Active plasmonics in optical interconnects
Modern electronic chips face a data gridlock because of limits on the transmission rates possible with electric signals in extremely small wires. Optical signals can carry a vast amount of information, but state-of-the-art conventional photonic waveguides are much larger than today’s nanoscale electronic components. Plasmonic waveguides can provide the solution. In these systems, light is coupled to electron oscillations at the surface of metals. Plasmonic circuits can confine light very tightly, can transmit optical and electric signals simultaneously, and are highly suited to active functions such as routing and manipulating of optical signals. Recent trends in plasmonics toward materials compatible with standard industrial semiconductor processes may lead to a new generation of plasmonic-electronic components and circuits.
In the middle of last century, humanity faced a problem using the telegraph and telephones to communicate because copper wires could not cope with the growing amount of electric signals to be transmitted between towns, cities, and continents. The solution was simple: use optical signals instead. Copper wires were replaced by optical fibers. Optical communications made possible the rise of the Internet and the World Wide Web at the turn of the century. Now, with computer-to-computer communications sorted out, another electric signal gridlock is looming inside the computer itself. Copper wires are failing once again, this time in the form of nanowires (‘interconnects’) at the other end of the communication chain, where extremely fast information exchange is required between and within microchips containing billions of electronic transistors. Powerful modern transistors often stay idle waiting for a signal to come from a memory unit or another chip. The small diameter of the wire prevents the fast flow of data by electric signals and there is no space for larger wires.
Light can come to the rescue once again, if researchers can develop efficient photonic interconnects that are small enough to fit on a chip. The problem is that a conventional optical fiber is typically at least 100μm in diameter—1000 times larger than the copper nanowires used on electronic chips today. Even with silicon photonics circuitry, in which infrared light travels through silicon itself, and which has the smallest photonic components so far, the silicon optical waveguides are an order of magnitude larger than copper interconnects.
Plasmonics can do better. This technology exploits phenomena that occur at the surface of a conductor. Free electrons in the conductor can support oscillations known as plasmons. At the surface, these oscillations can form plasmonic waves (more formally known as surface plasmon polaritons), a combination of electromagnetic waves (light) and the electron oscillations. The wavelength of a plasmonic wave is much shorter than that of ordinary light of the same frequency, which enables plasmonic waveguides to effortlessly provide subwavelength confinement of the optical field without needing any special arrangements. One can achieve plasmonic waveguides as small as modern electric interconnects. Furthermore, the plasmonic modes are highly sensitive to the properties of their surroundings, making it possible to build active components in which the optical signal propagating along the waveguide can be efficiently controlled with an electric field, another optical signal, or even a magnetic field. Another simple but important advantage is that the same circuitry can guide both electric and optical signals. Thus, electric control signals can be delivered along a photonic circuit without needing additional wiring. However, these advantages do not come for free. Plasmonic waveguides suffer from much higher propagation losses than occur in the dielectric (i.e., nonmetallic) waveguides of conventional and silicon photonics, although the importance of losses depends on the particular application. The advantage of plasmonic interconnects is their ability to seamlessly incorporate active functionalities, which are difficult to achieve in silicon nanophotonic circuitry.
There are many designs of plasmonic waveguides, providing different degrees of light confinement. The stronger the confinement, the higher the propagation loss. In evaluating a design, it is always necessary to consider the trade-off between these two factors, along with considerations of constraints imposed by one or another application, and the feasibility of implementing integrated active components, such as sources, detectors, and switches. Two types of plasmonic waveguide are particularly important for achieving both small mode size and active functionality: the so-called dielectric-loaded waveguides, in which a ridge of dielectric on the metal surface guides the plasmonic wave along the metal/dielectric interface, and metal-dielectric-metal waveguides, in which dielectric is sandwiched between two metal layers. The dielectric component of such waveguides can be polymer, glass, silicon, or indeed any other suitable nonmetallic material. With a proper choice of the material, one can design required functionalities. For example, strongly nonlinear integrated components for switching, modulation, and amplification of plasmonic signals can be achieved with doped polymers or semiconductors in this role.
