University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Publications from the Department of Electrical & Computer Engineering, Department of Electrical and Computer Engineering 2014 Tunable Plasmonic and Hyperbolic Metamaterials Based on Enhanced Nonlinear Response Christos Argyropoulos University of Texas at Austin, [email protected] Francesco Monticone University of Texas at Austin Nasim Mohammadi Estakhri University of Texas at Austin Andrea Alu University of Texas at Austin, [email protected] Follow this and additional works at: http://digitalcommons.unl.edu/electricalengineeringfacpub Part of the Computer Engineering Commons, and the Electrical and Computer Engineering Commons Argyropoulos, Christos; Monticone, Francesco; Estakhri, Nasim Mohammadi; and Alu, Andrea, "Tunable Plasmonic and Hyperbolic Metamaterials Based on Enhanced Nonlinear Response" (2014). Faculty Publications from the Department of Electrical and Computer Engineering. 410. http://digitalcommons.unl.edu/electricalengineeringfacpub/410 This Article is brought to you for free and open access by the Electrical & Computer Engineering, Department of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications from the Department of Electrical and Computer Engineering by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Hindawi Publishing Corporation International Journal of Antennas and Propagation Volume 2014, Article ID 532634, 11 pages http://dx.doi.org/10.1155/2014/532634 Research Article Tunable Plasmonic and Hyperbolic Metamaterials Based on Enhanced Nonlinear Response Christos Argyropoulos, Francesco Monticone, Nasim Mohammadi Estakhri, and Andrea Alù DepartmentofElectrical&ComputerEngineering,TheUniversityofTexasatAustin,Austin,TX78712,USA Correspondence should be addressed to Andrea Alu;` [email protected] Received 2 December 2013; Accepted 18 February 2014; Published 6 April 2014 Academic Editor: Giacomo Oliveri Copyright © 2014 Christos Argyropoulos et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We present here tunable and reconfigurable designs of linear and nonlinear plasmonic and hyperbolic metamaterials. Rich scattering features of multilayered composite nanoparticles are demonstrated, which include complex and exotic scattering signatures combining multiple dipolar Fano resonances and electromagnetic induced transparency (EIT) features. These dipole- dipole multi-Fano scattering responses can be further tuned through altering the plasmonic properties of the concentric layers or the permittivity of the core, for instance, by the presence of nonlinearities. Strong third-order nonlinear effects, such as optical bistability, may also be induced in the scattering response of nonlinear nanoparticles due to the highly enhanced and confined fields inside their core. Nonlinear hyperbolic metamaterial designs are also explored, which can realize tunable positive-to-negative refraction at the same frequency, as a function of the input intensity. Negative Goos-Hanchen¨ shift is demonstrated based only on the hyperbolic dispersion properties of these layered metamaterials without the usual need of negative index metamaterials. The Goos-Hanchen¨ shift may be tuned from positive-to-negative values, when the structure is illuminated with different frequencies. A plethora of applications are envisioned based on the proposed tunable metamaterials, such as ultrafast reconfigurable imaging devices, tunable sensors, novel nanotag designs, and efficient all-optical switches and memories. 1. Introduction fields [16], temperature [17, 18], liquid crystals [19, 20], graphene [21, 22], phase-change media [23, 24], and elec- Metamaterials are artificially constructed materials that can trooptical effects25 [ –27]. exhibit novel functionalities not available in nature. Unusual Nonlinear optical effects can also be strongly enhanced electromagnetic properties, such as negative refraction [1] with plasmonic metamaterial structures, mainly due to the and invisibility [2, 3], have been achieved with differ- highly localized fields obtained in these structures. For exam- ent metamaterial structures. These interesting properties ple, we recently demonstrated that strong optical bistability have been obtained with various metamaterial designs at can arise when nonlinear plasmonic waveguides operate at microwaves, terahertz, optical, and ultraviolet frequencies their cut-off frequency [28]. Furthermore, inspired by the [4–10]. Recently, increased attention has been dedicated to recent interest in the concept of Fano scattering resonances the study of reconfigurable and tunable metamaterials and [29], we