Photon Scanning Tunneling Microscope Research Articles

Quantitative photon tunneling and shear-force microscopy of planar waveguide splitters and mixers

A combined photon scanning tunneling and shear-force microscope has been developed to investigate the optical field distribution in a planar waveguide splitter and a multibranch mode mixer. The optical intensity distribution just above the surface of a planar waveguide is mapped with subwavelength resolution by a tapered optical fiber that probes the evanescent field. Simultaneously, the topography of the waveguide is recorded with subnanometer accuracy using a constant-distance feedback system based on shear-force detection with a tuning fork sensor. The experimental field patterns are quantitatively compared with field patterns simulated with the two-dimensional finite difference beam propagation method and a−mode solver. Good quantitative agreement between experiment and simulation is obtained. Moreover, the experiment reveals several details in the field distribution that results from incoupling conditions, mask imperfections, waveguide edges, and surface roughness. The surface effects in the optical field distribution are introduced by the use of the constant-distance feedback system.

Quantitative amplitude and phase measurement by use of a heterodyne scanning near-field optical microscope

A coherent photon scanning tunneling microscope is presented. The setup employs heterodyne interferometry, allowing both the phase and the amplitude of the optical near field to be measured. Experimental results of measurements on a standing evanescent wave reveal the high resolution that is obtainable with such an approach. In fact we have measured the amplitude and the phase of the near field, with a resolution of 1.6 nm between sample points.

Near-field and far-field optical properties of thin metallic films

Reflectivity measurements in an attenuated total internal reflection (ATIR) geometry and the variation of the transmitted intensity associated to the total electric field in a frustrated ATIR configuration using a photon scanning tunneling microscope (PSTM) of multilayered metallic films are presented as a function of the angle of incidence. Eigenmodes for total reflection and absorption associated with the optical properties of magnetic and nonmagnetic materials are theoretically and experimentally determined from the reflected and the transmitted spectra. Good agreement between theory and experiment demonstrates that PSTM is a powerful technique to characterize accurately and locally drastic variations of the intensity around optical modes of a multilayer system.

A Simulation of Near Field Optics by Three Dimensional Volume Integral Equation of Classical Electromagnetic Theory

A numerical simulation code for three dimensional problems of near-field optics has been developed using the volume integral equation with the moment method. The object is assumed to be continuous and macroscopic dielectric and can be treated by macroscopic Maxwell#x0027;s equations. The code can treat the large-scale moment method matrix that is obtained by the discretization of the volume integral equation. The resultant matrix equation is solved by an iteration method called the generalized minimum residual method with reasonable computational cost for simple problems of near field optics. Simulation of a simplified model of a scanning near-field optical microscope has been performed and basic polarization characteristics of the system have been investigated in detail. The code is also applied to the collection-mode of a photon scanning tunneling microscope, where the incident wave is the evanescent wave, and basic relation between near-field and far field i.e., output image, is recognized.

Phase mapping of optical fields in integrated optical waveguide structures

The phase evolution of optical waves in a waveguide structure has been studied with a heterodyne interferometric photon scanning tunneling microscope. Both phase and amplitude of the local optical field are measured with subwavelength resolution. Topographical maps of the waveguide surface are obtained simultaneously with the optical information. Unexpected phase patterns, with phase jumps and phase singularities, have been observed. The phase patterns can be fully understood by taking into account the total field that is the sum of the optical fields of the various modes. We show that with the unique spatial phase information, the relative field profiles and wave vectors of all the excited modes in a multimodal waveguide structure can be determined independently.

Photon scanning tunneling optical microscopy with a three-dimensional multiheight imaging mode

A photon scanning tunneling microscope with a three-dimensional multiheight mode has been developed for the mapping of optical field distributions in integrated optical waveguide structures. The optical field is measured at different heights above the waveguide surface. The multiheight measurements also contain the optical information gathered with the commonly used constant gap measurements in addition to the topography of the waveguide surface. With the multiheight method, the decay length of the evanescent field is readily determined as function of the in-plane coordinate. Moreover, the evanescent light can be distinguished from scattered light.

