Near-field Probe Research Articles

Nanoscale optical thermometry using a time-correlated single-photon counting in an illumination-collection mode

A nanoscale thermometry method called fluorescence near-field optics thermal nanoscopy (Fluor-NOTN) has been developed using near-field fluorescence imaging. This method can detect local temperature distributions with a nanoscale spatial resolution by measuring the fluorescence lifetimes of Cd/Se quantum dots (QDs) as a temperature probe. To increase the sensitivity of Fluor-NOTN, time-correlated single-photon counting (TCSPC) was introduced with a triple-tapered fusion-spliced near-field (TFN) optical fiber probe. This highly sensitive technique for measuring the fluorescence lifetime of QDs enabled the detection of low-level light signals with a picosecond time resolution at high-precision in an illumination-collection mode for Fluor-NOTN. The feasibility of this proposed method was experimentally verified by measuring the temperature dependence of the fluorescence lifetimes of the QDs by Fluor-NOTN using TCSPC with a TFN optical fiber probe with an aperture of 70 nm.

A simple model of the scanning near-field optical microscopy probe tip for electric field enhancement

A simple model of the scanning near-field optical microscopy probe tip for electric field enhancement

Long-Term Monitoring of Skin Recovery by Micromachined Microwave Near-Field Probe

The water content in the epidermis correlates with different pathologic states of the skin; thus its assessment can aid the diagnosis and monitoring of conditions such as inflammation, edema, burns, and skin cancer. A micromachined microwave near-field probe, operating from 90 to 106 GHz, which, in contrast to earlier used microwave probes, has a minimized sensing area of $0.6\,\,\text {mm} \times 0.5$ mm and an optimized sensing depth of 400 $\mu \text{m}$ in tissue, has been developed and technically characterized by the authors earlier. This letter reports on the long-term monitoring of sodium lauryl sulfate (SLS)-induced skin irritations with the micromachined microwave probe. Aqueous solutions with 1%, 2%, 5%, and 10% SLS were applied to the forearm of a volunteer for 24 h and microwave reflection measurements were taken before and during 11 days after the SLS application. For all SLS-treated spots the microwave absorption reached the highest levels of 4 to 7 days after SLS application and afterward converged toward baseline levels again. The observed biphasic progression of the microwave reflection signal agrees well with trends from the literature for capacitance measurements and for epidermal thickness and signal attenuation in optical coherence tomography after SLS exposure. The measurements indicate that the microwave probe is very suitable to determine changes in the water content in the epidermis and can aid in the diagnosis of pathologic conditions including skin cancer.

Resolving phase information of the optical local density of state with scattering near-field probes

We theoretically discuss the link between the phase measured using a scattering optical scanning near-field microscopy (s-SNOM) and the local density of optical states (LDOS). A remarkable result is that the LDOS information is directly included in the phase of the probe. Therefore by monitoring the spatial variation of the trans-scattering phase, we locally measure the phase modulation associated with the probe and the optical paths. We demonstrate numerically that a technique involving two-phase imaging of a sample with two different sized tips should allow to obtain the image the pLDOS. For this imaging method, numerical comparison with extinction probe measurement shows crucial qualitative and quantitative improvement.

Optical far-field super-resolution microscopy using nitrogen vacancy center ensemble in bulk diamond

We demonstrate optical far-field super-resolution microscopy using an array of nitrogen vacancy centers in bulk diamond as near-field optical probes. The local optical field, which transmits through the nanostructures on the diamond surface, is measured by detecting the charge state conversion of the nitrogen vacancy center. Locating the nitrogen vacancy center with a spatial resolution of 6.1 nm is realized with charge state depletion nanoscopy. The nanostructures on the surface of a diamond are then imaged with a resolution below the optical diffraction limit. The results offer an approach to build a general-purpose optical super-resolution microscopy technique and a convenient platform for high spatial resolution quantum sensing with nitrogen vacancy centers.

Phase transitions in hybrid SFS structures with thin superconducting layers

Features of a phase transition between 0 and π states in superconductor/ferromagnet/superconductor (SFS) Josephson structures with thin superconducting layers and a ferromagnetic barrier are studied experimentally and theoretically. The dependence of the critical temperature T c of a transition of the hybrid structure to a superconducting state on the thickness of superconducting layers d s is analyzed by a local method involving measurements of the nonlinear microwave response of the system by a near-field probe. An anomalous increase in the measured temperature T c at the reduction of the thickness d s is detected and is attributed to the 0-π transition.

