Phonon-Polaritonic Bowtie Nanoantennas - ACS Publications

Subscriber access provided by Queen Mary, University of London

Article

Phononic-Polaritonic Bowtie Nanoantennas: Controlling Infrared Thermal Radiation at the Nanoscale Tao Wang, Peining Li, Dmitry N. Chigrin, Alexander J. Giles, Francisco J Bezares, Orest J Glembocki, Joshua D Caldwell, and Thomas Taubner ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00321 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Photonics is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Phonon-Polaritonic Bowtie Nanoantennas: Controlling Infrared Thermal Radiation at the Nanoscale Tao Wang †, Peining Li †, Dmitry N. Chigrin *,†,††, Alexander J. Giles ‡,§, Francisco J. Bezares ǁ, Orest J. Glembocki ‡, Joshua D. Caldwell *,‡,¥, and Thomas Taubner *,† †

Institute of Physics (IA), RWTH Aachen University, Aachen 52056, Germany ‡

U.S. Naval Research Laboratory, Washington, DC, United States

§

NRC Postdoctoral Fellow (Residing at US. Naval Research Laboratory, Washington, DC) ‖

Departamento de Matemática-Física, Universidad de Puerto Rico – Cayey, Puerto Rico ††

¥

DWI Leibniz-Institute for Interactive Materials, Aachen 52074, Germany

Dept. of Mechanical Engineering, Vanderbilt University, Nashville, TN 37205 USA

*

Email: [email protected], [email protected], & [email protected]

aachen.de

ABSTRACT A conventional thermal emitter exhibits a broad emission spectrum with a peak wavelength depending upon the operation temperature. Recently, narrowband thermal emission was realized with periodic gratings or single microstructures of polar crystals supporting distinct optical modes. Here, we exploit the coupling of adjacent phonon-polaritonic nanostructures demonstrating experimentally that the nanometer-scale-gaps can control the thermal emission

ACS Paragon Plus Environment

1

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 28

frequency while retaining emission linewidths as narrow as 10 cm-1. This was achieved by using deeply sub-diffractional bowtie-shaped silicon carbide nanoantennas. Infrared far-field reflectance spectroscopy, near-field optical nanoimaging and full-wave electromagnetic simulations were employed to prove that the thermal emission originates from strongly localized surface phonon polariton (SPhP) resonances of nanoantenna structures. The observed narrow emission linewidths and exceptionally small modal volumes provide new opportunities for the user-design of near- and far-field radiation patterns for advancements in infrared spectroscopy, sensing, signaling, communications, coherent thermal emission, and infrared photo-detection.

TOC

KEYWORDS: thermal emission, surface phonon polariton, bowtie antenna, silicon carbide, polar crystal, nanophotonics


2

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Thermal emitters are used in a wide-array of applications covering a broad frequency range. At visible and near-infrared frequencies, thermal emitters are used for lighting (e.g. incandescent light bulbs), heating, and energy harvesting (e.g. thermophotovoltaic solar cells);1,2 while in the mid-infrared, they are utilized as light sources for infrared spectroscopy, sensing, and imaging3-5. Conventional thermal emitters are based on black- or gray-body radiation (e.g. silicon carbide glowbars), and give off a broadband spectrum covering a few 1000 cm-1. However, thermal emitters with a narrowband spectrum (< 50 cm-1) are highly desired, for example, in infrared photodetection6, non-dispersive infrared sensing7,8, and molecular spectroscopy8,9. In order to reduce the linewidth of thermal emission, concepts based on the resonant modes of photonic crystals10,11, plasmonic12-15, and phononic microantennas16-20 have been developed. Engineering of the optical modes within photonic crystals has recently resulted in thermal emission linewidths of approximately 10 cm-1.10,11 In contrast, thermal emission linewidths of 100 cm-1 have been realized within plasmonic perfect absorbers.12-14 However, with careful design of a plasmonic bandgap using gold gratings, reductions in linewidths to 10 cm-1 have also been shown.15 This reduction in linewidth is tied to the diffractive orders of the gratings though, thus the radiation pattern of the thermal emission and the linewidth become intimately entwined. A promising alternative to photonic crystals and surface plasmon polaritons (SPPs) is found in surface phonon polaritons (SPhPs), which are supported in polar crystals (e.g. SiC16-20) and have also been used for narrowband thermal emission. Unlike the SPP counterparts, SPhPs are only supported in a defined frequency range between the transverse (TO) and longitudinal (LO) optical phonons of the material, which for SiC occurs ~797-972 cm-1 (10.3-12.5 µm). This spectral region is referred to as the ‘Reststrahlen band’ and corresponds to the real part (ε1) of the dielectric function becoming negative and strongly dispersive (Fig. 1a).21,22 By changing the


