Strong Mode Coupling Research Articles

Observation of strong mode coupling in multimode step index silica optical fibers with fluorinated thermoplastic polymer cladding

Using the power flow equation, the state of mode coupling in step index multimode silica optical fiber with fluorinated thermoplastic polymer cladding is investigated in this work. Coupling coefficient is determined by employing our previously reported calculation method. The silica optical fiber with fluorinated thermoplastic polymer cladding shows much stronger mode coupling than the all plastic-clad silica fibers investigated in earlier works. Consequently, it takes much shorter fiber length for the equilibrium mode distribution or steady state distribution to develop (faster launch beam narrowing), which is of great interest in employing silica optical fiber with fluorinated thermoplastic polymer as a part of telecommunication, medical and sensory system.

Zero GVD slow-light originating from a strong coupling of one-way modes in double-channel magneto-optical photonic crystal waveguides.

We have studied the coupling effect of topological photonic states in a double-channel magneto-optical photonic crystal waveguide by introducing a two-stranded ordinary Al2O3 photonic crystal as the coupling layer. There exist both M1 (odd) and M2 (even) one-way modes simultaneously in the bandgap. Interestingly, M1 mode is always a fast-light mode with large group velocity (vg) and large group velocity dispersion (GVD) regardless what the radius (RA) of Al2O3 rods is. However, when RA is appropriate, M2 mode becomes a very slow-light mode exhibiting near-zero vg and zero GVD simultaneously. The physical reason of such slow-light is attributed to the strong coupling effect between the one-way edge modes in both sub-waveguides. Furthermore, the simulation results show that the robustness of both the fast- and slow-light modes are extremely strong against perfect electric conductor defect and the one-way transmittance is close to 100%. Besides, the PEC defect can cause significant phase delay. These results hold promise for many fields such as signal processing, optical modulation, and the design of various topological devices.

In Situ Monitoring of Electronic Structure in a Modal Strong Coupling Electrode under Enhanced Plasmonic Water Oxidation

Modal strong coupling between a localized surface plasmon resonance and Fabry–Perot cavity accelerates plasmon-induced oxygen evolution reactions (OERs). In this study, graphene-based electrochemic...

Enhancing plasmonic hot-carrier generation by strong coupling of multiple resonant modes.

Plasmon-induced hot carriers have recently attracted considerable interest, but the energy efficiency in visible light is often low due to the short lifetime of hot carriers and the limited optical absorption of plasmonic architectures. To increase the generation of hot carriers, we propose to exert multiple plasmonic resonant modes and their strong coupling using a metal-dielectric-metal (MDM) nanocavity that comprises an Au nanohole array (AuNHA), a TiO2 thin film and an Au reflector. Unlike common MDM structures, in addition to the Fabry-Pérot mode in the dielectric layer, AuNHA as the top layer is special because it excites the localized surface plasmon resonance (LSPR) mode in the Au nanoholes and launches the gap surface plasmon polariton (GSPP) mode in the Au reflector surface. The spatial field overlapping of the three resonance modes enables strong mode coupling by optimizing the TiO2 thickness, which leads to notably enhanced average IPCE (∼1.5%) and broadband photocurrent (170 μA·cm-2). This MDM structure would be useful for photochemistry and photovoltaics using sunlight.

Near-field engineering for boosting the photoelectrochemical activity to a modal strong coupling structure.

Near-field engineering is considered a significant strategy in constructing plasmonic nanostructures for efficient plasmonic chemistry. We demonstrate interfacial near-field engineering on a Au-NP/TiO2/Au-film (ATA) photoanode to improve the water oxidation efficiency. To tailor the near-field distribution, postdeposited Au on an ATA electrode (Au@ATA) is implemented using a facile constant potential electrolysis technique. As a result, the average photocurrent conversion efficiency of Au@ATA is approximately 1.3-fold higher than that of ATA.

Polarization Effects on Thermally Stable Latency in Hollow-Core Photonic Bandgap Fibers

Conveyance of light in air endows hollow-core optical fibers with remarkably low sensitivity of the propagation delay to temperature changes. This sensitivity was demonstrated to be further reduced and even made negative (crossing zero) in photonic bandgap type of hollow core fibers. When operating long lengths of this fiber close to the zero sensitivity wavelength, it was observed experimentally that there is a small residual variation in propagation delay which had no apparent correlation to imposed temperature changes. In this article, we analyze the polarization effects that give rise to this variation, showing that the highest level of practically achievable thermal stability of the latency is limited by polarization mode dispersion. We show measurements of differential group delay between polarization modes in long lengths of photonic bandgap fiber at various temperatures and focus on spectral regions where thermally stable latency is predicted and measured. Our experimental observations, corroborated by numerical simulations, indicate the presence of strong polarization mode coupling in the fibers in addition to birefringence. The detailed understanding gained through this study allows us to propose practically achievable (i.e., manufacturable) fiber designs with up to three orders of magnitude lower polarization mode dispersion at wavelengths where the latency is insensitive to thermal fluctuations. This paves the way to fibers with polarization independent and thermally stable latency to serve a multitude of applications.

