Near-field Optical Response Research Articles

Core-Shell Nanoparticle Resonances in Near-Field Microscopy Revealed by Fourier-Demodulated Full-Wave Simulations.

We present a detailed investigation of the near-field optical response of core-shell nanoparticles using Fourier-demodulated full-wave simulations, revealing significant modifications to established contrast mechanisms in infrared scattering-type scanning near-field optical microscopy (s-SNOM). Our work examined the complex interplay of geometrical and optical resonances within core-shell structures. Using a finite element method (FEM) simulation closely aligned with the actual s-SNOM measurement processes, we capture the specific near-field responses in these nanostructures. Our findings show that core-shell nanoparticles exhibit unexpected distinct resonance shifts and massively enhanced scattering driven by both the core and shell properties. This investigation not only advances the understanding of near-field interactions in complex nanosystems but also provides a refined theoretical framework to accurately predict the optical signatures of nanostructures with internal heterogeneity.

Giant Terahertz Birefringence in an Ultrathin Anisotropic Semimetal.

Manipulating the polarization of light at the nanoscale is key to the development of next-generation optoelectronic devices. This is typically done via waveplates using optically anisotropic crystals, with thicknesses on the order of the wavelength. Here, using a novel ultrafast electron-beam-based technique sensitive to transient near fields at THz frequencies, we observe a giant anisotropy in the linear optical response in the semimetal WTe2 and demonstrate that one can tune the THz polarization using a 50 nm thick film, acting as a broadband wave plate with thickness 3 orders of magnitude smaller than the wavelength. The observed circular deflections of the electron beam are consistent with simulations tracking the trajectory of the electron beam in the near field of the THz pulse. This finding offers a promising approach to enable atomically thin THz polarization control using anisotropic semimetals and defines new approaches for characterizing THz near-field optical response at far-subwavelength length scales.

Integrating collapsible plasmonic gaps on near-field probes for polarization-resolved mapping of plasmon-enhanced emission in 2D material.

Scanning near-field optical microscopy (SNOM) is an important technique used to study the optical properties of material systems at the nanoscale. In previous work, we reported on the use of nanoimprinting to improve the reproducibility and throughput of near-field probes including complicated optical antenna structures such as the 'campanile' probe. However, precise control over the plasmonic gap size, which determines the near-field enhancement and spatial resolution, remains a challenge. Here, we present a novel approach to fabricating a sub-20 nm plasmonic gap in a near-field plasmonic probe through the controlled collapse of imprinted nanostructures using atomic layer deposition (ALD) coatings to define the gap width. The resulting ultranarrow gap at the apex of the probe provides a strong polarization-sensitive near-field optical response, which results in an enhancement of the optical transmission in a broad wavelength range from 620 to 820 nm, enabling tip-enhanced photoluminescence (TEPL) mapping of 2-dimensional (2D) materials. We demonstrate the potential of this near-field probe by mapping a 2D exciton coupled to a linearly polarized plasmonic resonance with below 30 nm spatial resolution. This work proposes a novel approach for integrating a plasmonic antenna at the apex of the near-field probe, paving the way for the fundamental study of light-matter interactions at the nanoscale.

Plasmonic nanoresonator distributions for uniform energy deposition in active targets

Active targets implanted with core-shell-composition (CS) and nanorod-shaped (NR) plasmonic nanoresonators and doped with dyes were designed to ensure uniform energy deposition during illumination by two-counter propagating short laser pulses. The near-field enhancement, optical responses, and cross-sections were mapped above the concentration-Epump parameter-plane to inspect two different regions (I and II) with the potential to improve light-matter interaction phenomena. The distribution of steady-state absorption, as well as of the power-loss and power-loss density integrated until the complete overlap of the two short pulses was determined. The uniform distribution was adjusted to constrain standard deviations of the integrated power-loss distributions in the order of ∼10%. Dye doping of target-I/II implanted with uniform CS (NR) nanoresonator distributions results in larger absorption with increased standard deviation, larger power-loss, and power-loss density with decreased (decreased / increased) standard deviation. The adjustment allows larger absorption in CS-II and larger power-loss and power-loss density in CS-implanted targets, smaller standard deviation in targets-I for absorption, and in all targets for power-loss and its density. Larger dye concentration makes it possible to achieve larger absorption (except in adjusted NR-II), larger power-loss and power-loss density in all CS and in adjusted NR distributions, with decreased standard deviation in CS-implanted targets for all quantities and in NR-implanted targets for absorption. CS implantation results in larger absorption with a larger standard deviation, moreover allows larger power-loss in adjusted distributions and smaller standard deviation in power-loss quantities for larger concentration in both distributions and the same standard deviation for smaller concentration in adjusted distribution. Based on these results, adjusted CS distributions in targets doped with a dye of higher concentration are proposed.

