Sergei I. Bozhevolnyi

Configurational Resonances in Optical Near-Field Microscopy: A Rigorous Point-Dipole Approach

Abstract A theoretical analysis of the framework for near-field microscopy based on a microscopic description of the interaction between the dielectric probe and the surface is presented. The probe tip is assumed to be a point-like sphere and the surface (selvedge) is represented by point-like spheres placed on a two-dimensional lattice. The bulk is treated as a homogeneous continuum. Using a Greens function formalism we have established a set of self-consistent algebraic equations to describe the local field at the sites of the probe tip and the selvedge. All contributions including bulk reflection and many-body interactions have been taken into account. By means of a point-dipole approach we have for N dipoles solved the 3 N × 3 N linear algebraic equations exactly. The results have been compared with approximate solutions obtained by a Born series expansion. We have found that the approximate solutions are very different from the exact solution in the case of strong interaction. The approximate solutions tend to infinity only when the tip-surface distance decreases to zero, and the results depend strongly on the number of Born iterations performed. However, for the exact solution it is shown that there are regions close to the surface where an enhanced field can be induced by the probe, while, when the tip-surface distance decreases to zero the self-consistent field tends to zero. Thus, we have found resonance interactions between the probe tip and the surface. These resonances are referred to as configurational ones, since, for any given dipole polarizabilities, the system can be adjusted to resonance by varying the distances between the dipoles. The resonance conditions for some simple systems are presented in an explicit form. It is demonstrated that the resonance coupling of the field component perpendicular to the surface occurs at a bigger distance of the tip dipole from the surface than that of the parallel components. The relation between the polarizability of the probe tip and the resonance positions is considered. It is shown that in the absence of retardation and damping the resolution of the system can be as close to infinity as the tip-surface distance is close to the resonance value. The self-consistent field at the site of the probe is calculated for different distances between the tip and the surface and as a function of the position of the tip along the surface.

External-reflection near-field optical microscope with cross-polarized detection

An external-reflection scanning near-field optical microscope with the detected light polarized perpendicular to the polarization of the light coupled into the fiber is presented. When various metallic gratings are scanned, it is shown that the lateral and the depth resolutions of this microscope are better than 100 and 10 nm, respectively.

Control of the tip–surface distance in near-field optical microscopy

An experimental technique that makes use of the intensity of the interference pattern formed by light that propagates directly from the single-mode fiber tip and light that is reflected by the surface under anoblique angle of incidence is developed to control the tip-surface distance in near-field opticalmicroscopy. It is shown that by using another fiber as a detector with a polished edge placed at the surface near the fiber tip one can determine the tip-surface separation with an accuracy better than 15 nm at distances less than 1 µm. The technique proposed is used to investigate the influence of the shape of the tip in near-field measurements.

Self-consistent model for photon scanning tunneling microscopy: implications for image formation and light scattering near a phase-conjugating mirror

A macroscopic self-consistent model for photon scanning tunneling microscopy with an uncoated fiber tip is developed by use of integral plane-wave representations of the total electric field in two-dimensional geometry. The model framework allows one to treat the incident field with an arbitrary angular spectrum and the presence of a thin-layer medium with a subwavelength structure on the sample surface. Imaging with the photon scanning tunneling microscope and light scattering near a phase-conjugating mirror are considered with our model by use of numerical simulations. We show that the near-field optical images provided by the microscope can be quite different from the light intensity distributions that exist near the sample surface in the absence of the fiber tip. We demonstrate that the self-consistent field at the site of a scatterer placed in front of a phase-conjugating mirror can be significantly enhanced as a result of multiple phase conjugation of the scattered light.

Macroscopic self-consistent model for external-reflection near-field microscopy

The self-consistent macroscopic approach based on the Maxwell equations in two-dimensional geometry is developed to describe tip–surface interaction in external-reflection near-field microscopy. The problem is reduced to a single one-dimensional integral equation in terms of the Fourier components of the field at the plane of the sample surface. This equation is extended to take into account a pointlike scatterer placed on the sample surface. The power of light propagating toward the detector as the fiber mode is expressed by using the self-consistent field at the tip surface. Numerical results for trapezium-shaped tips are presented. We show that the sharper tip and the more confined fiber mode result in better resolution of the near-field microscope. Moreover, it is found that the tip–surface distance should not be too small so that better resolution is ensured.

