Coupled Nanomechanical Graphene Resonators: A Promising Platform for Scalable NEMS Networks

Abstract

Arrays of coupled nanoelectromechanical resonators are a promising foundation for implementing large-scale network applications, such as mechanical-based information processing and computing, but their practical realization remains an outstanding challenge. In this work, we demonstrate a scalable platform of suspended graphene resonators, such that neighboring resonators are persistently coupled mechanically. We provide evidence of strong coupling between neighboring resonators using two different tuning methods. Additionally, we provide evidence of inter-resonator coupling of higher-order modes, demonstrating the rich dynamics that can be accessed with this platform. Our results establish this platform as a viable option for realizing large-scale programmable networks, enabling applications such as phononic circuits, tunable waveguides, and reconfigurable metamaterials.

1. Introduction

Networks of coupled NEMS resonators have attracted recent attention for the promise of mechanical computing [1,2,3] applications and for the study of fundamental physics, including metamaterial [4,5,6,7] and collective dynamics [8,9]. To continue to scale the size and prospects of coupled NEMS resonator networks, we need to develop robust platforms that host strong coupling and are scalable in size and tunability. Suspended graphene resonators offer many properties [10,11] that could be essential for achieving large 2D tunable arrays [12], such as intrinsic nonlinearities [13] that enable network dynamics [6,14,15,16] and multiple forms of energy transduction [17] for tuning options [18,19]. Persistent phototuning [19] has also recently been demonstrated, opening the possibility for scalable tuning of large-scale graphene networks.

Many coupling schemes have been established to host tunable strong coupling between resonators by means of parametric [20,21] and electromechanical [22,23] coupling. However, these methods are limited in scalability because they are not persistent and often require individually addressing each resonator, which limits the dimension of scalability in these platforms. One coupling means that is persistent and scalable in 2D is direct mechanical strain coupling though a shared clamping point [24,25,26], bridge [9], or substrate [22]. Mechanical strain coupling has been demonstrated between spatially distinct graphene nanoribbons [27,28] but has been limited to a 1D linear chain. However, direct strain coupling has been utilized to construct a 2D network of coupled pillars [29], hinting that strain coupling may enable other material platforms to scale in 2D as well.

In this work, we present a 2D platform [16] that hosts persistent strain coupling between suspended graphene resonators. We show evidence of strong coupling between two and three resonators of varying sizes and locations, establishing the viability of the platform for diverse array-based resonator applications. Additionally, this platform accommodates rich dynamics with inter-resonator coupling of higher-order modes, which we observed between the second-order mode of a driven resonator and the fundamental mode of its neighbor. With this platform, we establish a means in which the unique properties of 2D suspended graphene resonators can be accessed to enable a broad range of large-scale network applications.

2. Materials and Methods

To optimize the strain coupling between neighboring suspended graphene resonators, we designed a pillar array substrate [16,30,31,32] as the base structure for the network. The shared membrane between pillars would provide a mechanism for mechanical strain coupling between spatially distinct resonators. We selected pillar parameters that would lead to strong coupling by using finite element analysis (FEA) to determine the eigenfrequencies of the symmetric (Figure 1a) and antisymmetric (Figure 1b) modes of resonator pairs. We fabricated the platform by patterning Si/SiO₂ substrates with pillar arrays and using a wet transfer method to suspend the graphene. Throughout the arrays, we intermittently omitted specific pillars to create resonators of different sizes and resonator pairs; see Figure 1c. We were able to suspend the transferred graphene almost fully on the denser arrays, as in Figure 1c. However, the suspension yield varied across the sample (see Appendix A for additional FEA and fabrication details).

In this work, we used an optical drive/probe method to thermally drive and measure the out-of-plane motion of the resonators. We use two methods of tuning to demonstrate the versatility of this platform. Our first tuning method was to thermally tension the driven resonator only by applying a power offset to the modulated drive laser. Our second tuning method was to electrically tension all resonators on the sample by applying a bias between the graphene and the Si substrate [17]. Our optical apparatus also enabled scanning interference microscopy [33] (SIM) with fast scanning mirrors to raster the probe and collect spatial images of local areas of amplitude and phase (see Appendix A for additional details).

