Diffraction-based tracking of surface plasmon resonance enhanced transmission through a gold-coated grating

Surface plasmon resonance enhanced transmission through metal-coated nanostructures represents a highly sensitive yet simple method for quantitative measurement of surface processes and is particularly useful in the development of thin film and adsorption sensors. Diffraction-induced surface plasmon excitation can produce enhanced transmission at select regions of the visible spectrum, and wavelength shifts associated with these transmission peaks can be used to track adsorption processes and film formation. In this report, we describe a simple optical microscope-based method for monitoring the first-order diffracted peaks associated with enhanced transmission through a gold-coated diffraction grating. A Bertrand lens is used to focus the grating's diffraction image onto a CCD camera, and the spatial position of the diffracted peaks can be readily transformed into a spectral signature of the transmitted light without the use of a spectrometer. The surface plasmon peaks appear as a region of enhanced transmission when the sample is illuminated with p-polarized light, and the peak position reflects the local dielectric properties of the metal interface, including the presence of thin films. The ability to track the position of the plasmon peak and, thus, measure film thickness is demonstrated using the diffracted peaks for samples possessing thin films of silicon oxide. The experimental results are then compared with calculations of optical diffraction through a model, film-coated grating using the rigorously coupled wave analysis simulation method.     

Multicolor surface plasmon resonance imaging of ink jet-printed protein microarrays

We report a technique that utilizes surface plasmon resonance dispersion as a mechanism to provide multicolor contrast for imaging thin molecular films. Illumination of gold surfaces with p-polarized white light in the Kretschmann configuration produces distinct reflected colors due to excitation of surface plasmons and the resulting absorption of specific wavelengths from the source light. In addition, these colors transform in response to the formation of thin molecular films. This process represents a simple detection method for distinguishing between films of varying thickness in sensor applications. As an example, we interrogated a protein microarray formed by a commercial drop-on-demand chemical ink jet printer. Submonolayer films of a test protein (bovine serum albumin) were readily detected by this method. Analysis of the dispersion relations and absorbance sensitivities illustrate the performance and characteristics of this system. Higher detection sensitivity was achieved at angles where red wavelengths coupled to surface plasmons. However, improved contrast and spatial resolution occurred when the angle of incidence was such that shorter wavelengths coupled to the surface plasmons. Simplified optics combined with the robust microarray printing platform are used to demonstrate the applicability of this technique as a rapid and versatile, high-throughput tool for label-free detection of adsorbed films and macromolecules.     

Ultraviolet surface plasmon-coupled emission using thin aluminum films

Surface plasmon-coupled emission (SPCE) is the directional radiation of light into a substrate due to excited fluorophores above a thin metal film. To date, SPCE has only been observed with visible wavelengths using silver or gold films. We now show that SPCE can be observed in the ultraviolet region of the spectrum using thin (20 nm) aluminum films. We observed directional emission in a quartz substrate from the DNA base analogue 2-aminopurine (2-AP). The SPCE radiation occurs within a narrow angle at 59 degrees from the normal to the hemicylindrical prism. The excitation conditions precluded the creation of surface plasmons by the incident light. The directional emission at 59 degrees is almost completely p-polarized, consistent with its origin from surface plasmons due to coupling of excited 2-AP with the aluminum. The emission spectra and lifetimes of the SPCE are those characteristic of 2-AP. Different emission wavelengths radiate at slightly different angles on the prism providing intrinsic spectral resolution from the aluminum film. These results indicate that SPCE can be used with numerous UV-absorbing fluorophores, suggesting biochemical applications with simultaneous surface plasmon resonance and SPCE binding assays.     

Evanescent-wave holography by use of surface-plasmon resonance

We report a method for evanescent-wave holography using surface-plasmon resonance from the illumination light. The device we have made consists of three layers: a prism of high refractive index, a thin metallic film, and a grating. Evanescent waves generated by the surface plasmons are diffracted with a prerecorded grating to reconstruct a three-dimensional image. The possibility of white-light illumination and the application to a flat display system with waveguides in the proposed method are discussed.     

