# Phase Change Material-Driven Tunable Metasurface for Adaptive Terahertz Sensing and Communication in 6G Perceptive Networks

Yat-sing To, Jiachen Du, Cyril Decroze, Laure Huitema, Aurelian Crunteanu, Hang Wong

► To cite this version:

Yat-sing To, Jiachen Du, Cyril Decroze, Laure Huitema, Aurelian Crunteanu, et al., Phase Change Material-Driven Tunable Metasurface for Adaptive Terahertz Sensing and Communication in 6G Perceptive Networks. Advanced Functional Materials, inPress, 10.1002/adfm.202515085, hal-05263504

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# Phase Change Material-Driven Tunable Metasurface for Adaptive Terahertz Sensing and Communication in 6G Perceptive Networks

Yat-Sing To, Jiachen Du, Cyril Decroze, Laure Huitema, Aurelian Crunteanu,*
and Hang Wong*

Terahertz (THz) wireless technology offers unprecedented capabilities in sensing, imaging, and communication for 6G perceptive networks. Recent reconfigurable THz metasurfaces enable adaptive beam manipulation, supporting diverse functionalities like frequency, polarization, spatial, and temporal adjustments for rapid communications and object tracking. However, these are hindered by multilayer complexity, insertion losses, and scalability challenges. Here, it is overcome these constraints by realizing a pioneering single-layer, optically activated tunable metasurface incorporating Germanium Telluride (GeTe) material, acquiring multifunctional THz sensing, imaging, and communication within a unified platform for the first time. GeTe's low-power, non-volatile switching facilitates dynamic reconfiguration, eliminating bulky bias networks of traditional THz metasurfaces. This measured metasurface delivers an adaptive sub-THz communication channel with extensive coverage and enhanced passive object detection via wide frequency-dispersive scanning. Leveraging phase-change material-driven tunability, this antenna technique enables efficient, adaptive 6G connectivity and high-precision localization, with transformative potential for low Earth orbit networks, smart cities, and advanced Internet of Things (IoT).

## 1. Introduction

The evolution of wireless technologies has experienced a remarkable paradigm shift characterized by integrating multiple functionalities within cohesive systems. Traditionally, sensing and communication systems operated as distinct entities, requiring specialized hardware, dedicated spectrum resources, and individual processing capabilities.[1] This dissociation often led to inefficiencies and limitations in performance. However, rising demands for greater spectrum efficiency, simplified system architectures, and enhanced overall performance have propelled the development of Integrated Sensing and Communication (ISAC) systems.[2] ISAC systems represent a sophisticated technological advancement, encompassing innovative features such as dual-functional waveform design, which enables the system to perform both sensing and communication tasks using a single device.[3-5] Additionally, ISAC incorporates joint resource allocation techniques that optimize available spectrum and computational resources and shared signal processing architectures that facilitate efficient data handling.[6,7] This integration of sensing and communication functions is particularly significant in the emerging 6G wireless networks. The shift toward ISAC addresses the demands of advanced wireless communications. It enhances the potential for innovative applications in various fields, such as autonomous vehicles,[8] smart cities,[9] and the Internet of Things (IoT).[10] Most of the ISAC demonstrations primarily depended on dedicated peripheral sensors or specific communication frequencies to assess multifunctionality. This approach raises concerns about system integrity. Therefore, it is essential to pursue a comprehensive strategy that emphasizes low interface complexity and high operational flexibility, particularly for high-frequency applications. Alongside the advancements in ISAC, Reconfigurable Intelligent Surfaces (RIS) have emerged as a promising technology capable of dynamically manipulating electromagnetic waves in real time. Using an extensive array of programmable meta-elements, RIS can adjust incident waves' phase, amplitude, and polarization, creating

<footnote>Y.-S. To, J. Du, H. Wong
State Key Laboratory of Terahertz and Millimeter Waves
City University of Hong Kong
83 Tat Chee Avenue, Hong Kong SAR999077, China
E-mail: hang.wong@cityyu.edu.hk</footnote>

<footnote>C. Decroze, L. Huitema, A. Crunteanu
XLIM Research Institute
CNRS/University of Limoges
Limoges 87060, France
E-mail: aurelian.crunteanu@xlim.fr</footnote>

<footnote>L. Huitema
Institut Universitaire de France (IUF)
Paris 75005, France</footnote>

<footnote>ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202515085</footnote>

<footnote>© 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>

<footnote>DOI: 10.1002/adfm.202515085</footnote>
a software-defined electromagnetic environment.[11,12] This re-
configurability allows for tailored signal propagation, improved
coverage, and enhanced link reliability, addressing fundamental
challenges within both sensing and communication frameworks.
The current paradigm of the RIS-empowered ISAC is centered
on the microwave and millimeter wave spectrum, facilitated by
a selection of digital coding metasurfaces.[13–20] This PIN diode-
based RIS, which boasts a well-established design and robust
control circuitry, utilizes multi-dimensional diversity to support
both sensing and communication functions. By incorporating
wideband and spatial diversity, the RIS is equipped to conduct
direction of arrival (DoA) estimation as well as object sensing ap-
plications. By integrating this information, beam tuning can be
executed to enhance communication quality.[13–16] Furthermore,
through the adoption of space-time coding techniques and spa-
tial partitioning schemes, the RIS can achieve harmonic sensing
while simultaneously maintaining communication at the funda-
mental frequency, thereby enabling concurrent operation.[17–20]
The ability of RIS to create adaptable and controllable environ-
ments enables more effective implementation of ISAC systems,
fostering new opportunities for precision in sensing tasks and
improving communication reliability in complex scenarios.[21]

