Wide range bilayer aluminum nanowire grating sensors with robust reflective peaks Download: 646次
Attributed to the unique advantages of high sensitivity, label-free, and real-time, surface plasmon resonance (SPR) is broadly utilized as a sensing technology[1,2] in biomedicine[35" target="_self" style="display: inline;">–
So far, in SPR sensors including both the dielectric prisms and unilayer gratings types, surface plasmons are excited on the metal/analyte interface away from the incident plane, which leads to narrow ranges of incident angles or ambient refractive index[12] for maintaining sharp reflective dips. As the incident angle, wavelength, or refractive index deviates from the design values[1315" target="_self" style="display: inline;">–
In this work, an SPR sensor based on a bilayer metallic grating was proposed, where plasmonic polaritons were excited on the interface between the bilayer metallic grating and the analyte in the incident plane, which resulted in sharp reflective peaks. Both the simulations and experiments prove that these SPR peaks remain sharp under a wide range of incident angle, metal thickness, and refractive index, which present high signal visibility. The metallic gratings were fabricated by laser interfere lithography and then coated with aluminum (Al), which is suitable for mass production. All of these high performance characteristics of the bilayer Al grating contribute to a robust sensor with large process tolerance and low fabrication cost.
The schematic of the bilayer Al nanowire grating is illustrated in Fig.
Fig. 1. (a) Schematic diagram of the bilayer metallic nanowire grating. (b) The theoretical SPR wavelength changing with ambient refractive index and incident angle . (c) AFM image of the fabricated grating structure.
The SPR excited on the interface between Al grating and analyte obeys the following equation[16,17]:
The wave number of SPR is
Thus, the sensor sensitivity, i.e., wavelength shift per refractive index unit (RIU), is
Fig. 2. Simulated reflective spectra of the grating in ambient conditions. (a1) Reflected spectra changing with incident angle . (a2) The magnetic field of the resonant peak for and . (b1), (b2) Simulated reflected spectra changing with duty ratio with for bilayer and unilayer Al gratings. (c1), (c2) Reflected spectra changing with PR thickness and Al thickness with incident angle for bilayer Al grating. Other parameters are the same as that in Fig. 1 .
The influences of the duty ratio
Figures
Fig. 3. Simulated reflective spectra under different ambient refractive indices . The dashed line is the theoretical changing with . Other structure parameters are the same as that in Fig. 1 .
The measurement setup is shown in Fig.
Fig. 4. (a) Snapshot of the measurement setup and the reflected photos with of white light in ambient air, deionized water, and 80% sucrose solution, respectively. (b1)–(b3) Measured reflective spectra with ambient air, deionized water, and 80% sucrose solution, respectively. The black dashed lines are the calculated by Eq. (4 ). (c) Refractive index calculated by Eq. (4 ). The horizontal lines are the average values.
The measured reflected spectra for air, deionized water, and sucrose solution are shown in Figs.
In conclusion, based on bilayer metallic nanowire gratings, metal thickness insensitive and wide working range grating-coupled SPR sensors were proposed and demonstrated, which produce sharp reflective peaks. The SPR peaks are red-shifted dramatically as the ambient refractive index increases; meanwhile, the profile of the peaks remains sharp within a wide range of refractive indices of analyte and incident angles. The easy fabrication and robust performance of the ultra-thin bilayer structure metallic nanowire sensors make the practical tests more convenient and lower cost.
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Zhicheng Ye, Jun Zheng. Wide range bilayer aluminum nanowire grating sensors with robust reflective peaks[J]. Chinese Optics Letters, 2020, 18(5): 052401.