Letter pubs.acs.org/NanoLett
A Plasmonic Fano Switch Wei-Shun Chang,†,⊥ J. Britt Lassiter,‡,⊥ Pattanawit Swanglap,† Heidar Sobhani,§ Saumyakanti Khatua,† Peter Nordlander,‡,§ Naomi J. Halas,*,†,‡,§ and Stephan Link*,†,§ †
Department of Chemistry, ‡Department of Physics and Astronomy, and §Department of Electrical and Computer Engineering, Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States S Supporting Information *
ABSTRACT: Plasmonic clusters can support Fano resonances, where the line shape characteristics are controlled by cluster geometry. Here we show that clusters with a hemicircular central disk surrounded by a circular ring of closely spaced, coupled nanodisks yield Fano-like and non-Fano-like spectra for orthogonal incident polarization orientations. When this structure is incorporated into an uniquely broadband, liquid crystal device geometry, the entire Fano resonance spectrum can be switched on and off in a voltagedependent manner. A reversible transition between the Fano-like and non-Fano-like spectra is induced by relatively low (∼6 V) applied voltages, resulting in a complete on/off switching of the transparency window. KEYWORDS: Surface plasmon resonance, Fano resonance, active plasmonics, liquid crystals, single particle spectroscopy
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sensing modalities. For example, Fano minima can be used to dramatically enhance the Figure of Merit for localized surface plasmon resonance sensing of dielectric media,38−40 and the intense interparticle “hot spots” in the spectral region of Fano minima give rise to large surface enhanced Raman41 and IR absorption12 spectroscopic enhancements. Due to their narrow transparency window, one could also envision Fano resonance structures being used for filtering or color sorting42 applications in next-generation detection and display technologies. However, active Fano resonance-based applications beyond sensing will depend critically on our ability to control or modulate the Fano resonance by external means, ideally with minimal energy input. To date, several schemes for active modulation of plasmonic systems have been explored, typically by embedding structures in active media such as electro- and photochromic molecules,43−46 photoconductive solids,47−49 liquid crystals,50−54 and elastomers.55,56 Just as in sensing applications, the responsivity of a plasmonic nanostructure in active media is determined by its sensitivity, and this should give Fano resonant plasmonic structures a significant advantage in active plasmonics as well. Recently we demonstrated active and reversible control of the far-field scattering of individual nanorods, realizing a nanoparticle-based active plasmonic optical component. We showed that the polarization-dependent scattering of individual gold nanorods was rotated orthogonally to the initially scattered light by a voltagecontrolled in-plane alignment of a liquid crystal medium, independent of nanorod orientation.57
ollective oscillations of the conduction band electrons in metal nanostructures- known as surface plasmons1−7 have attracted significant attention for applications such as enhanced spectroscopies,8−11 nanoscale sensing,12−15 and subwavelength optical waveguiding.16−19 When adjacent nanoparticles couple to each other they can exhibit unique, hybridized plasmon modes.20 These hybridized modes can in turn interact by a variety of physical mechanisms, resulting in exotic lineshapes and interference phenomena such as Fano resonances.21 Fano resonances have been observed in numerous plasmonic systems such as metamaterials,22−24 ring-disk cavities,25 and coupled clusters of nanoparticles.26−36 In each of these structures, a Fano resonance occurs due to the interaction of a broad, superradiant mode and a narrow, subradiant mode.21,37 The superradiant, “bright” mode can couple efficiently to the incident optical field, while the subradiant, “dark” mode couples only very weakly, if at all. If a plasmonic structure has both a sub- and superradiant mode that overlap in energy, then, depending on geometry, the subradiant mode can be excited indirectly by near field coupling to the optically excited superradiant mode. This resulting energy exchange gives rise to a Fano resonance in the scattering spectrum of the cluster, with a line shape possessing a narrow transparency window corresponding to the energy of the subradiant plasmon mode. The unique properties of the plasmonic Fano resonance have drawn extraordinary interest because of their particularly promising potential for active plasmonics applications. Since its properties arise from interparticle coupling, the Fano resonance can be sensitively tuned and tailored by changes in a nearby or surrounding dielectric medium, as well as by changes in geometry. In fact, due to its strong sensitivity to the local environment, plasmonic Fano resonant structures are currently the subject of intense interest for enhancing a range of © XXXX American Chemical Society
