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Micro/Nano Lithography

Focusing surface plasmon polariton waves

Analogs of diffractive and refractive optics in free space can be developed to manipulate surface plasmon waves.
30 January 2008, SPIE Newsroom. DOI: 10.1117/2.1200801.0946

Surface plasmon polaritons (SPPs) are electromagnetic surface waves formed through strong interaction between electromagnetic field and free electron oscillations at a metal-dielectric (i.e., insulating) interface. The SPPs are highly confined at this location. Moreover, their wavelength can be shorter than that of light in surrounding media. These properties suggest potential applications in subwavelength electromagnetic wave guiding, biochemical sensing at functionalized surfaces without having to use fluorescent labels, and in-plane microscopy, perhaps delivering subwavelength resolution.

To realize the promise of SPP technologies, a comprehensive arsenal of devices for launching, detecting, guiding, imaging, focusing, and otherwise transforming SPP waves must be readily available. S-bends, Y-splitters, Mach–Zehnder interferometers, and waveguide-ring resonators for the SPP have already been demonstrated.1,2 Yet the challenge remains to excite and control propagating SPP fields in a systematic fashion, as is possible with optical fields both in free space and in dielectric waveguides. Once the basic optical manipulation of SPP has become routine, it will pave the way for more sophisticated devices, such as confocal microscopes with subdiffraction-limited resolution (i.e., higher than the resolution achieved in conventional imaging instruments).

Figure 1. (a) Schematic diagram of SPP FZP geometry and design parameters. (b) Scanning electron micrographs of an SPP FZP fabricated by depositing amorphous silicon over an aluminum/air interface. Also seen are integrated arrays of nanoholes in the aluminum film for SPP field excitation (left) and detection (right).

Figure 1 represents an adaptation of a free-space optical component known as the Fresnel zone plate (FZP) for focusing SPP waves.3 Focusing was previously obtained by coupling a laser beam to the SPP via an array of concentrically arranged circular metallic slits4,5 or other in-plane structures, by pairing the polariton with a converging beam via a 2D rectangular nanohole array,6 and by means of a lens-like dielectric refractive element placed directly on top of the metallic film.7 The approach shown Figure 1 differs in that it employs diffractive rather than refractive phenomena.

A conventional free-space FZP8 comprises concentric rings, alternately transparent and opaque, with boundaries at radial locations


where λ is the free-space incident wavelength and f is the focal distance. Light passing through the transparent regions interferes constructively to generate a primary focal spot at a distance f along its optical axis, as well as less-pronounced additional foci at distances f/3, f/5, f/7, and so on. For the SPP adaptation in Figure 1, we replace the wavelength λ in Equation (1) with the SPP wavelength λSPP, which is a function of dielectric permittivities of the metal and the surrounding dielectric (air in our experiment). The device is designed to operate at the optical frequency corresponding to free-space wavelength λ0=1.55μm (λSPP=1.547μm) and primary focal length f=80μm. The 12 opaque zones are constructed as 5μm-wide, 400nm-high blocks of amorphous silicon deposited over a 100nm-thick aluminum film. In a separate fabrication step, two rectangular nanohole arrays, visible in Figure 1(b) to the left and to the right of the FZP structure, are etched into the aluminum film. The nanohole arrays serve to couple far-field optical radiation into the SPP, and to couple the SPP into the far field for detection. The SPP wave is launched with a 200fs pulsed laser source focused through a 5× microscope objective onto the left-hand array. The far field is captured on the right with a 10× objective coupled to a CCD camera. Cross-polarizers are used to suppress the directly transmitted optical beam in favor of energy re-radiated through the SPP.6

Figure 2. (a) Measured SPP time-averaged intensity map over the nanohole array to the right of the FZP, showing so-called +1st-order diffractive focusing and –1st-order diffractive fringes. The dashed white line indicates the left edge of the nanohole array. The FZP is located at x=−110 μm. (inset) Schematic diagram of the FZP and the nanohole array. (b) Post-processed image obtained from (a) by compensating for radiative loss.

Figure 2 shows experimental results in terms of the output nanohole array. The white dashed line indicates the edge of the array. The FZP (not visible in the figure) is located 20μm to the left at x=−110μm. The primary focal point at x=−27μm, 83μm away from the FZP, can be clearly discerned. The raw intensity map in Figure 2(a) also attests to the rapid attenuation of SPP waves as they propagate from left to right across the nanohole array. The array is introduced only for visualization purposes, and in principle would not be necessary for in-plane Fourier optical applications such as those used for imaging, filtering, and telecommunications. It is therefore informative to compensate in post-processing for the radiation losses caused by this array. The compensation implemented in Figure 2(b) suggests that, had the detection nanohole array not been present, SPP intensity at the focal point could be expected to be about three times the intensity of the input SPP wave.

We note that the effectiveness of our FZP device is limited somewhat by the fact that even its opaque zones are partially transparent. This is due to the technical difficulty of fabricating silicon-based FZP higher than several microns using the lift-off method. The evanescent tail of SPP fields in air extends further than the height of the FZP, allowing a fraction of the incident SPP to ‘sail over’ the silicon barrier. Experimentally, about 30% transmission was observed through the opaque zones, a figure consistent with our finite-element numerical models.

We have demonstrated that analogs of diffractive and refractive optics in free space can be developed to manipulate surface waves such as SPPs. The basic SPP optical components are the necessary enablers for more sophisticated devices. SPP-focusing elements may find application in in-plane signal processing, microscopy, and instruments such as enhanced Raman scattering spectroscopy that require a high degree of field localization.

Yeshaiahu Fainman
Electrical and Computer Engineering Department (ECE)
University of California, San Diego (UCSD)
La Jolla, CA

Yeshaiahu Fainman is a Cymer Endowed Chair Professor in Advanced Optical Technologies of ECE at UCSD. His current research interests include near-field phenomena in optical nanostructures, nanophotonic devices, and their integration. He has contributed more than 160 manuscripts to refereed journals and over 280 conference presentations and proceedings. He is a fellow of the Optical Society of America, the IEEE, and SPIE, and a recipient of the Miriam and Aharon Gutvirt Prize.