d 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 and transport information, analogous to electronics and photonics. Photonics has nearly unlimited bandwidth and almost no loss or dispersion, making it invaluable for high-throughput long-haul information networks. However, photonics components are diffraction-limited to micron-scale sizes, ruling out dense integration with nanoscale electronics. Plasmonics is potentially poised to bridge the tens of nanometer scale of modern electronics to the micron scale world of photonics. Many interesting devices, such as a 50nm metal-insulator-metal (MIM) surface plasmon waveguide capable of guiding 1.5um light even around 90o corners, have been proposed and are being actively investigated. Such devices have great promise, but also must overcome the limits imposed by the lossy propagation of surface plasmons in metals. The propagation lengths of surface plasmon waves in the visible and near infrared regimes are typically on the order of hundreds of microns on unpatterned surfaces, but drop to under thirty microns in sub-wavelength MIM waveguides. The propagation length in a modern optical fiber is drastically larger, on the order of kilometers. The small propagation lengths of surface plasmons are a significant hurdle in the practical implementation of plasmonics; however, researchers throughout the world are actively pursuing geometries which take account for this loss and new designs to limit the loss as much as possible.
Surface plasmons have many exciting potential applications and rich underlying physics drawing heavily from solid-state physics and electrodynamics. Although known for over 50 years, there are still many unsolved theoretical and experimental problems in the field of surface plasmons. Additionally, with the use of modern nanofabrication techniques, a whole range of new plasmonic devices have become possible. Although not without their drawbacks, surface plasmon based devices have a plethora of laboratory and commercial applications--including the potential to revolutionize photonic-electronic integration wi