Photoinduced Force Mapping of Plasmonic Nanostructures

Mapping electric field components of superchiral field with photo-induced force

Circular dichroism (CD) of materials, difference in absorbance of left- and right-circularly polarized light, is a standard measure of chirality. Detection of the chirality for individual molecules is a frontier in analytical chemistry and optical science. The usage of a superchiral electromagnetic field near metallic structure is one promising way because it boosts the molecular far-field CD signal. However, it is still elusive as to how such a field actually interacts with the molecules. The cause is that the distribution of the electric field vector is unclear in the vicinity of the metal surface. In particular, it is difficult to directly measure the localized field, e.g., using aperture-type scanning near-field optical microscope. Here, we calculate the three-dimensional (3D) electric field vector, including the longitudinal field, and reveal the whole figure of the near-field CD on a two-dimensional (2D) plane just above the metal surface. Moreover, we propose a method to measure the near-field CD of the whole superchiral field by photo-induced force microscopy (PiFM), where the optical force distribution is mapped in a scanning 2D plane. We numerically demonstrate that, although the presence of the metallic probe tip affects the 3D electric field distribution, the PiFM is sufficiently capable to evaluate the superchiral field. Unveiling the whole figure of near-field is significantly beneficial in obtaining rich information of single molecules with multiple orientations and in analyzing the boosted far-field CD signals.

Visualizing ultrafast photothermal dynamics with decoupled optical force nanoscopy

The photothermal effect in nanomaterials, resulting from resonant optical absorption, finds wide applications in biomedicine, cancer therapy, and microscopy. Despite its prevalence, the photothermal effect in light-absorbing nanoparticles has typically been assessed using bulk measurements, neglecting near-field effects. Beyond standard imaging and therapeutic uses, nanosecond-transient photothermal effects have been harnessed for bacterial inactivation, neural stimulation, drug delivery, and chemical synthesis. While scanning probe microscopy and electron microscopy offer single-particle imaging of photothermal fields, their slow speed limits observations to milliseconds or seconds, preventing nanoscale dynamic investigations. Here, we introduce decoupled optical force nanoscopy (Dofn), enabling nanometer-scale mapping of photothermal forces by exploiting unique phase responses to temporal modulation. We employ the photothermal effect's back-action to distinguish various time frames within a modulation period. This allows us to capture the dynamic photothermal process of a single gold nanorod in the nanosecond range, providing insights into non-stationary thermal diffusion at the nanoscale.

Development of low-temperature and ultrahigh-vacuum photoinduced force microscopy

In this paper, we develop optical and electronic systems for photoinduced force microscopy (PiFM) that can measure photoinduced forces under low temperature and ultrahigh vacuum (LT-UHV) without artifacts. For our LT-UHV PiFM, light is irradiated from the side on the tip-sample junction, which can be adjusted through the combination of an objective lens inside the vacuum chamber and a 90° mirror outside the vacuum chamber. We measured photoinduced forces due to the electric field enhancement between the tip and the Ag surface, and confirmed that photoinduced force mapping and measurement of photoinduced force curves were possible using the PiFM that we developed. The Ag surface was used to measure the photoinduced force with high sensitivity, and it is effective in enhancing the electric field using the plasmon gap mode between the metal tip and the metal surface. Additionally, we confirmed the necessity of Kelvin feedback during the measurement of photoinduced forces, to avoid artifacts due to electrostatic forces, by measuring photoinduced forces on organic thin films. The PiFM, operating under low temperature and ultrahigh vacuum developed here, is a promising tool to investigate the optical properties of various materials with very high spatial resolution.

Photo-induced force microscopy (PiFM) - principles and implementations

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. In this review, we present various technical implementations of the PiFM approach. In addition, 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.

Enhancing Efficiency of Nonfullerene Organic Solar Cells via Using Polyelectrolyte-Coated Plasmonic Gold Nanorods as Rear Interfacial Modifiers

