Plasmonic Circuits for Manipulating Optical Information

Nanophotonics 2017; 6(3): 543–559 Review article Open Access Timothy J. Davis*, Daniel E. Gómez and Ann Roberts Plasmonic circuits for manipulating optical information DOI 10.1515/nanoph-2016-0131 “software-defined networks” [4, 5] necessary to simplify Received July 30, 2016; revised September 16, 2016; accepted network reconfigurability. As data in optical communica- September 22, 2016 tions can be encoded using amplitude, phase, intensity, wavelength, and polarization, direct serial operations Abstract: Surface plasmons excited by light in metal struc- between light signals can eliminate the complex optics- tures provide a means for manipulating optical energy at electronics-optics conversion, thereby maintaining the the nanoscale. Plasmons are associated with the collective encoding during processing. oscillations of conduction electrons in metals and play a However, there are good reasons electronics is used role intermediate between photonics and electronics. As for computation whereas optics is used for communica- such, plasmonic devices have been created that mimic tion [6]. Electronics is based on the movement of electrons, photonic waveguides as well as electrical circuits operat- with signals encoded using current or voltage. The opera- ing at optical frequencies. We review the plasmon technol- tion frequency of electrical circuits is limited by the rate ogies and circuits proposed, modeled, and demonstrated at which electrons can move, which in turn is governed over the past decade that have potential applications in by inductive and capacitive effects as well as resistive optical computing and optical information processing. losses associated with electron propagation in materi- Keywords: nanorods; optical computing; optical devices; als. Electronic systems are capable of strong non-linear optical logic devices; optical properties of nanostructures; behaviors, such as switching and state changes, because optical signal processing; plasmonics. of the strong interaction between electrons mediated by their electric fields. More fundamentally, electrons have PACS: 73.20.Mf; 42.79.Sz; 42.79.Ta; 42.79.-e; 78.67.Qa; mass and therefore can be confined in stationary states 78.67.-n. in small regions of space, which is important for memory. Electronic circuits are characterized by electron propagation through wires, resistance, capacitance, inductance, 1 Introduction and non-linear behavior as represented by transistors (Figure 1). Optical information processing (optical computing) has Photonics is based on the propagation of light. Unlike been an active topic of research for at least six decades electrons, the optical signal can be encoded directly on the already [1]. In its original form, Fourier transforms photon wave function, such as by modulating the polari- of coherent light distributions were performed using zation or the phase of the beam, which takes advantage lenses, enabling extremely fast and highly parallel data of the coherent nature of the wave. The carrier frequency processing such as correlations for object recognition. of an optical electromagnetic wave is exceptionally high, There has been renewed interest in all-optical methods in the region of hundreds of terahertz, enabling very large to process signals in communication networks [2, 3] with data transfer rates. However, photons interact very weakly the idea of using optical processing elements to enable with one another, if at all, so that all-optical modulation and switching are difficult to achieve [7]. With very intense beams, it is possible to drive non-linear changes in mate- *Corresponding author: Timothy J. Davis, School of Physics, rial properties, such as refractive index changes, that can University of Melbourne, Parkville, Victoria 3010, Australia, be used for modulation or by using electro-optical effects e-mail: [email protected] whereby an electrical signal changes the optical proper- Daniel E. Gómez: School of Applied Science, RMIT University, Melbourne, Victoria 3000, Australia ties of a material to modulate the light beam. Currently, Ann Roberts: School of Physics, University of Melbourne, Parkville, information in communications networks is processed by Victoria 3010, Australia converting the optical signal into an electronic one and ©2016, Timothy J. Davis et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. 