Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX
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Surface-Enhanced Thermal Emission Spectroscopy with Perfect Absorber Metasurfaces Franziska B. Barho,*,† Fernando Gonzalez-Posada,† Mario Bomers,† Aude Mezy,‡ Laurent Cerutti,† and Thierry Taliercio† †
IES, Université de Montpellier, CNRS, Montpellier, France Sikémia, F-34095 Montpellier, France
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ABSTRACT: Surface-enhanced spectroscopy techniques using plasmonic nanoantennas or metasurfaces help to reduce the detection limit for biochemical sensing. While infrared spectroscopy is an excellent tool to identify a molecular species, a typically expensive IR light source is needed. We report a surface enhanced spectroscopy technique based on the thermal emission of III−V semiconductor metasurfaces. The presence of a molecular species grafted on the surface modulates the emission spectrum analogously to the modulation achieved in surface-enhanced infrared absorption (SEIRA) spectroscopy. The vibrational fingerprint of the molecular species is detected due to the electromagnetic field enhancement obtained with a plasmonic metasurface. Because the metasurface acts simultaneously as radiation source and sensor chip, the experimental setup is simplified and therefore more compact and potentially more cost-efficient. This novel approach of surface-enhanced thermal emission spectroscopy (SETES) is appealing for miniaturized and integrated molecular sensing devices. KEYWORDS: metamaterials, thermal emission, surface-enhanced IR spectroscopy, perfect absorber, biochemical sensing, silanes
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broadband such as globars or narrowband such as the mentioned tunable QCL. Replacing these rather expensive sources by a simple metasurface emitter holds an obvious potential to reduce the experimental setup and goes one step forward to building compact biosensing devices. Coherent thermal emission from patterned polaritonic materials has first been evidenced in 2002 by Greffet et al.31 A reverse process to the excitation of surface polaritons by incoming radiation is at the origin of the observed coherent thermal radiation: At ambient temperatures, the theoretic spectrum of surface polariton modes can be excited by the energy available in the system. These modes can couple to outgoing radiation due to a periodic pattern on the surface. It is possible to precisely determine the emission properties of a nanostructured sample using Kirchhoff’s law, 32 which states the equality of absorptance and emittance, the ratio of absorbed, respectively emitted, to the incident radiant power. Metamaterial perfect absorbers were introduced around one decade ago.33 They are ideal candidates for efficient thermal emitters.34 Based on different materials and by scaling the characteristic dimensions, perfect absorbers for optical to micrometric wavelengths have nowadays been demonstrated.35−39 The perfect absorber principle has also been used for SEIRA.40−42
nfrared spectroscopy is a standard analysis technique for molecule detection and identification. While initially limited to sufficient quantities of analyte molecules, the surface enhancement effect using plasmonic nanostructures and metamaterials helps to downscale the necessary quantity and has been successfully applied for the detection of monolayers of chemical moieties,1−6 proteins,7−10 lipid membranes,11 or dust particles.12 Different strategies have been employed to reduce the detection limit, such as optimizing the nanoantenna geometrically to achieve highest field enhancement at a target wavelength,5,9,13−17 creating an adapted tuning ratio between the antenna resonance and the molecular vibration,18,19 exploiting nanoantenna systems with sharp Fano-like resonances20 or collective array resonances,21,22 or using alternative plasmonic materials with a tunable dielectric function to cover various spectral ranges.10,23−26 Comprehensive reviews have recently discussed these approaches for improving surfaceenhanced infrared absorption (SEIRA) spectroscopy.27,28 Besides this quest for better signal enhancement, some attention has been awarded to the experimental setup. The issues of acquisition time or the possibility of integration and miniaturization were addressed. Current approaches include the use of a quantum cascade laser (QCL), which is tuned across a small spectral range covering the molecular absorption line,29 or an imaging-based approach employing a pixelated metasurface that is operational without the need of a spectrometer.30 Typically, the reflectance or transmittance is measured, thus, necessitating an external IR light source, either © XXXX American Chemical Society
Received: February 13, 2019
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DOI: 10.1021/acsphotonics.9b00254 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Article
Figure 1. (a) Schematic of the perfect absorber metamaterial. ht, hs, and hm indicate, respectively, the mirror thickness, the GaSb spacer thickness, and the top layer thickness. (b) SEM image of the fabricated grating. (c) Sketch of the experimental setup. The abbreviations denote the following: PM, parabolic mirror; BS, beam splitter; SM, spherical mirror. The radiation path is indicated by the red beam.
Here, we focus on a perfect absorber or, inversely, an efficient thermal emitter for the mid-infrared spectral range. At these wavelengths, the room temperature blackbody emission reaches a maximum, as can be seen from Wien’s displacement 0.29 cm law:43 λmax = T(K) = 9.7 μm for T = 300 K. As demon-
metamaterial has two roles here: First, it serves as emitter to replace the IR radiation source, and second, it enhances the signal of the analyte molecule, like the plasmonic nanoantenna or metasurfaces used for SEIRA. We experimentally compare the reflectance and emittance spectra and find that SEIRA and surface enhanced thermal emission spectroscopy (SETES), described in the following, both give equal access to enhanced signatures of molecular vibrations. Quantitatively, the metamaterial used in this work gives rise to a lower differential signal level, but a higher signal-to-noise ratio for SETES compared to SEIRA. The III−V semiconductor-based perfect absorber metamaterial is functionalized with a layer of 11pentafluorophenoxyundecyltrimethoxysilane (PFTMS) to probe the molecular vibrations around 1000 cm−1. Absorbance band fitting is performed to extract the dielectric function of PFTMS, which is then introduced into rigorous coupled wave analysis (RCWA) simulations of the monolayer-coated metamaterial. The model confirms the surface-enhancement effect, enabling the detection of a monolayer. Experimental and theoretical proof is hence given for the SETES sensing scheme.
