Optical Nanoimaging Research Articles

Optical Nanoimaging of Surface Plasmon Polaritons Supported by Ultrathin Metal Films.

The physics of electrons, photons, and their plasmonic interactions change dramatically when one or more dimensions are reduced to atomic-level thicknesses. For example, graphene exhibits unique electrical, plasmonic, and optical properties. Likewise, atomic-thick metal films are expected to exhibit extraordinary quantum optical properties. Several methods of growing ultrathin metal films were demonstrated, but the quality of the obtained films was much worse compared to bulk films. In this work, we propose a new method of making ultrathin gold films that are close in their properties to bulk gold films. Excellent plasmonic properties are revealed by directly observing quasi-short- and quasi-long-range plasmons in such a film via scanning near-field optical microscopy. The results pave the way for the use of ultrathin gold films in flexible and transparent nanophotonics and optoelectronic applications.

Optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3.

Phonon polaritons (PhPs), collective modes hybridizing photons with lattice vibrations in polar insulators, enable nanoscale control of light. In recent years, the exploration of in-plane anisotropic PhPs has yielded new levels of confinement and directional manipulation of nano-light. However, the investigation of in-plane anisotropic PhPs at the atomic layer limit is still elusive. Here, we report the optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3 (5 atomic layers). We show that narrow α-MoO3 nanoribbons as thin as a few atomic layers can support anisotropic PhPs modes with a high confinement ratio (∼133 times smaller wavelength than that of light). The anisotropic PhPs interference fringe patterns in atomic layers are tunable depending on the PhP wavelength via changing the illumination frequency. Moreover, spatial control over the PhPs interference patterns is also achieved by varying the nanostructures' shape or nanoribbon width of atomically-thin α-MoO3. Our work may serve as an empirical reference point for other anisotropic PhPs that approach the thickness limit and pave the way for applications such as atomically integrated nano-photonics and sensing.

Polarization-insensitive plasmon nanofocusing with broadband interference modulation for optical nanoimaging

Polarization-insensitive plasmon nanofocusing with broadband interference modulation for optical nanoimaging

Multimodal (Non)Linear Optical Nanoimaging and Nanospectroscopy

This Perspective highlights recent advances in linear and nonlinear spectral nanoimaging. The described developments are motivated by the need to characterize molecular and material systems noninvasively with nanometer spatial and femtosecond temporal resolution. Indeed, the ability to image and chemically characterize heterogeneous interfaces with joint nano-femto resolution is a prerequisite to advancing our fundamental understanding of processes as diverse as heterogeneous catalysis, microbial communication, and energy flow in pristine/defect-containing low-dimensional quantum materials, to name a few. We describe pioneering work and recent demonstrations of (non)linear optical nanoimaging and nanospectroscopy, with an emphasis on high spatial resolution measurements conducted under ambient laboratory conditions.

Ultrastable tip-enhanced hyperspectral optical nanoimaging for defect analysis of large-sized WS2 layers.

Optical nanoimaging techniques, such as tip-enhanced Raman spectroscopy (TERS), are nowadays indispensable for chemical and optical characterization in the entire field of nanotechnology and have been extensively used for various applications, such as visualization of nanoscale defects in two-dimensional (2D) materials. However, it is still challenging to investigate micrometer-sized sample with nanoscale spatial resolution because of severe limitation of measurement time due to drift of the experimental system. Here, we achieved long-duration TERS imaging of a micrometer-sized WS2 sample for 6 hours in a reproducible manner. Our ultrastable TERS system enabled to reveal the defect density on the surface of tungsten disulfide layers in large area equivalent to the device scale. It also helped us to detect rare defect-related optical signals from the sample. The present study paves ways to evaluate nanoscale defects of 2D materials in large area and to unveil remarkable optical and chemical properties of large-sized nanostructured materials.

