1. Introduction
Surface plasmon resonance (SPR) phenomenon has been widely applied in biochemical sensing in recent decades, and this technology possesses a profound developing potential due to their unique advantages for real-time in-situ and label-free sensing [
1,
2]. The SPR-based sensors have exhibited high performance, especially the very small detection limit of physical and biochemical parameters, such as the refractive index of analyte solution, DNA, protein, and bacteria [
3,
4]. Lab-on-chip integration is the trend of SPR-based sensor for biochemical applications in the last decade [
4], coming with many new principles, excitations, configurations and materials related to SPR [
5].
The most common SPR sensors are based on Kretschmann prism configuration, which is bulky and not compatible with the current MEMS technology, and therefore not good for realizing miniaturized integrated sensing system [
6]. As another popular configuration, the fiber optic SPR biosensor require complex micromachining process and some high-performance equipment, it is difficult to achieve low cost and small size of the overall sensing system [
7,
8]. The silicon-on-insulator (SOI) rib waveguide with large cross-section, by contrast, can achieve low-loss propagation and high integration with the optical fiber communication systems and the (opto-) electronic systems, so it is able to provide a competitive development platform for the SPR sensing applications. However, the requirement for the evanescent field of waveguide or fiber to excite the SPR phenomenon is that the propagation constant of the guiding mode must match the wave vector of SPR [
9]. Obviously, for a SOI rib waveguide with large cross-section, the mode effective index is much larger than the refractive index of the general analyte solution, so this traditional SPR excitation is not feasible. In this paper, a new SPR excitation is proposed and analyzed.
In general, there are several metals can be used to excite the SPR, like gold (Au), aluminum (Al) or silver (Ag). As the most common SPR active metal, gold (Au) possesses better chemical stability and exhibits a larger shift in resonance wavelength than other metals, which leads to a very high sensitivity of SPR sensor [
10]. However, due to its high absorption of light, the resonance curve of Au SPR sensor is broader and bad for the performance of the sensor [
11]. By comparison, Al and Ag are known for their narrower spectral width of SPR curves, but they are easily oxidized when used in liquid or gaseous environments because of their poor chemical stability [
12,
13]. Moreover, Al is more economical than Au or Ag, and it can be deposited directly over the silicon layer so that other transitional material layers are not necessary. As compared to a single metallic layer, the bimetallic configuration with a few nanometers Au over Al make the SPR sensor exhibits better performance and effectively protects Al from oxidation [
11,
14]. In this article, the sensor performance with respect to Al and Au will be specifically analyzed and the bimetallic configuration will be employed.
Usually, angular and wavelength interrogation are the most prevalent detection approaches for the biochemical sensors based on SPR, so the high performance of sensors is guaranteed by the high resolution tunable laser sources and optical spectrometers or high-precision angle sweeping equipment. These external attachments are expensive and cumbersome; the whole measurement systems are not low cost and difficult to integrate, so an overwhelming majority of SPR sensors are confined in laboratories. With the advantages of the silicon microfabrication technology, a SOI rib waveguide array can be realized easily. Using an array of light sources with a sequence of wavelengths for the SPR sensor array, wavelength interrogation can be implemented and the resonance wavelength of SPR can be determined from the response of the photodetector array rather than the scanning and measurement of wavelength, therefore no spectrometer is needed. Similarly, angular interrogation can also be implemented by a suite of array elements with different angles. Making full use of sensor array is not only conducive to integration and package of the whole sensing system but also reduces the cost of production.
As the requirements of the biochemical
in-situ real-time sensing increase, the research of miniaturized and integrated high-performance SPR biochemical sensors has become necessary. This article proposes a micro integrated surface plasmon resonance (SPR) biochemical sensor platform based on silicon-on-insulator (SOI) rib waveguide with large cross-section. Following the introduction to the background of the research, the principles of operation are presented in
Section 2, including the structure configuration of SPR sensor, calculation model and three quantitative indicators of sensor performance. In
Section 3, the results of the investigation are analyzed and discussed. The conclusions of the research will be given in the last section.
