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Modulating light on the nanometer scale using polyvalent metal

A compact optical switch exploits the phase-transition properties of polyvalent metallic gallium to provide efficient control of light.
1 December 2011, SPIE Newsroom. DOI: 10.1117/2.1201111.003912

Active control of light is a crucial issue in developing modern circuits, especially those involving optical switches and modulators. Silicon (Si)-based devices need either high power or nonplanar structures because of the relatively weak nonlinear optical properties of Si. These requirements render them unsuitable for dense circuit component integration and hence limit their applications. As a promising, ultra-small-scale approach to the problem, surface plasmon polaritons (SPPs)—optical modes concentrated at the interface between metallic and dielectric (insulating) materials—boast a number of advantages, including small guiding wavelengths. Especially important for our purposes is that propagation of SPPs is extremely sensitive to minor changes in optical properties in the vicinity of the interface. Dynamically controlling this propagation can be realized either by applying a voltage1, 2 to the dielectric material or shining light on it.3,4

Recently, researchers have been increasingly drawn to the enhanced nonlinear effect of phase transitions in polyvalent metals, which can have different charges, such as gallium (Ga). The significant difference in optical properties between the two phases of Ga—α and metallic—make it a very suitable material for plasmonic switches. We exploited5 this feature in constructing an optical switch consisting of a simple metal-insulator-metal (MIM) plasmonic waveguide with a silicon nitride (Si3N4) core sandwiched between two Ga film layers.

Figure 1. (a) The simulated structure of the optical switch. (b and c) Simulation results of power distribution with α-gallium (Ga) and metallic Ga, respectively. (d) Power transmission of the plasmonic waveguide as a function of wavelength for different phases and crystalline directions of Ga (indicated by AB, AC, and so on). Si3N4: Silicon nitride. wd: Width of input and output silicon (Si) waveguides. wp: Width of the plasmonic waveguide. lp: Length of the plasmonic waveguide. d: Thickness of the metallic Ga, which is sandwiched between Si3N4 and α-Ga.

Figure 1(a) shows the simulated structure in which a Ga–Si3N4–Ga plasmonic waveguide is embedded between two Si dielectric waveguides. The simulations use the frequency-dependent dielectric constants of Ga for three main crystalline axes (a, b, and c). Figure 1(b) and (c) shows the power distribution when Ga is in the α-phase, CB (CB denotes the c-axis lying along the propagation direction and b-axis lying along the transverse direction), and metallic phase, respectively. The power transmission decreases from 24.8% (metallic phase) to 1.06% (α-phase, CB). Figure 1(d) shows the transmission of incident light at different wavelengths. The structural phase of Ga is clearly the main factor in determining the transmission level.

Figure 2. (a) Extinction ratios of the Ga–Si3N4–Ga plasmonic waveguide as a function of the thickness (d) of sandwiched metallic Ga. (b) Power transmission as a function of d.

Figure 3. (a) Proposed 3D structure. (b) Extinction ratio as a function of thickness of Si3N4or Ga film.

Another interesting property of Ga is the coexistence of different structural forms of the element, which is also known as the surface-mediated effect.6 Figure 1(a) shows a very thin layer of metallic Ga with thickness d developing at the interface of Si3N4 and α-Ga. As the external light intensity or temperature increases, the metallic Ga grows steadily thicker as well.6 We simulated the conditions under which this happens. We also showed how it will impact the transmission and extinction ratio of the amount of light passed on an oriented axis of the modulator.

Figure 2(a) and (b) shows how the extinction ratio of the waveguide and transmission, respectively, increase rapidly in the presence of metallic Ga tens of nanometers in thickness. At a wavelength of 1550nm, the extinction ratio reaches 9.98dB and transmission is improved by 10 times its original value with metallic Ga (40nm-thick). Figure 3(a) shows our proposed optical switch in a 3D configuration, which can be made in just a few standard semiconductor fabrication steps. One challenge of the process is the conformity of the deposited film—either Ga or Si3N4—thicknesses. Figure 3(b) shows that Ga film thickness is more critical: ±10% variation in thickness will cause a maximum 15% change in the extinction ratio.

In summary, we have proposed a compact, efficient optical switch based on a simple MIM waveguide. Since the phase transition of Ga is a surface-mediated effect, we also show that metallic Ga tens of nanometers thick sandwiched between Si3N4 and α-Ga greatly increases the power-transmission level. We are now working on new structures to further improve the performance of the switch.

This work is supported in part by the US Army under grant W911NF-10-1-0153 and the National Science Foundation under grant ECCS-1057381.

Wangshi Zhao, Zhaolin Lu
Rochester Institute of Technology
Rochester, NY

Wangshi Zhao received her BS in information science and electrical engineering from Zhejiang University, China. Currently, she is a PhD candidate in microsystems engineering. Her research focus includes nanoplasmonics and metamaterials.

Zhaolin Lu is an assistant professor in the Department of Microsystems Engineering, where he established the Nanoplasmonics and Metamaterials Laboratory. The focus of this laboratory is both the theoretical and experimental aspects of metamaterials, photonic crystals, active and passive nanophotonic elements, and their integration into optoelectronic subsystems. His research interests include passive and active nanoplasmonic devices, photonic crystal devices, and optical waveguides for application in next-generation optoelectronic systems, 3D metamaterials and applications, and 3D negative refraction imaging.

1. W. Cai, J. S. White, M. L. Brongersma, Compact, high-speed, and power-efficient electrooptic plasmonic modulators, Nano Lett. 9, pp. 4403-4411, 2009. doi:10.1021/nl902701b
2. T. Nikolajsen, K. Leosson, S. I. Bozhevolnyi, Surface plasmon polariton based modulators and switches operating at telecom wavelengths, Appl. Phys. Lett. 85, pp. 5833-5835, 2004. doi:10.1063/1.1835997
3. J. Dintinger, I. Robel, P. V. Kamat, C. Genet, T. W. Ebbesen, Terahertz all-optical molecule-plasmon modulation, Adv. Mater. 18, pp. 1645-1648, 2006. doi:10.1002/adma.200600366
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6. A. V. Krasavin, N. I. Zheludev, Active plasmonics: controlling signals in Au/Ga waveguide using nanoscale structural transformations, Appl. Phys. Lett. 84, pp. 1416-1418, 2004. doi:10.1063/1.1650904