Lack of analytical tools capable of providing nanoscale chemical characterization of biomembranes under ambient conditions in a non-invasive and label-free fashion limits our understanding of the vital cellular processes. Tip-enhanced Raman spectroscopy (TERS) is one of the very few analytical techniques capable of meeting this challenge by combining the single-molecule sensitivity of surface-enhanced Raman spectroscopy and nanoscale spatial resolution of scanning probe microscopy [1, 2]. In this talk, I will discuss how TERS can be applied in both atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) modes for correlative topographical and chemical analysis of biological membranes at the nanoscale. Specifically, I will present some recent studies from our laboratory to demonstrate the application of hyperspectral TERS imaging for nanoscale visualization of molecular disorder in supported lipid membranes [3], phase separation in mixed lipid (DPPC, DOPC and cholesterol) membranes and molecular complexity of a human cell membrane [4]. For example, in a Au(111) supported DPPC monolayer sample, we achieved reproducible STM-TERS imaging together with the correlative topography imaging for the first time. Furthermore, TERS images revealed the presence of structural defects in the DPPC monolayer and ratiometric analysis of the C-H stretching region revealed domains of high and low molecular disorder with ca. 20 nm spatial resolution. We went a step further and investigated the molecular complexity of a real-life human cell membrane using TERS. Hyperspectral AFM TERS imaging showed segregation of nanoscale phenylalanine, histidine, phosphatidylcholine, protein, and cholesterol-rich domains in a pancreatic cancer cell membrane ca. 20 nm spatial resolution. These results demonstrate the unique ability of TERS to provide label-free and non-destructive characterization of biomembranes with nanoscale resolution under ambient conditions. References [1]Kumar, N., Mignuzzi, S., Su, W., Roy D., Tip-enhanced Raman spectroscopy: Principles and applications. European Physical Journal Techniques and Instrumentation 2015, 2(1), 9. [2] Kumar, N., et al., Nanoscale mapping of newly-synthesised phospholipid molecules in a biological cell using tip-enhanced Raman spectroscopy. Chemical Communications, 2017, 53 (16), 2451-2454. [3] Pandey, Y., Kumar, N., Goubert, G., Zenobi, R., Nanoscale Chemical Imaging of Supported Lipid Monolayers using Tip‐Enhanced Raman Spectroscopy. Angewandte Chemie. 2021, 133, 19189-19194. [4] Mrđenović, D., Ge, W., Kumar, N., Zenobi, R., Nanoscale Chemical Imaging of Human Cell Membranes Using Tip-Enhanced Raman Spectroscopy. Angewandte Chemie International Edition, 2022, 61, e2022102.

In this talk, I will present ultrastable tip-enhanced Raman spectroscopy (TERS) setup that has a home-built feedback system to compensate possible drift in all three dimensions during hyperspectral Raman imaging. The ultrastable optical nanoimaging system enables long-duration and high resolution TERS imaging of large-sized nanostructured samples far beyond conventional TERS imaging systems. I will show applications of ultrastable TERS imaging of 2D material systems, in which nanoscale characterization of unique physical properties of atomic layer of 2D semiconductors and structural properties of self-assembled biomolecular nanoarchitectures on 2D materials is performed.

Tip enhanced Raman Scattering (TERS) imaging is gaining more and more popularity for the nanoscale spectroscopic characterization of novel 2D semiconductors and their vertical and lateral heterostructures. Gap mode TERS imaging when a thin sample is sandwiched between the TERS active SPM probe and plasmonic substrate like silver or gold, is the most popular and advantageous option due to strong enhancement of optical electric field in the tip-substrate cavity. Despite multiple examples of successful application of the gap-mode TERS imaging of nanoscale heterogeneities in 2D semiconductors such as transition metal dichalcogenides (TMD), the exact physics of complex interaction between the plasmons in the tip-substrate junction and excitons in TMDs remains poorly understood. One of the possible approaches towards probing complex resonant phenomena occurring in gap-mode TERS experimental conditions is performing TERS imaging with varied excitation wavelength [1]. I’ll demonstrate that the TERS spectra of WS2-WSe2 vertical heterostructures obtained with 785 nm, 632.8 nm, 671 nm and 593.8 nm excitation may differ significantly, the intensity of characteristic Raman bands of WSe2 at around 250-260 cm-1 being extremely low in case of 632.8 nm excitation but rising as we move to shorter or longer excitation wavelengths. I’ll discuss the advantages of concurrent two-color excitation TERS imaging as well as interesting counterintuitive variations of the intensity of TERS bands of the monolayer WS2 as the function of the excitation wavelength. Finally, I’ll demonstrate that the ratio of the intensity of the in-plane E2g and the out-of-plane A1g modes in the above- and below- the-band-gap non resonant TERS spectra of bilayer WS2 on silver inverses as we move from 532 nm to 785 nm excitation. Références [1] Andrey Krayev, Peng Chen, Humberto Terrones, Xidong Duan, Zhengwei Zhang, and Xiangfeng Duan The Journal of Physical Chemistry C 2022, 126, 11, 5218-522 DOI: 10.1021/acs.jpcc.1c10469