3. Results

We first employed our thermal-tensioning tuning method to modulate the coupling strength between two neighboring resonators. With the drive and probe lasers aligned over the region highlighted as R1 in Figure 2a, we probed for coupling by measuring a spectrograph of the amplitude, shown in Figure 2c. In the resulting spectrograph, we observed an avoided crossing of a lower frequency mode, ${\omega }_{-}$, and a higher frequency mode, ${\omega }_{+}$, at a power offset of $2.8$ V, implying the presence of coupling.

To confirm that the coupling was mechanical and to locate the coupled neighboring resonators, we took SIM images at both ${\omega }_{-}$ and ${\omega }_{+}$ with a drive power near the avoided crossing minimum to optimize each amplitude. For SIM measurements, we drove at a frequency slightly off resonance to avoid large heating fluctuations in amplitude and phase. The sample angle in the SIM images is depicted in Figure 2b. To compare the amplitude and phase of each active resonator, we analyzed a line cut of each SIM image through the center of each resonator. All reported uncertainty is calculated standard error from these linecuts. In the ${\omega }_{-}$ mode, we observed two distinct amplitude peaks, Figure 2e, that corresponded to the locations of R1 and R2, highlighted in Figure 2a. These two regions oscillated near in phase (R1 $=1.47\pm 0.03$ rad and R2 $=1.00\pm 0.04$ rad); Figure 2f, which is expected for the lower frequency mode of two coupled resonators, with slight differences likely due to heating fluctuations from scanning the probe laser. In the ${\omega }_{+}$ mode, we again observed two distinct amplitude peaks, Figure 2g, which corresponded to the same R1 and R2 locations. The two regions oscillated $~\pi$ out of phase (R1 $=2.47\pm 0.03$ rad and R2 $=-0.51\pm 0.03$ rad), Figure 2h, as expected for the higher frequency mode of two coupled resonators.

To verify the coupling between R1 and R2, we obtained a spectrograph of R2 by using the SIM image to reposition the probe laser over R2, while leaving the drive laser stationary to drive R1. In the resulting spectrograph, shown in Figure 2d, we again observed a lower and higher frequency mode that did not cross, with a minimum mode separation occurring at 2.9 V. This frequency was slightly higher than that for the R1 resonator, which may be due to heating and cooling effects associated with repositioning the probe laser. We also note that the avoided crossing curve shapes and amplitudes were less typical than observed in the R1 spectrograph, which may also be due to the probe repositioning. From the SIM spatial maps and the correlated avoided crossing curves, we conclude that the R1 and R2 resonators are coupled.

To determine the coupling strength, we calculated the minimum mode separation between ${\omega }_{-}$ and ${\omega }_{+}$ as

$$g={\omega }_{+}-{\omega }_{-}=\mathsf{\Delta }\omega$$

Based on the R1 avoided crossing spectrograph (Figure 2c), the coupling strength was $g/2\pi \approx 400$ kHz. Because this coupling strength exceeded the linewidths of the two modes ($~150$ kHz), this resonator pair was strongly coupled.

We then used our electrical tuning method to characterize the coupling between an additional set of two resonators. We first positioned the drive and probe laser over R1, highlighted in Figure 3a, and probed for coupling by measuring an amplitude spectrograph. In the resulting spectrograph, shown in Figure 3c, we observed an avoided crossing of two modes, ${\omega }_{-}$ and ${\omega }_{+}$, at 6.4 V, implying that R1 was strongly coupled to at least one neighboring resonator. In this avoided crossing, we also observed applied bias ranges in which each mode tuned very little. This behavior is likely evidence of phototuning [19], or a change in the charge neutrality point of the suspended graphene due to a redistribution of charge [34].

