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Abstract
We report a tunable terahertz (THz) metamaterial absorber (MA) actuated by thermomechanical bimaterial microcantilevers. The THz MA, which is suspended on a silicon substrate by the bimaterial microcantilevers, is a sandwich structure with a bottom Al ground plane, middle air and SiNx dielectric layers, and a top Al rotationally symmetric open split ring resonator. Upon application of a current, a Ti heating resistor integrated on the SiNx dielectric layer induces the bimaterial microcantilevers to bend, causing the air layer thickness to change, modulating the absorption of the THz MA. The tunable THz MA exhibited a relative modulation depth of absorption of 28.1% at 0.69 THz and a thermomechanical sensitivity of 0.12°/K. This tunable THz MA has potential applications in filtering, modulation, control, and THz imaging.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz (THz) radiation is electromagnetic radiation with frequency between the microwave and infrared regions. Currently, the limited intrinsic response of natural materials limits the development and application of THz radiation. The emergence of metamaterials has paved the way for the advancement of various types of THz devices and systems.
Metamaterials are artificially constructed electromagnetic materials [1], which have interesting electromagnetic responses not achieved by natural materials inducing many interesting properties, such as negative refractive index, electromagnetic wave cloaking, inverse Doppler effect, and artificial magnetism. Recent THz metamaterial research is greatly advancing towards realizing tunable THz metamaterial absorbers (MAs) that enable real-time control over the structural and optical properties of metamaterials. The absorption can be modulated by changing the electromagnetic parameters of materials and adjusting the size of single resonators by thermal [2,3], electric [4–6], magnetic [7,8], and optical [9,10] signals. Among the numerous solutions for tunable THz MAs, the hybridization of microelectromechanical systems (MEMS) and THz MAs has attracted considerable attention because such hybrid systems can be controlled electrically and have small size and enhanced electro-optic performance [11–18]. Moreover, hybrid MEMS and THz MAs exhibit markedly reduce thermal noise and energy consumption, which is crucial for sensing and imaging weak THz signals. Fangrong Hu et al. demonstrated a dynamically tunable THz MA based on an electrostatic MEMS actuator and an electrical dipole resonator array with a modulation of the central frequency and amplitude to approximately 10% and 20%, respectively [11]. Mingkai Liu et al. demonstrated an ultrathin tunable THz MA based on a MEMS-driven metamaterial by combining meta-atoms that support strongly localized modes with flat, suspended silicon nitride membranes that can be driven electrostatically [14]. These tunable THz MAs based on electrostatically driven MEMS have lower power consumption required for switching the state as well as in retention of the switched states, but usually have limited range of motion, which limits the operation of tunable THz MAs. However, the thermomechanically driven THz MAs [15,16] can realize a large range of motion with a low operating voltage or current.
In this paper, we propose a tunable THz MA actuated by thermomechanical bimaterial microcantilevers. By changing the thickness of the air layer of the sandwiched absorber by bending the bimaterial microcantilevers, a relative modulation depth of absorption of 28.1% at 0.69 THz was achieved. Our study provides an approach that can be further extended to benefit applications in THz spatial light modulation.
2. Design and simulation
The THz MA is a sandwich structure with an Al ground plane at the bottom, air and SiNx dielectric layers in the middle, and a rotationally symmetric Al open split ring resonator (SRR) at the top. The THz MA is suspended on a silicon substrate by two symmetric bimaterial microcantilevers with the same width, one acting as a deformation microcantilever and the other acting as a thermal isolation microcantilever. Figure 1 shows the schematics of the top and cross-section views of the single tunable THz MA. The deformation microcantilever consists of the Al layer and the SiNx dielectric layer, which will deform upon temperature changes owing to the very different thermal expansion coefficients of the two materials. The thermal isolation microcantilever is merely a SiNx layer with a low thermal conductivity, which can minimize the heat dissipation from the absorber to the substrate and reduce the total thermal conductance. The microcantilevers are anchored to a silicon substrate separately through two anchors and Al ground plane on the silicon substrate serves as the electrical line. The release holes are opened in the absorber to facilitate the removal of the polyimide (PI) sacrificial layer under the structure during the releasing step.
Fig. 1. Top view (a) and cross-section view (b) of the single tunable THz MA. The dimensions of the THz MA are as follows: L = 80 µm; L1 = 105 µm; Ld = 63 µm; Li = 32 µm; Wr = 4 µm; Ws = 10 µm; P = 120 µm.