Plasmon nanofocusing enables to create an intense nanoscale light source through super-focusing of plasmons propagating on a metallic tapered structure toward its apex. Among various unique characteristics of plasmon nanofocusing, the broadband property has recently been gaining attention in the nanophotonics field, owing to several successful applications such as broadband super-resolution spectral imaging. Here, we introduce a more active way to apply broadband plasmon nanofocusing, which is nanoscale optical switching of electronic states of photochromic materials by ultraviolet and visible plasmon nanofocusing. We will also introduce a few more new applications of broadband plasmon nanofocusing such as absorption sensing.

Recent studies on plasmon-assisted chemical reactions postulate that the hot carriers of plasmon-excited nanostructures may induce a non-thermal vibrational activation of metal-bound reactants. However, a quantitative validation at the level of molecular quantum states is currently missing. I will present our recent spectroscopic studies on metal-bound reactants, which directly proves the non-thermal vibrational molecular excitations caused by plasmons.

We have recently found that the interaction between metallic nanostructures exhibiting localized surface plasmon resonance in the mid-infrared wavelength region and molecular vibrational modes significantly reduces the fluorescence and surface-enhanced Raman scattering signals. In addition, in the photochemical reaction of photochromic molecules, a phenomenon in which the reaction is promoted or suppressed has been observed. The purpose of this study is to clarify the phenomenon that the spectroscopic characteristics and reaction characteristics change due to the interaction between plasmons and excited state molecules or molecular vibrational modes through ultrafast spectroscopy.

Deep ultraviolet (DUV) light plays a critical role in many spectroscopy and imaging techniques. To advance these techniques, it is essential to develop devices that can provide substantial field enhancement within the DUV range. This presentation will introduce metasurfaces composed of various dielectric materials. Specifically, we will showcase Si metasurfaces demonstrating plasmonic modes induced by the "photon-doping effect," resulting in significant field enhancement at 266 nm. Furthermore, we will discuss DUV metasurfaces made of low-loss dielectric material HfO2, which exhibit multiple high-quality-factor quasi-bound-state-in-the-continuum resonances. Potential applications of the reported metasurfaces will be discussed.

The origin and progression of a variety of leading health challenges, encompassing Alzheimer’s disease, heart disease, fibrosis and cancer, are directly linked to changes in the presence and orientation of fibrous matter in biological tissue. Fibrous biological tissue exhibits distinct anisotropic optical properties, which can be leveraged for selective imaging. However, these naturally occurring light-matter interactions are inherently weak, posing barriers to their visualization. Here, we leverage anisotropic, colorimetric metasurfaces to selectively visualize disease-relevant fiber density and orientation in biological tissue. We then investigate versatile fiber-affecting diseases where metasurfaces hold great potential to achieve rapid, precise and low-cost tissue diagnostics.

Single-molecule biosensors employing fluorescence microscopy enable direct counting of molecular targets such as nucleic acids and proteins. Whereas highly specific sandwich assays have been developed using fluorescence probes these assays provide intrinsically low single-molecule fluorescence signals. As a result, these sensors are not compatible with complex biological fluids such as blood serum, and they require research-grade optical microscopies. I will present our recent work on nanoplasmonic biosensors that exploit plasmon-enhanced fluorescence (PEF) thereby enabling single-molecule biosensing with a strongly enhanced signal-to-noise ratio. The biomarker concentration is quantified by the direct counting of single-molecule interactions, in this case for DNA. The strongly enhanced fluorescence signals enable single-molecule sensing directly in full blood serum with negligible background. Employing fluorescent probes with different emission wavelengths allows for multiplexed biosensing. The bright signals allow for single-molecule sensing in a highly miniaturized optical microscope. This further obviates the need for high numerical aperture objective lenses and paves the way to deployment of the sensors in healthcare and environmental applications.