3

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

material (such as SiC16-20,23-26, hexagonal BN27-30, SiO231, GaN32), SPhP resonances can be realized throughout the spectral range extending from roughly 6 to 325 µm33,34. Importantly, SPhPs exhibit optical lifetimes that are orders of magnitude longer than SPPs, thereby resulting in much lower optical losses, as observed in the exceptionally low values of the imaginary part (ε2) of the dielectric function in Fig. 1a. The combined virtues of low optical losses and the polaritonic dispersion allow the realization of narrowband thermal emitters with deeply subdiffractional geometries and exceptionally narrow, highly absorptive resonances. For instance, by using the propagating SPhP modes in SiC gratings16-19 and the localized SPhP resonances in SiC whiskers20, thermal emission with linewidths as narrow as 5 cm-1 have been observed. However, up to now, narrowband thermal emission is mainly controlled at the micrometer length-scale by the grating period10-12,14-19 or the antenna dimension13,20. In the realm of ultracompact optical sources and devices at infrared frequencies, there is a need to scale down the size of those narrow-band thermal emitters and to control their thermal emission at the nanometer scale. In addition, this provides the ability to independently control both the thermal emission frequency, linewidth and radiation pattern through the nanoantenna particle and array design. Here, for the first time, we experimentally demonstrate that nanometer-gaps within nanoscale 4H-SiC bowtie antennas can be used to control the narrowband thermal emission frequency throughout the Reststrahlen band with emission linewidths as narrow as 10 cm-1. Consistent with the results of Schuller et al.20, we also show that the emission polarization is determined by the geometry of the underlying nanoantennas. Using nanoscale imaging of the optical fields through the use of scattering-type scanning near-field optical microscopy (s-SNOM)25,30, we have mapped the local SPhP field distributions and have successfully correlated these to


4

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

electromagnetic simulations with good qualitative agreement. This correlation provides strong support that such simulations can offer the predictive design of the thermal emission spectra and near- and far-field radiation patterns. RESULTS AND DISCUSSION Design of SPhP bowtie-antennas with nanoscale gaps. To demonstrate and investigate the nanoscale control of thermal emission, we fabricated bowtie-shaped 4H-SiC nanopillars with nanoscale gaps (Fig. 1b-c), following the procedures outlined in Ref. 23 and 24. Each phononic bowtie antenna consists of two triangular nanopillars with a base W and height L (W = L = 600 nm) that are pointing at one another and have a height h ~ 1 µm (Fig. 1b). These nanoantennas were fabricated using electron beam lithography and reactive ion etching into a 4H-SiC semiinsulating substrate23,24 resulting in nanostructures protruding from and still physically attached to the underlying SiC substrate. In an effort to adjust the interparticle coupling and accordingly, the resonance (emission) frequency, the gap g between the bowtie tips was varied from nominally 0 (touching) to 90 nm in approximately 15 nm increments. SEM measurements determined g = 0, 25, 35, 50, 65, 80 and 95 nm at the top surface of the bowtie components. However, as shown from the SEM images, this gap is not uniform along the height, with up to a ~50% gap variation observed due to the etching process. For each gap size, bowtie-shaped nanopillars were arranged into a periodic array with a period p = 2 µm center-to-center and covered a total area of 50 × 50 µm2. The shape and relative position of bowtie antennas has direct implications on their coupling and resonance behavior. In comparison to the previously studied circular-shaped and cylindrical SPhP antennas (or resonators)23-26, the bowties possess sharp corners and small gaps at the nanometer scale. Thus they provide stronger local field enhancements and a higher sensitivity to


5

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

changes in gap,35-39 thereby providing a nanometric ‘knob’ to tune the emission spectra, while retaining the same overall form-factor. In the case of the incident electric field being aligned perpendicular to the bowtie long-axis, the two nanopillars act as two weakly interacting, triangular-shaped antennas. In contrast, with the incident field aligned parallel to this axis, the tips of the adjacent triangles in the bowtie antenna strongly couple and the resonance frequency, amplitude and linewidth depend directly upon the gap-size. Thus, not only does this geometry provide an anisotropic emission response, but it also provides the opportunity by changing the bowtie gap to independently control the emission in one orientation (parallel), while nominally leaving the other unchanged (perpendicular). In this Article we will focus on this parallel case, with a full spectroscopic analysis and polarization dependence to be presented elsewhere. Measurements of gap-dependent far-field reflection and thermal emission. To examine and understand the thermal emission spectra of the bowtie antennas, we first measured their reflectance spectra at both room (20°C) and elevated temperatures (350°C), with the latter also serving as the temperature for the collection of the thermal emission spectra (See Methods). In the reflectance spectra of the bowtie arrays with a 50 nm gap (Fig. 2a) at both room (black curve) and elevated 350°C (red curve) temperatures, four primary, localized SPhP modes (labeled A1 to A4) are observed. At 350°C, a spectral red-shift is induced due to 4H-SiC lattice expansion,16,20 however, otherwise the spectra remain nominally unchanged. The thermal emission spectrum (measured at 350°C, blue curve) of bowtie nanoantennas with the ~50 nm gap is presented in Fig. 2a, with the four resonances being correlated to those observed in the reflectance spectra. In addition, the polarization selection rules and approximate linewidths of the resonant modes were also retained in the far-field, clearly indicating that these nanostructures have the potential to serve as solid-state, narrow-band polarized light sources in


6

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

the infrared spectral domain. The resonance frequency and amplitude of the thermal emission show a clear dependence on the gap size, especially for the modes A2 and A3 (Fig. 2b). In particular, when g decreases from 95 to 0 nm, the resonance peak of mode A2 was found to shift ~10 cm-1, from 882 to 872 cm-1, while a similar ~9 cm-1 shift was found for A3, shifting from 905 to 896 cm-1. The relative emittance of A2 and A3 also depends upon g, with A3 dominating the emittance spectra at large gaps, with ~24% larger emittance than A2 at this widest gap. However, at g = 50 nm, the emittance of the A2 mode overtakes A3, eventually resulting in ~53% larger emittance for the A2 mode than A3 at g = 25 nm (see Supporting Information Fig. S2 and Fig. S3). While the amplitude of the thermal emission dictates the brightness of these optical sources, the monochromaticity is expressed by the emission linewidth. A good figure of merit for this, the quality factor (Q = ωres⁄∆ωres), also takes into account the emission frequency, and was quantified for the four resonances observed as a function of g. Here, ωres and ∆ωres represent the resonance frequency and linewidth, respectively. As shown in Fig. 2b, Q is nominally independent of the gap size. Quantitatively, the linewidths of the four emission modes were extracted using a Lorentzian lineshape and determined to be ~10 cm-1 for A1 and ~18 cm-1 for the other modes. This results in corresponding thermal emission Q-factors of ~ 80, 70, 50 and 55 for A1, A2, A3, and A4, respectively. The Q-factors of these four resonances are comparable to (or better than) thermal emission resonances in SiC gratings16-19 and SiC whiskers20. However, in contrast to those structures, here the narrow linewidths are obtained from isolated, deeply subdiffractional structures. While the four resonances observed in this sample overlap resulting in a broader overall spectral bandwidth than any single resonance, it is important to note that the number of resonances and their quality factors can be engineered by careful choice of individual