Received: July 14, 2012 Revised: August 22, 2012
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Here we demonstrate active modulation of the Fano resonance of a plasmonic cluster in a low-voltage liquid crystal device. An asymmetric octamer structure was designed to have strongly polarization-dependent scattering spectra, possessing a strong Fano resonance for one polarization orientation and no Fano resonance for the orthogonal polarization. We modulated the polarization of the scattered light using an applied voltage by integrating this octamer structure into a liquid crystal device geometry with planar, interdigitated electrodes. The modulation of the Fano resonance resulted from an orthogonal rotation of the far-field scattering spectrum when the liquid crystal medium underwent an electric field-induced transition from a homogeneous nematic phase to a twisted nematic phase. With this device we can turn on and/or off the Fano-like minimum with low (∼6 V) applied voltages. We first examined the asymmetric octamer structure, where a large, central, hemicircular disk was surrounded by an evenly spaced circular ring of seven smaller nanodisks (Figure 1A, inset). We have recently shown that for symmetric plasmonic clusters, the depth of the Fano minimum is controlled by the
size of the central disk relative to the size and number of the ring of coupled peripheral disks, and by tuning this geometry the Fano minimum can be reduced to zero.35 For the symmetric structure, the Fano resonance is independent of polarization direction. However, here we have broken symmetry by removing half of the large central disk, which results in a strongly polarization dependent response. Asymmetric octamers were patterned by electron beam lithography on an indium tin oxide (ITO) substrate, and were fabricated by depositing a 30 nm Au layer with a 2 nm Ti adhesion layer via electron beam evaporation. The sample was then spin-coated with a thin polyimide film in preparation for insertion into a liquid crystal cell. Polarization-resolved measurements of individual octamers were performed using transmission dark-field scattering spectroscopy (see Supporting Information for more details). Essentially, unpolarized white light was focused on the sample by a dark-field condenser and the scattered light from a single octamer cluster was collected by an objective, then passed through a polarizer before being spectrally dispersed by a spectrometer. The polarization-dependent scattering spectrum of a single asymmetric octamer is shown in Figure 1A. Here the blue curve represents the scattering spectrum of the octamer structure as collected through a polarization analyzer oriented at 0° (as defined by the blue arrow in Figure 1A with respect to the octamer, shown in the inset SEM image). The red curve represents the scattering spectrum of the cluster with the polarization of light oriented at 90° (Figure 1A, red arrow). At the 0° polarization angle the asymmetric octamer structure displays a strong Fano resonance with the characteristic transparency window at 800 nm, while at 90° no Fano resonance was observed. By rotating the analyzer through the oblique angles between 0 and 90° (black curves, Figure 1A), it is evident that the modes observed at 0 and 90° are the fundamental modes, because each of these angle-dependent spectra can be understood as weighted averages of the 0 and 90° cases (Supporting Information Figure S1). Therefore, we refer to the polarized spectra at 0° as “Fano-like” and at 90° as “non-Fano-like” in the following discussions. To understand the origin of this polarization-dependent behavior we performed electromagnetic simulations of this structure using the finite difference time domain (FDTD) method (Lumerical), utilizing the empirical dielectric function for gold,58 an index of refraction of n = 1.7 for the ITO substrate, and an effective refractive index for the finite (tens of nanometers) polyimide overlayer of n = 1.5. The simulated spectra shown in Figure 1B agree extremely well with our experimental observations of a strong Fano resonance line shape for polarization along the 0° direction, and no Fano resonance is observed for orthogonally polarized light. The charge distribution on the nanocluster, calculated at several key wavelengths, provides insight into the underlying plasmon modes of the cluster. For the 0° polarization (Figure 1C), a typical superradiant mode is observed in the charge distribution for the peaks at both 915 and 670 nm; here, the dipole moments of the center particle and the outer ring of particles are aligned. At the dip at 800 nm, however, the charge distribution is characteristic of a subradiant mode, where the dipole moments of the outer ring and the center particle are antialigned. This confirms that the spectral line shape observed in the 0° polarization is a Fano resonance caused by interference between the narrow subradiant mode at 800 nm and the broad superradiant mode. For 90° polarization (Figure
