Metamaterials, a recently developed category of engineered structural materials with pre-designed electromagnetic response units, exhibit properties such as negative refraction index, negative permittivity, negative permeability, negative dielectric constant, and reverse Doppler effect, setting them apart from conventional natural materials [[1], [2], [3], [4]]. As an artificial material, the essence of metamaterials lies in exploiting the coupling effects between subwavelength structural units and incident electromagnetic waves, thereby enabling the control of electromagnetic waves [[5], [6], [7], [8]]. As a result, metamaterials undergo extensive research and applications across various fields, including imaging [9], phase shifters [10], sensors [11], polarization converters [12], and orbital angular momentum control [13]. They have expanded the scope of research and application in absorbers.

However, similar to most metamaterial-based devices, the resonant frequency of the metamaterial absorber remains fixed once manufactured. Materials designed for single-frequency absorption fall short of meeting the demands for dynamic control, thereby restricting the applicability of the resulting absorber. The incorporation of tunable materials, such as vanadium dioxide (VO2) [14], liquid crystals (LCs) [[15], [16], [17], [18]], graphene [19], and perovskite [20], unlocks diverse and convenient pathways for controlling metamaterial absorber behavior. The characteristics of these active materials can be modified through external control options, such as optical [21], electrical [22,23], magnetic [24], and temperature [25]. Zhang et al. proposed a dynamically switchable and tunable bifunctional terahertz (THz) metamaterial absorber based on VO2 and graphene [26]. Their designed metamaterial absorber can be transformed into a tunable multiband absorber when VO2 is converted to the insulated state. Chen et al. introduced a tunable graphene-based metamaterial absorber at a dual THz band to achieve perfect absorption with adjustable resonant frequencies by modifying the geometric parameters of the graphene [27]. Wang et al. presented a dual-band absorber reaching absorption rates of 99.60% at 2.67 THz and 99.79% at 4 THz and a tunable absorption rate under light conditions using photosensitive silicon [28].

LCs are widely used control materials known for their low cost, stable performance, low insertion loss, and broad response range spanning from millimeter-wave to THz frequencies [[29], [30], [31], [32]]. As an anisotropic material exhibiting birefringence in the THz range, LCs are considered one of the ideal materials for tunable THz devices. Liu et al. demonstrated a tunable dual-band perfect metamaterial absorber working in the infrared band. The dual-band resonance frequencies of their proposed absorber exhibit continuous tunability by adjusting the refractive index of the LC, which can be controlled by applying external voltage [33]. The approach of utilizing external control options for the continuous modulation of LCs has gained broad applications. However, there is still ample room for stability and improving switching speeds. To tackle these challenges, sophisticated control techniques, like encoding, can be implemented in metamaterial devices [34]. For instance, Zhang et al. proposed a phase shifter based on digital coding, allowing the device to attain four different phase states at 120 GHz [35].

This paper presents a metamaterial absorber utilizing coded digital gratings and LCs. In contrast to previously reported absorbers that modulate absorption frequencies through voltage amplitude, the proposed structure achieves dynamic control over multiple frequency points by applying a switchable encoding mode to the grating. Voltage can be applied between the grating and resonant structure to create a vertical electric field or between two gratings to form a horizontal electric field. Reflective characteristics of the designed absorber were simulated and analyzed. Additionally, the sample underwent fabrication and testing using photolithography. The test results indicate that encoding different grating combinations and applying voltages to cause deflection of LCs in different fixed regions can achieve dynamic control over five states in the range of 116.5 GHz–121.8 GHz. It presents a novel method for switching absorption frequency points.