Exploiting Evanescent Field Polarization for Giant Chiroptical

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Exploiting Evanescent Field Polarization for Giant Chiroptical Modulation from Achiral Gold Half-Rings Kyle Warren Smith, Lauren Ann McCarthy, Alessandro Alabastri, Luca Bursi, Wei-Shun Chang, Peter Nordlander, and Stephan Link ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07060 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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Exploiting Evanescent Field Polarization for Giant Chiroptical Modulation from Achiral Gold HalfRings Kyle W. Smith†,‡,‖ Lauren A. McCarthy†,‡,‖, Alessandro Alabastri‡,§,, Luca Bursi‡,§, Wei-Shun Chang†,‡, Peter Nordlander‡,§,⊥, Stephan Link†, ‡,⊥* †

Department of Chemistry, Rice University, 6100 Main Street, MS 60, Houston, Texas 77005, United States ‡

Laboratory for Nanophotonics, Rice University, 6100 Main Street, MS 60, Houston, Texas 77005, United States

§

Department of Physics and Astronomy, MS 61, Rice University, 6100 Main Street, Houston, Texas 77005, United States

⊥Department

of Electrical and Computer Engineering, Rice University, 6100 Main Street, MS 366, Houston, Texas 77005, United States

KEYWORDS: evanescent fields, longitudinal field oscillations, nanoantenna, polarization discrimination, extrinsic chirality, planar chirality 1 ACS Paragon Plus Environment

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ABSTRACT: For applications seeking to realize on-chip polarization discriminating nanoantennas, efficient energy conversion from surface waves to far-field radiation is desirable. However, the response of individual nanoantennas to the particular polarization states achievable in surface waves, such as evanescent fields, has not yet been thoroughly investigated. Here, we report giant modulation of visible light scattering from achiral gold half-rings when switching between evanescent surface wave excitation produced from total internal reflection of left-handed and right-handed circularly polarized light. The effect is driven by a differing relative phase between the in-plane transverse and longitudinal field oscillations of the evanescent wave depending on the incident light handedness. As longitudinal field oscillations are not found in freespace excitation, this presents a fundamentally different mechanism for chiroptical responses as traditional mechanisms for circular dichroism only account for purely transversal field oscillations. Although the half-ring scattering modulation is dependent on the wave vector orientation, an orientation invariant response is also realized in planar chiral nanoantennas composed of eight half-rings in a rotationally symmetric arrangement, with up to 50% scattering modulation observed at 725 nm. Though both structures are found to produce scattering modulation when switching the handedness of free-space light, the distinct polarization properties of evanescent fields are shown to be strictly required to observe giant scattering modulation. These results ultimately deepen our understanding of the range of possible chiroptical effects in light-matter interactions.

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Metal nanoantennas have been under intense investigations due to their strong light-matter interactions and significant polarization sensitivities determined by their nanoscale structure.1-4 The optical responses of metal nanoantennas are dominated by coherent oscillations of conduction band electrons, or plasmons, which can be coupled to each other analogously to molecular hybridization.5 This idea has enabled the design of nanoantennas with intuitive polarization sensitive plasmons to both linearly6-9 and circularly polarized light.10, 11 Circular dichroism (CD) describes chiral matter’s differential extinction between left and right-handed circularly polarized light (LCP and RCP, respectively). Circular differential scattering has been developed as a technique to determine the CD properties of single chiral nanoparticle assemblies.10, 12 Circularly polarized light is obtained when there is a 90º phase delay between the two orthogonal, transverse components of free space light, producing a longitudinal spin vector with sign determined by the leading component. While handed molecules are sensitive to this phase delay, the overall difference in the phase of the excitation from one end of the molecule to the other is vanishingly small, resulting in relatively weak polarization sensitivity. However, the geometrical dimensions of metal nanoantennas can be similar to the wavelengths of visible and near-infrared light resulting in a non-uniform phase of the exciting field across the structure. This property is critical to CD in the Born-Kuhn model13 and contributes to the giant CD effects14-16 observed for chiral plasmonic nanoantennas.11, 17 Recently, it has been discussed that confined electromagnetic fields, such as evanescent fields produced by total internal reflection (TIR) possess non-transversal (longitudinal) field oscillations, giving rise to the property of transverse spin resulting from a phase delay between the transverse and longitudinal field components.18-20 Transverse spin has been responsible for notable chiroptical effects such as circular polarization dependent routing of light 21-25 and the photonic 4 ACS Paragon Plus Environment