Sufficient sunlight absorption and exciton generation are critical for developing efficient nonfullerene organic solar cells (OSCs). In this work, polyelectrolyte polystyrenesulfonate (PSS)-coated plasmonic gold nanorods (GNRs@PSS) were incorporated, for the first time, into the inverted nonfullerene OSCs as rear interfacial modifiers to improve sunlight absorption and charge generation via the near-field plasmonic and backscattering effects. The plasmonic GNRs effectively improved the sunlight absorption and enhanced the charge generation. Meanwhile, the negatively charged PSS shell ensured the uniform dispersion of the GNRs on the surface of the photoactive layer, optimized the interfacial contact, and further promoted the hole transport to the electrode. These concerted synergistic effects augmented the efficiency (10.11%) by nearly 20% relative to the control device (8.47%). Remarkably, the ultrathin (∼2.2 nm) organic layer on the surface of GNRs was closely examined by acquiring the carbon contrast image through energy-filtered transmission electron microscopy (EF-TEM), which clearly confirmed the coating uniformity from the side to end-cap of GNRs. The surface plasmon resonance (SPR) effect of the GNRs@PSS on the surface of the photoactive layer was unprecedentedly mapped by photoinduced force microscopy (PiFM) under the illumination of a tunable wavelength supercontinuum laser mimicking sunlight. Furthermore, investigations into the effect of size, surface coverage, and incorporation location of GNRs@PSS on the performance of OSCs revealed that the appropriate design and incorporation of the plasmonic nanostructures are crucial, otherwise the performance can be decreased, as evidenced in the case of front interface integration.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Efficient method for the calculation of the optical force of multiple nanoparticles based on the coupling theory of quasinormal modes

An efficient method is proposed for the calculation of the optical force of multiple nanoparticles. In this method, the optical force is calculated by integrating the Maxwell stress tensor (MST) over a closed surface encompassing the nanoparticle. The electromagnetic (EM) field required for evaluating the MST is computed with the coupling theory of quasinormal modes (QNMs), in which the EM field is expanded onto a small set of QNMs of each nanoparticle. Once these dominant modes, which are eigensolutions of source-free Maxwell equations with complex eigenfrequencies, are known, any variation of the interparticle distance, illumination polarization, or wavelength can be treated analytically. Comparisons with the full-wave numerical method demonstrate the accuracy and efficiency of the formalism. With the formalism, force maps are calculated at remarkable computation speed, providing a promising simulation tool for applications such as plasmon tweezer and photoinduced force microscopy.

Probing of coupling effect induced plasmonic charge accumulation for water oxidation

A key issue for redox reactions in plasmon-induced photocatalysis, particularly for water oxidation, is the concentration of surface-accumulating charges (electrons or holes) at a reaction site for artificial photosynthesis. However, where plasmonic charge accumulated at a catalyst's surface, and how to improve local charge density at active sites, remains unknown because it is difficult to identify the exact spatial location and local density of the plasmon-induced charge, particularly with regard to holes. Herein, we show that at the single particle level, plasmon-coupling-induced holes can be greatly accumulated at the plasmonic Au nanoparticle dimer/TiO2 interface in the nanogap region, as directly evidenced by the locally enhanced surface photovoltage. Such an accumulation of plasmonic holes can significantly accelerate the water oxidation reaction (multi-holes involved) at the interfacial reaction site, with nearly one order of magnitude enhancement in photocatalytic activities compared to those of highly dispersed Au nanoparticles on TiO2. Combining Kelvin probe force microscopy and theoretical simulation, we further clarified that the local accumulated hole density is proportional to the square of the local near-field enhancement. Our findings advance the understanding of how charges spatially distribute in plasmonic systems and the specific role that local charge density at reaction sites plays in plasmonic photocatalysis.

Observation of nanoscale opto-mechanical molecular damping as the origin of spectroscopic contrast in photo induced force microscopy

Infrared photoinduced force microscopy (IR-PiFM) is a scanning probe spectroscopic technique that maps sample morphology and chemical properties on the nanometer (nm)-scale. Fabricated samples with nm periodicity such as self-assembly of block copolymer films can be chemically characterized by IR-PiFM with relative ease. Despite the success of IR-PiFM, the origin of spectroscopic contrast remains unclear, preventing the scientific community from conducting quantitative measurements. Here we experimentally investigate the contrast mechanism of IR-PiFM for recording vibrational resonances. We show that the measured spectroscopic information of a sample is directly related to the energy lost in the oscillating cantilever, which is a direct consequence of a molecule excited at its vibrational optical resonance—coined as opto-mechanical damping. The quality factor of the cantilever and the local sample polarizability can be mathematically correlated, enabling quantitative analysis. The basic theory for dissipative tip-sample interactions is introduced to model the observed opto-mechanical damping.

