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Optoelectronics & Communications

Nanophotonics: sensing with surface plasmon polaritons

The results of an investigation into the excitation, propagation, and detection of surface plasmon polariton modes, at scales less than 100nm, have been applied to a biochemical sensor.
22 June 2006, SPIE Newsroom. DOI: 10.1117/2.1200604.0252

Nanophotonics is an emerging technology that involves the interaction of light with structures smaller than about 100nm. Thanks to recent fabrication advances, nanophotonic applications have been growing in number and diversity. The anticipated attainment of 16nm lithography resolution by 2020 would further advance the field by improving system integration. The increased resolution would lead to fabrication methods that are reliable, scalable, power-efficient, and cost-effective. Nanoscale component integration would be feasible, both from the top down (e.g., fabrication on wafers), and from the bottom up (e.g. building the system starting at the scale of atoms and molecules).

In recent efforts to develop nanophotonic waveguides, attention has focused on optical modes that are concentrated at the interface between metallic and dielectric materials. This interest was sparked by the discovery that the modes exhibit enhanced light transmission through nanohole arrays and regions of nanoscale surface corrugations.1–6 The modes—called surface plasmon polaritons (SPPs)—propagate along the interface, and have the potential for full optical confinement in 3D at nanoscales. These characteristics could lead to photonic devices smaller than optical refraction technologies can currently achieve. They might also help bridge the gaps between photonics, biochemical sensing, and CMOS-based electronics technologies.

Many potential SPP-based information technology applications will require a complete understanding of in-plane SPP pulse propagation, but previous studies have generally focused on steady-state behavior. To address this challenge, we developed a way to excite and visualize the propagating pulse. In our experiments,6,7 we used a 2D nanohole array to couple the optical field into a metallic film on a dielectric substrate.

To better understand the excitation and propagation process of the SPP modes, we constructed a laser-illuminated imaging apparatus6 shown schematically in Figure 1(a). A polarizer-analyzer pair is used to control the polarization state of the excitation field and the field in the image plane. The excitation is performed with a focused laser beam of wavelength λo at normal incidence. In contrast to typical far-field measurements, this apparatus images the metal film onto a CCD camera.

Shown in Figure 1(b) are SPP scattering at various period-to-wavelength ratios, aλ 0, and the location of the ratios on the dispersion map. The spectra are dominated by asymmetric, Fano-type line shapes at the various SPP excitation conditions. The transmitted field has two components: one that scatters through the small, sub-wavelength holes directly, and one stemming from the excited surface mode. The interference between these two components yields the amplitude extremes seem in Figure 1(b). The outlines of these major features of the dispersion map are well described by the relation:  where is the SPP wavevector,  is the in-plane component of the incident electromagnetic wavevector, and  and  are the reciprocal lattice vectors in the x and y directions, respectively. SPP modes are labeled in terms of the lattice momentum needed to excite a mode, which travels along the metal surface in the corresponding direction.

We recently applied our findings to construct a high-spectral-resolution surface plasmon-resonance (SPR) sensor that operates at and near normal incidence, and which facilitates high spatial-resolution imaging. The nanohole samples for our SPP sensor experiments were fabricated by depositing gold films on a glass substrate, followed by holographic lithography to achieve large usable areas (∼1cm2). Multiple exposures of SU-8 resist yielded a 2D array of nanoholes that were transferred onto gold film using ICP/RIE dry etching. The array fine control achieved about 200nm for the hole diameter. A polydimethylsiloxane mold with a microfluidic delivery channel measuring 1cm × 2mm × 100×m was bonded to the substrate by oxygen plasma.

Measurements were made using the setup shown schematically in Figure 2(a). When the polarizer and analyzer are parallel, we get Fano-type transmission lines, as in Figure 1(b), indicating a combination of scattering and resonance. When the polarizers are orthogonal, the background radiation is suppressed, and there is only a pure resonance (Lorentzian) transmission line. This line width is narrower in both frequency and phase, and thus can be used to effectively monitor changes in refractive index. We tested the sensor by using (+1,0) SPP modes to interrogate the over-layer fluid's refractive index, which was varied by changing the Na2CrO4 concentration in water. Figure 2(b) shows how the phase of the resonant transmission peak changed with the fluid's changing index of refraction at the interface. The salt concentration could have also been interrogated using wavelength changes.

Figure 1. (a) Shown is a schematic diagram of the SPP imaging system. The array illumination is from a laser of wavelength λ0 focused with a microscope objective. The resulting SPP mode is imaged on a CCD array using a 4F imaging system (MO and Lens). For the case shown, the incident +45° polarization is decomposed into orthogonal components that excite (1,0) modes at the array, which radiate in the forward and backward directions. The radiation is polarized at -45° to obtain the field in the image plane. MO1, MO2: Microscope objectives. ψP, ψA: Polarizer, analyzer angles. (b) Shown are spectral measurements of unpolarized zero-order transmittance for hole arrays in an aluminum film on a glass substrate. Data from arrays with different periods, a, were combined for the composite intensity image. The stitching frequencies appear as horizontal black lines, and data are replicated at negative wavenumbers for viewing. The SPP images correspond to four values of a/λ0. The polarizer-analyzer pair was arranged at 0°/90° degrees for the (±1, ±1) modes of the nanohole array with a/λ0 = 1.41, and at +45°/-45° for the (±1,0) and (0,±1) modes for a/λ0 = 1.03. Higher order modes were obtained at a/λ0 = 2. No SPP excitation is observed for a/λ0= 0.9.

Figure 2. (a) Shown is a diagram of a 2D nanohole-array-based sensor. The input and output polarization states of a tunable laser are controlled, providing variable spectral or angular Fano-type profiles. A microfludic channel transports the analyte fluid to the surface of the sensing area, and can tune the SPP resonance frequency by controlling the refractive index at the metal-dielectric interface. Shown in the upper left are scanning electron microscope images of a representative sample. (b) The resonance-peak position shift is plotted versus the fluidic over-layer refractive index change, which depends on salt concentration. The black line is a linear fit to the log-log data. Δn: Refractive index change. θ: Resonant phase peak shift.

Because an SPP mode's amplitude is greatest at the interface between the metal and dielectric layer, the sensor is extremely sensitive to perturbations there. This heightened sensitivity can be used to monitor chemical reactions of various types by attaching molecules to the surface and supplying probe molecules in the analyte fluid. The pure Lorentzian shapes of the structure's phase and spectral transmittance maximize the sensor resolution, which we show is of order 10-5 refractive index units (RIU). We estimate the potential of this system of order 10-6 RIU under optimal conditions, with a nonabsorbing over-layer.

In the future, the SPP combined with optofluidics will have a significant impact on applications such as nanoscale spectroscopy, tomography and metrology, live cell dynamics, sensing, and hyper-spectral imaging of biological species. The investigation of opto-plasmonic fields and learning to control them on a nanoscale have the potential to lead to new tools that can advance cell biology and medicine.

The authors acknowledge the support of NSF, DARPA, and AFOSR.

Shaya Fainman, Kevin Tetz
Dept. Electrical and Computer Engineering, University of California, San Diego
La Jolla, CA
Yeshaiahu (Shaya) Fainman is a professor of electrical and computer engineering at the University of California, San Diego. His research interests include nanophotonics, near-field optics, plasmonics, information processing with femtosecond pulses, quantum information processing, diffractive and nonlinear optics, adaptive optics, and multidimensional imaging. Fainman co-chaired a conference on optofluidics in 2006.