Showing papers on "Total internal reflection published in 2001"

Dynamical theory of x-ray diffraction

Abstract: This chapter presents the dynamical theory of the diffraction of X-rays by perfect crystals. The most important part is devoted to the case of plane waves (Section 5.1.2). The solutions of the propagation equation of plane waves in crystals are given in Section 5.1.3 using the concept of wavefields introduced by Ewald for X-rays in 1913 and by Bloch for electrons in 1928 (known in solid-state physics as Bloch waves). They are applied to the interpretation of the main properties of dynamical diffraction: anomalous transmission, standing waves and Pendellosung. The expressions for the diffracted intensity are given in both the transmission (Section 5.1.6) and the reflection (Section 5.1.7) cases. The last part (Section 5.1.8) concerns the diffraction of real and spherical waves, which is described in a qualitative way.

Substrate-guided optical beam expander

Abstract: The invention provides an optical device, including a light-transmitting substrate, optical means for coupling light into the substrate by total internal reflection, and a plurality of partially reflecting surfaces carried by the substrate, characterized in that the partially reflecting surfaces are parallel to each other and are not parallel to any of the edges of the substrate.

Resonant-enhanced evanescent-wave fluorescence biosensing with cylindrical optical cavities.

Abstract: We show that the artificial resonances of dielectric optical cavities can be used to enhance the detection sensitivity of evanescent-wave optical fluorescence biosensors to the binding of a labeled analyte with a biospecific monolayer. Resonant coupling of power into the optical cavity allows for efficient use of the long photon lifetimes (or equivalently, the high internal power) of the high-Q whispering gallery modes to increase the probability of photon absorption into the fluorophore, thereby enhancing fluorescence emission. A method to compare the intrinsic sensitivity between resonant cavity and waveguide formats is also developed. Using realistic estimates for dielectric cylindrical cavities in both bulk and integrated configurations, we can expect sensitivity enhancement by at least an order of magnitude over standard waveguide evanescent sensors of equivalent sensing geometries. In addition, the required sample volume can be reduced significantly. The cylindrical cavity format is compatible with a large variety of sensing modalities such as immunoassay and molecular diagnostic assay.

