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We report a breakthrough in observing living cells with infrared s-SNOM (scattering near-field microscope).(1, 2) To keep cells "happy" it is mandatory to avoid evaporation, oxygen attack, and mechanical distortion. We reported at IRMMW-THz 2019 that we successfully analyzed E. coli cells protected by graphene, but later had to realize that strong van der Waals forces squeeze cells and thus impair their metabolic functions.Here we demonstrate a new, satisfactory method of letting cells in nutrient broth adhere weakly to a biocompatible surface, SiN, where they can develop freely. We choose this material in form of an extremely thin (10 nm), free-standing membrane so that the s-SNOM tip can work on and probe through it from the other side at atmospheric conditions, while the tip-confined infrared near field reaches through this membrane largely unaffected.

Infrared fingerprint spectra can reveal the chemical nature of materials down to 20-nm detail, far below the diffraction limit, when probed by scattering-type scanning near-field optical microscopy (s-SNOM). But this was impossible with living cells or aqueous processes as in corrosion, due to water-related absorption and tip contamination. Here, we demonstrate infrared s-SNOM of water-suspended objects by probing them through a 10-nm thick SiN membrane. This separator stretches freely over up to 250 µm, providing an upper, stable surface to the scanning tip, while its lower surface is in contact with the liquid and localises adhering objects. We present its proof-of-principle applicability in biology by observing simply drop-casted, living E. coli in nutrient medium, as well as living A549 cancer cells, as they divide, move and develop rich sub-cellular morphology and adhesion patterns, at 150 nm resolution. Their infrared spectra reveal the local abundances of water, proteins, and lipids within a depth of ca. 100 nm below the SiN membrane, as we verify by analysing well-defined, suspended polymer spheres and through model calculations. SiN-membrane based s-SNOM thus establishes a novel tool of live cell nano-imaging that returns structure, dynamics and chemical composition. This method should benefit the nanoscale analysis of any aqueous system, from physics to medicine.

We investigated the excitation of surface plasmon polaritons on gold films with the metallized probe tip of a scattering-type scanning near-field optical microscope (s-SNOM). The emission of the polaritons from the tip, illuminated by near-infrared laser radiation, was found to be anisotropic and not circularly symmetric as expected on the basis of literature data. We furthermore identified an additional excitation channel via light that was reflected off the tip and excited the plasmon polaritons at the edge of the metal film. Our results, while obtained for a non-rotationally-symmetric type of probe tip and thus specific for this situation, indicate that when an s-SNOM is employed for the investigation of plasmonic structures, the unintentional excitation of surface waves and anisotropic surface wave propagation must be considered in order to correctly interpret the signatures of plasmon polariton generation and propagation.

We report progress towards sealing alive cells in their native, wet environment by single-layer graphene, where graphene stabilizes the cell by tightly conforming with its surface and thus enables topographic AFM maps of the cell’s surface. Furthermore, graphene’s transparency to infrared near-fields enables local chemical recognition, at nominally 20-nm spatial resolution, by scattering scanning mid-infrared microscopy (s-SNOM) and spectroscopy (nano-FTIR). Here we demonstrate first infrared nanoscopy results of the surface of native E. Coli cells that show spatial and spectral contrasts. Our study opens a new door into characterizing subcellular structures and components of cells and of fresh tissue sections. Infrared nanoscopy is certainly free of any need for labelling as in super-resolution fluorescence microscopy, and free of fixation and metallization procedures as in electron microscopy that kill and distort cells.