Until very recently, only silver and gold were considered to be suitable metals for plasmonic applications because they provide the smallest ohmic losses, which determine the propagation loss of plasmonic waveguides. The drive toward optical interconnect applications and, therefore, compatibility with semiconductor industry processes—ultimately, with complementary metal-oxide-semiconductor (CMOS) fabrication—has led to a re-evaluation of the plasmonic material base. Gold and silver are not CMOS-compatible and in general are not very attractive for use with semiconductors because they modify the semiconductors’ electronic properties. Aluminum and copper are CMOS-compatible, but have been regarded as too lossy. However, the interplay between the real and imaginary parts of the permittivity of alumninum appears to be favorable for certain geometrical configurations, such as silicon-loaded aluminum waveguides (i.e., a ridge of silicon on an aluminum surface), where propagation loss comparable to that of gold-based waveguides can be achieved. Further investigations into copper properties indicated that copper thin films fabricated by the CMOS process can have losses comparable to aluminum, significantly better than the values tabulated in the literature for conventional copper films. Researchers have already implemented several copper-based plasmonic components fabricated completely with CMOS processes.
The area of technology where plasmonics has an undisputed edge over other techniques is that of active nanophotonic components. Plasmonic modes are very sensitive to their environment (i.e., the dielectric material next to the metal), and if some electric or optical signals can control the refractive index of the dielectric, the response of the plasmonic device can be stronger than that of a conventional photonic device of the same geometry. For example, a plasmonic Mach-Zehnder interferometer or ring resonator (see Figures 1 and 2) will respond strongly because of the short wavelength and tight confinement of the plasmonic mode. Researchers have demonstrated thermo-optical, electro-optical, and all-optical components with a variety of plasmonic platforms. In one of the most important demonstrations, a group directly integrated a plasmonic thermo-optical modulator within silicon-photonic circuitry and tested it under the stringent conditions of optical network operation. In other work, Alexey Krasavin and I proposed one of the smallest plasmonic devices: a plasmonic electro-optical modulator that is fully CMOS-compatible and is just 25nm × 30nm × 100nm in size, similar to modern local electronic interconnects. It employs an indium tin oxide (ITO) layer for controlling plasmonic loss, with an applied electric field changing the concentration of free electrons in the material, a technique that is proven to be very efficient.
A very new trend in plasmonics, which may have significant implications in optical interconnects, is the discovery of nonmetallic materials that can play the role of the metal and support plasmonic excitations. These materials are various types of metal oxides and doped semiconductors with a free-electron concentration and electron mobility high enough to sustain surface plasmons at telecom frequencies and with low propagation loss. Examples include ITO, zinc oxide doped with aluminum or gallium, and titanium nitride. In some instances, the material’s performance arguably exceeds that of traditional metals for certain applications. Being semiconductor-type materials, they also provide opportunities to use standard semiconductor industry processes for fabricating plasmonic devices integrated with electronics.
These three recent developments—the use of aluminum and copper, the demonstration of numerous active devices, and plasmonic excitations in semiconductor materials—may prove invaluable for practical implementation of plasmonic-electronic circuitry. The particular choice of the plasmonic interconnect will depend on the required distance to be covered. For short-range local interconnections (i.e., within a small region on a chip), loss in nanoscale plasmonic waveguides will not be an issue, even at the very small mode size required by the density of integration on the chip (recall that losses worsen as the mode confinement gets tighter). For longer-range interconnection, losses become more important but the constraint on mode size is not as tight. The required mode size can then be achieved with silicon-photonic waveguides, and plasmonic components integrated within silicon-photonic circuitry will be crucial to provide low-energy active functions, light sources, and detectors. In conventional passive photonic circuitry, where routing and conditioning of signals is done in the electronic domain, the energy and space requirements for electronic-to-optical and optical-to-electronic signal conversion become a significant issue for high-rate data transmission. Thus, all-optical processing enabled by the enhanced nonlinearities in plasmonic components may be the answer in the quest for all-optical (transparent) photonic integrated circuits with low energy-per-bit requirements comparable to electronic processing energies.
All possible nanophotonic components have been demonstrated on a plasmonic platform: splitters, Mach-Zehnder interferometers, ring resonators, Bragg gratings—take any photonic component and you will find its plasmonic counterpart. Now they can be built using only CMOS-compatible materials with similar performance in terms of optical signal guiding. Integration of plasmonic components within standard photonic circuitry and with silicon photonics is on the research and development agenda and will boost practical introduction of plasmonics in real-life integrated photonic devices. Each potential application requires careful selection of an appropriate type of waveguide for building the relevant active devices. Ultimately, one can see plasmonic and plasmonic/photonic integrated circuits of various sizes forming the main components of biosensing devices and serving as ultrafast modulators and switches for optical networks.
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