Role of multipole moment of the probe in apertureless near-field optical microscopy

The model of apertureless near-field optical microscope is developed taking into account the multipole moment of probe. In the case of samples with small dielectric absorption, the multipole moments are shown to be responsible for the appearance of additional resonances in the spectrum of scattered signal. The influence of multipole moments is especially pronounced in the near-field microscopy with modulation of tip-sample distance. A good agreement of the theoretical results with experimental data in the case of resonant interaction of the probe and sample is demonstrated.     

Apertureless near-field optical microscopy via local second-harmonic generation

We describe an apertureless scanning near-field optical microscope (SNOM) based on the local second-harmonic generation enhancement resulting from an electromagnetic interaction between a probe tip and a surface. The imaging mechanisms of such apertureless second-harmonic SNOM are numerically studied. The technique allows one to achieve strongly confined sources of second-harmonic light at the probe tip apex and/or surface area under the tip. First experimental realization of this technique has been carried out using a silver-coated fibre tip as a probe. The experiments reveal a strong influence of the tip–surface interaction as well as polarization of the excitation light on images obtained with apertureless second-harmonic SNOM. The technique can be useful for studying the localized electromagnetic excitations on surfaces as well as for visualization of lateral variations of linear and nonlinear optical properties of surfaces.     

Balanced homodyning for apertureless near-field optical imaging

A quadrature optical detection technique, based on polarized balanced-homodyne interferometry, has been developed for specific application to apertureless near-field scanning optical microscopy (ANSOM). With such technique, multiplicative background interference, inficiating quantitative optical imaging in standard homodyne-based ANSOM, can be suppressed. Periodic modulation of interferometric optical phase, typically employed in heterodyne-based ANSOMs even to such purpose, is not needed in the present configuration. Homodyne detection also facilitates detection of harmonic components of the ANSOM optical signal at the probe/sample distance modulation frequency, necessary for near-field discrimination and suppression of artifacts. Furthermore, since amplitude signal is not affected by phase fluctuations generated in the optical path of the interferometer, an optical fiber could be included in one interferometer arm, to couple the ANSOM head to the detection system, obtaining improved versatility of the instrument. A demonstration of the interferometer performance is given by a test confocal optical scan of a mirror surface. This technique, as applied to near-field microscopy, is anticipated to provide absolute values of optical contrast not depending on background interference and topography artifacts.     

Nanoscale optical imaging of chromosomes with apertureless microscopy.

Near-field optical structure of the centromere region of undyed polytene chromosomes has been observed using an apertureless near-field optical microscope that detects the intensity of light scattered from an atomic force microscope tip under laser illumination. The centromere is of primary importance to the functioning of the chromosome in the cell during cell division. It is also particularly interesting for structural/optical studies since its DNA repeat sequences are highly conserved among organisms and it is possible that they play a part in the centromere self assembly (Clark and Wall 1996).     

Apertureless scanning near-field optical microscopy for ion exchange channel waveguide characterization

We report the characterization of an integrated Ag⁺/Na⁺ ion exchange waveguide realized in a silicate glass substrate using apertureless scanning near‐field optical microscopy. Our experimental set‐up is based on the combination of a commercial atomic force microscope with an optical confocal detection system. Thanks to this system, the topography and evanescent optical field at the waveguide top surface are mapped simultaneously. Also, the process of apertureless scanning near‐field optical microscopy image formation is analysed. In particular, fringe patterns appearing in the image reveal the intrinsic interferometric nature of the collected signal, due to interference between the field scattered by the tip end and background fields related to guide losses. We give a quantitative interpretation of these fringes. Evanescent intensity mapping on the sample surface allowed us to extract physical waveguide parameters. In particular, it shows an unambiguous multimode beat along the waveguide propagation axis. Furthermore, we show that analysis of this intensity profile reveals back‐reflection effects from the waveguide exit facet. The resulting standing waves pattern allows us to evaluate the eigenmode propagation constants.     

Near-field reflection backscattering apertureless optical microscopy: application to spectroscopy experiments on opaque samples, comparison between lock-in and digital photon counting detection techniques

An apertureless scanning near-field optical microscope (ASNOM) in reflection backscattering configuration is designed to conduct spectroscopic experiments on opaque samples constituted of latex beads. The ASNOM proposed takes advantage of the depth-discrimination properties of confocal microscopes to efficiently extract the near-field optical signal. Given their importance in a spectroscopic experiment, we systematically compare the lock-in and synchronous photon counting detection methods. Some results of Rayleigh's scattering in the near field of the test samples are used to illustrate the possibilities of this technique for reflection backscattering spectroscopy.     

