Selective Mode Excitation Research Articles

Selective excitation of localized acoustic skyrmion modes based on directional sound sources

Acoustic skyrmion modes are topological texture structures of velocity field vectors generated on the surface of acoustic structures. This protected vector distribution provides new dimensions for advanced sound information processing, transmission, and data storage. In this study, we design a combined structure of waveguides and spiral structures, using directional acoustic sources to excite waveguide mode transmission, thereby achieving selective excitation of localized acoustic skyrmion modes. Through theoretical analysis and numerical simulations, we studied the pressure field distribution and velocity field distribution excited by spin acoustic sources, Huygens acoustic sources, and Janus acoustic sources in this structure, demonstrating the directional transmission properties of acoustic surface waves and the selectively excited acoustic skyrmion modes in the combined structure. Numerical calculations reveal that when the spin acoustic source excites acoustic surface waves to propagate directionally along the waveguide, it selectively excites the acoustic skyrmion modes in the helical structure in the direction corresponding to the propagation. When the Huygens source excites acoustic surface waves to propagate directionally along the waveguide, it selectively excites acoustic skyrmion modes in the right or left direction. However, when the Janus source excites acoustic surface waves propagating directionally along the waveguide, it will selectively excite acoustic skyrmion modes in the upward or downward direction. This waveguide excitation method is a new means of exciting acoustic skyrmion modes, making the excitation of acoustic skyrmion modes more flexible. Moreover, this waveguide excitation method has significant application potential in more complex and larger-scale acoustic systems. The research results may promote the understanding of the symmetry in acoustic near-field physics, opening new pathways for using sound waves to manipulate particles, and potentially paving the way for the design of advanced acoustic devices.

(Invited) Unraveling the Impact of Different Thermal Quenching Routes on Luminescence Efficiency of Y3Al5O12:Ce3+ Phosphor: Insights from Mode-Selective Vibrational Excitation Experiments

Cerium doped yttrium aluminium garnet, Y3Al5O12:Ce3+, is the prototype material for solid-state white lighting and it still is an important white LED phosphor. However, fundamental understanding of the thermal quenching of luminescence, which leads to a pronounced reduction of the emission intensity under high-power light-emitting diode operation, remains to be obtained. Here we show, through a multi-technique approach based on photoluminescence, thermoluminescence and mode-selective vibrational excitation experiments that thermal quenching of luminescence in Y3Al5O12:Ce3+ is caused by a combined effect of thermal ionization, thermally activated concentration quenching, and thermally activated 5d - 4f crossover relaxation via electron-phonon coupling, and establish the general trends upon variation of the Ce3+ concentration and temperature. Thermal quenching below 600 K is primarily the result of concentration quenching and crossover relaxation, which can be suppressed by keeping the Ce3+ dopant concentration far below 0.7 mol%, whereas for temperatures above 600 K thermal ionization is the predominating quenching process. This new insight into the interplay between different thermal quenching processes provides design principles for optimizing the light emittance and colour stability of new phosphor materials used in white lighting devices characterized by certain operating temperatures.

Selective excitation of vibrations in a single molecule

The capability to excite, probe, and manipulate vibrational modes is essential for understanding and controlling chemical reactions at the molecular level. Recent advancements in tip-enhanced Raman spectroscopies have enabled the probing of vibrational fingerprints in a single molecule with Ångström-scale spatial resolution. However, achieving controllable excitation of specific vibrational modes in individual molecules remains challenging. Here, we demonstrate the selective excitation and probing of vibrational modes in single deprotonated phthalocyanine molecules utilizing resonance Raman spectroscopy in a scanning tunneling microscope. Selective excitation is achieved by finely tuning the excitation wavelength of the laser to be resonant with the vibronic transitions between the molecular ground electronic state and the vibrational levels in the excited electronic state, resulting in the state-selective enhancement of the resonance Raman signal. Our approach contributes to setting the stage for steering chemical transformations in molecules on surfaces by selective excitation of molecular vibrations.

