Modeling columnar thin films as platforms for surface–plasmonic–polaritonic optical sensing

Via exploitation of surface plasmon polaritons (SPPs), columnar thin films (CTFs) are attractive potential platforms for optical sensing as their relative permittivity dyadic and porosity can be tailored to order. Nanoscale model parameters of a CTF were determined from its measured relative permittivity dyadic, after inverting the Bruggeman homogenization formalism. These model parameters were then used to determine the relative permittivity dyadic of a fluid-infiltrated CTF. Two boundary-value problems were next solved: the first relating to SPP-wave propagation guided by the planar interface of a semi-infinitely thick metal and a semi-infinitely thick CTF, and the second to the plane-wave response of the planar interface of a finitely thick metallic layer and a CTF in a modified Kretschmann configuration. Numerical studies revealed that SPP waves propagate at a lower phase speed and with a shorter propagation length, if the fluid has a larger refractive index. Furthermore, the angle of incidence required to excite an SPP wave in a modified Kretschmann configuration increases as the refractive index of the fluid increases.     

Surface-plasmon-polariton waves guided by the uniformly moving planar interface of a metal film and dielectric slab

We explored the effects of relative motion on the excitation of surface-plasmon-polariton (SPP) waves guided by the planar interface of a metal film and a dielectric slab, both materials being isotropic and homogeneous. Electromagnetic phasors in moving and non-moving reference frames were related directly using the corresponding Lorentz transformations. Our numerical studies revealed that, in the case of a uniformly moving dielectric slab, the angle of incidence for SPP-wave excitation is highly sensitive to (i) the ratio β of the speed of motion to speed of light in free space and (ii) the direction of motion. When the direction of motion is parallel to the plane of incidence, the SPP wave is excited by p-polarized (but not s-polarized) incident plane waves for low and moderate values of β, while at higher values of β the total reflection regime breaks down. When the direction of motion is perpendicular to the plane of incidence, the SPP wave is excited by p-polarized incident plane waves for low values of β, but s-polarized incident plane waves at moderate values of β, while at higher values of β the SPP wave is not excited. In the case of a uniformly moving metal film, the sensitivity to β and the direction of motion is less obvious.     

Direct observation of a propagating, planar-waveguide surface mode in a discontinuous film of metal nanoparticles

We report the observation of p-polarized guided waves that propagate confined to the surface of a two-dimensional array of silver (Ag) nanoparticles of average particle diameter and film thickness of approximately 400 and 154 nm, respectively, and comparable interparticle spacing. We interpret resonant features in the attenuated total reflection angular spectrum as arising from the excitation of guided waves in our discontinuous samples. The excitation of these waves is a direct consequence of the interaction of the light field with the localized resonance of the conduction electrons in the individual metal nanoparticles.     

Surface-plasmon-assisted electromagnetic field enhancement in semiconductor quantum dots

The temporal development of incident electromagnetic plane waves across semiconductor quantum dots (QDs) is analyzed by the finite-difference time-domain method. By coating the QDs using thin metal films, surface plasmon polaritons (SPPs) can be created. As illustration, our modeling approach is applied to fluorescent multiphoton quantum dots made of cadmium sulphide of particular size (3.7 nm) and energy band gap (2.67 eV). When such a QD is coated by a metal film, a dipole-formed SPP is generated at the external surface of the coated QD by the incident electromagnetic wave with a photon energy of 1.34 eV corresponding to a two-photon process. When the thickness of the metal film is 0.37 nm, the peak intensity of the SPP oscillates through both the thin metal film and the core QD, resulting in an electromagnetic field inside the QD enhanced by a factor of 10, and thus an increased two-photon excitation that can be useful for bioimaging applications. Further increasing the metal film thickness blockades the SPP initially generated at the external surface of the coated QD from penetrating through the metal film, reducing the electromagnetic field inside the QD. PACS 73.22.-f; 78.67.Hc     

薄膜和半无限大介质间的近场辐射换热研究

研究了硼掺杂硅(记为Si-19)薄膜和半无限大物体(Si-19和SiC)在100 nm真空间距下的近场辐射换热随薄膜厚度的变化。研究结果表明,当半无限大物体和薄膜为相同的Si-19材料时,由于表面波激发并相互耦合,使得近场辐射换热随薄膜的厚度变化比较复杂。当半无限大物体为SiC材料时,由于表面波的耦合遭到破坏以及辐射体的高发射率频率区和吸收体的高吸收率频率区不匹配,导致表面波的激发对不同材料间的近场辐射换热的增强程度降低,因此在相同计算区域内热流密度随厚度的增加单调增加,没有出现极值点。     

Nonlinear excitation of surface plasmon polaritons by four-wave mixing

We demonstrate nonlinear excitation of surface plasmons on a gold film by optical four-wave mixing. Two excitation beams of frequencies omega(1) and omega(2) are used in a modified Kretschmann configuration to induce a nonlinear polarization at a frequency of omega(4wm)=2omega(1)-omega(2), which gives rise to surface plasmon excitation at a frequency of omega(4wm). We observe a characteristic plasmon dip at the Kretschmann angle and explain its origin in terms of destructive interference. Despite a nonvanishing bulk response, surface plasmon excitation by four-wave mixing is dominated by a nonlinear surface polarization. To interpret and validate our results, we provide a comparison with second-harmonic generation.     

