Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX
Observation of Giant Infrared Circular Dichroism in Plasmonic 2DMetamaterial Arrays Richard Knipper,†,§ Thomas G. Mayerhöfer,† Vladimír Kopecký, Jr.,‡ Uwe Huebner,† and Jürgen Popp*,†,§ †
Leibniz Institute of Photonic Technology (IPHT), Albert-Einstein-Straße 9, Jena, D-07745, Germany Institute of Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, Prague 2, CZ-12116, Czech Republic § Institute of Physical Chemistry and Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Helmholtzweg 4, Jena, D-07743, Germany ‡
S Supporting Information *
ABSTRACT: We report the observation of an enormous circular dichroism (CD) response in broadband surfaceenhanced infrared absorption sensing arrays. In our experiment, a 2D-metamaterial chiral cross-slit array adjusted for application in conventional measurement setups was used. The inherent chirality of the design results in a very strong CD absorbance of 0.83 at around 2.86 μm (3500 cm−1). This far surpasses our predictions based on the finite-difference time-domain simulations. This is probably the highest CD response ever reported for the mid-infrared region. Our observation was rigorously validated; thus most error sources were ruled out. Further, this Letter discusses our results in view of possible remaining artifacts that might have had an influence on our observations, as both chiral enantiomers and an achiral counterpart with similar structure were created and measured. A giant CD 2D-metamaterial might lead toward surface-enhanced vibrational circular dichroism spectroscopy, a field that is subject to lively discussions. KEYWORDS: chiral plasmonics, infrared spectroscopy, metamaterials, surface enhancement, vibrational circular dichroism ince its first description by Hartstein et al.,1 plasmonic enhancement in the IR range has moved from being a mere theoretical consideration to a potential tool for practical applications.2 In the last couple of years, plasmonic enhancement has promised to boost signal intensities far beyond the monolayer capacity already available by conventional methods such as attenuated total reflection3 and buried metal layer infrared (IR) reflection absorption spectroscopy,4 as well as the combination of both,5 interference-enhanced attenuated total reflection. Even though infrared absorption cross sections are typically much larger than those in Raman spectroscopy (RS), they are still 3 to 4 orders of magnitude lower than, for example, fluorescence,6 so a further boost of signal intensities by improving conventional methods cannot be expected. Naturally, the weak ∝E2 signal enhancement in surfaceenhanced IR spectroscopy (as compared to ∝E4 for surfaceenhanced RS) reduces applicability in “real-life” spectroscopic tasks, also due to the problems of hot-spots.7 By employing slotted membranes, one can utilize extraordinary optical transmission (EOT)8 to counteract this problem. The corresponding idea of subwavelength hole arrays (SHAs) with slits dates back to 2014,9 making this a rather recent approach. One year later, Mayerhöfer et al.10 presented an SHA based on a combination of EOT and perfect absorption.
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© XXXX American Chemical Society
Deviating from a simple array of a single slit, even before the works of Huck et al.,11 Cetin et al.9 published a study about Hshaped slits where the slit connecting the two parallel and vertically oriented slits was extended beyond the vertical slits. This system showed highly anisotropic plasmon resonances, one polarized along the horizontal slit and two in the vertical polarization direction, which allowed tuning the resonance frequencies of the three resonances independently. This design permits covering a broader range of the infrared spectrum. In addition, the plasmonic active layer is supported by a freestanding membrane, enabling measurements both in reflection as well as in transmission mode. While using a similar support structure, the slit structure employed in this work consists of two slits, one vertically oriented and the second 45° rotated clockwise or anticlockwise with the pivot at the center of both slits. When the slits are of different length, also two different plasmonic resonances occur; however, the resonances are not orthogonal to each other. More important, the unit cell structure, when embedded in an array, is chiral like early substrates shown by Decker et al.,12 as compared to achiral substrates also demonstrated.13−17 Received: December 4, 2017 Published: February 14, 2018 A
DOI: 10.1021/acsphotonics.7b01477 ACS Photonics XXXX, XXX, XXX−XXX
Letter
ACS Photonics
individual SHA gratings on the wafer was performed using the shaped electron-beam writer SB350 OS at an electron dose of 400 μC/cm2. The resist was developed for 60 s in AR600-546 (Allresist) and rinsed for 30 s in 2-propanol. Ar+-ion-beam etching opened the hard mask NiCr layer. The silicon wafer was structured backside using silicon wet-etching (KOH) to open up the membrane, which was then slitted via CHF3-RIE etching to structure the SiO2/Au/SiN sandwich. After removing the NiCr hard mask with wet etching, chips were separated to 1.5 × 1.5 cm2 by notching and breaking. The samples were cleaned in ethanol, blown dry, and then O2 plasma-cleaned. Resulting slits are 1700 nm in length for the primary and 1000 nm for the secondary slit, both with 50 nm width. Infrared Spectroscopy. The IR reflection spectra were recorded with a Vertex 80v FT-spectrometer (Bruker) with an attached HYPERION 2000 (Bruker) IR microscope equipped by a 15× Cassegrain IR objective (0.4 NA). The angle of incidence was
