Field Scanning Optical Microscopy Research Articles

Detection of surface enhanced Raman scattering active hotspot using near field scanning optical microscopy

Hotspots are high-intensity electromagnetic zones that form, for example, at the interstitials of plasmonic nanoaggregates, resulting in a considerable rise in the enhancement factor. However, it is inevitable to achieve specific nanometric geometry as well as a suitable technique to capture the details of hotspots. We report near-field surface-enhanced Raman scattering (SERS) spectroscopy of a well-defined gold nanoaggregate of a few nanoparticles adsorbed with a small number of target analytes. A spectrally and spatially resolved SERS measurement setup using an aperture near-field scanning optical microscope (a-NSOM) facilitated the direct observation of localized electromagnetic (EM) fields at the interstitials through SERS. Correlated optical image and corresponding nanometric geometry were captured through the home-built a-NSOM setup. Near-field SERS spectra were recorded at different sites of interest. It was evident that the interstitial positioned at the center of the tetramer provided the most intense Raman scattering, implying the possibility of a SERS-active hotspot therein. SERS bands of the spectrum of the Raman-active dye Rhodamine 6G recorded at the same hotspot coincided well with those reported so far. It was noteworthy that most of the SERS bands in such scenery got enhanced. Such direct observation with high spatial resolution is indispensable to understanding the origin of localized EM fields at “hotspots” and the EM enhancement factor in the SERS process. A finite-difference time-domain (FDTD) analysis was carried out to validate the results.

Numerical calculation of near-field in vicinity of three-dimensional nano-optical probes using boundary element method

The diffraction limit is one of the main obstacles in the development of microscopes to analyze the morphology and structure of materials. The main idea of near field scanning optical microscopy (NSOM) is to overcome the diffraction limit using sub-wavelength apertures. In this work, the near-field is simulated in the vicinity of three-dimensional nano-optical apertureless probes. For this purpose, the Helmholtz equation is solved using the boundary element method (BEM). The effects of different parameters on the near field generated in the vicinity of the optical probe are studied. These parameters consist of the length and radius of the probe, the size of the aperture, the angle of the tapered tip, and the geometry of the probe tip. The main advantages of the proposed method are the high accuracy, the very short calculation time, and the ability to calculate the near field inside and outside the optical probe without any approximation.

Microring Zone Structure for Near-Field Probes

Recent advances in Surface Plasmon Resonance (SPR) technologies have shown the possibility of transmission enhancement of localized modes propagating through sub-diffraction wide slits and apertures, resulting in the strong near-field focusing of metallic planar nanostructures. This work presents a new approach to the fabrication of high-resolution near-field optical probes using 3D lithography in combination with numerical finite difference time domain (FDTD) simulations. A narrow 500 nm depth of field focus area was observed both by numerical analysis and near field scanning optical microscopy (NSOM) measurements. Further research and optimization are planned in order to achieve subwavelength focal regions and increased signal intensities.

Near Field Differential Interference Contrast Microscopy

Near field scanning optical microscopy exploiting differential interference contrast enhancement is demonstrated. Beam splitting in the near field region is implemented using a dual color probe based on plasmonic color sorter idea. This provides the ability to illuminate two neighboring points on the sample simultaneously. It is shown that by modulating the two wavelengths employed in exciting such a probe, phase difference information can be retrieved through measuring the near field photoinduced force at the difference of the two modulation frequencies. This difference in frequency is engineered to correspond to the first resonant frequency of the cantilever, resulting in improved SNR, and sensitivity. The effect of both topographical and material changes in the proposed near field differential interference (NFDIC) technique are investigated for CNT and silica samples. This method is a promising technique for high contrast and high spatial resolution microscopy.

Surface plasmon coupled nano-probe for near field scanning optical microscopy.

Near-field scanning optical microscopy (NSOM) is a powerful tool for study of the nanoscale information of objects by measuring their near-field electric field distributions. The near-field probe, which determines NSOM system performance, can be either a scattering-type or an aperture-type. Both types have strengths and weaknesses. Here we propose and study a surface plasmon-coupled type nano-probe, which works as a hybrid scheme and could potentially combine the advantages of the two NSOM probe types. The key element of the proposed probe is a nanoparticle-on-film structure designed on a tapered fiber tip. On the one hand, the probe can yield the signals scattered in the near field by a nanoparticle with a scattering mechanism; on the other hand, the scattered signals can be transmitted by the metal film and coupled into the fiber via surface plasmon coupled emission, thus providing a collection mode similar to an aperture-type NSOM. This will lead to signal enhancement, while greatly suppressing background noise. This surface plasmon-coupled nano-probe thus has great potential for near-field optical microscopy applications.