Active metasurfaces incorporating semiconductor compo-
nents, such as varactor diodes and PIN diodes,[22,23] have been
regarded as the key enablers in RIS-empowered ISAC due to their
potential applications in multifunctional devices within the mi-
crowave frequency range. These devices utilize electrical stimuli
from a bias feed network to dynamically modulate their electro-
magnetic (EM) response. This modulation occurs due to fluctu-
ations in current distribution across the active elements, allow-
ing for real-time reconfigurability. However, this approach faces
several challenges when applied to higher frequency ranges, par-
ticularly in the millimeter-wave (mmW) and terahertz (THz) do-
mains. The primary issues arise from the relatively large size
of the semiconductor components, potential interference caused
by the bias network, and increased insertion losses that mani-
fest at these elevated frequencies. To address these limitations,
recent studies have increasingly focused on phase change ma-
terials (PCMs) to develop THz active metamaterials.[24–26] Due
to their distinct properties, PCMs provide a promising alterna-
tive for achieving multifunctional capabilities at high frequen-
cies. In contrast to traditional diodes, PCMs facilitate rapid and
reversible switching, are characterized by low power consump-
tion, and exhibit nonvolatility of their specific states (insulating
or conducting), while responsive to multiple stimuli. A notable
advancement in this area is the development of a THz polarizer
integrated with vanadium dioxide (VO₂), which has successfully
demonstrated the capability to switch polarization states of THz
waves by inducing topological changes in the grating structure,
manipulated along both the x- and y-directions.[27] By utilizing
the electrically addressable phase control of VO₂, a self-adaptive
metasurface has been realized for intelligent signal manipulation
with dynamic beam steering and power stabilization in the THz
band.[28] The VO₂ performs a phase transition at 68 °C between
a monoclinic insulator state to a rutile metallic one, which can
also be triggered thermally, electrically, or optically. However, this
insulator-to-metal transition in VO₂ is volatile; the material re-
covers its initial state upon removing the specific excitation.[27,28]
This implies that in the metallic activated state (ON state), like

in semiconductor-based devices, the VO₂-based device needs to
be continuously connected to a source of energy. Additionally,
chalcogenide PCMs have been effectively employed in design-
ing split-ring resonators, significantly altering the EM character-
istics of the structures in the THz frequency range.[29,30] The ma-
jor advantage of chalcogenide PCMs over the VO₂ phase change
is their non-volatile transition between the two states. The non-
volatile character of PCM-based devices is a unique key advan-
tage of this technology since the device does not require perma-
nent excitation (voltage bias, optical irradiation) to be maintained
in a specifically prepared state (ON or OFF), the only energy be-
ing required during reversible switching events. This innovative
application of materials enhances the functionality of metasur-
faces and opens up new possibilities for advanced THz technolo-
gies, positioning PCMs as a key focus for future research in this
field.

This article presents a wideband multifunctional reflective
metasurface based on PCM operating at the sub-THz spectrum
to demonstrate an alternative approach of achieving both com-
munication and sensing applications in a single device, as de-
picted in **Figure 1**. Based on the foundation of developing the
PCM-based metasurface,[31] the proposed metasurface consists
of an array of reflective meta-atoms integrating Germanium Tel-
luride (GeTe) material. Through the integration of metal and
GeTe patterns on the silica substrate, the conductive variation on
the meta-atoms pattern occurs by optical irradiation, thus effec-
tively modifying the incidence wave phase in a 1-bit phase differ-
ence of 180° ± 20° over the frequency range of 100 to 120 GHz
with high reflection coefficients of 0.8 dB. Owing to the recon-
figurability of the proposed meta-atom, coding reflective meta-
surfaces with different patterns based on the phase compensa-
tion technique[32] are demonstrated for terahertz beam form-
ing. Feeding by the linear polarized horn, the proposed meta-
surface is assigned designated coding patterns for demonstrat-
ing the wide-angle beamforming from 0° to ±45°. The mea-
sured radiation performances are obtained from the in-house far-
field THz measurement platform and compared with simulated
results for validation. Furthermore, the proposed metasurface
showcases dual functionalities, enabling the establishment of
sub-THz communication links and facilitating frequency-diverse
sensing. Experimental results demonstrate a broad coverage of
a 16 Quadrature Amplitude Modulation (QAM) 100 GHz com-
munication link at a symbol rate of 1 GS/s through the arbitrary
beam manipulation offered by the metasurface. Additionally, the
proposed metasurface achieves wide-angle passive object detec-
tion with a remarkable position precision of 1 mm by comple-
menting the frequency scanning range associated with differ-
ent coding patterns. The validated capabilities in both sensing
and communication suggest that this metasurface is a promis-
ing candidate and presents a novel approach for sub-THz ISAC
applications.

## 2. Results and Discussion

### 2.1. Wideband 1-Bit Functional Metasurface

The design and operational principle of the tunable meta-atom
are illustrated in **Figure 2a**. The detailed configuration of the
meta-atom is given in Note S1 (Supporting Information). A 1-bit
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Conceptual illustration of the proposed multifunctional coding metasurface for sub-terahertz (THz) near-field sensing and far-field communication. Crafted from functional materials, the metasurface can precisely manipulate reflected waves through optical activation. This capability facilitates a broad coverage communication environment with a dedicated high-gain signal channel. Furthermore, the metasurface enhances passive object detection through wide-angle scanning, which complements the frequency scanning range of different coding patterns.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Design and operating principle of the proposed 1-bit tunable meta-atom. a) The structural configuration of the meta-atom featuring stacked circular patches made of gold and GeTe built on the quartz glass substrate, specifically designed for 1-bit phase shifting. The phase tuning approach relied on the state change of the GeTe to imitate the size variation of the patch size, thus realizing the change of resonance on the meta-atom. b,c) Parametric study on the meta-atom radius R for analyzing the effect on the reflection coefficient – b) magnitude and c) phase. d) Optical activation of the meta-atom illustrating the reversible state change process between the crystalline state (State “1”) and amorphous state (State “0”). e,f) Finite-element simulations of the meta-atom reflection coefficient – e) magnitude and f) phase – in corresponding states.</figcaption></figure>
phase delay is realized through the size variation of the metallic pattern printed on the meta-atom. This variation influences the surface current induced by the incident field, leading to different propagation path lengths. A parametric study of its reflection performance was conducted to examine the characteristics of this meta-atom. Figure 2b depicts how the reflection coefficient changes with the radius in the frequency range of 100 to 120 GHz. Notably, as the radius increases from 150 to 600 µm, the reflection magnitude of the meta-atom remains within -0.15 dB, indicating that most of the incident energy is effectively reflected. Furthermore, the phase response (Figure 2c) indicates that by adjusting the radius within the 100 to 120 GHz frequency range, a phase difference of 180 degrees can be attained. This capability fulfills the phase requirements necessary for metasurface operation. According to the electrical properties of GeTe,[24,33] the GeTe patch acts as a conductor and isolator due to the high conductivity in the crystalline state ($\sigma = 410\ 000\ S\ m^{-1}$) and high relative permittivity in the amorphous state ($\epsilon_r = 22$). Compared to other PCM compositions (e.g., Ge2Sb2Te5-GST, SbTe, Ge-rich GST, etc.), GeTe (50:50 composition) has the advantage of both lower dielectric losses in the insulating state and, at the same time, lower conductivity in the metallic state,[33] making it a very adaptable composition when integrated into RF-switches or high-frequency devices.[24–26,29–31] Such a state transition of GeTe material results in the meta-atoms’ conductive variation, thus imitating the size variation of the metallic pattern for 1-bit phase shifting. The meta-atom with GeTe in the amorphous state is represented by state 0, while state 1 means that with GeTe in the crystalline state. As shown in Figure 2d, when the laser light passes through a proximity mask and illuminates the metasurface, only the meta-atoms in the void area are exposed to the laser and transition from the amorphous state to the crystalline state. A laser pulse with a maximum frequency of 20 Hz and a power of 90 mJ cm$^2$ is used to irradiate the GeTe layer in a dielectric state with a single pulse of 35 ns, initiating its transition to the crystalline/metallic state. The non-irradiating areas maintain their amorphous state. To change the distribution of meta-atom states on the surface, a second laser pulse with a power of 190 mJ cm$^2$ is employed to irradiate the metallic GeTe patterns and revert them through fast quenching to the amorphous state. Subsequently, the amorphous GeTe patterns can be irradiated with laser pulses of 90 mJ cm$^2$, based on the new distribution provided by an additional mask, leading to a change in the coding pattern. While this approach using the metasurface irradiation through specifically coded proximity mask is used here as a proof of concept, a more versatile solution is currently under development. It is implying a vectorial X/Y optical scanner directing a focused pulsed laser beam over the device surface and which have the ability to selectively transform, in a similar manner as described above, the topologies of the meta-atoms at much high raster scan frequencies (6–10 kHz).