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spin-Hall effect.26-28 Transverse spin has also recently been shown to manipulate light-matter interactions of atoms in a magnetic field29 and to direct the scattering of nanoparticles in tightly focused Gaussian beams.30 Despite this increasing research effort into matter’s sensitivity to transverse spin components of light, the dependence of the scattering intensity from individual nanoantennas on the polarization of evanescent waves has not been examined, hindering the full potential that nanoantennas have as components in photonics circuits. In this work, we report a single-particle mechanistic study of the strong polarization discriminating scattering properties of achiral gold half-rings when excited with evanescent fields. Numerical simulations reveal that the excitation of bright and dark modes in the half-rings sensitively depends on the phase delay between the transverse and longitudinal electric field oscillations found under these excitation conditions. As the relative phase delay of the in-plane field components depends on the incident light polarization, these nanoantennas display nearperfect scattering modulation when switching between LCP and RCP incident excitation. The effect is dependent on the orientation of the half-ring relative to the incident wave vector, k, but a planar chiral structural variant composed of eight half-rings in a rotationally symmetric arrangement was also fabricated and displays large scattering modulation for incident LCP and RCP light with no sensitivity to wave vector orientation. We furthermore tested the polarization sensitivity of both structures to free-space LCP and RCP excitation and observed responses of significantly smaller magnitude in both cases. Our results present a fundamentally different mechanism for chiroptical responses, one that relies on sensitivity to a phase delay between a transverse and longitudinal field oscillation, rather than traditional chiroptical activity, which relies on a phase delay only between two transverse field oscillations.


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RESULTS AND DISCUSSION Arrays of gold half-ring nanoantennas were fabricated using standard electron beam lithography (see methods section). The half-rings have a 100 nm inner radius of curvature and a 40 nm width (Figure 1 a and b). The nanoantenna scattering is nearly entirely modulated by evanescent waves produced by TIR of LCP and RCP light. The handedness associated with the scattering was determined by the orientation of the half-ring relative to the wave vector k and was inverted upon reversal of k. This effect was shown to be reproducible across two 4x4 arrays of half-rings that are mirror images of each other (Figure 1 c, d). The left array is composed of structures shown in Figure 1 a, which scatter under LCP excitation, while the right array is composed of structures shown in Figure 1 b, which scatter under RCP excitation. Measurements performed with the entire sample rotated by 180° yielded consistent results (Figure S1), demonstrating that only the orientation of the half-ring with respect to the incident wave vector, k, determines the dominant scattering handedness.


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Figure 1. Scattering of 100 nm radius gold half-ring nanoantennas under evanescent wave excitation. (a, b) Scanning electron microscopy (SEM) images of half-rings with different orientations relative to k. Scale bars: 200 nm. (c, d) Dark-field scattering microscopy images of two 4x4 arrays of half-ring nanoantennas, arranged in different orientations as shown in (a, b) and collected under LCP and RCP incident excitation, respectively. 200 nm diameter gold nanodisks in the center and bottom-left corner of the images were used as polarization invariant fiduciary markers. (e) Experimental geometry used to probe the polarization dependent scattering of metal nanoantennas. LP: Linear polarizer, QWP: Quarter wave-plate, 𝐸𝑖𝑠,𝑖𝑝 : Incident electric field s- and p-components, n1,2: refractive indices, 𝜑: incident angle, 𝐸𝑒 : Electric field of the evanescent wave, k: Wave vector of evanescent wave. Measured (f, g) and simulated (h, i) single particle dark-field 7 ACS Paragon Plus Environment