Near-Field IR Imaging and Spectroscopy of Interfaces and Interphases in Li-Ion Electrodes

Most Li-ion battery chemistries involve the creation or transformation of materials under non-equilibrium environments. Because these chemical energy storage systems operate beyond the thermodynamic stability limits of the electrolytes, reactions between electrodes and electrolyte result in the formation of new phases and interphases, which hinder further electrolyte decomposition and assure battery operation over the lifespan of the application. The basic physico-chemical properties and long-term stability of these surface layers known as the solid electrolyte interphase (SEI) determine the performance and durability of the entire system [1,2]. However, our understanding of how and why some materials function, and, equally importantly, why some materials with many ideal characteristics actually fail, is mainly rudimentary and empirical. The requirements for long-term stability of Li-ion batteries are extremely stringent and necessitate control of the chemistry at a wide variety of temporal and structural length scales. However, the spatial resolution of standard optical spectroscopic and imaging techniques is limited by diffraction to light beam sizes on the order of the wavelength of the source i.e., ∼10 μm for IR and∼500 nm in the visible range. The advent of near-field optical methods during the past decades has led to the development of new advanced techniques for chemical analysis. This presentation provides an overview of novel imaging and spectroscopic optical methodologies, which exploit micro and nano-manipulation techniques and single crystal model electrodes to provide sufficient sample definition suitable for advanced multifunctional far- and near-field optical scanning probes and electrochemical characterization techniques. The SEI chemistry is known to be highly heterogeneous at the nanoscale, and although many compounds have been suggested as contributing, their distribution, relative concentration and functionality remain poorly understood [2]. To address this challenge we used the apertureless near-field scanning optical microscopy (aNSOM) (3-9), which combines the high resolution of AFM with the chemical specificity of IR spectroscopy, to resolve the SEI chemistry at high (∼20 nm) spatial resolution. Examples of detailed in situ molecular characterization of graphite Sn and Si electrodes surface and bulk processes at the nano-level scale exceeding the diffraction limit will be discussed [7-9]. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, under contract no. DE-AC02-05CH11231.

Near-Field Imaging of Cell Membranes in Liquid Enabled by Active Scanning Probe Mechanical Resonance Control

Despite the power of far-field super-resolution microscopies for three-dimensional imaging of biomolecular structures and processes, its application is challenged in dense and crowded samples and for certain surface and membrane studies. Although near-field imaging with its ability to provide intrinsic subdiffraction limited spatial resolution at any optical modality, its application to biological systems has remained limited because of the difficulties of routine operation in liquid environments. Here we demonstrate stable and sensitive near-field scanning optical microscopy (NSOM) in a liquid based on a new mechanical resonance control and an optimization of the tip length, achieving a high quality factor (>2800) force sensing of the near-field probe. Through near-field imaging of the spatial distribution of epidermal growth factor receptors (EGFRs) on the membrane of A431 cancer cells as an example, we reveal nanoscale correlations between surface EGFR and intracellular organelle structures with ∼50 nm...

Directionally enhanced probe for side-illumination Tip enhanced spectroscopy

We demonstrate a high-performance apertureless near-field probe made of a tapered metal tip with a set of periodic shallow grooves near the apex. The spontaneous emission from a single emitter near the tip is investigated systematically for the side-illumination tip enhanced spectroscopy (TES). In contrast with the bare tapered metal tip in conventional side-illumination TES, the corrugated probe not only enhances strongly local excitation field but also concentrates the emission directivity, which leads to high collection efficiency and signal-to-noise ratio. In particular, we propose an asymmetric TES tip based on two coupling nanorods with different length at the apex to realize unidirectional enhanced emission rate from a single emitter. Interestingly, we find that the radiation pattern is sensitive to the emission wavelength and the emitter positions respective to the apex, which can result in an increase of signal-to-noise ratio by suppressing undesired signal. The proposed asymmetrical corrugated probe opens up a broad range of practical applications, e.g. increasing the detection efficiency of tip enhanced spectroscopy at the single-molecule level.

Quartz tuning fork based microwave impedance microscopy.

Microwave impedance microscopy (MIM), a near-field microwave scanning probe technique, has become a powerful tool to characterize local electrical responses in solid state samples. We present the design of a new type of MIM sensor based on quartz tuning fork and electrochemically etched thin metal wires. Due to a higher aspect ratio tip and integration with tuning fork, such design achieves comparable MIM performance and enables easy self-sensing topography feedback in situations where the conventional optical feedback mechanism is not available, thus is complementary to microfabricated shielded stripline-type probes. The new design also enables stable differential mode MIM detection and multiple-frequency MIM measurements with a single sensor.