Examination of the +1, −1 Surface Plasmon Mini-gap on a Gold Grating

Abstract

An energy gap in the excitation of surface plasmons is found for light at normal incidence to a gold grating. This gap occurs at the crossing of the plus and minus first order surface plasmons. It arises directly as a consequence of distortion of the grating from sinusoidality, the first harmonic of the grating providing coupling between the plus and minus one orders. Experiments have been performed using both wavelength scans, where at a fixed angle of incidence the wavelength of excitation is varied, and angle of incidence scans, where for a fixed wavelength the angle of incidence is varied a few degrees either side of normal to the grating. By fitting the angular dependent reflectivity scans using grating modelling theory the gold grating is characterized at all wavelengths. This then allows a detailed comparison of the theoretical dispersion curve with that obtained experimentally. The agreement for both p-polarized light (for angle dependence with the plane of incidence normal to the grating grooves) and for s-polarized light (angle dependence with the plane of incidence perpendicular to the grating grooves) is excellent. An apparent momentum gap in the lower energy branch of the dispersion curve, attributed to the loss of coupling strength, is found to move to the upper branch if the grating profile is inverted.     

Direct evidence for surface plasmon-mediated enhanced light transmission through metallic nanohole arrays

This paper provides direct evidence for the role of surface plasmons in the enhanced optical transmission of light through metallic nanoscale hole arrays. Near-field optical images directly confirmed the presence of surface plasmons on gold nanohole arrays with interhole spacings larger than the surface plasmon wavelength. A simple interference model provides an intuitive explanation of the two types of fringe wavelengths observed in the near-field optical images. Far-field spectroscopy revealed a surface plasmon band that contributed a factor > 8 to the transmission enhancement. Furthermore, silicon nanohole arrays did not exhibit any features in the near-field, which demonstrates that metallic materials are necessary for enhanced light transmission through nanohole arrays.     

Simultaneous observation of surface plasmons on both sides of thin silver films

Abstract

Both prism and grating coupling have been used to optically excite surface plasmons on the two sides of thin silver films. This allows the optical permittivity of the silver films to be determined simultaneously at the metal/air and the (protected) glass/metal interfaces. It is found that there is a significant difference between the permittivities for the two interfaces which depends upon the thickness of the metal film. This difference is tentatively attributed to the silver films being more porous at the air interface, although an alternative interpretation, based on possible changes in electron density through oxygen absorption by silver, is not excluded.     

Resonance quenching and guided modes arising from the coupling of surface plasmons with a molecular resonance

In this paper, we describe experimental and modeling results that illucidate the nature of coupling between surface plasmon polaritons in a thin silver film with the molecular resonance of a zinc phthalocyanine dye film. This coupling leads to several phenomena not generally observed when plasmons are coupled to transparent materials. The increased absorption coefficient near a molecular resonance leads to a discontinuity in the refractive index, which causes branching of the plasmon resonance condition and the appearance of two peaks in the p-polarized reflectance spectrum. A gap exists between these peaks in the region of the spectrum associated with the molecular resonance and reflects quenching of the plasmon wave due to violation of the resonance condition. A second observation is the appearance of a peak in the s-polarized reflection spectra. The initial position of this peak corresponds to where the refractive index of the adsorbate achieves its largest value, which occurs at wavelengths just slightly larger than the maximum in the molecular resonance. Although this peak initially appears to be nondispersive, both experimental data and optical modeling indicate that increasing the film thickness shifts the peak position to longer wavelengths, which implies that this peak is not associated with the molecular resonance but, rather, is dispersive in nature. Indeed, modeling shows that this peak is due to a guided mode in the film, which appears in these conditions due to the abnormally high refractive index of the film near the absorbance maximum. Results also show that, with increasing film thickness, numerous additional guided modes appear and move throughout the visible spectrum for both s- and p-polarized light. Notably, these guided modes are also quenched near the location of the molecular resonance. The quenching of both the plasmon resonance and the guided modes can be explained by a large decrease in the in-plane wave propagation length that occurs near the molecular resonance, which is a direct result of the film's large absorption coefficient.     

Surface plasmon resonance radiation from germanium

It is well known that when certain reflection gratings which are coated with different metal or dielectric-metal layers are illuminated by p-polarized light surface plasmon resonance radiation appears in the diffraction spectra in the form of polarization anomalies (Wood's anomalies). Surface plasmons can be excited in this way if the real part ε₁ of the dielectric constant of the metal layer has a value less than -1. This technique has been extended to semiconductors in the present work by coating a grating with Ge and investigating the diffracted light in the 4–6 eV region, where ε₁ for Ge is less than -1. Experimental results are presented which show that certain peaks in the observed spectra may be due to the expected resonance radiation.     