strated in previous work, the III−V materials InAsSb:Si on GaSb are an appealing choice for plasmonics and metamaterials in the mid-IR spectral range.44−46 Briefly, they offer the advantage of a tunable plasma frequency by the doping level and, hence, a higher flexibility of the operation wavelength. Furthermore, lattice matched heterostructures can be grown, and the materials are compatible with standard semiconductor fabrication technologies, unlike gold, which can diffuse from thin surface layers into the substrate and introduces deep-level defects.47 Recently, InAs devices have been grown on silicon substrates, thus, demonstrating the compatibility with silicon technology and potential cost reduction by avoiding III−V substrates.48 Using a layer system of InAsSb:Si as a metal-like material stacked with GaSb as spacer, near perfect absorption can be achieved.49 When the system is at resonance, the effective optical impedance equals the free space impedance so that reflection is suppressed. Simultaneously, transmission is avoided due to the optically thick heavily doped semiconductor (InAsSb:Si) metal-like ground plane. Hence, all incident radiation is absorbed within the layer structure. When unity absorptance is reached, Kirchhoff’s law states that emission is as high as the one of a perfect blackbody, in other words, unity emittance is obtained. In this work, we present an innovative spectroscopy technique applicable to molecular detection based on emission spectroscopy at a temperature of 65 °C. The plasmonic
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RESULTS AND DISCUSSION The metamaterial thermal emitter (MTE) used in the following consists of an optically thick (1 μm), metal-like heavily Si-doped InAsSb back reflector, a dielectric GaSb spacer, and a 100 nm thin top metal-like InAsSb:Si layer patterned into a grating with a period Λ = 640 nm (Figure 1a). A scanning electron micrograph of the surface is shown in Figure 1b. Figure 1c schematizes the experimental setup for the emission spectroscopy. The thermal emitter is placed in the focal plane of a 90° off-axis parabolic gold mirror which collimates and redirects the emitted light toward the input port of a Fourier-transform infrared (FTIR) spectrometer. A B
DOI: 10.1021/acsphotonics.9b00254 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Article
Figure 2. (a) Emittance (black) and reflectance (blue) spectra of the MTE. The radiation is polarized perpendicular to the grating, as schematized in the inset, where the green arrow indicates the direction of the electric field vector. (b) Emittance and reflectance parallel to the grating, as shown in the inset. Emittance is obtained using eq 1 from emission measurements of MTE and blackbody samples heated to 65 °C. Reflection measurements are performed at room temperature.
leading to a significant variation of the dielectric function with temperature, resulting hence in a shift between reflectance and emittance extrema.31,51 The MTE is characterized by a dual band emittance and reflectance in the polarization direction perpendicular to the grating. The two resonances in Figure 2a correspond to an antireflectance effect at 1038 cm−1, depending on the quarter-wavelength optical thickness of the dielectric spacer layer and a surface plasmon resonance close to 810 cm−1, presenting the emittance maximum for the investigated sample or, in other terms, a reflectance below 0.01. These resonances may hybridize as discussed elsewhere,49 giving a plasmonic character to the “quarterwavelength” resonance. In the opposite polarization, parallel to the grating’s grooves, the emission peak is linked to the quarter-wavelength optical thickness reflectance minimum. The two narrow spectral bands of emission cover an important range for molecule detection in the so-called fingerprint range from 500 to 1500 cm−1, useful for the identification of diverse chemical and biological moieties.27,52,53 To prove the concept of molecular sensing by surface-enhanced thermal emission spectroscopy, a selfassembled molecular layer of 11-pentafluorophenoxyundecyltrimethoxysilane (Sikémia, France; CAS-No. 944721-47-5), abbreviated PFTMS in the following, was immobilized on the MTE as detailed in the Methods section. At ambient conditions, native oxides form layers in the order of a few nanometer on the surface of GaSb, InAs and its derivative InAsSb.54 While a native oxide layer of around 3 nm thickness forms within 1 day on GaSb, only little increase of 1 nm in 3 years was evidenced in a long-time air oxidation study.55 Working with the surface oxide instead of against it has been proven to be an appropriate approach for surface functionalization of GaSb and InAsSb.6 For successful binding of a molecular species, it is essential to consider the stability of the surfaces in different solvents. The hydrolytic stability of InAs in neutral water was reported.56 However, GaSb is not stable in aqueous environment.17,57,58 A water-free surface chemistry is thus required to prevent the oxidation of GaSb in water, which is much faster and more detrimental than the air oxidation. Trimethoxysilane derivatives are an interesting approach for monolayer formation on metal oxides, and we demonstrate in the following the successful surface modification by silane surface chemistry. Trimethoxysilanes are moderately reactive monolayer forming agents and a covalent linkage between the substrate and the silane stabilizes the monolayer.59 The trifunctional molecules provide increased
polarizer is placed in the beam path after the interferometer in order to select the orientation-dependent emission from the anisotropic sample. The emission is collected by a liquid nitrogen cooled mercury−cadmium−telluride (MCT) detector in a spectral range from 4000 to 450 cm−1. The samples are heated on a Peltier element at 65 °C to increase the sample’s signal Isample compared to the background radiation. The intrinsic emission Ibackground of the optical elements inside the FTIR spectrometer, notably the beam splitter, is collected by measuring the radiation when the spectrometer’s input port, indicated in Figure 1c, is covered and is subtracted from all spectra. Due to the slight heating of the sample, its emission intensity is at a significantly higher level than the internal emission of the optics setup at room temperature. With the aim of molecule detection, it is, however, important to keep the sample temperature sufficiently low in order to neither break the molecules’ bonds to the metasurface nor to degrade temperature sensitive conformations of biomolecules. Lower heating to