Nano-FTIR Correlation Nanoscopy for Organic and Inorganic Material Analysis

Significance: Correlation scanning probe techniques to complement nanoscale IR measurements for next generation sample characterizationScattering-type Scanning Near-field Optical Microscopy (s-SNOM) is a scanning probe approach to optical microscopy and spectroscopy, bypassing the ubiquitous diffraction limit of light to achieve a spatial resolution below 20 nanometers. s-SNOM employs the strong confinement of light at the apex of a sharp metallic atomic force microscopy (AFM) tip to create a nanoscale optical hot-spot. Analyzing the scattered light from the tip enables the extraction of the optical properties of the sample directly below the tip and yields nanoscale resolved images simultaneous to topography [1]. In addition, the technology has been advanced to enable Fourier-Transform Infrared Spectroscopy on the nanoscale (nano-FTIR) [2] using broadband radiation from the visible spectral range to THz frequencies.Recently, the combined analysis of complex nanoscale material systems by correlating near-field optical data with information obtained by other scanning probe microscopy (SPM)-based measurement methodologies has gained significant interest. For example, the material-characteristic nano-FTIR spectra of a phase-separated polystyrene/low-density polyethylene (PS/LDPE) polymer blend verifies sharp material interfaces by measuring a line profile across a ca. 1 μm sized LDPE island. Near-field reflection/absorption imaging at 1500cm-1 of the ca. 50nm thin film allows to selectively highlight the distribution of PS in the blend and simultaneously map the mechanical properties like adhesion of the different materials [3,4].Further, we present results that correlate the near-field optical response of semiconducting samples like graphene (2D) or functional SRAM devices (3D) in different frequency ranges (mid-IR & THz) to Kelvin Probe Force Microscopy (KPFM) measurements. Thus, s-SNOM systems represent an ideal platform to gain novel insights into complex material systems by different near-field and AFM-based method.[1] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 362, 787 (2004).[2] F. Huth, et al., Nano Lett. 12, 3973 (2012).[3] B. Pollard, et al., Beilstein J. of Nanotechn. 7, 605 (2016).[4] I. Amenabar, et al., Nature Commun. 8, 14402 (2017).

Orientation-averaged light scattering by nanoparticle clusters: Far-field and near-field benchmarks of numerical cubature methods

The optical properties of nanoparticles can be substantially affected by their assembly in compact aggregates. This is a common situation notably for nanoparticles synthesised and self-assembled into rigid clusters in colloidal form, where they may be further characterised or used in spectroscopic applications. The theoretical description of such experiments generally requires averaging the optical response over all possible cluster orientations, as they randomly orient themselves over the course of a measurement. This averaging is often done numerically by simulating the optical response for several directions of incidence, using a spherical cubature method. The simulation time increases with the number of directions and can become prohibitive, yet few studies have examined the trade-off between averaging accuracy and computational cost. We benchmark seven commonly-used spherical cubature methods for both far-field and near-field optical responses for a few paradigmatic cluster geometries: dimers of nanospheres and of nanorods, and a helix. The relative error is rigorously evaluated in comparison to analytical results obtained with the superposition T-matrix method. Accurate orientation averaging is especially important for quantities relating to optical activity, the differential response to left and right circularly polarised light, and our example calculations include in particular far-field circular dichroism and near-field local degree of optical chirality.