544 T.J. Davis et al.: Plasmonic circuits for manipulating optical information processing. To date, most of the effort has been directed Electronics Wires to understanding the basic principles and demonstrat- Non-linearity Electron propagation Resistance Voltage & current ing devices in proof-of-principle experiments. The resur- Low frequency gence in the study of surface plasmons has been driven by Electron-electron interaction strong the optics community, and as such, most applications of Inductance plasmonics mimic configurations in optics or photonics, Capacitance such as waveguides, devices for converting polarization, Plasmonics optical filters, and so on [12]. However, plasmons repre- Electric charge oscillations on metal surfaces sent the extreme frequency limit of the classical skin effect Oscillations at optical frequencies well known in radio frequency engineering, which has led Propagate on wires as waves – maintain phase coherence Resonate on particles like electrical circuits to descriptions of plasmons in terms of electric circuits, Plasmon-plasmon interactions strong but linear antennas, and electrical filters. In this review, we examine the optical and electrical Photonics circuit descriptions of plasmonics and then present some Waveguide Diffraction Light propagation recent ideas on plasmonic systems that may have applica- Amplitude, phase, polarisation Very high frequency tions in optical-domain information processing. Photon-photon interaction weak Interference Figure 1: A comparison between electronic and photonic circuits. 2 A brief overview of surface The field of plasmonics lies in between. plasmons then applying electronic signal processing methods. This Research into SPPs has burgeoned in the last 16 years, is not a coherent process, and the phase information asso- driven largely by the emergence of methods for nanoscale ciated with the light beam must be decoded and converted structuring of metals [13–20]. Although strictly a quantum into an electrical signal. Moreover, photons have no mass quasi-particle composed of coherent charge oscillations, and therefore cannot remain stationary in space, which plasmons can be described classically using Maxwell’s is problematic for optical memory. Photonic circuits are equations of electromagnetism, which predicts the pres- characterized by waveguides and linear effects such as ence of propagating electromagnetic waves trapped at interference, diffraction, and resonance. the interface between a dielectric and a metal (Figure Intermediate between electronics and photonics lies 2). These are SPPs. They are the combined effect of the plasmonics. Surface plasmon polaritons (SPPs), often electron plasma at the metal surface coupled to the abbreviated to plasmons, are collective oscillations of polarization charges induced in the dielectric. Light with the conduction electrons driven by light at the surfaces a free-space wavenumber k0 = ω/c will create surface εε εε of metals [8, 9]. As such, the plasmons oscillate at optical plasmons with a wavenumber kkspp0=+md/( md), frequencies, around hundreds of terahertz, and are associ- which depends on the relative electric permittivity of ε ε ated with strong electric fields. The plasmons maintain the the metal m(ω) and that of the adjacent dielectric d. At ε ε ε phase relationships with the incident light and are there- optical frequencies, m < 0 is negative and large | m| d ε fore coherent excitations. As plasmons can be confined to so that kkspp0> d . This means that the plasmon wave- regions 100 times smaller than light focused to a diffrac- length is smaller than that of the light and the plasmon tion-limited spot [10], they provide a means to manipu- is unable to radiate into the dielectric, becoming trapped late optical energy at the nanoscale. This size regime lies at the surface (Figure 2A and B). Furthermore, it is then in between that of photonics, where device feature sizes quite difficult to excite surface plasmons as light from the are typically above 1000 nm, and that of electronics, for dielectric cannot be phase-matched to it. However, plas- which transistor feature sizes are now approaching 10 nm. mons radiate at discontinuities such as ridges or pits in Plasmons propagating on metal structures such as plane the surface or abrupt changes in the dielectric, and like- surfaces, grooves, or wires or even confined to small metal wise, light incident on these discontinuities can excite particles [where they are known as localized surface plas- surface plasmons (Figure 2C). These discontinuities can mons (LSPs)] have been described as light on a wire [11]. also reflect and scatter plasmons [20, 21]. It is fair to say that very little has been done using A thick metal film with dielectric materials adjacent plasmonics for optical computing or optical information to both surfaces will support two independent plasmon T.J. Davis et al.: Plasmonic circuits for manipulating optical information 545 A B Surface plasmon polaritons (SPP) 700 Dielectric Propagation direction 600 500 Glass − + − ++− Metal substrate 400 Plasmon on

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