Observation of interband collective excitations in twisted bilayer graphene

The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, particularly when the two layers are rotated relative to each other by a small angle (θ ≈ 1°), owing to the moiré potential induced by the twist. Here we probe the collective excitations of TBG with a spatial resolution of 20 nm, by applying mid-infrared near-field optical microscopy. We find a propagating plasmon mode in charge-neutral TBG for θ = 1.1−1.7°, which is different from the intraband plasmon in single-layer graphene. We interpret it as an interband plasmon associated with the optical transitions between minibands originating from the moiré superlattice. The details of the plasmon dispersion are directly related to the motion of electrons in the moiré superlattice and offer an insight into the physical properties of TBG, such as band nesting between the flat band and remote band, local interlayer coupling, and losses. We find a strongly reduced interlayer coupling in the regions with AA stacking, pointing at screening due to electron–electron interactions. Optical nano-imaging of TBG allows the spatial probing of interaction effects at the nanoscale and potentially elucidates the contribution of collective excitations to many-body ground states. Moiré potentials substantially alter the electronic properties of twisted bilayer graphene at a magic twist angle. A propagating plasmon mode, which can be observed with optical nano-imaging, is associated with transitions between the moiré minibands.

Optical nano-imaging via microsphere compound lenses working in non-contact mode.

Microsphere lens for nano-imaging has been widely studied because of its superior resolving power, real-time imaging characteristic, and wide applicability on diverse samples. However, the further development of the microsphere microscope has been restricted by its limited magnification and small field-of-view. In this paper, the microsphere compound lenses (MCL) which allow enlarged magnification and field-of-view simultaneously in non-contact imaging mode have been demonstrated. A theoretical model involving wave-optics effects is established to guide the design of MCL for different magnifications and imaging configurations, which is more precise compared with common geometric optics theory. Experimentally, using MCL to image the specimen with a tunable magnification from 2.8× to 10.3× is realized. Due to the enlarged magnification, a high-resolution target with 137 nm line width can be resolved by a 10× objective. Besides, the field-of-view of MCL is larger than that of a single microsphere and can be further increased through scanning working manner, which has been demonstrated by imaging a sample with ∼76 nm minimum feature size in a large area. Prospectively, the well-designed MCL will become irreplaceable components to improve the imaging performances of microsphere microscope just like the compound lens in the conventional macroscopic imaging system.

Realization of noncontact confocal optical microsphere imaging microscope.

A novel noncontact confocal optical microsphere imaging microscope (COMIM) is proposed to achieve high contrast optical nano-imaging at a high scanning speed. The developed setup obtains images with significantly higher contrast when compared with conventional bright-field microsphere nano-imaging, as well as with a commercial confocal laser scanning microscope (CLSM). With the image-by-image scanning scheme, COMIM takes only 11% time to complete a three-dimensional (3D) scanning image than using a commercial CLSM, while still offers an outstanding resolving power for the nano-features on the sample.

Modern Scattering-Type Scanning Near-Field Optical Microscopy for Advanced Material Research.

Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past decade, scattering-type scanning near-field optical microscopy (s-SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5-3000 µm) nanoimaging, and nanospectroscopy with fine spatial (<10 nm), spectral (<1 cm-1 ), and temporal (<10 fs) resolution. The history of s-SNOM is briefly introduced and recent advances which broaden the horizons of this technique in novel material research are summarized. In particular, this includes the pioneering efforts to study the nanoscale electrodynamic properties of plasmonic metamaterials, strongly correlated quantum materials, and polaritonic systems at room or cryogenic temperatures. Technical details, theoretical modeling, and new experimental methods are also discussed extensively, aiming to identify clear technology trends and unsolved challenges in this exciting field of research.

Graphene on meta-surface for super-resolution optical imaging with a sub-10 nm resolution.

Nowadays, wide-field of view plasmonic structured illumination method (WFPSIM) has been extensively studied and experimentally demonstrated in biological researches. Normally, noble metal structures are used in traditional WFPSIM to support ultra-high wave-vector of SPs and an imaging resolution enhancement of 3-4 folds can be achieved. To further improve the imaging resolution of WFPSIM, we hereby propose a wide-field optical nanoimaging method based on a hybrid graphene on meta-surface structure (GMS) model. It is found that an ultra-high wave-vector of graphene SPs can be excited by a metallic nanoslits array with localized surface plasmon enhancement. As a result, a standing wave surface plasmons (SW-SPs) interference pattern with a period of 11 nm for a 980 nm incident wavelength can be obtained. The potential application of the GMS for wide-field of view super-resolution imaging is discussed followed by simulation results which show that an imaging resolution of sub-10 nm can be achieved. The demonstrated method paves a new route for wide field optical nanoimaging, with applications e.g. in biological research to study biological processes occurring in cell membrane.