Near-field optical study of mesoscopic Au periodic samples: Effect of the polarization and comparison between different imaging modes

Laboratoire de Microstructures et de Microle´ctronique, 196 Avenue H. Ravera, Boiˆte Postale 107, 92195 Bagneux Cedex, France~Received 1 June 1999!This paper presents a complete study, theoretical as well as experimental, of an electromagnetic fieldscattered by subwavelength metallic pads, allocated in a periodic manner on a silica substrate. The simulationof the far field and near field is obtained with the differential method. When the sample is illuminated in totalinternal reflection, the simulations show that the amplification of the electromagnetic field above the Aumetallic pads depends on different parameters ~wavelength, polarization of light, angle of incidence, and indexof refraction!. In this paper, we only consider the effect of the probe-to-sample distance and of the polarizationof the illuminating light. As the experimental setup, we used the photon scanning tunneling microscope. If wecompare these results with the calculations carried out with the dielectric pads, we show that the amplificationis induced by the dielectric contrast between the metallic structures and their environment. Experimental resultsare presented in two different imaging modes. In the ‘‘constant intensity’’ mode, our experimental results arein excellent agreement with the simulations. Therefore, for a metallic sample analyzed in our experimentalconditions, it validates the assumption that the signal detected is proportional to the square modulus of theelectric field in the absence of the probe. We particularly show that if the polarization and the probe-to-sampledistance are suitably chosen, dramatic localizations of the electromagnetic field are observed. We then presentimages obtained in the so-called ‘‘two-wavelength’’ mode, where two light sources illuminate the sample. Thefirst beam is used for feedback regulation; it consequently allows a control of the tip motion, and permits us todetermine the reference surface. By using such feedback regulation for this first beam, we are able to comparequantitatively the effect of the polarization on the field distribution of the second beam. The results areconfirmed by the corresponding simulations.I. INTRODUCTION

Second-harmonic and sum-frequency imaging of organic nanocrystals with photon scanning tunneling microscope

Second-harmonic generation and sum-frequency generation with photon scanning tunneling microscopy and shear-force detection are used to map the nonlinear optical response and the surface topograph of N-(4-nitrophenyl)-(L)-prolinol crystals with a subdiffraction-limited resolution. The domain-size dependence of the spatial feature is obtained, which shows the local orientational distribution of the optical near field radiated by nonlinear nanocrystals and reveals the difference between nanoscopic and macroscopic second-order optical nonlinearities of molecular crystals.

High-resolution photon-scanning tunneling microscope measurements of the whispering gallery modes in a cylindrical microresonator

A detailed analysis of spatio-spectral photon scanning tunneling microscope scans of the light intensity inside a cylindrical microresonator has been carried out. By comparing the experimental results with theory, it is shown that the inclusion of spectral mode-beat phenomena is crucial for an accurate analysis of the modes present in the microresonator.

Local detection of the optical magnetic field in the near zone of dielectric samples

We present a study of the influence of the probe composition on the formation of constant-height photon scanning tunneling microscope images when observing a dielectric sample. Dramatic effects due to the metallization of the tip are presented and discussed in detail. We show how the recorded images can look quite different when the probe is dielectric or coated with gold. Comparison with numerical calculations indicate that the experimental signals are of electric or magnetic nature depending on the composition of the tip. For well-defined conditions, gold-coated tips provide images of the distribution of the magnetic field intensity associated with the optical near field. To get more insight into the origin of this effect, we have performed a careful analysis of the plasmon modes associated with a gold-coated dielectric cylinder. We conclude that the detection of the optical magnetic field is related to the conditions of excitation of circular symmetry plasmon, or ring plasmon, in the gold-coated tip.

Detection of the optical magnetic field by circular symmetry plasmons

We report on the influence of coating a sharpened optical fiber tip with Au when observing nanofabricated dielectric structures with a Photon Scanning Tunneling Microscope (PSTM) in constant-height mode. For well-defined incident wavelengths and coating thicknesses, we found that such tips detect the distribution of the magnetic field associated with the optical wave in the near-field zone. A simple tip model indicates that this phenomenon is related to the excitation of circular symmetry plasmons in Au coated tips.

Local observations of phase singularities in optical fields in waveguide structures

The phase evolution of light in an optical waveguide structure has for the first time been visualized with subwavelength resolution using a novel heterodyne interferometric photon scanning tunneling microscope. Phase singularities in the optical field of the waveguide have been observed. The phase singularities of charge one appear at locations where the modal field amplitude vanishes, due to the interference of various modes in the waveguide. Excellent agreement of the data with calculations has been obtained.