Evanescent Waves: From Newtonian Optics to Atomic Optics

Abstract: I. The Evanescent Field.- 1. Total Internal Reflection.- 1.1 The Electromagnetic Field at Total Internal Reflection.- 1.1.1 Snell's Law.- 1.1.2 Analysis of Total Internal Reflection on the Basis of Maxwell's Equations.- 1.1.3 Components of the Electric Field in the Second Medium in the z = 0 Plane.- 1.2 Flux of the Poynting Vector Associated with the Evanescent Field.- 1.3 Shifts of the Beams at Total Internal Reflection.- 1.4 Frustrated Total Internal Reflection.- 1.5 Resonant Tunneling Effect.- 1.6 Conclusion.- 2. Diffraction from an Aperture and Dipolar Radiation.- 2.1 Analysis of the Propagation of Light Through an Aperture.- 2.2 Diffraction of Light from a Circular Aperture.- 2.2.1 Diffraction from an Aperture in an Infinitely Thin Plane.- 2.2.2 Diffraction from a Circular Aperture in a Thick Screen.- 2.3 Coupling Between Several Apertures.- 2.4 Dipolar Emission.- 2.4.1 Expression of the Dipolar Field.- 2.4.2 Energy Emitted by a Dipole.- 2.5 Dipolar Emission in the Vicinity of a Surface.- 2.6 Conclusion.- 3. The Evanescent Field in Guided Optics.- 3.1 The Evanescent Field in Planar Optics.- 3.1.1 Analysis of Planar Waveguides.- 3.1.2 Production of Step-Index Planar Waveguides.- 3.2 Confined Waveguides.- 3.3 Optical Fibers.- 3.3.1 Ray-Optical Analysis of the Propagation in Optical Fibers.- 3.3.2 Modes of Step-Index Fibers.- 3.3.3 Modes of Inner-Cladding Fibers.- 3.3.4 Modes of Annular-Core Fibers.- 3.3.5 Modes of Graded-Index Fibers.- 3.3.6 Modes of Polarization-Preserving Fibers.- 3.4 Whispering-Gallery Modes.- 3.5 Band-Gap Photonics Waveguides.- 3.6 Conclusion.- Conclusion of Part I.- II. Delocalized Interaction with the Evanescent Field.- 4. Evanescent-Field Optical-Fiber Couplers.- 4.1 Types of Couplers.- 4.2 Fabrication Techniques of Evanescent-Field Fiber-Optic Couplers.- 4.2.1 Twist-Etched Fiber Couplers.- 4.2.2 Mechanically Polished Fiber Couplers.- 4.2.3 Fused-Tapered Fiber Couplers.- 4.2.4 Comparison Between the Different Types of Couplers.- 4.3 Analysis of the Coupling.- 4.3.1 Coupled Power Between Two Parallel Uniform Fibers.- 4.3.2 Step-Index Fibers.- 4.3.3 Inner-Cladding Fibers.- 4.3.4 Variable-Diameter Couplers.- 4.4 Spectral Filters and Spectral Multiplexers.- 4.5 Polarization Splitters.- 4.6 Production of Modal Filters.- 4.7 Devices Produced from Evanescent-Field Couplers.- 4.7.1 Optical-Fiber Gyroscope.- 4.7.2 Fiber Lasers.- 4.8 Conclusion.- 5. Integrated-Optical Evanescent-Field Couplers.- 5.1 Description of Integrated-Optical Couplers.- 5.2 Analysis of the Coupling Between Two Waveguides.- 5.3 Active Couplers.- 5.4 Coupling from a Fiber to a Planar Waveguide.- 5.5 Integration of a Waveguide and a Photodiode.- 5.6 Conclusion.- 6. Evanescent-Field Waveguide Sensors.- 6.1 General Points on Sensors.- 6.2 Fiber-Optic Sensors.- 6.2.1 Monitoring of a Chemical Reaction by Fluorescence Detection.- 6.3 Integrated-Optical Sensors.- 6.3.1 Analysis of the Sensitivity of Integrated-Optical Sensors.- 6.3.2 Creating the Sensing Region.- 6.3.3 Evanescent-Field Interferometric Sensors.- 6.3.4 Amplification of the Evanescent Field by a Multilayered System and Applications to Biosensors.- 6.4 Conclusion.- 7. Internal-Reflection Spectroscopy.- 7.1 Effect of Index Variations on Total Internal Reflection.- 7.1.1 Effective Thickness.- 7.1.2 Measurement of the Dielectric Constants in an Arbitrary Medium.- 7.2 Spectroscopy Devices Based on Total Internal Reflection.- 7.2.1 Description of Different Systems Generating Total Internal Reflection.- 7.2.2 Description of Internal-Reflection Spectroscopes.- 7.2.3 Quality of the Reflective Element.- 7.2.4 Constraints in the Preparation of the Samples.- 7.3 Atom Spectroscopy in the Vicinity of Interfaces.- 7.4 Conclusion.- 8. Evanescent-Wave Atom Optics.- 8.1 Atomic Interferences.- 8.2 Reflection of Atoms.- 8.3 Deflection of Atoms.- 8.3.1 Deflection Based on the Use of Evanescent Waves Generated at Total Internal Reflection.- 8.3.2 Deflection Based on the Use of the Evanescent Field of Whispering-Gallery Modes of a Sphere.- 8.4 Atom Guiding.- 8.5 Conclusion.- 9. Dark-Field Microscopy and Photon Tunneling Microscopy.- 9.1 Dark-Field Microscopy.- 9.1.1 Basic Principles.- 9.1.2 Description of the Dark-Field Microscope.- 9.1.3 Comparison between Dark-Field and Bright-Field Images.- 9.1.4 Dark-Field Microscopy and Fluorescence.- 9.2 Photon Tunneling Microscopy.- 9.3 Conclusion.- Conclusion of Part II.- III. Localized Interaction with the Evanescent Field.- 10. Scanning Tunneling Optical Microscopy.- 10.1 Fundamental Principles of the Scanning Tunneling Optical Microscope.- 10.2 Detection of the Near-Field in the Vicinity of a Plane Surface.- 10.3 Early Results in Scanning Tunneling Microscopy.- 10.4 Near-Field Study of Homogeneous Samples.- 10.4.1 Effects of the Polarization and Orientation of the Source.- 10.4.2 Effect of the Distance Between the Probe and the Surface.- 10.4.3 Effect of the Coherence of the Source.- 10.4.4 Effect of the Wavelength.- 10.4.5 Effect of the Probe.- 10.5 Near-Field Study of Non-Homogeneous Samples.- 10.6 Near-Field Study of Optical Waveguides.- 10.6.1 Observation of the Index Variations of a Waveguide.- 10.6.2 Detection of the Evanescent Field of Guided Modes.- 10.6.3 Near-Field Analysis of the Structure of Guided Modes.- 10.7 Local Near-Field Spectroscopies.- 10.8 Photon Scanning Tunneling Microscopy and Fluorescence.- 10.9 Near-Field Study of Surface Plasmons.- 10.10 Conclusion.- 11. Micro-Aperture Microscopy.- 11.1 Fundamental Principles of the Scanning Near-Field Optical Microscope.- 11.2 The Breaking of the Rayleigh Limit on Resolution for Microwave and Optical Frequencies.- 11.3 Description of the Scanning Near-Field Optical Microscope.- 11.4 Effects of the Physical Parameters on the Formation of the Images.- 11.4.1 Effect of the Polarization.- 11.4.2 Effect of the Wavelength.- 11.4.3 Effect of the Coherence of the Source.- 11.4.4 Effect of the Distance Between the Probe and the Surface.- 11.5 Local Fluorescence Detection.- 11.6 Near-Field Optics and Photolithography.- 11.7 Conclusion.- 12. Apertureless Microscopies.- 12.1 Near-Field Optical Microscope Based on the Local Perturbation of a Diffraction Spot.- 12.2 Scanning Interferometric Apertureless Microscope.- 12.3 Tetrahedral Probe Microscope.- 12.4 Local Probe Microscope Derived from the PSTM.- 12.5 Radiation Pressure Scanning Microscope.- 12.6 Conclusion.- Conclusion of Part III.- References.