Phonon-Polaritonic Bowtie Nanoantennas: Controlling Infrared Thermal Radiation at the Nanoscale

A conventional thermal emitter exhibits a broad emission spectrum with a peak wavelength depending upon the operation temperature. Recently, narrowband thermal emission was realized with periodic gratings or single microstructures of polar crystals supporting distinct optical modes. Here, we exploit the coupling of adjacent phonon-polaritonic nanostructures, demonstrating experimentally that the nanometer-scale gaps can control the thermal emission frequency while retaining emission line widths as narrow as 10 cm–1. This was achieved by using deeply subdiffractional bowtie-shaped silicon carbide nanoantennas. Infrared far-field reflectance spectroscopy, near-field optical nanoimaging, and full-wave electromagnetic simulations were employed to prove that the thermal emission originates from strongly localized surface phonon-polariton resonances of nanoantenna structures. The observed narrow emission line widths and exceptionally small modal volumes provide new opportunities for the user-design of near- and...

Numerical analysis of wide-field optical imaging with a sub-20\u2009nm resolution based on a meta-sandwich structure

Biological research requires wide-field optical imaging techniques with resolution down to the nanometer scale to study the biological process in a sub-cell or single molecular level. To meet this requirement, wide-field structured illumination method (WFSIM) has been extensively studied. The resolution of WFSIM is determined by the period of the optical interference pattern. However, in traditional WFSIM this period is diffraction limited so that pattern having periodicity smaller than 100 nm cannot be generated and as a result achieving an imaging resolution better than 50 nm is a great challenge. Here, we demonstrate a wide-field optical nanoimaging method based on a meta-sandwich structure (MSS) model. It is found that this structure can support standing wave surface plasmons interference pattern with a period of only 31 nm for 532 nm wavelength incident light. Furthermore, the potential application of the MSS for wide-field super-resolution imaging is discussed and the simulation results show an imaging resolution of sub-20 nm can be achieved. The demonstrated method paves a new route for the improvement of the wide field optical nanoimaging, which can be applied by biological researchers to study biological process conducted in cell membrane, such as mass transportation and others.

Tip-Enhanced Raman Spectroscopy: Technique and Recent Advances.

This review discusses a relatively new technique for optical nanoimaging at visible wavelength, known as tip-enhanced Raman spectroscopy (TERS). This technique relies on the enhancement and spatial confinement of light in the close vicinity of the apex of a plasmonic nanotip. The plasmonic nanotip can be positioned on the sample and controlled by a suitable scanning probe microscopy, such as atomic force microscopy or scanning tunneling microscopy. By raster scanning the nanotip, one can obtain nanoimages with high spatial resolution. While enhancement helps measuring weak phonon modes from a tiny volume of the sample, confinement delivers extremely high spatial resolution in nanoimaging. We will discuss the technique of TERS in more detail with several applications and review the recent advances.

Meta-nanocavity model for dynamic super-resolution fluorescent imaging based on the plasmonic structure illumination microscopy method.

Biological research requires dynamic and wide-field optical microscopy with resolution down to nanometer to study the biological process in a sub-cell or single molecular level. To address this issue, we propose a dynamic wide-field optical nanoimaging method based on a meta-nanocavity platform (MNCP) model which can be incorporated in micro/nano-fluidic systems so that the samples to be observed can be confined in a nano-scale space for the ease of imaging. It is found that this platform can support standing wave surface plasmons (SW-SPs) interference pattern with a period of 105 nm for a 532 nm incident wavelength. Furthermore, the potential application of the NCP for wide-field super-resolution imaging was discussed and the simulation results show that an imaging resolution of sub-80 nm can be achieved.