(Keynote) Dynamics of Electron Transfer in Enhanced Water Splitting Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) nanoparticles shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and semiconductor.[1]-[6] Recently, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[7],[8] However, in general Au-NPs loaded semiconductor photoelectrode, the insufficient absorption limits its solar energy conversion efficiency. Aiming at the enhancement of light absorption, in the present study, we apply the principle of modal strong coupling to plasmonic water splitting using Au-NPs/titanium dioxide (TiO2) thin-film/Au-film (ATA) photoelectrode.[9],[10] Furthermore, the dynamics of the hot electron transfer under the modal strong coupling conditions are further studied by transient reflection spectroscopy.Modal strong coupling between the Fabry–Pérot nanocavity mode of the TiO2thin-film/Au-film and the LSPR mode of the Au-NPs is induced when their resonant frequencies overlap. To increase the coupling strength in this modal strong coupling regime, a key feature of the ATA photoelectrode is partially inlaying of Au-NPs into the TiO2 nanocavity by several nanometers. Under a three-electrode system measurement with a saturated calomel electrode (SCE) as a reference electrode, a Pt wire as a counter electrode and an electrolyte of KOH (0.1 mol/dm3), we demonstrated that the action spectrum of incident photon to current conversion efficiency (IPCE) exhibited two bands, which almost corresponds to the absorption spectrum of ATA. The IPCE of ATA is extraordinarily enhanced as compared to that of Au-NPs/TiO2photoanode. Most importantly, under the modal strong coupling conditions, the internal quantum efficiency (IQE) of the photocurrent generation is also enhanced at the wavelengths of strong coupling induced hybrid branches. The increase in IQE implies the possibility of increasing the generation of hot electrons in the modal strong coupling regime. The plasmon-induced water splitting using a two-electrode system was also quantitatively measured. To understand the IQE enhancement under the strong coupling conditions, the dynamics of the hot electrons transfer from Au-NPs to TiO2 were further explored by transient measurement using a visible pump/infrared probe (at 3500 nm) femtosecond transient reflection spectrometers. An ultrafast hot electrons injection from Au-NPs to TiO2 was observed. Interestingly, under the strong coupling conditions, the hot electrons excited in the ATA photoelectrode that fabricated on Al film instead of Au film exhibited a much longer lifetime than that on Au film. The contributions for the long lifetime of the injected electrons to TiO2 are also discussed. References Nishijima, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett. 1, 2031 (2010). Zhong, K. Ueno, Y. Mori, X. Shi, T. Oshikiri, K. Murakoshi, H. Inoue, H. Misawa, Angew. Chem. Int. Ed., 53,10350(2014).Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed., 53, 9802 (2014).Oshikiri, K. Ueno, H. Misawa, Angew. Chem. Int. Ed., 55,3942 (2016).Nakamura, T. Oshikiri, K. Ueno, H. Misawa et al. J. Phys. Chem. Lett., 7, 1004 (2016).V. Hoang, K. Hayashi, K. Ueno, H. Misawa et al. Nat. Commun., 8, 771 (2017).Ueno, T. Oshikiri, H. Misawa, ChemPhysChem, 17, 199 (2016).Ueno, T. Oshikiri, Q. Sun, X. Shi, H. Misawa, Chem. Rev., 118, 2955 (2018). Shi, K. Ueno, T. Oshikiri, Q. Sun, K. Sasaki, H. Misawa, Nat Nanotechnol., 13, 953 (2018).Y. Cao, T. Oshikiri, X. Shi, K. Ueno, J. Li, H. Misawa, ChemNanoMat, 5, 1008 (2019).