Effects of fibre grating parameters on the spectral characteristics of cascaded long- and short-period gratings

Using coupled-mode theory combined with the transfer matrix method, this paper presents the impact of fibre grating parameters(grating period, grating length and average index modulation) and the separation between long-period fibre grating and fibre Bragg grating (FBG) on the reflection spectra of cascaded long-and short-period grating (CLBG). Two grating periods and two grating lengths have only impact on the two reflection peaks. The two resonance wavelengths shift towards the longer wavelength when the period of FBG decreases. Further, we study the sensing characteristics of coated CLBG. By analysing the relationship between the refractive index sensitivity and the thin film parameters (film refractive index and film thickness), the optimized parameters for the coated CLBG used as film sensor were obtained. Data simulation shows that the resolution of the refractive index of this coated CLBG film sensor is predicted to be 10^?6. This implies that the coated CLBG has an important application value in sensing and measurement field.     

Photopolarimeter based on two gratings recorded in thin organic films

A photopolarimeter based on two different kinds of diffraction gratings (a two-grating photopolarimeter) has been developed for real-time measurements of the four elements of the Stokes vector. The main elements of the device are a pure polarization grating and an ordinary transmission grating, both recorded by means of holographic techniques in thin films of organic materials. The first one consists of a diffraction grating recorded by two interfering opposite circularly polarized beams in a Langmuir-Blodgett film of an azo-compound material. The second component is a grating recorded by two interfering parallel circularly polarized beams in a thin film of a photosensitive polymer. Both gratings offer long time stability and good diffraction efficiency. Four photodiodes collect the first-order diffracted beams from these gratings, the output signals of which are read through an analog-to-digital converter by a PC. The optical alignment of the device is easy and the calibration is realized in a one-step procedure.     

Optical and photoluminescence studies of gold nanoparticles embedded ZnO thin films

Gold nanoparticles (AuNPs) embedded ZnO thin films were prepared by sandwiching a thin thermally evaporated Au film between two sputtered ZnO films. The films were characterized by high resolution transmission electron microscopy (HRTEM), glancing angle X-ray diffraction (GXRD), optical absorption and photoluminescence (PL) measurements. GXRD data exhibited peaks which were attributed to the reflections from various ZnO and Au planes. Size dependence of the plasmon absorption was studied by forming nanoparticles with various sizes. Optical absorption spectra showed strong absorption due to localized surface plasmons at about 608, 638 and 676 nm for films having average AuNPs sizes of 27, 40 and 67 nm respectively. AuNPs embedded ZnO film showed a strong reduction in the intensity of photoluminescence, which was prominent in the case of pure ZnO film. The rise in temperature at a single nanoparticle site was calculated to be 22 K for a particle size of 80 nm.     

Grating-coupler assisted infrared reflection absorption spectroscopy for the characterization of organic thin films

We demonstrate how grating-coupler assisted infrared reflection absorption spectroscopy can be used to simultaneously determine the chemical identity and relative thickness of organic thin films. With a grating substrate, a threshold anomaly associated with passing off of the -1 diffracted order occurs at grazing angles of incidence, resulting in a sharp absorbance in the infrared. The position of this peak is sensitive to the grating geometry as well as the dielectric environment near its surface. Thus, shifts in the peak position can be used to determine the relative thickness of adsorbed films or quantify molecular adsorption events. To illustrate the characteristics and sensitivity of this phenomenon, several samples were prepared and tested, including self-assembled alkanethiolate monolayers with 11-mercaptoundecanoic acid, 11-mercapto-1-undecanol, decanethiol, and a covalently linked layer of bovine serum albumin on a commercial, gold-coated grating. For these samples, the position of the threshold absorbance peak shifted to lower wavenumbers as film thickness increased, which is consistent with calculated shifts based upon an increasing refractive index at the interface. The sensitivity of this shift was measured to be 3.7 cm(-1) nm(-1). These results illustrate how a grating substrate can be exploited in a standard infrared reflectance measurement to provide additional information about the relative thickness of adsorbed surface films.     

Broadband Excitation and Emission of Surface Plasmons

Abstract

The well known advantages of using surface plasmons, in particular the high sensitivity to surface adsorbates, are nearly always compromised in practice by the use of monochromatic excitation and the consequent lack of proper spectroscopic information. This limitation arises from the angle/wavelength selective nature of the surface plasmon resonance. The work described here uses an elegant broadband excitation/decay scheme in a substrate(silica)-grating profiled photoresist-Ag film geometry. Laser radiation of wavelength 488 nm, incident through the silica substrate, excites by near-field coupling a broad band of surface plasmons at the photoresist-Ag interface within the spectral range of the photoresist fluorescence. With a judicious choice of grating period this mode can cross-couple to the mode supported at the Ag-air interface. This latter mode can, in turn, couple out to light by virtue of the same grating profile. The spectral distribution of the light emitted due to this three-step process has been studied as a function of the angle of emission and depth of the grating profiled surface for each polarization. It is found that the optimum emission efficiency occurs with a groove depth in the region of 65 nm. This is considerably greater than the optimum depth of 40 nm required for surface plasmon-photon coupling at a Ag-air interface or, in other words, for the last step of the process in isolation.     