The crystal structure of solid-state matter greatly affects its electronic properties. For example, in multilayer graphene, precise knowledge of the lateral layer arrangement is crucial, since the most stable configurations, Bernal and rhombohedral stacking, exhibit very different electronic properties. Nevertheless, both stacking orders can coexist within one flake, separated by a strain soliton that can host topologically protected states. Clearly, accessing the transport properties of the two stackings and the soliton is of high interest. However, the stacking orders can transform into one another, and therefore, the seemingly trivial question of how reliable electrical contact can be made to either stacking order can a priori not be answered easily. Here, we show that manufacturing metal contacts to multilayer graphene can move solitons by several μm, unidirectionally enlarging Bernal domains due to arising mechanical strain. Furthermore, we also find that during dry transfer of multilayer graphene onto hexagonal boron nitride, such a transformation can happen. Using density functional theory modeling, we corroborate that anisotropic deformations of the multilayer graphene lattice decrease the rhombohedral stacking stability. Finally, we have devised systematics to avoid soliton movement, and how to reliably realize contacts to both stacking configurations, which will aid to reliably access charge transport in both stacking configurations., F.R.G., F.W., D.P., J.L., and R.T.W. acknowledge funding from the excellence initiative Nanosystems Initiative Munich (NIM), the Center for Nanoscience (CeNS) and the Solar Technologies go Hybrid (SolTech) initiative. We additionally acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy − EXC-2111−390814868 (MCQST) and EXC 2089/1−390776260.“ (e-conversion). We also thank Leonid S. Levitov, Nicola Mazzar,i and Nicolas Mounet for discussions and Jochen Feldmann for using his scanning Raman setup. R.G.-A., M.P., and A.A. thank the Project FIS2016-76617-P of the Spanish Ministry of Economy and Competitiveness MINECO, the Basque Government under the ELKARTEK project (SUPER), and the University of the Basque Country (Grant No. IT-756-13) for partial funding of this work. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, A3 Foresight by JSPS and the CREST (JPMJCR15F3), JST.

Non-equilibrium photoinduced plasmons in a high-mobility graphene monolayer are investigated at infrared wavelengths. The success of metal-based plasmonics for manipulating light at the nanoscale has been empowered by imaginative designs and advanced nano-fabrication. However, the fundamental optical and electronic properties of elemental metals, the prevailing plasmonic media, are difficult to alter using external stimuli. This limitation is particularly restrictive in applications that require modification of the plasmonic response at sub-picosecond timescales. This handicap has prompted the search for alternative plasmonic media1,2,3, with graphene emerging as one of the most capable candidates for infrared wavelengths. Here we visualize and elucidate the properties of non-equilibrium photo-induced plasmons in a high-mobility graphene monolayer4. We activate plasmons with femtosecond optical pulses in a specimen of graphene that otherwise lacks infrared plasmonic response at equilibrium. In combination with static nano-imaging results on plasmon propagation, our infrared pump–probe nano-spectroscopy investigation reveals new aspects of carrier relaxation in heterostructures based on high-purity graphene.

We demonstrate 50-nm resolved near-field imaging at λ=500 μm for the first time, using a high-harmonic microwave circuit for emitting/detecting via free space to a standard s-SNOM, and map conductivity contrasts at near single-charge sensitivity.

At terahertz (THz) frequencies, scattering-type scanning near-field optical microscopy (s-SNOM) based on continuous wave sources mostly relies on cryogenic and bulky detectors, which represents a major constraint for its practical application. Here, we devise a THz s-SNOM system that provides both amplitude and phase contrast and achieves nanoscale (60-70nm) in-plane spatial resolution. It features a quantum cascade laser that simultaneously emits THz frequency light and senses the backscattered optical field through a voltage modulation induced inherently through the self-mixing technique. We demonstrate its performance by probing a phonon-polariton-resonant CsBr crystal and doped black phosphorus flakes.

Moiré patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a crystal of hexagonal boron nitride. As a result of the moiré pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe the dynamical response of the moiré graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice mini-bands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moiré superstructures. This novel form of collective modes is likely to be generic to other forms of moiré-forming superlattices, including van der Waals heterostructures.

Hexagonal boron nitride (h-BN) is a natural hyperbolic material, for which the dielectric constants are the same in the basal plane (epsilon^t = epsilon^x = epsilon^y) but have opposite signs (epsilon^t*epsilon^z < 0) from that in the normal plane (epsilon^z). Due to this property, finite-thickness slabs of h-BN act as multimode waveguides for propagation of hyperbolic phonon polaritons - collective modes that originate from the coupling between photons and electric dipoles in phonons. However, control of these hyperbolic phonon polaritons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN. Here we show by direct nano-infrared imaging that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene with hyperbolic phonon polaritons in h-BN, so that the eigenmodes of the graphene/h-BN heterostructure are hyperbolic plasmon-phonon polaritons. Remarkably, the hyperbolic plasmon-phonon polaritons in graphene/h-BN suffer little from ohmic losses, making their propagation length 1.5-2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon-phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN structures can be classified as electromagnetic metamaterials since the resulting properties of these devices are not present in its constituent elements alone., 20 pages, 3 figures in Nature Nanotechnology 2015

We investigated emission and propagation of polaritons in a two dimensional van der Waals material hexagonal boron nitride (hBN). Our specific emphasis in this work is on hyperbolic phonon polariton emission that we investigated by means of scattering-type scanning near-field optical microscopy. Real-space nano-images detail how the polaritons are launched in several common arrangements including: light scattering by the edges of the crystal, metallic nanostructures deposited on the surface of hBN crystals, as well as random defects and impurities. Notably, the scanned tip of the near-field microscope is itself an efficient polariton launcher. Our analysis reveals that the scanning tips are superior to other types of emitters we have investigated. Furthermore, the study of polariton emission and emission efficiency may provide insights for development of polaritonic devices and for fundamental studies of collective modes in other van der Waals materials.