Probing dark exciton navigation through a local strain landscape in a WSe2 monolayer

In WSe2 monolayers, strain has been used to control the energy of excitons, induce funneling, and realize single-photon sources. Here, we developed a technique for probing the dynamics of free excitons in nanoscale strain landscapes in such monolayers. A nanosculpted tapered optical fiber is used to simultaneously generate strain and probe the near-field optical response of WSe2 monolayers at 5 K. When the monolayer is pushed by the fiber, its lowest energy states shift by as much as 390 meV (>20% of the bandgap of a WSe2 monolayer). Polarization and lifetime measurements of these red-shifting peaks indicate they originate from dark excitons. We conclude free dark excitons are funneled to high-strain regions during their long lifetime and are the principal participants in drift and diffusion at cryogenic temperatures. This insight supports proposals on the origin of single-photon sources in WSe2 and demonstrates a route towards exciton traps for exciton condensation.

Dynamic Tuning of Moiré Excitons in a WSe2/WS2 Heterostructure via Mechanical Deformation.

Moiré superlattices in van der Waals (vdW) heterostructures form by stacking atomically thin layers on top of one another with a twist angle or lattice mismatch. The resulting moiré potential leads to a strong modification of the band structure, which can give rise to exotic quantum phenomena ranging from correlated insulators and superconductors to moiré excitons and Wigner crystals. Here, we demonstrate the dynamic tuning of moiré potential in a WSe2/WS2 heterostructure at cryogenic temperature. We utilize the optical fiber tip of a cryogenic scanning near-field optical microscope (SNOM) to locally deform the heterostructure and measure its near-field optical response simultaneously. The deformation of the heterostructure increases the moiré potential, which leads to a red shift of the moiré exciton resonances. We observe the interlayer exciton resonance shifts up to 20 meV, while the intralayer exciton resonances shift up to 17 meV.

Tuning of Optical Phonons in α-MoO3-VO2 Multilayers.

Merging the properties of VO2 and van der Waals (vdW) materials has given rise to novel tunable photonic devices. Despite recent studies on the effect of the phase change of VO2 on tuning near-field optical response of phonon polaritons in the infrared range, active tuning of optical phonons (OPhs) using far-field techniques has been scarce. Here, we investigate the tunability of OPhs of α-MoO3 in a multilayer structure with VO2. Our experiments show the frequency and intensity tuning of 2 cm-1 and 11% for OPhs in the [100] direction and 2 cm-1 and 28% for OPhs in the [010] crystal direction of α-MoO3. Using the effective medium theory and dielectric models of each layer, we verify these findings with simulations. We then use loss tangent analysis and remove the effect of the substrate to understand the origin of these spectral characteristics. We expect that these findings will assist in intelligently designing tunable photonic devices for infrared applications, such as tunable camouflage and radiative cooling devices.

Local near-field optical response of gold nanohole excited by propagating plasmonic excitations

The remote excitation of a localized surface plasmons (LSP) by the propagating plasmonic excitations, i.e., propagating surface plasmons (PSP), receives increasing attention in recent years. Here, we study the local near-field optical response of the gold nanohole, which support LSP, remotely excited by the PSP (PSP-LSP) and the results show that this optical response of the gold nanohole relates to the excitation field, the local near-field optical spectra show a redshift phenomenon, but the intrinsic frequency of LSP remains invariant. We attribute this spectral redshift phenomenon of the PSP-LSP to the dominating effects of the PSP. A weak dependence of the resonant wavelength peak on the size of a nanostructure has been observed. Moreover, it is found that the dephasing time of the PSP-LSP is longer than the case directly excited by the laser field, and it presents a decreased trend with the increasing distance from the PSP launching location. The results in this work can be used to improve the efficiency of remote excitations and provide a new path to prolong the dephasing time of the enhanced resonant field.

