Electromagnetically induced modification of gold optical properties

The reflection of light from a metal film, i.e., a mirror, is among the most fundamental and well-understood effects in optics. If the film thickness is greater than the wavelength, reflection is strong and is explained in simple terms by the Fresnel equations. For film thickness much less than the wavelength, reflection is far weaker and more exotic effects become possible. This is especially so if the light illuminating the film is pulsed at the femtosecond time scale. In this work, a phenomenon is proposed where few-femtosecond laser pulses temporarily modify a thin metal film's optical properties via processes that appear linear and classical in nature. By casting a pulsed standing-wave pattern across the metal surface, we consider the possibility that conduction electrons are redistributed to create temporary regions of partly enhanced or reduced density without the excitation of inter-band transitions. The process would constitute a temporary change to the conductivity of the metal, and thus, may be observable as changes to the metal's transmittance and reflectance. In regions where the density is enhanced (reduced), the transmittance is decreased (increased). The concept is termed Electromagnetically Induced Modification (EIM) and is premised on the fact that the pulse length is shorter than the relaxation time of the conduction electrons. An experiment is conducted to test the concept by measuring the change in reflectance and transmittance of gold films with thickness ranging from 20-300 Angstrom. The results show that the film's transmittance decreases only when the standing-wave pattern is present. As the pulse length is increased, or as the film thickness is increased, the changes disappear. The changes show little dependence on the pulse intensity as it is varied by a factor of two. To gain further insight, the Drude theory is used to develop a simplified model for EIM, which qualitatively agrees with the observations. However, neither the experiment nor the model can prove the validity of the EIM concept. As such, an assessment is made for the potential of alternative well-known processes to explain the observations.

Extraordinary transmission of gold-capped sphere arrays in mid-infrared range

We report an extraordinary transmission (EOT) of gold-capped silica microsphere monolayers in the mid-infrared range of 5-25 μm. The observed transmittance is significantly greater than that of a flat gold film with the same thickness, although the surface of the microsphere monolayer appeared to be completely covered with gold when observing from above. The calculations based on the finite difference time domain method indicate that light passes through the openings between the gold coating on the substrate and that on the microspheres. The EOT-type studied here occurs over the mid-infrared wavelength range, thus indicating that it is not attributable to the resonance of the surface plasmons. This type of EOT is absent in the visible and near infrared wavelength range, where gold does not function as an ideal metal. In addition, spectral modification originating from localized phonon polariton resonance (LPhPR) in silica microsphere is observed. LPhPR can be interpreted based on the analogy of the localized surface plasmon resonance in metallic nanospheres, in the visible or ultraviolet wavelength range.

Amplification of stimulated light emission in arrays of nanoholes by plasmonic absorption-induced transparency

Absorption induced transparency is an optical phenomenon that occurs in plasmonic nanostructures when materials featuring narrow lines in their absorption spectra are deposited on top of it. First reported in the visible range for metallic arrays of nanoholes, using dye lasers as covering, it has been described as transmission peaks unexpectedly close to the absorption energies of the dye. In this work, amplification of stimulated light emission is numerically demonstrated in the active regime of absorption induced transparency. Amplification can be achieved in the regime where the dye laser behaves as a gain material. Intense illumination can modify the dielectric constant of the gain material in a short span of time and thus the propagation properties of the plasmonic modes excited in the hole arrays, providing both less damping to light and further optical feedback that enhances the stimulated emission process.

Strong Plasmon-Exciton Coupling in Ag Nanoparticle-Conjugated Polymer Core-Shell Hybrid Nanostructures

Strong plasmon–exciton coupling between tightly-bound excitons in organic molecular semiconductors and surface plasmons in metal nanostructures has been studied extensively for a number of technical applications, including low-threshold lasing and room-temperature Bose-Einstein condensates. Typically, excitons with narrow resonances, such as J-aggregates, are employed to achieve strong plasmon–exciton coupling. However, J-aggregates have limited applications for optoelectronic devices compared with organic conjugated polymers. Here, using numerical and analytical calculations, we demonstrate that strong plasmon–exciton coupling can be achieved for Ag-conjugated polymer core-shell nanostructures, despite the broad spectral linewidth of conjugated polymers. We show that strong plasmon–exciton coupling can be achieved through the use of thick shells, large oscillator strengths, and multiple vibronic resonances characteristic of typical conjugated polymers, and that Rabi splitting energies of over 1000 meV can be obtained using realistic material dispersive relative permittivity parameters. The results presented herein give insight into the mechanisms of plasmon–exciton coupling when broadband excitonic materials featuring strong vibrational–electronic coupling are employed and are relevant to organic optoelectronic devices and hybrid metal–organic photonic nanostructures.