Background: Breast cancer represents the most common malignant disease in women worldwide. As currently systematic palliative treatment only has a limited effect on survival rates, the concept of health-related quality of life (HRQoL) is gaining more and more importance in the therapy setting of metastatic breast cancer. One of the major patient-reported outcomes (PROs) for measuring HRQoL in patients with breast cancer is provided by the European Organization for Research and Treatment of Cancer (EORTC). Currently, paper-based surveys still predominate, as only a few reliable and validated electronic-based questionnaires are available. Facing the possibilities associated with evolving digitalization in medicine, validation of electronic versions of well-established PRO is essential in order to contribute to comprehensive and holistic oncological care and to ensure high quality in cancer research. Objective: The aim of this study was to analyze the reliability of a tablet-based measuring application for EORTC QLQ-C30 in German language in patients with adjuvant and (curative) metastatic breast cancer. Methods: Paper- and tablet-based questionnaires were completed by a total of 106 female patients with adjuvant and metastatic breast cancer recruited as part of the e-PROCOM study. All patients were required to complete the electronic- (e-PRO) and paper-based versions of the HRQoL EORTC QLQ-C30 questionnaire. A frequency analysis was performed to determine descriptive sociodemographic characteristics. Both dimensions of reliability (parallel forms reliability [Wilcoxon test] and test of internal consistency [Spearman rho and agreement rates for single items, Pearson correlation and Kendall tau for each scale]) were analyzed. Results: High correlations were shown for both dimensions of reliability (parallel forms reliability and internal consistency) in the patient’s response behavior between paper- and electronic-based questionnaires. Regarding the test of parallel forms reliability, no significant differences were found in 27 of 30 single items and in 14 of 15 scales, whereas a statistically significant correlation in the test of consistency was found in all 30 single items and all 15 scales. Conclusions: The evaluated e-PRO version of the EORTC QLQ-C30 is reliable for patients with both adjuvant and metastatic breast cancer, showing a high correlation in almost all questions (and in many scales). Thus, we conclude that the validated paper-based PRO assessment and the e-PRO tool are equally valid. However, the reliability should also be analyzed in other prospective trials to ensure that usability is reliable in all patient groups. Trial Registration: ClinicalTrials.gov NCT03132506; https://clinicaltrials.gov/ct2/show/NCT03132506 (Archived by WebCite at http://www.webcitation.org/6tRcgQuou). [J Med Internet Res 2017;19(9):e322]

Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with metamaterials, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. Using scattering-type scanning near-field optical microscopy, we studied polaritonic phenomena in hBN. We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers [1]. Additionally, we carried out the modification of hyperbolic response in meta-structures comprised of a mononlayer graphene deposited on hBN [2]. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the “hyperlens” for subdiffractional focusing and imaging using a slab of hBN [3]. References [1] S. Dai et al., Science, 343, 1125 (2014). [2] S. Dai et al., Nature Nanotechnology, 10, 682 (2015). [3] S. Dai et al., Nature Communications, 6, 6963 (2015).

Self-detection is demonstrated as a simplifying new operation mode of scattering-type near-field optical microscopy (s-SNOM) in the mid infrared (6.2 μm wavelength). The back-scattered light from a tapping AFM tip is fed back into the quantum cascade laser used as illumination source such that recording of the laser voltage, at harmonics of the tapping frequency, generates amplitude and phase near-field images. This method is validated in a dual setup that simultaneously allows the usual homodyne operation mode which requires a detector and an asymmetric Michelson interferometer. [1] Detector-less self-detection-mode s-SNOMs should work also with semiconductor lasers in the visible and near-infrared, and most importantly, also in the THz region[2] which lacks fast, sensitive detectors.