3. Results and Discussion
3.2. Bimetallic SPR
Al is easily oxidized because its poor chemical stability and its oxidation will influence the performance of Al SPR sensors [
12,
13]. Making use of the bimetallic configuration (coating an ultra-thin layer of Au over Al) is a good solution to avoid this problem [
11,
12,
14]. Supposing 3 nm Au deposited over 37 nm Al, the bimetallic SPR curve is shown in
Figure 8. In this case, the sensitivity and
FWHM of bimetallic SPR sensor is larger slightly than that of single metallic SPR sensor with the same thickness of metal layer, but the
Contrast is almost unchanged. It is concluded that the bimetallic SPR sensor with 3 nm Au over 37 nm Al inherits the advantages of the single metallic SPR sensor with 40 nm Al, and the several nanometers Au can protect Al from oxidization.
Figure 8.
The comparison diagram of wavelength interrogation curve for single metallic SPR with 40 nm Al and bimetallic SPR curve with 3 nm Au over 37 nm Al.
Figure 8.
The comparison diagram of wavelength interrogation curve for single metallic SPR with 40 nm Al and bimetallic SPR curve with 3 nm Au over 37 nm Al.
For the bimetallic configuration with a few nanometers Au layer deposited over Al, the thicknesses of each metal layer have influence on the performance of the SPR sensor. It can be seen from
Figure 9 that the bimetallic SPR sensor exhibits highest sensitivity
Sn = 4.458 × 10
4 nm/RIU with 20 nm Al and 5 nm Au, but smallest
DA = 5.95 μm
−1. In addition, the
Contrast achieves a maximum in thinner Al layer (about 22 nm) and has an overall downtrend. The thinner Al layer leads to higher sensitivity and
Contrast but lower
DA of SPR sensor, so there is a tradeoff between the sensitivity,
Contrast and
DA. On the other hand, with the thickness of Au increasing, the sensitivity of SPR sensor increases linearly, but the
DA and
Contrast become worse, as shown in
Figure 10. It is worth noting that the influence of Au layer thickness is smaller than that of Al, therefore, a few nanometer Au layer retains the sensor’s high performance along with protects Al from oxidation.
Figure 9.
The influence of the thickness of Al layer in the bimetallic SPR sensor with 3 nm or 5 nm Au.
Figure 9.
The influence of the thickness of Al layer in the bimetallic SPR sensor with 3 nm or 5 nm Au.
After comprehensive consideration, there is a trade-off decision that the limited thickness ranges of metal layer are from 25 nm to 35 nm for Al and from 2 nm to 5 nm for Au. As a typical example, the bimetallic SPR sensor with 3 nm Au over 32 nm Al possess a high sensitivity of 3.968 × 10
4 nm/RIU, a detection-accuracy of 14.7 μm
−1 and a
Contrast of 0.82. Supposing the resolution of the spectrometer connected to the output waveguide is 0.02 nm, the bimetallic SPR based on SOI rib waveguide can achieve a refractive index detection limit of 5.04 × 10
−7 RIU. The achieved detection limit is lower than that obtained by the sensor based on Young’s interference (9 × 10
−9 RIU) [
28], but is in the same order as the counterparts (10
−5–10
−7 RIU) that reviewed in reference [
3,
4].
Figure 10.
The influence of the thickness of Au layer in the bimetallic SPR sensor with 27 nm or 32 nm Al.
Figure 10.
The influence of the thickness of Au layer in the bimetallic SPR sensor with 27 nm or 32 nm Al.