Existing high-dimensional optical imaging techniques that record space and polarization cannot detect the photon’s time of arrival due to the limited speeds of electronic sensors. Here, the authors develop a single-shot ultrafast imaging modality to record light-speed high-dimensional events with picosecond resolution.

Direct Detection of Optical Forces of Magnetic Nature in Dielectric Nanoantennas

Optical forces on nanostructures are usually characterized by their interaction with the electric field component of the light wave, given that most materials present negligible magnetic response at optical frequencies. This is not the case however of a high-refractive-index dielectric nanoantenna, which has been recently shown to efficiently support both electric and magnetic optical modes. In this work, we use a photoinduced force microscopy configuration to measure optically induced forces produced by a germanium nanoantenna on a surrounding silicon near-field probe. We reveal the spatial distribution, character, and magnitude of the generated forces when exciting the nanoantenna at its anapole state condition. We retrieve optical force maps showing values of up to 20 pN, which are found to be mainly magnetic in nature, according to our numerical simulations. The results of this investigation open new pathways for the study, detection, and generation of magnetic light forces at the nanometer scale.

Direct Chemical Imaging of Ligand-Functionalized Single Nanoparticles by Photoinduced Force Microscopy

Chemical characterizations of biochemically functionalized single nanoparticles are necessary to optimize the nanoparticle surface functionality in recently advanced nanobiological applications but have not yet been fully explored because of technical difficulties. Exploiting the photoinduced force exerted on a light-illuminated nanoscale tip, nanoscale mid-infrared hyperspectral images with a 10 nm spatial resolution of a monolayer ligand-functionalized single gold nanoparticle under ambient and environmental conditions are presented. We extend our study to the diagnosis of nanoscale heterogeneous chemical contaminants which come from a particle functionalization process but are undetectable in conventional ensemble-averaged imaging technique. High sensitivity and high spatial resolution are achieved via the strongly localized tip-enhanced force at the junction between the gold-coated tip and the functionalized nanoparticle in photoinduced force microscopy, which far exceeds the capability of the conventional methods. The present study paves a new way to directly detect heterogeneous nanochemicals at the single-component level, which is necessary to evaluate nanomaterial safety in biomedical applications.

Simultaneous Optical Photothermal Infrared (O-PTIR) and Raman Spectroscopy of Submicrometer Atmospheric Particles

Physicochemical analysis of individual atmospheric aerosols at the most abundant sizes in the atmosphere (<1 μm) is analytically challenging, as hundreds to thousands of species are often present in femtoliter volumes. Vibrational spectroscopies, such as infrared (IR) and Raman, have great potential for probing functional groups in single particles at ambient pressure and temperature. However, the diffraction limit of IR radiation limits traditional IR microscopy to particles > ∼10 μm, which have less relevance to aerosol health and climate impacts. Optical photothermal infrared (O-PTIR) spectroscopy is a contactless method that circumvents diffraction limitations by using changes in the scattering intensity of a continuous wave visible laser (532 nm) to detect the photothermal expansion when a vibrational mode is excited by a tunable IR laser (QCL: 800-1800 cm^-1 or OPO: 2600-3600 cm^-1). Herein, we simultaneously collect O-PTIR spectra with Raman spectra at a single point for individual particles with aerodynamic diameters <400 nm (prior to impaction and spreading) at ambient temperature and pressure, by also collecting the inelastically scattered visible photons for Raman spectra. O-PTIR and Raman spectra were collected for submicrometer particles with different substrates, particle chemical compositions, and morphologies (i.e., core-shell), as well as IR mapping with submicron spatial resolution. Initial O-PTIR analysis of ambient atmospheric particles identified both inorganic and organic modes in individual sub- and supermicrometer particles. The simultaneous IR and Raman microscopy with submicrometer spatial resolution described herein has considerable potential both in atmospheric chemistry and numerous others fields (e.g., materials and biological research).

Nanoscale spectroscopic origins of photoinduced tip-sample force in the midinfrared

Photoinduced force at tip–sample junction provides nanoscale spectroscopic information with label-free and far-field background-free manner. This approach, spectronanoscopy through force detection, shows higher sensitivity and 1,000 times better spatial resolution than conventional ensemble averaged infrared microscopy, even under ambient and environmental conditions. Unfortunately, the origin of this promising photoinduced force effect is sometimes unclear because the force has 2 independent physical aspects: One is the electromagnetic effect related to induced dipoles in tip and sample, and the other one is the thermodynamic effect related to thermal heating of sample. Here, we reveal how the light illumination results in the 2 kinds of photoinduced forces at the tip–sample junction and provide quantitative interpretation of nanoscale spectroscopic measurements.