Giant bistable lateral shift owing to surface-plasmon excitation in Kretschmann configuration with a Kerr nonlinear dielectric

We investigate the reflection of a TM-polarized light beam from a Kretschmann configuration with a Kerr nonlinear dielectric. It is found that there exists a hysteretic response between the lateral shift of the reflected beam and the intensity of the incident beam. In contrast to the lower switch threshold, the higher switch threshold of optical bistability is more sensitive to the variations of the angle of incidence and the thickness of metal film. It is also found that the peak value of the lateral shift is strongly dependent on the thickness of metal film.     

On the Focusing of Surface Plasmonic Waves on the Free Surface of a Metal Film

Optics and Spectroscopy - The excitation and focusing of a surface plasmonic wave on the free surface of a metal film in the Kretschmann scheme have been considered based on the theory of...     

Metallic rugate structures for near-perfect absorbers in visible and near-infrared regions

Metallic rugate structures are theoretically investigated for achieving near-perfect absorption in the visible and near-infrared regions. Our model builds on nanoporous metal films whose porosity (volume fraction of voids) follows a sinewave along the film thickness. By setting the initial phase of porosity at the top surface as 0, near-perfect absorption is obtained. The impacts of various structural parameters on the characteristic absorption behaviors are studied. Furthermore, multiple peaks or bands with high absorption can be achieved by integrating several periodicities in one structure. The rugate absorbers show near-perfect absorption for TE and TM polarizations and large incident angles.     

Nonlinear s-polarized surface plasmon polaritons

We have examined the dispersion relations for s-polarized surface plasmon polaritons, guided by (a) the interface between a semi-infinite metal and dielectric medium, and (b) a metal film bounded by semi-infinite dielectric media for situations in which one or more of the dielectric media are characterized by an intensity-dependent refractive index. We found that s-polarized waves satisfy the dispersion relations for very thin metal films bounded by nonlinear dielectric media. These waves exist only for power levels above a threshold that depends on the material parameters. We also comment on the experimental feasibility of observing these waves.     

Surface-plasmon wave at the planar interface of a metal film and a structurally chiral medium

The solution of a boundary-value problem formulated for a modified Kretschmann configuration shows that a surface-plasmon wave can be excited at the planar interface of a sufficiently thin metal film and a nondissipative structurally chiral medium, provided the exciting plane wave is p-polarized. An estimate of the wavenumber of the surface-plasmon wave also emerges thereby.     

Simultaneous excitation of cavity resonance and surface plasmon resonance in Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Ag layer structure

Simultaneous excitation of cavity resonance (CR) and surface plasmon resonance (SPR) was observed in the angular spectrum by substituting Ag/Al₂O₃/Ag layers for the metal film in a Kretschmann structure. Two reflective valleys, elicited respectively by CR and SPR, appeared at different positions in the angular spectrum. The former is the sum of enhanced transmission of CR and absorption of the metal, expressed in the reflection spectrum and extremely insensitive to the changes of the surface environment (refractive index). The latter behavior is like that when two metal films are stuck together: it has almost the same resonance depth and width, and is extremely sensitive to the changes of the surface environment. Moreover, two SPR peaks could be excited simultaneously at one angle but with different wavelengths in the frequency spectrum, which is not seen in traditional Kretschmann structures.     

Investigation of the near field distribution in circular nanostructures using Stokes polarization states

In this paper, the near field distribution patterns formed from nanostripe corral and half spiral are investigated. Various near field distribution patterns are generated owning to the interference of propagating surface plasmon waves that emerged from the nanoslits or nanostripe. The half spiral nanoslits are illuminated with Stokes polarizations. Each polarization state shows a different field pattern at different locations on the surface of metal film. This is due to the excitation of surface plasmon waves at different parts of the nanostructures when illuminated with different types of polarization states. The same Stokes polarization states are also illuminated on a nanostripe corral structure. In this case, dipolar field distributions are observed when illuminated with different linear polarization states, while optical vortices are observed for circular polarization. It is believed that these interesting field patterns due to different arrangements of nanostructures could be used for near field imaging and polarization sensing.     

