Surface plasmons can circumvent certain limitations imposed by conventional optics

Oscillations of conduction electrons in a metal trap photon energy on the surface, and can squeeze through subwavelength holes.

In the days of bulk optics, surface plasmons were just another curious physical effect of largely academic interest. Plasmons are resonant oscillations of free electrons at the surface of a conductive metal that electromagnetic waves can excite under certain conditions. Plasmons are tricky to excite, occupy a very small area, and can’t travel far, so for a long time they had little practical import beyond certain sensing applications.

That has changed with the development of techniques for fabricating micro- and nanoscale structures that alter the interaction of light with electrons. These structures can control optical interactions on a subwavelength scale for applications including sensing, nonlinear devices, optical storage, and signal processing. So the once-obscure field has become a hot research area.

Basics plasmon physics

The concept of the plasmon comes from the interaction of an electromagnetic wave with a plasma. The electric field of the wave applies force to both ions and electrons in the plasma, but it moves the light electrons much farther than the much more massive ions, so in practice the field is considered to change the distribution of only the electrons.

Figure 1. Surface plasmons propagate along a metal surface, with an electric field in a dielectric material and electric charges moving in the plasmon at metal surface. The electric field in the dielecteric declines exponentially with distance from the surface. The magnetic field of the light is in the surface plane.

A similar effect occurs in conductive metals, in which electrons drift freely, forming a kind of plasma within the solid, although at a much higher density than an ionized gas. An electromagnetic wave that strikes the surface of such a metal can induce free electrons at the surface to oscillate, creating a surface plasmon, if the resonance and coupling conditions are met-the details depend on the properties of the metal and of the dielectric that transmits the light. It takes ultraviolet photons to excite surface plasmons in most metals, but visible light can excite plasmons in the most conductive metals, including silver, gold, and the alkali metals. Metallic nanoparticles also can exhibit surface-plasmon phenomena.

A surface plasmon is actually a hybrid state ­including a photon and an exciton-an electron-hole pair. This mixed state is called a polariton, so strictly speaking a surface plasmon is really a surface-plasmon polariton. In this hybrid state, with the electric field oscillating normal to the surface above the metal and charge carriers moving within the metal, the ­electric-field strength drops exponentially with distance from the surface, like an evanescent wave, helping confine the plasmon to the surface (see Fig. 1). ­Absorption in the metal limits plasmon transmission along the surface. Silver is the best metal for use at visible wavelengths; it can propagate plasmons for up to 100 µm in the visible and nearly 1 mm in the 1.5-µm ­fiber transmission window.

Surface plasmons are small compared to photons, so they concentrate an optical signal to subwavelength dimensions. This raises the electric-field intensity, enhancing nonlinear interactions. Their compact dimensions also open the prospect of building subwavelength surface-plasmon circuits far smaller than purely optical integrated circuits.

Sensing applications

Surface plasmons have been recognized for decades, but they were not ­initially identified as responsible for their first application, surface-­enhanced ­Raman scattering. Electrochemist Martin ­Fleischmann, now better known for his cold-fusion research, discovered the effect in the early 1970s when he began studying the Raman spectra of electrode surfaces. He tested a variety of materials, and found that using roughened silver surfaces enhanced the Raman signal by as much as a million-fold. Only later was the effect linked to surface plasmons, which concentrated the electromagnetic field to multiply the intensity of the nonlinear Raman signal.

FIGURE 2. Surface-plasmon sensors can be found in three different configurations. Light enters a prism and interrogates a metal layer in contact with the test material (top); sensing is based on attenuated total internal reflection. Light incident on a metal grating with the test material on top, and diffracted into different orders (center). Light from an optical waveguide excites a plasmon in a metal film in contact with both the optical waveguide and the test material (bottom).

Sensing applications based on resonances of surface plasmons became well established over the past two decades, and remain a center of interest. The sensors generally rely on coupling light at an angle into thin films of gold or silver and measure changes in the light scattered from the surface (see Fig. 2). They can be used for real-time detection of specific materials such as biomolecules that are present at low concentrations. A 1999 review paper cited well over a hundred papers, and mentioned a number of commercial applications.1

A recent trend in plasmon-­resonance sensing is the development of high-resolution microscopy and measurement. One recent example is ­profiling a strongly focused light beam with submicrometer resolution by using it to excite surface plasmons on a thin gold film.2

Nanostructured metal surfaces

The new interest in surface plasmons owes much to the development of ­photonic-bandgap materials, writes William L. Barnes and colleagues at the University of Exeter (England) in a Nature review.3 In transparent materials, subwavelength periodic structures create photonic bandgaps that block light at certain wavelengths. Likewise, metallic nanostructures such as arrays of bumps that are periodic in two dimensions can scatter plasmon waves that are twice the length of the array spacing, effectively creating a plasmon bandgap. ­Periodic nanostructures on the surface allow plasmons to propagate at the edge of the blocked band, and allow light at the edge of the band to excite plasmons when incident at a much wider range of angles.

As with photonic-bandgap materials, the development of surface plasmons is headed in many directions at once. Researchers are working on understanding how plasmons interact with nanostructured surfaces, and how these interactions can manipulate light. Most of the work is being done with metal films only 10 to 200 nm thick, which so far appear to have the best properties for plasmon propagation.