To verify the functionality of the proposed meta-atom, its simulated model is investigated in the full-wave electromagnetic simulator HFSS (See Experimental Section). The reflection coefficients in different states are extracted through the excitation of the linear-polarized incident waves to verify the capability of 1-bit phase shifting. The reflection responses of the proposed meta-atom in 2 discrete phase states are investigated (Figure 2e,f). As seen from the reflection magnitude (Figure 2e), the proposed

meta-atom achieves and maintains a significantly high value of 0.8 in both states over the frequency range, which implies that most of the energy is reflected from the incident waves correspondingly in a wide bandwidth. Observing the reflection phase response (Figure 2f), the proposed meta-atom provides a stable 180° phase difference for a wide bandwidth ranging from 100 – 120 GHz. Sequentially, the relative phases of the proposed meta-atom in states are found in the reflection phase response to determine the metasurfaces' coded phase distribution. As observed in phase responses, the reflection phases at 100 GHz are 73.71° and 267.58° for state 0 and state 1, respectively. Accordingly, the reflection phases of the proposed meta-atom in each state are regarded as the coding criteria $\phi_{coding}$ for determining the coding sequence on the metasurface aperture.[32] Thanks to the axially symmetric structure of the meta-atom, it exhibits insensitivity to the polarization of the incident field. This indicates that both the magnitude and phase responses remain unchanged regardless of the polarization of the incoming wave. The reflection coefficients of the meta-atom in relation to either linearly or circularly polarized incident waves are detailed in Figures S2 and S3 (Supporting Information).

## 2.2. Wideband Arbitrary Beam Manipulation

To validate the reconfigurability of the proposed meta-atom, a reflective metasurface with a diameter of $D = 27.2$ mm based on 20 × 20 meta-atoms is demonstrated and investigated in the full-wave simulator, HFSS, for terahertz beam forming through three different scenarios (Figure 3). The coding patterns on the metasurface aperture determine the desired beam manipulation of the proposed metasurface. Based on the phase compensation technique, as discussed in Note S3 (Supporting Information), the phase distribution of the proposed metasurface aperture associated with each beamforming scenario is calculated and quantized into a 1-bit coding sequence based on coding criteria. It is unavoidable that there are phase differences between the required compensation phase, $\phi_{mn}$, and the coding phase, $\phi_{coding}$, due to the nature of the coding meta-materials, known as the quantization error, denoted as $\Delta\phi_{mn} = \phi_{coding} - \phi_{mn}$. These phase errors can lead to a degradation in radiation performance, affecting factors such as realized gains, side-lobe levels, cross-polarization levels, and aperture efficiency. To minimize the phase differences and reduce the effect of the quantization error, the coding state of the meta-atom is properly assigned based on the reflection phase closest to the calculated compensation phase. Consider there is 1-bit phase state of the meta-atom implying each state covers the range of 180° ($\pi$) of the calculated compensation phase, the reflection response of each meta-atom state $\angle r^{State0}$ and $\angle r^{State1}$ serves as the baseline, while the compensation phase $\phi_{mn}$ fall within the range of $\angle r^{State1/State0} \pm 90°$ will be discretized to a binary 1-bit phase states, State 0 and State 1 respectively. For the computed compensation phase $\phi_{mn}$ within the range of 164° < $\phi_{mn}$ ≤ 344°, the meta-atom is represented by State 1. Otherwise, it is regarded as a State 0 meta-atom. The pixels in purple represent the meta-atom in state 0, while meta-atoms in state 1 are represented in yellow. According to the configuration of the proposed reflective metasurface system, all the coded
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Simulated models and fabricated prototypes of the proposed metasurface for different beam manipulation scenarios. a) Beam directing to $\theta_r = 0°$, $\Phi_r = 0°$ for an incident beam with $\theta_i = -30°$, $\Phi_i = 90°$, b) Beam tilting at $\theta_r = 30°$, $\Phi_r = 90°$ for an incident beam with $\theta_i = 0°$, $\Phi_i = 0°$, and c) Beam tilting at $\theta_r = 45°$, $\Phi_r = 90°$ for an incident beam with $\theta_i = 0°$, $\Phi_i = 0°$. For simplicity, the coding patterns for beam directing at $\theta_r = 0°$ beam tilting at $\theta_r = 30°$ and $\theta_r = 45°$ are denoted as pattern #1, pattern #2, and pattern #3, respectively, in the following contents.</figcaption></figure>

metasurfaces are illuminated by the linear-polarized incidence from the stub-loaded horn with the F/D ratio of 0.64 for the optimal aperture efficiency, where the focal distance of F = 17.4 mm is defined as the distance originating from the aperture center of the feeding horn to the center of metasurface. The detailed calculation of the aperture efficiency for the metasurface is shown in Note S4 (Supporting Information). For the proof of concept, the prototypes of the different coded metasurfaces for linearly polarized beam manipulation are fabricated with specified coding patterns #1 to #3.