Analysis of the measured signals in apertureless near-field optical microscopy

We present an analytical model able to explain the optical signal recorded during our experimental approach curves in the infrared at a wavelength lambda=10.6 microm, with a home-made apertureless near-field scanning optical microscope ANSOM. This model uses classical electrodynamics to calculate the scattering cross section of the oscillating tip, considered as a dipole, and its dielectric image in the sample as a function of the tip-sample separation from the near-field to the far-field regime. The dipoles are placed in a non-uniform electric field because of the standing wave arising from the interference between the incident and the specular laser beams. We also added a background field coming from a scatterer on the surface in order to account for zeroing of the optical signal for particular tip-sample separation and interference patterns.     

Apertureless near-field scanning Raman microscopy using reflection scattering geometry

The combination of near-field scanning optical microscopy and Raman spectroscopy provides chemical/structural specific information with nanometer spatial resolution, which are critically important for a wide range of applications, including the study of Si devices, nanodevices, quantum dots, single molecules of biological samples. In this paper, we describe our near-field Raman study using apertureless probes. Our system has two important features, critical to practical applications. (1) The near-field Raman enhancement was achieved by Ag coating of the metal probes, without any preparation of the sample, and (2) while all other apertureless near-field Raman systems were constructed in transmission mode, our system works in the reflection mode, making near-field Raman study a reality for any samples. We have obtained the first 1D Raman mapping of a real Si device with 1s exposure time. This is a very significant development in near-field scanning Raman microscopy as it is the first demonstration that this technique can be used for imaging purpose because of the short integration time. In addition, the metal tips used in our set-up can be utilized to make simultaneous AFM and electrical mappings such as resistance and capacitance that are critical parameters for device applications.     

A single gold particle as a probe for apertureless scanning near-field optical microscopy

We report on the fabrication, characterization and application of a probe consisting of a single gold nanoparticle for apertureless scanning near-field optical microscopy. Particles with diameters of 100 nm have been successfully and reproducibly mounted at the end of sharp glass fibre tips. We present the first optical images taken with such a probe. We have also recorded plasmon resonances of gold particles and discuss schemes for exploiting the wavelength dependence of their scattering cross-section for a novel form of apertureless scanning near-field optical microscopy.     

A novel fabrication method for fluorescence-based apertureless scanning near-field optical microscope probes

We report a novel method for the fabrication of probes with localized sub-wavelength fluorescing media at their extremities. We present our first results and discuss future plans to extend this technique to the systematic fabrication of fluorescent probes for apertureless scanning near-field optical microscopy.     

A novel fabrication method for fluorescence-based apertureless scanning near-field optical microscope probes

We report a novel method for the fabrication of probes with localized sub-wavelength fluorescing media at their extremities. We present our first results and discuss future plans to extend this technique to the systematic fabrication of fluorescent probes for apertureless scanning near-field optical microscopy.     

An evanescent-field optical microscope

The classic diffraction limit of resolution in optical microscopy (～γ/2) can be overcome by detecting the diffracted field of a submicrometre-size probe in its near field. The present stage of this so-called scanning near-field optical microscopy (SNOM) is reviewed. An evanescent-field optical microscope (EFOM) is presented in which the near-field regime is provided by the exponentially decaying evanescent field caused by total internal reflection at a refractive-index transition. A sample placed in this field causes a spatial variation of the evanescent field which is characteristic for the dielectric and topographic properties of the sample. The evanescent field is frustrated by a dielectric probe and thus converted into a radiative field. In our case the probe consists either of an etched optical fibre or of a highly sharpened diamond tip. The probe is scanned over the sample surface with nanometre precision using a piezo-electric positioner. The distance between probe and sample is controlled by a feedback on the detected optical signal. The resolution of the microscope is determined by both the gradient of the evanescent field and the sharpness of the tip. Details of the experimental set-up are discussed. The coupling of the evanescent field to the submicrometre probe as a function of probe-sample distance, angle of incidence and polarization has been characterized quantitatively. The observed coupling is generally in agreement with presented theoretical calculations. Microscopy has been performed on a regular latex sphere structure, which clearly demonstrates the capacity of the evanescent-field optical microscope for nanometre-scale optical imaging. Resolution is typically 100 nm laterally and 10 nm vertically. The technique is promising for biological applications, especially if combined with optical spectroscopy.     

Evanescent field excitation and measurement of dye fluorescence in a metallic probe near-field scanning optical microscope

We introduce a method of dye fluorescence excitation and measurement that utilizes a near-field scanning optical microscope (NSOM). This NSOM uses an apertureless metallic probe, and an optical system that contains a high numerical aperture (NA) objective lens (NA= 1.4). When the area which satisfies NA < 1 is masked, the objective lens allows for the rejection of possible transmitted light (NA < 1) through the sample. In such conditions, the focused spot consists of only the evanescent field. We found that this NSOM system strongly reduces the background of the dye fluorescence and allows for the measurement of the fluorescence intensity below the diffraction limit of the excitation source.     

