Aperture Scanning Near-field Optical Microscope Research Articles

Scanning near-field optical microscopy measurements and simulations of regularly arranged silver nanoparticles

Silver nanoparticles on a glass substrate are experimentally investigated by aperture scanning near-field optical microscopy (a-SNOM). To understand the experimental results, finite-element-method simulations are performed building a theoretical model of the a-SNOM geometry. We systematically vary parameters like aperture size, aluminum-coating thickness, tip cone angle, and tip-surface distance and discuss their influence on the near-field enhancement. All these investigations are performed comparatively for constant-height and constant-gap scanning modes. In the end, we establish a reliable and stable optical model for simulating a-SNOM measurements, which is capable of reproducing trends observed in experimental data.

Application of a quantum cascade laser aperture scanning near-field optical microscope to the study of a cancer cell.

This work reports the first images obtained by combining an infrared aperture scanning near-field optical microscope (SNOM) with a quantum cascade laser (QCL). The future potential of this set-up is demonstrated by a preliminary study on an OE33 human oesophageal adenocarcinoma cell in which the cell is imaged at 1751 cm-1, 1651 cm-1, 1539 cm-1 and 1242 cm-1. In addition to the 1651 cm-1 image, three other images were acquired within the Amide I band (1689 cm-1, 1675 cm-1 and 1626 cm-1) chosen to correspond to secondary structures of proteins. The four images obtained within the Amide I band show distinct differences demonstrating the potential of this approach to reveal subtle changes in the chemical composition of a cell.

Near-field surface plasmon field enhancement induced by rippled surfaces.

The occurrence of plasmon resonances on metallic nanometer-scale structures is an intrinsically nanoscale phenomenon, given that the two resonance conditions (i.e., negative dielectric permittivity and large free-space wavelength in comparison with system dimensions) are realized at the same time on the nanoscale. Resonances on surface metallic nanostructures are often experimentally found by probing the structures under investigation with radiation of various frequencies following a trial-and-error method. A general technique for the tuning of these resonances is highly desirable. In this paper we address the issue of the role of local surface patterns in the tuning of these resonances as a function of wavelength and electric field polarization. The effect of nanoscale roughness on the surface plasmon polaritons of randomly patterned gold films is numerically investigated. The field enhancement and relation to specific roughness patterns is analyzed, producing many different realizations of rippled surfaces. We demonstrate that irregular patterns act as metal–dielectric–metal local nanogaps (cavities) for the resonant plasmonic field. In turn, the numerical results are compared to experimental data obtained via aperture scanning near-field optical microscopy.

Near-Field Raman Spectroscopy with Aperture Tips.

In this paper, we review nano-Raman techniques based on aperture scanning near-field optical microscopy (SNOM). Fundamentals of SNOM and aperture-tip-based near-field Raman spectroscopy and their applications in key technical issues, including Raman signal intensity and collection time, are introduced. Recent advances in the tip design are discussed, and applications of the aperture-SNOM-based nano-Raman technique are presented. We attempt to identify the most pressing open questions in this field. We believe that, by improving the power transmission efficiency and combining the local field enhancing technique with the tip-enhanced spectroscopy, the performance of aperture-SNOM can be significantly improved. Its nanometer-scale excitation volume and the consequent low background make the aperture-tip technique feasible for many important samples that cannot be measured by other optical nanospectroscopies.

Magnetic domain structure investigation of Bi: YIG-thin films by combination of AFM and cantilever-based aperture SNOM

We present the results of magnetic domain structure investigation by combination of atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). Special hollow-pyramid AFM cantilevers with aperture was used. This combination allows us use same probe for both topography and domain structure visualization of Bi -substituted ferrite garnet films of micro- and nano-meter thickness. Samples were excited through aperture by tightly focused linearly polarized laser beam. Magneto-optical effect rotates polarization of transmitted light depend on domain orientation. Visualization of magnetic domains was performed by detecting cross polarized component of transmitted light. SNOM allows to obtain high resolution magnetic domain image and prevent sample from any disturbance by magnetic probe. Same area SNOM and MFM images are presented.

