mons of a whole range of wavelengths can be excited. However, due to the high energy of the electrons, they have a very large momentum and it is difficult to precisely control the coupling: even a slight spread in energy or angle will result in a very broad range of surface plasmon wavelengths being excited. Additionally, only surface plasmons far along the dispersion curve, where kspp is largest, are generally excited. A more precise method to launch surface plasmons is through coupling with an incident electromagnetic wave (photon).
As mentioned previously, direct excitation of surface plasmons by free-space photons is not achievable because kspp is always greater than ko; this can be seen in figure 1, where the surface plasmon dispersion relation always lies to the right of the free space dispersion curve.
This can be overcome by back-side illumination through a material with a higher index of refraction (n), where the far-field radiation has a larger wave-vector (k=nko). This type of coupling geometry is shown in figure 2, and is known as a Kretschmann-Raether coupler. A surface plasmon will be efficiently excited when k|| = nkosin(&theta) = kspp. Kretschmann couplers are commonly used in experiments, but are limited to very thin films such that the high-k photon is able to tunnel through the film and couple to a surface plasmon on the lower-index surface.
A more general approach to launching surface plasmons with light is the use of structured surfaces that are able to impart momentum on the photon, enabling it to couple to the surface plasmon mode. Anything from a single sub-wavelength disk or slit, to rectangular or sinusoidal diffraction gratings are used for this type of coupling. A thorough overview of surface plasmon coupling and patterned and rough surfaces is given by Raether.
Surface plasmons have many interesting applications such as optical measurements of films, chemicals, and biological agents bound to a metallic surface; confinement and guiding of light below the classical diffraction limit; and enhanced light-matter interaction such as surface-enhanced Raman spectroscopy (SERS). A few of the more interesting applications from the literature and industry are outlined below.
Surface plasmon resonance sensors based on Kretschmann couplers can be used to make precise optical measurements at a metal-dielectric interface. The basic idea behind these detectors is encapsulated in the coupling condition discussed above for the Kretschmann geometry: k|| = nkosin(&theta) = kspp. Since the surface plasmon is tightly bound to the metal-dielectric interface, any changes in that interface will translate directly into changes in the surface plasmon wave-vector kspp. This will cause a corresponding change in the optimal excitation angle for the Kretschmann coupler; by measuring this angle, the change in kspp can be deduced, allowing for the change in the refractive index to be computed. These detectors are sensitive enough that they can measure a change in film thickness of just a single monolayer, or refraction index changes (for example, due to chemical binding events on a bio-functionalized gold surface) as small as 10-6 RIU (refractive index units). For example, the commercial sensiQ detector from Nomadics is capable of measuring binding events of just one picogram of protein/mm2.
More recently, there has been an explosion of interest in surface plasmons on nanoscale patterned surfaces. Using modern nanofabrication facilities, it is possible to make structures with feature sizes much smaller than the wavelength of visible light (&sim 500nm). For example, enhanced transmission through arrays of sub-wavelength slits has been experimentally demonstrated and has promising applications in photolithography and high-density optical data storage. Because surface plasmons can be launched at optical frequencies with much smaller wavelengths, it is possible to manipulate the flow of light on the scale of tens of nanometers. This has led to the coining of the term "plasmonics", where surface plasmons are used to manipulate an