Near-field Optical Microscopy Images Research Articles

Substrate-enhanced photothermal nano-imaging of surface polaritons in monolayer graphene

Surface polaritons comprise a wealth of light–matter interactions with deep sub-wavelength scale confinement of electromagnetic modes. However, their nanoscale localized dissipation and thermalization processes are not readily accessible experimentally. Here, we introduce photothermal force microscopy to image surface plasmon polaritons (SPPs) in monolayer graphene through their non-radiative SiO2 substrate dissipation. We demonstrate the real-space SPP imaging via photo-induced atomic force detection, and from comparison with scattering-type scanning near-field optical microscopy imaging attribute the force response to substrate dissipation-enhanced thermal expansion. This work illustrates that nano-optical tip–sample induced dissipative forces facilitate a direct mechanical detection of surface polariton interactions with monolayer sensitivity.

Effect of sample anisotropy on scanning near-field optical microscope images

Scattering-type scanning near-field optical microscopy (s-SNOM) has been widely used to characterize strongly correlated electronic, two dimensional, and plasmonic materials, and it has enormous potential for biological applications. Many of these materials exhibit anisotropic responses that complicate the extraction of dielectric constants from s-SNOM measurements. Here, we generalize our recently developed approach for retrieving the near-field scattering signal from isotropic systems and apply it to anisotropic dielectrics. Specifically, we compare our theoretical results with experimental measurements on modestly anisotropic sapphire that exhibit strong resonances at the infrared frequency range. Good agreement with the experimental result is found. Our result is important for understanding the near-field response of low damping, anisotropic polaritonic states in dielectric media.

Visibility of subsurface nanostructures in scattering-type scanning near-field optical microscopy imaging.

Detecting structures below a cover film at the nanoscale resolution is of essential importance. In this work, we explored factors affecting subsurface material contrast and structural visibility in scattering-type scanning near-field optical microscopy (s-SNOM). A kind of multilayered reference samples containing different buried structures was fabricated and applied for s-SNOM imaging. The dependence of near-field optical contrast on structure geometry, dimension and cover thickness was investigated. Results demonstrate that distinguishing the buried slit pattern is easier than the circular hole with the same critical dimension. The s-SNOM can sense material difference under a more than 100 nm thick polymethyl methacrylate layer and it has a subsurface spatial resolution better than 100 nm.

Hybrid AlGaInAs/Si Fabry–Pérot lasers with near-total mode confinements * * Project supported by the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (No. QYZDJ-SSW-JSC002), the NSFC/RGC Joint Project (No. 61431166003), and the National Natural Science Foundation of China (No. 61377105).

We have proposed and demonstrated hybrid AlGaInAs/Si Fabry–Pérot (FP) lasers, with the FP cavity facet covered by the p-electrode metal for enhancing mode confinement. Continuous-wave lasing is obtained at room temperature with a threshold current of 45 mA for the hybrid FP laser with a cavity length of 415 μm and a width of 7 μm. Near-field optical microscope images indicate an efficient output emission from the underneath evanescently-coupled silicon waveguide. Furthermore, single-mode lasing with a side-mode suppression-ratio of 29 dB and a threshold current of 16 mA is realized for the 150 μm-long hybrid FP laser.

Tunable Optical Antennas Based on Metallic Nanoshells with Nanoknobs

We investigate optical properties of a new complex plasmonic nanostructure, which consists of a spherical metallic nanoshell and a small metallic nanoparticle ("nanoknob") situated on its surface. The plasmon resonance wavelength of the entire structure is guided by the geometrical and material properties of the nanoshell whereas the electromagnetic field of the incident light is localized and enhanced near the "nanoknob". The idea is supported with electromagnetic modeling and near-field optical microscopy imaging. In addition, we proposed and demonstrated a new method of nanoparticle precise manipulation under electron beam, which could be used in fabrication of such plasmonic structures and other nanosized elements.

Novel approaches for scanning near-field optical microscopy imaging of oligodendrocytes in culture

Newborn rat oligodendrocyte cultures were investigated by scanning near-field optical microscope (SNOM), a versatile new tool able to map cell membranes in 3D and simultaneously obtain images of the cytoplasm. Topography, error, transmission and reflection signals were acquired to describe cell morphology with nanometer-scale resolution. Oligodendrocytes were studied as a model because their extensive membrane processes (typical of their physiological role in myelination) made them particularly suitable to test the sensitivity of the new method. Furthermore, we combined a classical histochemical method with SNOM, to identify specific intracellular proteins at high definition. In particular, with this technique, cytoskeleton elements of oligodendrocytes, such as microtubules, were observed with tubulin antibodies. Images obtained with SNOM were also compared with those from conventional scanning electron microscopy (SEM) and optical microscopy. Our results showed that SNOM allowed to observe cell nanostructures otherwise undetectable all together with other microscopies. In conclusion, SNOM, combined with rapid and non-invasive methods of specimen preparation, appears to be a powerful tool that can offer new possibilities in the field of neuroscience imaging at nano-scale level.