Lateral Magnetic Near‐Field Imaging of Plasmonic Nanoantennas With Increasing Complexity

The design of many promising, newly emerging classes of photonic metamaterials and subwavelength confinement structures requires detailed knowledge and understanding of the electromagnetic near-field interactions between their building blocks. While the electric field distributions and, respectively, the electric interactions of different nanostructures can be routinely measured, for example, by scattering near-field microscopy, only recently experimental methods for imaging the magnetic field distributions became available. In this paper, we provide direct experimental maps of the lateral magnetic near-field distributions of variously shaped plasmonic nanoantennas by using hollow-pyramid aperture scanning near-field optical microscopy (SNOM). We study both simple plasmonic nanoresonators, such as bars, disks, rings and more complex antennas. For the studied structures, the magnetic near-field distributions of the complex resonators have been found to be a superposition of the magnetic near-fields of the individual constituting elements. These experimental results, explained and validated by numerical simulations, open new possibilities for engineering and characterization of complex plasmonic antennas with increased functionality.

Mapping Magnetic Near-Field Distributions of Plasmonic Nanoantennas

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning near-field optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution.

Extraordinary Optical Transmission Brightens Near-Field Fiber Probe

Near-field scanning optical microscopy (NSOM) offers high optical resolution beyond the diffraction limit for various applications in imaging, sensing, and lithography; however, for many applications the very low brightness of NSOM aperture probes is a major constraint. Here, we report a novel NSOM aperture probe that gives a 100× higher throughput and 40× increased damage threshold than conventional near-field aperture probes. These brighter probes facilitate near-field imaging of single molecules with apertures as small as 45 nm in diameter. We achieve this improvement by nanostructuring the probe and by employing a novel variant of extraordinary optical transmission, relying solely on a single aperture and a coupled waveguide. Comprehensive electromagnetic simulations show good agreement with the measured transmission spectra. Due to their significantly increased throughput and damage threshold, these resonant configuration probes provide an important step forward for near-field applications.

Near-field microscopy with a single-photon point-like emitter: Resolution versus the aperture tip?

We discuss theoretically the concept of spatial resolution in near-field scanning optical microscopy (NSOM) in light of a recent work [Opt. Express 17 (2009) 19969] which reported on the achievement of active tips made of a single ultrasmall fluorescent nanodiamond grafted onto the apex of a substrate tip and on their validation in NSOM imaging. Since fluorescent nanodiamonds tend to decrease steadily in size, we assimilate a nanodiamond-based tip to a point-like single photon source and compare its ultimate resolution with that offered by standard metal-coated aperture NSOM tips. We demonstrate both classically and quantum mechanically that NSOM based on a point-like tip has a resolving power that is only limited by the scan height over the imaged system whereas the aperture-tip resolution depends critically on both the scan height and aperture diameter. This is a consequence of the complex distribution of the electromagnetic field around the aperture that tends to artificially duplicate the imaged objects. We show that the point-like tip does not suffer from this “squint” and that it rapidly approaches its ultimate resolution in the near-field as soon as its scan height falls below the distance between the two nano-objects to be resolved.

Performance of Scanning Near-Field Optical Microscope Probes with Single Groove and Various Metal Coatings

We investigate the performance of a simple corrugated aperture scanning near-field optical microscope (SNOM) probe with various cladding metals. The probes have only one corrugation, however, they offer increased transmission over both uncorrugated probes and those with many grooves. Enhancement of light throughput results from excitation of surface plasmons at the corrugation at the core–cladding interface. We show how the choice of metal influences radiation properties of grooved probes.