7

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

nanostructure geometry. For example, a single, narrowband thermal emission peak can be obtained in a cylindrical nanopillars of SiC23,24 (see Supporting Information Fig. S4). Further research might be necessary to identify an optimal configuration of the nanoantennas that would provide the largest quality factor, emission spectral properties and far-field radiation pattern for a given application, although the results provided here indicate that this design can benefit from careful nanoscale control of the interparticle separations. Numerical simulations and near-field optical characterization. In order to identify the physical nature of the resonances in the thermal emission spectra we have further analyzed the experimental and simulated reflectance spectra. To fit the experimental data, we considered the anisotropy of SiC in the simulations. The overall spectral shifts induced through the modification of bowtie gap size is most clearly understood through comparison of the series of experimental and simulated (See Methods) reflectance spectra presented in Fig. 3a and b, respectively. The simulations show a good agreement with the experimental results, confirming the gap-size dependences of the main reflectance dips (Fig. 3c). While the positions of A1 and A4 appear nominally independent of the gap size (A1 ~ 864 cm-1, A4 ~ 960 cm-1), A2 and A3 display distinct red-shifts as the gap is reduced, with the shifts being consistent with thermal emission spectra described above. Upon close inspection, it is clear that the ‘A2’ resonance is actually two overlapping SPhP modes centered ~ 880 - 890 cm-1 at 95 nm gaps and undergo a ~ 5 cm-1 redshift with decreasing bowtie gap down to 35 nm. Further gap reduction to 25 nm induced another ~ 5 cm-1 red-shift to ~ 875 cm-1. This drastic increase in the rate of the resonance spectral shift is consistent with the well-known plasmonic coupling or plasmon hybridization effects typically discussed in terms of plasmonic ‘hot-spots’.40,41 To our knowledge, SPhP-based ‘hot-spots’ have not be reported. Such highly localized optical fields have been shown in plasmonic systems to


8

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

drastically enhance light matter interactions, for instance enabling single molecule detection via the surface enhanced Raman scattering effect or surface enhanced fluorescence35. Upon further reduction of the bowtie gap the red-shift is reversed, as the further reduction from 25 nm to 0 nm results in a blue shift for the A2 mode and no further shift for A3 are observed. This may be related to nonlocal or quantum effects of SPhPs similar to close-to-touching plasmonic bowtie antennas.37,42 Different from the quantum plasmonic resonance,37,42-44 free electrons are not responsible for the SPhP resonance, thus field screening effects and/or new theories based on the coupling of phonon-polaritonic vibrations between near-touching SPhP resonators are desired to fully understand the possible quantum effects of SPhPs. In addition to the experimental and simulated reflectance, we also provide the simulated surface-charge densities (Fig. 3d) and the electric field distributions (Fig. 4a) for those resonance modes. Through comparison of near-field optical amplitude and phase images (Fig. 4b-d) recorded by using a s-SNOM25,30,45-47 (details in Methods) to these calculated spatial plots, their validity can be corroborated. Due to restrictions of the laser-covered wavelength range (See Methods), the s-SNOM images were collected at the closest possible frequency to each resonance for the modes A2 - A4, while the A1 mode was outside of our accessible spectral window. Such near-field modal distributions (Fig. 3d and Fig. 4) offer direct insight into the physical origin of the resonances; that is, they provide the necessary understanding of the observed resonance to explain the observed spectral shifts with gap size and enable the eventual control of the modal properties of the thermal emission radiation patterns in the near- and farfield. For the resonance peak A1 (~ 864 cm-1), the surface charge density is fairly evenly distributed across the nanopillar top surface, with the strongest localization of charge occurring about the


9

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 28

base of the structure. This charge distribution is also symmetric about the gap (Fig. 3d), with a large uniform charge distribution on the substrate that balances the opposing charge present in the pillars. Such a charge density distribution is consistent with the ‘monopolar’ resonance mode for cylindrical nanopillars23,24 and is a result of the presence of a negative permittivity substrate. As can be inferred from the surface charge density distribution of this A1 mode, almost no interaction across the gap is expected and thus the resonance frequency should not depend upon the bowtie gap size, consistent with the spectral data (Fig. 3a-b). In contrast to A1, the resonance peak A2 (~ 890 cm-1) exhibits a strong dependence upon the gap size and the surface charge is found to be primarily distributed on the top surface and along the closest facing sidewalls of the bowtie nanopillars. This local charge distribution is consistent with a dipole resonance established in each triangular nanopillar along the long-axis of the bowtie, being anti-symmetric across the bowtie gap (Fig. 3d). In the simulated field distribution (Fig. 4a, 886 cm-1) and the s-SNOM image recorded at 888 cm-1 (Fig. 4b, close to the resonance A2, 880-890 cm-1), the nanoantenna yields stronger near fields at the gap and the corners of the bowtie, indicating that this mode consists of strongly coupled, localized near-fields at the center of the gap. Such a coupled dipolar field distribution is consistent with optical modes observed in planar plasmonic bowtie antennas35-37, and the spectral red-shift of the A2 mode in the reflectance spectra, resulting in ‘hot-spot’ formation and ultra-small mode volume on the scale of (λ/20)3. Such hot-spots could potentially find additional utility in the design of substrates for surface enhanced infrared absorption (SEIRA). We note that the experimentally recorded field distribution (Fig. 4b) is not as symmetric as the simulated results (Fig. 4a). It is assumed that this may be due to the non-zero tip influence (e.g. the Si tip exhibiting a high refraction index) under a large illumination angle (about 60 degrees off-normal) in the measurements. Further systematic


