Nanometer-Scale Heterogeneous Interfacial Sapphire Wafer Bonding for Enabling Plasmonic-Enhanced Nanofluidic Mid-Infrared Spectroscopy

Abstract

As one of the most effective surface-enhanced infrared absorption (SEIRA) techniques, metal–insulator–metal structured metamaterial perfect absorbers possess an ultrahigh sensitivity and selectivity in molecular infrared fingerprint detection. However, most of the localized electromagnetic fields (i.e., hotspots) are confined in the dielectric layer, hindering the interaction between analytes and hotspots. By replacing the dielectric layer with the nanofluidic channel, we develop a sapphire (Al2O3)-based mid-infrared (MIR) hybrid nanofluidic-SEIRA (HN-SEIRA) platform for liquid sensors with the aid of a low-temperature interfacial heterogeneous sapphire wafer direct bonding technique. The robust atomic bonding interface is confirmed by transmission electron microscope observation. We also establish a design methodology for the HN-SEIRA sensor using coupled-mode theory to carry out the loss engineering and experimentally validate its feasibility through the accurate nanogap control. Thanks to the capillary force, liquid analytes can be driven into sensing hotspots without external actuation systems. Besides, we demonstrate an in situ real-time dynamic monitoring process for the acetone molecular diffusion in deionized water. A small concentration change of 0.29% is distinguished and an ultrahigh sensitivity (0.8364 pmol–1 %) is achieved. With the aid of IR fingerprint absorption, our HN-SEIRA platform brings the selectivity of liquid molecules with similar refractive indexes. It also resolves water absorption issues in traditional IR liquid sensors thanks to the sub-nm long light path. Considering the wide transparency window of Al2O3 in MIR (up to 5.2 μm), the HN-SEIRA platform covers more IR absorption range for liquid sensing compared to fused glass commonly used in micro/nanofluidics. Leveraging the aforementioned advantages, our work provides insights into developing a MIR real-time liquid sensing platform with intrinsic IR fingerprint selectivity, label-free ultrahigh sensitivity, and ultralow analyte volume, demonstrating a way toward quantitative molecule identification and dynamic analysis for the chemical and biological reaction processes.

Progress in wafer bonding technology towards MEMS, high-power electronics, optoelectronics, and optofluidics

“…Jikai Xu et al from Harbin Institute of Technology and Zhihao Ren et al from the National University of Singapore jointly develop a sapphire direct bonding method using the mixed O 2 /NH 3 / H 2 O plasma treatment. [153,157] They show a dynamic monitoring platform actuated by the capillary force, as shown in Figure 7(j). Robust atomic bonding with a nanometer-scale interface is achieved.…”

“…Although the sapphire-based sensing platform has presented ultrahigh sensitivity in molecular detection, the transparency window is only up to 5.2 mm in MIR. [153,159] For the biochemical molecules whose fingerprints are in the range of 7-10 mm, sapphire substrates will have strong infrared absorption. Therefore, new bonding methods for LiF 2 , MgF 2 , CaF 2 , and BaF 2 substrates must be developed to promote further investigations of the plasmonic-enhanced nanofluidic MIR spectroscopy.…”

High-Contrast and Compact Integrated Wavelength Diplexer Based on Subwavelength Grating Anisotropic Metamaterial for 1550/2000 nm

“…In the past decades, silicon photonics of mid-infrared (MIR) (2-20 μm) has developed fast for its huge potential in biological, chemical, and industrial sensing and detection [12][13][14][15][16]. In particular, the wavelength of 2000 nm has great potential to be the new telecommunication window.…”

Mid-infrared Nanoantennas as Ultrasensitive Vibrational Probes Assisted by Machine Learning and Hyperspectral Imaging

“…[25][26][27] However, the narrow gap ruins the sensing performance by a decrement in the active area and increases fabrication cost. Therefore, other approaches like hybrid 2D materials [28][29][30] and homo/heterogeneous bonding 31,32 are developed to bypass the fabrication using lithography of desired nanogap and to achieve a large area electromagnetic hot spots for sensing. Additionally, the loss engineering method is proposed to tune the coupling condition between PNA and molecular vibration by adjusting the loss of antenna from specific structures to achieve optimized enhancement.…”

“…Additionally, the loss engineering method is proposed to tune the coupling condition between PNA and molecular vibration by adjusting the loss of antenna from specific structures to achieve optimized enhancement. 32,33 In addition to sensitivity, the detection range is another critical FOM of PNA sensors, reflecting the number of fingerprint absorption peaks that can be captured. Thanks to the sharp resonance peaks of PNA, the enhancement becomes the maximum only when molecule fingerprint absorption peaks match with the PNA resonance, which is a very narrow bandwidth.…”