Beam pen lithography

Lithography techniques are currently being developed to fabricate nanoscale components for integrated circuits, medical diagnostics and optoelectronics. In conventional far-field optical lithography, lateral feature resolution is diffraction-limited. Approaches that overcome the diffraction limit have been developed, but these are difficult to implement or they preclude arbitrary pattern formation. Techniques based on near-field scanning optical microscopy can overcome the diffraction limit, but they suffer from inherently low throughput and restricted scan areas. Highly parallel two-dimensional, silicon-based, near-field scanning optical microscopy aperture arrays have been fabricated, but aligning a non-deformable aperture array to a large-area substrate with near-field proximity remains challenging. However, recent advances in lithographies based on scanning probe microscopy have made use of transparent two-dimensional arrays of pyramid-shaped elastomeric tips (or 'pens') for large-area, high-throughput patterning of ink molecules. Here, we report a massively parallel scanning probe microscopy-based approach that can generate arbitrary patterns by passing 400-nm light through nanoscopic apertures at each tip in the array. The technique, termed beam pen lithography, can toggle between near- and far-field distances, allowing both sub-diffraction limit (100 nm) and larger features to be generated.

Numerical simulation of the near-field distribution of light spot of aperture pyramid-type optical probe with a metal tip

The finite-difference time-domain method (FDTD) is employed to investigate the near-field distribution of the light spot of aperture pyramid-type optical probe with a metal tip. The factors that have some influence on the electric field distribution such as the length，radius and position departed from aperture of the tip are investigated. The mechanism of electromagnetic wave transmitting from the aperture to metal tip by way of plasmon is discussed. Our calculation results are consistent with the results obtained by Taminiau with different methods. How and why Pohl etc. have obtained images with high resolution in the 20—25 nm range between 1984 and 1986 are analyzed. The imaging mechanism of their experiment system is discussed. Numerical simulation demonstrates that the imaging mechanism of Pohls experiment system may belong to a scattering-type near-field optical microscope (S-SNOM) rather than an aperture scanning near-field optical microscope (A-SNOM). This paper can help us to find optimized tip design in the future.

Numerical simulation of near-field fluorescence correlation spectroscopy using a fiber probe

Numerical simulation of fluorescence correlation spectroscopy (FCS) based on near-field scanning optical microscopy (NSOM) was performed. A finite-difference time-domain method was employed to calculate the electric field distribution in the vicinity of the NSOM aperture and the Brownian motion of nanoparticles was reproduced by a Monte Carlo simulation. The physical validity of the simulation result was confirmed by comparing it with a theoretical calculation using a Gaussian observation volume, which is applied to FCS based on conventional optical microscopy.

Optical and electrical characterization at the nanoscale with a transparent probe of ascanning tunnelling microscope

A new type of scanning probe microscope, combining features of the scanning tunnellingmicroscope, the scanning tunnelling luminescence microscope with a transparent probe andthe aperture scanning near-field optical microscope, is described. Proof-of-conceptexperiments were performed under ultrahigh vacuum conditions at varying temperature onGaAs/AlAs heterostructures.

Hollow-pyramid based scanning near-field optical microscope coupled to femtosecond pulses: A tool for nonlinear optics at the nanoscale

We describe an aperture scanning near-field optical microscope (SNOM) using cantilevered hollow pyramid probes coupled to femtosecond laser pulses. Such probes, with respect to tapered optical fibers, present higher throughput and laser power damage threshold, as well as greater mechanical robustness. In addition, they preserve pulse duration and polarization in the near field. The instrument can operate in two configurations: illumination mode, in which the SNOM probe is used to excite the nonlinear response in the near field, and collection mode, where it collects the nonlinear emission following far-field excitation. We present application examples highlighting the capability of the system to observe the nonlinear optical response of nanostructured metal surfaces (gold projection patterns and gold nanorods) with sub-100-nm spatial resolution.

Numerical analysis of the spectral response of an NSOM measurement

Near-field Scanning Optical Microscopy (NSOM) is a powerful tool for investigating optical field with resolution greater than the diffraction limit. In this work, we study the spectral response that would be obtained from an aperture NSOM system using numerical calculations. The sample used in this study is a bowtie nanoaperture that has been shown to produce concentrated and enhanced field. The near- and far-field distributions from a bowtie aperture are also calculated and compared with what would be obtainable from a NSOM system. The results demonstrate that it will be very difficult to resolve the true spectral content of the near-field using aperture NSOM. On the other hand, the far-field response may be used as a guide to the near-field spectrum.

