Surface plasmons are electromagnetic surface waves that propagate at the interface between a conductor and a dielectric (usually metal-air interface). The most appealing feature of surface plasmons is the capability to concentrate light on two-dimensional platform and produce very high local-field intensity, providing a vital route to construct on-chip optical devices and manipulate light-matter interactions. In parallel to the flourish of metamaterials and metasurfaces in free-space, the development of meta-optics has also brought a renewed interest to the manipulation of surface plasmons.
The main idea of meta-optics is constructing subwavelength artificial structures to support specific resonances or modes, in which the effective optical parameters including permittivity, permeability, and chirality can be flexibly tailored by three-dimensional bulk metamaterials and the local scattering responses including phase, amplitude, and polarization can be arbitrarily tailored by two-dimensional flat metasurfaces.
Recently, The founding Associate Editor of Photonics Insights, Prof. Weili Zhang of Oklahoma State University, and the Center for Terahertz Waves of Tianjin University jointly contribute a comprehensive review paper entitled "Meta-Optics Inspired Surface Plasmon Devices", which was published in Photonics Insights, Volume 2, Issue 1. 2023. (Quan Xu, Yuanhao Lang, Xiaohan Jiang, Xinyao Yuan, Yuehong Xu, Jianqiang Gu, Zhen Tian, Chunmei Ouyang, Xueqian Zhang, Jiaguang Han, Weili Zhang. Meta-optics inspired surface plasmon devices[J]. Photonics Insights, 2023, 2(1): R02).
In this paper, they discuss recent progress on meta-optics inspired surface plasmon devices in detail. First, they briefly introduce the fundamentals of SPs and meta-optics and review the experimental setups for SP characterizations. Then, they respectively review the SP devices for coupling, tailoring, and scattering of SPs, including both traditional and nascent methods. Finally, conclusions and outlooks for future developments are presented.
Coupling devices
A key step to implement SP-related applications is converting propagating light into SPs. Traditional coupling methods include prism, diffraction and focusing, which mainly consider the momentum mismatch between the free-space light and SPs in visible and infrared frequencies. At terahertz and microwave frequencies, proper designs should be applied to enhance the confinement of SPs, such as coating a thin dielectric film at the metal interface or constructing subwavelength periodic structures on metal surfaces to apply the spoof SPs concept. In recent years, phase discontinuity metasurfaces (gradient phase metasurfaces), arranging a set of ultrathin meta-atoms as phase shifters at the interface has also attract great attention to manipulate the radiation wavefront of incident light through interaction with these meta-atoms.
This depends on the resonant phase modulation brought by the subwavelength periodic structures. The azimuth angle changes can introduce additional geometric phase modulation into the orthogonal polarization conversion of the incident circularly polarized light, which makes the surface plasmon coupling exhibit selective excitation related to the spin of the incident light.
For the purpose of feeding on-chip plasmonic systems, unidirectional and asymmetric couplers that couple the incident free-space light into SPs toward specific directions have long been pursued. By properly choosing the period of the indentations, SP with specific wavelength emerging from the slit to the grating side can be selectively backscattered. The interference of this reflected SPs with the that excited from slit to the right side can be tuned by adjusting the separation between the slit and the grating.
Constructing asymmetric grooves or illuminating oblique beams on slits are also be employed to realize unidirectional and asymmetric coupling. Another commonly used design strategy is utilizing the interference between two subwavelength resonators. By tailoring the relative phase at resonance and the separation between two subwavelength resonators, SPs can be steered to predominantly propagate along one specific direction.
Another attractive design flexibility of metasurfaces is taking their responses to the angle, polarization, frequency, and orbital angular momentum of incident beam into consideration, as thus to achieve multiplexed coupling of SPs from different kinds of free-space light.
Figure 1. Surface plasmon couplers based on meta-optics. (a) The gradient phase metacoupler. (b) The geometric phase metasurface. (c) Coupled modes for asymmetric SP excitations. (d) Polarization-controlled SP coupling. (e) Frequency-division multiplexing coupler. (f) Focusing of SPs based on orbital angular momentum division multiplexing.
On-chip tailoring devices
In addition to simply coupling the free-space light into on-chip SP mode, it is also of great interests to tailor the on-chip behaviors of coupled SPs. In particular, the ability to tailor the wavefront of the SPs in the coupling process would greatly simplify the design and fabrication for most integrated plasmonic devices. The most straightforward method is constructing specially shaped SP couplers, where the shape-induced propagation phase could thus determine the wavefront of coupled SPs.
Long slits that milled through the metal film are also one of the most used elements in constructing SP couplers such as the plasmonic vortex lens. It takes the form of a set of split curved slits, which are designed to produce SP vortices with specific vortex topological charges. In addition to the grooves and slits that milled in the metal film, ridges or plasmonic masks composed by metal structures on the metal film are also commonly used in SP coupling and tailoring.
