Near-field optical observations of surface plasmon wave interference at subwavelength hole arrays perforated in Au film

We image optical near-field patterns at subwavelength circular hole arrays in Au film by using scanning near-field optical microscopy in near-infrared wavelengths.Periodical oscillation features are found in the near-field images at the air/Au interface and exhibit two typical kinds of standing wave oscillation forms at the wavelengths corresponding to the transmission minimum and maximum in the transmission spectrum,and the latter one originates from the excitation and interference of a surface plasmon wave at the metallic hole arrays.Our work indicates that monitoring optical near-field patterns can help to reveal many interesting properties of surface plasmon waves at metallic nanostructures and understand their underlying physical mechanisms.     

Lattice plasmon mode excitation via near-field coupling

The optical response of metal nanoparticles can be modified through near-field or far-field interaction,yet the lattice plasmon modes(LPMs)considered can only be excited from the latter.Here instead,we present a theoretical evaluation for LPM excitation via the near-field coupling process.The sample is an arrayed structure with specific units composed of upper metal disks,a lower metal hole and a sandwiched dielectric post.The excitation process and underlying mechanism of the LPM and the influence of the structure parameters on the optical properties have been investigated in detail by using a finite-difference time-domain(FDTD)numerical method.Our investigation presented here should advance the understanding of near-field interaction of plasmon modes for LPM excitation,and LPMs could find some potential applications,such as in near-field optical microscopes,biosensors,optical filters and plasmonic lasers.     

Mid-infrared optical resonances in quantum dot photodetectors coupled with metallic gratings with different aperture diameters

The paper is devoted to optical testing of mid-infrared Ge/Si photodetectors obtained by stacking of self-assembled Ge quantum dots in multilayer structures, which are near-field coupled to the adjacent nanoplasmonic arrays of subwavelength holes in metallic films. It is shown that photocurrent and near-field spectra consist of several sets of peaks, which are attributted to surface plasmon waves, localized surface plasmon modes or diffractive Rayleigh anomaly depending on the hole diameter and the angle of incidence θ. We find that for small holes the greatest contribution to the photocurrent enhancement is due to the excitation of the surface plasmon-polariton waves for all θ. As the hole diameter is increased and becomes comparable with the array periodicity, the normal-incident photoresponse improvement is provided by the Rayleigh anomaly. With the increase of incident angle, the photocurrent enhancement is supposed to arise from coupling of the localized shape resonance and propagating plasmon modes.     

Coexistence of localized and delocalized surface plasmon modes in percolating metal films

Near-field intensity statistics in semicontinuous silver films over a wide range of surface coverage are investigated using near-field scanning optical microscopy. The variance of intensity fluctuations and the high-order moments of intensity enhancement exhibit local minima at the percolation threshold. This reduction in local field fluctuations results from resonant excitation of delocalized surface plasmon modes. By probing the modification of the critical indices for high-order moments of intensity enhancement caused by the delocalized states, we provide the first experimental evidence for the coexistence of localized and delocalized surface plasmon modes in percolating metal films.     

Ultrafast dynamics of surface plasmon polaritons in plasmonic metamaterials

Using near-field scanning optical microscopy and ultrafast laser spectroscopy, we study the linear optical properties of subwavelength nanoslit and nanohole arrays in metal films, which are prototype structures for novel plasmonic metamaterials. Near-field microscopy provides direct evidence for surface plasmon polariton (SPP) excitation and allows for spatial imaging of the corresponding SPP modes. By employing spectral interferometry with ultrashort 11-fs light pulses, we directly reconstruct the temporal structure of the electric field of these pulses as they are transmitted through the metallic nanostructures. The analysis of these data allows for a quantitative extraction of the plasmonic band structure and the radiative damping of the corresponding SPP modes. Clear evidence for plasmonic band gap formation is given. Our results reveal that the coherent coupling between different SPP modes can result in a pronounced suppression of radiative SPP damping, increasing the SPP lifetime from 30 fs to more than 200 fs. These findings are relevant for optimizing and manipulating the optical properties of novel nano-plasmonic devices. PACS 42.70.Qs; 07.79.Fc; 42.25.-p     

Phase singularity of surface plasmon polaritons generated by optical vortices

We demonstrate an experimental result that shows the phase singularity of surface plasmon waves generated by the direct transform of optical vortices at normal incidence focused on a structureless metal surface. The near-field two-dimensional intensity distribution near the focal plane is experimentally examined by using near-field scanning optical microscopy and shows a good agreement with the finite-difference time-domain simulation result. The experimental realization demonstrates a potential of the proposed excitation scheme to be reconfigured locally with advantages over structures milled into optically thick metallic films for plasmonics applications involving plasmonic vortices.     

