Hybrid plasmonic metasurfaces

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Hybrid plasmonic metasurfaces

Evan S. H. Kang, Mina Shiran Chaharsoughi, Stefano Rossi and Magnus Jonsson

The self-archived postprint version of this journal article is available at Linköping University Institutional Repository (DiVA):

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-160892

N.B.: When citing this work, cite the original publication.

Kang, E. S. H., Shiran Chaharsoughi, M., Rossi, S., Jonsson, M., (2019), Hybrid plasmonic metasurfaces, Journal of Applied Physics, 126(14), 140901. https://doi.org/10.1063/1.5116885 Original publication available at:

https://doi.org/10.1063/1.5116885

Copyright: AIP Publishing

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Hybrid Plasmonic Metasurfaces

Evan S. H. Kang, Mina Shiran Chaharsoughi, Stefano Rossi and Magnus P. Jonsson*

Laboratory of Organic Electronics, Department of Science and Technology,

Norrköping 60174, Sweden

*magnus.jonsson@liu.se Abstract

Plasmonic metasurfaces based on ensembles of distributed metallic nanostructures can absorb, scatter and in other ways shape light at the nanoscale. Forming hybrid plasmonic metasurfaces by combination with other materials opens up for new research directions and novel applications. This perspective highlights some of the recent advancements in this vibrant research field. Particular emphasis is put on hybrid plasmonic metasurfaces comprising organic materials and on concepts related to switchable surfaces, light-to-heat conversion, and hybridized light-matter states based on strong coupling.

1. Introduction

Light interacts strongly with metal nanostructures via resonant excitation of plasmons,

which are collective oscillations of the conduction electrons in the nanostructures.1

The nanostructures are typically on the order of tens to hundreds of nanometers in size and effectively act as antennas for light. So-called plasmonic metasurfaces are 2D arrays of such optical nanoantennas, distributed on a substrate to provide properties

beyond the mere average response of the materials they are made from.2,3_{The recent}

large interest in plasmonic metasurfaces and nanoantennas is related to their ability

to control light at the nanoscale.4_{They can focus optical fields to nanoscale ‘hot}

spots’5–7_{and abruptly modify optical wavefronts.}3,8,9_{Furthermore, they can generate}

vibrant structural colours10–15_{or convert light to heat in nanoscopic volumes.}16–21_{As a}

result of all these functionalities, plasmonic metasurfaces find application in an

exceptionally wide range of areas, including biosensing,22–38_{energy conversion,}33,39–47

display technologies,14,48–51_{photocatalysis,}52–55_{ultrathin optical components,}8,10,56–59

and many more.60–72

Focus of this perspective is put on recent research that combines plasmonic metasurfaces with complementary materials and concepts, forming so-called hybrid

This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.

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plasmonic metasurfaces. Such hybrid combinations can provide novel properties and

functions that are difficult or even not possible to achieve by the original components

when used separately. Potential applications range from new types of displays14,48,73–

77_{and sensor systems}78–81_{to different energy conversion concepts,}39,45,46,54,82_including

devices that can harvest energy from light fluctuations.83_{An additional branch of}

hybrid plasmonics relates to coupling of molecules to the vacuum field of plasmonic nanocavities, which forms hybrid light-matter energy states and hybrid systems with a

whole new set of additional interesting possibilities.69,70,84_{This perspective covers by}

no means all exciting research on hybrid plasmonics, but primarily uses our recent contributions as examples to present the current state of the field, and as basis to discuss new possibilities and future directions. For an in-depth overview of hybrid plasmonics including inorganic systems, see for example the recent review by Jiang et

al.85_{We focus particularly on systems that utilize organic functional materials and on}

three directions: dynamic systems, heat management and strong coupling.