Figure 3: Beaming light from a single subwavelength hole surrounded by an array of concentric grooves is shown in a micrographic image of the pattern in a silver film (top left); the transmission spectrum as a function of angle and wavelength, showing the emission is concentrated in a narrow beam (top right); a cross section of the beam produced (bottom left); and intensity at the peak transmission wavelength drops with distance from the center of the beam (bottom right). (Courtesy Thomas Ebbersen, from Lezec et al.)

One interesting example is the transfer of energy through metal films containing subwavelength holes in an array with regular spacing that is also smaller than a wavelength. Normally, subwavelength holes transmit little light, but in 1998 Thomas Ebbesen of the University Louis Pasteur (Strasbourg, France) found that, thanks to plasmon arrays in thin metal films, the arrays of subwavelength holes transmitted light at resonant wavelengths much more efficiently than predicted by diffraction theory.4 Dimensions of the nanostructure determine the resonant wavelength (see Fig. 3).

Other groups soon adapted the concept to make conductive films that also transmitted a reasonable amount of light. Films with arrays of subwavelength holes have been used as conductors on semiconductor and organic light-emitting diodes (LEDs) and on vertical-cavity surface-emitting lasers (VCSELS).

Even a single subwavelength hole can transmit more light than expected if the output side of the hole is surrounded by a suitable periodic nanostructure (see Fig. 4). In this example, made by H. J. Lezec of Ebbesen’s group in Strasbourg, a central 250-nm hole in a 300-nm silver film is surrounded by concentric rings on the output surface. This structure selectively transmits red light at 660 nm in a beam with half-angle of three degrees. The transmitted intensity peaks strongly at 660 nm, a wavelength slightly longer than the 600-nm spacing of the rings. Writing in Science, Lezec’s group called the subwavelength aperture with surrounding structure “miniature phased-array antennas in the optical regime,” with narrow transmission angle in a limited band of wavelengths. They suggest the devices might be useful for reducing beam divergence of LEDs and diode lasers.5

Integrated plasmon devices

The compact size of plasmons opens the possibility of integrated plasmon circuits with component densities far higher than conventional integrated optics. Input optical signals would be converted into plasmons, processed in that form, and converted back into light. The small size of plasmons would enhance nonlinear effects for processing the signals, but major challenges remain in developing and integrating components, particularly the nonlinear elements needed for switching signals.

The first metal-film plasmon waveguides were demonstrated several years ago, but major challenges remain in reducing their attenuation and integrating them with optics. Last year a Caltech group reported plasmon waveguides with loss of 1.2 dB/µm. The waveguides were made of silver films fabricated alongside silicon waveguides using standard semiconductor technology. Insertion loss for transferring light from the silicon to the silver was 3.4 dB.6 A team at Spectralis (Nepean, Ontario) this year demonstrated a family of waveguide components including S bends, Y junctions, and couplers in gold films embedded in a homogeneous dielectric material.7 Others have demonstrated the counterparts of mirrors and beamsplitters.8 Much more is in the offing.

Figure 4: Light transmission by arrays of submicrometer holes is shown with color-coded transmission curves. The size and spacing of holes selects the peak transmission wavelength. The blue light is from an array of 180-nm holes with 450-nm spacing, and the red light from an array of 225-nm holes with 550-nm spacing. Courtesy Thomas Ebbersen from Barnes et al., 2003.)

Surface-plasmon techniques may aid in the development of other optical devices. Last year a team from Caltech and Nichia increased LED emission by depositing a thin layer of silver or aluminum just 10 nm above the light-emitting indium gallium nitride (InGaN) quantum-well layer. The authors predicted that by increasing efficiency, surface plasmons “would lead to a new class of very bright LEDs, and highly efficient solid-state light sources.” 9

Theoretical studies suggest new ways to enhance surface-plasmon effects. Only a few metals can support surface plasmons in solid thin films. However, an analysis published last year shows that arrays of submicrometer holes should allow thin films of other metals to support surface plasmons when properly excited.10

Outlook

Well established in sensing, surface-plasmon resonances are expanding into other areas, aided by the development of micro­structured materials. By operating on subwavelength scales, surface plasmons are opening new doors in optical devices, effects, and integration. Surface plasmons offer ways to circumvent important limitations imposed by conventional optics in areas such as beam divergence and the density of component integration. It’s too early to say what will prove practical, but interesting developments are certain.❑

REFERENCES

1. J. Homola,, S. S. Yee, and G. Gauglitz, Sensors and Actuators B 54, 3 (1999).

2. H. Ditlbacher et al., Optics Lett. 29, 1408 (Jun 15, 2004).

3. W. T. Barnes, A. Dereux, and T.s W. Ebbesen, Nature 424, 824 (Aug. 14, 2003).

4. T. W. Ebbeson et al., Nature 391, 667 (Feb. 12, 1998).

5. H. J. Lezec et al., Science 297, 820 (Aug. 2, 2002).

6. M. Hochberg, T. Baehr-Jones, C.Walker, and A. Scherer, Optics Exp. 12, 5481 (Nov. 1, 2004).

7. R. Charbonneau et al., Optics Exp. 13, 977 (Feb. 7, 2005).

8. J. R. Krenn et al., J. Microscopy 209, 167 (March 2003).

9. K. Okomoto et al., Nature Materials 3, 601 (September 2004)

10. J. B. Pendry, L. Martin-Moreno, and F. J. Garcia-Vidal, Science 305, 847 (Aug. 6, 2004).