Optical Near-field Enhancement Research Articles

Effect of near-field coupling on far-field inelastic scattering imaging of gold nanoparticles

The optical near-field enhancement induced by coupling between noble nanoparticles andthe substrate has been studied by a far-field imaging method. The longitudinalmode of the incident laser is revealed to contribute to the coupling. The far-fieldimages of individual gold nanoparticles exhibit a peanut-shaped pattern; thesewere constructed by the intensity of inelastically scattered light. The couplingbetween gold nanoparticles and the silicon substrate leads to the patterned image.By tuning the separation between the gold nanoparticles and substrate usingSiO2 layers of different thickness, the coupling efficiency decreases with the thickness of theSiO2 layer.

Nearfield surface enhanced spectroscopy using targeted nanoparticle deposition

Surface enhanced spectroscopy is demonstrated by depositing gold nanoparticles on a surface using an atomic force microscope. A modified dip pen nanolithography process is used to place particles directly on to a target. Near-field optical enhancement is demonstrated for Raman and infrared spectroscopies and is applicable for fluorescence spectroscopy. This approach provides localized, near-field spectroscopy with subdiffraction limit resolution and a general method for fabricating plasmonic device structures.

Optical Antenna Properties of Scanning Probe Tips: Plasmonic Light Scattering, Tip−Sample Coupling, and Near-Field Enhancement

The optical near-field distribution and enhancement near the apex of model scanning probe tips are calculated within the quasistatic approximation. The optical tip−sample coupling sensitively depends on both the tip and sample material. This, in addition to the tip−sample distance and apex geometry, is found to affect the spatial resolution that can be obtained in scattering near-field microscopy (s-SNOM). A pronounced structural plasmon resonant behavior is found for gold tips, which redshifts upon tip−sample approach on the length scale given by the tip radius. This near-field tip−sample coupling also allows for surface plasmon excitation in the sample. With the critical dimensions of the tip apex in the range of 10 to several 10s of nanometers, the results are found to be in good agreement with experiment and more rigorous theoretical treatments.

Finite-difference time-domain analysis of hemi-teardrop-shaped near-field optical probe

A hemi-teardrop-shaped probe designed to produce an intense optical near-field to a subwavelength spatial dimension is presented. The influence of probe shape on optical near-field enhancement is investigated using the finite-difference time-domain method. The numerical results show that the resonant wavelength can be controlled by adjusting the sharpness of the object.

Large-area parallel near-field optical nanopatterning of functional materials using microsphere mask

Large-area parallel near-field optical nanopatterning on functional material surfaces was investigated with KrF excimer laser irradiation. A monolayer of silicon dioxide microspheres was self-assembled on the sample surfaces as the processing mask. Nanoholes and nanospots were obtained on silicon surfaces and thin silver films, respectively. The nanopatterning results were affected by the refractive indices of the surrounding media. Near-field optical enhancement beneath the microspheres is the physical origin of nanostructure formation. Theoretical calculation was performed to study the intensity of optical field distributions under the microspheres according to the light scattering model of a sphere on the substrate.

Plasmonic effects in near-field optical transmission enhancement through a single bowtie-shaped aperture

In this paper, the enhanced optical transmission through a special type of aperture of a bowtie shape is investigated through near-field imaging and finite-difference numerical analysis. Under linear polarizations in two orthogonal directions, the optical near fields of the bowtie aperture and comparable square and rectangular apertures made in gold and chromium thin films are measured and compared. The bowtie aperture is able to provide a nanometer-sized optical spot when the incident light is polarized across the bowtie gap and delivers a considerable amount of light. Localized surface plasmons are clearly observed in the near-field images for both bowtie and rectangular apertures in gold, but invisible in chromium. Finite-difference time-domain calculations reveal that, depending on the polarization of the incident light, the unique optical properties of the bowtie aperture are a result of either the optical waveguide and the coupled surface plasmon polariton modes existing in the bowtie gap or the coupling between the two open arms of the bowtie aperture.

Influence of the interaction between metal particles on optical properties of Ag-Si_3N_4 composite films I Experiment

Ag-Si3N4 composite films with metal fractions of 20% and 33% were prepared by magnetron sputtering. The Ag particles inside the composite films were less than 5 nm in radius. The optical absorption and near-field enhancement properties of the composite films were measured. On the plots of optical absorption, the extra absorption was observed as well as the surface plasmon resonance absorption. The intensity of the extra absorption depended on the particle size and metal fraction. Moreover, the composite films with higher extra absorption had better near-field enhancement properties. These phenomena were possibly related with the interaction between Ag particles in composite films.

Influence of the interaction between metal particles on optical properties of Ag-Si_3N_4 composite films II Two-particle approximation

The interaction between metal particles was calculated based on the discrete dipole method. The calculated results revealed that the multiple interactions in a collection of metal particles are dominated by the interaction between the neighboring particles. Thus, a two-particle approximation was deduced and applied to estimate the optical absorption and near-field enhancement property of Ag-Si3N4 composite films. The calculated results indicated that the interaction between Ag particles will induce extra absorption near the surface plasmon resonance peak and improve the near-field enhancement property of composite films. Calculated results were consistent with the experimental results.