Parametric resonance of Josephson plasma waves: A theory for optically amplified interlayer superconductivity in YBa2Cu3O6+x

Non-linear interactions between collective modes play a definitive role in far out of equilibrium dynamics of strongly correlated electron systems. Understanding and utilizing these interactions is crucial to photo-control of quantum many-body states. One of the most surprising examples of strong mode coupling is the interaction between apical oxygen phonons and Josephson plasmons in bilayer YBa$_2$Cu$_3$O$_{6+x}$ superconductors. Experiments by Hu et al (2014). and Kaiser et al. (2014) showed that below Tc, photo-excitation of phonons leads to enhancement and frequency shifts of Josephson plasmon edges, while aboveTc, photo-excited phonons induce plasmon edges even when there are no discernible features in the equilibrium reflectivity spectrum. Recent experiments by Van Hoegen et al. (2019) also observed parametric generation of Josephson plasmons from photo-excited phonons both below Tc and in the pseudogap phase. In this paper we present a theoretical model of phonon-plasmon three wave interaction arising from coupling between the oxygen motion and the in-plane superfluid stiffness. Analysis of the parametric instability of plasmons based on this model gives frequencies of the most unstable plasmons that are in agreement with experimental observations. We also discuss how strong parametric excitation of Josephson plasmons can explain pump induced changes in the TeraHertz reflectivity of YBa$_2$Cu$_3$O$_{6+x}$ in the superconducting state, including frequency shifts and sharpening of Josephson plasmon edges, as well as appearance of a new peak around 2THz. An interesting feature of this model is that overdamped Josephson plasmons do not give any discernible features in reflectivity in equilibrium, but can develop plasmon edges when parametrically excited. We suggest that this mechanism explains photo-induced plasmon edges in the pseudogap phase of YBa$_2$Cu$_3$O$_{6+x}$.

Intrinsically Ultralow Thermal Conductivity in Ruddlesden-Popper 2D Perovskite Cs2PbI2Cl2: Localized Anharmonic Vibrations and Dynamic Octahedral Distortions.

Fundamental understanding of the correlation between chemical bonding and lattice dynamics in intrinsically low thermal conductive crystalline solids is important to thermoelectrics, thermal barrier coating, and more recently to photovoltaics. Two-dimensional (2D) layered halide perovskites have recently attracted widespread attention in optoelectronics and solar cells. Here, we discover intrinsically ultralow lattice thermal conductivity (κL) in the single crystal of all-inorganic layered Ruddlesden-Popper (RP) perovskite, Cs2PbI2Cl2, synthesized by the Bridgman method. We have measured the anisotropic κL value of the Cs2PbI2Cl2 single crystal and observed an ultralow κL value of ∼0.37-0.28 W/mK in the temperature range of 295-523 K when measured along the crystallographic c-axis. First-principles density functional theory (DFT) analysis of the phonon spectrum uncovers the presence of soft (frequency ∼18-55 cm-1) optical phonon modes that constitute relatively flat bands due to localized vibrations of Cs and I atoms. A further low energy optical mode exists at ∼12 cm-1 that originates from dynamic octahedral rotation around Pb caused by anharmonic vibration of Cl atoms induced by a 3s2 lone pair. We provide experimental evidence for such low energy optical phonon modes with low-temperature heat capacity and temperature-dependent Raman spectroscopic measurements. The strong anharmonic coupling of the low energy optical modes with acoustic modes causes damping of heat carrying acoustic phonons to ultrasoft frequency (maximum ∼37 cm-1). The combined effect of soft elastic layered structure, abundance of low energy optical phonons, and strong acoustic-optical phonon coupling results in an intrinsically ultralow κL value in the all-inorganic layered RP perovskite Cs2PbI2Cl2.

Localizing resonant magnetic perturbations for edge localized mode control in KSTAR

An external 3D magnetic perturbation typically drives a resonant response at the rational surfaces from the core to the edge of tokamak plasmas, due to strong mode coupling and amplification. This paper presents a method to isolate the edge from core resonant fields using the ideal perturbed equilibrium code and to design an edge-localized resonant magnetic perturbation (RMP) for effective edge localized mode (ELM) control. A robust feature of the edge-localized RMP is the curtailed response to the field at the low-field-side (LFS) midplane, as opposed to typical RMPs which strongly resonate with the LFS fields. This emphasizes the importance of off-midplane coils to improve ELM control without provoking a large core response that could lead to devastating instabilities. The conceptual design of new ELM control coils based on the edge-localized RMP in KSTAR shows how this new insight can be utilized to enhance the efficiency of our ELM suppression capabilities. Simple window-pane coils matching the edge-localized resonant mode structure substantially expand in the ELM suppression window beyond the existing coil. Further optimization using the flexible optimized coils using space-curves code leads to additional enhancement in the edge-localized control.