Strongly coupled evenly divided disks: a new compact and tunable platform for plasmonic Fano resonances

Plasmonic artificial molecules are promising platforms for linear and nonlinear optical modulation at various regimes including the visible, infrared and terahertz bands. Fano resonances in plasmonic artificial structures are widely used for controlling spectral lineshapes and tailoring of near-field and far-field optical response. Generation of a strong Fano resonance usually relies on strong plasmon coupling in densely packed plasmonic structures. Challenges in reproducible fabrication using conventional lithography significantly hinders the exploration of novel plasmonic nanostructures for strong Fano resonance. In this work, we propose a new class of plasmonic molecules with symmetric structure for Fano resonances, named evenly divided disk, which shows a strong Fano resonance due to the interference between a subradiant anti-bonding mode and a superradiant bonding mode. We successfully fabricated evenly divided disk structures with high reproducibility and with sub-20 nm gaps, using our recently developed sketch and peel lithography technique. The experimental spectra agree well with the calculated response, indicating the robustness of the Fano resonance for the evenly divided disk geometry. Control experiments reveal that the strength of the Fano resonance gradually increases when increasing the number of split parts on the disk from three to eight individual segments. The Fano-resonant plasmonic molecules that can also be reliably defined by our unique fabrication approach open up new avenues for application and provide insight into the design of artificial molecules for controlling light-matter interactions.

Direct molecular-level near-field plasmon and temperature assessment in a single plasmonic hotspot

Tip-enhanced Raman spectroscopy (TERS) is currently widely recognized as an essential but still emergent technique for exploring the nanoscale. However, our lack of comprehension of crucial parameters still limits its potential as a user-friendly analytical tool. The tip’s surface plasmon resonance, heating due to near-field temperature rise, and spatial resolution are undoubtedly three challenging experimental parameters to unravel. However, they are also the most fundamentally relevant parameters to explore, because they ultimately influence the state of the investigated molecule and consequently the probed signal. Here we propose a straightforward and purely experimental method to access quantitative information of the plasmon resonance and near-field temperature experienced exclusively by the molecules directly contributing to the TERS signal. The detailed near-field optical response, both at the molecular level and as a function of time, is evaluated using standard TERS experimental equipment by simultaneously probing the Stokes and anti-Stokes spectral intensities. Self-assembled 16-mercaptohexadodecanoic acid monolayers covalently bond to an ultra-flat gold surface were used as a demonstrator. Observation of blinking lines in the spectra also provides crucial information on the lateral resolution and indication of atomic-scale thermally induced morphological changes of the tip during the experiment. This study provides access to unprecedented molecular-level information on physical parameters that crucially affect experiments under TERS conditions. The study thereby improves the usability of TERS in day-to-day operation. The obtained information is of central importance for any experimental plasmonic investigation and for the application of TERS in the field of nanoscale thermometry.

Near-Field Optical Identification of Metallic and Semiconducting Single-Walled Carbon Nanotubes**Supported by the National Natural Science Foundation of China under Grant Nos. 11574204 and 11774224, and the National Key Research and Development Program of China (2016YFA0302001). Z.S. acknowledges the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and additional support from a Shanghai talent program.

Single-walled carbon nanotubes (SWCNTs), due to their outstanding electrical and optical properties, are expected to have extensive applications, such as in transparent conductive films and ultra-small field-effect transistors (FETs). However, those applications can only be best realized with pure metallic or pure semiconducting SWCNTs. Hence, identifying and separating metallic from semiconducting SWCNTs in as-grown samples are crucial. In addition, knowledge of the type of an SWCNT is also important for further exploring its new properties in fundamental science. Here we report employing scanning near-field optical microscopy (SNOM) as a direct and simple method to identify metallic and semiconducting SWCNTs on SiO2/Si substrates. Metallic and semiconducting SWCNTs show distinct near-field optical responses because the metallic tubes support plasmons whereas the semiconducting tubes do not. The reliability of this method is verified using FET testing and Rayleigh scattering spectroscopy. Our result demonstrates that the SNOM technique provides a reliable, simple, noninvasive and in situ method to distinguish between metallic and semiconducting SWCNTs.

Near-field imaging of the multi-resonant mode induced broadband tunable metamaterial absorber.