Molecular methylation detection based on terahertz metamaterial technology

Terahertz wave has a good ability to identify biomolecules due to its fingerprint spectrum characteristics. However, the minimum detectable limit of terahertz technology by the conventional tablet pressing method is on the order of milligrams, which cannot meet the application requirements of low concentration detection in the biomedical field-near or below micrograms. Here, we proposed a method to enhance the detection sensitivity by designing a metamaterial chip with the absorption-induced transparency (AIT) effect, which can enhance the interaction between terahertz waves and biomolecules and lower the detectable limit. Taking 7-methylguanine (7-MG) as an example, based on its terahertz characteristic absorption peak, we designed a split-ring resonator (SRR) metamaterial chip, which has the advantages of high sensitivity, unlabeled detection, fast response and simple measurement. Its quantitative detection limit can reach 6.30 μg, which is about 500 times smaller than that of the traditional tablet pressing method (2.95 mg). In addition, for methylated and unmethylated substances, the chip exhibits different frequency shifts, which also realizes the qualitative identification effectively. These results provide a reference for the rapid and accurate diagnosis of diseases associated with molecular methylation in clinical medicine.

Extracting Electronic Transition Bands of Adsorbates from Molecule-Plasmon Excitation Coupling

The coupling between molecular electronic and particle plasmon excitations can result in various intriguing outcomes depending on how strongly or weakly the excitations couple to compete with their respective decay rates. In this work, using methylene blue and thionine dyes as model systems, we show that the electronic absorption band of resonant adsorbates can be determined with submonolayer sensitivity from the weak molecule-plasmon excitation coupling that results in the attenuation on the plasmonic absorption band. The extracted spectra are strongly similar to the absorption spectra of the corresponding molecules in solution, apart from the expected spectral red-shift and broadening. Interestingly, the adsorption isotherms determined on the basis of the magnitude of the attenuation correlate linearly with that determined from the adsorbate-induced plasmon resonance red-shift. The results demonstrate that in the weak coupling regimes the plasmon modes can be considered as an environment that supplies energy to and takes energy from the adsorbates.

Optics of exciton-plasmon nanomaterials

This review provides a brief introduction to the physics of coupled exciton-plasmon systems, the theoretical description and experimental manifestation of such phenomena, followed by an account of the state-of-the-art methodology for the numerical simulations of such phenomena and supplemented by a number of FORTRAN codes, by which the interested reader can introduce himself/herself to the practice of such simulations. Applications to CW light scattering as well as transient response and relaxation are described. Particular attention is given to so-called strong coupling limit, where the hybrid exciton-plasmon nature of the system response is strongly expressed. While traditional descriptions of such phenomena usually rely on analysis of the electromagnetic response of inhomogeneous dielectric environments that individually support plasmon and exciton excitations, here we explore also the consequences of a more detailed description of the molecular environment in terms of its quantum density matrix (applied in a mean field approximation level). Such a description makes it possible to account for characteristics that cannot be described by the dielectric response model: the effects of dephasing on the molecular response on one hand, and nonlinear response on the other. It also highlights the still missing important ingredients in the numerical approach, in particular its limitation to a classical description of the radiation field and its reliance on a mean field description of the many-body molecular system. We end our review with an outlook to the near future, where these limitations will be addressed and new novel applications of the numerical approach will be pursued.

Realization of Fractal-Inspired Thermoresponsive Quasi-3D Plasmonic Metasurfaces with EOT-Like Transmission for Volumetric and Multispectral Detection in the Mid-IR Region