3.3. Transmission Simulation of SPR Sensor
According to the structure configuration shown in
Figure 1, and employing the resonance parameters from above analytical method, the bimetallic SPR sensor with 3 nm Au over 32 nm Al is simulated by using the 2D-FDTD with PMLs, and its optical transmission distribution is shown in
Figure 11. Here, the TE-polarized fundamental mode of the SOI rib waveguide acted as the source incident into the input waveguide, and the half-angle of the V-shaped bend waveguide θ = 22.65
°, which was calculated by the above analysis method with the effective model at resonance wavelength of 1550 nm.
Figure 11.
The electric intensity map of the bimetallic SPR sensor with 3 nm Au over 32 nm Al in the 2D-FDTD simulation at an operating wavelength of 1550 nm. The mesh grid in the regions of trench-based waveguide bends is set to 5 nm, and the mesh grid of metal layer is set to 1 nm.
Figure 11.
The electric intensity map of the bimetallic SPR sensor with 3 nm Au over 32 nm Al in the 2D-FDTD simulation at an operating wavelength of 1550 nm. The mesh grid in the regions of trench-based waveguide bends is set to 5 nm, and the mesh grid of metal layer is set to 1 nm.
It can be seen obviously that the SPR phenomenon was excited by the guiding mode at the metal-analyte interface. Unfortunately, only about 25 percent of the light energy was absorbed by SPR, which is not consistent with the Contrast = 0.82 that calculated from above analytical method. There are several aspects that lead to this situation. Firstly, because of the simplified effective model, the analytical method cannot take the spatial angular distribution of the mode source into account, which is the main reason for the large difference of Contrast. Secondly, the FDTD method itself is a discrete numerical solver for Maxwell equations, and the EIM is employed to realize 2D-FDTD to simulate propagation of the waveguide mode and excite SPR phenomenon, which undoubtedly will bring some errors. Thirdly, the dispersion model has some errors inevitably, especially the effective refractive index of the guiding mode. In addition, these above reasons lead to a certain errors of SPR resonance position, which will also cause the deviation of the signal contrast.
Even so, we can draw a conclusion exactly that the SPR phenomenon can be excited at the waveguide bends by the guiding mode, and the resonance parameters from the N-layer transfer matrix method are consistent with the 2D-FDTD simulation methods within a reasonable range of error. Therefore, the foregoing analysis using the analytical method is reliable.
3.4. SPR Sensor Arrays
Based on silicon microfabrication technology, an array can be used to implement the wavelength interrogation, as shown in
Figure 12. Light with different wavelength from each element of the source array is coupled into the input waveguide of each element of the SPR sensor array through a standard single-mode fiber. A photodetector array is integrated at the end of the output waveguides and detects the output signal. For a set of operating wavelengths, the wavelength response of the SPR sensors can be obtained from the photodetector array, and the resonance wavelength of SPR can be determined, so the parameters of the analyte solution can be estimated.
Figure 12.
Schematic of SPR sensor array for wavelength interrogation.
Figure 12.
Schematic of SPR sensor array for wavelength interrogation.
As the output signal is produced by the photodetector array and the operating wavelength is discrete, the refractive index based detection limit of this SPR sensor array is determined by the wavelength resolution (δλ) of the adjacent sensor element, which is closely related to the relative power measurement accuracy (
IR) of the photodetector [
29]. The wavelength resolution (δλ) of adjacent sensor element can be obtain from the SPR resonance curve of almost continuous wavelength interrogations, using the N-layer transfer matrix method with a very small calculating increment of wavelength, as shown in
Figure 13. Generally, the relative power measurement accuracy can reach a valve of 0.01 dB, the wavelength resolution of adjacent sensor element of SPR sensor array with 3 nm Au over 32 nm Al can be found, δλ = 0.44 nm. With the high sensitivity of 3.968 × 10
4 nm/RIU, the refractive index variation of 1.14 × 10
−5 RIU can be detected.
Figure 13.
The wavelength interrogation curve for the bimetallic SPR curve with 3 nm Au over 32 nm Al.
Figure 13.