1. Hybrid systems for dynamic plasmonic metasurfaces and reflective displays The use of plasmonic nanostructures for colour generation dates back to the at least

the 4th_century,86_{and currently forms an emerging technology for ink-free colour}

production.13,15,87–91_{Advantages include possibility for high resolution and}

chromaticity, excellent stability over time, and environmental friendliness due to low consumption of materials compared with traditional colouration based on dye or

pigment.15,49,88,92_{The mechanism of plasmonic colouration relates to the nature of the}

plasmon excitation itself, which is a resonant phenomenon that occurs preferentially for certain frequencies of light. The plasmon resonance frequency, and hence colour, depends on many factors, including shape, size and distribution of the nanostructures,

as well as on the complex permittivity of the metal (𝜀m(𝜔)) and the permittivity of the

surrounding (𝜀_𝑠). In order to illustrate the resonant nature of plasmonic interactions

and their dependence on different factors, we present the polarizability 𝛼i(𝜔) for a

single ellipsoidal metal nanoparticle. In the quasi-static approximation for particles

with dimensions much smaller than the wavelength we have:93

𝛼_i(𝜔) = 𝑉 𝜀m−𝜀s

𝜀_s+𝐿_i[𝜀_m−𝜀_S] (1)

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where 𝑉 is the particle volume and 𝐿_i is an axis-specific geometrical factor, fulfilling

𝐿1+ 𝐿2+ 𝐿3 = 1.94 The optical extinction upon excitation along axis i is then given by

𝜎_i(𝜔) = 𝑘Im[𝛼i], where 𝑘 is the wave number of the incident light.93 For metals with

low imaginary permittivity, resonance (vanishing denominator) occurs approximately

at 𝜀_m = −𝜒_𝑖𝜀_𝑠, where we have introduced 𝜒_i = (1 − 𝐿_i)/𝐿_i. 𝜒_i is 2 for a sphere and

can vary significantly for other shapes, but remains positive. These features illustrate both that plasmon resonances can be tuned by geometry and also why plasmon excitation occurs specifically for metals, which can provide the negative real permittivity needed to fulfil the resonance criterion. In addition, arrays of nanoparticles and other more complex systems provide further degrees of freedom to locally control resonances and colours. This was utilized to reproduce colour

photographs with extreme resolution,15_{as well as to create polarization-dependent}

colour images.90,95

While plasmonic metasurfaces can be designed to shine in more or less any colour, it is more challenging to change their properties after fabrication. The main reason is that the materials that the plasmonic structures are based on, often gold or silver, have well-defined optical properties that are not easily modified. Besides fixed permittivity, the most common plasmonic materials and structures also do not support modulation of particle shape back and forth. On the other hand, being able to tune plasmonic metasurfaces in situ, and dynamically control optical fields and interactions at the

nanoscale could lead to entirely new scientific directions and novel applications.96_As

example, ultrathin flat metasurface lenses based on abrupt wavefront engineering could find many additional uses if they could be dynamically tuned, as recently

demonstrated for a stretchable metasurface featuring dielectric nanoantennas.97

Substantial efforts have recently been focused on achieving dynamic control of the

optical response of plasmonic metasurfaces.96_{Some interesting approaches utilized}

permittivity-modulation in materials such as ultrathin gold films,98_transparent

conducting oxides99–101_{or graphene,}102–104_{or tuning based on more exotic materials}

like polycyclic aromatic hydrocarbons105,106_{and phase-change materials.}107,108_Our

group also recently contributed by introducing organic conductive polymers as new

plasmonic materials with redox-tuneable properties.109_{Other routes focused on}

varying geometrical factors, such as periodicity of nanostructures in plasmonic

arrays110,111_{or gap size for particle dimers.}112_{In addition to stretching using}

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elastomeric substrates,110_{researchers have also explored electromechanical actuation}

for reconfigurable plasmonics.113_{Yet further directions for dynamic control of}

plasmonic systems have involved liquid crystals,14,114–117_{magnetoplasmonic}

systems,71,72,118,119_{and electrovariable nanoplasmonics at liquid-liquid interfaces.}67

More information on dynamic plasmonic systems and active metasurfaces can be

found in the recent review by Shaltout et al.96

One important application area for dynamic plasmonic metasurfaces is reflective

displays (electronic paper, e-paper).48_{Electronic displays are already responsible for a}

substantial fraction of our energy consumption and the global use of displays is inevitably increasing. Since the energy needed to drive emissive displays cannot be reduced indefinitely, we need alternative and complementary types of systems. Reflective displays can save energy by not emitting light, but instead controlling how ambient light (sun light, indoor lighting, etc.) is reflected to produce text and images. Besides energy savings, reflective displays also come with additional advantages, such that they can be used in sunny conditions. Plasmonic metasurfaces are interesting for reflective displays since they can provide control of reflected colours while maintaining