State-selective optical near-field resonant ionization spectroscopy of atoms near a dielectric surface

Spin-sensitive optical near-field microscopy and spectroscopy are proposed based on the study on the conserved quantities in optical near-field interactions of atoms with dielectric surfaces. A two-step photoionization spectra of Cs atoms resolving hyperfine structures are demonstrated near a planar dielectric surface by using evanescent waves. These techniques of state/spin-selective excitation and highly sensitive detection, combined with the techniques of optical pumping, will open up possibilities of space- and polarization-sensitive detection of optical near‐fields using atomic probes. This novel method provides us with a useful technique for the observation of polarization nature of the optical near-field and controlling the spin states of mesoscopic electronic systems.     

Apertureless scanning near-field magneto-optical microscopy of magnetic multilayers

We imaged magnetic domains in Pt/Co/Pt multilayers using an apertureless scanning near-field optical microscope operating in reflection mode. As the magneto-optical effects are weak for this kind of structure, a polarization modulation technique with a photoelastic modulator was used to reveal the contrast between magnetic domains. In the case of a Pt/Co/Pt trilayer structure, a strong improvement in lateral resolution is observed compared with far-field magneto-optical images and good sensitivity is achieved. In the case of a Pt/[Co/Pt]Pt multilayer structure, stripe domains of 200 nm width could be resolved, in good agreement with images obtained by magnetic force microscopy on the same structure.     

Theoretical comparison of illumination and collection mode images of magneto-optical dots

We compare theoretical images of the same sample obtained with two different scanning near-field optical microscopes. The sample is a two-dimensional periodic array of magnetic sub-micrometric dots. The magnetization is perpendicular to the sample plane (polar magnetization). The first configuration is a scanning tunnelling optical near-field microscope (STOM) where the tip is used in the detection mode and the sample is illuminated by total internal reflection. The second configuration is an inverted STOM: the tip is used in the emission mode and the diffracted field is far-field detected in one direction. We present the models used to describe the two configurations and then explain the main lines of the formalism used to calculate the diffracted fields by a magneto-optical sample.     

A scanning near-field optical microscope having scanning electron tunnelling microscope capability using a single metallic probe tip

A new microscope system that has the combined capabilities of a scanning near-field optical microscope (SNOM) and a scanning tunnelling microscope (STM) is described. This is achieved with the use of a single metallic probe tip. The distance between the probe tip and the sample surface is regulated by keeping the tunnelling current constant. In this mode of operation, information about the optical properties of the sample, such as its refractive index distribution and absorption characteristics, can be disassociated from the information describing its surface structure. Details of the surface structure can be studied at resolutions smaller than the illumination wavelength. The performance of the microscope is evaluated by analysing a grating sample that was made by coating a glass substrate with gold. The results are then compared with the corresponding SNOM and STM images of the grating.     

Theoretical comparison of illumination and collection mode images of magneto-optical dots

We compare theoretical images of the same sample obtained with two different scanning near-field optical microscopes. The sample is a two-dimensional periodic array of magnetic sub-micrometric dots. The magnetization is perpendicular to the sample plane (polar magnetization). The first configuration is a scanning tunnelling optical near-field microscope (STOM) where the tip is used in the detection mode and the sample is illuminated by total internal reflection. The second configuration is an inverted STOM: the tip is used in the emission mode and the diffracted field is far-field detected in one direction. We present the models used to describe the two configurations and then explain the main lines of the formalism used to calculate the diffracted fields by a magneto-optical sample.     

Evanescent field excitation and measurement of dye fluorescence in a metallic probe near-field scanning optical microscope

We introduce a method of dye fluorescence excitation and measurement that utilizes a near-field scanning optical microscope (NSOM). This NSOM uses an apertureless metallic probe, and an optical system that contains a high numerical aperture (NA) objective lens (NA = 1.4). When the area which satisfies NA < 1 is masked, the objective lens allows for the rejection of possible transmitted light (NA < 1) through the sample. In such conditions, the focused spot consists of only the evanescent field. We found that this NSOM system strongly reduces the background of the dye fluorescence and allows for the measurement of the fluorescence intensity below the diffraction limit of the excitation source.     

Comparative imaging of a bacterial surface-located GFP fusion protein by epifluorescence and scanning near-field optical microscopy

IcsA is an autotransporter protein that plays a role in the virulence of Shigella bacteria. We have examined the cellular localization of a fusion of an IcsA fragment to the green fluorescent protein (GFP) expressed in Escherichia coli using a dual epifluorescence and scanning near-field optical microscope. By combining the data obtained from far-field with near-field microscopy of the same sample, discrimination between surface-bound fusion proteins and fusion proteins located in the cellular cytoplasm becomes possible. Furthermore, and for the first time, the inherent advantages in resolution of the near-field images provides highly specific details of the location of a GFP fusion protein on a bacterial cell surface.