Refractive index sensing is one of the representative techniques for biochemical sensing applications. By monitoring the intensity variation and the resonant wavelength shift of the refractive index sensing structure induced by the biochemical reactions, we are able to realize real-time detection.Recently, terahertz (THz) sensing technique has attracted extensive interests because the THz fingerprints of bio-molecules and chemical substances could greatly improve the sensitivity for detection and sensing.
So far, most THz sensors rely on electromagnetic resonant structure with strong field confinement such as metamaterials. However, the sensitivity is limited due to the long wavelength of THz wave and the evanescent wave sensing mechanism.Now, a novel refractive index sensor design based on the integration of microfluidic channel into metamaterial absorber is proposed by Professor CHEN Qin's research group at Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences. Benefit from its unique sensing mechanism based on resonant field rather than the usual evanescent wave, sensitivity as high as 3.5 THz/RIU is achieva ble in the frequency range of 4-9 THz. The related results were published on Laser & Photonics Reviews.
Fig. 1 The schematic of metamaterial absorber integrated microfluidic sensor.(Image by SINANO)
Metamaterial absorber, a metal microstructure/dielectric/metal film stacked structure, is artificial electromagnetic composite material that can trap the incident light. As shown a usual way to use metamaterial absorber for sensing in Figure 2b, microfluidic channels are placed on top of metamaterial absorber. Therefore, the spatial overlap between the localized field and the analyte in the channel is small as seen in Fig. 2b, limiting the sensitivity. Chen et al developed a special packaged sensor by integrating microfluidic channel into metamaterial absorber, as shown in Fig. 1 and Fig. 2d. The intermediate hollow layer of metamaterial absorber has dual functions, i.e. the dielectric layer of absorber and the microfluidic channel. Large spatial overlap between liquid analytes in the channel and highly confined local resonant modes is archieved (see Fig. 2e). Consequently, the sensitivity, defined as resonant peak shift with refractive index changes, is greatly improved, due to the enhancement of THz waves-matter interaction (compared to Fig. 2c and 2f).
Fig. 2 (a) and (d) are the cross-sections of two types of metamaterial integrated microfluidic sensor. (b) and (e) are resonant field profiles of the two sensors, white dashed squares indicate microfluidic channel. (c) and (f) are reflection spectra at different liquid refractive index.(Image by SINANO)
The experimental results agree well with the design. Compared to optical sensing techniques based on evanescent waves, the corresponding performance increases remarkably. The fabrication process of the proposed sensor is simple with standard low-cost fabrication techniques and therefore it is excellent for a wide range of applications. Moreover, this mechanism and the device design based on localized resonant mode are not limited to THz waves. It can also be extended to visible and near-infrared region, which boosts the optical refractive index sensing techniques into the applications of biochemical detection and material analysis.