The wavelength interrogation curve for the bimetallic SPR curve with 3 nm Au over 32 nm Al.
Similarly, the angular interrogation can also be implemented by a suite of array elements with different angles. Using the same calculating method with wavelength interrogation, the sensitivity of angular interrogation for a uniparted bimetallic SPR sensor with 3 nm Au over 32 nm Al is 19 degree/RIU. Considering the difficulty of processing, the minimum angle difference between the adjacent SPR sensor elements is assumed to be 0.001°, thus the refractive index detection limit of the SPR sensor angular array is 5.3 × 10−5 RIU. Although this detection limit value is much lower than that obtainable by the single SPR sensor (5.04 × 10−7 RIU), the optical detection unit at the output port is the low-cost and integrated photodetector array instead of the expensive optical spectrum analyzer. In addition, some numerical fitting algorithms can be used to determine the resonant wavelength or the resonant angle accurately, such as the polynomial fitting, so the accuracy of the sensor can be more accuracy.
3.5. Discussion
The proposed SPR sensor is based on the SOI rib waveguide with large cross-section, and the SPR phenomenon is excited at the waveguide bend by the guiding mode. According to the above analyses and simulations, it is found that the proposed SPR sensor have the ability to perform bulk sensing with high sensitivity, which can be used to detect the concentration changes of the analyte solution, acting as a chemical sensor. If some suitable receptor molecules are immobilized at the surface of metallic layer, this SPR sensor can also be used to detect biological reactions with surface sensing.
Compared with conventional SPR sensors, the biggest difference is that the configuration structure of this proposed structure SPR sensor is very unique, so the processing quality may affect the actual performance of the sensor. At first, the angular V-shaped structure leads to the waveguide is slant and the guiding mode of SOI rib waveguide will be reflected three times, so there is a high demand for the processing of waveguide and the trenches. Secondly, because the SPR interface locates on a deeply etched facet at the end wall of the V-shaped waveguide bend, it is difficult to homogeneously deposit the metallic layer. Moreover, as the metallic layer structure is very thin, especially the several nanometers gold film, the surface roughness may not meet the requirements of the effective excitation of the SPR. In practice, we can etch the rib waveguides at SOI wafer with ordinary lithography processes, then cleave the sensor units to make the deeply etched facet of the V-shape waveguide bending to be able to deposit the metal layers by the vacuum evaporate plating technology. At present, these processes are being implemented by our research group.
For the arrays of SPR sensors, the consistency and the angular precision of each element also affect the performance of SPR sensors. Therefore, a higher precision processing quality is needed to get a higher sensor performance, and the processing costs will be more. In addition, the package of the overall sensing system will introduce some errors, such as the coupling loss of the standard single-mode optical fibers with the input waveguides, output waveguides, laser light sources and spectrometers. Similarly, the integrated effects of the photodetector array with the output waveguide of the array element cannot be ignored.
The proposed SPR sensor not only exhibits high sensitivity and detection-accuracy, but also benefits to the micro-integration, which provides a new possibility for the SPR sensor to commercial applications. Due to high coupling efficiency of the large cross-section SOI rib waveguide with standard single-mode glass optical fiber, the proposed SPR sensor can be conveniently integrated with optical fiber communication systems and (opto-) electronic systems. For a uniparted SPR sensor, the input and output waveguide can be remotely connected with a high-resolution tunable laser source and an infrared spectrometer through communication optical fibers, therefore the proposed SPR sensor has the potential to realize remote sensing, in-situ real-time detecting and possible application in internet of things. In addition, making use of the advantage of the silicon microfabrication technology, the SPR sensor array with discrete operating wavelengths is used to implement wavelength interrogation without the expensive and bulky spectrometer, and it can also be realized to detect some biochemical reactions. In a word, this proposed SPR sensor is theoretically analyzed and simulated in this paper, but the further experimental research is urgently needed, and this is the next task of our research team.