In this presentation, the new findings of our work will be introduced. Amino acid-dependent chirality evolution can widen the current understanding of chirality evolution in nature, and the use of helicoid structures with nanogaps incorporated with surface-enhanced Raman scattering (SERS) can open a new avenue for chiral spectroscopic analysis.

Surface-enhanced Raman scattering (SERS) is known to be a powerful spectroscopic tool enabling trace-level detection of organic molecules on a plasmonic surface in aqueous systems without spectral interference of water, owing to the insensitivity of SERS to liquid water. SERS detection of interfacial water has been limited in some specific conditions, and the underlying mechanism about such an anomaly is poorly understood. In this talk, we will present low-frequency SERS observation of fast Debye relaxation of water, hydrogen-bond vibrations, and intramolecular OH vibrations in the wide frequency range covering from the terahertz (THz) region to the mid-infrared (MIR) region, which demonstrates that breaking of hydrogen bond networks of water is essential to make liquid water detectable in SERS.

We employ the anti-Stokes surface-enhanced Raman scattering to show that the reactants in plasmon-catalyzed photochemical reactions are excited to overtone vibrational quantum states. We use the light intensity and wavelength dependence of the overtone quantum population to identify the non-thermal contribution to the overall activation and the activation mechanism.

We fabricate SERS sensors by inkjet printing of colloidal Au nanoparticles on porous paper substrates. Using a modified commercial inkjet-printer, SERS sensors are prepared with multiple printing passes. SERS response is correlated with their diffuse reflectance characteristics. Chemical analyte detection is only possible with both SERS and diffuse reflectance measurements from sensors that has been subjected to more than 5 printing passes. This suggests that the simpler diffuse reflectance measurement can be used as an alternative method to characterize and optimize the SERS performance of the printed sensors. Sensors with a higher number of printing passes exhibit a much stronger SERS response from strong adsorbing analytes such as benzenethiol molecule. We compare the performances of 8 and 15 printing passes sensors with the commercially available paper-based sensors, p-SERS. We calculate their Relative Enhancement Factor (REF) by comparing their performances to the first order phonon vibration of Si, which serves as a reference standard. Lastly, we demonstrate the use of such sensors for the detection of chemical aerosols.

Raman spectroscopy has been used to sense molecule thanks to the specific vibration mode. However, the process is inefficient and strongly correlated with local electric field. The field oscillation in a confined nanostructures of metal can induce localized surface plasmon resonances (LSPR) that can induce high electric field. Therefore, Raman signal can be enhanced, so called surface-enhanced Raman scattering (SERS). Furthermore, to enhance light-matter interaction, we aim to engender electromagnetic coupling between LSPR in metal and cavity modes in dielectric. We proposed a Au-NPs/p-NiO/Au film (ANA) heterostructure to introduce strong coupling and evaluate the effect toward Raman sensing. We design and simulate the possible structure to realize the optical benefit. The heterostructure is experimentally realized via air-liquid interface self-assembly, fulfilling the ultrastrong coupling condition with Rabi splitting energy larger than 650 meV. The demonstrations brought out the bright future of plasmonic hybrid platform that can be further improved for sensing application.

Plasmon-enhanced Electronic and Vibrational Raman Spectroscopy in multiresonant nanolaminate nano-optoelectrodes enables monitoring of voltage-driven Faradaic and non-Faradaic processes at electrode-electrolyte interfaces, paving the way for advances in optical voltage biosensing, hybrid electronics-photonics signal transduction, and interfacial electrochemical monitoring.

Tip-enhanced Raman Spectroscopy (TERS) is a nanoscale chemical analysis and imaging method with a spatial resolution of <10 nm, even at ambient conditions. TERS relies on the enhancement of the local electromagnetic field by a plasmonic metal nanostructure that is scanned over the sample by means of a scanning probe microscope, using either AFM or STM feedback. Analogous to SERS, the local electromagnetic field of Raman scattered light is enhanced by many orders of magnitude in TERS. However, the study of sensitive materials with TERS is not straightforward, partially due to sample degradation. The working principle, experimental realization, and capabilities of TERS will first be presented. We will then discuss recent applications of TERS to the spatially resolved chemical analysis and imaging of molecular systems. Examples from recent TERS studies in our laboratory will be chosen, focusing on catalysis, catalyst poisoning, active sites and reactive intermediates on catalyst surfaces, as well as their spatial arrangements leading to reaction.