Figure 1. Fano resonance of a plasmonic octamer cluster. (A) Experimental scattering spectra of a single asymmetric octamer on an ITO substrate and covered with polyimide. The blue and red lines represent the scattering spectra with detection polarization at 0 (blue) and 90° (red), respectively. Black lines are the polarized spectra with the polarization varying from 15 to 75° in 15° increments. (Inset) SEM of a representative octamer. (B) Simulated scattering spectra of an octamer with excitation polarization at 0 (blue) and 90° (red) as defined in (A). (C) Simulated charge distributions of an octamer excited with polarization at 0° for several characteristic wavelengths. (D) Simulated charge distributions when excited with polarization at 90°. B
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(Supporting Information Figure S3), while the liquid crystal had no effect on the scattering spectrum. At room temperature, 5CB is in the homogeneous nematic phase with its director oriented parallel to 0°, as defined in the inset of Figure 1A, determined by the directional rubbing of the polyimide film prior to the assembly of the device. By applying a voltage (∼6 V, 1 kHz) to the IDA electrodes, a transition from a homogeneous nematic phase to a twisted nematic phase of the liquid crystal was induced. This phase transition was observed using a bright-field polarization microscope (Figure 2C). The liquid crystal device was placed between a polarizer (P) and an analyzer (A) with orthogonal orientations such that the nematic director of the liquid crystal device was parallel to the polarization direction of the polarizer. White light from a halogen lamp was passed through the polarizer and focused onto the device. The transmittance image was collected by an oil-immersion objective, passed through the analyzer, and was imaged using a SLR camera (Canon). Figure 2D,E displays the transmittance images of the liquid crystal device with voltage off (Voff) and on (Von), respectively. With no applied voltage (Voff), the transmittance image was completely dark, because the nematic director was homogeneous throughout the liquid crystal cell and aligned with the polarization direction of the polarizer. In this case, the polarization of the incident light was not altered as it passed through the liquid crystal cell but was blocked by the analyzer oriented orthogonally with respect to the polarizer. In contrast, when the voltage was applied to the IDA electrodes (Von), the nematic director in the plane of the electrodes was rotated to align with the direction of the electric field (perpendicular to the Voff direction) while at the opposing surface the nematic director remained aligned with the rubbing direction of the polyimide. As a result, in the Von state, the liquid crystal director was changed continuously from 0 to 90° in the direction of light propagation through the liquid crystal cell, resulting in a twisted nematic phase.57 This caused the polarization direction of the incident light to be changed upon passing through the liquid crystal device, creating the bright transmittance image observed in Figure 2E. The dark stripes in the image are the IDA electrodes. By performing polarized scattering spectroscopy of individual octamers in the device, we observed a reversible modulation of the Fano resonance upon switching between the Voff and Von states (Figure 3). The dark-field microscope geometry used for this measurement is illustrated in Figure 3A. An unpolarized white light source was focused onto the sample by a dark-field condenser (Zeiss, N.A. = 1.4) to excite the plasmon resonances of the octamer. The light scattered by the octamer passed through the liquid crystal device, was collected by an oilimmersion objective (Zeiss, N.A. = 0.7), and detected using a liquid nitrogen cooled CCD camera attached to a spectrometer. Because a dark-field collection geometry was used, only the light scattered by the octamer structure was collected. A polarization analyzer placed in front of the spectrometer ensured that the polarization dependence of the scattering spectrum could be observed. Figure 3B,C shows the scattering spectra of a single octamer detected with the analyzer oriented at 0 (Figure 3B) and 90° (Figure 3C). These spectra were normalized postcollection with the spectrum of a nearby region of the device where no octamer was present in order to eliminate any effects of light scattering by the liquid crystal medium. The solid (dashed) lines represent the spectrum observed when the liquid crystal device was in the Voff (Von)