To map the configuration of resonators coupled to R1, we took SIM images at ${\omega }_{-}$ and ${\omega }_{+}$ with zero applied voltage and an orientation as depicted in Figure 3b. In the ${\omega }_{-}$ mode, we observed two distinct amplitude peaks, Figure 3e, that corresponded to the regions, highlighted as R1 and R2 in Figure 3a,b. These two regions differed significantly in size ($\mathrm{A}\mathrm{r}\mathrm{e}{\mathrm{a}}_{R1}\approx 4\times \mathrm{A}\mathrm{r}\mathrm{e}{\mathrm{a}}_{R2}$). Because the frequency of the fundamental mode is inversely proportional to the width of the square membrane, it is possible that additional tensioning from the drive laser on R1 aligned the two individual resonance frequencies, resulting in the coupling of R1 and R2. In the ${\omega }_{-}$ mode, R1 and R2 oscillated near in phase (R1 $=1.79\pm 0.07$ rad and R2 $=2.26\pm 0.02$ rad); Figure 3f. In the ${\omega }_{+}$ mode, we observed two distinct regions of amplitude, Figure 3g, that corresponded to the same R1 and R2 regions and oscillated $~\pi$ out of phase (R1 $=1.53\pm 0.05$ rad and R2 $=-1.82\pm 0.04$ rad); Figure 3h.

We confirmed the coupling between R1 and R2 by measuring a spectrograph of R2, Figure 3d, which revealed an avoided crossing between 6.4 V and 7.0 V. Based on the R1 avoided crossing curve, we calculated the coupling strength between R1 and R2 to be $g/2\pi \approx 200$ kHz. Considering estimated linewidths of $~120$ MHz at the avoided crossing minimum, we conclude this resonator pair was strongly coupled.

Our pillar platform offers a high potential for 2D scalability, which we observed in the coupling between three adjacent resonators. We measured this coupling by aligning the drive and probe lasers over R2, highlighted in Figure 4a, and sweeping the drive frequency to locate resonance. In the resulting amplitude curve, shown in Figure 4c (middle), we observed two closely spaced but distinct peaks.

To determine if these two peaks signified hybridized modes, we took SIM spatial images at ${\omega }_{-}$ and ${\omega }_{+}$ and with the orientation depicted in Figure 4b. In the ${\omega }_{-}$ mode, we observed three distinct amplitude peaks, Figure 4d, that all oscillated nearly in phase as seen in Figure 4e (R1 $=0.23\pm 0.13$ rad, R2 $=0.20\pm 0.03$ rad, and R3 $=0.41\pm 0.04$ rad), implying coupling between a total of three resonators. Again, the observed oscillating regions, highlighted in Figure 4a,b, differed in size with the largest resonator, R2, subject to additional tensioning from the applied drive laser. In the ${\omega }_{+}$ mode, we again observed three distinct amplitude peaks, Figure 4f, in the same R1, R2, and R3 regions as ${\omega }_{-}$. In this mode, the two neighboring resonators, R1 and R3, oscillated out of phase with the driven R2 resonator (R1 $=-0.23\pm 0.17$ rad, R2 $=2.08\pm 0.02$ rad, and R3 $=-0.62\pm 0.18$ rad), Figure 4g. Although we may expect that three strongly coupled resonators will exhibit three hybridized modes, when driving the middle resonator, it is possible that only two modes can be resolved (see Appendix B).

To confirm this coupling, we measured an amplitude spectrum of R1, shown in Figure 4c (upper), and R3, shown in Figure 4c (lower). In all three amplitude spectra, we observe two peaks at about the same ${\omega }_{-}$ and ${\omega }_{+}$ frequencies. We therefore conclude that this cluster of three resonators—R1, R2, and R3—are weakly to strongly coupled.

Our pillar platform offers unique 2D coupling dynamics between resonators including the inter-resonator coupling of higher-order modes. We detected higher-order mode coupling by measuring the amplitude spectrum of R1, highlighted in Figure 5a. In the spectrum, Figure 5c (upper), we measured a single peak ${\omega }_0$, close to 14 MHz, and two closely paced peaks, ${\omega }_{-}$ and ${\omega }_{+}$, near 17 MHz, implying that coupling may occur at a higher-order mode of R1.