absolute reflection.48_{This is important because reflective displays are limited to using}

only the amount of light that is available from natural lighting. Motivated by this, we have explored switchable hybrid plasmonic metasurfaces for reflective displays in colour. These systems combine colourful high-reflective plasmonic metasurfaces with switchable electrochromic conducting polymer materials. The optical properties of conducting polymers can be controlled electrochemically via the redox state of the material. In brief, the redox state determines the density of charge carriers along the backbone of the conjugated polymer, which affects both the electrical conductivity

and optical transparency of the materials.120–123_{Combined with advantages such as}

low-cost, sustainability, easy processing and patterning, this has made electrochromic

polymers popular for reflective labels and displays.124–128_{One limitation, however, is}

that these electrochromic materials typically lack control of colour and primarily enable monochromic tuning. Recent research circumvented this by combining electrochromic polymers with colourful plasmonic metasurfaces, which thereby could

be turned into reflective or transmissive RGB pixels.73,75–77_{Such hybrid systems have}

potential to enable energy-efficient e-paper in colour. To facilitate low-cost sustainable devices compatible with large-scale use, we developed coloured plasmonic

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metasurfaces based on aluminium and copper instead of the more commonly used

materials gold and silver.77_{The main structure comprised an aluminium mirror and a}

plasmonic copper nanohole film, separated by an aluminium oxide spacer layer (see Fig. 1A). Resonance positions and colour of the optical nanocavity could be controlled by varying the spacer thickness, enabling red, green and blue metasurfaces (Fig. 1B) and the possibility to accurately reproduce colour images. The addition of nanoholes in the top mirror of the optical nanocavity allowed an enhancement of the colouration (except for red pixels), exploiting the resonant plasmonic excitation of the nanostructures. While these metasurface images are static, we introduced dynamic switching by screen-printing a thin film of the electrochromic polymer PEDOT:PSS (poly[3,4-ethylenedioxythiophene] doped with polystyrenesulfonate) on top of the metasurfaces. Based on its redox-tuneable optical transmission, the electrochromic layer allowed the reflection from the metasurface to be repeatedly turned on and off (see Fig. 1C-E). The electrochromic polymer provides bistability (relatively stable in both its transparent and opaque states), making the hybrid plasmonic system promising for reflective colour displays requiring ultralow energy-consumption, for use in applications ranging from billboards to smart labels and packaging. Future systems

may benefit from other types of polymers and metasurface designs,73,120_{as well}

innovative means of production.129_{There are also interesting systems that utilize}

classical optical microcavities instead of plasmonic metasurfaces, including tuneable

devices based on microelectromechanical systems130_{and phase-changing mirrors.}131

Brightness, contrast, chromaticity, viewing angles and power consumption are

some of the key parameters for colour displays48_{. While a wide range of techniques to}

obtain structural colour is available, the tunability of those structures remain challenging. In particular, the response times might be long (especially for electrochemical, chemical and phase change methods) or there could be issues with long-term stability. Brightness is also a crucial aspect, because reflective displays are restricted to working with the incident light. Many systems have the disadvantage of having a relatively low overall reflection efficiency. This is particularly problematic in light of RGB subpixel systems, for which the reflected intensity for a given colour cannot exceed 33% of the incident light. Single pixels that are tuneable throughout the entire visible range could be a promising solution to this issue.

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Figure 1. Plasmonic metasurfaces for reflective displays. (A) Schematic of the plasmonic metasurface architecture. (B) Optical micrographs of green, red and blue metasurfaces. (C) Schematic of the switchable metasurface system. (D) and (E) show spectra and optical images upon switching of the metasurfaces. Reprinted with permission from Xiong et al. Switchable Plasmonic Metasurfaces with High Chromaticity Containing Only Abundant Metals, Nano Lett. 17, 7033 (2017). Copyright 2017 American Chemical Society.