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scattering spectra of half-rings under LCP (blue lines) and RCP (green lines) incident excitation and their difference (black lines). The evanescent fields were produced by adjusting the angle of incidence of a white light source onto a prism such that the critical angle required for TIR is exceeded. A linear polarizer and quarter wave-plate were used to control the handedness of the incident excitation (Figure 1 e). The scattering from the half-rings is dominated by a single peak near 600 nm (Figure 1 f, g). LCP and RCP incident excitations produce the minimum and maximum scattering, depending on the halfring orientation, with s- and p-polarizations producing intermediate scattering intensities (Figure S2). Electromagnetic simulations of the half-rings under evanescent field excitation verify the large differential scattering (Figure 1 h, i) when switching between LCP and RCP light. While all half-rings were nominally fabricated with the same design parameters, minor differences in fabrication lead to slightly different spectral responses. However, the overall polarization sensitivity remained highly reproducible (Figure S3). Associated charge plots of the half-ring shown in Figure 1 a under LCP and RCP incident excitation, calculated at the peak scattering intensity, revealed the fundamental modes associated with each polarization (Figure 2 a, b). These modes show the formation of light-induced charge density on the “internal” and “external” surfaces of the half-ring, giving rise to radially oriented dipole distributions. The charge density under LCP excitation on the external (internal) surface is positive (negative) without any node, while under RCP excitation, two nodes in the charge density appear, both on the internal and external surfaces. This induced dipole moment distribution can be sketched as three dipoles along the half-ring, one at the center and two near the edges.


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Figure 2. Fundamental half-ring modes excited by TIR of LCP and RCP light. (a, b) Calculated charge plots of the gold half- ring with the orientation shown in Figure 1 a, as viewed from the glass, excited with evanescent waves produced through TIR of LCP (blue) and RCP (green) light. (c, d) Representation of the 𝐸𝑒𝑦 and 𝐸𝑒𝑧 field components of the evanescent waves produced through TIR of LCP (blue) and RCP (green) light with a wavelength of 600 nm. An accurately scaled gold half-ring schematic is overlaid to highlight and visually elucidate the relative phase of the fields experienced across the nanostructure. Black arrows correspond to the predicted direction of the three dipoles observed in the calculated charge plots.


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In the two charge plots, the dipoles differ only in their relative phase. The charge plot produced with TIR of LCP light shows a “bright mode”, which results in far-field scattering. The bright mode has all three dipoles radially in-phase. Conversely, TIR of RCP light shows a “dark mode” with minimal far-field scattering. The central dipole is out-of-phase with respect to the dipoles at the edges so that the total dipole moment of the mode is dramatically reduced, producing almost no scattering. This picture is consistent with calculated absorption spectra, which showed small differences for incident LCP and RCP light (Figure S4). The bright and dark modes can be intuitively understood by examining the half-ring in relation to the evanescent fields produced by TIR of LCP and RCP light (Figure 2 c, d). The evanescent field components are determined by the incident wave polarization represented by their s- and pcomponents (𝐸𝑖𝑠 and 𝐸𝑖𝑝 ), the incident angle (𝜑) and the ratio of refractive indices at the interface (N=n1/n2), 𝑇

𝐸𝑒 = [𝐸𝑒𝑥 𝐸𝑒𝑦 𝐸𝑒𝑧 ] = [sin(𝜑)𝑁 2 (1 + 𝑟𝑝 )𝐸𝑖𝑝 (1 + 𝑟𝑠 )𝐸𝑖𝑠 (−cos 𝜑)(1 − 𝑟𝑝 )𝐸𝑖𝑝 ]

𝑇

(1)

where 𝐸𝑒 is the evanescent field with x, y, and z components, 𝑟𝑠 and 𝑟𝑝 are Fresnel phase factors which are functions of 𝜑 and N, and T indicates the transpose.31 𝐸𝑒𝑧 describes the field oscillation along the direction of propagation, though paradoxically the evanescent wave maintains its status as a transverse wave.32 TIR of LCP light (𝐸𝑖𝑠 = −𝑖 ∗ 𝐸𝑖𝑝 ) and RCP light (𝐸𝑖𝑠 = +𝑖 ∗ 𝐸𝑖𝑝 ) results in evanescent fields with the phase of 𝐸𝑒𝑦 shifted by 180°. The x and z components remain unchanged in phase and magnitude (Figure S5). Switching between RCP and LCP excitation therefore results in a phase delay between the in-plane field components, 𝐸𝑒𝑦 and 𝐸𝑒𝑧 (Figure 2 c, d). More details regarding the calculation of the evanescent field polarization are given in the SI. The two dipoles at the ends of the half-ring are nearly aligned with the z-axis and are excited dominantly through the 𝐸𝑒𝑧 field component. The entire length of the half-ring from one end to 10 ACS Paragon Plus Environment