1D), only one broad mode is found at 690 nm. The charge distribution at 690 nm shows that this is a typical superradiant mode. At 800 nm, the charge distribution reveals the same superradiant mode, confirming that there is no subradiant mode present for this polarization, and thus no Fano resonance. Therefore, the observed polarization-dependent scattering spectra can be explained by the asymmetry of this octamer structure because it supports a subradiant mode only for the 0° polarization and not for the 90° polarization. The octamer nanostructures were patterned using electronbeam lithography and assembled into a liquid crystal device composed of an ITO glass substrate with the octamer nanostructures and a coverslip separated by glass beads (∼6 μm) (shown schematically in Figure 2A). As mentioned above,
Figure 2. Geometry of the liquid crystal device. (A) Side view of the liquid crystal device which was composed of an ITO coated glass slide and a coverslip spaced by glass beads (GB). The octamers (gold clusters) were located on the ITO glass slide and covered by a thin polyimide film (PI, pink layer). The IDA electrodes (gold bars) were placed on the coverslip. (B) Top view of the IDA electrodes. (C) Experimental scheme for the transmittance measurements. P, polarizer; A, analyzer. (D,E) Transmittance images of the liquid crystal device for a crossed polarizer/analyzer setting with Voff (D) and Von (E), respectively. The nematic director was aligned parallel to the direction of the polarizer.
a thin polyimide film (Nissan Chemical) was spin-cast onto the ITO substrate already patterned with octamer structures. The polyimide film serves to align the subsequent liquid crystal layer and in order to do so was rubbed unidirectionally, by mechanical means, parallel to 0° (Figure 1A). To switch the liquid crystal direction in the sample plane, gold interdigitated array (IDA) electrodes were fabricated on the coverslip by photolithography (Figure 2B).57 The height of the electrodes was 60 nm and the separation between electrodes was 10 μm. After spin-casting the glass beads onto the coverslip, the liquid crystal device was carefully assembled under a microscope to ensure spatial overlap between the areas of the octamers and the IDA electrodes. This allowed for a significant number of octamers to be located in the gaps between the electrodes (Supporting Information Figure S2). The rubbing direction of the polyimide film was aligned parallel to the axis of the electrodes. The nematic liquid crystal 4-cyano-4-n-pentylbiphenyl (5CB, TCI America) was inserted into the cell by capillary action while heating the sample to the temperature range where the 5CB is in the isotropic phase. After 5CB was inserted into the cell, it was cooled very slowly to room temperature and sealed with epoxy glue. Coating the octamers with the polyimide film resulted in a redshift of ∼100 nm C
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Figure 4. Effect of the liquid crystal medium on the polarized octamer scattering spectra when applying a voltage. (A,B) Simulation of the octamer spectra as observed after passing through the liquid crystal device for the Fano-like (A) and non-Fano-like (B) cases. The spectra are identical for orthogonal polarizations when comparing Voff and Von, indicating a 90° rotation of the scattering spectra with applied voltage. (C,D) Corresponding experimental scattering spectra of the octamer in the liquid crystal device for the condition showing the Fano-like spectrum (C) and non-Fano-like spectrum (D), as taken from Figure 3. In agreement with the simulations, each spectrum was reproduced in the orthogonal polarization state when the voltage was applied to the liquid crystal device.
Figure 3. Active modulation of the Fano resonance. (A) Experimental scheme of the dark-field spectroscopy (see the Supporting Information for details). (B,C) Scattering spectra of an individual octamer with Von and Voff detected for polarization directions of 0 (B) and 90° (C). The thickness of this liquid crystal device was measured to be 7.5 μm, which was slightly larger than the glass bead spacers, possibly due to bead size variations or the presence of small aggregates of glass beads. The switching voltage was 6 V.
