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Wavelength-Flattened Directional Coupler Based Mid-Infrared Chemical Sensor Using Bragg Wavelength in Subwavelength Grating Structure

1
Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore
2
Institute of Microelectronics, Agency for Science, Technology and Research (A*STAR), Singapore 138634, Singapore
3
Center for Intelligent Sensors and MEMS (CISM), National University of Singapore, Singapore 117576, Singapore
4
Graduate School for Integrative Science and Engineering, National University of Singapore, Singapore 117456, Singapore
5
School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
*
Authors to whom correspondence should be addressed.
Nanomaterials 2018, 8(11), 893; https://doi.org/10.3390/nano8110893
Received: 8 October 2018 / Revised: 17 October 2018 / Accepted: 23 October 2018 / Published: 1 November 2018
(This article belongs to the Special Issue From Nanomaterials to Intelligent Nanosystems)
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Abstract

In this paper, we report a compact wavelength-flattened directional coupler (WFDC) based chemical sensor featuring an incorporated subwavelength grating (SWG) structure for the mid-infrared (MIR). By incorporating a SWG structure into directional coupler (DC), the dispersion in DC can be engineered to allow broadband operation which is advantageous to extract spectroscopic information for MIR sensing analysis. Meanwhile, the Bragg reflection introduced by the SWG structure produces a sharp trough at the Bragg wavelength. This sharp trough is sensitive to the surrounding refractive index (RI) change caused by the existence of analytes. Therefore, high sensitivity can be achieved in a small footprint. Around fivefold enhancement in the operation bandwidth compared to conventional DC is achieved for 100% coupling efficiency in a 40 µm long WFDC experimentally. Detection of dichloromethane (CH2Cl2) in ethanol (C2H5OH) is investigated in a SWG-based WFDC sensor 136.8 µm long. Sensing performance is studied by 3D finite-difference time domain (FDTD) simulation while sensitivity is derived by computation. Both RI sensing and absorption sensing are examined. RI sensing reveals a sensitivity of −0.47% self-normalized transmitted power change per percentage of CH2Cl2 concentration while 0.12% change in the normalized total integrated output power is realized in the absorption sensing. As the first demonstration of the DC based sensor in the MIR, our device has the potential for tertiary mixture sensing by utilizing both changes in the real and imaginary part of RI. It can also be used as a broadband building block for MIR application such as spectroscopic sensing system. View Full-Text
Keywords: nanophotonics; photonics sensors; mid-infrared; waveguide coupler; subwavelength grating nanophotonics; photonics sensors; mid-infrared; waveguide coupler; subwavelength grating
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Dong, B.; Hu, T.; Luo, X.; Chang, Y.; Guo, X.; Wang, H.; Kwong, D.-L.; Lo, G.-Q.; Lee, C. Wavelength-Flattened Directional Coupler Based Mid-Infrared Chemical Sensor Using Bragg Wavelength in Subwavelength Grating Structure. Nanomaterials 2018, 8, 893.

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