Split ring resonator as a nanoscale optical transducer for heat-assisted magnetic recording.

Split ring resonators (SRR) are optical nanostructures that have received a lot of attention for their ability to support magnetic resonance and for their potential use as materials with negative dielectric constant. In this work, we design SRRs as near-field transducers (NFT) for generating a nanoscale hotspot in heat-assisted magnetic recording (HAMR), which is considered a candidate for the next-generation data storage technology. The underlying mechanisms for the generation of hotspot and the dependence on wavelength and geometry of the SRR structure are studied. Optical and thermal performance of SRRs functioning as NFTs in a HAMR device are evaluated. These structures were fabricated using focused ion beam milling. The focusing capability of the SRR is experimentally demonstrated using a scattering near field scanning optical microscope.

Differential reflectance contrast technique in near field limit: Application to graphene

We report on differential reflectance contrast (DRC) sub-microscopic images measured of graphene layers exfoliated on a SiO2/Si substrate by using a near field scanning optical microscope (NSOM) with a spatial resolution of 40 nm. In general, high-quality mechanically exfoliated graphene flakes have sizes of some micrometers and exhibit a distribution of different thicknesses; thus an approach to characterize the topography of the flakes in the sub-micrometric regime is fundamental. DRC in the near field limit is a very useful technique to characterize the flakes in the sub-microscopic scale. The DRC signal is obtained by taking the numerical difference between the reflectivity coming from a region with no graphene (substrate) and a region containing a graphene layer. It is shown by a multiple reflection model (graphene/SiO2/Si) and spectroscopic ellipsometry measurements that the optical contrast in such system can be modulated by changing the thickness of the SiO2 layer or/and the wavelength of the incident light. The results open the possibility to use this optical technique for the thicknesses characterization in the sub-micrometer scale of 2D materials.

On-chip beam positioning sensor via frequency locked cascaded ring resonators

We demonstrate an approach for on-chip beam positioning with a position accuracy of up to 100 nm. This approach is based on tracking the resonance of two adjacent microring resonators that are implemented on a silicon on insulator chip. We demonstrate the functionality of our approach by illuminating the chip through a Near Field Scanning Optical Microscope tip and monitoring the shift of the microring resonances due to the thermo-optic effect. We also discuss the contribution of different effects such as free carrier absorption and dispersion to the resonance shift.

Plasmon augmented two photon absorption in a strongly coupled nano-molecular hybrid

A long-standing goal in optical sciences has been to strengthen nonlinear optical effects at lower light powers or pulse energies. In this perspective, strongly-coupled hybrids which offer significant third order optical nonlinearities at low powers are currently a topic of global research. Herein, we report a detailed analysis of third order nonlinear photonic characteristics of the fabricated nanohybrid system. The far field and near field optical coupling between the constituents of the hybrid were investigated using Near field scanning optical microscopy and Raman spectroscopy respectively. While the optical nonlinearity (i.e. two photon absorption in this case) has been elucidated using Z-scan spectroscopy. The experimental outcomes reveal in the linear as well as the nonlinear regime the dye and the Au nanoislands represent a strongly interacting system. A 400% enhancement in the two photon absorption coefficients of an organic dye Eosin Yellow is observed when hybridized with Au nanoislands. The results are explained in terms of the energy transfer from the dye to the hybridized Au nanoislands film. The investigations of such mechanisms, may bring forth exciting advances, not only in terms of strongly interacting photons, but also shed light on the impact of energy transfer mechanisms on third order nonlinearity of nano-molecular hybrids.

Design and Simulation of a New Coaxial Probe for Near Field Scanning Optical Microscopy

Design of the new photonic devices having attractive optical properties such as subwavelength confinement of light has a numerous applications in developing nanoscale sensors, photonic integrated circuits, solar cells and etc. Abbe’s diffraction limit restricts the use of subwavelength scale photonic elements due to their size and therefore new approaches are required in order to achieve desirable nanometer scale properties of these devices. Delivering light energy to nanometer scale regions can be achieved by surface plasmon polariton (SPP) waves that propagate in metal-dielectric interface. The paper presents results in FDTD (finite-difference time domain) simulation of a new coaxial probe of nanometer-scale size.Based on FDTD simulations of the coaxial probe field localization and enhancement in the region around the tip of coaxial probe has been obtained. Configuration of the probe consists of the truncated Au cone as a core, truncated dielectric (SiO2) cone in the middle and outer metallic cladding. Simulations of the probe revealed that increasing the radius of Au core structure could lead to the enhanced field around the tip. Additionally, increasing radius of outer cladding layer doesn’t affect field enhancement around the tip. Different configurations of the probe have been analyzed in the range of visible spectrum of the source and as a result the most promising configuration has been selected

The light-matter interaction of a single semiconducting AlGaN nanowire and noble metal Au nanoparticles in the sub-diffraction limit.