Theoretical model of a planar waveguide refractive index sensor assisted by a corrugated long period metal grating

A theoretical model of a novel planar integrated refractive index sensor based on surface plasmon-polariton (SPP) excitation with a corrugated metal long period grating (LPG) is presented and comprehensively investigated. The main principle of the operation this device is based on co-directional energy transfer by means of a corrugated metal LPG between a p-polarized guided mode propagating in a waveguide layer and the SPP propagating a metal layer separated from the waveguide layer by a buffer. The corrugated LPG is engraved in the metal layer in contact with the sensed medium. The power transmitted through the LPG in the guided mode serves as an input signal for an interrogation unit. This device is free from any moving parts and can be simply integrated into any planar waveguide system. Our sensor simulations are based on the local-normal-mode transfer matrix method and performed in telecom wavelength range.     

表面等离子体共振控制镜面反射率

对金属介电常数随温度变化的计算进行了修正,提出了利用表面等离子体共振(SPR)实现温度控制镜面反射率的方法。在Kretschmann结构中的金属膜上涂覆热光系数较大的聚合物材料,考虑该结构中各种材料介电常数以及金属膜厚度随温度的变化,利用特征矩阵法进行了数值模拟,得到SPR反射率曲线随温度的变化。模拟结果显示,当波长为532nm的p偏振光分别以70°和75°入射时,在10℃～90℃范围内调节温度,可实现反射率在52.8%～41.5%和31.1%～18.8%范围内的调节。     

Gradient-index narrow-bandpass filter fabricated with glancing-angle deposition

Glancing-angle deposition (GLAD) is a fabrication method capable of producing thin films with engineered nanoscale porosity variations. GLAD can be used to create optical thin-film interference filters from a single source material by modification of the film refractive index through control of film porosity. We present the effects of introducing a layer of constant low density into the center of a rugate thin-film filter fabricated with the GLAD technique. A rugate filter is characterized by a sinusoidal refractive-index profile. Embedding a layer of constant refractive index, with a thickness equal to one period of the rugate index variation, causes a narrow bandpass to appear within the filter's larger stop band. Transmittance measurements of such a gradient-index narrow-bandpass filter, formed with titanium dioxide, revealed an 83% transmittance peak at a vacuum wavelength of 522 nm, near the center of the stop band, with a FWHM bandwidth of 15 nm.     

Characterization of polarization states of surface plasmon polariton modes by Fourier-plane leakage microscopy

We demonstrate that the polarization states of guided wave surface plasmon polariton (SPP) modes can be unambiguously identified by introducing a linear polarizer in the optical path of the light within a leakage-based microscope. We show the use of Fourier-plane leakage-based microscopy as a polarization characterization method to study the polarization states of SPP modes excited in plasmonic waveguides. Our results indicate that the inclusion of a linear polarizer provides additional image processing capabilities to leakage-based microscopes.     

Electromagnetic effect in enhanced infrared absorption of adsorbed molecules on thin metal films

The electric field strength within a homogeneous metal layer on a Ge hemi-cylindrical prism in the Kretschmann configuration has been calculated for infrared light using the Fresnel formula. To approximate the non-homogeneous island-like or porous nature of very thin metal films the dielectric constant of the metal layer was derived by the Bruggeman effective medium approximation. The dependence of the calculated intensity on the film thickness and the angle of incidence is in good agreement with the experimental observations of enhanced infrared absorption of adsorbed species. Combined with the observed short range effect of the enhancement this implies that the enhancement is due to actual high electric fields within the pores or between islands of the metal film which are proportional to the calculated effective field.     

Morphological effects on surface-plasmon-polariton waves at the planar interface of a metal and a columnar thin film

A theoretical study predicts that surface-plasmon-polariton (SPP) waves may propagate along the interface of a columnar thin film (CTF) and a metal over a range of propagation directions relative to the morphology of the CTF. The range of propagation directions depends on the tilt of the columns in the CTF. The phase speed of the SPP wave varies mainly as a function of the tilt of the CTF columns. Both the confinement of the SPP wave to the interface and the decay of the SPP wave along the direction of propagation depend strongly on the direction of propagation relative to the morphologically significant plane of the CTF. The greater the columnar tilt in relation to the interface, the shorter is the range of propagation. Because of CTF porosity and the ability to engineer this biaxial dielectric material, the CTF–metal interface may be more attractive for sensor applications than the traditional dielectric–metal interface used for SPP-wave-based sensors.     

Excitation of spin-helicon waves in sodium and potassium

The excitation of bound spin-helicon waves in the electron Fermi liquid of alkaline metals with variable field incident on a plane surface is analysed. The practicability of a spin-wave resonance in a semi-infinite metal (but not in a thin film, as usual) is demonstrated. The reflection coefficient is calculated and the resonance conditions are found.