We use a paradigmatic mathematic model known as Sierpiński fractal to reverse-engineer artificial nanostructures that can potentially serve as plasmonic metasurfaces as well as nanogap electrodes. Herein, we particularly demonstrate the possibility of obtaining multispectral extraordinary optical transmission-like transmission peaks from fractal-inspired geometries, which can preserve distinct spatial characteristics. To achieve enhanced volumetric interaction and thermal responsiveness within the framework, we consider a bilayer, quasi-three-dimensional (3D) configuration that relies on the unique approach of combining complementary and noncomplementary surfaces, while avoiding the need for multilayer alignment on the nanoscale. We implement an improved version of the model to (1) increase the volume of quasi-3D nanochannels and enhance the lightening-rod effect of the metasurfaces, (2) harness cross-coupling as a mechanism for achieving better sensitivity, and (3) exploit optical magnetism for pushing the resonances to longer wavelengths on a miniaturized platform. We further demonstrate vertical coupling as an effective route for ultimate miniaturization of such quasi-3D nanostructures. We report a wavelength shift up to 1666 nm/refractive index unit and 2.5 nm/°C, implying the usefulness of the proposed devices for applications such as dielectrophoretic sensing and nanothermodynamic study of molecular reactions in the chemically active mid-IR spectrum.

Transfer and control of the orientation of 3D nanostructures fabricated by nanoskiving

This communication describes the transfer and stacking of slabs of epoxy containing nanostructures fabricated by nanoskiving. Removal of the sacrificial layer and epoxy matrices produces 3D nanostructures via control over the position and alignment of each sequential layer. The process uses mild etchants and transfers the nanoskived features without wrinkling or damage. We demonstrate the utility of this method of transfer with aligned and intersecting nanowires and arrays of nano-holes and crescents.

Electronically tunable extraordinary optical transmission in graphene plasmonic ribbons coupled to subwavelength metallic slit arrays

Subwavelength metallic slit arrays have been shown to exhibit extraordinary optical transmission, whereby tunnelling surface plasmonic waves constructively interfere to create large forward light propagation. The intricate balancing needed for this interference to occur allows for resonant transmission to be highly sensitive to changes in the environment. Here we demonstrate that extraordinary optical transmission resonance can be coupled to electrostatically tunable graphene plasmonic ribbons to create electrostatic modulation of mid-infrared light. Absorption in graphene plasmonic ribbons situated inside metallic slits can efficiently block the coupling channel for resonant transmission, leading to a suppression of transmission. Full-wave simulations predict a transmission modulation of 95.7% via this mechanism. Experimental measurements reveal a modulation efficiency of 28.6% in transmission at 1,397 cm⁻¹, corresponding to a 2.67-fold improvement over transmission without a metallic slit array. This work paves the way for enhancing light modulation in graphene plasmonics by employing noble metal plasmonic structures.

Graphene-plasmon optical modulators with broadly tunable operating frequencies are sought for photonic applications. Here, Kim et al. demonstrate tunable mid-infrared transmission that utilizes resonant absorption in graphene ribbons to modulate the extraordinary optical transmission effect in metallic slit arrays.

Waveguide and Plasmonic Absorption-Induced Transparency

Absorption-induced transparency (AIT) is one of the family of induced transparencies that has emerged in recent decades in the fields of plasmonics and metamaterials. It is a seemingly paradoxical phenomenon in which transmission through nanoholes in gold and silver is dramatically enhanced at wavelengths where a physisorbed dye layer absorbs strongly. The origin of AIT remains controversial, with both experimental and theoretical work pointing to either surface (plasmonic) or in-hole (waveguide) mechanisms. Here, we resolve this controversy by carefully filling nanoholes in a silver film with dielectric material before depositing dye on the surface. Our experiments and modeling show that not only do plasmonic and waveguide contributions to AIT both exist, but they are spectrally identical, operating in concert when the dye is both in the holes and on the surface.

High-Throughput Fabrication of Resonant Metamaterials with Ultrasmall Coaxial Apertures via Atomic Layer Lithography

We combine atomic layer lithography and glancing-angle ion polishing to create wafer-scale metamaterials composed of dense arrays of ultrasmall coaxial nanocavities in gold films. This new fabrication scheme makes it possible to shrink the diameter and increase the packing density of 2 nm-gap coaxial resonators, an extreme subwavelength structure first manufactured via atomic layer lithography, both by a factor of 100 with respect to previous studies. We demonstrate that the nonpropagating zeroth-order Fabry-Pérot mode, which possesses slow light-like properties at the cutoff resonance, traps infrared light inside 2 nm gaps (gap volume ∼ λ(3)/10(6)). Notably, the annular gaps cover only 3% or less of the metal surface, while open-area normalized transmission is as high as 1700% at the epsilon-near-zero (ENZ) condition. The resulting energy accumulation alongside extraordinary optical transmission can benefit applications in nonlinear optics, optical trapping, and surface-enhanced spectroscopies. Furthermore, because the resonance wavelength is independent of the cavity length and dramatically red shifts as the gap size is reduced, large-area arrays can be constructed with λresonance ≫ period, making this fabrication method ideal for manufacturing resonant metamaterials.

