Abstract
Chip-scale chemical detection is demonstrated by using mid-Infrared (mid-IR) photonic circuits consisting of amorphous silicon (a-Si) waveguides on an epitaxial barium titanate (BaTiO3, BTO) thin film. The highly c-axis oriented BTO film was grown by the pulsed laser deposition (PLD) method and it exhibits a broad transparent window from λ = 2.5 μm up to 7 μm. The waveguide structure was fabricated by the complementary metal–oxide–semiconductor (CMOS) process and a sharp fundamental waveguide mode has been observed. By scanning the spectrum within the characteristic absorption regime, our mid-IR waveguide successfully perform label-free monitoring of various organic solvents. The real-time heptane detection is accomplished by measuring the intensity attenuation at λ = 3.0–3.2 μm, which is associated with -CH absorption. While for methanol detection, we track the -OH absorption at λ = 2.8–2.9 μm. Our monolithic Si-on-BTO waveguides establish a new sensor platform that enables integrated photonic device for label-free chemical detection.
Highlights
To overcome these challenges, we propose a monolithic amorphous silicon (a-Si) on BTO platform for mid-IR integrated photonic devices and sensing applications because of following advantages. i
Unlike other ferroelectrics, such as LiNbO3, BTO has the potential to be integrated on a Si wafer through various thin film deposition technologies, such as pulsed laser deposition (PLD), molecular beam epitaxy (MBE), and chemical vapor deposition (CVD), which enables the integration between functional oxides and Si photonics29–33. iv
The waveguide structure and the compositions of the Si-BTO-LAO multilayer were characterized by a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX)
Summary
The fabricated mid-IR sensing device was inspected by a SEM with EDX. Figure 1(a) and (b) are the top and the side SEM images of a 10 μm wide a-Si on BTO waveguide. Fundamental modes with similar ellipsoid intensity distribution are clearly resolved in the Si layer over λ = 2.5 μm to λ = 3.2 μm, while the evanescent fields in the air (z > −0.5 μm) and inside the BTO layer (z < −1.5 μm) increase as the mid-IR shifts to longer wavelengths. The evanescent fields increase as the wavelength increases from λ = 2.5 μm to 3.2 μm These results indicate our waveguide sensor will exhibit a higher sensitivity when it operates with TM polarization light as well as at a longer wavelength. Methanol and heptane were selected as the analytes to evaluate the performance of our waveguide sensors due to their strong characterstic absorptions existing in the mid-IR regime.
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