Terahertz CMOS High Sensitivity Sensor based on Hybridized Spoof Surface Plasmons Resonator

1. Introduction

Millimeter-wave and terahertz (THz) sensing have attracted increasing interest[1,2,3,4]. The low photon energy in the THz frequency band allows for nondestructive detection, and can penetrate dielectric materials, making it suitable for high-sensitivity detection and sensing applications[2,3]. However, in the high-frequency range of millimeter-wave, it is difficult to ensure accuracy due to the high loss tangent value of traditional dielectric substrate. Consequently, great interests have been focused on CMOS technology during the past few years[1,5]. It can not only reduce the bulky size but also decrease the high loss[6].

Surface plasmons (SPs) have attracted much attention for their ability to confine the field in the metal-dielectric interface in the optical and far-infrared frequency band[7,8,9,10,11,12,13]. Research in this area has expanded from the optical regime to the microwave and terahertz regime driven by the pioneering works[14,15,16,17]. According to the propagation characteristics, surface plasmons can be divided into two types, surface plasmon polaritons (SPPs), and localized surface plasmons (LSPs) which are localized near the metal particles[18].

By introducing periodic structures such as periodic sub-wavelength corrugation metal strips, spoof surface plasmon polaritons (SPPs) and spoof localized surface plasmons (SLSPs) can be utilized to propagate signals or constrain electric fields. Based on these, a planar ring resonator in the terahertz frequency has been designed and simulated, which has been experimentally tested in the microwave frequency band and holds promising potential in integrated plasmonic circuits and systems[11].

The hybridization of plasmons brings a highly sensitive resonance characteristic to SLSPs, which makes it a good candidate for sensors and detectors[19,20,21,22,23,24,25,26]. A hybridized SLSPs resonator was implemented by a concentric structure[19]. The design shows advantages of compact size, high-quality factor(Q-factor), and high environmental sensitivity. As a result, a small change in the surrounding environment can affect the resonant modes[24].

In this work, we propose a novel hybridized square quadrilateral resonator structure based on SLSPs fabricated using CMOS technology. This hybridized resonator structure consists of two coupled SLSPs resonators, a core resonator and a cavity resonator. Gratings are incorporated in both the outer and inner LSPs resonators, which are tightly interconnected to reduce the electrical size , with a total area of 126*126 $\mu m^2$. Experimental measurements show a Q-factor of 56.7 at 194 GHz. In addition, we performed pharmaceutical testing experiments on three medicinal solutions: glucose, Chuanbei, and Sanchi . The results conclusively demonstrate that the hybridized SLSPs resonator offers exceptional sensitivity for medicinal detection in the terahertz frequency range, indicating a substantial potential as a chip-based test strip.

2. Materials and Methods

To investigate the characteristics of the hybridized SLSPs resonator, we first simulated the cavity SLSPs resonator for comparison. As illuminated in Figure 1, the cavity resonator has a regular quadrilateral structure, with metallic spokes arranged periodically around the resonator. The attached spokes are located inside the cavity and extend inward. On the one side of the cavity resonator, the length of the attached spokes decreases gradually from middle to both sides. And the resonator is fed by non-contact coupling excitation, which is composed of a microstrip line terminated by a stub. A ground-signal-ground (GSG) pad is positioned on the left of the excitation to connect external probe. By tuning the length of the terminal stub $L_1$ and the gap between the resonator and the stub, coupling efficient of the feeding could be adjusted effectively.

An electromagnetic full-wave simulation was performed as depicted in Figure 2. The cavity SLSPs resonator was simulated to obtain the reflection coefficient, as well as the $E_z$-field distributions on the xoy plane. In Figure 2(a), two obvious resonances labelled m1 and m2 are observed at 173.6GHz and 234GHz, corresponding to the $E_z$-field distributions on the xoy plane presented in Figure 2 (b) and (c). From the $E_z$-field distribution, mode m1 could be categorized as asymmetric mode, when mode m2 is a dipole mode, demonstrating that SLSPs resonant mode is successfully excited. Especially, the Q-factor of mode m2 can be calculated by the following equation:

$$\begin{array}{c}Q={\displaystyle \frac{f_0}{f_{3dB}}}\end{array}$$

(1)

where $f_0$ is the center frequency, and $f_{3dB}$ is 3dB bandwidth. The Q-factor of dipole mode is 53.