Transmission enhancement of Ag nanoparticle aggregates in azo-polymer films

Ag nanoparticle aggregates in azo-polymer films were investigated by using scanning near-field optical microscopy. The near-field optical images show that transmission enhancement happened for these metal aggregates. Far-field experiment results show that the photo-induced isomeration speed of the azo-polymer molecules was enhanced when doped with Ag nanoparticle aggregates. The mechanism of the transmittance enhancement and speed enhancement was discussed from the viewpoint of the excitation of the surface plasmon of Ag nanoparticle aggregates. PACS 68.37.Uv; 42.70.Jk; 78.67.Bf; 71.36.+c     

Rigorous transmittance analysis of a sub-wavelength Sb thin film lens

We quantitatively analysed the factors contributing to the optical transmission enhancement of a sub-wavelength Sb thin film lens, using the finite-difference time-domain (FDTD) method. The results show that the transmission enhancement of the dielectric with a Gaussian distributed refractive index loaded in a sub-wavelength circular hole is not only due to the high refractive index dielectric, but also due to the specific distributions of refractive index. It is the first study about the effects of the refractive index distribution on the transmission of a sub-wavelength aperture. This kind of lens has practical applications in the very small aperture lasers and for near-field optical storage and lithography. PACS 42.79.Ag; 42.25.Bs; 42.25.Fx     

Surface plasmon mediated near-field imaging and optical addressing in nanoscience

We present an overview of recent progress in "plasmonics". We focus our study on the observation and excitation of surface plasmon polaritons (SPPs) with optical near-field microscopy. We discuss in particular recent applications of photon scanning tunnelling microscope (PSTM) for imaging of SPP propagating in metal and dielectric wave guides. We show how near-field scanning optical microscopy (NSOM) can be used to optically and actively address remote nano objects such as quantum dots. Additionally we compare results obtained with near-field microcopy to those obtained with other optical far-field methods of analysis such as leakage radiation microscopy (LRM).     

Fabrication of a Near-Field Optical Fiber Probe with a Nanometric Metallized Protrusion

We have developed a novel probe with a nanometric metallized protrusion extending through a subwavelength aperture to increase optical near-field excitation and collection efficiencies. The apex diameter of the fabricated metallized protrusion was 35 nm. The Intensity distribution of the optical near-field at the apex of the probe was measured by scanning another probe across the apex, and it was observed that strong optical near-field was generated at the apex of the metallized protrusion. The width of the intensity distribution was 150 nm including instrumental resolution. Probes with spherical and ellipsoidal metallized protrusion were also fabricated, by which enhancement of the optical near-field is expected due to localized plasmon excitation.     

Near-field spectroscopy of surface plasmons in flat gold nanoparticles

We use near-field interference spectroscopy with a broadband femtosecond, white-light probe to study local surface plasmon resonances in flat gold nanoparticles (FGNPs). Depending on nanoparticle dimensions, local near-field extinction spectra exhibit none, one, or two resonances in the range of visible wavelengths (1.6-2.6 eV). The measured spectra can be accurately described in terms of interference between the field emitted by the probe aperture and the field reradiated by driven FGNP surface plasmon oscillations. The measured resonances are in good agreement with those predicted by calculations using discrete dipole approximation. We observe that the amplitudes of these resonances are dependent upon the spatial position of the near-field probe, which indicates the possibility of spatially selective excitation of specific plasmon modes.     

The influence of wire shape on surface plasmon mode distribution

The influence of the nanowire shape on the excitation of surface plasmon polaritons at metallic nanowire arrays is studied numerically. For a system of silver nanowires housed on a polymer substrate, nanowires with rectangular and elliptical cross sections are compared. It was found that in the case of rectangular nanowires the excitation efficiency is higher for surface plasmons at the polymer–metal interface than for surface plasmons at the air–metal interface. Conversely, in the case of elliptical nanowires the air–metal plasmon modes are stronger. Further, it is noted that the nanowire shape directly influences the position of the surface plasmon resonance.     