10

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

s-SNOM experiments are required to interpret the observed asymmetric near-field distribution. However, the close qualitative agreement between experiment and theory provides strong evidence that the electromagnetic simulations provide an accurate picture of the near-field distributions and thus can be useful for predictive design of infrared thermal emitters. As in the case of the A2 resonance, a similar, if more continuous, spectral red-shift with decreasing bowtie gap size is observed for A3 (~ 910 cm-1). However, the charge density distributions are indeed quite different. In this case, the charge is highly localized near the base of the antenna close to the substrate. Analogous to A2, the distributions between the two triangular nanopillars of the bowtie are also anti-symmetric across the gap (Fig. 3d), again inferring a strong near-field coupling between the two closely spaced nanostructures with ultrasmall mode volume. Such field coupling across the bowtie gap is also seen in the simulated field distribution (910 cm-1, Fig. 4a). The s-SNOM image at 905 cm-1 (Fig. 4b, close to the resonance A3 at 910 cm-1) shows that the near fields are localized towards the edges on the top surface of the bowtie. Compared to A2, the mode A3 has a weaker near field amplitude (Fig. 4a-b), which corresponds to the lower reflectance dips (Fig. 3a-b) and thermal emission signal of the A3 mode at g = 50 nm (Fig. 2b). Although the simulated and recorded amplitudes are different, both the simulated and recorded phase are indeed similar. This could be understood by the fact that the laser wavelength used for the s-SNOM is slightly off the resonance. For such a sharp resonance, the slightly shifted wavelength modifies the amplitude with a less pronounced influence upon the phase. Finally, the mode A4 (~ 960 cm-1) does not exhibit any significant shift with the changing bowtie gap, including a lack of dependence for the peak amplitude as well. In this case, the charge distribution is found to be primarily confined along the antenna sidewalls outside the


11

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

bowtie gaps (Fig. 3d). The fields are also uniformly distributed across the top surface of the bowtie as shown in the simulated and measured field distributions (Fig. 4a-b). As very little charge and field are localized along the bowtie sidewall within the gap, it is not surprising that this mode exhibits very little dependence upon changes in the gap size. The observed modal behaviors of the bowtie antennas are indeed very sensitive to the small change of the gap size. As shown in Fig. 4c and d, we clearly observe variations of the field distributions in different bowties (such as I and II labelled in Fig. 4c) from the same array, as they exhibit slight variations in gap due to the non-uniform fabrication. This further confirms that the resonance frequency and near-field distributions of the antenna modes in our bowties can be strongly controlled by small changes of the bowtie gap size, and infer that better uniformity would result from a further reduction of the observed linewidth. CONCLUSIONS In conclusion, we have demonstrated the realization of narrowband, polarized, solid-state thermal emitters based on periodic arrays of SiC bowtie nanoantennas. By varying the gap size between the triangular nanopillars that make up the bowtie nanoantennas, direct control of the coupling strength of certain SPhP modes was observed and thus has experimentally proven nanoscale control of the narrowband thermal emission spectrum. By correlating the thermal emission spectra to the far-field reflectance, scanning near-field imaging and numerical simulations, the narrowband thermal emission can be understood as the result of the correspondingly narrowband SPhP resonances of the SiC bowtie nanoantennas. These observations imply that localized SPhP resonators can provide an avenue towards low-power, polarized, close to monochromatic thermal emitters useful for molecular sensing, SEIRA spectroscopy, free-space communications and infrared signaling.


12

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

The design of such compact infrared sources also provides access to solid-state, narrowband infrared sources in spectral ranges where currently none exist and could therefore result in currently unanticipated advances in applied spectroscopy and optical engineering. By changing the geometric shape (such as cylinder nanopillar), height, or the interparticle spacing, such narrowband thermal emission may be tuned throughout the entire Reststrahlen band of a given material.23-27 By photoinjection of excess free-carriers48 and phase change materials49, it is possible to actively control the SPhP-based narrowband thermal emission. This, therefore, provides a direct path towards the realization of amplitude or frequency modulation of these narrowband thermal emitters that can enable an even broader palette of applications, specifically for IR communications or beacons. Furthermore, by using other polar crystals, our thermal emitters can in principle be designed to cover the frequency range between 6 and approximately 325 µm (50 to 0.92 THz).33,34 The long-wavelength limit on such approaches will of course be dictated by the obvious limitations in the thermal energy available at any chosen spectral frequency.