2. Bring up the heat with hybrid plasmonics

Plasmonic metasurfaces can be used as light-triggered nanoscale heat sources, which has been studied extensively and found use in many applications, ranging from photothermal therapy and solar autoclaving to energy harvesting and plasmon-driven

biomolecular thermophoresis.17,19,24,132–136_{Other examples include heat management}

of windows and novel routes for antifogging ski goggles.137_{In fact, it is challenging to}

avoid heat losses in plasmonic systems, which has triggered exploration of alternative low-loss dielectric nanoantennas for use in applications where losses pose a

problem.138_{The field of thermoplasmonics instead utilize plasmonic heating}

favourably in various novel concepts and applications.139–142_{The phenomenon is}

related to Joule heating from the optically-induced current in the metal, with local heat power density 𝑞 at arbitrary position 𝑥 inside the metal given by:

𝑞(𝑥) =1

2Re (𝐽⃗

∗_{(𝑥) ∙ 𝐸⃗⃗(𝑥))} ₍₂₎

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where 𝐸⃗⃗ is the electric field generated by the plasmonic excitation, and 𝐽⃗ is the complex amplitude of the electronic current density. This expression can be modified to:

𝑞(𝑥) =1

2𝜀0𝜔Im(𝜀m)|𝐸⃗⃗(𝑥)|

2

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where 𝜀0 is the vacuum permittivity.136,140 Equation (3) illustrates that the heat power

is proportional to the imaginary component of the metal’s permittivity and to the square of the electric field generated inside the metal nanostructure. The heat generation is due to non-radiative decay of plasmons via absorption and the total heat generation, or heat source 𝑄, for a nanostructure is given by:

𝑄 = 𝜎_𝑎𝑏𝑠𝐼 (4)

where 𝜎_𝑎𝑏𝑠 is the absorption cross section of the nanostructure and 𝐼 is the incident

light irradiance.

Notably, heat generation is different from temperature increase. The increase in temperature for a system upon plasmonic excitation is not only determined by the generated heat, but governed by the balance between generated heat and heat dissipated to the surrounding environment. Hence, one should consider the complete system, including surrounding environment and nearby nanostructures, when designing thermoplasmonic systems. As example, a plasmonic nanodisk typically generates less heat (hence, absorb less light) compared with a single plasmonic nanohole in a metal film, but illuminating the nanodisk can still result in larger local temperature increase, because the nanohole system more efficiently dissipates the

heat through the continuous metal film.143_{To further illustrate the importance of}

accounting for the whole system, the situation can be the opposite for arrays of nanostructures, for which metal nanohole arrays can provide superior heating and temperature increase over nanodisk arrays, primarily because the metal film no longer

acts as an effective heat sink.144_{For plasmonic arrays, the presence of nearby}

nanostructures highly affects the temperature increase of each single structure. In fact, the major contribution to the temperature increase for any given particle may come from collective heating from neighbouring particles rather than from heat generated

by the particle itself.16,145_{For more information and application of plasmonic heating}

we refer to recent review articles on the topic.17,136,146

Plasmonic metasurfaces can be made transparent in the visible spectral region

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while generating heat from absorption of the NIR tail of the solar spectrum.147_Such

systems form novel routes for thermal management in buildings via transparent windows with optimized energy transmission. Xu et al. recently combined this concept with electrochromic materials to create hybrid plasmonic tuneable windows, showing that the photothermal properties could also reduce the tendency of the windows

being attacked by microorganisms.75_{This forms an example of the many concepts and}

applications enabled by hybrid thermoplasmonic systems.148–150

Hybrid thermoplasmonics allows for novel concepts for energy harvesting and

radiation sensing.80,81,83,144,151,152_{Our group has explored different directions within}

this area, including the combination of plasmonic heating with ionic

thermoelectrics144,153_{and with organic pyroelectrics.}79,83_{Based on the latter, we}

designed a hybrid plasmonic metasurface for harvesting of energy from light

fluctuations.83_{The concept utilized a gold nanodisk array as thermoplasmonic}

metasurface, combined with a thin organic layer with pyroelectric properties (see Fig. 2). If the molecules in the organic layer are properly aligned (polarized), the pyroelectric material (here poly[vinylidenefluoride-co-trifluoroethylene], P[VDF-TRFE]) could translate temporal thermal fluctuations to electrical signals over the thin film. The phenomenon is related to the temperature-dependence of the permanent dipole moment over the thin film. Changes in temperature modulate the charge density on the material interfaces, in turn inducing a compensating current through an external circuit. For our hybrid thermoplasmonic device, the thermal fluctuations resulted from fluctuations in illumination intensity and corresponding thermoplasmonic heat generation. The devices could harvest energy and produce electricity from fluctuating