Furthermore, the recent progress in micro/nano-fabrications has empowered the realization of subwavelength SP couplers such as metallic slit resonators and metal-insulator-metal resonators. These resonators strongly interact with the incident light, and when they reradiate energy, part of the light will be coupled into SP excitations with in-plane dipolar feature. Due to their compact physical size and robust excitation feature, such dipole sources have been widely adopted as building blocks in constructing functional SP devices.
Considering the near-field scale of SPs, plasmonic vortices are the vortex configurations formed by SPs with partial properties of optical vortices possessing plasmonic orbital angular momentum, and unique near-field properties. Researchers have carried out a series of works about the detailed spatiotemporal evolution of plasmonic vortices, including ultrafast spatiotemporal dynamics, spin-orbit mixing, orbital angular momentum multiplication of plasmonic vortices and manipulation of the spatiotemporal dynamics of plasmonic vortices with the same topological charge.
Plasmonic vortex-based devices are mostly two-dimensional on-chip devices and thus have the advantage of integrating in the environment with compact constraints. Benefiting from the deep subwavelength structured SPs field, it can be useful for optical micromanipulation, for example, plasmonic vortices can act as optical tweezers to drive microparticles on metal surfaces.
In addition to tailor SPs in the coupling process, it is also of great interests to achieve SP tailoring in the propagation process, which directly relates to the on-chip light information processing and on-chip optical system building. The most commonly used method is constructing structures on the metal surface and utilizing them to reflect or scatter SPs in the surface platform.
Another key core of the meta-optics is tailoring the dispersion of an artificial surface, which can also tailor the propagation behaviors of SPs. At the same time, metasurfaces can also be designed as decoupling devices that scatter on-chip surface plasmon into free-space light. For example, surface plasmon can be efficiently decoupled into free space using phase gradient metasurfaces, enabling focusing, holography and structured light generation.
Figure 2. On-chip tailoring devices based on meta-optics. (a) Polarization-controlled surface plasmon holography. (b) The spatiotemporal dynamics of plasmonic vortices. (c) Simultaneous control of excitation phase and amplitude in the propagation process. (d) Decoupler for free-space focusing, vortex and holography.
Applications
The local field enhancement and sub-wavelength characteristics of surface plasmon make it suitable for special application environments with precision, compactness and integration requirements. Meta-optics also promotes the development of surface plasmon based devices. The logic gate devices and meta-waveguide based on surface plasmon are highly compact and controllable, which provide a new paradigm for on-chip optical information transmission and processing.
Focused or vortex surface plasmon has important applications in the field of small particle manipulation, and meta-optics devices also provide new methods for accurate and fast multidimensional manipulation. Due to the plasmonic spin-Hall effect, the structured light field formed by surface plasmon has special dynamic and electromagnetic properties. Meta-optics also provides a platform with high design freedom for the study of plasmonic light-matter interaction and the construction of optical topological quasiparticles. In the coupling process of free space light, part of the information is also transferred to the surface plasmon.
At present, the detection devices of polarization and angular momentum based on surface plasmon have shown excellent resolution ability and the unique advantage of complete measurement in compact environment. Meta-optics lays the foundation for their miniaturization, modularization and integration in the future.
Figure 3. Meta-optics devices based on surface plasmon. (a) The SEM images, experimental and simulation results of the logic gates based on surface plasmon. (b) Schematic view of the plasmonic tweezer. (c) The schematic of overall design and measurement of the polarization components. (d) The results of plasmonic vortex interferometers for the measurements of the spin and orbital angular momentums.
Conclusion
This article reviewed the evolution of meta-optics inspired SP devices, including coupling devices, on-chip tailoring devices, decoupling devices, and some meta-optics empowered nascent SP applications. The novel physical mechanisms raised from meta-optics in conjunction with the development of micro/nano-fabrication technology have revolutionized the design concepts of traditional SP devices and sparked the invention of new ones.
Particularly, the scopey of SP-related studies has been considerably expanded, from the traditional optical frequencies to a much broader electromagnetic spectrum involving terahertz and microwave frequencies. The developed strategies for the coupling and manipulating of the SP modes could provide valuable guidance for designing devices in other systems, such as generalized surface waves, topological surface states, waveguides or quantum systems.
This review article has been highly praised by Professor Sergey I. Bozhevolnyi, Director of the Center for Nanooptics at the University of South Denmark. He and Assistant Professor Ding Fei have co-written a commentary article entitled "Enriching Surface Plasmons with Metesurfaces". "This comprehensive review is timely and covers a wide range of interesting topics in SP devices based on metasurfaces, which we believe will greatly help young researchers enter this fascinating field.", they said.
This review article has received high praise from Prof. Sergey I. Bozhevolnyi, the head of Centre for Nano Optics at University of Southern Denmark. He and Prof. Fei Ding have jointly written a commentary article titled "Enriching Surface Plasma with Metasurfaces". They said, "This comprehensive review is timely and covers a wide range of interesting topics in SP devices based on metasurfaces, which we believe will greatly help young researchers enter this fascinating field".