METHODS. FTIR reflectance and thermal emission measurement. The reflectance and thermal emission spectra were taken with a Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer in combination with a Hyperion 2000 infrared microscope. All spectra were collected using the same, custom-modifed Bruker microscope-based FTIR spectrometer similar to the setup in Ref. 20. A thermal stage (Linkam FTIR 600, Linkam Scientific Instruments, UK) was used to heat the sample with a controllable heating rate and maintain a stable temperature with variations < 0.1°. A 15× (numerical aperture NA=0.4) Cassegrain objective with a weighted


13

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 28

average incidence (or collection) angle of about 17° is used for the thermal emission and reflectance measurements in Fig. 2 and Fig. 3. Both the reflectance and the thermal emission spectra of the SiC bowtie-shaped nanopillar arrays could be sequentially measured without changing spatial position or aperture size. The FTIR reflectance and thermal emittance spectra were recorded using a MCT (mercury cadmium telluride) detector. During all experiments, knife-edge apertures were set to form a measurement area about 60 × 60 µm2. For reflection measurements, a polarizer was inserted in the optical path before the sample to polarize the light linearly with the electrical field vector along the long axis of the 4H-SiC bowtie antennas. All reflectance spectra were collected in reference to a flat gold mirror at the same conditions (temperature, polarization and aperture size). For the case of thermal emission, the polarizer parallel to the long axis of the bowtie antenna was also used in the emission collection path. The emittance spectra have been recorded with 400 scans and a resolution of 4 cm-1, while the reflectance spectra have been recorded with 400 scans and a resolution of 1 cm-1. As all resonance linewidths were found to be in excess of 10 cm-1, the different spectral resolution did not negatively impact the comparison. Full-wave numerical simulations. Full wave 3D simulations have been performed using a commercial solver, CST Studio Suite. Plane-wave excitation in combination with Floquet Mode Ports have been used to model experiment setup and to calculate a reflectance spectrum and field distributions. Periodic boundary conditions have been used in the lateral direction. Reflectance spectra calculations have been performed for an incident angle of 17° to match experiments, while the field profiles are calculated for an incident angle of 60° to better match the experimental conditions used in the s-SNOM measurements. To simulate the material response


14

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

of the 4H-SiC, an anisotropic uniaxial model of its dielectric function50 has been extracted and fitted to experimental data (Supporting Information S1). Near-field optical characterizations. Our s-SNOM (from Neaspec) is based on a tappingmode atomic force microscope (AFM) in which a sharp, usually metallic tip (radius ~30 nm) works as a scanning optical antenna for probing high-resolution near-field optical information.25,30,45-47 However, it has been demonstrated that, when the s-SNOM is used to characterize a resonant optical antennas, the metallic tip can distort the actual field distribution of the antenna mode due to the strong tip-antenna interaction45. To avoid this distortion, we instead use a dielectric Si tip to probe the actual near-fields of the SiC bowtie antenna25,30,45-47. The Si tip is illuminated by a tunable mid-infrared p-polarized CO2 laser (spanning from 885 - 1087 cm-1, Edinburgh Instruments) and the backscattered light from the tip is collected. The p-polarized incident light is parallel to the bowtie antenna, whose direction is marked by an arrow in Fig. 4c. Since the tip-induced polarizability is dominated along the out-of-plane (z) direction, the s-SNOM probes primarily the z-component of electric fields, namely Ez. To suppress background scattering from the tip shaft and the sample, the tip vibrates with an amplitude of about 50 nm at a tapping frequency ~270 KHz, and the detector signal is demodulated by higher harmonic n (for the experiments shown in Fig. 4, n = 2). By interferometric detection, the near-field optical amplitude sn and phase φn are obtained. The maximum scan range in the z-direction of our s-SNOM is 2.5 µm, which allows for imaging of the local field distribution on the top surface of the ~1 µm high bowtie structures. We present the measured field distributions (amplitude s2 and phase φ2) on the top surface of a typical bowtie antenna (from the array with g = 50 nm) in Fig. 4b. The fields from the substrate in the area outside the bowtie top surfaces are masked by the dark-blue color in an effort to highlight the


15

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

near-fields on the bowtie top surfaces (see Supporting Information, Fig. S5 for the complete image). ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website: (S1) Dielectric constants of 4H-SiC. (S2) Emittance ratio between the A2 and A3 modes. (S3) Simulated absorption spectra of the SiC bowtie nanoantennas. (S4) Thermal emission spectra with a single resonance peak from the SiC nanopillars. (S5) The SNOM image with and without mask. AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] & [email protected] # T. Wang and P. Li contributed equally to this work. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS This work was supported by the Excellence Initiative of the German federal and state governments, the Ministry of Innovation of North Rhine-Westphalia. This work was also supported by the Defense Acquisition Program Administration of South Korea and the Agency for Defense Development as a part of a basic research program under the contract UD110099GD. Work for NRL coauthors was funded by the Office of Naval Research and


16

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

administered by the NRL Nanoscience Institute. A.J.G. and F.J.B. acknowledge the NRC/ASEE Postdoctoral Fellowship Programs at NRL.