Experimental studies of surface plasmon polariton band gap effect

Surface plasmon polaritons (SPPs) propagation at a gold film surface covered by periodic arrays of ∼40‐nm‐high scatterers arranged in a triangular lattice of different periods containing straight line defects is studied using collection scanning near‐field optical microscopy. The results reveal the dependence of the SPP band gap (SPPBG) effect manifested via the SPP reflection and guiding (along line defects) on the parameters of the surface structures (period, filling factor and lattice orientation). We find that the SPPBG effect is stronger along ΓK direction for all investigated periodic structures. Our results demonstrate that the SPPBG effect becomes less pronounced with a decrease of the filling factor and disappears for a filling factor close to 0.2. We also observe that the centre of the SPPBG shifts towards longer wavelengths with an increase of the lattice period.

Optical paramagnetic polarizability of mesoscopic particles : a study of local field corrections

Abstract The local-field correction to the paramagnetic optical polarizability of a cubic mesoscopic particle is studied within the framework of the transverse self-field approximation. Assuming the electrons to be confined in a flat potential bounded by infinitely high barriers, rigorous results for the paramagnetic polarizability are established. In turn these results are used to study cases where a limited number of electrons participate in the dynamics. Local-field resonances in the frequency spectrum are identified and discussed. Special emphasis is devoted to a study of the so-called self-coupling approach, in which only the radiative interaction between the up and down-transitions of a given pair of levels is kept. Numerical results for the frequency dependence of the optical polarizability of cubic GaAs semiconductor quantum dots with side lengths 126 A, 200 A, and 271 A are presented.

Near/far-field investigations of the interaction between surface waves and nanoparticles

(a)PhysiquedelaMatie`reCondense´e,Universite´ deNice,06108NiceCedex2,France(b)InstituteofSemiconductorPhysics,ProspektNauki45,03028Kiev-28,Ukraine(c)AalborgUniversity,Pontoppidanstraede103,DK-9220AalborgOst,Denmark(d)UniversityofCalifornia,Irvine,California92697,USA(Received March 6, 2001; in revised form June 7, 2001; accepted September 13, 2001)Subject classification: 61.46.+w; 73.20.Mf; 78.35.+c; 78.67.Bf; S4; S10.1Results of experimental studies of the scattering of electromagnetic surface waves by metal nano-particles (NPs) located at different distances of a silver film and carried out by both near- and far-field optical techniques are presented for the first time, as far as we know. SNOM and AFM tech-niques were applied to investigate near-field scattering and topographical images of the outer inter-face. In the far-field, the half-space distribution of the normalized scattered light intensity has beenmeasured and shows, as well as SNOM, an increase of the scattered intensity with NPs–silver dis-tance. These results can be attributed to the variation of the space distribution of the electric fieldof the surface wave.

Surface plasmon polariton waveguiding in random surface nanostructures

In this study, guiding of surface plasmon polaritons excited at a gold film surface along corrugation‐free channels in regions that are covered with randomly located surface scatterers, is considered using near‐field microscopy for imaging of surface plasmon polariton intensity distributions at the surface. In the wavelength range 713–815 nm, we observed complete inhibition of the surface plasmon polariton propagation inside the random structures composed of individual (≈ 70 nm high) gold bumps (and their clusters) placed on a 55 nm thick gold film with a bump density of 75 µm−2. We demonstrate well‐defined surface plasmon polariton guiding along corrugation‐free 2 µm wide channels in random structures and, in the wavelength range 738–774 nm, low‐loss guiding around 20° bends having a bend radius of ≈ 15 µm.

Diagram Method for Exact Solution of the Problem of Scanning Near-Field Microscopy

A method is presented for calculating near-field images of nanoobjects from the intensity distributions measured using the scanning near-field optical microscopy technique. The method is based on a formally exact solution of the self-consistent local-field equation, which was derived using the diagram technique for summation of infinite series. It is shown that the self-consistent fields calculated with and without considering the dielectric substrate differ significantly. Near-field images of simple geometric objects—parallelepipeds with various side ratios—are calculated.