Photon scanning tunneling microscopy of tailor-made photonic structures

Optical field distributions around individually fabricated subwavelength scatterers mapped with a photon scanning tunneling microscope are presented. The photonic structures are produced from ridge waveguides using focused-ion-beam milling. This flexible technique allows us to make single holes and slits of sizes down to 30 nm. A quantitative analysis of the observed optical pattern due to interference between incoming and reflected light yields insight about subwavelength scatterers in waveguides. We conclude that light scattering into high-loss modes of the waveguide occurs.

Quasi interference of perpendicularly polarized guided modes observed with a photon scanning tunneling microscope

The simultaneous detection of TE- as well as TM-polarized light with a photon scanning tunneling microscope leads to a quasi-interference pattern of these mutually perpendicular polarized fields. This interference pattern has been observed in the optical field distribution as a function of both position and wavelength. Comparison of experimental data with simulations confirms the interference of mutually orthogonal fields. This quasi interference is caused by conversion of the linearly polarized light of both modes into elliptically polarized light by a fiber probe.

Spectroscopy with the Photon Scanning-Tunneling Microscope

Spectroscopy with the Photon Scanning-Tunneling Microscope

Two-photon fluorescence imaging and spectroscopy of nanostructured organic materials using a photon scanning tunneling microscope

Photon scanning tunneling microscopy and spectroscopy using femtosecond two-photon excitation are demonstrated. The measurement of both intensity dependence and spectral dependence is performed on a two-photon chromophore. A subdiffraction-limited resolution is obtained, and the domain-size dependence of spatial and spectral features is observed, which indicates the high degree of molecular order in the isolated nanoparticle. It is shown that the light confinement due to a quadratic dependence of the fluorescence intensity leads to an optical contrast enhancement with a coated probe.

Visualizing the whispering gallery modes in a cylindrical optical microcavity

Whispering gallery modes in cylindrical integrated optics microcavities have, for what is to our knowledge the first time, been mapped with a photon scanning tunneling microscope. Optical images were obtained with a spatial resolution of 50 nm. By combination of information on the spatial optical distributions with wavelength-dependent measurements, an unexpectedly rich variety of intracavity phenomena, such as polarization conversion and interference of copropagating and counterpropagating modes, could be directly observed. A quantitative comparison of the experimental data with computer simulations results in a comprehensive understanding of the various whispering gallery modes inside the microcavity.

Transfer functions in collection scanning near-field optical microscopy

It is generally accepted that, if in collection near-field optical microscopy the probe—sample coupling can be disregarded, a fiber probe can be considered as a detector of the near-field intensity whose size can be accounted for via an intensity transfer function. We show that, in general, this perception is wrong and it is impossible to introduce such a transfer function for the detected signal. Instead, we introduce an amplitude coupling function that relates the near-field amplitude and the amplitude of a mode guided in a probe fiber toward a detector. Different experimental configurations are considered with respect to the relation between near-field optical images and the corresponding intensity distributions. Our conclusions are supported with numerical simulations and experimental results obtained by using a photon scanning tunneling microscope with an uncoated fiber tip.

Direct observation of localized surface plasmon coupling

We report on the direct observation of localized surface plasmon coupling using a photon scanning tunneling microscope. The surface plasmons are excited in gold nanostructures tailored by electron beam lithography. Electromagnetic energy transfer from a resonantly excited nanoparticle to a nanowire, which is not directly excited by the incident light is observed. Our experimental results appear to be in good agreement with theoretical computations based on Green's dyadic technique.

Influence of the sample-probe coupling on the resolution of transmitted near-field optical image

Numerical simulation of photon scanning tunneling microscopy is presented to study the near-field distribution in the vicinity a dielectric surface with one-dimensional sub-wavelength structures. Multiple scattering between the probe tip and the sample has been taken into account implicitly by matching electromagnetic boundary conditions at interfaces. The near-field intensity in transmission mode through two ridges on surface has been modeled in order to analyze the resolution of the system. The effects on the signal by the sample-tip coupling, the polarization of the incident light, and the angle of incidence are investigated. We find that the capability to recognize the feature will be improved when the tip–object interaction is strong.