Eigenfrequency loci crossings, veerings and mode splittings of two cantilevers coupled by an overhang

Eigenfrequency loci veering, which indicates strong mode coupling and sometimes mode localization, is a much sought-after property in the applications of mass sensing and opto/electromechanics. A weak physical coupling is the mechanism responsible for the eigenfrequency loci veering and overhang is a widely used structure to realize such mechanism. A continuum model, which is more accurate and straightforward as compared with the discrete models, is presented for the structure of two overhanged cantilevers. The eigenvalue problem formulation based on this continuum model leads to a direct computation of the eigenfrequencies, which does not involve any numerical discretization procedure. A comprehensive study on the eigenfrequency loci veerings and mode splittings of the overhanged structure is presented. The influences of various parameters on the eigenfrequency loci crossing and veerings are also systematically studied. An efficient optimum design tool for the eigenfrequency loci veering of an overhanged structure is provided by the continuum model together with a direct computation method.

Extraordinary Multipole Modes and Ultra-Enhanced Optical Lateral Force by Chirality.

Strong mode coupling and Fano resonances arisen from exceptional interaction between resonant modes in single nanostructures have raised much attention for their advantages in nonlinear optics, sensing, etc. Individual electromagnetic multipole modes such as quadrupoles, octupoles, and their counterparts from mode coupling (toroidal dipole and nonradiating anapole mode) have been well investigated in isolated or coupled nanostructures with access to high Q factors in bound states in the continuum. Albeit the extensive study on ordinary dielectric particles, intriguing aspects of light-matter interactions in single chiral nanostructures is lacking. Here, we unveil that extraordinary multipoles can be simultaneously superpositioned in a chiral nanocylinder, such as two toroidal dipoles with opposite moments, and electric and magnetic sextupoles. The induced optical lateral forces and their scattering cross sections can thus be either significantly enhanced in the presence of those multipoles with high-Q factors, or suppressed by the bound states in the continuum. This work for the first time reveals the complex correlation between multipolar effects, chiral coupling, and optical lateral force, providing a distinct way for advanced optical manipulation.

Flexibly tunable surface plasmon resonance by strong mode coupling using a random metal nanohemisphere on mirror

Abstract We propose a unique random metal nanohemisphere on mirror (NHoM) structure to tune the surface plasmon (SP) resonance in a flexible manner. The SP resonance peak was split into two peaks owing to the strong coupling between the SP mode in the metal nanohemisphere and the mirror image mode generated in the metal substrate. This phenomenon is based on the fact that the strong coupling and the induced electromagnetic effects are similar to those pertaining to the Rabi splitting, Fano resonance, and electromagnetically induced transparency, thus providing quantum effect analogies. These phenomena have recently attracted increased attention and have been studied with nanocavities fabricated with top-down nanotechnologies. Compared with previous reports, NHoM structures can be fabricated in a much easier manner and are tunable in rather wider wavelength regions without nanofabrication technologies. The SP resonance peaks were enhanced, sharpened dramatically, and tuned flexibly, based on the optimization of the thickness of the spacer layer between the metal hemisphere and metal substrate. Experimental results were reproduced and were explained based on finite difference time domain (FDTD) simulations. These phenomena have never been observed previously on similar nanosphere on mirror (NSoM) because nanohemispherical structures were required. The NHoM nanocavity structure has a quality factor >200 that is surprisingly high for the localized SP mode of nanoparticles. Flexible tuning of the SP resonance with the use of NHoM is envisaged to lead to the development of new applications and technologies in the field of plasmonics and nanophotonics.

Design Considerations for Frequency Shifts in a Laterally Finite FBAR Sensor in Contact With the Newtonian Liquid.

Mode-coupled vibrations in a thickness-shear (TSh) mode and laterally finite film bulk acoustic resonator (FBAR) with one face in contact with Newtonian (linearly viscous and compressional) liquid are investigated. With boundary conditions and interface continuity conditions, exact dispersion curves in FBAR sensors contacting with two kinds of liquids are obtained, and they are compared with the dispersion curves in a bare sensor without liquid contact. Frequency spectra, describing mode couplings between the main TSh modal branch and undesirable modal branches, are calculated by employing weak boundary conditions at lateral free edges constructed based on the variational principle. Mode shapes of mechanical displacements in both the sensor and liquid layer are presented, and mode transformations are observed due to the liquid contact and lateral edge effect. The effect of liquid thickness on frequency spectra is also studied. Numerical results reveal that the generation of shear wave in the liquid layer results in the shifts of spectrum curves along the frequency axis and hence it is the main factor of frequency shifts of FBAR sensors. The compressional wave causes the shifts of spectrum curves along the lateral aspect ratio axis. Then for a given FBAR sensor, the liquid thickness changes could also cause frequency shifts. Therefore, desirable vibration modes should be chosen based on the frequency spectra to avoid strong mode couplings and to eliminate frequency shifts induced by the liquid thickness changes in real applications.