Tungsten and Molybdenum Disulfide flakes were investigated using tip-enhanced Raman spectroscopy, nanomechanical measurements and wide-field Raman Microscopy with Stochastic optical reconstruction Post processing. These microscopy approaches are well adapted to understand how the surface of 2D materials develop and how their mechanical properties can reveal hidden structures leading to a better understanding of their formation. Raman and luminescence Wide field microscopies with Stochastic optical reconstruction are also of interested to reveal photophysical processes at the edges of the flakes with a well adapted temporal resolution.

Field-enhanced biosensing has attracted considerable attention due to its capability of highly sensitive biomolecular detection and analysis. Here, we demonstrate a variety of nanogap-enhanced spectroscopies (fluorescence, Raman, infrared absorption) of biomolecular complexes using plasmonic and dielectric nanostructures. Plasmonic nanogaps between gold nanoparticles and gold nanoholes enabled us to perform plasmonic digital biosensing with the limit of detection (LOD) down to sub-fM level. All-dielectric nanogap antennas also allowed us for highly sensitive fluorescence immunosensing.

The convergence of optical spectroscopy with scanning probe microscopy (SPM) provides a near-field sensitive platform that circumvents the limitations of light diffraction and can measure highly localized heterogeneity and phenomena. In ultrahigh vacuum (UHV) and at cryogenic temperatures the spatial resolution of these methods can be pushed towards the atomic scale. Here, efforts to use UHV scanning tunneling microscopy (STM) coupled with tip-enhanced Raman spectroscopy (TERS) to investigate the behavior and chemistry of molecular adsorbates on pristine and modified noble metal substrates are highlighted. Spectroscopic imaging provides the ability to visualize highly localized phenomena, while vibrational fingerprints derived from light-matter interactions can be used to understand their effects on a molecule’s or material’s structure. This can provide insight into adsorbate-substrate interactions crucial to the growth of nanostructures through surface chemistry, as well as the functionalization or modification of 2D materials. Moving beyond UHV-STM-TERS, ongoing work to develop a new versatile cryogenic UHV-SPM platform for other measurements will also be discussed. Specifically, STM-induced luminescence and the coupling of infrared light into the tip-sample junction provide the opportunity to investigate otherwise inaccessible properties of materials that include light-matter interactions and quasiparticles at the atomic scale.

Metamaterial assisted illumination nanoscopy (MAIN) has recently been proven to be a promising approach for linear super-resolution 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 14-fold imaging resolution improvement. The MAIN technology represents a unique solution for both fluoresce and label-free super-resolution imaging with low phototoxicity and potentially high-speed.

Optical spectroscopy is a powerful tool for chemical analysis, providing a wealth of information on structure, dynamics, and physical properties of matter. However, the diffraction limit of light hampers to directly resolve nanoscale structures. This physical limitation can be overcome by near-field optics. In particular, localized surface plasmon resonance of metal nanostructures gives rise to strong confinement and enhancement of electromagnetic fields, enabling ultrasensitive optical spectroscopy. I will discuss our recent development on atomic-scale optical spectroscopy at surfaces using a combination of low-temperature scanning tunneling microscopy and quantum plasmonics.

Near field scanning optical microscopy (NSOM) has evolved into a rich field of study with many different variants over the past 25 years. In this talk we review recent results from an entirely new modality of microscopy where optical imaging is performed by measuring the dipole-dipole interaction force (or the trapping force) between an optically driven sample and its mirror image in the tip. Photo induced force microscopy is capable of measuring both the linear and non-linear optical response of a sample on the nanoscale. We shall review recent data both in the visible and in the mid infrared range of the spectrum.

Upon illumination of the nanojunction formed between an atomically sharp tip and the sample, both the tip and specimen experience heating, followed by thermal expansion. Whereas sample expansion forms the basis of photothermal infrared (PTIR) microscopy, the effect of tip expansion remains unclear. In this contribution we model and measure the tip heating and expansion effect and quantify its contribution under conditions typical for light-based scan probe microscopy.