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the other, along the k direction, is ~280 nm, which corresponds to ~0.5λ. Thus, the two ends experience a ~180° phase difference of 𝐸𝑒𝑧 , resulting in the opposite orientation of these two dipoles. Since 𝐸𝑒𝑧 is equivalent in phase and magnitude for LCP and RCP light, the dipole moments at the edges are similar for both polarizations. The primary difference between the charge plots induced by LCP and RCP excitations is the phase of the dipole moment located approximately at the center of the half-ring, aligned along the y-axis. The relative phase of this dipole shifts by 180° between LCP and RCP light, as shown by the charge density plots (Figure 2 a, b) and in agreement with the evanescent wave polarization (Figure 2 c, d). We therefore conclude that the near-perfect polarization discriminating response is the result of the different relative phases of the 𝐸𝑒𝑦 and 𝐸𝑒𝑧 field oscillations produced by TIR of LCP and RCP light. The half-rings were observed to have a distinctly different scattering response to non-TIR, darkfield (free-space light) excitation (Figure 3 a, b). Free-space excitation was achieved by angling the incident light onto the prism below the critical angle required for TIR. The scattering from the half-rings under non-TIR excitation was dominated by a single peak near 580 nm and when switching between LCP and RCP light, the peak intensity modulated only ~50%. Electromagnetic simulations of the half-rings under non-TIR excitation verify the reduced polarization sensitivity of the half-rings under free-space excitation compared to that of the half-rings under TIR excitation (Figure 3 c, d).


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Figure 3. Half-ring differential scattering with dark-field, non-TIR excitation. (a, b) Experimental and (c, d) simulated single particle scattering spectra with LCP and RCP free-space excitation. The response is well-mirrored for the different orientations of the half-rings. The inset shows the orientation of the half-ring relative to the incident wave vector k. We attribute the observed polarization sensitivity of the half-rings under free-space excitation to extrinsic chirality, wherein achiral systems in a fixed orientation relative to incident light can generate CD signals.33-35 In other words, extrinsic chirality is observed when a structure in a defined reference frame is sensitive to the handedness of the exciting circularly polarized light. The polarization sensitivity observed under free-space excitation is clearly distinct from that presented in Figure 1 for the TIR case (Figure S6). Furthermore, electromagnetic simulations of the peak scattering intensity of the half-ring as a function of the incident angle verify that once the critical angle is met and exceeded, the scattering modulation when switching between LCP and RCP excitation increases dramatically (Figure S7). As only the angle of incidence was varied in both the simulations and measurements, we therefore conclude that the near-complete polarization 12 ACS Paragon Plus Environment

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discrimination of the half-rings to TIR of LCP and RCP is due to the distinct polarization properties found in evanescent fields, not present in free-space excitation. In particular, under TIR excitation, the half-rings are sensitive to a phase delay between a transverse and a longitudinal field oscillation. This behavior is a fundamentally different property from CD, as CD is caused solely by a sensitivity to differing relative phases between two transverse oscillations. The sensitivity of the half-rings to TIR of LCP and RCP light is also an extrinsic effect, sensitive to the mutual orientation of the structure and the k vector. The half-rings were studied with TIR excitation from an annular-symmetric dark-field condenser (Figure 4 a), which is analogous to studying the averaged response over all half-ring orientations with respect to a single incident k vector. As in extrinsic chirality, the averaged response, dominated by a peak near 560 nm, over all half-ring orientations is identical for incident LCP and RCP excitations (Figure 4 b). We expect that the blue-shift in resonance wavelength relative to that with a single incident k vector (Figure 1 f, g) is due to higher energy mode excitations resulting from the different relative orientations of the half-ring and k. Additionally, we verified that full rings with a radius of 100 nm behave as approximately the sum of two half-rings of opposing orientations, resulting in no scattering modulation with a single incident k vector (Figure S8).


