% at GHz regime [21]. Cen et al. propose a tunable absorber in the far infrared and THz regime, based on a patterned graphene containing periodic elliptical hollows [22]. By both the active/passive tuning such as period, geometric parameters or electric doping, the absorption can be modified in a versatile way.
4. Structure-modified optical transmission
Despite with size smaller than the wavelength, the well-designed nanostructure can substantially perturb the propagation behaviour. For example, greatly enhanced transmission of light through a subwavelength can be achieved through an aperture in an otherwise opaque metallic film which has been patterned with a regularly repeating periodic structure [42]. Also, the wavefront of a light beam can be feasible engineered through Pancharatnam-Berry phase and thus manipulate the propagation at far field [43].
In this feature issue, Kang et al. report the thickness-dependent optical transmission for a plasmonic nano-hole [25]. By virtue of the Fano interference, a transition from suppressed to enhanced transmission is observed as the thickness of the metallic film varies. Eskandari et al. propose a 2D non-magnetic waveguide coupler with ideally 100% efficiency for TM waves, taking the unparalleled power of transformation optics [26]. Zhen et al. report a fibre-based optical mode filter by the coupling between the cladding mode and the surface plasmon polariton (SPP) [27]. The dissipation in the metallic nanowires filters out the corresponding mode in the fibre, driving the loss as a beneficial factor in the configuration.
5. Novel non-metallic plasmonic materials
Instead of widely-used noble metals such as Au and Ag, alternative plasmonic materials are preferred for practical systems and/or overcoming the limitations of traditional plasmonic materials (such as the inevitable loss for the noble metal) [44]. Transition metal nitrides, such as titanium nitride (TiN) and zirconium nitride (ZrN), have optical properties close to that of noble metals, which is involved in applications such as plasmonic interconnects, optical sensing, particle trapping, thermophotovoltaic and heat-assisted magnetic recording [44–46]. Transparent conducting oxides, such as indium tin oxide (ITO), aluminium-doped zinc oxide (AZO) and gallium-doped zinc oxide (GZO), are CMOS compatible, tunable and are suitable for use in modulation applications and harmonic generation [45,47,48].
In this feature issue, Britton et al. realise the tunability of dispersion in titanium nitride from visible to infrared by the addition of O2 and/or Si, which induces a broadband response owing to the double-plasmon resonance [23]. Ye et al. report a grating metalens based on silicon nitride (SiNx), a material with CMOS compatibility and feasible fabrication [24]. Similarly to Ref. [23], the permittivity can be easily tuned by a deposition process, introducing a new degree of freedom for the optical design.
6. New types of metasurface
As a two dimensional metamaterial, metasurfaces offer a platform for versatile applications far beyond its original function in achieving abnormal reflection and refraction [49–51], ranging from optical hologram [52], optical harmonic generation [53] to structural color generation [54] and ultrathin planar lenses [55].
In this feature issue, Li et al. utilise the geometric phases introduced by the orientation of meta-atoms to realise a coding metasurface at THz regime [28]. Peng et al. propose a multi- functional metasurface with simultaneous switchable absorption and polarisation conversion ability [29]. Taking the advantages of the feasible tunability of the conductivity of graphene, a tri-layer metasurface composed of two layers of graphene and one layer between composed of patterned gold is design to work bi-functionally. Fu et al. propose a double-layer metallic metasurface that can support both the electric and magnetic Fano resonant modes in the microwave regime [30].
7. For quantum and topological photonics
The nano-structured systems provide a versatile platform for quantum optics. For example, an all-dielectric metasurface was reported to image multiple projections of quantum states with a single metasurface, enabling a robust reconstruction of amplitude, phase, coherence, and entanglement of multiphoton polarization-encoded states [56]. A plasmonic system composed of a gold film and a gold nanoparticle leveraged the optical confinement below nm3, enabling nanoscale nonlinear quantum optics on the single-molecule level [57]. And waveguides formed by photonic crystals with excellent light confinement are routinely used for guiding quantum light [58].
In this feature issue, Alajlan et al. reports the strong coupling between Germanium vacancy centre in diamond and a gallium phosphide nanobeam cavity with Q/V value beyond 108 [31].
Meanwhile, photonic crystals and metamaterials are widely utilised in the field of topological photonics, a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behaviour of light [59]. Gyromagnetic photonic crystal operating at microwave frequencies was realized with a topological non-trivial Chern number [60]. Later on, topological systems were realised in bianisotropic metamaterials [61].
In this feature issue, Zhang et al. report a topological phase transition from a square- lattice photonic crystal composed of gyro-electric rods [32]. As the variation of the orientation angle of the rods, the system enters a topologically non-trivial phase as confirmed by the Chern’s number. Based on the theoretical calculations, they propose several devices including insulator, isolator and beam splitter.
8. Conclusion
This feature issue focuses on the progress in materials science and nanofabrication that boosts the development of novel concepts and device designs in nanophotonics. We believe that newly developed materials such as 2D materials (not only including graphene [62] and transition metal dichalcogenide [63] but also newly-developed magnetic ones [64]) and the perovskites [65] may lead to a new generation of nanophotonic devices. Meanwhile, embedding the novel physics concept such as Fano resonance [66], bound states in the continuum [67] and supersymmetry [68] offers new possibilities for the extension of the functionality. With continued exploration of new materials and a deeper understanding of the underlying physics in their interaction with light, we expect to see a significant impact of new plasmonics, photonic crystal or metamaterial based optical systems across multiple disciplines.
Acknowledgement
G. Li would like to the support from Dr. C. Liu for preparing the feature issue introduction.
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