(Invited) Time-Resolved in Situ Electrochemical SFG Study of Hot Electron Transfer from Metal Electrodes to Adsorbates

here have been recent reports of light enhanced electrocatalysis on metal electrodes, including H2 generation and CO2 reduction. The mechanism of such process remains unclear. One possible mechanism is selective excitation of adsorbate vibrational modes by rapid hot electron transfer and back transfer between the metal and the adsorbate. Although this mechanism has been studied for adsorbate on metal surfaces under vacuum conditions, it is unclear whether similar processes occur under electrochemical conditions and can be tuned by electrochemical bias. Using time-resolved vibrational sum frequency generation (VSFG) spectroscopy, we directly measure hot electron induced vibrational dynamics of adsorbates on metal electrodes. For CO adsorbed on Au electrodes, we show that excitation of the metal leads to the generation of vibrationally excited CO molecules. Detailed analysis reveals that the formation and decay of the excited adsorbates follows the electronic temperature of the electron, indicating the direct coupling of adsorbate vibration modes with hot electrons in the electrodes, likely through ultrafast hot electron transfer and back transfer induced vibrational excitation. This result suggests the possibility of plasmon (or light)-enhanced electrochemistry on metal electrodes. Ongoing studies are systematically examining how the hot electron transfer process depends on the nature of the electrode and applied bias.

Resolving plasmon-mediated high-order multiphoton excitation pathways in dolmen nanostructures using ultrafast nonlinear optical interferometry.

The multiphoton excitation pathways of plasmonic nanorod assemblies are described. By using dolmen structures formed from the directed assembly of three gold nanorods, plasmon-mediated three-photon excitation is resolved. These high-order multiphoton excitation channels were accessed by resonantly exciting a hybrid mode of the dolmen structure that was resonant with the 800-nm carrier wavelength of an ultrafast laser system. Rotation of the exciting field polarization to a non-resonant configuration did not generate third-order responses. Hence, the multiphoton excitation and resultant non-equilibrium electron distributions were generated by structure- and mode-selective excitation. Correlation between high-order and resonant plasmon excitation was achieved through sub-cycle time-resolved interferometric detection of incoherent nonlinear emission signals. The results illustrate the advantages of nonlinear optical interferometry and Fourier analysis for distinguishing plasmon-mediated processes from those that do not require plasmon excitation.

Spatially Controlled Phase Modulation - Selective Higher-Order Mode Excitation in 3D Nanoprinted on-Chip Hollow-Core Waveguides

Spatially Controlled Phase Modulation - Selective Higher-Order Mode Excitation in 3D Nanoprinted on-Chip Hollow-Core Waveguides

Reducing Artefacts in FRF-based Damage Detection Using Novel Mode Extraction Approach for Guided Ultrasonic Waves

Damage in thin-walled structures can be detected by guided ultrasonic wave (GUW) based structural health monitoring systems. The application of phased arrays enables the scanning of large-scale structures from a single position. However, physical wave focusing requires a lot of effort. This can be significantly reduced if frequency response functions (FRFs) are used. They enable the calculation of virtual response signals for virtual focusing on any position. Although this technique offers an energy-efficient and fast possibility of damage detection, it has the disadvantage of artefacts that occur due to the multimodal nature of GUW, as damage detection is generally designed for single-mode signals. These artefacts are often reduced by mode-selective excitation/sensing or by subtracting a baseline measurement. This work presents a concept to combine an existing FRF-based damage detection algorithm and a previously presented method for mode extraction. It enables the extraction of GUW mode components from broadband, temporally sampled, single-input single-output sensor data during signal processing on the basis of the respective dispersion relations. The aim is to reduce artefacts without using mode-selective excitation/sensing or baseline measurements. A finite element simulation of GUW propagation in an isotropic structure is used to demonstrate the advantages and limitations of this approach. The simulations include multimodal and single mode evaluation to point out the added value of performing mode extraction prior to damage detection. It is supposed that the successful extraction of the mode components from the temporally sampled data results in a decreasing amplitude of the occurring artefacts compared to the multimodal case, while the case of mode-selective excitation apparently results in no artefacts. The presented concept of adding mode extraction to damage detection algorithms can lead to an increase in performance of FRF-based phased array systems. Artefacts that would lead to false detection of damage are reduced inherently during signal processing which eliminates the need for mode-selective excitation/sensing or baseline measurements.

