Abstract
In this paper, we propose a compact optical gas sensor based on the widespread silicon-on-insulator (SOI) technology, operating in the near-infrared (NIR) region around the 1.55 µm wavelength. The sensor employs a loop-terminated Mach–Zehnder interferometer (LT-MZI) with a slot waveguide and a strip waveguide for the sensing arm and the reference arm, respectively. For the same arm length, the LT-MZI can achieve a detection limit two times lower than that of the conventional MZI. Different sensor components were designed, and the optimum dimensions were obtained using finite-difference eigenmode (FDE) and finite-difference time-domain (FDTD) solvers. With a sensing arm length of only 150 μm, our sensor achieves a device sensitivity of 1070 nm/RIU and a figure-of-merit (FOM) as high as 280.8 RIU−1 at the 1.55 μm wavelength. Higher values of FOM can be attained by employing a longer sensing arm. The whole sensor is subjected to air cladding; thus, there is no need for oxide deposition and a further lithography step for sensing-area patterning. The sensor is well suited for low-cost fabrication and large-scale production. Finally, the same LT-MZI device with strip and slot arms but with oxide cladding was fabricated and characterized. The measurements were in good agreement with the electromagnetic (EM) simulation results, ensuring the reliability of our proposed design.
Highlights
Gas sensing is rapidly growing in importance due to its essential role in various fields and for a wide range of applications such as the detection of hazardous and toxic gases, industrial inspection, and environmental monitoring [1]
Miniaturizing the footprint of these sensors using a complementary metal-oxide semiconductor (CMOS)compatible technology is of great importance, to enable their integration with other optical and electrical functions, allowing compact devices to be produced on a mass scale using low-cost fabrication [6,7]
Plasmonic sensors based on surface plasmon–polariton (SPP) waves, which propagate along a metal surface, have been proposed [24,25,26,27,28,29]
Summary
Gas sensing is rapidly growing in importance due to its essential role in various fields and for a wide range of applications such as the detection of hazardous and toxic gases, industrial inspection, and environmental monitoring [1]. Plasmonic sensors based on surface plasmon–polariton (SPP) waves, which propagate along a metal surface, have been proposed [24,25,26,27,28,29] Such sensors can achieve large wavelength sensitivities due to the high light–matter interaction that can be obtained using a plasmonic waveguide, especially a plasmonic slot. Evanescent field absorption gas sensors, which detect the imaginary part of the refractive index k, can be selective by working around the absorption fingerprint of the gas [36,37,38,39,40] These fingerprints are in the mid-infrared region where the photonic platforms are not as mature as in the near infrared. We propose a highly compact RI gas sensor using an SOI loop-terminated Mach–Zehnder interferometer (LT-MZI) and utilizing a slot waveguide in the sensing arm. For the same interferometer arm length, the LT-MZI exhibits twice the FOM of the conventional MZI, and an enhanced detection limit
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