Various electrochemical TERS approaches were developed to probe electrochemical processes at the nanoscale, in real time and over a wide potential window (around 800 mV). EC-TERS was successfully applied to the investigation of the transformation mechanism of different molecular layer architectures, i.e. self-assembled monolayers (SAMs) on gold electrodes and grafted layers via controlled electro-reduction of aryldiazonium salts. A bimolecular electrochemical reaction path, difficult to evidence from the electrochemical response only, is proposed in light of the EC-TERS analysis. The comparison among the in situ reactivity of different layer structures allowed establishing structure-reactivity relationships.

We home-build a multimodal hyperspectral micro/nanoscopy (MHNano) which is combined optical spectroscopy with scanning probe microscopy to characterize the linear and nonlinear optics, e.g., tip-enhanced (two-photon) photoluminescence (TEPL/TE2PPL). MHNano enables optical measurements with scales from 100s micrometers to a few nanometers in a single platform. In this work, we demonstrate the spatial resolution of TEPL and TE2PPL with sub-5 nm from CdSe/ZnS semiconductor quantum dots (QDs) using sputtered plasmonic gold probe under ambient conditions. The capability of TEPL/TE2PPL paves the way for (non) linear photoluminescence-based spectral nanoimaging of realistic/heterogeneous molecular and material systems.

Plasmonics utilize light-matter interaction at a dimension smaller than the wavelength of light. Due to their flexible shapes and geometries, metal-based plasmonic materials can be a reliable tool for obtaining desired plasmonic near- and far-field properties. In this talk, I will first introduce the highly controlled synthesis of gold nanocubes. The strategy relies on surface-protecting agents-dependent growth kinetics control and shape-selective flocculation process, which yielded exquisite control over the shape with an ultrahigh yield exceeding 98%. In this manner, structural fine-tuning is translated into controllability and high uniformity of optical signals. Adapting the concept of plasmon hybridization theory, I also developed plasmonic cube-in-cube nanoparticles where the coupling between core and shell supports super-radiant plasmon modes with an increased radiative damping rate. As a result, the cube-in-cube nanoparticles showed the highest photoluminescence quantum yield reported for metallic nanostructures without photobleaching or photoblinking.

We proposed a new method for remote-plasmonic enhanced Raman scattering (RPERS) spectroscopy that overcomes the limitations of surface-enhanced Raman scattering (SERS) spectroscopy. RPERS uses long-range molecular-plasmon coupling to achieve high sensitivity and quantification in enhanced Raman spectroscopy. The authors constructed RPERS substrates consisting of Ag nanoislands and columnar silica structures, which demonstrated a 2x10^7 enhancement in Raman scattering for Rhodamine 6G molecules, even when the metal nanostructures and the analyte molecules were over 100 nm apart. The RPERS substrates also exhibited improved reproducibility, stability, and sensitivity compared to conventional SERS substrates.

Room-temperature optical manipulation of small molecules could be an important tool to explore novel way of chemical reaction pass. One of the interesting targets is to control of molecular motion beyond the limit of thermal fluctuation. It could result in distinct kinetics in chemical reaction. Especially for molecule systems in solution, electrochemical potentials of molecules are also modified by the deviation from normal solvated states under equilibrium. To increase optical force for a single molecule trapping, it has been recognized that resonant excitation of molecules should be controlled under the light illumination. Strongly interacting molecules with solid surfaces at electrified interfaces show the exotic behaviour of electronic excitation by localized surface plasmon. In this talk, we emphases that surface-enhanced Raman scattering can be used to evaluate the resonant excitation of target molecules at interfaces. Under such excitation, the surface diffusion of small molecules can be controlled by the optical force generated by the intensity gradient of a highly localized electric field.

Excitons in two-dimensional transition metal dichalcogenides (2D TMDCs) exhibit extraordinarily rich properties and offer enormous opportunities for the development of a variety of photonic, optoelectronic, and quantum devices. To fully utilize the rich properties of the 2D excitons, the development of effective means for control of their emission performances is highly desired. Plasmonic nanostructures possess large scattering and absorption cross-sections at their plasmon resonance wavelengths. They can concentrate light down to the nanometer region. Their plasmon wavelengths can be synthetically adjusted over a broad spectral region. Plasmonic nanostructures therefore offer an attractive and powerful means for the manipulation of the 2D excitons. In the recent years, we have made great effort on understanding the interaction between localized plasmons and the 2D excitons as well as on the control of the 2D excitons. We have observed strong coupling between magnetic plasmons and the 2D excitons in (Au nanocup)-on-(WSe2 monolayer/multilayer) hybrid structures. We have created localized strain on WS2 monolayer and detected the emissions from the localized 2D excitons. We have further constructed high-quality Fabry–Pérot nanoresonators by use of circular Au nanodisks supported on WS2 flakes. The cavity modes can strongly modulate the exciton emissions from the WS2 flaks.