The advancement of quantum photonic technologies relies on the ability to precisely control the degrees of freedom of optically active states. First, motivated by recent evidence showing that nanowrinkles generate strain-localized room-temperature emitters, we demonstrate a method to intentionally induce wrinkles with collections of stressors. We show that long-range wrinkle direction and position are controllable with patterned array design, forming quantum emitters as evidenced by cryogenic anti-bunched emission. Next, we realize real-time, room-temperature tunable strong plasmon-exciton coupling in 2D semiconductor monolayers enabled by a general approach that combines strain engineering plus force- and voltage-adjustable plasmonic nanocavities. We show that the exciton energy and nanocavity plasmon resonance can be controllably toggled in concert by applying pressure with a plasmonic nanoprobe, allowing in operando control of detuning and coupling strength, with observed Rabi splittings >100 meV. We identify distinct polariton bands and dark polariton states, and map their evolution as a function of nanogap and strain tuning.

We have studied on the control of molecular properties in the spaces of various optical cavities, such as plasmonic metal nanostructures and Fabry-Perot mirrors. Single-molecule observations based on surface-enhanced Raman scattering (SERS) spectroscopy are used for the findings on unique resonant enhancements of SERS through exotic electronic excitation processes at electrified interfaces. Plasmonic surface lattices and Fabry–Perot mirrors allow the formation of electronic and vibrational strong coupling states, respectively, showing unique properties of molecule polaritons, even under dark conditions, i.e., without photo-illumination.

To broaden the spectral access of surface enhanced Raman spectroscopy (SERS) and improve the spectral sensitivity to collective molecular and lattice structural dynamics, low-frequency SERS in the terahertz region (LF-SERS, THz-SERS) is at the forefront of current development. In this spectral region, prominent spectral continuum backgrounds, sometimes strongly asymmetrical, are often observed. In this talk, we report our finding and analysis of these spectral continuum backgrounds unique to the THz region. We survey the physical origins of spectral continuum from molecular and lattice relaxational motions in liquid and solid phases as well as electronic Raman scattering processes. We show the observed asymmetrical background originates from the inherent asymmetry of the Bose-Einstein factor, which become apparent in the low frequency region. Accounting for this asymmetrical factor not only retrieves the symmetrical spectral response, but also provides a way to analyze the physical origins of the signal photons and local temperature.

Plasmonics deals with understanding and manipulating the interaction between light and matter at a scale that is significantly smaller than the wavelength of light (e.g., metal nanoparticles), and chemical nanoplasmonics is mainly about the study and use of nanoscale chemistry for advancing plasmonics and the use of plasmonics to address key issues and challenges in chemistry and other related fields. Designing, synthesizing and controlling metal nanostructures with a superhigh precision for a large number of structures are the keys to the reliable and widespread use of plasmonic nanostructures in chemistry, materials science, optics, nanoscience, biotechnology and medicine. Here, I will share the design, synthetic strategies and characterization results of molecularly tunable and structurally reproducible plasmonic nanostructures including metal nanogap structures, multi-component metal nanoparticles and gold nanocatenanes with strong, controllable and quantifiable plasmonic signals (e.g., quantitative surface-enhanced Raman scattering). I will then show their potential in addressing some of important challenges in plasmonics, biosensing, bioimaging and therapeutics, and discuss h

The controllable manipulation of bandgap, radiative emission, and energy transfer in low-dimensional quantum materials provides a versatile platform for a range of quantum photonic devices. Moreover, the understanding and precise regulation of nanoscale behaviors exhibited by excitonic quasiparticles, such as excitons and trions, in low-dimensional semiconductors are paramount for the development of highly efficient nano-excitonic devices. In this study, we introduce a tip-induced nano-spectroscopic approach for dynamically controlling light-matter interactions at the nanoscale. We then demonstrate a series of tip-induced nano-engineering experiments exhibiting plasmon-exciton interactions in quantum dots and atomically thin semiconductors. Our research shows a novel strategy for the creation of robust, tunable, and ultracompact nano-excitonic devices utilizing low-dimensional semiconductors.

