Mid-infrared Gas Sensing Using Graphene Plasmons Tuned by Reversible Chemical Doping

Highly confined plasmon modes in nanostructured graphene can be used to detect tiny quantities of biological and gas molecules. In biosensing, a specific biomarker can be concentrated close to graphene, where the optical field is enhanced, by using an ad-hoc functional layer (e.g., antibodies). Inspired by this approach, in this paper we exploit the chemical and gas adsorption properties of an ultrathin polymer layer deposited on a nanostructured graphene surface to demonstrate a new gas sensing scheme. A proof-of-concept experiment using polyethylenimine (PEI) that is chemically reactive to CO2 molecules is presented. Upon CO2 adsorption, the sensor optical response changes because of PEI vibrational modes enhancement and shift in plasmon resonance, the latter related to polymer-induced doping of graphene. We show that the change in optical response is reversed during CO2 desorption. The demonstrated limit of detection (LOD) of 390 ppm corresponds to the lowest value detectable in ambient atmosphere, which can be lowered by operating in vacuum. By using specific adsorption polymers, the proposed sensing scheme can be easily extended to other relevant gases, for example, volatile organic compounds.Peer ReviewedPostprint (published version

Scalable and tunable periodic graphene nanohole arrays for mid-infrared plasmonics

Despite its great potential for a wide variety of devices, especially mid-infrared biosensors and photodetectors, graphene plasmonics is still confined to academic research. A major reason is the fact that, so far, expensive and lowthroughput lithography techniques are needed to fabricate graphene nanostructures. Here, we report for the first time a detailed experimental study on electrostatically tunable graphene nanohole array surfaces with periods down to 100 nm, showing clear plasmonic response in the range ~1300-1600 cm-1 , which can be fabricated by a scalable nanoimprint technique. Such large area plasmonic nanostructures are suitable for industrial applications, for example, surface-enhanced infrared absorption (SEIRA) sensing, as they combine easy design, extreme field confinement, and the possibility to excite multiple plasmon modes enabling multiband sensing, a feature not readily available in nanoribbons or other localized resonant structures.Peer ReviewedPostprint (published version

Tunable complete optical absorption in multilayer structures including without lithographic patterns

Controlling the spectral transmission, reflection, and absorption properties of optical structures is of great interest for many applications in photonics. Particularly, perfect absorbers over a wide frequency (wavelength) range are desirable for thin-film thermal emitters, thermo-solar cells, photodetectors, and smart windows. Up to date, several mechanisms have been proposed to achieve nearly 100% absorption in various frequency ranges of the electromagnetic spectrum; starting from microwaves to near infrared (NIR) and visible. One of the first demonstrations of a structure that was absorbing with nearly 100% efficiency was proposed by Landy et al. in 2008,[1] where metamaterial resonator arrays were used to achieve narrowband and highly resonant absorption of GHz and THz waves. The narrowband character of the resonances can be an advantage when absorbers with high quality factor are required and wavelength selectivity is desirable. However, there are many applications that need broadband absorption. To this end great efforts have been made during the last decade, for instance by mixing multiple resonances in a many-fold resonator, which can lead to, e.g., dual band[2] or multiband[3-9] resonant absorption. Unfortunately fabrication of these structures requires sophisticated techniques such as micro- or nano-lithography, severely limiting their scalability and increasing the cost of the absorber.Peer ReviewedPostprint (author's final draft

Tunable complete optical absorption in multilayer structures including without lithographic patterns

Controlling the spectral transmission, reflection, and absorption properties of optical structures is of great interest for many applications in photonics. Particularly, perfect absorbers over a wide frequency (wavelength) range are desirable for thin-film thermal emitters, thermo-solar cells, photodetectors, and smart windows. Up to date, several mechanisms have been proposed to achieve nearly 100% absorption in various frequency ranges of the electromagnetic spectrum; starting from microwaves to near infrared (NIR) and visible. One of the first demonstrations of a structure that was absorbing with nearly 100% efficiency was proposed by Landy et al. in 2008,[1] where metamaterial resonator arrays were used to achieve narrowband and highly resonant absorption of GHz and THz waves. The narrowband character of the resonances can be an advantage when absorbers with high quality factor are required and wavelength selectivity is desirable. However, there are many applications that need broadband absorption. To this end great efforts have been made during the last decade, for instance by mixing multiple resonances in a many-fold resonator, which can lead to, e.g., dual band[2] or multiband[3-9] resonant absorption. Unfortunately fabrication of these structures requires sophisticated techniques such as micro- or nano-lithography, severely limiting their scalability and increasing the cost of the absorber.Peer Reviewe

Mid-Infrared Pyroresistive Graphene Detector on LiNbO3

Mid-infrared (mid-IR) photo-detection has been recently growing in importance because of its multiple applications, including vibrational spectroscopy and thermal imaging. We propose and demonstrate a novel pyro-resistive photo-detection platform that combines a ferroelectric substrate (a z-cut LiNbO3 crystal) and a graphene layer transferred on top of its surface with electrical connections. Upon strong light absorption in the LiNbO3 substrate and the subsequent temperature increase, via the pyroelectric effect, polarization (bound) charges form at the crystal surface. These causes doping into graphene which in turn changes its carrier density and conductivity. In this way, by monitoring the graphene electrical resistance one can measure the incident optical power. . Detectivities of about 10^5 cm sqrt(Hz)/W in the 6 to 10 microns wavelength region are demonstrated.We explain the underlying physical mechanism of the pyro-resistive photo-detection and propose a model that reproduces accurately the experimental results. We also show that up to two orders of magnitude larger detectivity can be achieved by optimising the geometry and operating in vacuum, thus opening the path to a new class of mid-IR photo-detectors that can challenge classical HgCdTe devices, especially in real applications where cooling is to be avoided and low cost is crucial

Hypothermia for moderate or severe neonatal encephalopathy in low-income and middle-income countries (HELIX): a randomised controlled trial in India, Sri Lanka, and Bangladesh

Copyright (c) 2021 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license