Metamaterials (MMs) [1,2] are periodic or aperiodic artificial materials composed of subwavelength metal/dielectric microstructures. Due to their unusual physical characteristics, many exciting phenomena have emerged, such as negative refractive index [3], polarization transformation [4,5], invisibility cloaks [6], holographic imaging [7], circular dichroism [8,9], and others. As a two-dimensional (2D) version of MMs, metasurfaces [10,11] have further significant advantages of low loss, lightweight, and high integration compared with bulky MMs.

In recent years, active metasurfaces have emerged as a primary developmental trend, expanding the dimensionality of multifunctionality. Reconfigurable intelligent metasurfaces (RIS), with deep learning generation networks [12,13] further possess the capability to flexibly adjust EM properties and achieve multifunctional control of EM waves within the wavelength thickness. Reported reconfigurable approaches mainly include mechanically tunable, thermally tunable, and electrically tunable methods. Mechanically tunable metasurfaces primarily achieve modulation of EM responses by applying external forces to reconfigure the geometric shapes of their microstructures. A tunable metasurface capable of providing mechanical and self-deformation flexibility with different operating modes, thereby achieving beam steering and splitting between left-handed and right-handed circularly polarized waves, was designed by Lim et al. [14]. Cai et al. [15] proposed a cascaded metasurface system employing independent rotational control of dielectric layers to achieve dynamic terahertz wavefront steering and polarization manipulation through effective Jones-matrix modulation. Although straightforward, this method might require complex mechanical systems and is often characterized by delayed response times. The principles of thermally tunable and electrically tunable metasurface (ETM) are essentially similar. It primarily involves incorporating specialized functional materials, such as liquid crystals (LC) [16], graphene [17,18], or transparent conducting oxides (TCO) [19,20] into metallic microstructures. By varying the temperature, dynamic parameters of refractive index (permittivity) or conductivity of these functional materials can be altered. Liu's team designed a kind of thermal terahertz (THz) split-ring resonator (SRR) metasurface based on liquid crystals [21]. A phase-change metasurface that utilizes the temperature properties of VO2 films is also designed by Yang et al. The polarization conversion efficiency can be switched between approximately 75 % and 0.5 % at low and high temperatures [22]. Generally, this thermal technique exhibits a slower response rate and it is still difficult to guarantee precise temperature control. Gold SRRs were integrated on a GaAs semiconductor substrate realize 50 % increase in the modulation rate of THz transmission with a reverse gate bias voltage ranging from 0 to 16 V [23]. An ETM designed by LEEB et al. utilizes MoS2 integrated with metal nanoantennas, with resonance coupling transitions between excitons and surface plasmon polaritons by adjusting the gate bias voltage [24]. In contrast to preceding methodologies, electrical control offers distinct advantages, including rapid response times, precise control accuracy, and strong anti-interference, which significantly promote on-chip integration with diverse microelectronic devices.

In the microwave region ETM designs, lumped elements (such as PIN diode, varactor, capacitor) are often employed where the bias voltage is applied. Sievenpiper et al. [12] demonstrated 2D beam steering using a varactor-tuned impedance surface, achieving ±40° scanning through LC resonance control. However, this approach causes reflection magnitude to decrease as the phase shifts. Chen et al. [13] developed a tunable Huygens' metalens using varactor diodes, achieving continuous phase tuning from 0° to 360° with relatively high transmission amplitude. Other approaches like graphene-varactor hybrid metasurface [25] achieved dual-tunability (frequency and amplitude of the resonance) without phase control. Most done by others can realize amplitude-only, phase-only metasurfaces, which considerably limit the degree of freedom for full-wave manipulation. A kind of varactor-based metasurface [26] enables reconfigurable beamforming, but its phase-amplitude coupling stemming from the varactor's capacitance-resistance (C–R) variations fundamentally restricts its independent control. To realize simultaneous phase and amplitude control, dual-diode reconfigurable design [27,28] and cascaded multi-layer method [29] were introduced consecutively. With complex sequential biasing, the reflection amplitudes covered from 0.45 to 0.92, 0.16 to 0.67 and 0.08 to 0.4, respectively. Notably, an innovative time-domain digital coding metasurface (TDCM) [30] accomplishes independent and precise harmonic amplitude/phase control across an ultra-wideband through modulation of coding sequences' duty cycles and time delays, yet still fail to provide wide-range amplitude modulation. Up to date, it is still elusive to accomplish simultaneous and independent control of amplitude and phase, together with a large tuning range/depth although it is highly demanded.

In this paper, a type of ETM with independent control of amplitude and phase is proposed. We first design the structure of the metasurface, followed by the simulation and analysis of its operational mechanism, demonstrating its capability for independent amplitude modulation and phase modulation. The reflection characteristics are analyzed in detail through the real-time modification of equivalent lumped parameters. Oblique incidence situations are also analyzed. Subsequently, theoretical analysis and simulation of encoding metasurfaces further demonstrate its versatility in controlling EM wave beams. Finally, a 4 × 2 unit cell array (with dimensions of 89 × 64 mm2) was fabricated, and its performance was validated through experimental measurements in a waveguide setup.