The inverse Faraday effect is associated with light-induced magnetism. In nonmagnetic materials, the magnetic field scales with intensity; an electric field both produces surface charges and imparts momentum to those charges. The angular momentum of surface currents may induce a magnetic field, that is highly dependent on the shape nano-geometry. Here, we measure the Inverse Faraday Effect on nonmagnetic plasmonic nanodisks. We explore the effect of nanodisk aspect ratio. When the disk is thin, the plasmon resonance significantly red-shifts, which coincides with electron spillout.

Here, we show nano-Raman and nano-photoluminescence imaging of moire lattices in twisted TMD bilayers. The results show a strong optical intensity in the Raman and PL signals from the moiré domain walls. Comparing with AFM imaging, our results show exact agreement of with boundaries of the moiré cells. Further, nano-Raman spectroscopy of the domain walls provides a direct optical measure of the relative strain. In related work, we investigate strong coupling between TMD excitons and a tunable plasmonic nanocavity formed between a plasmonic tip and gold substrate. We demonstrate the ability to continuously tune between weak and strong coupling conditions where both upper and lower polariton bands are observed.

In this talk, I will discuss three “dark” aspects of symmetry-broken plasmonic nanocavities, namely 1) light-induced electromagnetic asymmetry [Nature Communications 2021, 12, 4326], 2) plasmon-induced optical magnetism [Laser & Photonics Reviews 2020, 14(9), 200068] and 3) optical magnetism driven multiphoton luminescence [Nano Letters 2021, 21(6), 2453-2406].

Label-free optical observation of transparent nano-objects is challenging. Here, we demonstrate PlAsmonic NanO-apeRture lAbel-free iMAging (PANORAMA) that addresses existing issues for both SPR and LSPR imaging. PANORAMA involves a standard commercial bright-field microscope without the need for laser or interferometric detection. PANORAMA can image, size, count, and monitor a single polystyrene nanoparticle down to 25 nm at a millisecond timescale. The extrapolated size limit of detection is sub-10 nm. Higher imaging speed is achievable with high-speed cameras. Molecular imaging is envisioned with functionalized substrates for single nanoparticle analysis for extracellular vesicles (e.g., exosomes) and pathogens (e.g., viruses).

Plasmon-based single live-cell nanowire endoscopy is a promising novel tool for understanding biological processes at a single-cell level. This technique has realized the highly sensitive sensing of Raman signals from site-specific areas in a single live cell with less invasiveness by applying the unique plasmonic properties of a metal nanowire to an endoscopic probe. This technique gives us essential information, such as the interaction between DNA and fluorescence dye inside a nucleus, or site-specific intracellular local pH. Furthermore, this technique has great potential to realize intracellular materials delivery systems by engineering the surface of nanowire probes.

Scanning ion-conductance microscopy (SICM) is a scanning probe imaging technique that specifically for studying samples under controlled liquid environments. With the advance in near-field optics, tip-enhanced Raman spectroscopy (TERS) combines the nanoscale spatial resolution imaging with the single-molecule chemical sensitivities of surface-enhanced Raman spectroscopy. Our goal is to translate the Ångström-scale spatial resolution of ultrahigh vacuum TERS to liquid environments. Therefore, we developed a SICM for TERS and applied it for interrogation of materials under ionic environments. We anticipate this new chemical imaging method will enable a more detailed understanding of soft biological and emerging energy materials in electrolyte solutions.