Scanning probe technologies offer unparalleled insights into nanoscale light-matter interactions but are often hindered by the complex interplay of concurrent light-induced effects. To overcome this limitation, we have recently developed a technique known as decoupled optical force nanoscopy (Dofn). Dofn uniquely separates light-induced forces—such as optical gradient, photoacoustic, and photothermal effects—by employing a designated modulation and demodulation of the phases of measured optical forces. In my presentation, I will discuss how Dofn enables the precise probing of nanoscale chirality emerging from nanostructures, providing new avenues for understanding and manipulating light-matter interactions at the nanoscale.

We leverage the Morpho and Papilio Blumei butterfly wings and the Chalcothea Smaragdina beetle shell, as a novel imaging platform to elucidate the fibrous properties of biological tissue. Unstained and fixed murine breast cancer tissue sections are studied with high and low collagen content. By interfacing histological tissue sections with the studied nature-derived structural color system, we achieve selective structural coloration of the tissue based on its fibrous properties in a contact-free and label-free manner. Using Jones calculus and color theory, we define key figures of merit which quantify the anisotropic optical properties of the fibrous tissue for the tissue-structural color system and for the tissue on its own. The enhancement of the tissue optical anisotropy with the nature-derived metasurfaces will then be assessed, showcasing the potential of this technique in various diagnostic applications.

We present a novel approach to designing high-quality bound states in the continuum (BIC) in dielectric metasurfaces using magnetic dipole resonances coupled to a mirror, enabling strong light–matter interaction without the need for asymmetric geometry or polarization dependence. This simplifies fabrication and enhances practical applications, demonstrated by a record-breaking third harmonic generation efficiency. Additionally, we showcase superior modulation characteristics with a pulsewidth-limited switching time of 100 fs and a remarkably low energy consumption, facilitated by the narrow photonic resonance of our metasurface. Our findings offer significant potential for optical sensing, quantum photonics, and nonlinear devices.

Metamaterial assisted illumination nanoscopy (MAIN) has recently been proven to be a promising approach for super-resolution fluorescence microscopy with exceptionally large improvement factors in imaging resolution. In this talk, I will present our most recently results of MAIN using the start-of-the-art hyperbolic material, and achieving super-resolution for label-free imaging. The metamaterial assisted illumination technique represents a unique solution for both fluoresce and label-free super-resolution imaging with low phototoxicity and potentially high-speed.

The polarization-sensitivity of an optical array - a periodic assembly of subwavelength resonators - has been hypothesized to be correlated to the degree of asymmetry of its individual resonators. However, the lack of a global quantifying measure of asymmetry makes the purposeful design of asymmetric structures intractable. This work takes a bottom-up approach to investigate how the controlled violation of two-dimensional symmetry in regular polygon-shaped nanoscale resonators affects the polarization sensitivity of lattice resonant silicon nitride nano-optical arrays. By focusing on visible light, these arrays serve as color sensors for diverse polarized light domains. While rectangle-shaped resonators lack chirality for sensing varying chiral light orientations, removing corners enables the selective enhancement of near-field electric and magnetic moments in response to clockwise- and counterclockwise-orientations of circularly polarized light. This selective enhancement of near-field electric and magnetic moments leads to unique far-field spectra and enables the development of a chiral light orientation color sensor.

Intracellular delivery of biologically active molecules is an essential tool in modern biotechnology for both fundamental research and therapeutic applications. One of the primary approaches to current intracellular delivery is carrier-based methods, and nanotechnology has greatly contributed to achieving practical carrier-based intracellular delivery. Nanoparticles have emerged as particularly promising vehicles for intracellular delivery. Another mainstream delivery system involves membrane disruption methods using physical techniques such as electrical, mechanical, and thermal stimulation. Micro- and nanoneedles are among the promising tools in membrane disruption methods. In our presentation, we introduce novel approaches for intracellular delivery using a plasmon-based nano-needle. We demonstrate intracellular nitric oxide (NO) gas delivery by coating photosensitive NO release metal-organic framework (NOF-1) on the surface of a plasmonic metal nanowire, achieving time-controlled NO release. Due to the plasmon waveguide phenomenon occurring on the surface of a metal nanowire, we successfully demonstrate remotely controlled NO release from our NOF-1 coated nanowire.