Near-field probing of Bloch surface waves in a dielectric multilayer using photonic force microscopy

The potential of photonic force microscopy (PFM) for probing the optical near-field in the vicinity of a dielectric multilayer is demonstrated. An experimental study of Bloch surface waves (BSWs) using PFM is described in detail. The applied technique is based on measuring the BSW-induced gradient force acting on a probe particle combined with precise control of the distance between the particle and the multilayer surface. The BSW-induced potential profile measured using PFM is presented. The force interaction between the probe and the BSW evanescent field is numerically studied. The results indicate that a polystyrene particle with a diameter of 1 μm does not significantly perturb the BSW field and can be used to probe the optical near-field intensity in an elegant, noninvasive manner.

Photoconductive Terahertz Near-Field Detectors for Operation With 1550-nm Pulsed Fiber Lasers

We present a new generation of photoconductive (PC) terahertz (THz) near-field probes based on freestanding Beryllium-doped low-temperature-grown InGaAs/InAlAs cantilevers. The PC probes are compatible to optical gating with a femtosecond laser having a center wavelength of 1550 nm. Therefore, they are well suited for a cost-efficient direct integration with fiber-coupled THz time-domain spectroscopy (TDS) systems. The photoconductor material features electron lifetimes of 500 fs allowing for broadband detection up to 2 THz, which is comparable to existing LT-GaAs-based near-field detectors, which require 800-nm wavelength excitation. We demonstrate the detector operation in a state-of-the-art fiber-based TDS system with fast and coherent data acquisition and show obtained sheet-resistance mappings of conductive thin-films featuring subwavelength resolution.

Near-Field Spectroscopy and Imaging of Subwavelength Plasmonic Terahertz Resonators

We present the temporal evolution of the terahertz (THz) field leading to the excitation of plasmonic resonances in carbon microfibers. The field evolution is mapped in space and time for the 3/2 wavelength resonance using a subwavelength aperture THz near-field probe with an embedded THz photoconductive detector. The excitation of surface waves at the fiber tips leads to the formation of a standing wave along the fiber. Local THz time-domain spectroscopy at one of the standing wave crests shows a clear third-order resonance peak at 1.65 THz, well described by the Lorentz model. This application of the subwavelength aperture THz near-field microscopy for mode mapping and local spectroscopy demonstrates the potential of near-field methods for studies of subwavelength plasmonic THz resonators.

Signal of microstrip scanning near-field optical microscope in far- and near-field zones.

An analytical model of interference between an electromagnetic field of fundamental quasi-TM(EH)00-mode and an electromagnetic field of background radiation at the apex of a near-field probe based on an optical plasmon microstrip line (microstrip probe) has been proposed. The condition of the occurrence of electromagnetic energy reverse flux at the apex of the microstrip probe was obtained. It has been shown that the nature of the interference depends on the length of the probe. Numerical simulation of the sample scanning process was conducted in illumination-reflection and illumination-collection modes. Results of numerical simulation have shown that interference affects the scanning signal in both modes. However, in illumination-collection mode (pure near-field mode), the signal shape and its polarity are practically insensible to probe length change; only signal amplitude (contrast) is slightly changed. However, changing the probe length strongly affects the signal amplitude and shape in the illumination-reflection mode (the signal formed in the far-field zone). Thus, we can conclude that even small background radiation can significantly influence the signal in the far-field zone and has practically no influence on a pure near-field signal.

Microsphere-based cantilevers for polarization-resolved and femtosecond SNOM

We present a cantilever-based near-field probe with integrated Mie scattering dielectric SiO2 microsphere (MSDM) for near-field optical imaging as well as femtosecond spectroscopy applications. In contrast to the state-of-the-art transmissive near-field probes, the MSDM reveals a transmission of almost unity known from far-field microscopy configuration. For proper handling, the microsphere is integrated at the apex of a conventional pyramidal aperture tip carried by an atomic force microscopy cantilever. It proved to be mechanically robust during the scanning process even if operating it in the contact mode. The spherical symmetry provides on the one hand a well-defined mechanical contact point with the sample irrespective of its inclination angle to the sample surface. On the other hand, the symmetry of the device preserves the polarization of light proving to be useful for the investigation of the polarization dependent behavior of plasmonic nanostructures. The high transmission combined with low dispersion renders spectroscopic investigations on the femtosecond timescale with a moderate lateral resolution. Second order autocorrelation experiments on a BBO crystal reveals a time resolution well below \(100\, {\hbox{fs at }} 191\, {\hbox{nm}}\) spatial resolution.