Role of light intensification by cracks in optical breakdown on surfaces.

Abstract: The intensity distribution of an initially plane light wave incident on planar and conical surface cracks is calculated numerically by using a wave propagation computer code. The results show that light intensity enhancements caused by interference of internal reflections at the crack and the surface are very sensitive to the light polarization, the beam angle of incidence, and the crack geometry (e.g., crack width and orientation with the surface). The light intensity enhancement factor (LIEF) can locally reach 2 orders of magnitude for conical cracks of ideal shape. The electric field direction relative to the crack surfaces determines the light intensity profile around the crack. For normal-incidence illumination on the output surface, total internal reflection at the crack and the surface can occur and leads to higher LIEFs. For identical geometry and illumination conditions, a crack located on the entrance surface of an optic generates electric field enhancements that are weaker than those on the exit surface. As cracks on polished surfaces are randomly oriented, the probability for large intensity enhancements to occur is high. The model is able to predict quantitatively the magnitude of surface laser-induced damage threshold drop and damage propagation enhancement in dielectric materials that are due to cracks.

Periodic light coupler gratings in amorphous thin film solar cells

Abstract: Efficient light trapping structures for amorphous hydrogenated silicon (a-Si:H) solar cells have been realized using periodically structured aluminum doped zinc oxide (ZnO:Al) with periods between 390 and 980 nm as a transparent front contact. Atomic force microscopy, optical reflection, and diffraction efficiency measurements were applied to characterize solar cells deposited on such gratings. A simple formula for the threshold wavelength of total internal reflection is derived. Periodic light coupler gratings reduce the reflectance to a value below 10% in the wavelength range of 400–800 nm which is comparable to cells with an optimized statistical texture. Diffraction efficiency measurements and theoretical considerations indicate that a combination of transmission and reflection gratings contribute to the observed reduction of the reflectance.

Optical binding of particles with or without the presence of a flat dielectric surface

Abstract: Optical fields can induce forces between microscopic objects, thus giving rise to different structures of matter. We study theoretically these optical forces between two spheres, either isolated in water, or in the presence of a flat dielectric surface. We observe different behavior in the binding force between particles at large and at small distances (in comparison with the wavelength) from each other. This is due to the great contribution of evanescent waves at short distances. We analyze how the optical binding depends on the size of the particles, the material composing them, the wavelength, and, above all, the polarization of the incident beam. We also show that depending on the polarization the force between small particles at small distances changes its sign. Finally, the presence of a substrate surface is analyzed, showing that it only slightly changes the magnitudes of the forces, but not their qualitative nature, except when one employs total internal reflection, in which case the particles are induced to move together along the surface.