When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can contribute to the measured photoinduced force simultaneously. Of particular interest are the instantaneous force between the induced dipoles in the tip and in the sample, and the force related to thermal heating of the junction. A key difference between these 2 force mechanisms is their spectral behavior. The magnitude of the thermal response follows a dissipative (absorptive) Lorentzian line shape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Because the 2 interactions are sometimes comparable in magnitude, the origin of the chemical selectivity in nanoscale spectroscopic imaging through force detection is often unclear. Here, we demonstrate theoretically and experimentally how the light illumination gives rise to the 2 kinds of photoinduced forces at the tip–sample junction in the midinfrared. We comprehensively address the origin of the spectroscopic forces by discussing cases where the 2 spectrally dependent forces are entwined. The analysis presented here provides a clear and quantitative interpretation of nanoscale chemical measurements of heterogeneous materials and sheds light on the nature of light–matter coupling in optomechanical force-based spectronanoscopy.

Dipole modelling for a robust description of subdiffractional polariton waves

The nanophotonics of van der Waals (vdW) materials relies critically on the electromagnetic properties of polaritons defined on sub-diffraction length scales. Here, we use a full electromagnetic Hertzian dipole antenna (HDA) model to describe the hyperbolic phonon polaritons (HP²s) in vdW crystals of hexagonal boron nitride (hBN) on a gold surface. The HP² waves are investigated by broadband synchrotron infrared nanospectroscopy (SINS) which covers the type I and type II hyperbolic bands simultaneously. Basically, polariton waves, observed by SINS, are assigned to the resultant electric field from the summation over the irradiated electric fields of dipoles distributed along the crystal edge and at the tip location and a non-propagating field. The values of polariton momenta and damping extracted from the HDA model present excellent agreement with theoretical predictions. Our analysis shows that the confinement factor of type I HP²s exceeds that of the type II ones by up to a factor of 3. We extract anti-parallel group velocities (v_g) for type I (v_g,typeI = -0.005c, c is the light velocity in a vacuum) in relation to type II (v_g,typeII = 0.05c) polaritonic pulses, with lifetimes of ∼0.6 ps and ∼0.3 ps, respectively. Furthermore, by incorporating consolidated optical-near field theory into the HDA model, we simulate real-space images of polaritonic standing waves for hBN crystals of different shapes. This approach reproduces the experiments with a minimal computational cost. Thus, it is demonstrated that the HDA modelling self-consistently explains the measured complex-valued polariton near-field, while being a general approach applicable to other polariton types, like plasmon- and exciton-polaritons, active in the wide range of vdW materials.

All-optical tunable plasmonic nano-aggregations for surface-enhanced Raman scattering

Interparticle forces play a crucial role in nanoparticle-based nanoscience and nanoengineering for synthesizing new materials, manipulating nanoscale structures, understanding biological processes and ultrasensitive sensing. Complicated by the fluid-dynamical and chemical nature of the liquid environment of nanoparticles, previous attempts are limited to electromagnetic and chemical methods. Alternatively, optically induced forces provide a convenient and fabrication-free route to manipulate nanoparticles at the nanoscale. Here we demonstrate a new double laser trapping scheme for metallic nano-aggregation by inducing strong near-field optical interparticle forces without any chemical agents or complicated fabrication processes. These induced optical forces arising from strong localized plasmon resonance strongly depend on the interparticle separation well beyond the diffraction limit and the polarization of the incident laser field. We examine such sub-resolved interparticle separation in trapped nanoaggregates by measuring surface-enhanced Raman scattering, and further demonstrate the single-molecule sensitivity by implementing such nanostructures. This new technique opens a new avenue for all-optical manipulation of nanomaterials as well as ultra-sensitive bio-chemical sensing applications.