Absorption-induced transparency metamaterials in the terahertz regime

Contrary to what might be expected, when an organic dye is sputtered onto an opaque holey metal film, transmission bands can be observed at the absorption energies of the molecules. This phenomenon, known as absorption-induced transparency, is aided by a strong modification of the propagation properties of light inside the holes when filled by the molecules. Despite having been initially observed in metallic structures in the optical regime, new routes for investigation and applications at different spectral regimes can be devised. Here, to illustrate the potential use of absorption-induced transparency at terahertz, a method for molecular detection is presented and supported by a theoretical analysis.

Template-Stripped Tunable Plasmonic Devices on Stretchable and Rollable Substrates

We use template stripping to integrate metallic nanostructures onto flexible, stretchable, and rollable substrates. Using this approach, high-quality patterned metals that are replicated from reusable silicon templates can be directly transferred to polydimethylsiloxane (PDMS) substrates. First we produce stretchable gold nanohole arrays and show that their optical transmission spectra can be modulated by mechanical stretching. Next we fabricate stretchable arrays of gold pyramids and demonstrate a modulation of the wavelength of light resonantly scattered from the tip of the pyramid by stretching the underlying PDMS film. The use of a flexible transfer layer also enables template stripping using a cylindrical roller as a substrate. As an example, we demonstrate roller template stripping of metallic nanoholes, nanodisks, wires, and pyramids onto the cylindrical surface of a glass rod lens. These nonplanar metallic structures produced via template stripping with flexible and stretchable films can facilitate many applications in sensing, display, plasmonics, metasurfaces, and roll-to-roll fabrication.

Nanoscale Single-Element Color Filters

Visible-light filters constructed from nanostructured materials typically consist of a metallic grating and rely on the excitation of surface plasmon polaritons (SPPs). In order to operate at full efficiency, the number of grating elements needs to be maximized such that light can couple more efficiently to the SPPs through improved diffraction. Such conditions impose a limitation on the compactness of the filter since a larger number of grating elements represents a larger effective size. For emerging applications involving nanoscale transmitters or receivers, a device that can filter localized excitations is highly anticipated but is challenging to realize through grating-type filters. In this work, we present the design of an optical filter operating with a single element, marking a departure from diffractive plasmonic coupling. Our device consists of a ZnO nanorod enclosed by two layers of Ag film. For diffraction-limited light focused on the nanorod, narrow passbands can be realized and tuned via variation of the nanorod diameter across the visible spectrum. The spectral and spatial filtering originates from scattering cancellation localized at the nanorod due to the cavity and nanorod exhibiting opposite effective dipole moments. This ability to realize high-performance optical filtering at the ultimate size introduces intriguing possibilities for nanoscale near-field communication or ultrahigh resolution imaging pixels.

Absorption-induced scattering and surface plasmon out-coupling from absorber-coated plasmonic metasurfaces

Interactions between absorbers and plasmonic metasurfaces can give rise to unique optical properties not present for either of the individual materials and can influence the performance of a host of optical sensing and thin-film optoelectronic applications. Here we identify three distinct mode types of absorber-coated plasmonic metasurfaces: localized and propagating surface plasmons and a previously unidentified optical mode type called absorption-induced scattering. The extinction of the latter mode type can be tuned by controlling the morphology of the absorber coating and the spectral overlap of the absorber with the plasmonic modes. Furthermore, we show that surface plasmons are backscattered when the crystallinity of the absorber is low but are absorbed for more crystalline absorber coatings. This work furthers our understanding of light–matter interactions between absorbers and surface plasmons to enable practical optoelectronic applications of metasurfaces.

Plasmonic surfaces are used as two-dimensional metamaterials for light manipulation on nanoscale, and their optical properties can be further tuned by coating. Here the authors report a new absorption-induced scattering mode in a silver nanoparticle array coated with semiconducting organic absorbers.