Motion of electrons is at the heart of any chemical transformation, photo-induced charge or energy transfer process in molecules. Contemporary techniques in ultrafast science have the capability to generate and trace the consequences of this motion in real-time, but not in real-space. Scanning tunnelling microscopy (STM), on the other hand, can locally probe the valence electron density in molecules, but cannot provide by itself dynamical information at this ultrafast time scale. In my talk, I will show you how dynamics of coherent superposition of valence electron states generated by < 5 femtosecond long carrier-envelope-phase (CEP) stable laser pulses, can be locally probed with angstrom-scale spatial resolution and 300 attosecond temporal resolution simultaneously, at the single orbital-level with the help of an STM, defying the previously established fundamental space-time limits [1-4]. Electronic motion in molecules is usually coupled with atomic motion, especially in molecules undergoing photo-induced charge/energy transfer, structural or chemical transformation. In order to understand this coupling, we have recently realized femtosecond broadband coherent anti-Stokes Raman spectroscopy (CARS) in an STM and it has enabled tracking of coherent atomic motions in a single graphene nanoribbon with sub-angstrom scale spatial, meV energy and ~30 fs temporal resolution, simultaneously [5-6]. Time-resolved CARS implemented in an STM is the key to probing both electronic and atomic motion at the same time. 1. Garg et al. Nature 359-363, 538 (2016). 2. Gutzler, Garg et al. Nature Reviews Physics 3, 441-453 (2021). 3. Garg et al., Nature Photonics, 16, 196-202 (2022). 4. Garg and Kern. Science 367 (6476), 411-415 (2020). 5. Luo et al. Nano Letters 22 (13), 5100-5106 (2022). 6. Luo et al. Under Review (2022). arXiv:2210.02561

In the first block, we will explain how the nearfield manipulates capture and release mechanisms. With increasing field strengths, the rate constant of the capture kinetics increases while the release kinetics decrease, favouring the former over the latter. As a result, the SM capture state is more likely and more stable than the release state above a specific nearfield strength threshold. In the second block, we will explain how the steering of two physical features can extend the effectiveness of the PBJ trapping: (i) the target molecule’s optical resonance and (ii) the localised surface plasmon resonance energy of the nanogap.

Free electrons have the ability to create and probe optical excitations with an unsurpassed combination of spatial and temporal resolutions, which are now approaching the submillielectronvolt/subnanometer level. Combined with ultrafast light pulses, temporal resolution is also possible, with solid prospects to reach the attosecond regime. In this presentation, we will review recent examples of the characterization of plasmonic structures with an unsurpassed combination of space-energy resolution by means of free electrons. Combined with the plasmonic enhancement produced by hotspots in nanostructured metallic structures, free electrons can also be used to probe optical modes with atomic spatial resolution, as well will illustrate with recent experimental examples, for which a tutorial theoretical description will be discussed.

Optical second-harmonic generation (SHG) is not only a powerful structural tool owing to its sensitivity toward crystallographic symmetry,[1] but also a coherent probe because of its instantaneous but non-dissipative response. Two-dimensional (2D) transition metal dichalcogenides (TMDs) are ideal media for SHG because of their strong excitonic resonances in the NIR and visible range. The atom-thick sheet-like geometry eliminates the need for phase matching in SHG and enables novel properties to be designed through artificial stacking. In this talk, we will present our recent findings on SHG interference in 2D TMD heterocrystals with a thickness of ~1 nm.[2,3] All 2D crystal samples were prepared by mechanical exfoliation and analyzed with a polarized and interferometric micro-spectroscopy setup powered by a tunable femtosecond Ti-sapphire laser. Using spectral phase interferometry, we directly showed that SHG in heterobilayers of MoS2/WS2 is governed by optical interference between two coherent SH fields that are phase-delayed differently in each crystal.2 We also quantified the frequency-dependent phase difference between the two, which agreed with polarization-resolved data and first-principles calculations on complex susceptibility. The second part of the talk will focus on the remarkable modulation of SHG intensity and phase induced by van der Waals-like interlayer interactions in MoSe2/WS2.3 We will discuss the potential causes of these effects and our plans for future investigations to elucidate the origins. The second-harmonic analogue of Young’s double-slit interference, demonstrated in this work, showcases the potential of custom-designed parametric generation through atom-thick nonlinear optical materials. References [1] Y. Li, Y. Rao, K. F. Mak, Y. You, S. Wang, C. R. Dean and T. F. Heinz, “Probing Symmetry Properties of Few-Layer MoS2 and h-BN by Optical Second-Harmonic Generation”, Nano Lett. 2013, 13, 3329 [2] W. Kim, J. Y. Ahn, J. Oh, J. H. Shim and S. Ryu, “Second-Harmonic Young's Interference in Atom-Thin Heterocrystals”, Nano Lett. 2020, 20, 8825 [3] W. Kim, G. Jeong, J. Oh and S. Ryu, “Second-Harmonic Generation in 2D Heterostructures Strongly Modulated by Interlayer Coupling”, in preparation