To validate the expected radiation performance of the fabricated samples, a far-field measurement (Figure 4a) is conducted to confirm their beam-forming ability with respect to the radiation pattern and realized gain. The detailed far-field measurement setup is given in Experimental Section. Figure 4b depicts the reflection coefficients and realized gains of the proposed metasurfaces as a function of frequency. The proposed metasurfaces demonstrate effective impedance matching, achieving a -10 dB impedance bandwidth across the full frequency range of 90–140 GHz. This indicates that the metasurfaces are well-suited for compatibility with the feeding source, ensuring that a significant portion of the incident energy is transmitted to free space. The maximum measured realized gains of 23.55, 22.95, and 21.9 dBi are achieved with 3 dB gain bandwidths of 22.61%, 22.6%, and 33% for patterns #1, #2, and #3, respectively. It can be observed that the realized gain is inversely proportional

to the beam-tilted angle, which correlates with the bit-resolution of the metasurface. As the tilt angle increases, the required phase variation of the coding patterns will rapidly change across the aperture, and 1-bit phase resolution is insufficient for defining the phase front. Meanwhile, the deviated main beam direction in the out-of-band frequencies resulted in discrepancies between the simulated and measured realized gain. The simulated and measured radiation patterns of the proposed metasurfaces in the elevation plane (yoz-plane) at the operating frequency of 110 GHz are depicted in Figure 4c for patterns #1, #2, and #3, respectively. A well-defined directional beam can be observed on the azimuthal plane and pointed to the desired steering angles of 0°, 30°, and 45°. Meanwhile, both radiated beams in each coding function achieve a sidelobe level of 12 dB and a low cross-polarization level of 30 dB. The radiation patterns' results agreed with the desired beam manipulations and indicated the significant radiation performance of the proposed metasurface. Figure 4d illustrates the beam angle variation with the measured normalized radiation patterns of the proposed metasurface over the operating frequencies. It can be observed that the range of beam angle variation is limited to ±10° over the frequencies of interest from 90 to 140 GHz. The beam formed in the metasurface with pattern #1 varies from 7° to -8°, while both metasurfaces with patterns #2 and #3 provide a beam variation of 20°, varying from 39° to 18° and 54° to 34°, respectively. Such a phenomenon is caused by the frequency dispersion of the proposed metasurfaces.
<figure><img src="image_5.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Far-field radiation performance of the proposed metasurface used for arbitrary beam manipulation. a) Illustration and image of an in-house designed far-field sub-THz measurement platform. b) Comparison between simulated (blue lines) and measured (red lines) reflection coefficient and realized gain as a function of frequency ranging from 90 to 140 GHz. c) Simulated and measured azimuthal radiation patterns at the operating frequency of 110 GHz. d) Measured normalized radiation pattern as a function of theta (θ) and frequency ranging from 90 to 140 GHz.</figcaption></figure>

metasurface, which primarily arises due to the dispersive nature of the metasurface's phase response and the chromatic aberration in the calculated phase profiles. The compensated phase distribution on the metasurface aperture is only optimized for the operating frequency of 110 GHz, while the propagating phase of the incident wave impinged on the metasurface varies over the measured frequency domain. The incident wave with varying phase experiences the identical phase distribution over the

frequencies and thus results in the beam angle variation of the radiation patterns. This effect of frequency dispersion occurs in the proposed metasurface with patterns #1, #2, and #3, though varying degrees, while the dispersion behavior of the proposed metasurface is detailly depicted in Figure S6 (Supporting Information). The beam angle variation due to the frequency dispersion will be further utilized for the wide-angle sensing application based on frequency diversity.
## 2.3. Wide-Angle Frequency-Diverse Sensing for Passive Object

Sub-THz and THz spectra have attracted considerable attention in recent decades due to their unique properties and potential applications in imaging and sensing systems. This spectral region presents several advantages over traditional microwave frequency bands and optical techniques, including non-ionizing radiation, the capability to penetrate non-metallic materials, and enhanced sensitivity to molecular vibrations.[34,35] Integrating THz technology with advanced sensing systems opens up new possibilities across various fields, ranging from improved security screening[36] to innovative biomedical imaging.[37] Current solutions for THz and sub-THz sensing and imaging systems encompass a variety of innovative technologies and methodologies aimed at boosting performance. These include silicon-based THz transmitters and receivers utilizing complementary metal oxide semiconductor (CMOS) and silicon germanium (SiGe) heterojunction bipolar transistor (HBT) technologies to enhance the efficiency and power of THz wave generation,[38] as well as high-performance sub-THz planar antenna arrays and lens antennas offering cost-effectiveness and ease of fabrication.[39–41] Despite these advancements, significant challenges remain among current sub-THz and THz sensing technologies, including high costs, bulkiness, and low spatial resolution, which limit their effectiveness and applicability across various domains. In this section, drawing from the computational image theory outlined in Note S6 (Supporting Information), the proposed metasurface, designed as an accessible, cost-effective, and high-precision sensing system, is introduced to validate its object detection capabilities. This system harnesses frequency diversity through various coding patterns (Figure 5a). For the proof of concept, the sensing system is specifically aimed at detecting metallic objects measuring 1 mm × 1 mm (0.33λ₀) on the imaging plane located at z = 60 mm (20λ₀) (Figure 5b). Additional information on the sensing experiment is provided in Experimental Section. The frequency dispersion across the metasurface allows frequency scanning of the high-gain reflected beam within a range of 20° for each metasurface with different coding patterns. With its tunable beam manipulation capabilities, the metasurface, employing different coding patterns, enables wide-range beam scanning on the near-field imaging plane, enhancing the detection coverage for scattering objects.