We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding two orders of magnitude increase in the value of in-plane component of incident wavevector q compared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO2 substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage., Comment: 12 pages, 4 figures

Probing conductivity in a contactless way with nanoscale resolution is a pressing demand in such active fields as quantum materials, superconductivity, and molecular electronics. Here, we demonstrate a laser- and cryogen-free microwave-technology-based scattering-type scanning near-field optical microscope powered by an easily aligned free-space beam with a tunable frequency up to 0.75 THz. It uses Schottky diode components to record background-free amplitude and phase nano-images, for the first time in the terahertz range, which is uniquely sensitive for assessing conduction phenomena. Images of Si with doped nanostructures prove a conductance sensitivity corresponding to 1016 cm−3 mobile carriers, at 50 nm spatial resolution.

Nanostructures are at the heart of ever-shrinking electronic and photonic devices. The engineering of nanocomposite materials, building blocks, and conduction properties necessitate advanced microscopy tools to assess critical dimensional, compositional, structural, and conduction properties for analysis and quality control. Here we demonstrate with industrial bipolar and metal-oxide-semiconductor (MOS) devices that mid-IR scattering-type near-field optical microscopy (s-SNOM) has the potential to probe all of these parameters, and thereby excels electron and other scanning-probe microscopies. Within a single IR image, all relevant components such as Al, Ti, TiN, Si, Si3N4, and SiO2 are positively identified by material-specific amplitude and phase contrasts in addition to local conductivity in the form of mobile-carrier concentration, here over the range 10–10 cm. We image cross sections of the devices routinely prepared for failure analysis at 30 nm resolution, and this limit could be pushed below 10 nm in the course of future development. Resolution combined with high specificity to material and conductivity properties makes IR s-SNOM a promising tool beyond the microelectronics field for chemical nanotechnology, molecular electronics, photonics, and bioanalytics. IR spectroscopy has tremendous merit in the chemical and structural analyses of materials and in conduction assessment, but until now the limited spatial resolution has prevented its application in industrial failure analysis and quality control. Instead, scanning electron microscopy (SEM) is employed, which offers nanoscale resolution and sensitivity to materials, particularly when combined with Auger, energy-dispersive X-ray (EDX), or wavelength-dispersive X-ray (WDX) spectroscopies. However, only qualitative doping information with decoration-etched samples can be obtained. Transmission electron microscopy (TEM) offers elemental sensitivity in combination with EDX or electron energy loss spectroscopy (EELS), but it suffers from complicated and time-consuming sample preparation. Moreover, the feature sizes of semiconductor structures have already been reduced below the minimum thickness of TEM samples so that details can no longer be imaged. Scanning probe microscopy (SPM) provides topography, and in the extended versions of scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM), also doping but poor material sensitivity. We introduce IR s-SNOM as a method of choice, as SNOM generally extends SPM by the optical near-field interaction between tip and sample, and it thus enables the power of optical spectroscopies, such as fluorescence, Raman, or IR, to be exploited with nanoscale spatial resolution. In particular, IR s-SNOM has a demonstrated specificity to chemical composition, material category, structural variation, and electrical conduction. Our IR s-SNOM technique is based on atomic force microscopy (AFM) as described previously; imaging relies on a commercial, Pt-coated, cantilevered tip operating in tapping mode (frequency X ≈ 30 kHz) with a 20–30 nm radius of curvature. The tip is illuminated by a focused CO2 laser beam at a wavelength k = 10.7 lm. The backscattered light is analyzed interferometrically to record both amplitude and phase. Pure near-field image contrast is attained by pseudoheterodyne interferometric detection at a harmonic frequency nX, with n>1, yielding amplitude sn and phase!n signals simultaneously. According to a point-dipole model, these parameters relate to the complex dielectric value of the sample, e= e1 + ie2. Thus, s-SNOM enables us to distinguish the different materials and doping levels that can be described by a point-dipole model. In this model the field E scattered by the tip is approximated by a dipolar near-field interaction between the tip apex and the sample surface, yielding

We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At sub-nanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nano-imaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene: yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers., 27 pages, 11 figures

Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. In such materials, light propagation is unusual leading to novel and often non-intuitive optical phenomena. Here we report infrared nano-imaging experiments demonstrating that crystals of hexagonal boron nitride, a natural mid-infrared hyperbolic material, can act as a ‘hyper-focusing lens’ and as a multi-mode waveguide. The lensing is manifested by subdiffractional focusing of phonon–polaritons launched by metallic disks underneath the hexagonal boron nitride crystal. The waveguiding is revealed through the modal analysis of the periodic patterns observed around such launchers and near the sample edges. Our work opens new opportunities for anisotropic layered insulators in infrared nanophotonics complementing and potentially surpassing concurrent artificial hyperbolic materials with lower losses and higher optical localization., United States. Air Force Office of Scientific Research (Grant FA9550-11-1-0225)

Quite unexpectedly, THz and infraredspectroscopy has now a real chance to solveproblems in the nanosciences. This rests ona new microscope technique that overcomesthe Abbe diffraction limit, by using thenear field of a metal antenna in closeproximity to a scanned sample surface. HereI briefly summarize present activities inthe microwave, mid-infrared and visiblespectral ranges. It seems straightforwardand highly desirable to fill the existinggap between about 20 GHz and 20 THz, andattain spatial resolution of 10 nm andbelow also in this important part of theelectromagnetic spectrum.

Abbe’s diffraction limit did in the past prevent the analytical tool of vibrational-infrared spectroscopy from reaching beyond a spatial resolution of λ/2. This limit has been overcome with the invention of near-field proximal probes. Here we emphasize scattering-type scanning near-field infrared microscopy (s-SNIM) which offers two unique advantages for vibrational-absorption-based microscope contrast: (i) spatial resolution of

Optical near fields exist close to any illuminated object. They account for interesting effects such as enhanced pinhole transmission or enhanced Raman scattering enabling single-molecule spectroscopy. Also, they enable high-resolution (below 10 nm) optical microscopy. The plasmon-enhanced near-field coupling between metallic nanostructures opens new ways of designing optical properties and of controlling light on the nanometre scale. Here we study the strong enhancement of optical near-field coupling in the infrared by lattice vibrations (phonons) of polar dielectrics. We combine infrared spectroscopy with a near-field microscope that provides a confined field to probe the local interaction with a SiC sample. The phonon resonance occurs at 920 cm(-1). Within 20 cm(-1) of the resonance, the near-field signal increases 200-fold; on resonance, the signal exceeds by 20 times the value obtained with a gold sample. We find that phonon-enhanced near-field coupling is extremely sensitive to chemical and structural composition of polar samples, permitting nanometre-scale analysis of semiconductors and minerals. The excellent physical and chemical stability of SiC in particular may allow the design of nanometre-scale optical circuits for high-temperature and high-power operation.

Controlling the domain size and degree of crystallization in organic films is highly important for electronic applications such as organic photovoltaics, but suitable nanoscale mapping is very difficult. Here we apply infrared-spectroscopic nano-imaging to directly determine the local crystallinity of organic thin films with 20-nm resolution. We find that state-of-the-art pentacene films (grown on SiO2 at elevated temperature) are structurally not homogeneous but exhibit two interpenetrating phases at sub-micrometre scale, documented by a shifted vibrational resonance. We observe bulk-phase nucleation of distinct ellipsoidal shape within the dominant pentacene thin-film phase and also further growth during storage. A faint topographical contrast as well as X-ray analysis corroborates our interpretation. As bulk-phase nucleation obstructs carrier percolation paths within the thin-film phase, hitherto uncontrolled structural inhomogeneity might have caused conflicting reports about pentacene carrier mobility. Infrared-spectroscopic nano-imaging of nanoscale polymorphism should have many applications ranging from organic nanocomposites to geologic minerals., The grain boundaries between two coexisting phases in organic semiconductor pentacene are expected to obstruct charge transport in its thin-film devices. Westermeier et al. use infrared-spectroscopic nano-imaging to show an interlocking morphology, which is uncorrelated with its grain structures.

Pump-probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump-probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here we circumvent this deficiency and introduce pump-probe infrared spectroscopy with ~20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO2 at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 femtosecond (fs) time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances., Comment: 27 pages (including Supporting Information), 8 figures

Light distributions near resonant metal nanoparticles are recorded by a scattering-type scanning near-field optical microscope (s-SNOM), for the first time with a sub-particle-size resolution (

Optical phase contrast has for the first time been observed on a nanometer scale, with a near-field microscope of scattering type that maps the complete optical field of amplitude and phase. Backed by quasielectrostatic theory, we demonstrate the significance and experimental accessibility of even complex optical constants on a subwavelength scale. Further, our method can separate the near-field response from background artifacts and thus is expected to enable nanoscale optical mapping of even topography-rich objects such as resonant clusters and macromolecules.