Photo-Induced Force Microscopy by Using Quartz Tuning-Fork Sensor

We present the photo-induced force microscopy (PiFM) studies of various nano-materials by implementing a quartz tuning fork (QTF), a self-sensing sensor that does not require complex optics to detect the motion of a force probe and thus helps to compactly configure the nanoscale optical mapping tool. The bimodal atomic force microscopy technique combined with a sideband coupling scheme is exploited for the high-sensitivity imaging of the QTF-PiFM. We measured the photo-induced force images of nano-clusters of Silicon 2,3-naphthalocyanine bis dye and thin graphene film and found that the QTF-PiFM is capable of high-spatial-resolution nano-optical imaging with a good signal-to-noise ratio. Applying the QTF-PiFM to various experimental conditions will open new opportunities for the spectroscopic visualization and substructure characterization of a vast variety of nano-materials from semiconducting devices to polymer thin films to sensitive measurements of single molecules.

Integrated optofluidic micro-pumps in micro-channels with uniform excitation of a polarization rotating beam

We report an integrated optofluidic micro-pump with a pair of mirrored stirrers of circulating micro-beads in a micro-channel, driven by plasmon-assisted optical manipulation with the excitation of a polarization rotating beam. H-shaped apertures (HSAs) on a gold surface produce strong near-field hot spots when they are illuminated with a light beam polarized parallel to the long axis of "H." With the rotating of excitation polarization, loops of HSAs with gradually varied orientations can produce the circulation of hot spots, which can further trap micro-beads and make them go around in circles. A different sequence of HSAs can produce a different direction and phase of bead rotation, even under uniform excitation. A pair of mirrored circulations of micro-beads in a micro-channel can induce very effective directional flow. Through numerical modeling, we find that a group of non-synchronized multi-phase mirrored circulations can produce a very uniform flow rate with a speed of more than 10 micrometers per second. These micro-pumps can be heavily integrated and activated by a single beam, while the flow direction of each pump can be regulated, even under a uniform excitation. Our design proposes a new approach for the flow pumping in micro- and nanofluidic devices.

Exclusive Magnetic Excitation Enabled by Structured Light Illumination in a Nanoscale Mie Resonator

Recent work has shown that optical magnetism, generally considered a challenging light-matter interaction, can be significant at the nanoscale. In particular, the dielectric nanostructures that support magnetic Mie resonances are low-loss and versatile optical magnetic elements that can effectively manipulate the magnetic field of light. However, the narrow magnetic resonance band of dielectric Mie resonators is often overshadowed by the electric response, which prohibits the use of such nanoresonators as efficient magnetic nanoantennas. Here, we design and fabricate a silicon (Si) truncated cone magnetic Mie resonator at visible frequencies and excite the magnetic mode exclusively by a tightly focused azimuthally polarized beam. We use photoinduced force microscopy to experimentally characterize the local electric near-field distribution in the immediate vicinity of the Si truncated cone at the nanoscale and then create an analytical model of such structure that exhibits a matching electric field distribution. We use this model to interpret the PiFM measurement that visualizes the electric near-field profile of the Si truncated cone with a superior signal-to-noise ratio and infer the magnetic response of the Si truncated cone at the beam singularity. Finally, we perform a multipole analysis to quantitatively present the dominance of the magnetic dipole moment contribution compared to other multipole contributions into the total scattered power of the proposed structure. This work demonstrates the excellent efficiency and simplicity of our method of using Si truncated cone structure under APB illumination compared to other approaches to achieve dominant magnetic excitations.

Optical-Force-Dominated Directional Reshaping of Au Nanodisks in Al-Au Heterodimers

The optical reshaping of metallic nanostructures typically requires intense laser pulses to first approach or achieve melting, followed by surface-tension-dominated reshaping, transforming the original nanostructures into more spherical morphologies. Here, we report the directional optical reshaping of the Au nanodisk of an Al-Au heterodimer in the illuminated junction of an atomic force microscope (AFM). Both the heightening and the repositioning of the Au nanodisk component are induced, reducing the gap between the two nanodisks. There are three contributors to this process: the photothermal softening of the Au lattice, the optical force applied to the Au nanodisk by the Al nanodisk, and the optical force from the nearby AFM tip. The asymmetric reshaping of the heterodimer is observable structurally, through electron microscopic imaging, and through changes in the heterodimer optical response. This optical-force-directed shape manipulation may have potential applications in nanofabrication, optically induced nanomanufacturing, sensing, and quality control.