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Figure 4. Half-ring scattering with annular symmetric excitation (a) SEM image of the half-ring with arrows representing the annular symmetry of the wave vectors produced with a dark-field condenser. Scale bar: 200 nm. (b) Single particle scattering spectra of a half-ring with incident LCP and RCP excitation. In an effort to produce an analogous intrinsic response we fabricated nanoantennas composed of eight half-rings, connected in a rotationally symmetric manner (see methods section), which we call pinwheels (Figure 5 a, b). The pinwheel is a planar chiral structure. The core of the structure produces an asymmetry such that the direction of half-ring curvature can be assigned as clockwise or counter-clockwise as one traces a half-ring from the end to the core. The field traced out by the 𝐸𝑒𝑦 and 𝐸𝑒𝑧 components rotates with either a clockwise or counter-clockwise directionality, depending on the phase delay between the components. The alignment or misalignment of the direction of curvature of the electric field with the curvature of the half-rings in a pinwheel results in differential scattering produced by incident TIR of LCP and RCP light (Figure 5).


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Figure 5. Pinwheel nanostructure differential scattering with TIR excitation. (a, b) SEM images of pinwheel enantiomers which are composed of eight half-rings connected in a rotationally symmetric pattern. Arrows indicate the annular symmetric incident excitation produced with a condenser. Scale bars: 200 nm. Measured (c, d) and simulated (e, f) single particle dark-field scattering spectra of each pinwheel enantiomer show a significant scattering modulation that is well mirrored for the enantiomers. The experimental data was collected using an annular symmetric dark-field condenser excitation, while the simulated data was generated using a single incident k. The pinwheels were observed to have a broadband differential scattering response with a maximum difference of ~50% at 725 nm (Figure 5 c, d). Electromagnetic simulations verify the observed broadband differential response (Figure 5 e, f). The behavior is robust across a 4x4 array 15 ACS Paragon Plus Environment

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of pinwheels and mirrored by the two enantiomers (Figure S9). The differential scattering is consistent with prism excitation from a single incident direction and when exciting in an annular symmetric geometry using a dark-field condenser under TIR conditions (Figure S10). It is important to recall that no differential scattering is observed from single gold half-rings, which lack planar chirality, when excited using the annular symmetric dark-field condenser (Figure 4). The differential scattering of the pinwheels with free-space, non-TIR, LCP and RCP excitation was observed to be reduced in magnitude by ~50% from that observed with annular symmetric TIR excitation (Figures S11 and S12). The sensitivity of the pinwheels to free-space circularly polarized light is unsurprising, and is in accordance with the reports on planar chirality.36-38 However, the significantly greater polarization discrimination observed under TIR excitation conditions reveals a sensitivity to polarization states found only in confined electric fields. Therefore, we conclude that the pinwheels are particularly sensitive to planar rotational field motion, which can be found in evanescent fields, but not in free-space light. This sensitivity represents a fundamentally different property than CD, based not on helical field motion (traditional CD), but instead on rotational field motion.

CONCLUSION In conclusion, we have exploited the distinct polarization properties of evanescent waves to produce a near-perfect modulation of the scattering from gold half-ring nanoantennas excited through TIR of LCP and RCP light with a single incident wave vector. This result was shown to be dependent on the relative orientation of the structure and the wave vector. However, we extended this basic geometry to pinwheel nanoantennas with planar chirality to produce an orientation independent effect with 50% scattering modulation. To the best of our knowledge, the 16 ACS Paragon Plus Environment

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50% differential scattering response is the largest intrinsic linear chiroptical response at visible wavelengths from a plasmonic nanostructure yet reported. The distinct polarization properties of evanescent waves were shown to be strictly required for observing polarization dependent responses of these magnitudes, as the differential scattering when switching between LCP and RCP for both the half-ring and the pinwheel structures was significantly reduced when excited with free-space light. This result provides a fundamentally different mechanism for chiroptical responses, as it requires a phase delay between transverse and longitudinal electric field oscillations, not found in free-space light, whereas traditional mechanisms of CD only require structural sensitivity to a relative phase difference between transverse field oscillations. This work advances the potential of on-chip optical components for high efficiency polarization discrimination with sub-wavelength footprints, particularly in cases where conversions between surface waves and far-field radiation is desirable.