SNOM on cell thin sections: observation of Jurkat and MDAMB453 cells

In this study a comparison of scanning near-field optical microscopy with a traditional, well-known microscopic technique like transmission electron microscopy is discussed. To establish a reliable and comparable method for high-resolution scanning near-field optical microscopy imaging of biological samples, the attention is focussed on cell sections. In particular, we present a study of ultrathin sections of Jurkat T-cells and MDAMB453 cells. We show the relationship among the scanning near-field optical microscopy (topographic and optical) images and the kind of embedding medium (resin), the sections thickness and the staining of the sample. For a complementary investigation atomic force microscopy measurements are carried out, as well. The study reveals that scanning near-field optical microscopy technique on opportunely prepared thin sections can be applied successfully for investigation of the interior of the cells. Scanning near-field optical microscopy and transmission electron microscopy allow to obtain different, however comparable, and complementary information of the cell sample.

Scanning near‐field optical microscope based on a double resonant fibre probe montage and equipped with time‐gated photon detection

We discuss scanning near-field optical microscope based on original double resonant montage of a fibre probe onto the tuning fork and proprietary electronics capable of fast and precise measurements of the resonant frequency and the quality factor of sensor dithering. Special emphasis is given on the pulsed excitation/gated detection of optical signal. This option as well as the possibility of fast scanning facilitates a lot the problem of single fluorescence centres detection. To illustrate the performance of this microscope, we present first true single-molecule fluorescence resonance energy transfer scanning near-field optical microscope images of single CdSe nanocrystals on glass slide surface and observation of an optical 'pseudoresolution' of densely packed 100-nm-diameter transfluorescent spheres in noisy conditions.

Near‐ and far‐field verification of electro‐optic effect enhancement on a tunable lithium niobate photonic crystal

In this paper, we present the optical response of a tunable lithium niobate photonic crystal (PC) using the electro-optic effect of the material. The band gap tunability is 300 times higher than what one could expect for a bulk lithium niobate device of the same characteristics. Theoretical calculations based on the finite-difference time domain technique have allowed us to determine the physical origin of this enhanced electro-optic coefficient. Indeed, the effective second-order susceptibility in the LN nanostructure increases, giving rise to an ultra-compact low-voltage photonic crystal modulator when it operates at its band edge. In addition, the theoretical and far-field transmission results are confirmed by near-field optical microscopy images of the structure at different excitation voltages.

Scanning near-field optical microscopy images of microradiographs stored in lithium fluoride films with an optical resolution of λ∕12

Here we show a new, simple method to observe soft x-ray microradiographs of biological material. A thin film of lithium fluoride (LiF) works as image detector, storing the microradiograph obtained exposing biological samples to extreme ultraviolet and soft x-ray radiations. To read the stored image, collecting the optically stimulated visible luminescence emitted by the LiF active color centers locally produced by the x rays, a scanning near-field optical microscope is used with an optical aperture of 50nm, i.e., λ∕12, where λ is the wavelength of the collected photoluminescence.

Imaging of domain wall motion in small magnetic particles through near-field microscopy

We report magneto-optical scanning near-field optical microscopy images for 4×4×0.08μm3 and 16×16×0.08μm3 low magnetic anisotropy Co70.4Fe4.6Si15B10 particles. Measuring magneto-optical differential susceptibility we acquired images of the domain wall movement driven by an applied magnetic field with a spatial resolution better than λ∕4. For the 4×4×0.08μm3 sized particle, a sequence of 27 magneto-optical differential susceptibility images reveals the evolution of the magnetic domain structure between positive and negative saturation fields passing through the four-domain flux-closure magnetization structure. On the 16×16×0.08μm3 particle, we studied the role of the oscillating driving field on the susceptibility distribution. Comparing the different magneto-optical differential susceptibility sequences, the role of the shape anisotropy on the field induced domain wall movement is evidenced. Micromagnetic simulations were used to provide a better understanding of the domain wall movement.

Near-field imaging of surface plasmon on gold nano-dots fabricated by scanning probe lithography.

