A surface-plasmon sensor is a sensor that detects a medium in contact with a metal surface by using the surface-plasmon resonance generated at the interface between the metal and the medium to be measured. Such devices are frequently employed as biosensors and chemical sensors. There are several types of such sensors [1–3], including immunoassays, electrochemical sensors, optical sensors, and piezoelectric sensors. Immunological tests are laboratory-based, and they require concentration and purification prior to analysis, expensive equipment, and slow measurement times. Electrochemical sensors, optical sensors, and piezoelectric sensors can solve these problems. Electrochemical sensors can perform high-sensitivity and real-time measurements, but the electrochemical reactions may be affected by the time required for the measurement process—which may result in modifications of the electrode surface or the substance to be measured—so accurate sensing may not be possible. Piezoelectric sensors also are highly sensitive and capable of real-time measurements.
In contrast, optical sensors can perform non-destructive measurements and can detect multiple analytes, although some sensors have problems such as concentration-range limitations. Surface-plasmon sensors, however—which perform sensing based on the refractive index of the contacted medium—can easily measure the transmittance or reflectance peaks that are sensitive to the medium’s refractive index. They also are capable of detecting target analytes. Refractive-index sensors such as the minimum-deflection-angle method, the V-block method, and the Abbe refractometer are superior to other refractive-index sensors in terms of their high measurement accuracy, ease of use for measurements, wide measurement range, and ability to perform measurements with a small amount of sample.
Among the several types of surface-plasmon sensors, the most common include the Kretschmann configuration [4–15], the diffraction-grating method [16–27], localized surface-plasmon-resonance sensors that use metal nanoparticles [28, 29], and optical-fiber sensors. The various sensing methods include measurements in fluids using plasmonic waveguides [30–36] and nanostructured absorbers [37, 38]. In the present study, we focus on the diffraction-grating method. This method is superior to others in that surface plasmon resonances can occur under multiple conditions and that an appropriate surface-plasmon resonance angle can be obtained for different measurement system and measurement medium. In the late 1990s, Kano et al. [16] and Homola et al. [17] succeeded in using the diffraction-grating method to detect liquid media with the low refractive indices of 1.3 to 1.4.
On the contrary, media with refractive indices as high as 1.5 or higher may contain various harmful substances, and if a sensor can detect such media, it can be employed as a simple sensor for water pollution and soil contamination. In the Kretschmann arrangement, the angle of incidence is large, and measurement is difficult, by using a high-refractive-index medium such as GaP as the substrate, it is possible to detect measurement media with high refractive indices [13]. Using this method, it is also possible to perform sensing under conditions that facilitate measurements at low angles. We have recently succeeded in using a metal diffraction grating to fabricate a surface-plasmon sensor that can detect media with refractive indices up to 1.7, and we have used it successfully to perform measurements [20, 21]. However, this method requires an optical system that can control the angle of incidence precisely, which can be a problem. In the present study, we have therefore focused on normal incidence, where the wavenumber of the surface plasmon is zero, and the Kretschmann arrangement cannot be used for measurements at normal incidence. Localized propagating surface plasmon polaritons (hereafter referred to as non-propagating modes) are generated under the following conditions. It has been shown that anomalous transmission occurs when propagating surface plasmons occur in metal nanostructures [39, 40]. If we can obtain transmitted light that is sufficiently strong compared to that obtained using normal propagating surface plasmons, we can expect not only to improve sensor performance, but also to simplify measurements, due to the use of normal incidence. In addition, since a rotating mechanism is not used, the size of the device can be reduced.
Based on the background discussed above, our goal in this research was to create a surface-plasmon sensor using non-propagating modes. To achieve this goal, we first performed simulations using the rigorous coupled-wave analysis method (RCWA method). Guided by the results of these simulations, we designed such a sensor and fabricated a sample device using electron-beam lithography and lift-off. We subsequently evaluated the optical characteristics of the fabricated device. In addition, to clarify the behavior of the non-propagating modes, we used the RCWA method to obtain transmittance maps and the distributions of the electric-field vector.