illumination produced by leaves swinging in the wind.83

Hybrid thermoplasmonic metasurfaces also enable new types of self-powered light and heat sensors, which could be suitable for electronic skin applications in robotics and healthcare. Our recent approach combined plasmonic heating (of a gold nanohole array) with a new concept we called thermodiffusion-assisted

pyroelectrics.79_{A pyroelectric film provided rapid transient signals upon changes in}

temperature (induced by direct heating or by plasmon-induced heating upon

irradiation), while a thermoionic gel154_{capacitively coupled to the pyroelectric part}

contributed by also providing stable signals at equilibrium. The response was not only rapid, but the stable signal was also found significantly enhanced compared with the

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value expected from the pure thermoelectric response.

The hybrid thermoplasmonics concepts presented above exemplify that optical losses in plasmonic systems can be advantageous and enable novel applications and solutions when combined with other materials in hybrid systems. There are also challenges remaining that need further work, not least regarding improving efficiency of the thermoplasmonic-based energy harvesting concept in order to move from proof-of-concept to more practical useful devices. In that regard, sensor applications have already shown promise in terms of performance. Compared with light-induced heating by non-plasmonic materials, we believe future work on hybrid thermoplasmonics will benefit from further utilizing the spectral tunability of plasmonic systems, such as designing transparent devices heated by the infrared tail of the sun. The ultralow thickness of plasmonic metasurfaces forms another strength that could be particularly valuable in low-weight applications, for example, in certain robotics and space applications.

Figure 2. (A) Schematic illustration of a hybrid plasmonic device for producing electricity from light fluctuations. (B) Power density generated by a non-polarized hybrid device (green line) and a polarized hybrid device (black line) upon controlled light fluctuations (from light fluctuations produced using simulated sun light and a leaf as shutter, using a 9 MΩ resistor as load). (C) Same as in (B) but for random light

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fluctuations produced by letting the leaf swing in the wind of a fan. Copyright 2018 Wiley. Used with permission from Chaharsoughi et al. Switchable Plasmonic Metasurfaces with High Chromaticity Containing Only Abundant Metals, Advanced Optical Materials 6, 1701051 (2018).

3. Hybrid systems based on strong coupling

The examples of hybrid plasmonic metasurfaces above combine features provided by a plasmonic metasurface (e.g. colouration, light-induced heating) with functionalities provided by a second system (e.g. modulation of transparency, generation of electricity), thereby enabling novel devices and applications. Another promising research direction instead focuses on how molecules and plasmonic cavities affect each other on the fundamental level due to their mere proximity. Placing molecules close to a plasmonic structure or other optical cavity forms an exciting route to control the functions of molecules without changing their structure, enabling exotic concepts

such as long-range energy transfer,155–157_{low-threshold polariton lasing,}158

Bose-Einstein condensation,159_{and superfluidity.}160_{The concept is based on coupling}

between molecules (or other entities with strong transition dipole moments) and a plasmonic cavity via spontaneous exchange of energy between molecular transitions and cavity resonances. If the coupling is sufficiently strong, the optical and molecular resonances hybridize to new light-matter states. This regime is referred to as strong

coupling, which splits the initial molecular transition ( ℏωmolecule ) into two new

polariton states ( P± ) that are separated by the vacuum Rabi splitting ( ℏΩR , see

illustration in Fig. 3A). The vacuum Rabi splitting is determined by the number of molecules contributing to the coupling (𝑁), the molecular transition dipole moment

(𝑑⃗) and the vacuum electric field of the plasmonic cavity (𝐸⃗⃗):161

ℏΩ_R= 2√N d⃗⃗ ∙ E⃗⃗⃗ (5)