Surface-enhanced magneto-optics in metallic multilayer films

Abstract: The magneto-optical properties of noble-metal--ferromagnetic-metal multilayer thin films have been investigated as a function of the incidence angle, including the total reflection range, in the polar, longitudinal, and equatorial geometries, and for different values of the photon energy in the near-infrared and visible spectrum. The experimental and theoretical results are obtained on a Au/Co/Au model system. They demonstrate that the resonant coupling of the p component of the light electric field with the gold surface plasmon, which occurs in the total reflection range, yields a strong enhancement of the magneto-optical response and signal-to-noise ratio of the system for the three magnetization directions. This resonant coupling and the resulting enhancement of the relevant magneto-optical quantities are achieved for any photon energy in the near infrared and visible range simply by tuning the incidence angle. The efficiency of this enhancement effect is shown to increase towards the infrared region of the spectrum following the rise of the quality factor of the surface plasmon resonance.

Multimode resonances in square-shaped optical microcavities.

Abstract: Square-shaped two-dimensional optical microcavities (micro-cavities) were investigated for possible applications as filters for dense wavelength-division multiplexing. Multimode cavity resonances were observed in the elastic scattering of approximately 200-microm square-shaped micro-cavities in fused silica. Based on a two-dimensional k-space representation, we accounted for the multimode spectrum by different normal modes with rays confined by total internal reflection. The cavity-mode trajectories need not be closed after each round trip. Single-mode spectra are expected from smaller square-shaped micro-cavities.

Arrayed silicon prism coupler for a terahertz-wave parametric oscillator.

Abstract: Using room-temperature parametric oscillation of a LiNbO3 crystal pumped by a Q-switched Nd:YAG laser with a simple configuration, we have realized a widely tunable coherent terahertz- (THz-) wave source in the range between 1 and 3 THz. Inasmuch as the THz wave is affected by total internal reflection at the crystal edge, we used a Si prism coupler to couple out the THz wave. We introduce an arrayed Si-prism coupler that increases the efficiency and decreases the diffraction angle. By use of the arrayed-prism coupler, there is a sixfold increase in coupling efficiency and a 40% decrease in the far-field beam diameter, compared with the use of a single-prism coupler. We discuss the negative effect of the free carriers at the Si-prism surface that is excited by the scattered pump beam, and the positive effect of cavity rotation on the unidirectional radiation of the THz wave from a Si prism.

Frustrated total reflection: the double-prism revisited.

Abstract: Geometrical optics prohibits any penetration of light into an optically rarer medium in the case of total reflection. When sandwiching, however, the rarer medium between optically denser media, a transmitted beam can be observed in the third medium. The experiment is often realized by a double-prism arrangement [1]; the effect is called frustrated total reflection due to the enforced transmission. Amazingly, the reflected and transmitted beams are shifted with respect to geometrical optics as conjectured by Newton [2] and experimentally confirmed by Goos-H\"anchen 250 years later [3]. However, inconsistent results on the spatial shifts have been reported [4-7]. Here we report on measurements of the Goos-H\"anchen shift in frustrated total reflection with microwaves. We found an unexpected influence of the beamwidth and angle of incidence on the shift. Our results are not in agreement with both previous experiments [6,7] and theoretical predictions [8-10]. The topic of frustrated total reflection is important for both fundamental research and applications [11-13].

Attenuated Total Internal Reflection Infrared Mapping Microspectroscopy Using an Imaging Microscope

Abstract: Attenuated total internal reflection (ATR) infrared mapping microspectroscopy using an infrared microscope and a focal plane array is investigated and reported. The study demonstrates the advantages of conducting ATR microspectroscopy using a focal plane array detector. These benefits include the rapid acquisition of molecular specific images, ease of sample preparation, and increased spatial resolution. An experimental determination of the spatial resolution found that the combined system operates very close to the diffraction limit, and a 4× magnification factor associated with the germanium internal reflection element was realized. Experiments conducted on several polymer samples and a biological sample demonstrate the future viability of the method.

Optical reproduction system

Abstract: A lens system which has a first optical boundary with a radius of curvature R, a second optical boundary located substantially a distance R from the first boundary, and a third optical boundary nearer to the second optical boundary than R. Secondly, a lens system providing optical field limitation using total internal reflection. Also, an array of lenses for reproduction, capture and display of three dimensional images discussed.

