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High-sensitivity refractive index sensors using coherent perfect absorption on graphene in the Vis-NIR region Chawei Li, Jinlin Qiu, Jun-Yu Ou, Qing Huo Liu, and Jinfeng Zhu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00523 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019
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ACS Applied Nano Materials
High-sensitivity refractive index sensors using coherent perfect absorption on graphene in the VisNIR region Chawei Li1,2, Jinlin Qiu1, Jun-Yu Ou3, Qing Huo Liu4 and Jinfeng Zhu1,2* 1School
of Electronic Science and Engineering, Xiamen University, Xiamen 361005, China
2Shenzhen
Research Institute of Xiamen University, Shenzhen 518057, China
3Optoelectronics
Research Centre and Centre for Photonic Metamaterials, University of
Southampton, Highfield, Southampton, SO17 1BJ, UK 4Department
of Electrical and Computer Engineering, Duke University, Durham, North
Carolina 27708, USA
KEYWORDS: graphene, nanophotonics, refractive index sensing, coherent perfect absorption, plasmonics, metamaterial
ABSTRACT: Plasmonic structures with sophisticated nanofabrication have revolutionized the ability to trap light on the nanoscale and enable high-sensitivity refractive index sensing. Previous theoretical research has indicated that the sensitivity and figure of merit around the wavelength of
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1μm for a plasmonic sensing system can be up to 13,000 nm/RIU and 138, respectively. In order to improve the sensing performance, we propose a graphene-based non-plasmonic sensor with the sensitivity over 440,000nm/RIU at the wavelength of 1μm, which is 33 times more than the theoretical result of plasmonic sensors. Our graphene sensor is a nanofabrication-free design with perfect light confinement within a monolayer of graphene. Meanwhile, its figure of merit is up to the scale of thousands, which is also much higher than plasmonic sensors. Our scheme uses a simple dielectric structure with a monolayer of graphene and shows a great potential for low-cost sensing with high performance.
1. INTRODUCTION Optical refractive index sensors with high sensitivity are widely used in the fields of chemistry, biology, medicine and material engineering, and they have attracted a plenty of attention in the nanotechnology community. In the past decade, optical sensors using various plasmonic nanostructures have been extensively studied due to their high sensitivity.1-3 The basic mechanism of plasmonic sensing is to excite charge density oscillations propagating along the dielectric/metal interface.4 The spectral response related to these oscillations is sensitive to the environmental refractive index, so that plasmonic sensors can detect a very tiny variation of the refractive index. Theoretical research on a plasmonic sensing system has demonstrated the sensitivity and figure of merit (at the wavelength of 1μm) up to 13,000 nm/RIU and 138, respectively.5 With the aim to improve the performance of plasmonic sensing systems, the scheme based on metamaterials has
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ACS Applied Nano Materials
been adopted with the development of micro- and nano-fabrication in nanophotonics. A. V. Kabashin et al. has used plasmonic nanorod metamaterials for a high sensitivity over 30,000nm/RIU and a FOM up to 330 at the wavelength of 1.28μm.6 K. V. Sreekanth et al has reported a plasmonic sensing platform based on hyperbolic metamaterials, which has the sensitivity and FOM up to 30,000nm/RIU and 590 around the wavelength of 1.3μm, respectively.7 Even though the use of metamaterial concept has efficiently elevated the sensitivity and FOM of plasmonic sensors, their preparation often requires a sophisticated and expensive micro- or nanofabrication. In addition, other non-plasmonic structures have also been investigated to improve the sensing performance, like slot waveguide ring resonators, photonic crystal cavities and slabs.8-10 These systems have obviously increased the FOM or quality factor, but they hardly obtain higher sensitivity than the plasmonic systems. Therefore, high performance optical sensors with a simple structure and a lower cost are still quite in demand. Recently, 2D materials have become an important role for developing future optical devices due to their unique light-matter interaction.11,12 As the most popular 2D material, graphene has attracted a lot of optical investigations from ultraviolet (UV) to terahertz.13-16 A promising optical property of graphene is that it can support surface plasmon polaritons (SPP) and near-field enhancement for the spectral range from mid-infrared to terahertz, which enable graphene to replace metals in plasmonic applications.17-20 The mid-infrared spectra for graphene are well suited for optical sensing because they reflect many molecular fingerprints which can identify different biochemical building blocks of life, such as proteins, lipids, and DNA,21 and there have been plentiful efforts made by using graphene plasmonic effects to realize various sensing applications in mid-infrared range.22-24 However, in the range from the visible to near infrared (NIR), intrinsic graphene is unable to support plasmonic effects, and it can be regarded as a lossy conductive
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surface.25 In this condition, utilizing plasmonic effects of graphene for high performance optical sensing is no longer feasible for the visible and NIR region, in which the optical detecting configurations are usually simpler and much more economical than in the mid-infrared range. Therefore, in order to realize graphene-based optical sensing with high performance in the visible and NIR region, an innovative design based on nanophotonics of 2D materials has become a quite important research topic. In this work, a simple all-dielectric photonic structure for high performance optical sensing has been proposed, in which graphene is sandwiched between a lossless dielectric and a liquid sample. By utilizing optical total reflection and single-channel coherent perfect absorption, the use of metallic materials is not necessary and the complicated nanofabrication is also avoided. By a series of systematic design and optimization, we have achieved multiple extremely sensitive coherence modes with a maximum sensitivity of 440,000nm/RIU around the wavelength of 1μm and a maximum FOM up to the scale of thousands, which indicate that the proposed optical sensor is more advanced and promising than conventional plasmonic sensors due to its nanofabrication-free configuration and ultra-sensitive performance. 1. METHODOLOGY In this study, we propose an ultra-sensitive refractive index sensor with a coating of monolayer graphene. The proposed configuration consists of a lossless dielectric hemispherical prism, a liquid sample layer, and a graphene layer sandwiched between them. The liquid sample is injected through a microfluidic channel and confined in a circular container, as shown in Figure 1. The sample liquid is exposed to the air. The refractive index of the air, prism and sample liquid are nair, n1 and n2, respectively. The light beam obliquely illuminates from the prism to the central region of the prism/graphene interface at an incident angle θ1. Based on our previous work,26 when the
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conditions of nair
