A tunable omni-directional sensing platform: strong light-matter interactions enabled by graphene

In this theoretical work, we report on voltage-controllable hybridization of electromagnetic modes arising from strong interaction between graphene surface plasmons and molecular vibrations. The interaction strength depends strongly on the volume density of molecular dipoles, the molecular relaxation time, and the molecular layer thickness. Graphene offers much tighter plasmonic field confinement and longer carrier relaxation time compared to noble metals, leading to Rabi splitting and hybridized polaritonic modes at three-orders-of-magnitude lower molecular densities. Electrostatically tunable carrier density in graphene allows for dynamic control over the interaction strength. In addition, the flat dispersion band above the light line arising from the deep confinement of the polaritonic modes gives rise to the omnidirectional excitation. Our approach is promising for practical implementations in infrared sensing and detection.

Graphene is a monolayer of carbon atoms constructing a two-dimensional honeycomb structure, and it has an excellent carrier mobility and a very high thermal conductivity. Remarkably, it has been experimentally demonstrated that a monolayer graphene exhibits an exotic optical properties. To be specific, the plasmonic dispersion relation of a transverse magnetic graphene plasmon is electronically tunable by adjusting carrier density in graphene with external gate bias, and graphene plasmonic nano cavities have been utilized to modulate mid-infrared light. In this thesis, we present how to efficiently modulate mid-infrared light by combining graphene plasmonic ribbons with noble metal plasmonic structures. First, we propose and demonstrate electronically tunable resonant perfect absorption in graphene plasmonic metasurface enhanced by noble metal plasmonic effect, which results in modulating reflecting light. In this device, we improve coupling efficiency of free-space photons into graphene plasmons by reducing wavevector mismatching with a low permittivity substrate. In addition, the graphene plasmonic resonance is significantly enhanced by plasmonic light focusing effect of the coupled subwavelength metallic slit structure, which results in strongly fortifying resonance absorption in the graphene plasmonic metasurface. In the proposed device, theoretical calculation expects that perfect absorption in the graphene plasmonic metasurface is achievable with low graphene carrier mobility. We also present an analytical model based on surface admittance in order to fully understand how this enhancement occurs. In the second device, we propose and demonstrate a transmission type light modulator by combining graphene plasmonic ribbons with subwavelength metal slit arrays. In this device, extraordinary optical transmission resonance is coupled to 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. This phenomenon is also interpreted by anti-crossing between the graphene plasmonic resonance in the ribbons and the noble metal plasmonic resonance in the subwavelength metal slit arrays. Finally, we devise a platform to demonstrate graphene plasmonic resonance energy transport along graphene plasmonic ribbons. In this device, two metal-insulator-metal waveguides are connected by a subwavelength metal slit, and graphene plasmonic ribbons are located inside this slit. Due to the large impedance mismatch at the junction, light coupling efficiency across the junction is poor. If the graphene plasmonic ribbons are tuned to support strong graphene plasmonic resonances, the light energy can be transferred via graphene plasmons along the ribbons, and it leads to significant improvement in the light coupling efficiency across the junction. In addition to enhanced light coupling efficiency, we also present how to totally suppress the transmission by inducing a Fano resonance between a non-resonant propagation mode across the junction and a resonant graphene plasmonic transport mode, which can be utilized to efficiently modulate light in a noble metal plasmonic waveguide with the graphene plasmon resonance energy transfer.