Polarization dependent near-field speckle of random gold films

The near-field speckle of random gold nanostructures is experimentally and theoretically investigated. The topography and the intensity distribution, under polarized illumination, are obtained with an aperture scanning near-field optical microscope. The sharpness of the experimental images entitles us to perform a fine statistical analysis based on the autocorrelation function (ACF). The intensity ACF clearly exhibits an anisotropic behavior and subwavelength oscillations, depending on the incident polarization. A comparison with a numerical simulation exhibits the influence of the surface topography on the anisotropic near-field speckle.

Implementation of a bimorph‐based aperture tapping‐SNOM with an incubator to study the evolution of cultured living cells

We present the implementation of a tapping-mode aperture scanning near-field optical microscope (Tapping-SNOM) to a Binder CB incubator (Istituto di Struttura della Materia, Rome, Italy). The microscope operates in the intermittent contact mode using a nonbent optical fibre allowing to reduce the perturbation exerted on the sample, while the incubator maintains a constant temperature, humidity and CO(2) level. This instrument can maintain and analyse in a controlled environment different samples, both organic and nonorganic. In particular, the Tapping-SNOM can study different cell lines at nanometric resolution and in physiological buffer, following the evolution of the living cells almost indefinitely. We will present several examples of the capabilities of the tapping scanning near-field optical microscope in the study of different lines of living cells, showing corresponding topographical, optical or phase-lag images of the live samples, evidencing the excellent stability, versatility and resolution of the system.

Focussed ion beam machined cantilever aperture probes for near‐field optical imaging

Near-field optical probe is the key element of a near-field scanning optical microscopy (NSOM) system. The key innovation in the first two NSOM experiments (Pohl et al., 1984; Lewis et al., 1984) is the fabrications of a sub-wavelength optical aperture at the apex of a sharply pointed transparent probe tip with a thin metal coating. This paper discusses the routine use of focussed ion beam (FIB) to micro-machine NSOM aperture probes from the commercial silicon nitride cantilevered atomic force microscopy probes. Two FIB micro-machining approaches are used to form a nanoaperture of controllable size and shape at the apex of the tip. The FIB side slicing produces a silicon nitride aperture on the flat-end tips with controllable sizes varying from 120 nm to 30 nm. The FIB head-on drilling creates holes on the aluminium-coated tips with sizes down to 50 nm. Nanoapertures in C and bow tie shapes can also be patterned using the FIB head-on milling method to possibly enhance the optical transmission. A transmission-collection NSOM system is constructed from a commercial atomic force microscopy to characterize the optical resolution of FIB-micro-machined aperture tips. The optical resolution of 78 nm is demonstrated by an aperture probe fabricated by FIB head-on drilling. Simultaneous topography imaging can also be realized using the same probe. By mapping the optical near-field from a bow-tie aperture, optical resolution as small as 59 nm is achieved by an aperture probe fabricated by the FIB side slicing method. Overall, high resolution and reliable optical imaging of routinely FIB-micro-machined aperture probes are demonstrated.

Mapping local field enhancements at nanostructured metal surfaces by second‐harmonic generation induced in the near field

We report on an aperture scanning near-field optical microscope in which femtosecond pulses are coupled to a hollow-pyramid aperture sensor. Such probe displays high throughput and preserves pulse duration and polarization, enabling the achievement of sufficiently high peak power in the near field to perform nonlinear optics on the nanoscale. We use the system to observe the nonlinear optical response of nanostructured metal surfaces with sub-100-nm spatial resolution. We study second-harmonic generation from gold nanoparticles both isolated and in high-density patterns, highlighting a strong dependence of the generation efficiency on the shape and on the fine structure of the nanoemitter. In particular, we present results on closely packed gold triangles as well as on nanoellipsoids with different local surface plasmon resonances.