Metamaterial absorbers with tunability have broad prospects for mid-infrared absorption applications. While various methods have been proposed to control absorption, how to analyse and present the physical image of absorption mechanism in depth is still expected and meaningful. Here, we present experimental spatial near-field distributions of a multi-resonant mode induced broadband tunable metamaterial absorber by using near-field optical microscopy. The absorber is constructed by a metal double-sized unit cell and a metallic mirror separated by a thin Ge2Sb2Te5 (GST) spacer. To clearly obtain the physical images, we used a hybrid unit cell consisting of four square resonators to produce two absorption peaks at 7.8 μm and 8.3 μm. The resonance central-wavelength exhibits a redshift while switching the GST thin film from amorphous to crystalline phase. The near-field amplitude and phase optical responses of the absorber are directly observed at absorption frequencies when GST is in both phases, respectively. This work will pave the way for the fundamental science field and inspire potential applications in optical tunable absorption control.

Near-field infrared microscopy of graphene on metal substrate

Graphene plasmons, collective oscillation modes of electrons in graphene, have recently attracted intense attention in both the fundamental researches and the applications because of their strong field confinement, low loss and excellent tunability. The dispersion of graphene plasmons can be significantly modified in the system of graphene on metal substrate, in which the screening of the long-range part of the electron-electron interactions by nearby metal can lead to many novel quantum effects, such as acoustic plasmons, quantum nonlocal effects and renormalization of band structure. Scattering-type scanning near-field optical microscopy (s-SNOM) which consists of a laser coupled to the tip of an atomic force microscopy (AFM), is an effective technique to directly probe plasmons in two-dimensional materials including graphene, and the graphene plasmons can be observed visually by real-space imaging. But so far the detailed s-SNOM studies of graphene/metal system have not been reported. One potential challenge is that the near-field response of highly conductive metal substrate may partially or entirely obscure that of graphene, making it difficult to further explore graphene by using s-SNOM. Here in this paper, we report the direct observation of near-field optical response of graphene in a graphene/metal system excited by a mid-infrared quantum cascade laser. From a close examination of the data of graphene/Cu compared with that of h-BN/Cu, we are able to identify experimental features due to the near-field response of graphene. Surprisingly, two completely different behaviors are observed in the s-SNOM data for different graphene samples on Cu substrates with similar surface step geometries. These results suggest that the near-field response of graphene/metal system is not completely dominated by the metal substrate, and that two completely different near-field response behaviors of graphene may be attributed to their intrinsic properties affected by metal substrates themselves rather than surface step geometries of metal substrate. In addition, following this approach it is possible to distinguish the near-field optical responses of graphene from that of graphene/metal system. Our work reveals the clear signatures of the near-field optical response of graphene on metal substrate, which provides the foundation for probing plasmons in these systems by using the s-SNOM and understanding many novel quantum phenomena therein.

Review of Experimental Setups for Plasmonic Photocatalytic Reactions

Plasmonic photocatalytic reactions have been substantially developed. However, the mechanism underlying the enhancement of such reactions is confusing in relevant studies. The plasmonic enhancements of photocatalytic reactions are hard to identify by processing chemically or physically. This review discusses the noteworthy experimental setups or designs for reactors that process various energy transformation paths for enhancing plasmonic photocatalytic reactions. Specially designed experimental setups can help characterize near-field optical responses in inducing plasmons and transformation of light energy. Electrochemical measurements, dark-field imaging, spectral measurements, and matched coupling of wavevectors lead to further understanding of the mechanism underlying plasmonic enhancement. The discussions herein can provide valuable ideas for advanced future studies.

Effect of Film Thickness on the Far- and Near-Field Optical Response of Nanoparticle-on-Film Systems

We study the near-field and far-field optical responses of nanoparticle-on-film systems using single-nanoparticle spectroscopy and numerical simulations. We find that the optical spectra contain th...

(Invited) Nano-FTIR Correlation Nanoscopy for Organic and Inorganic Material Analysis

Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) is a scanning probe approach to optical microscopy and spectroscopy, bypassing the ubiquitous diffraction limit of light to achieve a spatial resolution below 20 nanometers. s-SNOM employs the strong confinement of light at the apex of a sharp metallic atomic force microscopy (AFM) tip to create a nanoscale optical hot-spot. Analyzing the scattered light from the tip enables the extraction of the optical properties (dielectric function) of the sample directly below the tip and yields nanoscale resolved images simultaneous to topography [1]. In addition, the technology has been advanced to enable Fourier-Transform Infrared Spectroscopy on the nanoscale (nano-FTIR) [2] using broadband radiation from the visible spectral range to THz frequencies. Recently, the combined analysis of complex nanoscale material systems by correlating near-field optical data with information obtained by other scanning probe microscopy (SPM)-based measurement methodologies has gained significant interest. For example, the material-characteristic nano-FTIR spectra of a phase-separated polystyrene/low-density polyethylene (PS/LDPE) polymer blend verifies sharp material interfaces by measuring a line profile across a ca. 1 µm sized LDPE island (Fig1). Near-field reflection/absorption imaging at 1500cm-1 of the ca. 50nm thin film allows to selectively highlight the distribution of PS in the blend and simultaneously map the mechanical properties like adhesion of the different materials [3,4]. Further, results will be presented that correlate the near-field optical response of semiconducting samples like graphene (2D) or functional SRAM devices (3D) in different frequency ranges (mid-IR & THz) to Kelvin Probe Force Microscopy (KPFM) measurements. Thus, s-SNOM systems represent an ideal platform to gain novel insights into complex material systems by different near-field and AFM-based method. [1] F. Keilmann, R. Hillenbrand, Phil. Trans. R. Soc. Lond. A 362, 787 (2004). [2] F. Huth, et al., Nano Lett. 12, 3973 (2012). [3] B. Pollard, et al., Beilstein J. of Nanotechn. 7, 605 (2016). [4] I. Amenabar, et al., Nature Commun. 8, 14402 (2017). Figure caption: Fig 1. Near-field correlation nanoscopy of a thin PS/LDPE polymer film, highlighting the phase separation of the materials by nano-FTIR measurements as well as the different mechanical properties of the polymers. Figure 1

Fundamental Limits to Near-Field Optical Response over Any Bandwidth

We develop an analytical framework to derive upper bounds to light--matter interactions in the optical near field, where applications ranging from spontaneous-emission amplification to greater-than-blackbody heat transfer show transformative potential. Our framework connects the classic complex-analytic properties of causal fields with newly developed energy-conservation principles, resulting in a new class of power--bandwidth limits. These limits demonstrate the possibility of orders-of-magnitude enhancement in near-field optical response with the right combination of material and geometry. At specific frequency and bandwidth combinations, the bounds can be closely approached by canonical plasmonic geometries, with the opportunity for new designs to emerge away from those frequency ranges. Embedded in the bounds is a material "figure of merit," which determines the maximum response of any material (metal/dielectric, bulk/2D, etc.), for any frequency and bandwidth. Our bounds on local density of states (LDOS) represent maximal spontaneous-emission enhancements, our bounds on cross density of states (CDOS) limit electromagnetic-field correlations, and our bounds on radiative heat transfer (RHT) represent the first such analytical rule, revealing fundamental limits relative to the classical Stefan--Boltzmann law.

Tip-Enhanced Second Harmonic Generation: an Approach for Hyper-Raman Spectroscopy

The near-field optical response generated at the apex of a scanning probe in tip-enhanced Raman spectroscopy (TERS) enhances and confines the interaction of light and matter to a few nanometers, thus breaking the diffraction limit of light while preserving the wealth of chemical and structural information. With the way paved to deploy Raman spectroscopy to surface nanostructures, we now investigate the most favorable experimental conditions and possible limitations for tip-enhanced hyper-Raman spectroscopy (TEHRS). In this two-photon-one phonon process, detected around the second harmonic of the excitation wavelength, selection rules different from those of the conventional Raman effect apply, thus holding promise for an almost comprehensive vibrational signature of the sample. TEHRS signal intensity critically depends on the second harmonic generation (SHG) efficiency. The aim of this work is to outline the experimental conditions that are best suited to obtain TEHRS signals in the challenging context of an extremely low scattering cross section. Using a silver tip and non-centrosymmetric PbTiO3 film on platinum substrate as a sample system in our simulations, we show a strong dependence of the intensity of SHG on the polarization of the incident light, the tip radius, the presence of a non-centrosymmetric sample, and the excitation power. We further discuss the effect of tip-sample distance for early-stage (proof of concept) TEHRS investigations and found 40% of total power dissipation for 1 nm distance.