For changing the temperature of the bimaterial microcantilevers, a heating resistor is integrated on the SiNx dielectric layer. Ti has a thermal conductivity of 21.9 W/(m·K), which is an order of magnitude lower than that of Au and Al, and electrical conductivity of 2.31 × 106 S/m, which also meets the requirements of the electrical interconnection. Hence, Ti is used as the material of the heating resistor and the interconnection line. The Ti heating resistor is designed with a fold line structure to increase the heating power with a line width of 5 µm. It is connected to the Al electrical line on the Si substrate by the Al layer of the deformation microcantilever and the Ti electrical interconnection on top of the thermal isolation microcantilever. When a current is applied to the Ti heating resistor, the increase in temperature of the THz MA causes the bimaterial microcantilevers to bend. This results in a change in the thickness of the air dielectric layer, allowing the modulation of the absorption of the THz MA.
Individual 36 × 36 MAs are arranged into an array for subsequent tests, as shown in Fig. 2(a), where adjacent MAs in each column share an anchor and the MAs in every row have the same electrical line in common. The electrical lines are connected so that the entire array is actuated simultaneously through two pads. The equivalent circuit of the entire array is shown in Fig. 2(b), where each MA is equivalent to a resistor and all MAs are connected in parallel. The overall dimension of the MA array is 4.32 mm × 4.32 mm with a fill factor of 64.9%.
To evaluate the performance of the tunable THz MA, we determined the thermomechanical sensitivity and modulation depth of absorption. The thermomechanical sensitivity STθ is defined as the bending angle of the microcantilever Δθ as a result of a unit temperature change ΔTj: ${S_{T\theta }} = \frac{{\Delta \theta }}{{\Delta {T_j}}}$. Given the length of the deformation microcantilever Ld, the thermal expansion coefficients of the two materials α1 and α2, the thermomechanical sensitivity can be calculated by [19],
Another important parameter of the tunable THz MA is the modulation depth of absorption. The absorption spectra at different bending angles were simulated through a commercially available finite-difference time-domain software. In the simulation, both the absorber surface and microcantilever were included in a unit cell and the wave is normal incident into the absorber surface with the electric field perpendicular to the x-direction. And the dielectric layer is composed of the SiNx layer and the air layer because of the suspended structure. The conductivity of Al film and Ti film are set to 2.7 × 107 S/m and 2.31 × 106 S/m, respectively. The permittivity ɛ = 7.5 and loss tanδ = 0.025 are used to model the SiNx layer, and the dielectric constant of air is set to 1.
The simulated absorption spectra and peak absorption of the designed tunable THz MA at different bending angles are shown in Fig. 3(a) and 3(b), respectively. As shown in Fig. 3(b), when the bending angle decreases from 12° to 8° the peak absorption increases from 81.7% to 99.5%; when the bending angle further decreases to 4°, the absorption decreases to 68.5%. The relative modulation depth was calculated to be 31.2%. To understand the reason that the peak absorption is achieved at the bending angle of 8° for the THz MA, an effective impedance model was used [20]:
Fig. 3. Simulated (a) absorption spectra, (b) peak absorption and (c) the magnitude of the normalized effective impedance spectra at different bending angles. (d)The simulated absorption spectra at different angles of polarization under TE mode for bending angle of 8°.
We also investigated the polarization sensitivity of the THz MA by simulating the absorption spectra at different angles of polarization under TE mode for the bending angle of 8°. As denoted in Fig. 3(d), there is a significant drop in peak absorption at α = 60° compared to normal incidence, but it does not change much at α = 30°. It means that the response of the THz MA depends on the polarization of the incident wave.
For investigating the absorption mechanism of the designed tunable THz MA in more detail, the electric field distributions above the top surface at different bending angles were simulated. As shown in Fig. 4(a) and 4(b), the incident electric field couples with the open SRR mainly at the gap position, and the resonance is an LC mode resonance. The electric field strength at the gap at the bending angle of 8° is stronger than that at the bending angle of 4°, and their difference corresponds to the change in the absorption.
Fig. 4. The electric field distributions above the top surface at different bending angles of (a) 4° and (b) 8°.