Ultrastrong plasmon-exciton coupling in metal nanoprisms with J-aggregates

In this Letter, ultrastrong plasmon-exciton coupling in an Ag nanoprism-J-aggregate hybrid nanostructure is reported. A localized surface plasmon wavelength of Ag nanoprisms is tunable starting from 400 to 1100 nm. Because of the large electric field localization at the corners of the nanoprisms, the observed Rabi splitting energy is higher than the previously reported Rabi splitting energies using metal nanoparticles. A giant Rabi splitting energy of more than 400 meV corresponding to ~19% of the j-band energy has been observed, thus indicating the ultrastrong coupling regime. The hybrid nanostructure of nanoprism-J-aggregate is easy to prepare in large quantities and it can be uniformly assembled on solid substrates.

Strong light-molecule coupling on plasmonic arrays of different symmetry

The strong coupling of porphyrin J-aggregates to plasmonic nanostructures of different symmetry is investigated. The nanostructures of higher symmetry show the strongest interaction with the molecular layer, suggesting that surface plasmon mode degeneracy plays an important role in the coupling efficiency. At high coupling strengths a new, weakly dispersive mode appears which has recently been predicted theoretically to be due to long-range energy transfer between molecules mediated by surface plasmons. These findings point to new ways for optimizing strong coupling and thereby realize its full potential for molecular and material science.

Strong coupling between molecular excited states and surface plasmon modes of a slit array in a thin metal film

We demonstrate strong coupling between molecular excited states and surface plasmon modes of a slit array in a thin metal film. The coupling manifests itself as an anticrossing behavior of the two newly formed polaritons. As the coupling strength grows, a new mode emerges, which is attributed to long-range molecular interactions mediated by the plasmonic field. The new, molecular-like mode repels the polariton states, and leads to an opening of energy gaps both below and above the asymptotic free molecule energy.

Plasmonic systems unveiled by Fano resonances

We show in detail how a derivation of Fano theory can serve as a new paradigm to study, understand, and control the interaction of nano-objects with light. Examples include a plasmonic crystal, a dolmen-type structure sustaining dark and bright plasmon modes, and a nanoshell heptamer. On the basis of only three coupling factors, a straightforward analytical formula is obtained, only assuming a plasmonic resonance for the continuum, and retaining the nonclassical character of the original formalism. It allows one to predict, reproduce, or decompose Fano interferences solely in terms of the physical properties of the uncoupled nanostructures when available, without the need of additional fitting parameters.

Polarization-induced tunability of localized surface plasmon resonances in arrays of sub-wavelength cruciform apertures

We demonstrate experimentally that by engineering the structural asymmetry of the primary unit cell of a symmetrically nanopatterned metallic film the optical transmission becomes strongly dependent on the polarization of the incident wave. By considering a specific plasmonic structure consisting of square arrays of nanoscale asymmetric cruciform apertures we show that the enhanced optical anisotropy is induced by the excitation inside the apertures of localized surface plasmon resonances. The measured transmission spectra of these plasmonic arrays show a transmission maximum whose spectral location can be tuned by almost 50% by simply varying the in-plane polarization of the incident photons. Comprehensive numerical simulations further prove that the maximum of the transmission spectra corresponds to polarization-dependent surface plasmon resonances tightly confined in the two arms of the cruciform aperture. Despite this, there are isosbestic points where the transmission, reflection, and absorption spectra are polarization-independent, regardless of the degree of asymmetry of the apertures.

Design of a perfect black absorber at visible frequencies using plasmonic metamaterials

The design and fabrication of a plasmonic black absorber with almost 100% absorbance spanning a broad range of frequencies from ultraviolet (UV) to the near infrared (NIR) is demonstrated. The perfect plasmonic absorber is achieved by a combination of a metal film with suitable metal/dielectric nanocomposites. Our fabrication technique is simple, versatile, cost-effective, and compatible with current industrial methods for solar absorber production.

Hybridization induced transparency in composites of metamaterials and atomic media

We report hybridization induced transparency (HIT) in a composite medium consisting of a metamaterial and a dielectric. We develop an analytic model that explains HIT by coherent coupling between the hybridized local fields of the metamaterial and the dielectric or an atomic system in general. In a proof-of-principle experiment, we evidence HIT in a split ring resonator metamaterial that is coupled to α-lactose monohydrate. Both, the analytic model and numerical calculations confirm and explain the experimental observations. HIT can be considered as a hybrid analogue to electromagnetically induced transparency (EIT) and plasmon-induced transparency (PIT).