Analog tunable gratings driven by thin-film piezoelectric microelectromechanical actuators

We present a microfabricated grating whose period can be tuned in analog fashion to within a fraction of a nanometer. The tunable angular range is more than 400 microrad in the first diffracted order. The design concept consists of a diffractive grating defined onto a 400-nm membrane, with the membrane subsequently strained in the direction perpendicular to the grating grooves by thin-film piezoelectric actuation. The strain-tuned grating device was fabricated with microelectromechanical processes, utilizing both surface and bulk micromachining. The fabricated piezoelectric film achieved a measured dielectric constant of 1200. Device characterization yielded grating period changes up to 8.3 nm (0.21% strain in the membrane) at 10 V and a diffracted angular change of 486 microrad, in good agreement with the theory. Uniformity across the actuated grating and out-of-plane deflections are characterized and discussed.     

Field control of magneto-optic gratings

The Faraday or Kerr rotation of the plane of polarization of light incident on adjacent magnetic domains has opposite sense. Thus light transmitted through a film containing stripe domains or reflected from it is diffracted, as by a grating. The period of such a grating can be controlled by the application of a uniform magnetic field. We report on experimental and theoretical work that explores the range of grating field control under quasistatic conditions, using real films, specifically     

Double Fano-type resonances in heptamer-hole array transmission spectra with high refractive index sensing

Nanohole arrays or individual nanohole oligomers in metallic films have attracted intense attention due to their unique optical properties such as extraordinary optical transmission or Fano resonance. However, the nanohole oligomer array still remains largely unexplored. In this work, we numerically investigate the heptamer-hole arrays in an optically thin silver film, which can support double Fano-type resonances in the transmission spectra. The two Fano-type transmissions arise from the interference between the non-resonant direct transmission through holes and the resonant indirect scatterings based on the excitations of surface plasmons polaritons (SPPs, set up by the array periodicity) and a sub-radiant localized surface plasmon resonance (LSPR, arising from the anti-bonding hybridization between the central and the surrounding holes). Because of their different physical mechanisms, the two Fano resonances can be tuned independently. In addition, the LSPR-related Fano resonance shows an ultra-high sensitivity to surrounding dielectric medium with a figure of merit of 25 due to its sub-radiant feature, far larger than the SPP-related Fano resonance, offering tremendous potentials for plasmonic biosensors.     

Nanoparticle-enhanced diffraction gratings for ultrasensitive surface plasmon biosensing

Ultrasensitive surface bioaffinity sensors are created by the adsorption of gold nanoparticles onto gold diffraction gratings. An enhanced diffraction obtained in a surface plasmon resonance geometry is observed due to the optical coupling of the planar surface plasmons in the grating to the localized surface plasmons in the gold nanoparticles. As a first example, these nanoparticle grating biosensors are employed to detect unmodified DNA at a concentration of 10 fM.     

Tunable subradiant lattice plasmons by out-of-plane dipolar interactions

Plasmonic nanostructures concentrate optical fields into nanoscale volumes, which is useful for plasmonic nanolasers, surface enhanced Raman spectroscopy and white-light generation. However, the short lifetimes of the emissive plasmons correspond to a rapid depletion of the plasmon energy, preventing further enhancement of local optical fields. Dark (subradiant) plasmons have longer lifetimes, but their resonant wavelengths cannot be tuned over a broad wavelength range without changing the overall geometry of the nanostructures. Also, fabrication of the nanostructures cannot be readily scaled because their complex shapes have subwavelength dimensions. Here, we report a new type of subradiant plasmon with a narrow (～5 nm) resonant linewidth that can be easily tuned by changing the height of large (>100 nm) gold nanoparticles arranged in a two-dimensional array. At resonance, strong coupling between out-of-plane nanoparticle dipolar moments suppresses radiative decay, trapping light in the plane of the array and strongly localizing optical fields on each nanoparticle. This new mechanism can open up applications for subradiant plasmons because height-controlled nanoparticle arrays can be manufactured over wafer-scale areas on a variety of substrates.