Figure 5c shows that the measured E-field distribution on the scanning plane varies across different frequencies. The frequency dispersion is validated using the proposed metasurface with distinct coding patterns as the converged beam moves horizontally along the scanning plane. The metasurface with pattern #2 offers, through the whole 90–140 GHz frequency dispersion domain, a scanning range of 35 to 55 mm, while the remaining range is complemented by the metasurface with pattern #3, which spans from 45 to 75 mm. Beam-tuning capabilities effectively cover the overall frequency-scanning area from 35 to 75 mm. However, it is essential to note that limited instances of E-field distribution can be used for computational imaging calculations. Due to inconsistencies in the phase difference of the meta-atoms across frequencies, high sidelobe levels are observed in the measured distribution at the bandwidth boundaries. This inconsistency can result in distortion of the imaging results. The coverage of the proposed sensing system is closely

<table><caption>Table 1. Normalized sensing results of the proposed sensing system.</caption><thead><tr><th></th><th colspan="10">Object position [mm]</th></tr></thead><tbody><tr><th>Actual</th><td>40</td><td>41</td><td>42</td><td>43</td><td>44</td><td>45</td><td>46</td><td>47</td><td>48</td><td>49</td><td>50</td></tr><tr><th>Measured</th><td>38.5</td><td>39</td><td>41</td><td>43</td><td>44</td><td>44.5</td><td>45.5</td><td>45.5</td><td>46</td><td>46.5</td><td>47.5</td></tr><tr><th>Actual</th><td>50</td><td>51</td><td>52</td><td>53</td><td>54</td><td>55</td><td>56</td><td>57</td><td>58</td><td>59</td><td>60</td></tr><tr><th>Measured</th><td>51</td><td>51.5</td><td>52.5</td><td>54.5</td><td>57</td><td>57</td><td>57.5</td><td>57</td><td>59</td><td>61</td><td>61.5</td></tr></tbody></table>

linked to the frequency scanning of the metasurface within its operating bandwidth. Figure 5d presents the measured results from the proposed sensing system, with the target object positioned at various locations. The results show a strong correlation with expectations, with high intensity recorded at 35, 40, 45, and 50 mm for the pattern #2 sample and at 55, 60, 65, and 70 mm for the pattern #3 sample along the imaging plane. An analysis of the precision of the proposed sensing system is conducted by comparing the detected peaks in the normalized sensing results (Figure 5e). The measured positions are indicated by dots representing where the maximum normalized magnitude occurred, and the actual data is shown as a dashed line. The results indicate that the system can accurately capture the object’s position with a precision of 1 mm. When comparing the measured positions to the expected values in Table 1, a high level of accuracy is achieved by limiting the sensing deviation to 3 mm (1λ₀). For a comprehensive analysis of the proposed sensing system performance, a detailed discussion on the resolution of the proposed system is given in Note S7 (Supporting Information). Overall, the experimental findings provided strong validation for the frequency-diverse sensing capability of the proposed system. These results underscore the significant potential of the tunable metasurface in facilitating accurate detection across a wide frequency range. The metasurface utilizes the high-precision sensed data to adaptively steer communication beams toward users or navigate around obstacles. For instance, the metasurface’s sensing capability can estimate the characteristics of the sub-terahertz channel, such as path loss, multipath effects, and the presence of obstacles, by scanning the environment. This capability ensures consistent signal quality by adapting to moving targets, ultimately enhancing throughput and reducing latency.

## 2.4. Wide-Coverage THz Communication Link Establishment

As the exponential increase in wireless data traffic and the rising demand for higher data rates, sub-THz and THz communication, operating within the frequency range of 100 GHz to 3 THz, has emerged as a promising frontier for next-generation wireless systems, particularly for 6G and beyond, potentially enabling data rates of up to terabits per second (Tbps) and ultra-low latency communications.[42] However, several fundamental challenges must be addressed to implement THz communications, including high path loss, atmospheric absorption, and hardware limitations.[43] Techniques such as phased-array antennas and dielectric lenses are widely used to mitigate propagation loss in THz wireless environments.[44] Nevertheless, the complexity of feeding networks and the limited coverage area due to narrow beamwidth remain significant hurdles to the widespread adoption of THz communication technologies. Therefore, addressing
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Demonstration of wide-angle frequency-diverse sensing for passive object detection. a) Illustration of the sensing principle based on the proposed metasurface with wide-angle frequency scanning via the complementation of coding patterns b) Experimental Setup of the proposed sensing system capturing the position of the metallic scattering object on the scanning plane c) Measured E-field distribution of the proposed metasurface with different coding patterns on the imaging plane across the frequency from 90 to 140 GHz, validating the characteristics of the frequency dispersion d) Measured sensing results of the proposed sensing system with target located at various position. It illustrates the scattering intensity of the target on the scanning plane, where the red spot indicates the object position with the highest scattering intensity. e) Normalized sensing intensity of the proposed sensing system across various object positions, showing the discrepancy between the real object position and measured object positions.</figcaption></figure>

these beamforming techniques is critical for advancing the devel-
opment of THz communication systems. The metasurface has
emerged as a promising technology for THz wireless communi-
cations, demonstrating significant potential due to its versatility
and scalability.[45,46] In the sub-THz band, a passive metasurface
utilizing transmissive C-shaped units has been successfully de-
veloped to multiplex orbital angular momentum (OAM) beams,
thereby achieving an increased communication channel with a
data rate of 10 Gbps.[45] Additionally, by incorporating an elec-
tronically controllable mechanism, a space-time coding metasur-
face has been introduced to facilitate the integration of commu-

nication transmitters with dynamic beam-steering systems for
sub-THz operations.[46] Current efforts to implement metasur-
faces in THz communication systems have highlighted their ca-
pabilities. However, the necessity for a bias network and multi-
layer structures has constrained their scalability and compatibil-
ity with high-frequency system integration. This section validates
the proposed metasurface through a wireless communication ex-
periment at the operating frequency of 100 GHz, offering an
alternative beamforming solution for wide-coverage, high-gain
sub-THz communications (Figure 6a); see Experimental Sec-
tion for detailed experimental setup for the communication link
<figure><img src="image_10.png" /><figcaption>**Figure 6.** Demonstration of wide-coverage sub-THz communication link. a) Experimental Setup of the sub-THz communication link establishment b) Measured power spectrum of 16 QAM signal with the metasurface, feeding source, and metal reflector at the carrier frequency of 100 GHz, showing the power enhancement offered by the metasurface c) Measured constellation diagram and eye diagram of 16 QAM signal with a symbol rate of 1 GS/s transmitted by the feeding source, the metal reflector and the metasurface with different coding patterns, verifying the improvement of the signal quality and noise suppression provided by the metasurface.</figcaption></figure>
<table><caption>**Table 2.** Measured SNR of 16 QAM signal with the metasurface, feeding source and metal reflector.</caption><thead><tr><th>Distance</th><th>300 mm [100λ<sub>0</sub>]</th><th>600 mm [200λ<sub>0</sub>]</th><th>900 mm [300λ<sub>0</sub>]</th></tr></thead><tbody><tr><td>Feeding source</td><td>21.21 dB</td><td>21.25 dB</td><td>18.47 dB</td></tr><tr><td>Metal Reflector</td><td>21.95 dB</td><td>19.76 dB</td><td>17.81 dB</td></tr><tr><td><b>Pattern #1</b></td><td>26.91 dB</td><td>22.59 dB</td><td>19.99 dB</td></tr><tr><td><b>Pattern #2</b></td><td>26.92 dB</td><td>23.68 dB</td><td>20.92 dB</td></tr><tr><td><b>Pattern #3</b></td><td>26.23 dB</td><td>26.53 dB</td><td>24.31 dB</td></tr></tbody></table>