REFERENCES 1. Kraemer, D.; Poudel, B.; Feng, H.-P.; Caylor, J. C.; Yu, B.; Yan, X.; Ma, Y.; Wang, X.; Wang, D.; Muto, A.; McEnaney, K.; Chiesa, M.; Ren, Z.; Chen, G. High-performance flatpanel solar thermoelectric generators with high thermal concentration. Nature Mater. 2011, 10, 532–538. 2. Lenert, A.; Bierman, D. M.; Nam, Y.; Chan, W. R.; Celanović, I.; Soljačić M.; Wang, E. N. A nanophotonic solar thermophotovoltaic device. Nature Nanotechnol. 2014, 9, 126–130. 3. Huth, F.; Schnell, M.; Wittborn, J.; Ocelic N.; Hillenbrand, R. Infrared-spectroscopic nanoimaging with a thermal source. Nature Mater. 2011, 10, 352–356. 4. Huth, F.; Govyadinov, A.; Amarie, S., Nuansing, W.; Keilmann, F.; Hillenbrand, R. NanoFTIR absorption spectroscopy of molecular fingerprints at 20 nm spatial resolution. Nano Lett. 2012, 12, 3973–3978. 5. Amenabar, I.; Poly,S.; Nuansing, W.; Hubrich, E. H.; Govyadinov, A. A.; Huth, F.; Krutokhvostov, R.; Zhang, L.; Knez, M.; Heberle, J.; Bittner A. M.; Hillenbrand, R. Structural

analysis

and

mapping

of

individual

protein

complexes

by

infrared

nanospectroscopy. Nature Commun. 2013, 4, 2890. 6. Rogalski, A. History of infrared detectors. Opto-Electron. Rev. 2012, 20, 279-308. 7. Hodgkinson, J.; Tatam, R. P. Optical gas sensing: a review. Meas. Sci. Technol. 2013, 24, 012004.


17

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 28

8. Inoue, T.; De Zoysa, M.; Asano, T.; Noda, S. Filter-free nondispersive infrared sensing using narrow-bandwidth mid-infrared thermal emitters. Appl. Phys. Express 2014, 7, 012103. 9. Inoue, T.; De Zoysa, M.; Asano, T.; Noda, S. Realization of narrowband thermal emission with optical nanostructures. Optica 2015, 2, 27-35. 10. De Zoysa, M.; Asano, T.; Mochizuki, K.; Oskooi, A.; Inoue, T.; Noda, S. Conversion of broadband to narrowband thermal emission through energy recycling. Nature Photon. 2012, 6, 535–539. 11. Inoue, T.; De Zoysa, M.; Asano, T.; Noda, S. High-Q mid-infrared thermal emitters operating with high power-utilization efficiency. Opt. Express 2016, 24, 15101-15109. 12. Ikeda, K.; Miyazaki, H. T.; Kasaya, T.; Yamamoto, K.; Inoue, Y.; Fujimura, K.; Kanakugi, T.; Okada, M.; Hatade, K.; Kitagawa, S. Controlled thermal emission of polarized infrared waves from arrayed plasmon nanocavities. Appl. Phys. Lett. 2008, 92, 021117. 13. Liu, X.; Tyler, T.; Starr, T.; Starr, A. F.; Jokerst, N. M.; Padilla, W. J. Taming the blackbody with infrared metamaterials as selective thermal emitters. Phys. Rev. Lett. 2011, 107, 045901. 14. Dao, T. D.; Chen, K.; Ishii, S.; Ohi, A.; Nabatame, T.; Kitajima, M.; Nagao, T. Infrared perfect absorbers fabricated by colloidal mask etching of Al–Al2O3–Al trilayers. ACS Photonics 2015, 2, 964–970. 15. Biener, G.; Dahan, N.; Niv, A.; Kleiner, V.; Hasman, E. Highly coherent thermal emission obtained by plasmonic bandgap structures. Appl. Phys. Lett. 2008, 92, 081913. 16. Greffet, J. J.; Carminati, R.; Joulain, K.; Mulet, J. P.; Mainguy, S.; Chen, Y. Coherent emission of light by thermal sources. Nature 2002, 416, 61-64.


18

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

17. Dahan, N.; Niv, A.; Biener, G.; Gorodetski, Y.; Kleiner, V.; Hasman, E. Enhanced coherency of thermal emission: Beyond the limitation imposed by delocalized surface waves. Phys. Rev. B 2007, 76, 045427. 18. Hasman, E.; Kleiner, V.; Dahan, N.; Gorodetski, Y.; Frischwasser K.; Balin, I. Manipulation of thermal emission by use of micro and nanoscale structures. J. of Heat Transfer 2012, 134, 031023. 19. Arnold, C.; Marquier, F.; Garin, M.; Pardo, F.; Collin, S.; Bardou, N.; Pelouard, J. L.; Greffet, J. J. Coherent thermal infrared emission by two-dimensional silicon carbide gratings. Phys. Rev. B 2012, 86, 035316. 20. Schuller, J. A.; Taubner T.; Brongersma. M. L. Optical antenna thermal emitters. Nature Photon. 2009, 3, 658-661. 21. Adachi, S. Optical constants of crystalline and amorphous semiconductors. New York: Springer Science & Business Media, 2013. 22. Fox, M. Optical properties of solids. New York: Oxford University Press, 2010. 23. Caldwell, J. D.; Glembocki, O. J.; Francescato, Y.; Sharac, N.; Giannini, V.; Bezares, F. J.; Long, J. P.; Owrutsky, J. C.; Vurgaftman, I.; Tischler, J. G.; Wheeler, V. D.; Bassim, N. D.; Shirey, L. M.; Kasica, R.; Maier, S. A. Low-loss, extreme subdiffraction photon confinement via silicon carbide localized surface phonon polariton resonators. Nano Lett. 2013, 13, 36903697. 24. Chen, Y.; Francescato, Y.; Caldwell, J. D.; Giannini, V.; Maß, T. W.; Glembocki, O. J.; Bezares, F. J.; Taubner, T.; Kasica, R.; Hong, M.; Maier, S. A. Spectral tuning of localized surface phonon polariton resonators for low-loss mid-ir applications. ACS Photonics 2014, 1, 718-724.