METHODS Electron Beam Lithography All samples were fabricated using electron beam lithography. Half-rings were fabricated with an interior radius of 100 nm, and an arm width of 40 nm. The half-rings were fabricated in two 4x4 arrays with one array rotated 180° relative to the other. The full-rings were fabricated with an interior radius of 100 nm and a width of 40 nm in a single 4x8 array. The pinwheels were fabricated with 8 half-ring arms, each with an interior radius of 100 nm and an arm width of 40 nm. Each half-ring composing the pinwheels had a span angle of 150°, which ensured that the tips of the constituent half-rings did not contact. The pinwheels have C8 rotational symmetry and were 17 ACS Paragon Plus Environment

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fabricated in two 4x4 arrays with one array as the mirror image of the other. All structures have a height of 40 nm with a 2 nm Ti adhesion layer. All patterns were written with a Nanometer Pattern Generation System, which directly interfaces with a scanning electron microscope. Indium tin oxide coated glass slides (Delta Technology LTD CG-50IN-S107) were used as the substrate. Slides were cleaned through sequential sonication for ten minutes in solutions of Liquinox®, water and ethanol. Slides were then dried with N2. A polymethyl methacrylate resist (Microchem, PMMA 950 A4) layer was deposited by spin coating at 3000 rpm for 60 seconds followed by baking for five minutes at 180o C. Patterns were written with a FEI Quanta 650 scanning electron microscope with the Nanometer Pattern Generation System (NPGS) using a beam voltage of 30 kV, a beam current of 40 pA, and a working distance of 7 mm. Samples were immersed in 1:3 methyl isobutyl ketone (MIBK)/isopropanol solution for 70 seconds before being rinsed in isopropanol and dried with N2. An electron beam evaporator was used to deposit a 2 nm Ti adhesion layer followed by 40 nm of gold. Excess material was removed by soaking the sample in acetone overnight followed by gentle sonication and drying with N2.

Single Particle Scattering Spectroscopy Single particle scattering measurements were performed with a homebuilt inverted dark-field microscope. Light from a quartz tungsten halogen lamp (Newport 66884) was passed through a fiber optic cable and coupled into a home-built rail system. The light was filtered with a 304-785 nm bandpass filter (Thorlabs FGS550) and collimated with a planar convex lens (Thorlabs AC254030-A) and passed through polarization optics. LCP and RCP excitation experiments were performed by placing a quarter wave retarder after a linear polarizer and setting the fast axis to +/45º relative to the linear polarization axis. The half-ring experiments were performed with optics 18 ACS Paragon Plus Environment

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tailored for the bluer spectral region with an effective wavelength range of 465-610 nm (linear polarizer: Thorlabs LPVISC100; quarter-wave plate: Edmund Optics, #46-558). The pinwheel experiments were performed with optics tailored for the redder spectral region with an effective wavelength range of 610-850 nm (linear polarizer: Thorlabs LPVIS100; quarter-wave plate: Edmund Optics, #63-935). The polarized light was focused with an additional planar convex lens (Thorlabs AC254-030-A) onto an equilateral fused silica prism. TIR conditions were achieved with an input angle of 55º relative to the normal of the sample plane. Free-space, high-angle, dark field excitation was maintained with an input angle of ~38º.

This light illuminated the

lithographically fabricated structures, and the scattered light was collected by either a 50x magnification air-space objective (Zeiss, numerical aperture of 0.8), for TIR experiments, or a 40x magnification air-space objective lens (Zeiss, numerical aperture of 0.6), for dark-field experiments. The scattered light was passed from the objective lens to the body of an inverted dark-field microscope (Zeiss Axio Observer m1) and was then directed to a hyperspectral detection system that has been described previously.10,

39

Briefly, output light was collected by a

spectrograph (Acton SpectraPro 2150i) with an adjustable input slit aperture. The dispersed light was detected by a back-illuminated CCD camera (Princeton Instruments Pixis 400). The CCD camera and spectrograph were fixed on an automated scanning stage driven by a linear actuator (Newport LTA-HL). The data collection was performed in a hyperspectral fashion, with the detector collecting multiple, spectrally-resolved image slices as the spectrograph was scanned across the field of view. Condenser illumination based experiments were performed using a slightly different experimental setup. Light from a tungsten-halogen lamp (Zeiss, Axioline HAL 100) was polarized using the same optics described above. The light was then focused onto the sample using an oil19 ACS Paragon Plus Environment

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immersion dark-field condenser (Zeiss, adjustable numerical aperture 0.7 to 1.4). For TIR experiments, the condenser was set to have annular symmetric incidence with an incident angle of 68º. The scattered light was collected by a 50x magnification air-space objective lens (Zeiss, numerical aperture of 0.8). For high-angle excitation experiments, the condenser was set to have four incident wave vectors with an incident angle of 36º. The scattered light was collected by a 40x magnification air-space objective (Zeiss, numerical aperture of 0.6). Scattered light from the sample was then directed to the body of the microscope and detected in the same way as described for prism TIR excitation.