Near-field nanoprobing using Si tip-Au nanoparticle photoinduced force microscopy with 120:1 signal-to-noise ratio, sub-6-nm resolution

We propose using a Si tip-Au nanoparticle (NP) combination system in photoinduced force microscopy (PiFM) to fundamentally improve its accuracy in the nanoscale characterization of light-matter interaction. Compared to conventional PiFM with Au-coated tips, such Si tip and Au NP combination enables superior photo-induced force detection while overcoming the tip-induced anisotropy by Au-coating. We map the near-field distribution of Au NPs in different arrangements achieving 120 signal-to-noise ratio and sub-6-nm resolution, even surpassing the tip-curvature limitation; we also map the azimuthally polarized beam profile showing an excellent symmetry. The proposed approach is essential to the promising single molecule spectroscopy.

Photoinduced Tip-Sample Forces for Chemical Nanoimaging and Spectroscopy

Control of photoinduced forces allows nanoparticle manipulation, atom trapping, and fundamental studies of light-matter interactions. Scanning probe microscopy enables the local detection of photoinduced effects with nano-optical imaging and spectroscopy modalities being used for chemical analysis and the study of physical effects. Recently, the development of a novel scanning probe technique has been reported with local chemical sensitivity attributed to the localization and detection of the optical gradient force between a probe tip and sample surface via infrared vibrationally resonant coupling. However, the magnitude and spectral line shape of the observed signals disagree with theoretical predictions of optical gradient forces. Here, we clarify this controversy by resolving and analyzing the interplay of several photoinduced effects between scanning probe tips and infrared resonant materials through spectral and spatial force measurements. Force spectra obtained on IR-active vibrational modes of polymer thin films are symmetric and match the material absorption spectra in contrast to the dispersive spectral line shape expected for the optical gradient force response. Sample thickness dependence shows continuous increase in force signal beyond the thickness where the optical dipole force would saturate. Our results illustrate that photoinduced force interactions between scanning probe tips and infrared-resonant materials are dominated by short-range thermal expansion and possibly long-range thermally induced photoacoustic effects. At the same time, we provide a guideline to detect and discriminate optical gradient forces from other photoinduced effects, which opens a new perspective for the development of new scanning probe modalities exploiting ultrastrong opto-mechanical coupling effects in tip-sample cavities.

Selective excitation and imaging of ultraslow phonon polaritons in thin hexagonal boron nitride crystals

We selectively excite and study two new types of phonon-polariton guided modes that are found in hexagonal boron nitride thin flakes on a gold substrate. Such modes show substantially improved confinement and a group velocity that is hundreds of times slower than the speed of light, thereby providing a new way to create slow light in the mid-infrared range with a simple structure that does not require nano-patterning. One mode is the fundamental mode in the first Restrahlen band of hexagonal boron nitride thin crystals on a gold substrate; the other mode is equivalent to the second mode of the second Restrahlen band of hexagonal boron nitride flakes that are suspended in vacuum. The new modes also couple efficiently with incident light at the hexagonal boron nitride edges, as we demonstrate experimentally using photo-induced force microscopy and scanning near-field optical microscopy. The high confinement of these modes allows for Purcell factors that are on the order of tens of thousands directly above boron nitride and a wide band, with new perspectives for enhanced light-matter interaction. Our findings demonstrate a new approach to engineering the dispersion of polaritons in 2D materials to improve confinement and light-matter interaction, thereby paving the way for new applications in mid-infrared nano-optics.

Ultra-confined mid-infrared resonant phonon polaritons in van der Waals nanostructures

Hexagonal boron nitride has been proposed as an excellent candidate to achieve subwavelength infrared light manipulation owing to its polar lattice structure, enabling excitation of low-loss phonon polaritons with hyperbolic dispersion. We show that strongly subwavelength hexagonal boron nitride planar nanostructures can exhibit ultra-confined resonances and local field enhancement. We investigate strong light-matter interaction in these nanoscale structures via photo-induced force microscopy, scattering-type scanning near-field optical microscopy, and Fourier transform infrared spectroscopy, with excellent agreement with numerical simulations. We design optical nano-dipole antennas and directly image the fields when bright- or dark-mode resonances are excited. These modes are deep subwavelength, and strikingly, they can be supported by arbitrarily small structures. We believe that phonon polaritons in hexagonal boron nitride can play for infrared light a role similar to that of plasmons in noble metals at visible frequency, paving the way for a new class of efficient and highly miniaturized nanophotonic devices.