19

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

25. Wang, T.; Li, P.; Hauer, B.; Chigrin, D. N.; Taubner, T. Optical properties of single infrared resonant circular microcavities for surface phonon polaritons. Nano Lett. 2013, 13, 50515055. 26. Gubbin, C. R.; Martini, F.; Politi, A.; Maier, S. A.; De Liberato, S. Strong and coherent coupling between localized and propagating phonon polaritons. Phys. Rev. Lett. 2016, 116, 246402. 27. Caldwell, J. D.; Kretinin, A. V.; Chen, Y. G.; Giannini, V.; Fogler, M. M.; Francescato, Y.; Ellis, C. T.; Tischler, J. G.; Woods, C. R.; Giles, A. J.; Hong, M.; Watanabe, K.; Taniguchi, T.; Maier, S. A.; Novoselov, K. S. Sub-diffraction, volume-confined polaritons in the natural hyperbolic material: hexagonal boron nitride. Nature Commun. 2014, 5, 5221. 28. Dai, S.; Fei, Z.; Ma, Q.; Rodin, A. S.; Wagner, M.; McLeod, A. S.; Liu, M. K.; Gannett, W.; Regan, W.; Watanabe, K.; Taniguchi, T.; Thiemens, M.; Dominguez, G.; Neto, A. H. C.; Zettl, A.; Keilmann, F.; Jarillo-Herrero, P.; Fogler, M. M.; Basov, D. N. Tunable phonon polaritons in atomically thin van der waals crystals of boron Nitride. Science 2014, 343, 1125. 29. Dai, S.; Ma, Q.; Andersen, T.; Mcleod, A. S.; Fei, Z.; Liu, M. K.; Wagner, M.; Watanabe, K.; Taniguchi, T.; Thiemens, M.; Keilmann, F.; Jarillo-Herrero, P.; Fogler, M. M.; Basov, D. N. Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material. Nature Commun. 2015, 6, 6963. 30. Li, P.; Lewin, M.; Kretinin, A. V.; Caldwell, J. D.; Novoselov, K. S.; Taniguchi, T.; Watanabe, K.; Gaussmann, F.; Taubner, T. Hyperbolic phonon-polaritons in boron nitride for near-field optical imaging. Nature Commun. 2015, 6, 7507. 31. Hafeli, A. K.; Rephaeli, E.; Fan, S. H.; Cahill, D. G.; Tiwald, T. E. Temperature dependence


20

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

of surface phonon polaritons from a quartz grating. J. Appl. Phys. 2011, 110, 043517. 32. Feng, K.; Streyer, W.; Islam, S. M.; Verma, J.; Jena, D.; Wasserman, D.; Hoffman, A. J. Localized surface phonon polariton resonances in polar gallium nitride. Appl. Phys. Lett. 2015, 107, 081108. 33. Caldwell, J. D.; Lindsay, L.; Giannini, V.; Vurgaftman, I.; Reinecke, T. L.; Maier, S. A.; Glembocki, O. J. Low-loss, infrared and terahertz nanophotonics with surface phonon polaritons. Nanophotonics 2014, 2192-8614. 34. Caldwell, J. D.; Vurgaftman, I.; Tischler, J. G.; Glembocki, O. J.; Owrutsky, J. C.; Reinecke, T. L. Atomic-scale photonic hybrids: towards designer and multifunctional nanophotonics. Nature Nanotechnol. 2016, 11, 9–15. 35. Kinkhabwala, A.; Yu, Z.; Fan, S.; Avlasevich, Y.; Müllen, K.; Moerner, W. E. Large singlemolecule fluorescence enhancements produced by a bowtie nanoantenna. Nature Photon. 2009, 3, 654–657. 36. Koh, A. L.; Fernández-Domínguez, A. I.; McComb, D. W.; Maier, S. A.; Yang J. K. W. Highresolution mapping of electron-beam-excited plasmon modes in lithographically defined gold nanostructures. Nano Lett. 2011, 11, 1323-1330. 37. Duan, H.; Fernández-Domínguez, A. I.; Bosman, M.; Maier, S. A.; Yang, J. K. W. Nanoplasmonics: classical down to the nanometer scale. Nano Lett. 2012, 12, 1683-1689. 38. Gonzalez, F.J.; Boreman, G. D. Comparison of dipole, bowtie, spiral and log-periodic IR antennas. Infrared Physics & Technology 2005, 46, 418–428. 39. Cuadrado, A.; Silva-López, M.; González, F. J.; Alda, J. Robustness of antenna-coupled distributed bolometers. Optics Letter 2013, 38, 3784–3787. 40. Nordlander, P.; Oubre, C.; Prodan, E.; Li, K.; Stockman, M. I. Plasmon hybridization in