Photo-induced force microscopy (PiFM) is a scan probe technique that offers images with spectroscopic contrast at a spatial resolution in the nanometer range. PiFM utilizes the non-propagating, enhanced near field at the apex of a sharp tip to locally induce a polarization in the sample, which in turn produces an additional force acting on the cantilevered tip. This photo-induced force, though in the pN range or less, can be extracted from the oscillation properties of the cantilever, thus enabling the generation of photo-induced force maps. Since its inception in 2010, the PiFM technique has grown into a useful nano-spectrocopic tool that has expanded its reach in terms of imaging capabilities and applications. Nonetheless, the precise physical mechanism underlying the generation of photo-induced force is still a matter of ongoing research and debate, and a definitive explanation has yet to be established. In this talk, we discuss the physical origin of the PiFM signal, highlighting the contributions from dipole–dipole forces as well as forces that derive from photo-thermal processes.

This conference presentation was prepared for SPIE Optics + Photonics, 2023.

S-SNOM has transformed into a key actor for nanoscale imaging and spectroscopy. Extension of its abilities beyond the mere infrared absorption would be interesting for numerous technological developments. Here, we use s-SNOM to study mid-IR to VIS sum-frequency generation (SFG), in which the sample’s SFG cross-section is enhanced by orders of magnitude by the microscope’s tip. Combining mid-IR and VIS illuminations on gold nanostructures and studying several molecular vibrations, we clarify the contribution of the tip onto this nonlinear optics process. Our work could also lead to a novel near-field microscopy modality where non-linear optical properties could be probed at the nanoscale.

The difficulties identifying the underlying mechanisms in biomineralization are compounded by the lack of imaging techniques which simultaneously provide the desired chemical and elemental sensitivity to both identify sparse nanoscale organics and inorganic crystal structure across a large field of view. Here, we combine nanoscale secondary ion mass spectroscopy (nano-SIMS) and infrared scattering-scanning near-field optical microscopy (IR s-SNOM) to gain simultaneous elemental and chemical nano-images across 10’s of micrometer scales. We map the loss of order within the biomineralized material to gain insight in its formation processes whereby amorphous phases assemble into well-ordered macrostructures.

Strong coupling is essential for the investigations like thresholdless lasing, single-photon switching and quantum emitter. However, Light–matter interaction is hard to realize in the free space due to the unmatching between the light wavelength and the emitter. Herein, a strong coupling between the magnetic plasmons and excitons is investigated on (Au nanocup)-on-(WSe2 monolayer/multilayer) hybrid structures. The asymmetric morphology of Au nanocups allows both electric and magnetic plasmon resonances on the individual particles. By varying the opening of Au nanocups from sideward and upward to downward orientations, resonance coupling from weak to strong is achieved, accompanied by a large split mode shown up in the dark-field scattering spectrum. The detuning between the magnetic plasmons and the excitons in the WS2 monolayer was modulated by varying the size of downward-orientated Au nanocups. A Rabi splitting up to 106 meV was realized at the zero detuning.

We will implement 1 nm resolution microscopy with chemical analysis and single molecule sensitivity by using picosecond lasers to excite stimulated Raman scattering (SRS) and measuring the photon force induced on an atomic force microscopy (AFM) tip. With our pulsed lasers system compared to constant power illumination at the same average power, we can significantly increase the signal level up to 2,000 times more. Additionally, we have created finite element models using ANSYS Multiphysics software to study the optical enhancement as a function of Raman shift, considering air and water environments, and key parameters such as tip radius and materials.

This study describes parametric modeling and prototype simulations using absorption spectra for NIR-SWIR absorbing dyes, which are obtained by inverse analysis of diffuse-reflectance spectra. Absorption spectra of NIR/SWIR-absorbing dyes obtained by inverse spectral analysis provides information for estimating their dielectric response functions. Sufficient sensitivity of absorption spectra relative to inverse spectral analysis implies that estimated dielectric response functions can be used for estimating reflectance from dye formulations on substrates, e.g., fabrics.