Two-dimensional arbitrary nano-manipulation on a plasmonic metasurface

In this Letter, we report on a plasmonic nano-ellipse metasurface with the purpose of trapping and two-dimensional (2D) arbitrary transport of nanoparticles by means of rotating the polarization of an excitation beam. The locations of hot spots within a metasurface are polarization dependent, thus making it possible to turn on/off the adjacent hot spots and then convey the trapped target by rotating the incident polarization state. For the case of a metasurface with a unit cell of perpendicularly orientated nano-ellipses, the hot spots with higher intensities are located at both apexes of the nano-ellipse whose major axis is parallel to the direction of polarization. When the polarization gradually rotates to its counterpart direction, the trapped particle may move around the ellipse and transfer to the most adjacent ellipse, due to the unbalanced trap potentials around the nano-ellipse. Clockwise and counterclockwise rotation would guide the particle in a different direction, which makes it possible to convey the particle arbitrarily within the plasmonic metasurface by setting a time sequence of polarization rotation. As confirmed by the three-dimensional finite-difference time-domain analysis, our design offers a novel scheme of 2D arbitrary transport with nanometer accuracy, which could be used in many on-chip optofluidic applications.

Wavelength-Dependent Optical Force Imaging of Bimetallic Al-Au Heterodimers

Many important applications of nanometer-scale metallic complexes arise from the light-induced, near-field interactions between their component structures. Here we examine the near-field interactions in bimetallic Al-Au plasmonic nanodisk heterodimers, where the coupling between the primitive plasmons of nanostructures composed of two different metals is studied. Understanding the correlations between nanoparticle morphology and near-field optical properties, particularly for nanostructures composed of two different metals, requires spectrally resolved near-field spatial information. An ideal tool for such investigations is the recently developed photoinduced force microscopy, where the electromagnetic forces between an optically excited plasmonic nanostructure and an adjacent scanning nanoscale tip are measured. Using this approach, we visualize the wavelength-dependent near-field interactions in these bimetallic heterodimers. This system provides a prime example of the diabatic, antenna-reactor picture of plasmon coupling where for a given wavelength the more resonant primitive "driving" plasmon induces a response, the "forced" plasmon, in the off-resonant component. We critically examine spectrally resolved tip-nanostructure forces, comparing experiment with theory, for tips and nanoscale structures of realistic dimensions relative to frequently used approximations for tip geometries. The contrasting effects of dielectric versus metallic tips on acquired spectral force profiles are also examined.

Contrast and imaging performance in photo induced force microscopy

We numerically analyze PiFM's lateral and vertical (subsurface) imaging performance in the visible and IR regimes. The lateral spatial resolution and subsurface imaging capabilities are limited by the field spatial confinement near the tip apex, which is directly proportional to the excitation wavelength. In addition, we show that near-field optical force exerted on the tip due to sample molecular resonance is indeed in the detectable range. Moreover, driving sample on (off) resonance reveals high (low) contrast. The strength of the optical forces is assessed for metal (gold), polymers (Polystyrene and Polymethylmethacrylate), and solid (SiC). By increasing tip-coating thickness from 5 nm to 35 nm, the gap-field enhancement decreases to about 40%. In IR, force spectrum over an absorption band is predominantly following the real part of the polarizability, as predicted by dipole-dipole approximation.

Dual-Scattering Near-Field Microscope for Correlative Nanoimaging of SERS and Electromagnetic Hotspots

Surface-enhanced Raman spectroscopy (SERS) enables sensitive chemical studies and materials identification, relying on electromagnetic (EM) and chemical-enhancement mechanisms. Here we introduce a tool for the correlative nanoimaging of EM and SERS hotspots, areas of strongly enhanced EM fields and Raman scattering, respectively. To that end, we implemented a grating spectrometer into a scattering-type scanning near-field optical microscope (s-SNOM) for mapping of both the elastically and inelastically (Raman) scattered light from the near-field probe, that is, a sharp silicon tip. With plasmon-resonant gold dimers (canonical SERS substrates) we demonstrate with nanoscale spatial resolution that the enhanced Raman scattering from the tip is strongly correlated with its enhanced elastic scattering, the latter providing access to the EM-field enhancement at the illumination frequency. Our technique has wide application potential in the correlative nanoimaging of local-field enhancement and SERS efficiency as well as in the investigation and quality control of novel SERS substrates.

Multimodal imaging and photothermal therapy were simultaneously achieved in the core–shell UCNR structure by using single near-infrared light

Core–shell nanostructures consisting of plasmonic materials and lanthanide-doped upconversion nanoparticles (UCNPs) show promising applications in theranostics including bio-imaging, diagnosis and therapy.