To further investigate this coupling, we took SIM images at drive frequencies of ${\omega }_0$, ${\omega }_{-}$, and ${\omega }_{+}$. Figure 5b depicts the angle of SIM imaging, which we also overlay on the SIM amplitude images, Figure 5d,f,h. In the ${\omega }_0$ mode, Figure 5d,e, we observed a region of high amplitude ($~{10}^{-4}$V) with near constant phase, corresponding to the R1 region highlighted in Figure 5a,b. We interpret this mode to be the fundamental mode of R1. In the ${\omega }_{-}$ mode, we observed three distinct regions of amplitude, Figure 5f. Two of the peak amplitude regions occurred within R1, with mode boundary drawn as a solid line through the R1 resonator in Figure 5b. The phase between the two amplitude peaks within the R1 region, Figure 5g, differed by $~\pi$ (upper half $=0.30\pm 0.10$ rad and lower half $=-2.46\pm 0.07$ rad), as expected for the second-order mode of a 2D graphene drumhead resonator [33]. The third amplitude peak corresponded to the region highlighted as R2 in Figure 5a,b. R2 oscillated near in phase with the upper half of the R1 resonator (R2 $=0.45\pm 0.38$ rad), Figure 5g. This phase pattern creates the least amount of curvature in the membrane for the case of coupling a fundamental mode to a second-order mode and is therefore expected to correspond to the lower energy state.

In the ${\omega }_{+}$ mode, we again observed three distinct amplitude peaks: Figure 5h. In this mode, R2 oscillated out of phase with the upper half of the R1 resonator and near in phase with the lower half (R2 $=1.96\pm 0.37$ rad, upper half of R1 $=-0.93\pm 0.07$ rad, and lower half of R1 $=2.49\pm 0.18$ rad); Figure 5i. This phase pattern creates more curvature in the membrane and is therefore expected to result in a higher energy state. Higher-order mode coupling between these two resonators may be possible due to the differing sizes of R1 and R2.

To confirm this coupling, we measured an amplitude spectrum of R2, shown in Figure 5c (lower). In this spectrum, we observed a small peak ($~4\times {10}^{-5}$ V) near the fundamental mode of R1 and two much larger peaks ($~9\times {10}^{-4}$ V) around ${\omega }_{-}$ and ${\omega }_{+}$, with slight shifts due to heating. Due to the amplitude difference, and the homogeneous phase behavior of R2 observed in both the ${\omega }_{-}$ and ${\omega }_{+}$ modes, we expect that the small peak near the fundamental mode of R1 does not correspond to a resonance of R2. We therefore conclude that the mode splitting observed around 17 MHz is due to coupling between the second-order mode of the driven resonator R1 and the fundamental mode of R2.

4. Discussion

We have demonstrated the tunability of our resonator platform by tensioning the resonators both thermally and electrically to collect avoided crossing spectrograph. Thermally driving the resonators often led to coupling between resonators asymmetric in size, which may be advantageous for tuning neighboring resonators at different rates under a universally applied back gate. This platform also has the potential for scalable phototuning [19], which would enable persistent and individual tensioning of suspended graphene regions without the need for individual back gates. Additionally, if the shared membrane between neighboring resonators were to be tensioned with any of the discussed methods, it may be possible to tune coupling strength between resonators.

The additional evidence of inter-resonator, higher-order mode coupling highlights the possibility of rich dynamics that are an asset unique to this graphene platform. Coupling between higher-order modes of spatially separate resonators can be utilized as means of turning coupling on and off between different areas of the network or to achieve coupling between resonators of different sizes without additional thermal tensioning.

Although we focused our analysis on strong coupling, the SIM spatial maps exhibit oscillating regions corresponding to areas of weak coupling. Weak coupling is an important network parameter, as it is essential for realizing many oscillator-based phenomena [8]. Although weak coupling is often difficult to measure [35], our SIM spatial imaging technique illuminates weakly coupled regions, enabling a more accurate model of the network.

5. Conclusions

In conclusion, we have presented evidence of coupling between two resonators, three resonators, and resonators of with a higher-order mode in a pillar-based graphene NEMS network platform. This platform thus enables persistent 2D mechanical strain coupling, with the unique properties of graphene offering potential for scalable tunability. With this platform, we can achieve large-scale arrays with persistent coupling for applications such as computing schemes [1,2,3], experimentation of tunable metamaterial [6,7,36,37], and physical simulation of natural and artificial networks [38,39].