Manipulating terahertz guided wave excitation with Fabry-Perot cavity–assisted metasurfaces

Metasurfaces are emerging as powerful tools for manipulating complex light fields, offering enhanced control in free space and on-chip waveguide applications. Their ability to customize refractive indices and dispersion properties opens up new possibilities in light guiding, yet their efficiency in exciting guided waves, particularly through metallic structures, is not fully explored. Here, we present a new method for exciting terahertz (THz) guided waves using Fabry-Perot (FP) cavity-assisted metasurfaces that enable spin-selective directional coupling and mode selection. Our design uses a substrate-free ridge silicon THz waveguide with air cladding and a supporting slab, incorporating placed metallic metasurfaces to exploit their unique interaction with the guided waves. With the silicon thin layer and air serving as an FP cavity, THz waves enter from the bottom of the device, thereby intensifying the impact of the metasurfaces. The inverse-structured complementary metasurface could enhance excitation performance. We demonstrate selective excitation of TE00 and TE10 modes with directional control, confirmed through simulations and experimental validations using a THz vector network analyzer (VNA) system. This work broadens the potential of metasurfaces for advanced THz waveguide technologies.

Design and characterization of a self-matching photonic lantern for all few-mode fiber laser systems

We model and demonstrate a self-matching photonic lantern (SMPL) device, which is designed to address the constraint of limited transverse modes generated by fiber lasers. The SMPL incorporates a FMF into the array at the input end of a traditional photonic lantern. The few-mode fiber at the output end is specifically configured to align with the few-mode fiber at the input, therefore named as SMPL. This paper details the design and fabrication of the SMPL device, validated by both simulation and experiment. The 980nm fundamental mode, injected via 980nm single-mode fibers, selectively excites corresponding higher-order modes at the few-mode port of the SMPL. Additionally, 1550nm fundamental and higher-order modes injected at the input end into the SMPL device demonstrates mode preservation and low-loss transmission characteristics. The SMPL is well-suited for developing a ring laser system, enabling selective excitation of 980nm pump light modes and facilitating closed-loop oscillation and transmission of 1550nm laser.

Selective Excitation of Exciton-Polariton Condensate Modes in an Annular Perovskite Microcavity.

Exciton-polariton systems composed of a light-matter quasi-particle with a light effective mass easily realize Bose-Einstein condensation. In this work, we constructed an annular trap in a halide perovskite semiconductor microcavity and observed the spontaneous formation of symmetrical petal-shaped exciton-polariton condensation in the annular trap at room temperature. In our study, we found that the number of petals of the petal-shaped exciton-polariton condensates, which is decided by the orbital angular momentum, is dependent on the light intensity distribution. Therefore, the selective excitation of perovskite microcavity exciton-polariton condensates under all-optical control can be realized by adjusting the light intensity distribution. This could pave the way to room-temperature topological devices, optical cryptographical devices, and new quantum gyroscopes in the exciton-polariton system.

Broadband chaos generation in a distributed-feedback laser by selecting residual side modes.

Chaotic dynamics with spectral broadening is experimentally obtained by selective excitation of residual side modes in a distributed-feedback (DFB) laser. For the single-mode laser that emits only at the main mode when free-running, feedback to a residual side mode is introduced via a fiber Bragg grating (FBG). The FBG feedback suppresses the main mode, selectively excites the residual side mode, and generates broadband chaotic dynamics. Such a chaos of the residual side mode has a broad electrical bandwidth reaching at least 26 GHz, which corresponds to a significant broadening by over 50% when compared with the main mode. The dynamics are attributed entirely to the one selected mode without invoking multimode interactions. The wavelength is tunable beyond 10 nm by using different FBGs. Through avoiding multimode interactions, this approach of broadband chaos generation is potentially simple to model and thus promising for applications.

Selective excitation of high-order modes in two-dimensional cavity resonator integrated grating filters.

The selective spatial mode excitation of a bi-dimensional grating-coupled micro-cavity called a cavity resonator integrated grating filter (CRIGF) is reported using an incident beam shaped to reproduce the theoretical emission profiles of the device in one and subsequently two dimensions. In both cases, the selective excitation of modes up to order 10 (per direction) is confirmed by responses exhibiting one (respectively two) spectrally narrowband resonance(s) with a good extinction of the other modes, the latter being shown to depend on the parity and order(s) of the involved modes. These results pave the way toward the demonstration of multi-wavelength spatially selective reflectors or fiber-to-waveguide couplers. Also, subject to an appropriate choice of the materials constituting the CRIGF, this work can be extended to obtain mode-selectable laser emission or nonlinear frequency conversion.