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene has been found to support plasmons in a wide range from infrared to terahertz. The confinement of plasmons in graphene is stronger than that on metallic surface. Moreover, the plasmon properties can be dynamically adjusted by doping or grating graphene. In this study, a composite structure comprised of graphene and subwavelength grating is proposed. Highly confined plasmons in graphene are excited by using a diffraction grating with guided mode resonance effect. The wave vector of plasmonic wave in graphene is far larger than that of light in vacuum. To excite plasmons in graphene with a freespace optical wave, their large difference in wave vector must be overcome. Optical gratings are widely used to compensate for wave vector mismatches. A diffraction wave generated by the grating structure can overcome the large wave vector difference and excite surface plasmons. The guided-mode resonance can greatly enhance the intensity of the diffraction field and the coupling efficiency between graphene and incident light. When the phase matching between illuminating wave and a guide mode supported by grating is achieved, guided-mode resonance effect occurs. A nearly 100% diffraction efficiency peak in the reflection or transmission spectrum occurs at a certain wavelength. In this study, the influences of graphene and grating structure on the local characteristics (the surface electric field Ex/Ein, quality factor Q, and effective mode area Seff) of surface plasmons are investigated. The effects of the structural parameters (the thickness of the buffer layer T2, the grating period p, the carrier mobility , and the Fermi level EF) on localization properties are analyzed by the finite element method (COMSOL). The results reveal that the localizations of the surface plasmons in the graphene surface is significantly improved at the certain parameters. 1) The increase of T2 will reduce the intensity of electric field on graphene (Ex/Ein), but the quality factor will obtain a certain increase. The excition of highly confined SPPs needs to improve Q and keep the intensity of Ex/Ein, so in this study T2 = 10 nm. 2) By adjusting the quality factor of SPPs can be improved significantly without changing the resonance frequency ( = 0.7 m2(Vs), Qmax = 1793). 3) Small changes in p and EF will make the resonance peak shift obviously, and the electric field on graphene is greatly enhanced (p = 235 nm, Ex/Ein = 3154; EF = 0.72 eV, and Ex/Ein = 3968). Strong localization leads to strong light-matter interaction, and thus the proposed structure has the potential to be used as sensors with high sensitivity and high-efficiency nonlinear optical devices, greatly expanding the application of graphene in nano optics.

Plasmons in metal surfaces and clusters have been extensively studied due to their potential applications in sensing, imaging, light harvesting and optical metamaterials. Graphene is a semimetal with tunable conductivity and hence can support plasmons as well. In addition to the tunability, graphene plasmons have relatively weak damping due to the high carrier mobility. In this talk, I’ll present our recent progress on the studies of plasmon excitations in graphene micro- and nano-structures and their behavior in an external magnetic field. The collective motion of Dirac fermions, which are relativistic with zero rest mass, shows peculiar properties with a tunable “plasmon mass”. We showed strong light-matter interaction in the terahertz frequency regime and demonstrated graphene plasmonic terahertz filters and polarizers with graphene/insulator stacks. Localized plasmons in graphene nano-structures go beyond terahertz frequencies. As an atomically thin plasmonic material which is sensitive to the surrounding environment, strong coupling between plasmons in graphene and substrate surface polar phonons has been unambiguously identified in the mid-infrared regime. A new plasmon damping channel through the emission of graphene optical phonons has been revealed for high frequency plasmons. Our study paves the way for graphene applications in photonics, optoelectronics and metamaterials, especially in the terahertz frequency regime.  

Triplet-triplet annihilation upconversion (TTA-UC) is a technique that converts low-energy light into higher-energy light and has garnered significant attention for its wide range of potential applications. In TTA-UC, porphyrins are commonly used as sensitizer molecules, but the method faces several limitations. These include a narrow excitation wavelength range, restricted by the molecules’ spectroscopic properties, and a low molar absorption coefficient due to weak interactions with light. This study aims to significantly enhance TTA-UC performance by achieving strong interactions between light and sensitizer molecules through localized surface plasmon (LSP) resonance. It is well-known that strong light-matter interactions can give rise to upper (UP) and lower (LP) polariton states. By utilizing these states, it is anticipated that the excitation wavelength range can be extended, and excitation efficiency can be improved through these enhanced interactions. To realize this, we employed a plasmonic structure composed of a 2D assembly of metal-polymer thin films and metal nanoparticles, designed to exhibit pronounced LSP resonance. This structure also facilitates macroscopic observation of upconverted emission through strong light-molecule interactions. Additionally, based on synthetic methods from metal-organic frameworks, we fabricated an aggregate structure of sensitizer molecules (Pd porphyrin derivatives) and integrated them with the plasmonic structure. These composite systems exhibited spectral characteristics indicating strong interactions between light and excitons in the sensitizer molecules, as evidenced by anticrossing behavior. Furthermore, the upconverted emission was found to be significantly amplified, demonstrating the potential of this approach for enhancing TTA-UC performance.

