We report the construction and testing of a chirped diffraction grating, which serves as a substrate for surface plasmon-enhanced optical transmission. This grating possesses a spatial variation in both pitch and amplitude along its surface. It was created by plasma oxidation of a curved poly(dimethoxysilane) sheet, which resulted in nonuniform buckling along the polymer surface. A goldcoated replica of this surface elicited an optical response that consisted of a series of narrow, enhanced transmission peaks spread over the visible spectrum. The location and magnitude of these transmission peaks varied along the surface of the grating and coincided with conditions where surface plasmons were excited in the gold film via coupling to one or more of the grating's diffracted orders. A series of measurements were carried out using optical diffraction, atomic force microscopy, and normal incidence optical transmission to compare the grating topology to the corresponding optical response. In addition, the impact of a thin dielectric coating on the transmission response was determined by depositing a thin silicon oxide film over the grating surface. After coating, wavelength shifts were observed in the transmission peaks, with the magnitude of the shifts being a function of the film thickness, the local grating structure, and the diffracted order associated with each peak. These results illustrate the ability of this surface to serve as an information-rich optical sensor whose properties can be tuned by control of the local grating topology.

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. C oupling of light with nanostructured objects leads to a variety of unique and potentially useful optical phenomena. 1 Some of the more interesting examples involve the coupling of light to nanostructured metal surfaces, which can lead to what is known as enhanced or extraordinary optical transmission. 2 The origins of enhanced transmission through metal films can be traced to the excitation of surface plasmons (SPs) in the nanostructured metal interface. 1b The high sensitivity of these SPs to the local dielectric conditions at the metal interface can be exploited in sensor development. 3 Examples of nanostructurebased plasmonic sensing include nanostructures consisting of nanohole arrays, 4 single nanometric holes, 5 nanoslit arrays, 6 and various grating-type and diffractive nanostructures. 7 A variety of fabrication strategies can be used to create nanostructured optical elements ranging from electron beam lithography to colloidal nanosphere lithography. 4a,8 Beyond these specialized methods, one can exploit the features of commercially available diffraction gratings as nanostructured elements. Indeed, optical sensors and SP-based sensing platforms that exploit gratings have become increasingly popular. 9 Gratings represent an inherently informationrich substrate due to SPs appearing not only in the directly reflected and transmitted peaks, but also in the various diffracted orders. 10 In addition, the SP response is highly tunable on the basis of the si...

We report grating-coupled surface plasmon resonance measurements involving the use of dispersion images to interpret the optical response of a metal-coated grating. Optical transmission through a grating coated with a thin, gold film exhibits features characteristic of the excitation of surface plasmon resonance due to coupling with the nanostructured grating surface. Evidence of numerous surface plasmon modes associated with coupling at both front (gold/air) and back (gold/substrate) grating interfaces is observed. The influence of wavelength and angle of incidence on plasmon coupling can be readily characterized via dispersion images, and the associated image features can be indexed to matching conditions associated with several diffracted orders at both the front and back of the grating. These features collapse onto a set of global dispersion curves when plotted as peak energy versus the grating wavevector, with feature locations clustered according to the refractive index values of the neighboring dielectric material, either air or polycarbonate. Coating of the grating with multilayer arachidic acid films via Langmuir-Blodgett deposition results in red-shifting of some, but not all, of the plasmon features. The magnitude of the shift is a function of the film thickness, wavelength, and angle of incidence. Dispersion images clearly depict the red-shifting and also broadening of the front side features with increasing film thickness. In contrast, little change is observed in features associated with the back-side of the grating. The nature and magnitude of the interaction between the plasmon modes appearing at the front and back sides of the grating are discussed and analyzed in terms of the predicted interactions determined via optical modeling calculations.

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 sand 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. ABSTRACT: 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 ac...

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