Polymerase chain reaction (PCR) is considered as the gold standard for nucleic acid detection, but the employed Peltier block also make commercial PCR system bulky, time and energy consumable. Photonic PCR can be a good candidate for replacing the traditional heating strategy because of its excellent photothermal properties. Bifunctional plasmonic magnetic nanoparticles (PMNs) combined the photothermal and magnetic properties, endowing it excellent potential in photonic PCR. PMNs are prepared by gold coating outside the magnetic core, which can be used as the nanoheater for heating PCR solution to 90°C in several seconds upon the irradiation of infra-red light. After PCR, the PMNs can be separated by magnet easily from solution for reducing fluorescence quenching. Furthermore, our PPT-RTPCR platform based on the photothermal effect of PMNs for RNA detection shows limit of detection (LOD) as low as 1.05 copies/μL, which is comparable with the reference commercial PCR at 0.92 copies/μL.

We introduce wearable chemical sensors based on flexible surface-enhanced Raman spectroscopy (SERS) substrates. These wearable SERS sensors, composed of gold- or silver-coated polyvinyl alcohol nanomesh, facilitate non-invasive, on-site detection of the wearer's biofluids through highly sensitive, label-free, multiplexed chemical analysis. In this talk, a recent breakthrough in this field, namely the development of highly scalable SERS sensors is presented. The excellent performance of the sensors, namely, high sensitivity, flexibility, stretchability, durability, reproducibility, uniformity, and wearability, are discussed. Furthermore, we show their diverse applications ranging from food safety testing to health monitoring.

Plasmonic nanogaps can intensify electromagnetic (EM) fields in their confined narrow areas, providing superior optical enhancement such as surface-enhanced Raman scattering (SERS). Although a narrower nanogap is more beneficial in magnifying the EM field, the highest SERS efficiency occurs in ~1 nm nanogap due to the quantum tunneling effect in sub-nm nanogap. We synthesized ~1-nm plasmonic nanogap embedded nanoparticles with nanobridges, producing a strong SERS signal. The nanogaps can be controlled by DNA sequence, DNA grafting density, and solution condition. Nanogap thickness, the number of nanobridges, and surface morphology can control the EM field within ~1-nm intra-nanogap, which determines the SERS feature of the nanoprobes.

Research in plasmonics has triggered intriguing studies of nonlinear photoemission from metallic nanostructures during the past decade. This has led to new avenues in electron microscopy, including the generation of attosecond electron pulse trains [1] and the probing of the electromagnetic near fields with attosecond resolution. Recent demonstrations of the photoemission of strongly Coulomb-correlated few-electron number states from metal nanotips open up interesting perspectives for enhancing the nonlinearity of multiphoton electron emission by increasing the electron number. Here, we explore this route in interferometric studies of multiphoton emission from sharp metal tips using few-cycle near-infrared pulses at 2 µm. We demonstrate multiphoton emission with up to 19th order and essentially single peak autocorrelation traces with sub-fs peak width. Studies of the carrier-envelope phase dependence of the photoemission suggest generation of isolated attosecond electron pulses in the multiphoton regime.

Free electrons have strong potential as tools that allow us to sample and modify quantum optical fields. Thanks to recent advancements in electron microscope technology and the manipulation of free electrons, including the synthesis of femtosecond electron wave packets that are synchronized with femtosecond light pulses, we are now able to control the longitudinal and transverse wave functions of free electrons and their interaction with nanoscale optical excitations. In this presentation, we will offer an overview of the fundamental principles ruling the interactions between free electrons, light, and photonic nanostructures. We will place particular emphasis on exploring the quantum aspects of these interactions, including a variety of intriguing challenges and emerging possibilities. Furthermore, we will discuss potential applications such as noninvasive spectroscopy and microscopy, the potential for the measurement of nonlinear optical responses with nanoscale spatial detail, the manipulation of density matrices related to free electrons and optical modes within small-scale specimens, and optical manipulation of electron beams.