establishment. The proposed approach employs a single-layer, optically activated GeTe metasurface, enabling low-power, non-volatile phase control without complex bias networks, and represents the first demonstration of wireless communication using a phase-change material-based metasurface in the sub-THz spectrum. During the experiment, a 16 QAM scheme is employed with a symbol rate of 1 GS/s. By utilizing the beamforming capabilities of the proposed metasurface, we measured transmitted signal power and spectrum at various reflection angles corresponding to pattern #1, pattern #2, and pattern #3 of the metasurface to demonstrate the broad coverage of the wireless communication environment. Additionally, measurements were taken at different far-field distances of 0.3m (100λ₀), 0.6m (200λ₀), and 0.9m (300λ₀) to exhibit the signal strength enhancement compared with controlled experiments using communication links from the feeding source and the metal reflector with the same dimensions.

Figure 6b depicts the measured power spectrum of 16 QAM signals across various distances. The measured power spectrum indicates a square waveform with a communication bandwidth of 1.1 GHz ≈100 GHz, determined by the full width at half maximum (FWHM) of the measured power spectrum. This represents a communication bandwidth, allowing for communication over this range with the 16-QAM modulation at the communication rate of 4 Gbps, is successfully realized. To evaluate the analysis of the effective communication bandwidth in this experiment setup, the experiment using the pattern #2 metasurface for the 16-QAM modulated signal is performed at various symbol rates of 1, 5, and 10 GS s⁻¹ over a distance of 600 mm, provided with a detailed discussion in Note S8 (Supporting Information). Accordingly, the measured Signal-to-Noise Ratio (SNR) is depicted in Table 2 for a comprehensive analysis of the measured 16 QAM signal power spectrum with the metasurface, feeding source, and metal reflector at various distances. Compared to the power spectrum of the feeding source and metal reflector, a significant power enhancement of 3 to 7 dB was observed when the communication system used the proposed metasurface at distances of 300 and 600 mm. However, at a distance of 900 mm, the power level achieved with the proposed metasurface dropped to the same level as that of the feeding source and metal reflector. This change occurs because the transmission power is insufficient to overcome atmospheric absorption under this communication setup, which lacks a power amplifier. The measured constellation diagram and eye diagram of the feeding source and the metasurface, featuring different coding patterns at a measured distance of 600 mm, are shown in Figure 6c. The measured constellation diagrams and eye diagrams at various distances are

<table><caption>**Table 3.** Measured EVM of 16 QAM signal with the metasurface, feeding source and metal reflector.</caption><thead><tr><th>Distance</th><th>300 mm [100λ<sub>0</sub>]</th><th>600 mm [200λ<sub>0</sub>]</th><th>900 mm [300λ<sub>0</sub>]</th></tr></thead><tbody><tr><td>Feeding source</td><td>6.54%</td><td>6.42%</td><td>8.96%</td></tr><tr><td>Metal Reflector</td><td>6.03%</td><td>7.69%</td><td>9.55%</td></tr><tr><td><b>Pattern #1</b></td><td>3.33%</td><td>5.50%</td><td>7.45%</td></tr><tr><td><b>Pattern #2</b></td><td>3.39%</td><td>4.83%</td><td>6.69%</td></tr><tr><td><b>Pattern #3</b></td><td>3.63%</td><td>3.53%</td><td>4.52%</td></tr></tbody></table>

included in Figures S12 and S13 (Supporting Information). By comparing the constellation diagrams of the feeding source and the metasurface, it can be observed that the received signal level from the metasurface is more concentrated and aligns well with the 16 levels of the ideal constellation in 16 QAM at either transmitting angle. This improvement is particularly noticeable at distances of 300 and 600 mm. However, at a distance of 900 mm, the constellation diagrams of the metasurface become more dispersed, approaching those of the feeding source. The eye diagram of the metasurface distinctly displays four signal levels in both the in-phase and quadrature signals across different distances. Compared to the feeding source and the metal reflector, the metasurface exhibits clearer and wider vertical eye openings in the eye diagrams. This finding indicates better signal integrity and reduces inter-symbol interference (ISI) provided by the proposed metasurface. The measured Error Vector Magnitude (EVM) of the 16 QAM signal was assessed with the metasurface, feeding source, and metal reflector at various distances, as shown in Table 3. EVM indicates the error ratio between the ideal signal level and the actual received signal level, serving as a crucial indicator of communication performance. For the proposed metasurface, the EVM values of the 16 QAM modulated channel ranged from 3.33% to 7.45%, while the feeding source and the metal reflector exhibited EVM values between 6.03% and 9.55% across the different measured distances. It is evident that the metasurface enhanced the communication link, improving EVM by 2% to 4% at various transmitting angles and distances. Throughout the experiment, the horn antenna excels in line-of-sight (LOS) scenarios but lacks adaptability to optimize signal paths, potentially leading to energy waste in misaligned links, while the metal plate, functioning as a passive reflector, produces broad, non-directional reflections with significantly reduced signal strength at the receiver. In contrast, the metasurface, which utilizes functional material-based tuning, facilitates active beamforming with a steering range of ±45°, achieving greater received power and lower error rates concerning SNR and EVM, respectively. The metasurface's tunable nature ensures optimal signal alignment and minimizes distortion, making it superior to both the metal plate and the horn antenna for sub-THz communications.