`Apertureless' probe tips have a much higher resolution potential compared to the traditional aperture tips of scanning near-field optical microscopes (SNOM), yet when illuminated by a laser focus a large amount of unwanted background scattering occurs both at the probe shaft and at the sample. Here we study in detail how this background can be suppressed by dithering the probe–sample distance, and thereby demonstrate how to enhance the optical near-field contrasts. We find from theory that the coupling of probe dipole and its image in the sample causes a steep increase of scattering cross-sections at small probe–sample distances. This strongly non-linear behavior produces higher harmonics when modulating the distance. Demodulation at higher harmonics, therefore, enables an effective probe tip `sharpening' and improves both resolution and image contrast. This effect is experimentally confirmed by imaging purely dielectric contrast of a topographically flat pn+-nanostructured semiconductor, realizing λ/100 resolution at 10 μm infrared wavelength.

In this paper, we present the first gas-absorption spectra measured with an all-electronic terahertz spectrometer. This instrument uses phase-locked microwave sources to drive GaAs nonlinear transmission lines that produce picosecond pulses, enabling measurement of broad-band spectra. By sweeping the fundamental excitation, however, the spectrometer can also measure single lines with hertz-level precision, a mode of operation not readily available with optoelectronic terahertz techniques. Since this system is based on integrated circuits, it could ultimately function as an inexpensive gas-sensing system, e.g., for vehicle emissions, an application for which we analyze the sensitivity of a prototypical system.

We explore the performance of a scanning near-field infrared microscope, which works by scattering tightly focused CO2 laser radiation (lambda = 10 microm) from the apex of a metallized atomic force microscope tip. The infrared images of test samples prove a spatial resolution of 30 nm and are free of topographical and inertial artefacts, thus they should be of great interest for practical applications. We also observe that the infrared contrast vanishes when the input beam polarization is orthogonal to the tip axis, in accordance with theoretical expectations for a mechanism of longitudinal field interaction.

Identification of chemical compounds by vibrational spectroscopy at infrared wavelengths requires macroscopic samples: the spatial resolution is diffraction-limited to a scale of about half the wavelength, or about five micrometres. The scanning near-field optical microscope1,2, however, can reveal sub-wavelength detail because it uses near-field probing rather than beam focusing. Here we demonstrate the use of the aperture-less approach to scanning near-field optical microscopy3,4,5,6 to obtain contrast in vibrational absorption on a scale of about 100 nanometres, about one-hundredth of a wavelength. We record infrared scattering from the tip of an atomic force microscope scanned over a composite polymer film. At the boundary between different polymers we observe contrast changes owing to changes in vibrational absorption. The contrast is strongly enhanced in the near field of the probe tip, which we interpret as evidence of surface-enhanced infrared absorption7. When extended to multi-wavelength operation, this approach should enable imaging of chemical composition at nanometre resolution.

We calculate electromagnetic (EM) fields in tapered metal cutoff waveguides by solving a simple system of two coupled differential equations which describe the incremental local reflection of the waveguide mode as the diameter changes gradually. The results give physical insight into aperture probes used in scanning near-field optical microscopy (SNOM). The method is an easily installed tool for SNOM optimization.

laser radiation from an AFM tip. In the microscopic images we find and identify a new type of AFM-induced artifact (crosstalk via the tapping amplitude). Minimizing this by proper scan parameters we obtain evidence of true infrared contrast. The results demonstrate the material-sensitive potential of infrared-spectroscopic imaging and a spatial resolving power of better than 100 nm.

A novel scanning-probe setup is reported that maps the complex transmission of microwaves. Response to topographic features as small as 15 nm was observed. The sharpened coaxial probe tip serves as a microwave antenna and, at the same time, as a tunnel tip to warrant precise distance control by STM (scanning tunneling microscope) feedback. The instrument can be applied to map the microwave conductivity of, e.g. thin films or low-dimensional semiconductors. Consequences for the development of an infrared microscope are outlined.

By scanning a metal grid sample between two coaxial transmission line tips, we resolve Δx ≈ 10 μ m features using up to λ = 20 m wavelengths, achieving wavelength-relative resolution Δx λ −7 in both magnitude and phase. As an example of the technique's quite general application to conductivity-contrast microscopy, we measure the local transmittance of a GaAs AlGaAs heterostructure versus magnetic field. We view this work as a proof-of-principle experiment which justifies scaling down the coaxial tip geometry to the sub-micrometer regime.