We obtained scanning near-field optical microscopy images to study the excitation of surface plasmons on metallic dots fabricated using scanning probe lithography. Gold nano-dots were fabricated by applying electric voltages to conducting probes installed in an atomic force microscope using the mechanism of field-induced diffusion and nano-oxidation plus Au-coating. High spatial resolution of scanning near-field optical microscopy revealed a 'bifold' pattern of surface plasmon mode on fabricated Au dots in the polarization direction of incident light. We found that scanning near-field optical microscopy imaging combined with scanning probe lithography is able to provide a systematic study of surface plasmon excitation on nano-metallic structures.

Photonic nanopatterns of gold nanostructures indicate the excitation of surface plasmon modes of a wavelength of 50–100 nm by scanning near‐field optical microscopy

Scanning near-field optical microscopy images of metal nanostructures taken with the tetrahedral tip (T-tip) show a distribution of dark and bright spots at distances in the order of 25-50 nm. The images are interpreted as photonic nanopatterns defined as calculated scanning near-field optical microscopy images using a dipole serving as a light-emitting scanning near-field optical microscopy probe. Changing from a positive to a negative value of the dielectric function of a sample leads to the partition of one spot into several spots in the photonic nanopatterns, indicating the excitation of surface plasmons of a wavelength in the order of 50-100 nm in metal nanostructures.

Imaging of photonic nanopatterns by scanning near-field optical microscopy

We define photonic nanopatterns of a sample as images recorded by scanning near-field optical microscopy with a locally excited electric dipole as a probe. This photonic nanopattern can be calculated by use of the Green’s dyadic technique. Here, we show that scanning near-field optical microscopy images of well-defined gold triangles taken with the tetrahedral tip as a probe show a close similarity to the photonic nanopattern of this nanostructure with an electric dipole at a distance of 15 nm to the sample and tilted 45° with respect to the scanning plane.

Single Nanowire Lasers

Ultraviolet lasing from single zinc oxide nanowires is demonstrated at room temperature. Near-field optical microscopy images quantify the localization and the divergence of the laser beam. The linewidths, wavelengths, and power dependence of the nanowire emission characterize the nanowire as an active optical cavity. These individual nanolasers could serve as miniaturized light sources for microanalysis, information storage, and optical computing.

Different contrast mechanisms induced by topography artifacts in near-field optical microscopy

Topography artifacts can affect the elastic scattering optical images in near-field optical microscopy. Moreover, such effects can completely mask the information on the optical properties of the samples, yielding unexpectedly high lateral resolutions. In this letter, we provide experimental evidence of how changes involving the collection geometry and the roughness of the sample can lead to radical modifications of the contrast induced by the artifacts to the optical map. An explanation of the different regimes is provided by means of approach curves in which the optical signal is detected as a function of the probe/sample distance.

Near-field optical microscope images of a dielectric flat substrate with subwavelength strips

In near-field scanning optical microscopy (NSOM), optical images are calculated for flat-surface samples with an optical index distribution. This makes it possible to estimate NSOM images that are free from perturbations in the topographical information of the sample. A realistic NSOM is simulated, including an aperture probe and a three-dimensional field. The calculations reveal the imaging characteristics of both the dielectric and opaque samples. We found that the NSOM images particularly under s-polarized illumination should be analyzed with the knowledge of the point spread function to avoid misinterpreting the detected images. Differences in images due to two orthogonal polarizations and incident angle of illumination are also examined.

Electrolytic formation of nanoapertures for scanning near-field optical microscopy

Aperture probes for near-field optical microscopy were produced by controlled all solid state electrolysis. Control of both the ionic current and light transmission provided reproducible probe tips with aperture diameters in the sub-50 nm range and flat end faces. High resolution scanning near-field optical microscopy images were obtained with these probes. As a by-product, the formation of an electrolytic nanometer-sized contact was observed.

Theoretical study of scanning near-field microscope

Numerical calculations of the scanning near-field optical microscope images of assemblies of dielectric spheres is reported. It is shown that the reported images obtained experimentally of a triangular lattice of polystyrene spheres can be reproduced in good agreement. Other possible images for various parameters are also discussed.

Influence of secondary tip shape on illumination-mode near-field scanning optical microscopy images

We report illumination-mode near-field optical microscopy images of individual 80–115-nm-diameter Au particles recorded with metal-coated fiber probes. It is found that the images are strongly influenced by the metal-coating thickness. This dependence is consistent with theoretical models, which are in good agreement with the experimental images. Probes with thick coatings (∼λ/2) produce images consisting of three extrema, owing to a resonancelike polarization of the probe end. Probes with thinner coatings generally produce simpler images. However, in some cases the images contain wavelike structures that are due to interference between direct radiation from the tip and propagating tip fields scattered by the particles.