Microwave impedance of a tunnel junction in the theory of a near-field microscope with atomic resolution

A theory is constructed for a near-field microwave microscope operating under tunneling breakdown between a probe and a conducting sample. Its informative characteristics are determined by the probe impedance, which is formed from the capacitive impedance Z p of the near-field probe–sample interaction and tunnel junction impedance Z t. A technique whereby the impedance Z p is calculated using coaxial geometry of the probe is developed. Some properties of the impedance Z t defined via the developed theory and published experimental data are investigated.

Guiding, bending, and splitting of coupled defect surface modes in a surface-wave photonic crystal

We experimentally demonstrate a type of waveguiding mechanism for coupled surface-wave defect modes in a surface-wave photonic crystal. Unlike conventional spoof surface plasmon waveguides, waveguiding of coupled surface-wave defect modes is achieved through weak coupling between tightly localized defect cavities in an otherwise gapped surface-wave photonic crystal, as a classical wave analogue of tight-binding electronic wavefunctions in solid state lattices. Wave patterns associated with the high transmission of coupled defect surface modes are directly mapped with a near-field microwave scanning probe for various structures including a straight waveguide, a sharp corner, and a T-shaped splitter. These results may find use in the design of integrated surface-wave devices with suppressed crosstalk.

Plasmonic gratings with nano-protrusions made by glancing angle deposition for single-molecule super-resolution imaging

Super-resolution imaging has been advantageous in studying biological and chemical systems, but the required equipment and platforms are expensive and unable to observe single-molecules at the high (μM) fluorophore concentrations required to study protein interaction and enzymatic activity. Here, a plasmonic platform was designed that utilized an inexpensively fabricated plasmonic grating in combination with a scalable glancing angle deposition (GLAD) technique using physical vapor deposition. The GLAD creates an abundance of plasmonic nano-protrusion probes that combine the surface plasmon resonance (SPR) from the periodic gratings with the localized SPR of these nano-protrusions. The resulting platform enables simultaneous imaging of a large area without point-by-point scanning or bulk averaging for the detection of single Cyanine-5 molecules in dye concentrations ranging from 50 pM to 10 μM using epifluorescence microscopy. Combining the near-field plasmonic nano-protrusion probes and super-resolution technique using localization microscopy, we demonstrate the ability to resolve grain sizes down to 65 nm. This plasmonic GLAD grating is a cost-effective super-resolution imaging substrate with potential applications in high-speed biomedical imaging over a wide range of fluorescent concentrations.

Novel Sensor Model Calibration Method for Resistively Loaded Diode Detectors

Miniaturized radio-frequency electromagnetic near-field broadband probes based on resistively loaded diode detectors have a limited linear dynamic range of $\approx$ 30 dB. Generic linearization schemes have been proposed and applied for continuous wave like or pulse-modulated signals to extend the dynamic range to over 50 dB. Modulation specific linearization has been proposed to also enable precise measurements of current wireless systems with complex modulation schemes. However, calibrations that require an experimental amplitude sweep for each signal have become impractical due to the growing number of wireless communication systems. In this paper, a novel sensor model calibration method has been developed and validated. It is based on calibration of the optimized sensor equivalent circuit model that was derived from the sensor response as a function of bandwidth, duty cycle, modulation schemes, data rate, and statistical distribution. It is shown that all the elements of the equivalent circuit model can be sufficiently accurate determined by the dynamic response to a set of ten generic signals. This model is then used to numerically determine the linearization parameters for any digitized communication signal. The method was tested on various probes for over 200 modulations, resulting in a linearity uncertainty of less than $k$ = 2) for a dynamic range of >50 dB. The proposed method will improve the precision of measurements, reduce calibration costs, increase the flexibility for application of diode-loaded sensors, and enable the use of real-time information for automated probe linearization during or after measurements.

Study of light emission and collection in a transparent dielectric cantilever-based near-field optical probe.

We report the design of a new type of scanning near-field optical microscopy probes combining the advantages of both tapered optical fibres type and cantilever type commercial scanning near-field optical microscopy probes. The material is an organomineral synthesized by the sol-gel method. This material matches mechanical and optical performances for such a scanning near-field optical microscopy probe fabrication. Numerical calculations were carried out using finite element method in order to study the optical transmission of the probe in emission and collection modes. The influence of the probe geometry on the intensity distribution in the vicinity of the aperture and in the extremity of the cantilever is studied in details.