Near field scanning optical microscopy (NSOM) is not only a tool for imaging of sub-diffraction limited objects but also a prominent characteristic tool for understanding the intrinsic properties of nanostructures. In order to understand light-matter interactions in the near field regime using a NSOM technique with an excitation of 532 nm (2.33 eV), we selected an isolated single semiconducting AlGaN nanowire (NW) of diameter ∼120 nm grown via a vapor liquid solid (VLS) mechanism along with a metallic Au nanoparticle (NP) catalyst. The role of electronic transitions from different native defect related energy states of AlGaN is discussed in understanding the NSOM images for the semiconducting NW. The effect of strong surface plasmon resonance absorption of an excitation laser on the NSOM images for Au NPs, involved in the VLS growth mechanism of NWs, is also observed.

Near-field scanning optical microscopy probes with heat sinks for higher power operations

The metal coating of a near field scanning optical microscope (NSOM) probe will melt when a few mW of laser light is introduced into the fiber. For higher power operation a process to coat the tip of the NSOM probe with epoxy without damaging the probe was developed. The tip protrudes no more than a few micron outside the epoxy surface such that the evanescent wave from the tip is still available for applications. The epoxy heat sink dissipates heat away from the NSOM probe keeping the metal coating cooler, which allows the input laser power to be increased to 21.4 mW at a wavelength of 405 nm without damaging the NSOM probe.

Near Field Investigation of a Plasmonic Lüneburg Lens

In this work, we investigate the interaction of surface plasmons with a plasmonic Luneburg lens using near field scanning optical microscopy. Gray-scale electron beam lithography is used to prepare a dome-shaped resist structure on top of a gold film. This particular shape yields the effective refractive index profile of a Luneburg lens for surface plasmons propagating at the film surface at an energy of $\hbar \omega ={1.72}$ eV. Next to the Luneburg lens a grating coupler is milled into the gold film with focused ion beam. The surface plasmons are launched to propagate through the lens and the near field pattern is scanned. We clearly identify a focal spot in the near field signal at the outer perimeter of the lens. In addition, we observe a beating pattern arising from further plasmon waves excited by higher orders of the grating coupler. The emergence of this beating pattern allows the detection of the plasmon’s wave fronts. An analytical model was used to retrieve the properties of the participating wave components. The measured near-field pattern could be modeled very well with electromagnetic simulations applying the effective refractive index approach.

20 Years of Corrosion Sensing and Microvisualization of Corrosion Processes

Localized corrosion processes continue to be a major cause for degradation failure in damaging fluids. Initiation stages of response remain difficult to characterize, arising in part because of the difficulty in being able to predict the location and onset of the attack. Propagation stages are seldom constant but change with time due to competing processes of dissolution and product deposition at the corrosion sites. The progress of the attack changes the local environmental concentrations which are especially important at buried interfaces that are isolated from bulk conditions. The present paper will describe some of the recent advances in optical, electrochemical, and microscopy that are directed at providing local information on “precursor sites” and vulnerable areas on metal and covering oxide surfaces. Previous studies of coupled reactions on reactive surfaces provide a general basis for the work to be described here. The objectives of the experimental techniques are: (1) to measure the local corrosion rate; (2) to locate breakdown sites, and; (3) to study the fundamental properties of passive films and how they are related to the stability of the films and damaging environments. The latter is of particular interest since it will allow one to predict why a particular precursor site is more vulnerable to breakdown as compared to neighboring areas. In locating precursor sites, entire regions of the corroding surface can be imaged concurrently or point-by-point using scanned probe or beam. Both classes will be discussed with their capabilities. The techniques used in our laboratory are: (1) the Quartz Crystal Microbalance (QCM); (2) Optical Fiber Micromirrors (FOP); (3) Confocal Laser Scanning Microscopy (CLSM); (4) Photoelectrochemical Microscopy (PEM); (5) Scanning Electrochemical Microscopy (SECM); (6) Scanning Photoelectrochemical and Electrochemical Microscopy (SPECM); (7) Near Field Scanning Optical and Spectroscopic Microscopy (NSOSM). The experimental setup for each and the capability help to reveal the advantages that are offered by them. Optical sensing by the QCM and optical fiber micromirrors have been used to measure corrosion rates, with sensitivity of fractions of an atomic layer. Similar to them but with the capability to image a larger reactive surface is the CLSM to determine local attack rates at various surface sites. The NSOSM is valuable to obtain both vertical and lateral super resolution images of reacting surfaces of about 60 nanometers. The techniques support the development of miniature corrosion sensors devoted to monitoring reactive surfaces in remote and occluded regions, coupled with wireless transmission.