The fact that ℏΩ_R is proportional to d makes organic materials favourable for strong

coupling since they typically have strong transition dipole moments.162_{Introducing the}

expression for the vacuum electric field further gives:69,163

ℏΩ_R= 2√Nd√_2εεℏω

0V (6)

where we have assumed that the molecules and the cavity are properly aligned for

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maximum coupling. 𝑉 is the mode volume of the optical cavity, ℏω is the

resonance energy, ε0 is the vacuum permittivity, ε is the relative permittivity of the

surrounding and ℏ is the reduced Planck’s constant. The fact that ℏΩR increases

with decreasing 𝑉 makes plasmonic systems particularly suitable for strong coupling due to their capability to squeeze optical fields into ultrasmall mode volumes. Indeed, Chikkaraddy et al. recently utilized a plasmonic nanocavity to achieve strong coupling

even for single molecules (𝑁 = 1) at room temperature.164_{For systems that instead}

involve many molecules (𝑁 > 1), we note that the coupling process not only produces the two polariton states, but also (𝑁 − 1) dark states that do not couple to light and

that remain at the original energy level of the molecules (see Fig. 3A).165_{For the many}

practical applications that involve a large number of coupled molecules, these dark subradiant states thus heavily outnumber the two radiant upper and lower polariton states. The roles of dark states in strong coupling applications are not yet fully

understood and forms an interesting area for further research.69_{Recent reports, for}

example, suggest that also the dark states may possess polaritonic properties such as

delocalized character.166_{We also note that Eq. 6 does not contain the intensity of any}

light source, illustrating that the polariton formation originates from coupling of the

molecules to the vacuum electromagnetic field of the cavity.84_{The phenomenon can}

thereby be exploited for non-optical applications as well. To mention some interesting examples, researchers have explored influence of strong coupling on chemical

reactivity,167–170_{ground state thermodynamics,}171,172_{work functions,}173_and

long-range transport of charges.64

As mentioned above, single plasmonic nanocavities can enable coupling of few or even single molecules owing to their small mode volumes. Plasmonic metasurfaces composed of ensembles of metal nanostructures can instead provide resonances that are delocalized over larger areas and thereby be interesting for different macroscopic

applications of strong coupling.70,174_{Compared to more commonly used Fabry Perot}

cavities, such plasmonic metasurfaces are not closed by mirrors, and thereby provide physical access to the strongly coupled molecules. On the other hand, plasmonic metasurfaces can also have features that for some applications may be undesirable, such as low quality factors and vacuum electric fields that are localized at metal interfaces and less uniform compared with more conventional microcavities. Metal nanohole arrays form a class of plasmonic metasurfaces that can provide delocalized

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plasmonic modes, via excitation of surface plasmon polaritons (SPPs) propagating at

the interfaces of the metal film.175–177_{We have studied plasmonic nanohole arrays for}

various applications,79,144,178_{and recently also explored hybrid systems based on metal}

nanohole films coupled to organic J-aggregates.179–181_{Based on clear anticrossing}

behaviour and Rabi splitting proportional to the square root of molecular concentration, we could conclude that the hybrid system was in the strong coupling

regime, with Rabi splitting reaching several hundred milli-electron volts.179

Furthermore, the polariton modes inherited the delocalized nature of the original plasmons of the nanohole films. Interestingly, we also found that the spectral positions

of the upper and lower polaritons (P+ and P-) did not match peaks appearing in optical

extinction spectra (compare the blue curves in Figure 3B). Compared with the polariton positions, as determined by integrating sphere absorption measurements, the extinction peaks (hence, transmission dips) were all significantly red-shifted. This result pinpoints an important aspect of systems comprising both resonant states and non-resonant continuum states, which leads to Fano interference that modifies

transmission and reflection spectra.177,182–184_{In that sense, we note that the same}

behaviour occurs also for bare (non-hybrid) plasmonic nanohole metasurfaces.184

Depending on details like hole dimensions and film thickness, resonances can be centred in the middle between transmission peaks and dips, highlighting the importance of using absorption measurements (e.g. using integrating sphere) to identify resonances in these systems.

Plasmonic nanoparticle arrays form another interesting class of metasurfaces that can provide optical modes with delocalized nature. In contrast to metal nanohole arrays, a metal particle array naturally does not contain continuous metal interfaces for propagation of conventional SPPs. However, certain nanoparticle arrays can still sustain delocalized surface lattice resonances (SLRs) through localized plasmon

resonances that are diffractively coupled to the array.174,185,186_{Proper choice of}

nanoparticle size, shape and periodicity can lead to extremely high quality factors and correspondingly narrow resonances compared with those of other plasmonic

systems.174,187_{Interestingly, SLRs share characteristics of surface plasmon polaritons}

on metal-dielectric interfaces, and can propagate over several periods in the array.188

Regarding hybrid systems, coupling of SLRs to molecular excitons189,190_{can lead to}

long-range spatial coherence lengths of micrometers also for the strongly coupled

