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Transparent Conducting Oxides for Electro-Optical Plasmonic Modulators

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Transparent Conducting Oxides for Electro-Optical Plasmonic Modulators Nanophotonics 2015; 4:165–185 Review Article Open Access Viktoriia E. Babicheva*, Alexandra Boltasseva, and Andrei V. Lavrinenko Transparent conducting oxides for electro-optical plasmonic modulators DOI 10.1515/nanoph-2015-0004 1 Introduction Received January 21, 2015; accepted February 3, 2015 Abstract: The ongoing quest for ultra-compact optical The advent of broadband optical signals in telecommuni- devices has reached a bottleneck due to the diffraction cation systems led to unprecedented high bit rates that limit in conventional photonics. New approaches that pro- were unachievable in the electronic domain. Later on, vide subwavelength optical elements, and therefore lead photonics presented a new platform for high-speed data to miniaturization of the entire photonic circuit, are ur- transfer by implementing hybrid electronic-photonic cir- gently required. Plasmonics, which combines nanoscale cuits, where data transmission using light instead of elec- light confinement and optical-speed processing of signals, tric signals significantly boosted the data exchange rates has the potential to enable the next generation of hy- on such photonic/electronic chips [1–3]. Different silicon- brid information-processing devices, which are superior based photonic passive waveguides have recently been to the current photonic dielectric components in terms of proposed and realized as possible optical interconnects speed and compactness. New plasmonic materials (other for electronic chip communications [4]. Nevertheless, the than metals), or optical materials with metal-like behav- main building block in the next generation networks ar- ior, have recently attracted a lot of attention due to the chitecture is an effective electro-optical modulator. promise they hold to enable low-loss, tunable, CMOS- There are several different physical effects for light compatible devices for photonic technologies. In this re- modulation as well as different material platforms, which view, we provide a systematic overview of various com- can be grouped according to the three main mechanisms: pact optical modulator designs that utilize a class of the thermo-optical, electro-optical, and electro-absorption. most promising new materials as the active layer or core— The first case is connected with changes in the opti- namely, transparent conducting oxides. Such modulators cal parameters with temperature. In spite of the principal can be made low-loss, compact, and exhibit high tunabil- speed limitation due to heat dissipation issues, some on- ity while offering low cost and compatibility with existing chip devices with a reasonable operation speed (predicted semiconductor technologies. A detailed analysis of differ- switching time of 1 µs) were demonstrated [5]. ent configurations and their working characteristics, such Much faster devices can be realized using the electro- as their extinction ratio, compactness, bandwidth, and optical mechanism, where variations in the complex re- losses, is performed identifying the most promising de- fractive index are induced by an electric field or current. signs. The nomenclature of electro-optical effects contains the linear electro-optic effect (Pockels effect), the quadratic Keywords: modulators; electro-optical materials; waveg- electro-optic effect (Kerr effect), and carrier concentration uide modulators; nanocircuits; plasmonics; surface plas- change effect. The Pockels effect isa χ(2) effect and can- mons; active plasmonics; transparent conducting oxides; not be exhibited by materials with centrosymmetric lat- epsilon-near-zero materials tices, like silicon. However, under distortion of the lat- tice by strain, such an effect appears [6]. The Kerr effect, where changes in the refractive index are proportional to *Corresponding Author: Viktoriia E. Babicheva: DTU Fotonik – Department of Photonics Engineering, Technical University of Denmark, Oersteds Plads 343, DK-2800 Kgs. Lyngby, Denmark and DTU Fotonik – Department of Photonics Engineering, Technical and ITMO University, Kronverkskiy, 49, St. Petersburg 197101, Rus- University of Denmark, Oersteds Plads 343, DK-2800 Kgs. Lyngby, sia, E-mail: [email protected] Denmark Alexandra Boltasseva: School of Electrical & Computer Engineer- Andrei V. Lavrinenko: DTU Fotonik – Department of Photonics ing and Birck Nanotechnology Center, Purdue University, 1205 West Engineering, Technical University of Denmark, Oersteds Plads 343, State Street, West Lafayette, IN 47907-2057 USA DK-2800 Kgs. Lyngby, Denmark © 2015 Viktoriia E. Babicheva et al., licensee De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. 