Liquid crystal quenched orientational disorder at an AFM-scribed alignment surface.

A polyimide substrate was scribed using the stylus of an atomic force microscope, then covered with a nematic liquid crystal. The fiber from a near field scanning optical microscope was immersed into the liquid crystal and rastered approximately 80 nm above the surface, thereby obviating smearing effects that occur in thicker samples. By appropriate averaging of multiple data sets, a histogram of the "frozen-in" director deviation Δφ from the average easy axis was obtained, having a full-width-half-maximum of ∼0.02 rad. Additionally, the spatial autocorrelation function of Δφ was extracted, where the primary correlation length was found to be comparable to, but larger than, the liquid crystal's extrapolation length. A secondary characteristic length scale of a few μm was observed, and is thought to be an artifact due to material ejection during the scribing process. Our results demonstrate the utility of nanoscale imaging of the interface behavior inside the liquid crystal.

Independently analyzing different surface plasmon polariton modes on silver nanowire.

In this paper, surface plasmon polariton (SPP) modes on silver nanowire (AgNW), with different field symmetric, are studied by different near field methods, respectively. In the experiment, the excitation and detection of SPPs are performed by two probes of near field scanning optical microscope (NSOM) simultaneously, which realizes the study of SPPs in complete near field. By controlling the experimental conditions, two of the fundamental SPP modes are detected separately and their intensity distributions on AgNW are given by the NSOM images. In the discussion, creeping wave (CW) is introduced to analyze the experimental results, which improves the coincidence of the experimental results and the theoretical calculations. A detailed characterization of SPPs modes in near field, which gives a further insight into optical properties of AgNW, will benefit integrated optical circuits.

Examination of the Electrolytic Polishing Metal Probe for the Cell Trap by an Electric Field

Generally, the metal probe for NSOM (Near field scanning optical microscopy) or STM (Scanning Tunneling Microscope) was made with gold or tungsten. However, they were not suitable for the cell trap in our research for the reasons of cost, hardness, etc. In our research, these problems were solved by choosing brass as a material of a probe. Since the probe production by electrolytic polishing can change the shape of the top, tip angle, and taper length etc, we can propose a probe suitable for a cell trap. Therefore, in this examination, we propose the brass probe by electrolytic polishing with low cost and sufficient hardness for cell trap.

Transient temperature response of near field scanning optical microscope probes under pulsed illumination

Time resolved thermoreflectance imaging with a nanosecond laser is applied to study the transient thermal response of near field scanning optical microscope (NSOM) probes driven by either microsecond (μs) or nanosecond (ns) laser pulses. It is found that when the laser energies are the same, μs and ns laser pulses show similar trend of close to adiabatic heating during the laser pulse and close to conductive cooling after the laser pulse. The peak temperature is almost linear with respect to input laser energy for the energy selected. The peak temperature of the NSOM probe is increased when the aperture size of the probe is smaller and when the incident laser wavelength is shorter. However, compared with μs laser pulses, small physical damages appear around the NSOM aperture, which gradually decreases the optical-thermal energy conversion efficiency in the NSOM probe when ns laser pulses are applied.

NSOM Lithography for Organized Growth of Gap Nanowires

In this contribution, near field scanning optical microscope (NSOM) lithography is presented as a tool for organized growth of nanowires. Non contact mode of NSOM lithography was used to pattern planar structures in photoresist deposited on GaP substrate. In combination with lift-off technique, metal-catalyst particles on GaP substrate for subsequent growth of GaP nanowires were prepared. Different periodic and predefined arrangements of GaP nanowires were achieved.

Optical and electrical mappings of surface plasmon cavity modes

AbstractPlasmonics is a rapidly expanding field, founded in physics but now with a growing number of applications in biology (biosensing), nanophotonics, photovoltaics, optical engineering and advanced information technology. Appearing as charge density oscillations along a metal surface, excited by electromagnetic radiation (e.g., light), plasmons can propagate as surface plasmon polaritons, or can be confined as standing waves along an appropriately-prepared surface. Here, we review the latter manifestation, both their origins and the manners in which they are detected, the latter dominated by near field scanning optical microscopy (NSOM/SNOM). We include discussion of the “plasmonic halo” effect recently observed by the authors, wherein cavity-confined plasmons are able to modulate optical transmission through step-gap nanostructures, yielding a novel form of color (wavelength) selection.