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regime, indicating delocalized nature also of the hybrid polariton states.191_Compared

with plasmonic nanohole arrays, nanoparticle arrays contain much less metal per area (assuming same thickness) and therefore show lower material absorption and higher direct transmission. As a result, SLR systems show only minimal Fano interference effects and resonances can be determined by the positions of dips observed in

transmission spectra.184,192_{Nanohole arrays instead provide advantages such as}

superior light-to-heat conversion and capability to act as electrodes.144

Hybrid plasmonic metasurfaces with delocalized polariton states have been explored for exotic macroscopic optical and non-optical phenomena, where manipulation of transport in organic thin films form an interesting example. Orgiu et al. studied aromatic diimide-based organic semiconducting polymers coupled with plasmonic nanohole arrays and reported enhancement of charge transport due to

strong coupling.64_{Long-range polariton transport facilitated by strong coupling has}

been reported for non-plasmonic optical cavity systems193_{and enhancement of both}

exciton transport194_{and charge transport}195_{have been addressed theoretically.}

However, other reports did not observe enhancement in charge transport upon strong coupling, including for systems based on microcavities coupled to p-type organic

polymers,196_{carbon nanotubes,}197_{or high mobility ambipolar polymers,}198_{as well as}

for hybrid plasmonic metasurfaces based on SLRs coupled with ambipolar polymers.199

Hence, it is currently not known whether enhancement of charge transport is limited to only certain types of strongly coupled systems, to our knowledge so far only reported for plasmonic nanohole metasurfaces, or if the concept is more general and possible to utilize more widely. In turn, this makes effects of strong coupling on charge transport an interesting topic for further studies, including investigation of the roles of

dark states and their potential delocalized nature.69,166_{Overall, hybrid strongly coupled}

plasmonic metasurfaces is an intriguing research area with several interesting unexplored directions and phenomena that are not yet fully understood, while also holding great potential for important (room-temperature) applications, including

electrical pumping,197_{low-threshold polariton lasers,}200,201_{optical logic circuits}202_and

quantum polaritonic devices.203,204

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Figure 3. (A) Schematic diagram of strong coupling between molecule resonance and plasmon resonance, forming new hybrid polariton states separated by Rabi splitting of

ℏΩ_R. (B) Extinction (upper curves) and absorption (lower curves) spectra for TDBC (red), nanohole film (grey) and hybrid system (blue). Vertical dashed lines designate polariton resonance energies, as determined by peak positions in the absorption spectrum. Reprinted with permission from Kang et al. Strong Plasmon–Exciton Coupling with Directional Absorption Features in Optically Thin Hybrid Nanohole Metasurfaces, ACS Photonics. 5, 4046 (2018). Copyright 2017 American Chemical Society.

4. Summary

We hope that this perspective illustrates how hybrid plasmonic metasurfaces enable unique studies and applications beyond what is feasible with non-hybrid systems. While focused on three prime directions – somewhat biased to our own work – other plasmonic functionalities like hot electron production are also highly relevant

for hybrid plasmonic metasurfaces.205,206_{The perspective also focuses on systems}

where plasmonic metasurfaces are combined with organic materials, while interesting current and future research directions also features other materials, such as transition

metal dichalcogenides,207_MXenes,208,209_perovskites,210_{as well as inorganic}

electrochromic materials211_{and semiconductors.}41,85,212_{Likewise, not only}

plasmonic, but also dielectric metasurfaces based on high-index optical nanoantennas hold great promise for contributing to the field of hybrid metasurfaces, in particular

for applications requiring low losses.97,138,213,214_{Future studies and applications of}

hybrid metasurfaces may also cover wavelength ranges beyond the visible or near-infrared part of the spectrum, including the thermal emissivity range in the far

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infrared66,126,215_{as well as the THz range.}102,216_{These applications and studies could}

benefit particularly from the use of non-traditional plasmonic materials, including

transparent conducting oxides,100,217–219_graphene220–222_{and other 2D materials,}209,223

and even organic systems like redox-tuneable conductive polymers.109

Acknowledgements

The authors acknowledge funding from the Wenner-Gren Foundations, the Swedish Research Council, the Swedish Foundation for Strategic Research, and Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU o 2009 00971).

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