A platform for practical plasmonics

One of the best-known trends in technology is Moore’s law, which predicts that the number of transistors in a central processing unit (CPU) will double every two years. It has guided the long-term strategy of the semiconductor industry since 1965, but is quickly reaching its limits due to fundamental size constraints on transistors. Currently available CPUs have a gate length of approximately 20nm, or roughly 40 atoms, which cannot be reliably scaled much further without a significant impact on performance. To meet the demand for faster, smaller, andmore power-efficient electronics, additional technologies are needed. One of the leading alternatives is light, which has long been used to achieve high-speed, low-loss communication in fiber optics, and more recently has been used for external data transmission in consumer devices.1 However, photonic systems working at wavelength are difficult to integrate with an electrical chip because they cannot be smaller than roughly /2 (the ‘diffraction limit’). Given that the standard wavelength for telecommunication devices is 1550nm, integrated photonic systems are on the micron scale and 10–100 larger than today’s transistors. If the high-speed operation, broad bandwidth, and low loss properties of light could be brought onto a microprocessor, faster and more efficient computers could be achieved in addition to new integrated optical technologies for medicine, chemistry, biology, and more. Plasmonics squeezes light into dimensions much smaller than the diffraction limit by coupling it with the collective oscillations of electrons at the interface of a metal and a dielectric.2 While such waves, called surface plasmon polaritons (SPPs), can have extremely small mode sizes (down to a few nanometers), they are plagued by large and unavoidable losses (a trade-off in plasmonic systems). Nevertheless, active plasmonic devices Figure 1. A general illustration of the available schemes for integrated optical communication. Photonics, depicted as a silicon ridge waveguide on a silicon dioxide substrate, is diffraction limited in size and cannot obtain confinement much smaller than the micron scale for the telecommunication wavelength range. Plasmonics, depicted as a metalinsulator-metal slot waveguide carrying gap plasmons, can achieve extremely high modal confinement (< =10), but suffers from excessive loss. However, these structures are important for ultra-compact active plasmonic devices (switches, modulators, sensors, and so on). Low-loss plasmonics, depicted as a plasmonic strip waveguide carrying longrange surface plasmon polaritons (LR-SPPs), enables acceptable losses by enlarging the mode size. However, these plasmonic structures bring additional technological advantages such as a reduced coupling loss to active plasmonic devices, an extreme sensitivity to the metal-dielectric surface, the ability to support both electrical and optical signals, and more. We suggest that the additional technological advantages of plasmonic structures offset the lack of modal confinement, and make them more advantageous for use in integrated optical devices.