Guest Editorial Introduction to the Issue on Nanobiophotonics

The papers in this special issue focus on the topic of nanobiophotonics, an advanced field of modern science and biomedical nanotechnology. This new field continues to vastly expend with state-of-the-art developments across the entire spectrum of biomedical applications ranging from fundamental studies of light-nanobiomaterial interactions to clinical diagnostics and therapeutics with nanophotonics. In nanobiophotonics research areas, there has been great impetus recently for noninvasive imaging and sensing intracellular structures and functions as well as for obtaining quantitative information for light-tissue interactions at the cellular, intracellular and molecular level. The papers in this issue offer some of the latest leading-edge developments in nanobiophotonics. Some of these developments include advanced optical nanoimaging and nanosensing techniques based on nanoprobes enhanced imaging and sensing principles employing various highly effective nanobiomaterials such as nanoparticles, quantum dots, and plasmonic nanostructures. These techniques offer an effective and fast way for sensing and monitoring various biomedical quantities in-vivo with a nanoresolution beyond the diffraction limit. Other new developments include optical manipulation of nanoparticles, single molecule spectroscopy and imaging, integrated nanoprobe-enhanced diagnostics and therapeutics, and novel nanobiophotonics devices.

Cell structure imaging with bright and homogeneous nanometric light source.

Label-free optical nano-imaging of dendritic structures and intracellular granules in biological cells is demonstrated using a bright and homogeneous nanometric light source. The optical nanometric light source is excited using a focused electron beam. A zinc oxide (ZnO) luminescent thin film was fabricated by atomic layer deposition (ALD) to produce the nanoscale light source. The ZnO film formed by ALD emitted the bright, homogeneous light, unlike that deposited by another method. The dendritic structures of label-free macrophage receptor with collagenous structure-expressing CHO cells were clearly visualized below the diffraction limit. The inner fiber structure was observed with 120nm spatial resolution. Because the bright homogeneous emission from the ZnO film suppresses the background noise, the signal-to-noise ratio (SNR) for the imaging results was greater than 10. The ALD method helps achieve an electron beam excitation assisted microscope with high spatial resolution and high SNR.

Twofold photoswitching of NIR fluorescence and EPR based on the PMI–N–HABI for optical nanoimaging of electrospun polymer nanowires

A fluorescent dyad constituted by HABI and PMI with photo-induced NIR fluorescence and EPR has been reported and applied to optical nanoimaging of electrospun polymer nanowires.

Optical nanoimaging for block copolymer self-assembly.

One approach toward optical nanoimaging involves sequential molecular localization of photoswitchable fluorophores to achieve high resolution beyond optical limit of diffraction. Block copolymer micelles assembled from polystryrene-block-poly(ethylene oxide) block copolymers (PSt-b-PEO) are visualized in optical nanoimaging by staining the polystyrene blocks with spiropyrans (SPs). SPs localized in hydrophobic phase of block copolymer micelles exhibit reversible fluorescence on-off switching at alternating irradiation of UV and visible light. Phase-selective distribution of SPs in block copolymer micelles enables optical nanoimaging of microphase structures of block copolymer self-assembly at 50-nm resolution. To date, this is the sturdiest realization of optical nanoimaging with subdiffraction resolution for solution self-assembly of block copolymers.

Optical nano-imaging with electron beam excitation

Optical nano-imaging with electron beam excitation

A trident dithienylethene-perylenemonoimide dyad with super fluorescence switching speed and ratio.

Photoswitchable fluorescent diarylethenes are promising in molecular optical memory and photonic devices. However, the performance of current diarylethenes is far from satisfactory because of the scarcity of high-speed switching capability and large fluorescence on-off ratio. Here we report a trident perylenemonoimide dyad modified by triple dithienylethenes whose photochromic fluorescence quenching ratio at the photostationary state exceeds 10,000 and the fluorescence quenching efficiency is close to 100% within seconds of ultraviolet irradiation. The highly sensitive fluorescence on/off switching of the trident dyad enables recyclable fluorescence patterning and all-optical transistors. The prototype optical device based on the trident dyad enables the optical switching of incident light and conversion from incident light wavelength to transmitted light wavelength, which is all-optically controlled, reversible and wavelength-convertible. In addition, the trident dyad-staining block copolymer vesicles are observed via optical nanoimaging with a sub-100 nm resolution, portending a potential prospect of the dithienylethene dyad in super-resolution imaging.