Numerical Simulations The electromagnetic calculations were performed using the commercial software COMSOL Multiphysics. All the simulated structures were modeled in close agreement with the experimental geometrical parameters and placed in a 1000 nm thick (along the vertical direction) air layer (refractive index of 1), lying on the surface of a 400 nm thick (along the vertical direction) background medium layer. An effective value of 1.70 was used for the refractive index of the background medium, which represents an intermediate value between the refractive index of the glass (1.52) and that of the 120–160 nm thick ITO coating layer (1.88, see Figure S13 for more details). The 2 nm Ti adhesion layer was neglected in the calculations. The lateral extensions of the simulation domain were 600 nm for the 100 nm radius half-ring and the 100 nm radius fullring (see Figure S8) structures, but 1400 nm for the pinwheels. The simulation domain was truncated by exploiting 50 nm thick perfectly matched layers in all the spatial directions. The structures’ optical properties were described by using the Rakić permittivity for gold. 40 The incident radiation was represented by RCP and LCP light, reaching the structures through the 20 ACS Paragon Plus Environment

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background medium with an incident angle of 55°, above the critical angle needed for TIR, when simulating evanescent excitation (Figures 1h, 1i, 5e, 5f, S4, S8c, and S13). For the non-TIR excitation (Figures 3c, 3d), the incident angle used in the simulations was 35° to the surface normal, below the critical angle needed for TIR, which is 36º, when the background medium layer substrate is assigned an effective refractive index of 1.70. Absorption and scattering spectra were obtained by performing the electromagnetic simulations explicitly for a finite number of wavelengths, in the spectral region of interest, and then interpolating by means of cubic spline functions, as routinely done in frequency-domain finite element approaches. In particular, the calculated scattering spectra were obtained by integrating the Poynting vector – scattered by the structure – on the surface of a virtual hemisphere, enclosing the whole structure, located within the air layer and centered in the center of the structure’s surface that sits on the planar interface between air and background medium layers. Numerical simulations provided in Figures 1h, 3c, and 5e were explicitly performed for the structures represented in Figures 1a and 5a, while the spectra shown in Figures 1i, 3d, and 5f were obtained by assuming mirror symmetry. The charge density plots were obtained at the frequencies of selected resonance peaks.

ASSOCIATED CONTENT Supporting Information. Calculations of the evanescent wave polarization; Digital camera images of the half-ring array rotated 180°; Spectra of the achiral half-rings at additional polarizations; Variations in differential scattering among different half-rings; Simulated absorption spectra of the half-rings; Visual representations of the evanescent field produced through TIR of LCP and RCP light; Percent CDS of half-rings for TIR and dark-field excitations; Calculated peak scattering intensity of the half-rings under LCP and RCP excitation as a function 21 ACS Paragon Plus Environment

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of the incident angle; Experimental and simulated spectra of full-rings; Hyperspectral image of the 8-arm pinwheels demonstrating signal reproducibility across 32 pinwheel spectra; Comparison between spectra acquired with annular-symmetric excitation and single wave vector excitation for 8-arm pinwheels; Pinwheel nanostructure differential scattering with non-TIR, dark-field excitations; Percent CDS of pinwheels for TIR and dark-field excitation; Additional simulations performed at different refractive indices and at long wavelengths. This material is available free of charge via the Internet at http://pubs.acs.org. Gold_Half_Rings_SI.pdf The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions S.L., W.-S. C., K.W.S. and L.A.M. designed the research. K.W.S. and L.A.M. fabricated and measured the nanostructures. A. A. and L.B. performed the simulations and were supervised by P.N. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‖

These authors contributed equally.

ACKNOWLEDGMENT 22 ACS Paragon Plus Environment

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This work is funded by the Robert A. Welch Foundation (C-1664 to S.L. and C-1222 to P.N.) and the National Science Foundation (CHE1507745 to S.L.). K.W.S. and L.A.M. acknowledge that this material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program (1450681).


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Exploiting Evanescent Field Polarization for Giant Chiroptical

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