Noise and Distortion Reduction in OFDM Radio-Over-Fiber Link by Graded-Index Plastic Optical Fiber

We demonstrate that a graded-index plastic optical fiber (GI POF) enables higher-quality radio-over-fiber (RoF) transmission of an orthogonal frequency-division multiplexing (OFDM) signal, compared with a conventional multimode fiber, for short-distance household applications (< 100 m). It is shown that the improvement in OFDM signal transmission, using the GI POF, can be attributed to the reduction in noise and intermodulation distortion, owing to the external optical feedback from the transmission fibers. The low noise and low distortion in the signal transmission are mainly due to the strong mode coupling in the GI POF, which is closely related to the microscopic heterogeneous structures in core polymer materials. The developed GI POF will be invaluable for RoF networks in households as a solution for indoor broadband wireless communications in the upcoming 5G era.

Self-assembled plasmonics for angle-independent structural color displays with actively addressed black states

Nanostructured plasmonic materials can lead to the extremely compact pixels and color filters needed for next-generation displays by interacting with light at fundamentally small length scales. However, previous demonstrations suffer from severe angle sensitivity, lack of saturated color, and absence of black/gray states and/or are impractical to integrate with actively addressed electronics. Here, we report a vivid self-assembled nanostructured system which overcomes these challenges via the multidimensional hybridization of plasmonic resonances. By exploiting the thin-film growth mechanisms of aluminum during ultrahigh vacuum physical vapor deposition, dense arrays of particles are created in near-field proximity to a mirror. The sub-10-nm gaps between adjacent particles and mirror lead to strong multidimensional coupling of localized plasmonic modes, resulting in a singular resonance with negligible angular dispersion and ∼98% absorption of incident light at a desired wavelength. The process is compatible with arbitrarily structured substrates and can produce wafer-scale, diffusive, angle-independent, and flexible plasmonic materials. We then demonstrate the unique capabilities of the strongly coupled plasmonic system via integration with an actively addressed reflective liquid crystal display with control over black states. The hybrid display is readily programmed to display images and video.

Two-Dimensional Plate Theory for the Analysis of Coupling Vibrations in Shear Mode FBARs

The shear mode film bulk acoustic wave resonators (FBARs) have been developed rapidly in recent years due to the increased interest in biosensors. In this paper, the mode coupling effects which are unavoidable in real devices are theoretically analyzed and discussed. Mindlin's approximation method of expanding the displacement fields with power series along the plate thickness direction is employed and a system of two-dimensional plate equations is obtained. The accuracy of the plate theory is validated through the comparison of dispersion curves with the results calculated by three-dimensional exact theory. Free coupling vibrations of the cross-sectional plane of finite-sized shear mode FBAR plates are further studied with the derived two-dimensional theory. Results in this paper show that the presented plate theory is accurate and efficient for the structural analyses of shear mode FBARs. Besides, the selection of aspect ratios can be used to avoid strong mode couplings, which is helpful for device designs.

Light field imaging through a single multimode fiber for OAM-multiplexed data transmission

Multimode fibers (MMFs) can support a large number of spatial modes, increasing the amount of transmitted information, a feature that benefits fields such as endoscopic imaging or optical communications. However, the transport of information encoded in the phase through a standard MMF has not been achieved due to strong mode coupling, hindering direct data transfer using light beams endowed with orbital angular momentum (OAM). Here, we present a computational light field imaging technique to achieve OAM-multiplexed transmission through a commercial MMF. Such a technique enables the retrieval of the incident field from the speckle intensity at the distant end of the fiber. Based on the recovered field, the extraction of the transferred data by addressing all the OAM channels can be achieved. Experimentally, light field imaging through a MMF is demonstrated by recovering a structured light field and a phase picture. For optical transmission, the crosstalk between OAM channels is tested first, and then we demonstrate high-fidelity image data transfer via 24 multiplexed OAM channels through a commercial MMF.

