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Carbon nanotubes (CNT) are being intensively studied for many applications because of their unique properties, such as high electrical and thermal conductivities, excellent mechanical and chemical stability. One of the areas of CNT application is a bolometric detection of near- and mid-infrared (IR) radiation. The record near-IR bolometric performance of CNT devices is comparable to the performance of the commercial vanadium oxide detectors [1]. However, the low intrinsic absorption of CNT in the mid-IR range limits their applications for the detection in the crucial 3-5 µm and 8-12 µm regions. The phenomenon of surface plasmon excitation has been utilized to improve light harvesting efficiency of solar cells and to increase absorption of monolayer graphene [2]. Plasmons were recently observed on an individual CNT [3], but the excitation of a surface plasmon on the macroscopic CNT films have not been reported yet.
In the study, we experimentally demonstrate the 100%-enhanced bolometric response of a single-walled carbon nanotube (SWCNT) film in the vicinity of a mid-IR surface plasmon resonance. As a basis for the sample we use a pristine SWCNT film with the thickness of 400 nm, the width of 3 mm, and the length of 6 mm, suspended between two gold contacts. The femtosecond laser is used to drill 3- µm round holes which are arranged in a 2D square lattice with a period of 10 µm. Only one half of the film (3x3 mm2) is structured, while the second half is left unstructured to perform reference measurements. Reflectance and transmittance spectra of both parts of the film are measured with the Fourier-transform infrared spectrometer in the range 2-100 µm. Surface plasmon manifests itself as a Fano-type resonance in spectra of the structured part of the film at the wavelengths around 15 µm. The absorption of the structured part is enhanced in the same spectral region by 75% as compared with the unstructured part. Spectral dependence of the bolometric voltage sensitivity is measured using the sample as a detector of the spectrometer. It is shown that the voltage response of the structured part is enhanced in the vicinity of the surface-plasmon resonance by 100%. We show that the wavelength of the resonance and its magnitude can be controlled by tailoring the geometry of the sample. Numerical calculations by scattering-matrix method show that the central wavelength of the absorption resonance can be tuned in the range 5-25 µm. The maximum available enhancement of absorption is 240% as compared with the unstructured film.
We claim that the proposed method applies to achieve enhanced spectrally selective response for any CNT-based bolometer in both near- and mid-infrared ranges of the spectrum.
[1] G. E. Fernandes, J. H. Kim, A. K. Sood, J. Xu, Adv. Funct. Mater. 2013, 23, 4678.
[2] N. K. Emani, T.-F. Chung, A. V Kildishev, et al. Nano Lett. 2013, 14, 78.
[3] Z. Shi, X. Hong, H. A. Bechtel, et al. Nat. Photonics 2015, 9, 515
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