Ten years ago, I published an exhaustive review [1] of the physics and applications of long-range surface plasmon polaritons (LRSPPs) [1]. Close to 370 papers were cited where LRSPPs appeared as the main or peripheral focal point. The reviewed studies covered a very broad landscape, including modal characteristics, excitation, field enhancement, nonlinear interactions, molecular scattering, fluorescence, surface-enhanced Raman spectroscopy, optical interconnects, integrated structures, gratings, thermo-, electro- and magneto-optics, amplification, lasing, and (bio)chemical sensing. This thread, and indeed the field of plasmonics, have not stopped growing.

LRSPPs are TM-polarized optical surface waves that propagate with low loss over long ranges (centimeters) along a symmetrically clad thin metal slab or stripe. Typically, range extension factors of 10 to 1000 are achievable with LRSPPs, relative to conventional single-interface SPPs. The lower attenuation enables many functions, including interconnected and passive integrated optical structures, high-sensitivity biosensors, and the exploitation of a broad range of materials effects (e.g., electro-optic, thermo-optic, and optical gain), rendering tunable and active applications feasible. In this brief editorial, I highlight non-exhaustively some of the work carried out with LRSPPs since the publication of my review article.

Interconnects. One of the early applications proposed for LRSPPs was their exploitation in interconnects based on metal stripe waveguides. Such interconnects have the ability to simultaneously transmit optical and electrical signals. Work along this direction has continued to evolve. For instance, mechanically flexible structures supporting LRSPPs were demonstrated, as shown in Fig. 1(a) [2]. Here the authors propose arrays of single-mode metal stripes cladded by polymer that, once peeled from the substrate used as their fabrication platform, can be bent out of plane. Another direction consists of using alternative plasmonic materials [3] to form metal stripe waveguides, such as TiN [4]. Here the authors demonstrate the propagation of LRSPPs along TiN stripes deposited on sapphire. TiN deposited on such a substrate forms an epitaxial layer, as shown in Fig. 1(b), capable of guiding LRSPPs with an attenuation comparable to that of Au stripes. As a further example, metal stripe LRSPP waveguides were integrated into a microwave coplanar waveguide (CPW) as the ground stripes [5], as shown in Fig. 1(c). Here, the authors demonstrated the microwave and optical performance of the structure, as well as its ability to transmit microwave data at a rate of 12 Gbit/s on the fundamental quasi-TEM mode of the CPW, and optical data at a rate of 40 Gbit/s on LRSPPs, simultaneously and without crosstalk. The integration of plasmonic nanoantennas on a metal stripe providing polarization demultiplexing (coupling) has also been explored [6].

 

Figure 1. LRSPP waveguide structures. (a) Au stripes in polymer as a flexible interconnect. Reprinted with permission from [2]. Copyright 2018 Optical Society of America. (b) Epitaxial TiN film on sapphire as a LRSPP waveguide. Reprinted with permission from [4]. Copyright 2014 Optical Society of America. (c) Microwave coplanar waveguide with Au ground stripes operating simultaneously as LRSPP waveguides. Reprinted with permission from [5]. Copyright 2015 Optical Society of America. (d) Cross-section of a waveguide comprising a Au stripe covered with Cytop on a multilayer dielectric stack acting as a truncated 1D photonic crystal, supporting Bloch LRSPPs. Reprinted with permission from Fong et al., ACS Photonics 4, 593–599 (2017) [9]. Copyright 2017 American Chemical Society.

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Bloch LRSPPs. An interesting direction consists of replacing the dielectric substrate supporting a metal slab or stripe by a multilayer structure, operating as a truncated 1D photonic crystal that is designed so that the LRSPP propagates in the plane and within the bandgap of the structure as a Bloch LRSPP [7,8]. Advantageously such a multilayer stack enables the use of many good dielectrics to realize the structure, conferring the advantages of robustness and stability while ensuring that the field symmetry required for propagation is maintained. Such structures hold promise for sensing applications in fluidic and gaseous media. A metal stripe on a multilayer dielectric stack is shown in Fig. 1(d) [9]. Similarly, the use of asymmetric stratified layers to ensure field symmetry and propagation of LRSPPs holds promise [10]. The propagation of LRSPPs and long-range surface phonon polaritons on metamaterial structures has also been explored [11].

Sensors: The use of LRSPPs in sensing applications remains of very strong interest. This is due to the long optical interaction length of LRSPPs with the sensing medium enabled by its low attenuation, and the LRSPP’s large penetration depth into the sensing medium. The former leads to biosensors with a high overall sensitivity (higher than single-interface SPPs), and the latter enables probing deeper into the perpendicular direction. When used in a prism-coupled configuration, narrow linewidths are produced in angular or wavelength scans, which are easy to track. An interesting application of this concept consists of sensing living cells while monitoring their viability when subjected to toxins [12]. A further application consists in integrating fluorescence microscopy with LRSPP biosensors in a multimodal biodetection assay [13]. LRSPPs on metal stripes in integrated waveguide geometries excited via butt-coupling are also quite compelling, as the structures are very sensitive and compact and integrate the required microfluidics [14]. Such biosensors have been used in disease detection applications in complex fluids, including Dengue detection in patient plasma, leukemia detection in patient sera, and bacteria infection in urine [15]. LRSPPs have been used for the detection of ${\mathrm{H}}_2$ gas using a thin Pd sensing patch integrated with a Au LRSPP waveguide on a thin optically-non invasive free-standing membrane [16].

Amplification and lasing: The low attenuation of LRSPPs has motivated many studies involving their interaction with gain media. For instance, a laser dye in a reasonable concentration can have sufficient gain to overcompensate LRSPP attenuation, enabling amplification and oscillation. Stimulated emission into LRSPPs was observed and net gain produced as confirmed via amplifier cut-back [17] and ASE [18] measurements. LRSPP oscillators (lasers) have also been reported, as a Fabry-Perot structure [19] and a distributed feedback laser [20], the latter producing a highly coherent single-mode emission. Such structures are of strong interest as sources of surface plasmons.

Thus, the field of long-range plasmonics remains vibrant, as highlighted in this brief (non-exhaustive) editorial.