Resolution enhancement in standing-wave total internal reflection microscopy: a point-spread-function engineering approach

Abstract: The theoretical basis for resolution enhancement in standing-wave total internal reflection microscopy (SW-TIRM) is examined. This technique relies on the formation of an excitation field containing super-diffraction-limited spatial-frequency components. Although the fluorescence generated at the object planes contains high-frequency information of the object distribution, this information is lost at the image plane, where the detection optics acts as a low-pass filter. From the perspective of point-spread-function (PSF) engineering, one can show that if this excitation field is translatable experimentally, the high-frequency information can be extracted from a set of images where the excitation fields have different displacement vectors. We have developed algorithms to combine this image set to generate a composite image with an effective PSF that is equal to the product of the excitation field and the Fraunhofer PSF. This approach can easily be extended to incorporate nonlinear excitation modalities into SW-TIRM for further resolution improvement. We theoretically examine high-resolution imaging based on the addition of two-photon, pump–probe, and stimulated-emission depletion methods to SW-TIRM and show that resolution better than 1/20 of the emission wavelength may be achievable.

A Comparison of Through-the-Objective Total Internal Reflection Microscopy and Epifluorescence Microscopy for Single-Molecule Fluorescence Imaging

Abstract: The design and characterization of a wide-field, through-the-objective, Total Internal Reflection (TIR) microscope which has the sensitivity required for characterizing single molecules under ambient conditions is described. A careful comparison between TIR and epifluorescence imaging is made at the single-molecule level with equal incident intensities, using the laser dyes DCM and R6G in a polymer matrix as test samples. With these samples, measurements show that while the signal-to-background ratio (SBR) of single molecules using TIR is comparable with that for epifluorescence imaging over a range of illumination intensities, the signal-to-noise ratio (SNR) for TIR is superior and is the principal source of improved images which have previously been reported with this form of microscopy. The likely source of the improved SNR is simply the reduction in the relative size of photon shot-noise caused by the higher effective pumping intensity inherent in evanescent wave excitation. The technique's general applicability to a wider range of single-molecule studies is discussed. In addition, the effect on detected images produced by variations in the gain of an intensified CCD camera is characterized.

Optical switch with moveable holographic optical element

Abstract: An optical switch formed of a holographic optical element (HOE) disposed above a top surface of a substrate and moveable relative thereto is shown. Light is traveling through the substrate under total internal reflection, which creates an evanescent field extending beyond the reflecting surfaces of the substrate. The HOE is characterized, in one embodiment, by being formed from a plurality of strips that are moveable between a first position in which the strips are above the evanescent field and a second position in which the strips are inside the evanescent field. In the first position, the light in the substrate propagates unaffected by the HOE in a primary direction of propagation. In the second position, the light in the substrate is altered by the HOE and made to propagate in a reflected direction oblique to that of the primary direction of propagation.

Optical device and apparatus employing the same

Abstract: This invention improves the use efficiency of light emitted by a solid light emitter such as a light emitting diode, and realizes a desired directional pattern. On a front boundary surface of a mold resin 13 sealing a light emitter 12, there are formed a direct emission region 18 for emitting the light from the light emitter 12 and a total reflection region 19 for totally reflecting the light from the light emitter 12. The direct emission region 18 is convex lens-shaped. A light reflecting portion 20 having a concave mirror shape is disposed on a rear wall of the mold resin 13. A part of light emitted from the light emitter 12 is emitted forward by receiving an optical lens action when it passes through the direct emission region 18. Another part of the light emitted by from the light emitter 12 is totally reflected by the total reflection region 19, and is reflected by the light reflecting portion 20, and emitted forward from the total reflection region 19.

Evanescent field illumination devices and methods

Abstract: Devices and methods are disclosed comprising providing a sample holder having a plurality of sample wells. Inner and outer surfaces of at least one well are configured for total internal reflection of incident light.

Along-slope current generation by obliquely incident internal waves

Abstract: A series of numerical experiments is performed to investigate the breaking of obliquely incident internal waves propagating towards a bottom slope. The case of critical reflection is considered, where the angle between the wave group velocity vector and the horizontal matches the bottom slope angle. The flow evolution is found to be significantly different from the evolution observed previously in simulations of normally incident waves. The divergence of the Reynolds stress in the breaking zone causes a strong along-slope mean current, which changes the flow structure dramatically. The wave does not penetrate the current but breaks down at its upper surface as the result of a critical layer interaction. A continuously broadening mean along-slope current with an approximately constant velocity is produced. We propose a simple model of the process based on the momentum conservation law and the radiation stress concept. The model predictions are verified against the numerical results and are used to evaluate the possible strength of along-slope currents generated by this process in the ocean.

Three-dimensional total internal reflection microscopy.

Abstract: We investigate the inverse-scattering problem that arises in total internal reflection microscopy. An analytic solution to this problem within the weak-scattering approximation is used to develop a novel form of three-dimensional microscopy with subwavelength resolution.