This work investigates the linear and nonlinear optical force between a sharp tip and a sample that had been photo-excited by using the time-resolved pump-probe force microscopy. By measuring this force, we are able to study the nonlinear polarization of the material. Our findings reveal that the ground state bleaching of excitons in 2-D materials is influenced by the same excited state dynamics observed in optical pump-probe experiments. We have demonstrated that ultrafast pump-probe force microscopy is a non-optical detection technique that can achieve nano-femto scale resolution, thus enhancing the sensitivity of pump-probe experiments to the point where they may be used for single-molecule studies.

The interaction between electron beams and nanoscale optical fields offers unique ways of studying ultrafast processes in matter with an unpreceded spatiotemporal and spectral resolution, currently approaching a combined Å–fs–meV resolution. However, the electron kinetic energies commonly employed in most ultrafast electron microscopes (tens to hundreds of keV) far exceeds that of the optical fields (few eV), thereby resulting in small electron–light coupling and thus limiting the emergence of quantum effects beyond the perturbative regime; indeed, in such context the interaction is well described by the classical, point-electron and non-recoil approximations. Here, we theoretically investigate electron–light–matter interactions between optical modes and low-energy electrons with comparable energies, and find substantial quantum and recoil effects imprinted in the spectrum of surface-scattered electrons interacting with both confined, nanoscale optical modes or plane-wave photons.

Transition metal dichalcogenides (TMDs), which are layered compounds, have attracted attention as next-generation semiconductor materials due to their direct bandgap and photoluminescent properties when in monolayer form. In particular, the heterostructures of p-type semiconductor WSe2 and n-type semiconductor MoS2 exhibit Type II band offsets, leading to superior charge separation efficiency, thus offering potential applications in optoelectronic and light-emitting devices. However, their thin layers result in low light absorption efficiency and relatively low photoluminescence quantum yields, posing challenges for device construction utilizing TMD heterostructures. Enhancement of light absorption efficiency and quantum yield improvement can be achieved through electromagnetic field enhancement by localized surface plasmon resonances. This study elucidated carrier dynamics through time-resolved measurements to reveal the details of charge separation induced in TMD heterostructures and to investigate the impact of plasmon resonance on charge recombination rates and photoluminescence intensity.

Mid-infrared photothermal (MIP) microscopy is an advanced type of infrared (IR) spectroscopy that surpasses conventional IR microscopy in terms of sensitivity and spatial resolution. Rapid progress on this field has revealed the excellent applicability of MIP microscopy in super-resolution IR imagings of polymer surfaces to bio-materials. Nonetheless, further improving the MIP performance would open the possibility of analyzing chemicals in a new analytical system. In light of this, we have been actively investigating the use of colloidal plasmonic nanowires in MIP microscopy to enhance performance. High crystalline, wet-synthesized colloidal metallic nanowires with several nm in length show strong resonance with mid-IR laser light theough Fabry-Perot resonance, which reaults in a a resonant-surface-enhanced infrared absorption (SEIRA) effect. Utilizing the strong near-field light that is confined to the metallic nanowire, the enhanced MIP performance in terms of both spatial resolution and surface/detection sensitivity can be achieved in optical- and AFM-MIP microscopy. This invited talk will feature an update on the latest progress made on this topic.

Mid-infrared photothermal (MIP) microscopy is an emerging tool for high-resolution vibrational imaging of biological and material samples. In a MIP microscope, a pulsed IR beam induces a thermal lens of a liquid specimen or thermal expansion of a particle. The photothermal effects are probed by a visible beam. MIP imaging can be implemented in scanning mode or wide-field mode. In wide-field mode, we present a wide-field interferometric defocus-enhanced mid-infrared photothermal (WIDE-MIP) microscope, that addresses the unmet need for identification of single virus, including detection of viral proteins, DNA and RNA. In scanning mode, we demonstrate click-free MIP imaging of azide-tagged trehalose in live mycobacteria and reveal trehalose pathway dynamics without photobleaching. MIP mapping of unreacted azide after click reaction further reveals click reaction heterogeneity within a bacterium. Broader applications of azido photothermal probes (Apps) in unraveling carbohydrate utilization inside in yeast and mammalian cells are demonstrated.

