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.
Highlights
To implement industrial process control, security and surveillance, environmental analysis, and clinical/biomedical monitoring, numerous sensors with small footprints, high stability, low cost, and low power consumption are demanded
Besides many demonstrations of different types of sensors relying on in-plane configuration, such as spiral waveguide sensors [34,35,36], ring/disk resonator sensors [37,38], slot waveguide sensors [39,40,41], and plasmonics based sensors [42,43], directional coupler (DC) as a key component for light routing and power splitting has been explored as a nanophotonics sensor [44,45]
We report a compact wavelength-flattened DC (WFDC, or broadband DC) based sensor for the MIR
Summary
To implement industrial process control, security and surveillance, environmental analysis, and clinical/biomedical monitoring, numerous sensors with small footprints, high stability, low cost, and low power consumption are demanded. A SWG structure which engineers dispersion to allow broadband operation is incorporated in DC while it introduces a sharp trough at the Bragg wavelength for high sensitivity. As the first investigation of a DC based MIR sensor, our device can utilize both the absorption and RI change caused by analytes to potentially allow tertiary mixtures sensing It can be adopted as a promising broadband component for light routing and power splitting in MIR spectroscopic sensing systems. Unlike the linear dispersion presented in the slab mode, the effective RI of the floquet mode in SWG rises drastically as λ approaches the Bragg wavelength λB. Sepnesritpiveirtcyenotfa−ge0.o4f7C%H, −2C0l2.4c5o%nc,e−nt0r.a1t7io%n, iasnddem0%onTs/trIactehdanatg3e.6p6e8r μpmer,ce3n.6t7a6geμomf ,C3H.628C5lμ mco,nacnednt3ra.7ti2o7n is deμmmonrsetsrpaetecdtivaet l3y.6. 6T8hμismr,e3s.u6l7t6sμhmow, 3s.6t8h5atμtmh,eafnirdst3.d72e7rivμamtivreespoefctthiveelTy/.ITshpiescrtersuumlt sherovwes tahsatathe firgstododerivreafteivrenocfethfeorT/tIhsepescetlreucmtiosnervoef ssaesnasignogodwraevfeerlencgethforastheitselheicgtihonvoaflusensrientugrwnsavheliegnhgth assietsnshiitgivhitvyalwuehirleetuitrsnnsehairg-hzesreonsviatilvuietyrewtuhrilnesitnsenaer-azre-zreorsoevnasliutievirteytu. rns near-zero sensitivity
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