To illustrate the practical application of the sub-THz communication link, an image of the City University of Hong Kong campus is transmitted through the proposed metasurface-based communication system, as demonstrated in Video S1 (Supporting Information). The image data is modulated using a Quadrature Phase Shift Keying (QPSK) scheme at 100 GHz and is successfully transferred to the receiving end in real time, showcasing the stability of the sub-THz communication link established by the
<table><caption>**Table 4.** Comparison of state-of-the-art in RIS solutions.</caption><thead><tr><th rowspan="2">Refs.</th><th rowspan="2">Switching approach</th><th rowspan="2">Operating frequency</th><th colspan="2">ISAC compatibility</th><th colspan="3">System integration</th></tr><tr><th>Communication</th><th>Sensing</th><th>Complexity</th><th>Scalability</th><th>Power Consumption</th></tr></thead><tbody><tr><td>[13]</td><td>PIN diode</td><td>28GHz</td><td>QPSK<br>(3.125 Mbps)</td><td>DoA estimation</td><td>High<br>(Multi-layer)</td><td>Low<br>(element-wise electrical bias)</td><td>High<br>(biasing)</td></tr><tr><td>[14]</td><td>PIN diode</td><td>15-21GHz</td><td>16-QAM<br>(2.4 Gbps)</td><td>Passive object detection</td><td>High<br>(Multi-layer)</td><td>Low<br>(element-wise electrical bias)</td><td>High<br>(biasing)</td></tr><tr><td>[17]</td><td>PIN diode</td><td>10.3GHz</td><td>QPSK<br>(0.5 Mbps)</td><td>DoA estimation</td><td>High<br>(Multi-layer)</td><td>Low<br>(element-wise electrical bias)</td><td>High<br>(biasing)</td></tr><tr><td>[18]</td><td>PIN diode</td><td>10 GHz</td><td>QPSK<br>(N/A)</td><td>DoA estimation</td><td>High<br>(Multi-layer)</td><td>Low<br>(element-wise electrical bias)</td><td>High<br>(biasing)</td></tr><tr><td>[28]</td><td>VO<sub>2</sub></td><td>425 GHz</td><td>N/A</td><td>DoA estimation</td><td>Low<br>(Single layer)</td><td>Moderate<br>(Column-wise electrical bias)</td><td>High<br>(Volatile)</td></tr><tr><td>[46]</td><td>Varactor diode</td><td>81.25-94.65 GHz</td><td>BFSK/<br>QPSK/16-QAM<br>(12.5 Mbps)</td><td>N/A</td><td>Low<br>(Single layer)</td><td>Moderate<br>(Column-wise electrical bias)</td><td>High<br>(biasing)</td></tr><tr><td>This work</td><td>GeTe</td><td>90-140 GHz</td><td>16-QAM<br>(4 Gbps)</td><td>Passive object detection</td><td>Low<br>(Single layer)</td><td>High<br>(Optical Activation)</td><td>Low<br>(non-volatile)</td></tr></tbody></table>

proposed metasurface. In this series of communication link experiments, the proposed metasurface successfully established a sub-THz communication link modulated by 16 QAM at a symbol rate of 1 GS s⁻¹ and demonstrated a real-time image data transfer via the QPSK modulated channel. The results showed stable and significant performance across all three metasurface patterns at various far-field distances. This demonstrates the extensive capability of the proposed metasurface approach to facilitate a wide-coverage wireless communication channel with enhanced signal quality.

### 3. Conclusion

We proposed a novel tunable metasurface for beam manipulation in the THz regime. Benefiting from the characteristics of the GeTe material, whose state is switchable between crystalline/metallic and amorphous/insulating via optical stimuli, the proposed meta-atom was enabled to operate a 1-bit reflection phase change using the variation of radiating patch radius. As a demonstration, a reflective metasurface composed of the proposed meta-atom is presented for wideband arbitrary beam manipulation functions. The simulated and measured results indicated that a similar metasurface topology can form a directional beam at 0°, 30°, and 45° radiation from the oblique and normal incidence. The stable realized gain of 23 dBi was conserved over a wide bandwidth ranging from 100 to 120 GHz. Furthermore, the multifunctional property of the proposed metasurface has been verified in practical implementations to both i) establish sub-THz communication links and ii) facilitate frequency-diverse sensing. The proposed metasurface demonstrated the frequency-diverse sensing capability of the scattering object at the near-field distance of 60 mm and achieves accurate passive object detection with the sensing deviation limited to 3 mm in a wide frequency scanning range of ±60°. Additionally, the set of communication systems based on the proposed metasurface exhibited a broad coverage of a 16 QAM 100 GHz communication link at a symbol

rate of 1 GS s⁻¹, showing a 5 dB power enhancement and a maximum 4% EVM improvement compared to the feeding source. The overall results presented here particularly emphasize the advantages of the metasurface reconfigurable characteristics, with dual-function capabilities, which allow for adapting and enhancing its effectiveness in various applications, from sub-THz communication links to sensing, detection, and imaging.

To identify the significance of our proposed functional material-based metasurface, **Table 4** illustrates the comparison between our work and the state-of-the-art RIS solutions. Our proposed metasurface showcases significant advancements in state-of-the-art RIS solutions, particularly in their compatibility with ISAC applications. It is experimentally validated as a unified platform with two distinct functions: wide frequency-dispersive scanning for passive object detection and adaptive sub-THz communication channels that provide extensive coverage. Additionally, our design offers notable benefits in system integration, including reduced structural complexity due to its single-layer architecture, as opposed to traditional multilayer RIS designs, and enhanced scalability through a low-profile optical activation mechanism. The use of GeTe's low-power, non-volatile phase-change switching eliminates the necessity for bulky bias networks with sustained power supplies, resulting in superior energy efficiency and dynamic reconfiguration capabilities. Depending on the ISAC system's architecture and algorithms, our proposed metasurface provides the flexibility of simultaneous or sequential sensing and communication through diverse multiplexing techniques. In summary, the study demonstrates the promising role of this functional material-based metasurface in advancing the field of sub-THz ISAC applications.

### 4. Experimental Section

**Simulation Model:** The characteristics of the meta-atoms and the radiation performance of the metasurface were analyzed using Ansys HFSS.
The reflection amplitude and phase response of the meta-atom were assessed with periodic boundaries applied in both the x- and y-directions, simulating a planar periodic structure. Floquet ports along the z-direction facilitated wave excitation. Furthermore, the radiation characteristics of the metasurface model were obtained through the Finite Element-Boundary Integral (FE-BI) hybrid algorithm solver, which enhances solving efficiency, particularly for electrically large models.