(Invited) Enhanced Photochemical Reactions Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices due to their spectrum tunability. We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and the semiconductor photoelectrode.[1]-[7] Previously, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[8],[9] However, for the monolayer of Au-NPs on the semiconductor the insufficient absorption limited its solar energy conversion efficiency.Recently, we reported Au-NPs/TiO2/Au-film photoanode with a modal strong coupling between Fabry–Pérot nanocavity (FP-nanocavity) mode and LSPR of Au-NPscan enhance water splitting reaction.[10] In particular, it should be noted that in addition to the absorption increment, the internal quantum efficiency (IQE) is enhanced under strong couplingconditions.Additionally, we investigated the efficiency of hot-electron transfer under modal strong coupling conditions by monitoring the photocurrent generated at a plasmonic photoanode. We explored the effect of the modal strong coupling on the incident photon-to-current conversion efficiency (IPCE) and IQE in the presence of triethanolamine (TEOA) as a sacrificial electron donor to accelerate the surface reaction enough.[11] The absorptionspectrum showed distinct dual bands,which corresponded to the strong-coupling-induced splitting ofenergy levels into upper and lower branches. The IPCE was dramaticallyenhanced as the TEOA concentration increased, and finally, theIPCE reached a maximum of ca. 4%. Additionally, both hybrid modes formed bythe modal strong coupling contributed to the hot-electron transfers and photocurrentgeneration in the presence of TEOA because the IPCE action spectracan be separated into two peaks. Furthermore, the integrated IQE, which was obtained for wavelengths from 500 to 800 nm, was enhanced by approximately 5 times upon the addition of 1 vol% of TEOA and reached 3%.

(Keynote) Dynamics of Electron Transfer in Enhanced Water Splitting Under Modal Strong Coupling Conditions

Metallic nanoparticles such as gold (Au) and silver (Ag) nanoparticles shows light absorption and scattering at the arbitrary wavelength of visible and near-infrared regions based on localized surface plasmon resonances (LSPRs). LSPRs which are collective oscillations of conductive electrons give rise to the enhancement of near-field and are expected as a light harvesting optical antenna for light energy conversion devices We have successfully developed the plasmon-induced artificial photosynthesis systems such as water splitting and ammonia synthesis systems as well as solid-state plasmonic solar cells based on the principle of plasmon-induced charge separation between gold nanoparticles (Au-NPs) and semiconductor.[1]-[6] Recently, the plasmon-induced charge separation has received considerable attention as a novel strategy for solar energy conversion.[7],[8] However, in general Au-NPs loaded semiconductor photoelectrode, the insufficient absorption limits its solar energy conversion efficiency. Aiming at the enhancement of light absorption, in the present study, we apply the principle of modal strong coupling to plasmonic water splitting using Au-NPs/titanium dioxide (TiO2) thin-film/Au-film (ATA) photoelectrode.[9],[10] Furthermore, the dynamics of the hot electron transfer under the modal strong coupling conditions are further studied by transient reflection spectroscopy.Modal strong coupling between the Fabry–Pérot nanocavity mode of the TiO2thin-film/Au-film and the LSPR mode of the Au-NPs is induced when their resonant frequencies overlap. To increase the coupling strength in this modal strong coupling regime, a key feature of the ATA photoelectrode is partially inlaying of Au-NPs into the TiO2nanocavity by several nanometers. Under a three-electrode system measurement with a saturated calomel electrode (SCE) as a reference electrode, a Pt wire as a counter electrode and an electrolyte of KOH (0.1 mol/dm3), we demonstrated that the action spectrum of incident photon to current conversion efficiency (IPCE) exhibited two bands, which almost corresponds to the absorption spectrum of ATA. The IPCE of ATA is extraordinarily enhanced as compared to that of Au-NPs/TiO2photoanode. Most importantly, under the modal strong coupling conditions, the internal quantum efficiency (IQE) of the photocurrent generation is also enhanced at the wavelengths of strong coupling induced hybrid branches. The increase in IQE implies the possibility of increasing the generation of hot electrons in the modal strong coupling regime. The plasmon-induced water splitting using a two-electrode system was also quantitatively measured. To understand the IQE enhancement under the strong coupling conditions, the dynamics of the hot electrons transfer from Au-NPs to TiO2were further explored by transient measurement using a visible pump/infrared probe (at 3500 nm) femtosecond transient reflection spectrometers. An ultrafast hot electrons injection from Au-NPs to TiO2was observed. Interestingly, under the strong coupling conditions, the hot electrons excited in the ATA photoelectrode that fabricated on Al film instead of Au film exhibited a much longer lifetime than that on Au film. The contributions for the long lifetime of the injected electrons to TiO2are also discussed.