166 Ë Viktoriia E. Babicheva et al. the square of electric field, has broader presentation in ma- graphene sheets both in the optical or terahertz ranges has terials including silicon [7]. Although, both electro-optical gained massive attention [18–22]. effects are fast, they are rather weak. The strongest Pockels Overall, from the material point of view, the integrated effect is exhibited by 3LiNbO , which is a classical material silicon platform is the most established. However, to com- for electro-optical modulators, but hardly can be used in pete with the compactness of conventional electronic ele- the subwavelength devices. The modulation speed of de- ments, photonic solutions have to overcome the diffraction vices based on such effects is estimated to be about 100 limit of light by implementing nanoscale optical compo- GHz or more [8, 9]. nents. The refractive index changes induced by a change in One avenue that offers subwavelength light confine- the carrier concentration have been also the focus of sig- ment is to employ surface plasmon polaritons (SPPs) [23– nificant research attention for decades [3, 7, 10]. Gigahertz 34]. These are surface waves existing on metal-dielectric operating frequencies in silicon on-chip modulators were interfaces. SPPs are tightly confined to an interface and al- demonstrated a decade ago [11], and recently, the speed low for the manipulation of light at the nanoscale, thus has been pushed up to 40 Gbit/s in a Mach–Zehnder in- leading to a new generation of fast on-chip devices with terferometer scheme [12]. The carrier concentration can be unique capabilities. In particular, they could offer a higher altered in different ways, namely accumulation, injection, bandwidth and reduced power consumption [35–37]. or depletion of free carriers, each method having its pros To provide the basic nanophotonic circuitry function- and cons [3]. alities, elementary circuit components such as waveg- The focus on the electro-absorption mechanism was uides, modulators, sources, amplifiers, and photodetec- created recently, when it was shown that the absorption tors are required [35–41]. Among them, a waveguide is the factor (in other words, the imaginary part of the refractive key passive component connecting all elements in the cir- index) can be quickly and efficiently altered by an external cuit. Various designs of plasmonic waveguides have been electric field. Such an effect appears due to the distortion proposed, aiming to achieve the highest mode localization of wave functions of holes and electrons in the bandgap while maintaining reasonable propagation losses [42–58]. followed by a slight red-shift of the bandgap. Named as However, because of the high intrinsic losses of plasmonic the Franz–Keldysh effect in bulk semiconductors or the materials, signal propagation is highly damped even for quantum-confined Stark effect in thin heterostructure lay- the best (with the highest DC conductivity) metals such as ers, the effect was utilized in modulator designs [3, 13– silver and gold. Thus, the ultimate goal is to reduce mate- 16] exposing performance comparable (28 Gb/s in [13]) to rial losses utilizing new plasmonic materials. photonic-based electro-optic devices. Estimates that a ger- Recently, efforts have been directed towards develop- manium quantum well can potentially operate with a mod- ing new material platforms for integrated plasmonic de- ulation speed up to 100 GHz with the sufficiently high elec- vices [59–66]. Transparent conducting oxides (TCOs), for tric field intensity are presented in [14]. instance, indium tin oxide (ITO) and aluminum-doped Naturally, the modulation mechanism can be pro- zinc oxide (AZO), are oxide semiconductors convention- jected onto a material platform to be employed in de- ally used in optoelectronic devices such as flat panel dis- vices. Electronic fabrication lines prefer the group IV ma- plays and in photovoltaics. The optical response of TCOs terials: silicon and germanium. III–V semiconductors ex- is governed by free electrons, whose density is controlled hibit even higher nonlinearities and larger bandgaps; how- through the addition of n-type dopants. The free carrier ever, their integration with the silicon-on-insulator plat- concentration in TCOs can be high enough so that TCOs form still brings a lot of challenges. Polymers with nonlin- exhibit metal-like behavior in the near-infrared (NIR) and ear inclusions are considered as cheap alternatives to the mid-infrared (MIR) ranges, and can be exploited for sub- conventional modulator materials, especially for filling wavelength light manipulation (Figure 1). Moreover, in nanoscaled gaps and slits in waveguides. Such small vol- contrast to the optical properties of noble metals, which umes intensify light-matter interactions due to the high- cannot be tuned or changed, the permittivity
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