Metasurfaces offer distinct advantages，such as compact dimension and low loss，making them widely employed in various applications including metalens，holograms，and optical vortex manipulation ^［1-4］. Traditionally，metasurface structures are fixed upon array configuration，limiting their adaptability. The growing demand for tunable metasurfaces has emerged with advancements in metasurface technology. The introduction of phase-change materials（PCMs），exemplified by the germanium-antimony-telluride（GST）family and vanadium dioxide（VO₂），has facilitated the development of tunable metadevices ^［5-6］. PCMs undergo reversible transitions between amorphous and crystalline states，enabling the creation of multifunctional metalens，optical switches，and beam-steering devices ^［7-9］.

Notably，the integration of liquid crystals，diodes，and graphene in coding metasurfaces，where discrete phase shifts manipulate electromagnetic waves，has found extensive applications in tunable devices ^［10-13］. Moreover，coding metasurfaces utilizing PCMs have been widely adopted for dynamic applications. For example，Huang et al. designed a coding metasurface with switchable beam deflection and focusing based on GST ^［14］，while Lin et al. proposed a coding metasurface based on germanium telluride（GeTe）for dynamic modification of terahertz beams ^［15］. However，traditional PCM-based metasurfaces are constrained to only two tunable states，pre- and post-phase change. Recent attention has shifted towards miniaturized and highly integrated devices，particularly on-chip devices ^［16-18］. It offers advantages for the manipulation of in-plane electromagnetic wave propagation. Liquid-based tunable metamaterials，leveraging the ease of liquid addition and removal，have been developed with diverse functionalities. CH₂I₂ with high refractive index has been utilized for aplanatic numerical aperture increasing lenses ^［19］. Switchable beam-steering has also been achieved through water immersion or drying ^［20］，and the filling height of water in integrated-resonant metaatoms is adjusted to tune the focal length of meta-lenses ^［21］. Metasurface with a strong polarization correlation shows different beam reflections in a liquid environment ^［22］. Water-immersion tuning scheme achieves the switch for both near- and far-field meta-display transformation ^［23］. Dynamic control of light fields is realized by flowing liquids with different refractive indices over nanostructures ^［24］. An environment-compliant tunable meta-optics is enabled by a liquid immersion tuning scheme ^［25］. Despite the versatility demonstrated in different states，the current approaches are limited in their maximal multiplexing channels.

In this work，we present a novel tunable coding metasurface designed for two-level control. Leveraging a combination of GSST and CH₂I₂ liquid，our metasurface regulates in-plane electromagnetic waves across both single and broadband wavelengths. The tunable function is inactive when GSST is in the crystalline state，resulting in an indistinguishable deflection angle for etched holes with or without CH₂I₂. Conversely，when GSST transitions to the amorphous state，the tunable function activates，allowing the conversion of the coding metasurface from 3-bit to 4-bit，accompanied by a change in deflection angle when the etched hole is filled with CH₂I₂. We also discuss the impact of the refractive index of the liquid. This two-level control mechanism introduces a new avenue for designing on-chip dynamic tunable devices.

Fig. 1（a）illustrates the schematic of our tunable coding metasurface，designed as a beam deflection device operating at a wavelength of 5.2 μm. The tunable functionality，denoted as ON or OFF，is contingent upon the state of GSST，while the specific deflection angle is determined by the presence or absence of CH₂I₂ liquid within the etched hole. The arrangement of the etched hole along the y-direction allows manipulation of the TE polarization（polarized along the y-direction）incident wave，propagating along the +x-direction. In the amorphous state of GSST，the deflection angle of the electromagnetic wave undergoes alteration upon filling or emptying the CH₂I₂ liquid within the hole. Correspondingly，the phase distribution of the xy-plane is depicted in Fig. 1（b）. This dynamic modulation of the deflection angle and phase distribution highlights the tunable nature of our coding metasurface，showcasing its ability to control electromagnetic waves based on the state of GSST and the presence of CH₂I₂ liquid.

The metaatom's schematic diagram，as depicted in Fig. 2（a），comprises the etched hole，GSST，and SiO₂ substrate. GSST is a kind of nonvolatile phase change materials，which phase state can maintain at room temperature. GSST has a large difference in the optical property between the crystalline and amorphous states ^［26］，making it a perfect candidate for the design of tunable metasurfaces. The GSST layer，with a thickness（H）of 2 μm，plays a crucial role in the overall functionality. The dimensions of the etched hole，denoted as length（L）and width（W），are carefully tuned to achieve the desired deflection function. The periodicity of the metaatom，extending along the y-direction，is established at P = 1 μm. It’s necessary to have further consideration on the experimental feasibility. The GSST film can be deposited onto the substrate by thermal co-evaporation，and the desired film can be achieved by controlling the ratio of evaporation rates of two isolated targets of Ge₂Sb₂Te₅ and Ge₂Sb₂Se₅. The GSST film can be etched via electron beam lithography and reactive ion etching. The GSST film can be changed to the crystalline state by hot-plate annealing at 250°C for 30 min ^［27］. The addition of CH₂I₂ liquid can by a pipettor and the removal of it can by vaporization ^［28］. Therefore，the practical experimental is possible for this work. In terms of material properties，the refractive indices of GSST differ in its amorphous and crystalline states，measuring 3.19 and 4.65，respectively，at the operational wavelength of 5.2 μm ^［26］. Additionally，CH₂I₂ liquid，a key component，exhibits a refractive index of 1.691 at the specified wavelength（5.2 μm）and temperature（20 ℃）^［29］. The high refractive index of CH₂I₂ can provide a wide range of phase control，making it a good choice to replace low refractive index water or oil for the design of the tunable metasurface. To determine the refraction angle（θ），we employ the generalized Snell's law ^［30］，expressed as θ = sin^-1（λ/（n_tΓ）），where λ represents the designed wavelength，n_t is the effective refractive index of the refraction medium，and Г signifies the super-period of the designed coding sequence. This formulation guides the modulation of the refraction angle based on the specified parameters，ensuring precise control over the metaatom's optical properties.