Then, a core SLSPs resonator was incorporated inside the cavity SLSPs resonator to form the hybridized SLSPs resonator, as shown in Figure 3. The shape of the core SLSPs resonator is also regular quadrilateral, the same as the cavity resonator. And spokes attached outside the metallic quadrilateral of core resonator interdigitate with the spokes on the cavity resonator. According to our previous work[19], strong coupling between interdigitated spokes will be generated. By altering the length of the spokes, the resonant characteristics will be changed accordingly. Figure 4(a) illustrates the S parameter of hybridized SLSPs resonator in comparison to the cavity resonator. Two resonant modes of hybridized resonator are observed and labelled M1 and M2, which are located at 168.8 GHz and 194.4 GHz, respectively. Comparing with the cavity resonator, both resonant modes of hybridized resonator exhibit red shift, indicating the ability reducing the electric size of the resonator. In Figure 4(b) and (c), electric distribution of modes M1 and M2 are presented, revealing that the cavity and core resonators resonate out-phase.

To study the resonance modes in detail, surface currents of mode m1, m2, M1, M2 are monitored and plotted in Figure 5. The surface current of the cavity SLSPs resonator are exhibited in Figure 5 (a) and (b). It can be observed that there are a pair of surface currents around the periphery of the cavity resonator, corresponding to the electric field oscillated out-phase of mode m1 and m2. In comparison, two pair of surface currents oscillated out-phase are also observed around the hybridized SLSPs structure. In Figure 5 (c) and (d), a pair of surface currents flow around the cavity resonator while another pair of currents flows around the core resonator. The overlapping of the currents increases the current path, which makes the red shift occur. Especially, the surface current intensity around the core resonator of mode M2 is stronger than that of mode M1, resulting a more significant red shift of mode M2.

3. Results

Complete S parameters from 170 GHz to 220 GHz were measured by the VectorStar broadband vector network analyzer ME7838G. The comparison between the simulation result and measured one is exhibited in Figure 6. The simulation and experimental results are located at 194.4 GHz and 194 GHz, respectively, with a quiet small frequency deviation. The measured Q-factor is 56.7, which is a fairly high level compared to the published resonator shown in Table 1.

To verify the highly sensitive biosensing properties of the SLSPs sensor, a series of experimental verifications were conducted for medicinal solution sensing. Three medicinal powders were made into solution for experiment: Sanchi, Chuanbei and glucose. Figure 7(a) presents the fabricated chip, where a solution was dripped on. When one solution measurement is completed, the chip will be cleaned by an ultrasonic cleaner. After the last solution is cleaned up, the chip will be dried thoroughly in air. Subsequently, next solution will be dripped on the chip, and the measurement will be conducted after the solution dried to a film. The process will be repeated until finishing the experiment.

Measured S parameter spectra are shown in Figure 7(b). It can be observed that compared to the S parameter in air, the spectra under tested samples exhibit a significant red shift, while maintaining a high Q-factor. Wherein, compared to air, the frequency shift for Chuanbei is 1.4GHz, and the one for Sanchi is 3.2GHz, when the one for glucose is 4GHz, demonstrating the potential of the resonator as a promising candidate for a variety of terahertz sensing applications.

Here, the results provide a qualitative analysis for biosensing. In the future, a quantitative measurement would be designed and implemented, allowing for the evaluation of sensing characteristics using figures of merit(FoM).

To highlight the superior features of the proposed SLSPs, comparison of resonators based on COMS technology are listed in Table 1. Comparing the reference 27 and 28 with the proposed SLSPs resonator, this work shows a quiet high Q-factor and maintains compact electrical size, showcasing a prominent performance in the terahertz band.

4. Conclusions

In this work, a novel approach is presented for achieving a high Q-factor compact resonator utilizing CMOS technology, based on the spoof localized surface plasmons. A quadrilateral hybridized SLSPs resonator is proposed and fabricated with a compacted electric size of 0.081λ×0.081λ. The measured Q-factor is as high as 56.7 in the THz frequency range. The pharmaceutical testing is also implemented on three medicine solutions: Sanchi, Chuanbei and glucose, and frequency shifts of 1.4GHz, 3.2GHz, 4GHz are observed. The hybridized SLSPs resonator provides a new solution for CMOS high sensitivity sensor in the terahertz band.