21

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

nanoparticle dimers. Nano Lett. 2014, 4, 899–903. 41. Halas, N. J.; Lal, S.; Chang, W. S.; Link, S.; Nordlander, P. Plasmons in strongly coupled metallic nanostructures. Chemical Reviews 2011, 111, 3913–3961. 42. Wiener, A.; Duan, H.; Bosman, M.; Horsfield, A. P.; Pendry, J. B.; Yang, J. K. W.; Maier, S. A.; Fernandez-Dominguez, A. I. Electron-energy loss study of nonlocal effects in connected plasmonic nanoprisms. ACS Nano 2013, 7, 6287-6296. 43. Tan, S. F.; Wu, L.; Yang, J. K. W.; Bai, P.; Bosman, M.; Nijhuis, C. A. Quantum plasmon resonances controlled by molecular tunnel junctions. Science 2014, 343, 1496-1499. 44. Zhu, W.; Esteban, R.; Borisov, A. G.; Baumberg, J. J.; Nordlander, P.; Lezec, H. J.; Aizpurua, J.; Crozier, K. B. Quantum mechanical effects in plasmonic structures with subnanometre gaps. Nature Commun. 2016, 7, 11495. 45. García-Etxarri, A.; Romero, I.; de Abajo, F. J. G.; Hillenbrand, R.; Aizpurua, J. Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes. Phys. Rev. B 2009, 79, 125439. 46. Schnell, M.; Garcia-Etxarri, A.; Huber, A. J.; Crozier, K.; Aizpurua, J.; Hillenbrand, R. Controlling the near-field oscillations of loaded plasmonic nanoantennas. Nature Photon. 2009, 3, 287-291. 47. Alonso-González, P.; Albella, P.; Golmar, F.; Arzubiaga, L.; Casanova, F.; Hueso, L. E.; Aizpurua, J.; Hillenbrand, R. Visualizing the near-field coupling and interference of bonding and anti-bonding modes in infrared dimer nanoantennas. Opt. Express 2013, 21, 1270-1280. 48. Spann, B. T.; Compton, R.; Ratchford, D.; Long, J. P.; Dunkelberger, A. D.; Klein, P. B.; Giles, A. J.; Caldwell, J. D.; Owrutsky, J. C. Photoinduced tunability of the reststrahlen band in 4H−SiC. Phys. Rev. B 2016, 93, 085205.


22

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

49. Li, P.; Yang, X.; Maß, T. W.; Hanss, J.; Lewin, M.; Michel, A. K. U.; Wuttig, M.; Taubner, T. Optically reversible switching of ultra-confined phonon polaritons with an ultra-thin phase change material. Nature Mater. 2016, 15, 870–875. 50. Tiwald, T. E.; Woollam, J. A.; Zollner, S.; Christiansen, J.; Gregory, R. B.; Wetteroth, T.; Wilson, S. R.; Powell, A. R. Carrier concentration and lattice absorption in bulk and epitaxial silicon carbide determined using infrared ellipsometry. Phys. Rev. B 1999, 60, 11464-11474.


23

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

FIGURES

Figure 1. Array of low-loss SiC-bowtie nanoantennas. (a) Dielectric permittivity (ε = ε1+ iε2) of the used 4H-SiC substrate: black lines, εo for the o-axis, red lines, εe for the e-axis (Supporting information S1). The ratio η (η = |ε2 / ε1|) is provided in the insert. (b) Sketch of one unit cell of the SiC bowtie array. (c) An SEM image of an array of nanoantennas with a large gap size, taken at 45o. (d) An SEM image of an array of nanoantennas with a small gap size, taken at 0o. All the scale bars indicate 1 µm.


24

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 2. IR Reflectance and thermal emission spectra of the SiC bowtie nanoantennas. (a) Room temperature (20°C), high temperature (350°C) reflectance, and high temperature (350°C) thermal emittance spectrum with four distinct modes (A1 to A4). The gap g is ∼ 50 nm. (b) Thermal emittance spectra for all fabricated gap sizes. In all measurements, the electric field is parallel to the antenna axis.


25

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

Figure 3. Reflectance spectra of the SiC bowtie nanoantenna arrays. (a) Experimentally obtained FTIR reflectance spectra (at room temperature) of arrays of the SiC-bowtie nanoantennas with different gap sizes, by keeping the antenna width W = 600 nm and length L = 600 nm. During the measurements, electric field parallel to the antenna axis has been used (TM polarization, as indicated in the insert). Four major dips (A1 - A4) are found in the measured spectra. (b) Simulated gap-dependent reflectance spectra of SiC-bowtie nanoantenna arrays. (c) The resonance peaks of mode A1 to A4 as a function of bowtie gap g. Open (closed) scatter points indicate the simulated (experimental) results. The dashed lines are a guide for the eye. (d) Simulated surface-charge density distribution of the SiC bowtie array with the gap size g = 50 nm at the modes A1 to A4 frequencies, respectively.


26

Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Figure 4. Calculated (a) and experimentally measured (b-d) near-field patterns of resonant modes in SiC bowties: (a) Calculated z-axis component of total electric fields (top, amplitude |Ez|; bottom, phase Arg(Ez)) calculated at a plane 10 nm above the bowtie (g = 50 nm). All the images are normalized to the same scale. For highlighting the fields on the bowtie top surfaces, the outer area is covered by a mask. Calculations are for three frequencies: 886 cm-1 (A2), 910 cm-1 (A3), 954 cm-1 (A4), corresponding to the numerical values of modes resonant frequencies. (b) Measured distribution of optical near fields (amplitude s2 and phase ϕ2, see Methods) on the top surface of representative bowtie antennas with g ∼ 50 nm at three wavelengths: 888 cm-1 (close to A2), 905 cm-1 (close to A3), 954 cm-1 (close to A4). The incident (p-polarized) light is from a line-tunable mid-infrared CO2 laser. These images are recorded using a dielectric Si tip to suppress the potential tip-influence as much as possible. The recorded images reveal distinct


27

ACS Photonics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 28

mode behaviors in the bowtie antennas. (c-d) The near-field amplitude s2 (c) and phase ϕ2 (d) of a bowtie antenna array with g ∼ 50 nm at the wave number of 888 cm-1. In (b-d), the white arrows indicate the direction of electric fields. All the white scale bars indicate 1 µm. The I and II in c indicate two bowtie antennas with different resonance patterns.


28

Phonon-Polaritonic Bowtie Nanoantennas - ACS Publications

Recommend Documents