We experimentally detect high-refractive-index media ( n > 1.5) using a surface plasmon resonance (SPR) sensor with a diffraction grating. While SPR sensors are generally based on the attenuated total reflection method using metal films, here, we focus on a method using a diffraction grating, which can detect relatively higher refractive-index media and is suitable for device miniaturization. In this study, we used the rigorous coupled-wave analysis method to simulate the dependence of the reflectance on an incident angle for media with refractive index values up to 1.700. In the experiment, a medium ( n = 1.660 - 1.700) was successfully detected using this grating. Under the conditions of the grating (period: 600 nm, Au thickness: 40 nm) using a red laser ( λ: 635 nm), a sharp decline in the reflectance and a rise in the transmittance at certain angles were confirmed, demonstrating the extraordinary transmission enabled by SPR. Because excitation angles changed with changes in the refractive index, we concluded that this method can be applied to sensors that detect high-refractive-index media.
Surface plasmon resonance (SPR) is widely used in various applications such as in the detection of infinitesimally sized materials and high-efficiency photoelectric conversions [
In this study, we focus on the diffraction grating method using the diffraction grating as the structure of the sensor. In the diffraction grating method, a metal diffraction grating is used instead of a thin metal film and the resonance is caused by the diffraction of light [
However, previous studies have focused primarily on materials with refractive indices less than 1.5. Many harmful media, such as halogen compounds (e.g., Br2: n = 1.640) and heavy metal compounds (e.g., C14H16Pb: n = 1.626), exhibit refractive indices greater than 1.5. The use of surface plasmon sensors for the safe detection of such media is considered advantageous because of their simple operation and ability to detect small amounts of a target. In this study, for the first time, we experimentally demonstrate the detection of a high-refractive-index medium (n = 1.660 - 1.700) using surface plasmon sensors. To this end, we fabricated a surface plasmon sensor with a metal diffraction grating and evaluated the optical characterization for various materials with refractive indices greater than 1.5.
We calculated the dependence of the reflectance on the incident angle using the rigorous coupled-wave analysis (RCWA) method for the case of a metal diffraction grating. The simulation model is illustrated in
The metal diffraction grating used in this study was fabricated using electron beam lithography and sputtering on a glass substrate. The resist was applied on a glass substrate and the pattern was formed via irradiation with an electron beam. Then, metal (Au) was sputtered and a one-layer metal diffraction grating was formed via removing the resist with a lift-off process. The metal thickness, period, and duty ratio of the diffraction grating fabricated in this study were 40 nm, 600 nm, and 0.5, respectively. Prior to electron beam lithography, the Au thickness is measured using spectroscopic ellipsometry (UVISEL, HORIBA) and a non-contact film thickness meter (FR-pOrtable, Theta Metrisis). By using various thickness samples, the thickness of Au is determined to be 40 nm. The size of the sample was 1.5 × 1.5 mm2. The scanning electron microscopy (SEM) image of the surface of the structure in
The method to measure the reflectance is illustrated in
angle in the case of the refractive index n = 1.700. Due to the limitation of the measurement system, the experiment is conducted between 15˚ and 30˚. The simulation results are shown by the black curve and the experimental measurement results are shown by the red curve. Experimentally, a sharp decline in the reflectance was observed at an angle of 25.6˚. This value is essentially equal to the excitation angle (26.2˚) expected from the simulation. It is speculated that the mismatch between experimental and numerical data is caused by the subtle difference between the simulation model and the actual structure. Moreover, further measurements using three other media showed declines in the reflectance at nearly the same angles as those predicted by the simulations. The four experimentally discovered excitation angles are shown in
The magnetic field distribution at the SPP excitation angle for a medium with n = 1.700 is shown in
We investigated SPR using a metal diffraction grating as a sensor to detect a high-refractive-index medium using four different materials with refractive indices varying from 1.660 to 1.700 to test our approach. For every medium, it was found that the experimentally obtained excitation angles were very similar to the SPP excitation
angles predicted by the numerical simulations. The angular transmittance and magnetic field distribution show that an extraordinary light transmission phenomenon is likely enabled by SPR. Furthermore, it was found that SPP excitation angles shifted to the higher angle side with slight increases in the refractive index, clearly resolving the change in the SPP excitation angle for different materials. Therefore, we conclude that it is possible to detect a medium with a refractive index as high as 1.700 using a SPR sensor with an Au diffraction grating.
This study was supported in part by the Murata Science Foundation and JSPS KAKENHI Grant Numbers 25600090, 26390082, and 15H03556. The authors would like to thank Enago (www.enago.jp) for the English language review.
Atsushi Motogaito,Shinya Mito,Hideto Miyake,Kazumasa Hiramatsu, (2016) Detecting High-Refractive-Index Media Using Surface Plasmon Sensor with One-Dimensional Metal Diffraction Grating. Optics and Photonics Journal,06,164-170. doi: 10.4236/opj.2016.67018