3. Experimental results and discussion
3.1 Fabrication methods and results
The array of the tunable THz MA was fabricated by using a PI sacrificial layer technique, as illustrated in Fig. 5. A 400 µm thick double polished silicon wafer was used as the substrate. Firstly, a 200 nm thick SiO2 layer was deposited as the insulating layer on the substrate using low-pressure chemical vapor deposition. Then, a 500 nm thick Al layer was sputtered and etched photolithographically as the ground plane and the electrical line. Next, a 1.8 µm thick PI was spin-coated as the sacrificial layer, and a 100 nm thick Ti layer was sputtered as the hard mask, followed by the anisotropic dry etching of the PI layer in oxygen plasma to define the anchors. The bottom Al electrical line was leaked out, and the Ti hard mask was etched away. Next, a 600 nm thick SiNx layer was deposited using plasma-enhanced chemical vapor deposition as the structural layer, and the electrical interconnection through-hole was defined by etching the SiNx using reactive ion etching (RIE). A 300 nm thick Ti layer was then sputtered and wet etched to define the electrical interconnection and the heating resistor. Subsequently, a 400 nm thick Al layer was sputtered and wet etched to define the metal layer on the deformation microcantilever and the top SRR. Afterward, the overall structure was defined by etching the SiNx layer using RIE. Finally, the structure was completely released by the isotropic dry etching of the PI sacrificial layer in an oxygen plasma. Figure 6(a) and 6(b) shows scanning electron microscopy (SEM) images of the fabricated tunable THz MA.
3.2 Test experiment and results
The thermomechanical sensitivity of the device was obtained by measuring the end displacement ΔZ of the microcantilever at different temperatures. The bending angle θ can be calculated by:
Here, the initial height Z0 is 19 µm and L is the length of the microcantilever, which is 95 µm. Figure 7(a) shows the bending angles with the temperature increasing from 25 °C to 70 °C. After a linear fitting of the measured data, the thermomechanical sensitivity of the device was then calculated to be 0.12°/K. The relationship between the bending angles and the currents is shown in Fig. 7(b), and the current response (Δθ/ΔΙ) of the device was measured to be 0.009°/mA.Fig. 7. Test results of the tunable THz MA. (a) Bending angles as functions of temperatures. (b) Bending angles as functions of currents.
A reflection-mode THz time-domain spectroscopy system was used to quantify the absorption properties of the tunable THz MA. During the measurements, a mirror was used to measure the reference reflection spectrum Eref(ω). Then, the reflection spectra Eref(ω) of the tunable THz MA under different applied currents were measured. The reflection spectrum was normalized as ${R_\textrm{i}}(\omega ) = {{{E_i}} \mathord{\left/ {\vphantom {{{E_i}} {{E_{ref}}}}} \right.} {{E_{ref}}}}$. Considering that the transmittance of the tunable THz MA is nearly zero, the absorption spectrum can be written as ${A_i}(\omega ) = 1 - {R_i}(\omega )$. Figure 8(a) shows the absorption spectra of the tunable THz MA under different applied currents. In the figure, a distinct absorption peak can be observed at 0.69 THz, which is a little higher than the simulated results. When the applied current increases from 0 to 1200 mA, the peak absorption increases slightly and then decreases. As shown in Fig. 8(b), the peak absorption of 95.6% is reached at 200 mA, and the peak absorption of 68.7% is reached at 1200 mA. The relative modulation depth was calculated to be 28.1%.
4. Conclusions
We demonstrated a tunable THz MA actuated by thermomechanical bimaterial microcantilevers has been investigated. The THz MA, which is a sandwich structure with an Al ground plane at the bottom, air and SiNx dielectric layers in the middle, and an Al rotationally symmetric open SRR on top, is suspended on a silicon substrate by the bimaterial microcantilevers. When a current is applied to the Ti heating resistor integrated on the SiNx dielectric layer, the increase in temperature of the THz MA causes the bimaterial microcantilevers bending. This results in a change in the thickness of the air layer and therefore modulating the absorption of the THz MA. The fabricated tunable THz MA achieves a relative modulation depth of absorption to 28.1% at 0.69 THz and a thermomechanical sensitivity of 0.12°/K. The experimental results reproduce the simulated results with reasonable agreement. The tunable THz MA can be further integrated with addressable circuits to realize the control of the single pixel in the array, which has potential applications in THz spatial light modulation.
Funding
National Natural Science Foundation of China (61935001).
Disclosures
The authors declare no conflicts of interest.
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