state. When the analyzer was oriented in the 0° direction (Figure 3B), a Fano-like spectrum was detected when the liquid crystal device was in the Voff state (solid blue line). By applying a voltage to the liquid crystal device (Von state, dashed blue line), the Fano resonance was switched off and the non-Fanolike spectrum was detected instead. We observed the same switching behavior when the analyzer was oriented at 90° (Figure 3C), except that the spectra of the Voff and Von states were reversed. In this case, the non-Fano-like spectrum was observed when the liquid crystal device was in the Voff state (red solid line) while the Fano-like signature was seen in the Von state (red dashed line). The observed switching behavior arises from a 90° rotation of the octamer’s scattering spectrum in the Von state. To substantiate this, we first modeled the light transmittance through a homogeneous nematic liquid crystal phase, representing the Voff state, and a twisted nematic liquid crystal phase, corresponding to the Von state, using Jones calculus (see Supporting Information). As illustrated in Supporting Information Figure S4, the polarization of broadband light passing through the liquid crystal device is invariant for Voff, while for Von the polarization is rotated almost perfectly by 90° over the entire wavelength range. On the basis of this electrooptical response, the working principle of our Fano switch device can be modeled quantitatively using the simulated polarization-dependent octamer scattering spectra from Figure 1B as the light input (Figure 4A,B). The Fano-like spectrum is observed when the liquid crystal device is in the Voff state and the analyzer set to 0°, but also in the Von state when the analyzer is in the 90° orientation (Figure 4A), matching the behavior observed experimentally (Figure 4C, replotted from Figure 3). An identical process occurs for the non-Fano-like spectrum embedded in the device as seen in Figure 4B, matching again very well with the experiment (Figure 4D). These results confirm that both the Fano-like and non-Fanolike spectra are rotated by 90°, independent of wavelength when a voltage induces a transition to a twisted nematic phase in our device. This in-plane liquid crystal phase transition makes our device unique because the orthogonal polarization
rotation is virtually independent of incident wavelength over a range of several hundred nanometers, as well as insensitive to device thickness (for devices thicker than 3 μm). A 90° polarization rotation could in principle also be achieved with a conventional half-wave plate configuration (e.g., a nematic liquid crystal with the director aligned 45° with respect to the incident polarization), but this would require the operating wavelength and device thickness to be precisely and carefully matched. The realization of this prototype Fano switch is very promising for potential applications. Several aspects of the Fano switch can still be improved. For example, a faster switching speed can be achieved with liquid crystals having a lower viscosity, by designing the switch to be based on a different liquid crystalline phase, or by reducing the thickness of the device. The operation voltage can be further reduced by using electrodes with smaller gaps and/or liquid crystals with higher dielectric anisotropies. Finally, the contrast of the modulation could be increased by designing a plasmonic structure with better overlap between the Fano-like and nonFano-like spectra so that the Fano dip is further maximized. In summary, we have demonstrated the on−off switching of the Fano resonance of a plasmonic cluster by its incorporation into a broadband polarization rotating liquid crystal device. An octamer with a half-disk as the central particle was designed to specifically achieve polarized Fano-like and non-Fano-like spectra at orthogonal polarization directions. Active switching of the Fano resonance resulted from the rotation of the polarized spectra by 90°, independent of wavelength over the entire spectral range used, in a novel, broadband liquid crystal device geometry. Active, low-voltage switching of Fano resonances represents an important step toward the realization of active plasmonic devices whose vivid optical and chemically inert properties are likely to generate applications in a broad range of photonic technologies, such as active filters, spectrum or polarization analyzers, and robust color displays. D
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ASSOCIATED CONTENT
S Supporting Information *
Description of the single particle measurements, reconstructed polarized spectra of the octamer using the orthogonal polarized Fano-like and non-Fano-like spectra, experimental and simulated spectra of an octamer in air, dark-field image of a liquid crystal cell composed of octamers and IDA electrodes, and Jones matrix description for the simulation of the liquid crystal transmittance spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (N.J.H.)
[email protected]; (S.L.)
[email protected]. Author Contributions ⊥
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the Office of Naval Research (N00014-10-1-0989) and the Robert A. Welch Foundation under Grants C-1664 (S.L.), C-1220 (N.J.H.), and C-1222 (P.N.). P.S. acknowledges support from the Royal Thai Government.
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