A Multiband Standing Wave Oscillator With Wave Pattern Tailoring for Selective Mode Excitation

A Multiband Standing Wave Oscillator With Wave Pattern Tailoring for Selective Mode Excitation

Selective Excitation of Dark Plasmon Modes Using Cylindrical Vector Beams Studied by Microscopic Imaging of Nonlinear Photoluminescence

Selective Excitation of Dark Plasmon Modes Using Cylindrical Vector Beams Studied by Microscopic Imaging of Nonlinear Photoluminescence

Stress Relaxation of Extremely-Thin-Body Ge-on-Insulator p-Type Metal-Oxide-Semiconductor Field-Effect Transistor Along and Observed By Oil-Immersion Raman Spectroscopy

Background and purpose Strain techniques and channel materials with high carrier mobility are the principal technology boosters for the realization of high-performance logic devices composed of group IV semiconductors such as silicon (Si). In particular, strain engineering plays an important role in improving the performance of metal-oxide semiconductor field-effect transistors (MOSFETs) from the viewpoint of reducing the effective carrier mass. It is important to evaluate the stress relaxation in group IV semiconductor devices for the optimization of the carrier mobility and the device structure. In this paper, we demonstrated the stress evaluation of extremely-thin-body Ge-on-insulator (ETB GOI) p-type MOSFET along the <110> and <100> longitudinal directions by oil-immersion Raman spectroscopy with the high-numerical-aperture (high-NA) lens to realize precise evaluation of strain states in the Ge channels. Experiments Biaxially-strained (001) GOI layer was prepared by applying optimized Ge condensation technique on epitaxially-grown Si/Si0.75Ge0.25/Si-on-insulator substrates with 4-hour slow cooling [1, 2]. After the removal of the thermal oxide, narrow channel patterning was performed by electron-beam (EB) lithography. GOI channel was further thinned down by the cyclic digital etching process (plasma oxidation and wet etching). Surface passivation process was then immediately performed to fabricate GeO x and Al2O3 by plasma oxidation and atomic layer deposition at 300oC, respectively. The source-drain regions were formed with EB evaporated Ni through a lift-off process and Al deposition was performed as the back gate electrode.We used the Raman spectrometer with spectrograph focal length of 2,000 mm. The numbers of grating grooves were 2,400 mm-1 and a visible laser was used as the incident excitation light source whose wavelength was 532 nm. To realize anisotropic stress evaluation assuming the channel region of the ETB GOI MOSFET, we used a high-NA immersion lens for the selective excitation of both LO and TO phonon modes [3]. The high-NA and refractive index n of the oil were approximately 1.4 and 1.5, respectively. Using a high-NA immersion lens, a z-polarized light component can be effectively obtained owing to the aperture angle. Therefore, TO phonons can be excited on the basis of Raman selection rules for (001)-oriented GOI [3, 4]. Results and discussion Figure 1 shows the channel width dependence of the anisotropic biaxial stress of ETB GOI MOSFET along the <100> longitudinal direction obtained by the oil-immersion Raman spectra. Anisotropic biaxial compressive stress at the GOI channel width direction of the GOI channels was observed even when the GOI channel width is wide (GOI channel width ~ 630 nm). The stress along the GOI channel length (the <100> longitudinal direction) is relatively high. These results are clearly different from the stress relaxation in the ETB GOI MOSFET along the <110> longitudinal direction. In previous studies, a clear stress relaxation at the GOI channel width direction was observed as the GOI channel width becomes narrower of the ETB GOI MOSFET along the <110> longitudinal direction (from channel width ~ 200 nm) [5]. The results observed in this study are generally well correlated with the results obtained in strain relaxation measurements of carbon-doped Si nanowires along <100> by X-ray diffraction with synchrotron radiation [6]. The difference in the strain relaxation mechanism depending on the GOI channel direction may affect the electrical properties, and we consider that it is useful to be physical elucidation of the carrier mobility enhancement. Acknowledgements This work was supported by JSPS KAKENHI Grant Number 22H00208, Japan. A part of this work was con-ducted at Takeda Sentanchi Supercleanroom, The University of Tokyo, supported by "Nanotechnology Platform Program" of MEXT, Japan, Grant Number JPMXP09F-20-UT-0007.