Sensor utilizing attenuated total reflection

Abstract: A sensor utilizing attenuated total reflection (ATR) is provided with a dielectric block, a thin film layer formed on a surface thereof, a light source for emitting a beam, and an optical system for making the beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection is satisfied at an interface between the dielectric block and the thin film layer. The sensor is further provided with a photodetection section for detecting the ATR, a differentiation section for differentiating signals output from the light-receiving elements of the photodetection section, in a direction in which the light-receiving elements are juxtaposed, and an adjustment section for optically expanding the width of a dark line, corresponding to the ATR, of the beam which falls on the photodetection section, so that the width of the dark line becomes greater than a pitch between the light-receiving elements.

Evanescent-wave coupled microcavity laser

Abstract: Disclosed is an evanescent-wave-coupled microcavity laser in which a gain medium (120) is positioned outside a circularly symmetric microcavity (110) having a size of a few tens of microns to a few hundreds of microns to generate a laser oscillation using a gain medium existing in the evanescent-field of a resonance mode. Particularly, a gain medium containing a semiconductor, atoms, molecules, or quantum dots is placed outside the microcavity where the evanescent-wave of the microcavity mode exists, to be excited by an electric or an optical pumping. Fluorescence irradiated from the excited gain medium is coupled with the evanescent-wave of the resonator mode to obtain a gain, so that amplification of light is triggered. The amplified light circulates inside the microcavity through total internal reflection to induce a stimulated emission of radiation from the excited gain medium in the field of evanescent-wave so that a stable laser oscillation is established. Particularly, the present invention includes the evanescent-wave-coupled microcavity lasers using the microspheres of extremely low energy loss, microdisks or microcylinders capable of being large-scale integrated.

Low voltage electro-optic polymer light modulator using attenuated total internal reflection

Abstract: An improved electro-optic (EO) polymer light modulator based on attenuated-total-internal-reflection (ATR) is demonstrated. The modulator consists of a prism–metal–EO-polymer–metal multilayer structure. An applied electric field across the EO polymer layer electrically modulates the energy coupling efficiency of incoming light into guided wave resonance at fixed angles. Compared with conventional ATR modulators based on surface plasmon resonance, the driving voltage for this modulator has been greatly reduced because of the newly chosen working interior angle. It also offers advantages in terms of insertion loss and aperture size over other techniques for the amplitude modulation of a collimated light beam.

Reflected image of a strongly focused spot

Abstract: In various experimental situations light is focused on or near an interface between dielectric media. For some techniques such as confocal microscopy and laser tweezers the focused light is incident from the optically denser medium. If the numerical aperture (NA) of the focusing lens is sufficiently large, some field components of the focused light undergo total internal ref lection (TIR) at the interface. Under these conditions the ref lected wave is phase shifted with respect to the incident wave and the apparent ref lection point appears to be displaced beyond the

Guiding Mechanisms in Dielectric-Core Photonic-Crystal Optical Waveguides

Abstract: We show that the main guiding mechanisms in dielectric-core photonic crystal optical waveguides are total internal reflection and distributed Bragg reflection. We also show that by placing a slab of semiconductor material between two photonic band gap (PBG) mirrors, we can obtain waveguide modes at frequencies out of the photonic bandgap. These modes are similar to the modes of a conventional dielectric slab waveguide. Using these modes, we can obtain very good coupling between a PBG waveguide and a dielectric slab waveguide with similar slab properties.

The role of tip plasmons in near-field Raman microscopy.

Abstract: The enhancement in electric field strength in the vicinity of a metal tip, through the excitation of plasma modes in the tip, is investigated using the finite difference time domain method; such tip enhancement has significant potential for application in scanning near-field Raman microscopy. To represent an experimentally realistic geometry the near-field probe is described by a conical metal tip with a spherical apex, with radii 20 nm and 200 nm considered, in close proximity to a glass substrate. Illumination through the substrate is considered, both at normal incidence and close to the critical angle, with the polarization in the plane of incidence. By modelling the frequency dependent dielectric response of the metal tip we are able to highlight the dependence on the scattering geometry of the nature of the electromagnetic excitations in the tip. In particular, the strongest electric field enhancement with the greatest confinement occurs for the excitation of modes localized at the tip apex, excited only for off-normal incidence. Bulk modes excited in the tip also produce enhancement, although over a larger area and with significantly less enhancement than that of the localized modes; however, the excitation of bulk modes is independent of the angle of incidence.