Strong interaction between terahertz (THz) and matter is a topic of paramount importance, considering continuously enhanced interest in THz photonics as well as condensed matter physics, which can lead to the observation of many linear and nonlinear phenomena in the THz regime. Here, we demonstrate a unique and novel metamaterial-based technique of strong THz matter interaction towards thin film sensing, where the analyte is sensed in between the stacked metasurfaces forming the Fano cavity. Sub-wavelength structures typically overcome the diffraction limit of any optical system, which also possess very high confinement of electromagnetic energy. Fano resonance possesses a sharp asymmetric line shape, low radiation loss and large tuning capability. In addition to them, the material under test is placed in between the metasurfaces to utilize the substantial energy confinement leading to strong light matter interaction, a scheme never explored before. By intelligently exploiting the above characteristics, we have demonstrated a novel way to detect both the refractive index (dielectric constant), thickness and loss factor of the material under test when placed between the array of the meta-resonators forming the Fano metamaterials. Our study revealed that the sensitivity and figure of merit (FOM) are strikingly different for dipole and Fano modes. A maximum sensitivity of >1 THz RIU−1 (1.76 × 105 nm RIU−1) and FOM of around 14.05 are achieved at the Fano mode. Additionally, our sensor shows better performance with decreasing spacer thickness (lesser the material, more the sensitivity). Moreover, the proposed device is passive towards typical ambience temperature variation, and is highly compact because of its stacked configuration. The demonstrated device can be extremely beneficial towards realizing ultra-sensitive meta-sensors and other miniaturized THz meta-photonic devices, and bio-chemical sensing where strong light matter interaction is mandatory.

In modern physics, the investigation of the interaction between light and matter is important from both a fundamental and an applied point of view. Cavity quantum electrodynamics (cavity QED) is the study of the interaction between light confined in a reflective cavity and atoms or other particles where the quantum nature of light photons is significant. The strong interaction between an exciton and cavity photon in a high-finesse microcavity can induce a hybrid light-matter eigenstate which is usually named as polariton in solid-state systems. This strong light-matter interaction can be achieved when this interaction is larger than all broadenings caused by other various factors e.g. electron phonon scattering and cavity loss. The polariton is now stimulating tremendous research interests due to its high potential in cavity quantum electrodynamics (QED) and the achievement of polaritonic devices. Moreover, when the interaction strength between an excitation and the cavity photon, quantified by vacuum Rabi frequency, becomes comparable to or larger than the corresponding electronic transition frequency in a cavity, the system can enter an ultrastrong coupling regime, which has been experimentally observed. In this regime, the standard rotating-wave approximation is no longer valid and the antiresonant term of the interaction Hamiltonian starts to play an important role, giving rise to exciting effects in cavity QED.

This work investigates the plasmonic properties of a twisted bilayer graphene (TBG) and talc heterostructure. Talc, a naturally occurring phyllosilicate, promotes p-type charging of graphene, supporting high charge mobility and strong interaction between graphene plasmons and talc's phonon polaritons. This interaction results in the formation of surface plasmon-phonon polariton (SP3) modes, which are detected using infrared scattering-type scanning near-field optical microscopy (IR s-SNOM) at room temperature. Notably, without the need for electrostatic gating, our study reveals confinement of SP3 modes in a TBG-talc heterostructure and a transition from surface plasmonic waves to the emergence of moiré superlattices, characterized by reflections at domain walls. These findings provide fresh insights into the coupling mechanisms in hybrid materials and suggest promising applications in nanoscale optoelectronics.