Figs. 2（b）to 2（f）collectively illustrate the four tunable states of the coding metasurface，offering insight into the phase shift and transmittance characteristics of the metaatoms. In the amorphous state of GSST，the phase shift of the metaatom is approximately π/4 and π/8 for the etched hole without or with CH₂I₂ liquid，respectively（Figs. 2（c）and 2（d））. According to the concept of coding metasurface，n-bit coding metasurfaces consist of 2ⁿ coding units with a phase difference of 2π/2^n［10-12］. Due to the smaller discrete phase difference，higher bit coding provides more precise control of electromagnetic waves and the spurious interference will be effectively suppressed ^［31-32］. This distinction allows us to define the metaatoms as 3-bit and 4-bit coding configurations，respectively. Conversely，in the crystalline state of GSST，the phase shift remains approximately π/4 for both configurations，regardless of the absence or presence of CH₂I₂（Figs. 2（e）and 2（f））. Consequently，in this crystalline state，the metaatoms are categorized as 3-bit coding. Notably，during the crystalline state of GSST，the tunable function of the coding metasurface is inactive，signifying that the tunable coding metasurface is in the OFF state.

The validation of our designed tunable coding metasurface was conducted through full-wave simulations. In the amorphous state of GSST，when the tunable function is active，the simulated deflection angles for the etched hole without or with CH₂I₂ liquid are 12.1° and 5.9°，respectively，as depicted in Fig. 3（a）. The theoretically calculated deflection angles closely match at 12.4° and 6.2°，respectively. This alignment highlights the dynamic changes in the deflection angle corresponding to the presence or absence of CH₂I₂ in the etched hole. Upon conversion of GSST into the crystalline state，the simulated deflection angles for the etched hole without or with CH₂I₂ liquid are 8.8° and 8.0°，as illustrated in Fig. 3（b）. The theoretically calculated deflection angles for both cases are consistent at 8.3°. Remarkably，the presence of CH₂I₂ has minimal impact on the deflection angle in this crystalline state. The phase distribution in the xy-plane for the four different states is presented in Figs. 3（c）~（f）. It is evident that the isophase of the wavefront propagating in-plane inclines to varying degrees in each state，resulting in distinct deflection angles in the far field. This observation reinforces the tunable nature of the coding metasurface，demonstrating its ability to dynamically control the direction of electromagnetic waves based on the state of GSST and the presence of CH₂I₂.

The potential influence of evaporation-induced fluctuations in the refractive index of CH₂I₂ liquid on deflection is investigated by simulating four different refractive indices in both amorphous and crystalline states of GSST. The results，presented in Figs. 4（a~d）and 4（e~h），demonstrate the robustness of the coding metasurface in maintaining a consistent deflection effect within a certain range of liquid refractive indices. For the amorphous state of GSST，as the refractive index of the liquid varies between 1.4，1.6，1.9，and 2.0，the simulated deflection angles are 6.8°，6.4°，5.5°，and 4.7°，respectively（Figs. 4（a~d））. Similarly，in the crystalline state of GSST，the simulated deflection angles for the same range of liquid refractive indices are 8.4°，8.4°，7.6°，and 6.4°，as shown in Figs. 4（e~h）. Notably，the coding metasurface exhibits a consistent and reliable deflection effect across different liquid refractive indices in both amorphous and crystalline states. Furthermore，the deflection angle decreases with an increase in the liquid refractive index，indicating the sensitivity of the deflection effect to variations in the refractive index of CH₂I₂ liquid within the specified range.

In addition，we delved into the operational bandwidth of the proposed tunable coding metasurface. When the device operates in the amorphous state of GSST，we found that the working wavelength of the metasurface can be expanded to a broadband range from 5 to 6 μm. The deflection angles of the etched hole without or with liquid at wavelengths of 5 μm，5.5 μm，5.7 μm，and 6 μm are illustrated in Fig. 5. At these specified wavelengths，the simulated deflection angle varies from 11.7° to 14.1° when the etched hole is without liquid，and from 5.5° to 6.8° when the etched hole is filled with liquid. According to the generalized Snell's law，the deflection angle is positive correlation with the working wavelength. Hence，the deflection angle is corresponding increases with the increase of working wavelength. This indicates that the deflection function remains valid across a broadband spectrum. The on-chip photonic device we designed exhibits robust performance，showcasing its suitability for applications requiring reliable and tunable deflection within a broad range of wavelengths.

In conclusion，our study successfully achieved two-level control of the coding metasurface，demonstrating versatility in both single and broadband wavelength applications. The tunable function of the on-chip device transitions from OFF to ON when GSST converts from the crystalline to the amorphous state. Under the OFF state，the deflection angle remains consistent whether the etched hole is without or with liquid. Conversely，under the ON state，the deflection angle is switchable based on the presence or absence of liquid in the etched hole. Notably，the deflection effect is robust and can be maintained regardless of changes in refractive index of liquid or operating wavelength，emphasizing the resilience of our on-chip photoelectric devices. Our findings open avenues for dynamic manipulation in on-chip devices，holding potential implications for the advancement of optical computational circuits，on-chip spectrometers，detectors，and other related technologies.