Surface plasmon delocalization by short-range correlations in percolating metal systems

Theoretical and experimental studies of the electromagnetic response of semicontinuous metal films are presented. A scaling theory is developed by solving the surface plasmon eigenproblem. The short-range correlations in the governing Kirchhoff Hamiltonian result in delocalization of the surface plasmon eigenmodes. Although their relative weight in the spectrum becomes asymptotically small for large systems, the existence of these delocalized states modifies the critical indices for the high-order field moments. This modification is confirmed experimentally by the near-field measurement of high-order intensity moments for percolating silver films on glass substrates. PACS 68.37.Uv; 73.20.Mf; 42.25.Dd; 78.67.-n     

二维亚波长金属小孔阵列的透射光增强效应

对金属薄膜上的二维亚波长小孔阵列的光透射增强现象进行了数值模拟，结果显示不仅实际金属薄膜上的小孔阵列结构具有透射增强效应，理想导体薄膜的相同结构也具有透射增强效应，但没有实际金属薄膜的增强明显-通过分析指出，这种小孔阵列的光透射增强效应是一种复杂的波导耦合效应-与金属薄膜上的表面电流一样，表面等离激元波具有将入射光能量从金属表面向小孔转移的作用，但不是透射增强的本质原因- 关键词： 表面电流 共振耦合 透射增强 表面等离激元     

Sub-wavelength probing and modification of photonic crystal nano-cavities

In this work we present a sizeable and reversible spectral tuning of the resonances of a two-dimensional photonic crystal nano-cavity by exploiting the introduction of a sub-wavelength size glass tip. The comparison between experimental near-field data and results of numerical calculations shows that the spectral shift induced by the tip is proportional to the local electric field intensity of the cavity mode. This observation proves that the electromagnetic local density of states in a microcavity can be directly measured by mapping the tip-induced spectral shift with a scanning near-field optical microscope. Moreover, a non-linear control on the cavity resonance is obtained by exploiting the local heating induced by near-field laser excitation at different excitation powers. The temperature gradient due to the optical absorption results in an index of refraction gradient which modifies the dielectric surroundings of the cavity and shifts the optical modes.     

Optical bistability in nonlinear subwavelength metal hole array involving Kerr materials

We investigate an optical bistability phenomenon in a structure consisting of a silver film with a hole array filling with Kerr medium with single wavelength. We explore the mechanism of optical bistability in our structures and find that the excitation condition of surface plasmon at this moment is related with that at last moment. The contrast of optical bistability can reach to the maximum at the wavelength of the transmission peak. Our structure can realize the function of the beam splitter.     

Plasmonic waveplate: incident polarization modulation

Abstract We demonstrate that the rectangular nanohole arrays perforated in a 100 nm gold film can be used to tune the polarization direction of the transmitted light with maximum rotation angle of about 30 degrees. Theoretical analysis with the three-dimensional finite-difference time-domain simulations indicates that this phenomenon is attributed to the excitation of the surface plasmon wave on the gold film surface and the resonance of localized surface plasmon in the hole. With multiple plasmon resonances, the plasmonic waveplate can realize multi-wavelength polarization modulation. Our results may be useful to understanding the physical mechanism of enhanced plasmon mediated transmission and potential applications in plasmonic optical components.     

Spectral scattering properties of a nanohole in a noble-metal film in the evanescent waves area

The effect of enhanced optical transmission through subwavelength holes and their arrays is used for multiple practical applications especially in optical antennas and local biosensors design. This effect is usually considered under excitation of plane wave propagating at the normal direction to the screen surface. In this work the effect of extreme transmission through the hole in the evanescent wave's area is in focus. The discrete sources method has been applied to analyse the spectral characteristics of light scattered by a cylindrical nanohole in a noble-metal film on a prism surface. The influence of the wavelength, incident angle, film materials and hole's filling on the scattering characteristics has been investigated. A close correlation between the effect of extreme transmission and the surface plasmon resonances has been detected.     

Near-field plasmonic coupling for enhanced nonlinear absorption by femtosecond pulses in bowtie nanoantenna arrays

Plasmonic bowtie nanoantennas (BNAs) can exhibit a strong enhancement of optical field, leading to large nonlinear effects. We investigated the nonlinear optical absorption of an array of BNA by femtosecond pulses, using the open-aperture Z-scan technique. The BNA array composed of paired gold nanotriangles was fabricated by nanosphere lithography. We experimentally demonstrated that upon decreasing the gap width, nonlinear absorption is enhanced due to both the enhancement of near-field coupling of nanoantennas and the minimum of the spectral detuning between the center wavelength of the laser for excitation and the localized surface plasmon resonances. The role of near-field resonant plasmonic coupling in BNA is analyzed theoretically and confirmed by our simulations.     

Carrier-diffusion-limited remote optical addressing of single quantum dots

We present a scheme for remotely addressing single quantum dots (QDs) by means of near-field optical microscopy that simply makes use of the polarization of light. A structure containing self-assembled CdTe QDs is covered with a thin metal film presenting sub-wavelength holes. When the optical tip is positioned some distance away from a hole, surface plasmons in the metal coating are generated which, by turning the polarization plane of the excitation light, transfer the excitation towards a chosen hole and induce emission from the underlying dots. In addition, our procedure gives valuable insight into the diffusion of photo-excited carriers in the QD plane that can put limits to the addressing scheme.