Graphene plasmon polaritons are the hybrids of Dirac quasiparticles and infrared photons, which allows strong light-matter interaction within the tightly confined mode volume. However, plasmon dissipation channels, such as dielectric losses of the substrates, edge scattering, inefficient coupling to free space photon, significantly reduce external quantum efficiency of graphene plasmon. In this work, we demonstrate >60% absorption in a metal-on-graphene hybrid plasmonic structure, with far-infrared transparent spacer material for managing the photon recycling through backside reflectors. The widely tunable infrared plasmon resonance between 10 and 16 μm is achieved through controlling the dimension of periodic metal microstructures. In combination with phase matched spacer thickness, more than 86% absorption is graphene plasmonics can be achieved.

The stability and the activity of electrocatalysts in fuel cells and other applications can be improved by the strong interaction between noble metal nanoparticles and transition metal oxide, a phenomenon known as “strong metal-support interaction (SMSI).” Understanding and harnessing this interaction could lead to the development of more efficient and durable electrocatalysts. When SMSI occurs, the d-band center of noble metal as the catalyst changes to the favored state for the catalytic reaction or stability. Naturally, the variation of the d-band center depends on the combination of noble metal elements and transition metal oxide species, and the optimum state of the d-band is determined by the reaction. We focused on the oxygen reduction reaction (ORR), which is the cathode reaction of polymer electrolyte fuel cells (PEFCs). Because PEFCs require highly active and durable electrocatalysts.Generally, in catalysts where SMSI occurs, the electron-poor metal oxide covers the electron-rich noble metal particles in the reduction environment. On the other hand, the covered noble metal particles recover by conducting the oxidation treatment. An essential point for this phenomenon is that noble metal particles are smaller enough than metal oxide as the support or substrate. Therefore, the metal oxide layer can cover the noble metal particles. Theoretically, if the oxide layer is a few nanometers thick, it is difficult to cover the noble metal particles because the oxide layer is insufficient around them. In other words, it is expected that the electron structure of catalyst particles may be dramatically changed.The aim of this study was to find the optimum combination between noble metal and oxide for ORR in cathode catalysts in PEFCs. As the first step, Ir nanoparticles were deposited on the TiO2 layer, which was a few nanometers thick, and its ORR activity was evaluated. Electrooxidation by potential scanning was employed as the oxidation method for a model electrode. Moreover, the prepared Ir/TiO2 model electrode was conducted in the thermal reduction treatments, and the XPS analysis was performed to investigate the change in its electron structure. Other combinations, for example, Pt/TiO2, Ir/CeO2, etc., will be also done. Ir/TiO2 model electrode was composed of a glassy carbon substrate, a 1 nm TiO2 middle layer made from titanium oxide nanosheet, and Ir nanoparticles on the top deposited using the arc plasma deposition method. The electrochemical measurements were conducted using a three-electrode cell with a working electrode, a carbon rod counter electrode, an RHE reference electrode, and 0.1 M HClO4 at room temp. The reduction method was calcination at 500ºC under the reduction environment. The XPS results indicated that the TiO2 middle layer suppressed the valence variation of Ir nanoparticles. In this study, it was confirmed that the effects on ORR by introducing the TiO2 middle layer were stabilization of ORR activity in the Ir/TiO2 model electrode and inhabitation of the dissolution of Ir nanoparticles. In the day's presentations, we will compare with other combination model electrodes and discuss the relationship between valence variation and ORR activities of several model electrodes.

Graphene has emerged as a promising material for active plasmonic devices in the mid-infrared (MIR) region owing to its fast tunability, strong mode confinement, and long-lived collective excitation. In order to realize on-chip graphene plasmonics, several types of graphene plasmonic waveguides (GPWGs) have been investigated and most of them are with graphene ribbons suffering from the pattern-caused edge effect. Here we propose a novel nanoplasmonic waveguide with a pattern-free graphene monolayer on the top of a nano-trench. It shows that our GPWG with nanoscale light confinement, relatively low loss and slowed group velocity enables a significant modulation on the phase shift as well as the propagation loss over a broad band by simply applying a single low bias voltage, which is very attractive for realizing ultra-small optical modulators and optical switches for the future ultra-dense photonic integrated circuits. The strong light-matter interaction as well as tunable slow light is also of great interest for many applications such as optical nonlinearities.