We demonstrate—for a typical polymer vibrational infrared line—that scattering-type “apertureless” optical near-field microscopy features a spectral signature that differs characteristically from far-field absorption. Theory predicts a dispersion-like amplitude spectrum (besides an absorption-like, bell-shaped phase spectrum). This signature is experimentally verified for a vibrational resonance of PMMA, by probing with a CO laser tuned from 5.5 to 6 μm. We apply this signature to identify PMMA in the near-field imaging of a nanostructured PMMA/PS polymer blend, at

The third‐harmonic generation of far‐infrared (FIR) laser radiation in n‐doped silicon was measured with full temporal resolution of the power fluctuations during the FIR laser pulse. Thus, the intensity dependence of the nonlinear coefficient χ(3) could be observed. For 2 MW incident power, corresponding to an intensity of 15 MW/cm2, at a wavelength of 676 μm a power conversion efficiency of 10−3 was reached.

In future fusion-oriented plasma devices alpha-particle diagnostics will be of great importance. It is generally recognized that collective Thomson scattering techniques show great potential in this field. For ion temperature measurements as well as for alpha-particle diagnostics a radiation source emitting in the submillimetre range is required. However, powerful radiation sources which meet the requirement of pulse durations in the millisecond range are not easily available at these wavelengths. A high frequency gyrotron, combined with an efficient frequency tripler could be an interesting alternative to the free-electron laser, the only source which has been considered up till now. We report experiments on third harmonic generation of radiation in n-doped Si, using a source with a wavelength of 676 mu m. A power of up to 2 kW could be generated at 225 mu m with a conversion efficiency of 0.1% While the onset of saturation could be observed, the power limit was due to surface breakdown effects rather than saturation.

We demonstrate Fourier transform infrared nanospectroscopy (nano-FTIR) based on a scattering-type scanning near-field optical microscope (s-SNOM) equipped with a coherent-continuum infrared light source. We show that the method can straightforwardly determine the infrared absorption spectrum of organic samples with a spatial resolution of 20 nm, corresponding to a probed volume as small as 10 zeptoliter (10(-20) L). Corroborated by theory, the nano-FTIR absorption spectra correlate well with conventional FTIR absorption spectra, as experimentally demonstrated with poly(methyl methacrylate) (PMMA) samples. Nano-FTIR can thus make use of standard infrared databases of molecular vibrations to identify organic materials in ultrasmall quantities and at ultrahigh spatial resolution. As an application example we demonstrate the identification of a nanoscale PDMS contamination on a PMMA sample.

Surface plasmons are collective oscillations of electrons in metals or semiconductors enabling confinement and control of electromagnetic energy at subwavelength scales. Rapid progress in plasmonics has largely relied on advances in device nano-fabrication, whereas less attention has been paid to the tunable properties of plasmonic media. One such medium-graphene-is amenable to convenient tuning of its electronic and optical properties with gate voltage. Through infrared nano-imaging we explicitly show that common graphene/SiO2/Si back-gated structures support propagating surface plasmons. The wavelength of graphene plasmons is of the order of 200 nm at technologically relevant infrared frequencies, and they can propagate several times this distance. We have succeeded in altering both the amplitude and wavelength of these plasmons by gate voltage. We investigated losses in graphene using plasmon interferometry: by exploring real space profiles of plasmon standing waves formed between the tip of our nano-probe and edges of the samples. Plasmon dissipation quantified through this analysis is linked to the exotic electrodynamics of graphene. Standard plasmonic figures of merits of our tunable graphene devices surpass that of common metal-based structures., 15 pages, 3 figures

For infrared spectroscopy of low-dimensional electron systems we suggest a geometry of critical-angle total internal reflection. An external polarizer then allows one to orient the electric field at the interface either parallel or perpendicular to the growth plane. As an example, obtained with a Ge coupling prism, we present a first observation of saturation of the infrared photon-drag effect in GaAs/GaAlAs quantum wells.

The small-signal amplification of a far-infrared p-Ge laser is measured with a gas laser and a fast detector, and is found to be surprisingly large (≈0.1 cm -1 ) and short-lived (≈0.2 μs), suggesting further ways to optimize this laser. Absorption by impurity and Drude processes is also observed. Saturation effects during self-oscillation indicate that neutral impurities are involved in the laser process.