reported giant all-optical scattering switches based nonlinear plasmonic devices, where even more exotic and on nonlinear core-shell plasmonic nanoparticles [30]. The breakthrough functionalities may be achieved [11, 12]. Sev- proposed nanoparticles exhibited purely dipolar Fano reso- eral reconfigurable and tunable metamaterial and plasmonic nances, in contrast to more conventional Fano resonances devices have been presented in the recent literature. Their based on the interaction and coupling of dipolar and higher- operation may be controlled with an applied voltage (varac- order scattering modes supported by complex subwavelength tors) [13, 14], electromagnetic forces [15], external magnetic plasmonic systems [31–35]. The concept of purely dipolar 2 International Journal of Antennas and Propagation ac2 ac4 ac3 ac2 ac1 ac1 a core core a Plasmonic shells (a) (b) Figure 1: Geometry of composite nanoparticles consisting of (a) two and (b) four plasmonic layers, respectively. The core is made of dielectric material. Fano resonances has also been extended to multilayered permittivity 2, which follows a similar Drude dispersion plasmonic nanoparticles, inducing Fano-comb scattering with a plasma frequency increased by Δ.Onewayto responses [36, 37]. Cloaking and resonant scattering states achieve this effect could be to modify the doping level of excited within the proposed plasmonic shells may be used, plasmonic semiconductors, similar to our previous work [37] respectively,asthedarkandbrightmodestoinducemultiple in which nanoparticles with four plasmonic shells based dipolar Fano-like scattering features. on aluminum-doped zinc oxide (AZO) semiconductors [41] In this paper, we further study the interesting scattering were utilized to produce Fano scattering combs. properties of multilayered plasmonic nanoparticles in order Following the analysis presented in [36], we calculate the to demonstrate tunable Fano-comb operation for sensing quasi-static conditions for resonant scattering and cloaking and nanotagging applications. Third-order optical nonlinear based on Mie theory [42]. The contour plots of the total materialsloadedinthecoreoftheseplasmoniccomposite SCS of this composite nanoparticle with two plasmonic layers nanoparticlesareshowntoinducelargebistabilityforeach are shown in Figures 2(a)–2(d) as a function of the aspect dipolar Fano resonance, due to the enhanced and strongly ratio and the wavelength of operation, where the plasma localized electric fields inside the core of the device. More- frequency of the second plasmonic layer is increased by (a) over, reconfigurable nonlinear hyperbolic layered metama- Δ = 750 THz, (b) Δ = 500 THz, (c) Δ = 250 THz, terial structures will be studied, which may achieve tunable and (d) Δ =50THz in each case, respectively. The red positive-to-negative refraction as a function of the input radi- and blue thick curves in these plots indicate the dispersion of ation intensity [38]. We will also demonstrate that hyperbolic quasi-static conditions for resonant scattering and cloaking, metamaterials can realize strong negative Goos-Hanchen¨ respectively. We fix the geometry picking a large aspect ratio shifts [39] without the need of negative refractive index meta- = 0.91 and calculate the dynamic normalized scattering 2 materials. Reconfigurable operation for Goos-Hanchen¨ shift response (SCS/ ),whichisshowninFigures2(e)–2(h) for will be presented based on layered metamaterial structures as each increased plasma frequency of the second plasmonic a function of the frequency of the impinging radiation. layer. The dimensions of the plasmonic layers are equal to 1 = / =55nm and 2 =60nm. In Figures 2(e)–2(h) it canbeseenthattwopurelydipolarsharpFanoresonancesare 2. Plasmonic Composite Nanoparticles formed in the scattering response of the composite plasmonic nanoparticle for different plasma frequencies of the second Two composite plasmonic nanoparticle geometries are con- layer. It is interesting that the second Fano resonance is sideredinthefollowing,asshowninFigure 1.First,we getting closer to the first as we reduce Δ,whereasthe calculate the scattering response of the geometry depicted first one stays almost unperturbed at the same wavelength in Figure 1(a), consisting of a dielectric core with radius position. This demonstrates the high tunability potential of =50 =2 nmandrelativepermittivity core and two the proposed double-layer nanoparticle design, especially for = / plasmonic coating layers. The aspect ratio 1 is the second Fano-like resonance. defined as the ratio between the radius of the dielectric core Next, the same nanoparticle