Direct, parallel nanopatterning of silicon carbide by laser nanosphere lithography

A technique to create nanopatterns on hard-to-machine bulk silicon carbide (SiC) with a laser beam is presented. A monolayer of silica (SiO2) spheres of 1.76-µm and 640-nm diameter are deposited on the SiC substrate and then irradiated with an Nd:YAG laser of 355 and 532 nm. The principle of optical near-field enhancement between the spheres and substrate when irradiated by a laser beam is used for obtaining the nanofeatures. The features are then characterized using a scanning electron microscope and an atomic force microscope. The diameter of the features thus obtained is around 150 to 450 nm and the depths vary from 70 to 220 nm.

Enhanced resolution induced by random silver nanoparticles in near-field optical disks

The super-resolution near-field structure (super-RENS) is a high-density near-field optical data storage medium which can achieve superior spatial resolution beyond the diffraction limit. Our previous studies found that enhanced local optical intensity occurred at the near fields of super-RENS disks, and the nonlinear near-field optical enhancement is related to the localized surface plasmons of silver clusters dissociated from the AgOx layer in the super-RENS disks. In this paper, we studied the near-field and far-field properties of AgOx-type super-RENS with different distributions of silver nanoparticles using finite-difference time-domain (FDTD) simulations. Highly localized enhancements are found between adjacent silver nanoparticles in the near fields. The far-field signals of different silver nanoparticles distributions confirm the super-resolution capability of AgOx-type Super-RENS disks, and a simplified Fourier optics model is used to describe the relation between highly localized near-field distributions and enhanced resolution of far-field signals.

Laser-assisted nanopatterning of aluminium using particle-induced near-field opticalenhancement and nanoimprinting

Laser-assisted nanopatterning of aluminium (Al) thin films using particle-induced near-field opticalenhancement and nanoimprinting has been investigated experimentally and theoretically. It isfound that nano pit arrays can be created on Al surfaces by laser irradiation (KrF excimer laser,λ = 248 nm) on an Al surface on which a monolayer of silica particles has been self-assembled.The influence of particle size and laser fluence on the structuring of Alsurfaces has been examined. Particles with various diameters of 0.97, 2.34 and5.06 µm were used in the experiment. Near-field optical enhancement and nanoimprinting wereidentified to explain the mechanisms for the formation of different structures on Al surfacesunder different laser fluences. A high frequency structure simulator (HFSS) was used tosimulate the optical field distribution in the particles attached on Al surfaces.

Optical near-field enhancement around lithographic metallic nanostructures using an azo-dye polymer: direct observation and realization of sub-wavelength complex structures

ABSTRACTWe report on the direct observation of optical near-field enhancement around metallic nanoparticles. We used an easy to set up approach which consists in irradiating a photosensitive azo-dye polymer film spin-coated on metallic nanostructures. Photoinduced topographical modifications of the polymer film surface are characterized after irradiation by atomic force microscopy (AFM). Comparisons between AFM images and numerical simulations show that these photo-induced topography agrees with the near-field intensity distribution around the nano-structures. The possibility of generating complex structures is demonstrated.

Metal-aperture surface emitting laser with nano metal particle for near-field optics

We demonstrate the optical near-field enhancement of a metal-aperture GaAs VCSEL with a nanometer-size Au particle. We achieved a record near-field intensity estimated from far-field measurements. The far-field output power from the nano-aperture was increased by a factor of 1.8 by inserting an Au nano-particle. The optical near-field was localized with a near-field power density of 5.7mW/µm2 and a spot size of 280nm. Also, we could avoid the polarization dependence of metal-aperture VCSELs by using a symmetric-shaped nano-particle in a metal aperture. This significant improvement may enable us to use the nano-aperture VCSEL for high-density optical storages.

Surface plasmon resonance on microaperture vertical-cavity surface-emitting laser with metal grating

We proposed a microaperture vertical-cavity surface-emitting laser (VCSEL) with metal grating for the enhancement of optical near-field induced by surface plasmon resonance. We carried out the two-dimensional modeling on the near-field intensity at a microaperture surrounded by a grating in a silver film. An increase in throughput through the microaperture was obtained by the control of a surface plasmon mode, which is strongly dependent on some structural parameters of the grating, such as the period and the depth of a grating, and so on. We also fabricated a microaperture VCSEL with concentric circular grating, exhibiting a large enhancement in peak intensity. The intensity is eight times larger than that of a single-aperture device without grating.

Optical near-field enhancement at a metal tip probed by a single fluorophore

The optical near-field in the vicinity of a metal tip is mapped using a single-molecule optical probe. We observe an enhancement of the fluorescence signal by a factor of 5.7±0.3, clearly larger than the fourfold enhancement that can arise from constructive interference if the tip acts as a simple mirror. Considering the tip apex as a nanoparticle of nonregular shape, we suggest that, in the case of gold tips, the enhancement is due to resonant plasmon excitation. Consistently, no enhancement has been observed using Pt/Ir tips.

Near-field optics: Direct observation of the field enhancement below an apertureless probe using a photosensitive polymer

We report the direct observation of the optical near-field enhancement at the nanometric extremity of a metallic probe for apertureless scanning near-field optical microscopy. Our approach consists in making the “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end via a photosensitive polymer. This distribution is coded by polymer surface topography which is characterized in situ by atomic force microscopy using the same probe. Results clearly reveal nanometric dots corresponding to local field enhancement below the tip end. The field enhancement is shown to be crucially dependent on the polarization state of the incident laser beam as well as the tip material and geometry. The experimental results are found to agree with the results of preliminary calculations. This experiment both constitutes a useful tool for investigating field enhancement below apertureless probes and has potential applications in nanophotolithography.