Mitigating stimulated Brillouin scattering in multimode fibers with focused output via wavefront shaping.

The key challenge for high-power delivery through optical fibers is overcoming nonlinear optical effects. To keep a smooth output beam, most techniques for mitigating optical nonlinearities are restricted to single-mode fibers. Moving out of the single-mode paradigm, we show experimentally that wavefront-shaping of coherent input light to a highly multimode fiber can increase the power threshold for stimulated Brillouin scattering (SBS) by an order of magnitude, whilst simultaneously controlling the output beam profile. The SBS suppression results from an effective broadening of the Brillouin spectrum under multimode excitation, without broadening of transmitted light. Strongest suppression is achieved with selective mode excitation that gives the broadest Brillouin spectrum. Our method is efficient, robust, and applicable to continuous waves and pulses. This work points toward a promising route for mitigating detrimental nonlinear effects in optical fibers, enabling further power scaling of high-power fiber systems for applications to directed energy, remote sensing, and gravitational-wave detection.

On the selective mode excitation of wide tunable MEMS capacitive resonator

On the selective mode excitation of wide tunable MEMS capacitive resonator

Hybrid Spin and Anomalous Spin-Momentum Locking in Surface Elastic Waves.

Transverse spin of surface waves is a universal phenomenon which has recently attracted significant attention in optics and acoustics. It appears in gravity water waves, surface plasmon polaritons, surface acoustic waves, and exhibits remarkable intrinsic spin-momentum locking, which has found useful applications for efficient spin-direction couplers. Here we demonstrate, both theoretically and experimentally, that the transverse spin of surface elastic (Rayleigh) waves has an anomalous sign near the surface, opposite to that in the case of electromagnetic, sound, or water surface waves. This anomalous sign appears due to the hybrid (neither transverse nor longitudinal) nature of elastic surface waves. Furthermore, we show that this sign anomaly can be employed for the selective spin-controlled excitation of symmetric and antisymmetric Lamb modes propagating in opposite directions in an elastic plate. Our results pave the way for spin-controlled manipulation of elastic waves and can be important for a variety of areas, from phononic spin-based devices to seismic waves.

Giant acceleration of polaron transport by ultrafast laser-induced coherent phonons.

Polaron formation is ubiquitous in polarized materials, but severely hampers carrier transport for which effective controlling methods are urgently needed. Here, we show that laser-controlled coherent phonon excitation enables orders of magnitude enhancement of carrier mobility via accelerating polaron transport in a prototypical material, lithium peroxide (Li2O2). The selective excitation of specific phonon modes, whose vibrational pattern directly overlap with the polaronic lattice deformation, can remarkably reduce the energy barrier for polaron hopping. The strong nonadiabatic couplings between the electronic and ionic subsystem play a key role in triggering the migration of polaron, via promoting phonon-phonon scattering in q space within sub-picoseconds. These results extend our understanding of polaron transport dynamics to the nonequilibrium regime and allow for optoelectronic devices with ultrahigh on-off ratio and ultrafast responsibility, competitive with those of state-of-the-art devices fabricated based on free electron transport.

Vibrationally Hot Reactants in a Plasmon-Assisted Chemical Reaction.

Recent studies on plasmon-assisted chemical reactions postulate that the hot electrons of plasmon-excited nanostructures may induce a non-thermal vibrational activation of metal-bound reactants. However, the postulate has not been fully validated at the level of molecular quantum states. We directly and quantitatively prove that such activation occurs on plasmon-excited nanostructures: The anti-Stokes Raman spectra of reactants undergoing a plasmon-assisted reaction reveal that a particular vibrational mode of the reactant is selectively excited, such that the reactants possess >10 times more energy in the mode than is expected from the fully thermalized molecules at the given local temperature. Furthermore, a significant portion (∼20%) of the excited reactant is in vibrational overtone states with energies exceeding 0.5 eV. Such mode-selective multi-quantum excitation could be fully modeled by the resonant electron-molecule scattering theory. Such observations suggest that the vibrationally hot reactants are created by non-thermal hot electrons, not by thermally heated electrons or phonons of metals. The result validates the mechanism of plasmon-assisted chemical reactions and further offers a new method to explore the vibrational reaction control on metal surfaces.