Interactions between single spins and photons are essential for quantum networks and distributed quantum computation. Achieving spin-photon interactions in a solid-state device could enable compact chip-integrated quantum circuits operating at gigahertz bandwidths. Many theoretical works have suggested using spins embedded in nanophotonic structures to attain this high-speed interface. These proposals implement a quantum switch where the spin flips the state of the photon and a photon flips the spin state. However, such a switch has not yet been realized using a solid-state spin system. Here, we report an experimental realization of a spin-photon quantum switch using a single solid-state spin embedded in a nanophotonic cavity. We show that the spin state strongly modulates the polarization of a reflected photon, and a single reflected photon coherently rotates the spin state. These strong spin-photon interactions open up a promising direction for solid-state implementations of high-speed quantum networks and on-chip quantum information processors using nanophotonic devices.

The exploitation of strong light-matter interactions in chiral plasmonic nanocavities may enable exceptional physical phenomena and lead to potential applications in nanophotonics, information communication, etc. Therefore, a deep understanding of strong light-matter interactions in chiral plasmonic-excitonic (plexcitonic) systems constructed by a chiral plasmonic nanocavity and molecular excitons is urgently needed. Herein, we systematically studied the strong light-matter interactions in gold nanorod-based chiral plexcitonic systems assembled on DNA origami. Rabi splitting and anticrossing behavior were observed in circular dichroism spectra, manifesting chiroptical characteristic hybridization. The bisignate line shape of the circular dichroism (CD) signal allows the accurate discrimination of hybrid modes. A large Rabi splitting of ∼205/∼199 meV for left-handed/right-handed plexcitonic nanosystems meets the criterion of strong coupling. Our work deepens the understanding of light-matter interactions in chiral plexcitonic nanosystems and will facilitate the development of chiral quantum optics and chiroptical devices.

The infrared optical response of noble metals is traditionally considered perfect electrical conductor (PEC)-like due to the noble metals’ exceptionally large electron concentrations, and thus large (and negative) real permittivity. While PEC-like behavior is ideal for a broad range of applications, for instance mirrors, gratings, and wavelength- (and macro-) scale resonators and antennas, the utility of noble metals for nanoscale (sub-diffraction-limit) physics at long wavelengths is limited. However, in ultra-low volume (dilute) metal films, such as those with nanometer-scale thicknesses or lithographic dilution (subwavelength perforation), the thin films’ sheet conductivity is massively reduced, enabling light to penetrate and interact with the films much more efficiently. This avails the infrared of a host of opportunities for noble-metal-based plasmonics, with the potential for nanoscale (deep subwavelength) confinement and strong light-matter interaction, otherwise prohibited with noble metals in this wavelength range. In this perspective, we review the recent advances in dilute metal films for near- and mid-infrared photonics and plasmonics, and discuss the advantageous properties of these optical thin films for potential applications in sensors, detectors, sources, and nonlinear and quantum optics.

Two-dimensional (2D) materials and their vertically stacked heterostructures have attracted much attention due to their novel optical properties and strong light-matter interactions in the infrared. Here, we present a theoretical study of the near-field thermal radiation of 2D vdW heterostructures vertically stacked of graphene and monolayer polar material (2D hBN as an example). An asymmetric Fano line shape is observed in its near-field thermal radiation spectrum, which is attributed to the interference between the narrowband discrete state (the phonon polaritons in 2D hBN) and a broadband continuum state (the plasmons in graphene), as verified by the coupled oscillator model. In addition, we show that 2D van der Waals heterostructures can achieve nearly the same high radiative heat flux as graphene but with markedly different spectral distributions, especially at high chemical potentials. By tuning the chemical potential of graphene, we can actively control the radiative heat flux of 2D van der Waals heterostructures and manipulate the radiative spectrum, such as the transition from Fano resonance to electromagnetic-induced transparency (EIT). Our results reveal the rich physics and demonstrate the potential of 2D vdW heterostructures for applications in nanoscale thermal management and energy conversion.