Nanospectroscopy, using the apertureless scattering method, has become a standard method for achieving spectral analysis of surfaces with spatial resolution approaching 10 nm. The method involves light scattered from the tip of an atomic force microscope (AFM) in combination with a Michelson-type interferometer and infrared detector. A high spectral radiance source (such as a laser) is necessary for overcoming the extremely small scattering efficiency. Examples include the quantum cascade laser (QCL), although having somewhat limited spectral coverage. Here we describe our capabilities at the National Synchrotron Light Source II (NSLS II) using a truly broadband (continuum) source of synchrotron infrared to perform FTIR spectroscopy into the far-IR. A key component for achieving this is a long-wavelength MCT photoconductor operating at T=4.2K and reaching wavelengths beyond 50 microns (i.e. below 200 cm-1). Results showing polaritons in 2D materials as well as semiconductor transport characterization will be included. *This work supported by the U.S. DOE under contract DE-SC0012704 at NSLS-II and BNL. The Neaspec SNOM used in this work provided courtesy of Attocube GmbH.

We have been studying the spectral properties of strong coupling states between metallic nanostructures and excitons. We have reported on the modulation and dynamics of the near-field spectral properties of exciton-plasmon strong coupling systems. However, it has been difficult to quantitatively evaluate the phase relaxation times experimentally because the central wavelength of the exciton is far from the central wavelength of the Ti:sapphire laser. Therefore, in this study, we fabricated an exciton-plasmon strong coupling system using an exciton with a central wavelength near 800 nm and aimed to quantitatively clarify the relationship between the near-field spectral characteristics and the phase relaxation dynamics of the system. Peak splitting was observed both in the far-field and near-field spectra, indicating the successful formation of a strong coupling state. We are now trying to measure the phase relaxation dynamics of the strong coupling system.

Dark-field microscopy (DFM) is a widely used imaging tool due to its high-contrast capability in imaging label-free specimens. However, traditional DFM requires optical alignment to block the oblique illumination and the resolution is diffraction-limited to wavelength scale. In this work, we present a single frame super resolution method using plasmonic dark-field microscopy (PDF) and deep learning image reconstruction algorithm. Based on our framework, we demonstrated more than 2.5 times resolution enhancement on various objects. We highlight the potential of our technique as a compact alternative of traditional DFM with enhanced spatial resolution.

This study continues examining the topological dependence of diffuse reflectance for NIR-SWIR absorbing dyes on substrates. The dependence is examined by case studies using composite-material layers that include IR absorbing dyes on fabric substrates. Understanding the topological dependence of diffuse reflectance can assist in determining optimal composite-material configurations for specific reflectance specifications, which can include UV protecting materials.

Super-resolution fluorescence imaging techniques have emerged as a pivotal solution to examine sub-cellular processes. Among these techniques, Localized Plasmonic Structured Illumination Microscopy (LPSIM) has shown remarkable potential to achieve three-fold resolution improvement and video imaging speed by leveraging plasmonic nanoantenna arrays. However, the conventional design process for these arrays is hindered by time-consuming trial-and-error simulations and limited design degree of freedom. We introduce a hybrid inverse design framework that combines deep learning and genetic algorithm-based optimization to enhance the capabilities of LPSIM. Our approach yields optimized nanoantenna arrays that demonstrate superior reconstruction quality, offering robustness against noise, and requiring fewer measurements. The proposed method not only streamlines the efficient design process for LPSIM nanoantenna arrays, but also opens new avenues for exploring and optimizing plasmonic nanostructures for a wide range of applications beyond imaging.

Localized surface plasmon was formed with plasmonic nanohole arrays for measurement of the Raman spectrum in polystyrene beads. For the selective acquisition of enhanced Raman signals, we used the surface-enhanced Raman spectroscopy (SERS) system employing particle manipulation within the electrode chip. This integration involves the amalgamation of metal nanostructures and microelectrodes to assess the electrophysiological characteristics of microparticles. The manipulation of particle movement, contingent upon their electrical properties such as conductivity and permittivity, can be controlled through modulation of the frequency of the applied alternating current field. In our integrated system, Raman spectroscopy can be conducted with enhanced intensity. Real-time measurement of enhanced Raman scattering is achieved by controlling the direction of force exerted on single particle and selectively trapping it on the nanostructured surfaces.