We report that three main constituents of nanosystems—metals, semiconductors, and dielectrics—can be categorically distinguished by their specific optical near-field contrast at 633 nm wavelength. The decisive property is the local dielectric constant as we show by calculations based on dipolar coupling theory. Experiments with Au/Si/PS(polystyrene) nanostructures using an apertureless scattering-type near-field optical microscope yield optical images at 10 nm resolution, with clear material contrast close to predicted levels.

Methods for imaging of nanocomposites based on X-ray, electron, tunneling or force microscopy provide information about the shapes of nanoparticles; however, all of these methods fail on chemical recognition. Neither do they allow local identification of mineral type. We demonstrate that infrared near-field microscopy solves these requirements at 20 nm spatial resolution, highlighting, in its first application to natural nanostructures, the mineral particles in shell and bone. "Nano-FTIR" spectral images result from Fourier-transform infrared (FTIR) spectroscopy combined with scattering scanning near-field optical microscopy (s-SNOM). On polished sections of Mytilus edulis shells we observe a reproducible vibrational (phonon) resonance within all biocalcite microcrystals, and distinctly different spectra on bioaragonite. Surprisingly, we discover sparse, previously unknown, 20 nm thin nanoparticles with distinctly different spectra that are characteristic of crystalline phosphate. Multicomponent phosphate bands are observed on human tooth sections. These spectra vary characteristically near tubuli in dentin, proving a chemical or structural variation of the apatite nanocrystals. The infrared band strength correlates with the mineral density determined by electron microscopy. Since nano-FTIR sensitively responds to structural disorder it is well suited for the study of biomineral formation and aging. Generally, nano-FTIR is suitable for the analysis and identification of composite materials in any discipline, from testing during nanofabrication to even the clinical investigation of osteopathies.

We use ultra-resolving terahertz (THz) near-field microscopy based on THz scattering at atomic force microscope tips to analyze 65-nm technology node transistors. Nanoscale resolution is achieved by THz field confinement at the very tip apex to within 30 nm. Images of semiconductor transistors provide evidence of 40 nm (λ/3000) spatial resolution at 2.54 THz (wavelength λ = 118µm) and demonstrate the simultaneous THz recognition of materials and mobile carriers in a single nanodevice. The mobile carrier contrast can be clearly related to near-field excitation of THz-plasmons in the semiconductor regions. The extraordinary high sensitivity of our microscope provides THz near-field contrasts from less than 100 mobile electrons in the probed volume.

A far‐infrared rotating‐analyzer ellipsometer which uses a step‐tunable, optically pumped gas laser as its light source is described herein. As polarizers novel metal grids with 10 000:1 polarization contrast were used. The instrument determines the complex dielectric function in the spectral range between 10 and 150 cm−1. A cryostat allows both reflection and transmission measurements from 10 to 330 K. Measurements of the birefringence of crystalline quartz, of both the carrier density and the scattering frequency of doped semiconductors, and of the low energy excitations of high‐TC YBaCuO ceramics are presented herein.

We present time-resolved cyclotron resonance spectra of holes in p-germanium measured during a single magnetic field pulse by using a rapid-scanning, fiber-coupled THz time-domain spectroscopy system.

We demonstrate 300 cm−1 wide spectra in the 800–1400 cm−1 range taken from single pixels of about 20 nm size. Technical issues of coupling a fiber-laser-powered coherent spectrometer with a standard near-field microscope will be discussed, together with results on the phonon-resonant response of numerous crystals. Quantitative amplitude and phase spectra will be compared with theory.

A Mach-Zehnder interferometer illuminated by a CO(2) laser at wavelengths selectable between 9.1 micro and 11.3 micro is described. With a field of view 22 mm in diameter two-dimensional interferograms are recorded. The method uses liquid crystals for ir-optical image conversion. With the small laser used, an exposure time of 1/40 sec is required, while the steady state resolution limit of the detector is a radiation flux of less than 5 mW/cm(2). The interference pattern of an argon plasma jet is shown.

With ever shrinking dimensions in microelectronics, the conductivity performance of charge carriers approaches physical limits and demands tighter control. We show that near-field microscopy carried out at sufficiently long infrared wavelengths—below the plasma frequency—selectively detects and characterizes subsurface mobile carriers with 30 nm resolution, timely for next generation chips as well as for fundamental research, e.g., on low-dimensional electron systems.