**Prototype Fabrication:** The fabrication of the prototypes started with a substrate with similar dimensions to the reflective metasurface. The backside of the substrate is metalized with a 1-µm thick gold as a ground plane. On the front surface, a GeTe layer was fabricated using DC magnetron sputtering, and the PCM patterns are further defined using optical photolithography followed by reactive ion etching. Finally, gold patterns were defined and fabricated on the GeTe patterns using lithography (lift-off technique) and electron-beam evaporation of a 1-µm thick gold layer. For the laser irradiation process, a contact mask patterned with the desired coded pattern is placed on the metasurface, and the exposed GeTe units are optically activated through laser beam raster scanning of the whole metasurface area (KrF* excimer laser operating at 248 nm with a pulse duration of 35 ns). Thus, metasurfaces with the desired coding pattern were fabricated according to the distribution defined by the contact irradiation mask. The method effectively showcases the advantages of the optical activation scheme for PCM and highlights the high-frequency performance of the PCM material itself, positioning them as agile components within reconfigurable metasurface devices. By optimizing the irradiation process with a rapid laser scanning system, it could achieve switching times as low as tens of milliseconds, making them suitable for practical RIS applications.

**Far-Field Radiation Measurement:** An in-house designed far-field THz measurement platform was proposed and illustrated in Figure 4a. It consists of the following components: a fabricated prototype fed by the feeding horn and a vector network analyzer (Agilent N5245A VNA) wideband module (OML V08VNA2-T/R) for transmission, a Mi-wave WR-8 standard gain horn and a VNA wideband module for reception, and a motorized rotor for stable and precise angular movement of the prototype under test. After passing through the transmitting horn, the electromagnetic wave signal reaches the reflective metasurface at a focal distance of 17.4 mm. The metasurface under test was attached to the 3D-printed fixture. It was made of polylactic acid (PLA) with a dielectric constant of 2.7 to reinforce the robustness of the metasurface sample and provide the accurate alignment between the metasurface and the feeding horn. Both the reflective metasurface and the transmitting module were fixed on a high-precision rotating motor and placed at a far-field distance of 0.7m. The receiving horn and wideband module can receive electromagnetic wave signals from various angles of the reflective metasurface through the rotating motor. Due to the constraints of the rotating motor, which has a limited rotation angle of 180° on the azimuthal plane, the scanning ranges of the reflective metasurfaces are adjusted correspondingly to complete the far-field patterns scanning in different beam manipulation scenarios.

To assess the gain performance of the metasurface, we utilize a gain comparison method. This approach involves comparing the power levels received by a standard gain horn and the metasurface under test, both situated in the same test environment. Initially, the metasurface was precisely aligned with the receiving horn to capture the maximum radiation from the metasurface's main lobe. Then, the power received, $P_{DUT}$, was measured by the receiving horn, which was a Mi-wave WR-8 standard gain horn. Subsequently, under the same conditions, the metasurface prototype was replaced with the standard gain horn as the transmitting antenna and again measures the power received, $P_{SGH}$, at the receiving side. Since the gain of the standard gain horn, $G_{SGH}$, is provided in its datasheet, we can determine the gain of the metasurface, $G_{DUT}$, using the Friis transmission formula.

$$G_{DUT}(dB) = G_{SGH}(dB) + P_{DUT}(dB) - P_{SGH}(dB) \quad (1)$$

**Frequency-Diverse Sensing Experiment:** The sensing demonstration was carried out through a specialized high-frequency near-field scanning system (Figure 5b). It consists of the following components: a transmitting horn antenna and a VNA extension module (OML V08VNA2-T/R)

on the transmitting side, a receiving Mi-wave WR-8 waveguide probe and a VNA extension module supported by a 3-axis motorized stage on the receiving side, a scattering object made of copper foil, and an Agilent N5245A VNA. The copper foil with a dimension of 1 mm × 1 mm was regarded as the scattering object and was supported by the foam board, which had a dielectric constant of air.

The first step of frequency-diverse sensing will be conducted with the near-field measurement platform to measure the system response matrix $H$ of the metasurface on the sensing plane. The metasurface reflects the energy from the feeding horn with the transmitting module, while the probe connected with the receiving module captures the near-field plane data at a distance of 60 mm. The 3-axis motorized stage was set to provide precise movement while recording the near-field data on the scanning plane with a size of 30 × 80 mm. Each near-field scanning plane across the frequencies consisted of 61 × 161 field data points, achieving a resolution of 0.5 mm. Then, the scattering object was placed in front of the metasurface at the same distance of 60 mm as the near-field scanning plane, and the power scattered by the object at each 1 mm displacement was captured by the receiving horn. The 2-axis motorized stage underneath the object provides accurate movement of the object. The sensing results were calculated with MATLAB 2024.

**Sub-THz Communication Experiment:** Figure 6a depicts the schematic diagram and illustration of the sub-THz communication experiment. The developed communication system was dedicated to establishing 100 GHz communication links at various far-field distances. The baseband signal was generated and modulated in the desired modulation scheme via the Agilent M8194A arbitrary waveform generator on the transmission side. The root-raised cosine filter was applied to the modulated signal with the roll-off factor $\alpha$ of 0.1. Considering the 16-QAM modulation scheme implemented in this experiment, the communication bandwidth for the modulated signal can be determined as:

$$BW = Symbol \ rate \times (1 + \alpha) \quad (2)$$

For up-converting the signal on the baseband to the sub-THz band, the modulated signal was sent to the AT-Microwave 90–140 GHz subharmonics mixer (AT-SHM-90140D) as an intermediate frequency (IF) signal, while the Agilent E8257D signal generator with SAGE V-band power amplifier (SBP-5037532512-1515-E1) provides the 50 GHz signal at +8 dBm as a local oscillator (LO) signal for driving the mixer. The IF signal in the DC to 10 GHz range is upconverted to the THz frequency band using harmonics through the THz mixer. This process utilizes the second harmonic, which was generated by doubling the frequency of the LO signal and resulting in the desired radio frequency (RF) signal. It implies that the frequency of the RF signal for the sub-harmonic mixer is governed by:

$$RF = IF \pm 2LO \quad (3)$$

Finally, the RF signal was carried out by the feeding horn of the metasurface and reflected in a variety of directions ($\theta_r$) corresponding to the metal reflector or the patterns of the metasurface. In the configurations involving metasurfaces and metal reflectors, the center of either the metasurface or the metal reflector aperture was aligned with the midpoint of the receiving horn at various far-field distances. In contrast, the LOS communication experiment requires the direct alignment of the centers of both the transmitting and receiving horn antennas separated at the same far-field distances used in the metasurface measurement. Throughout all measurement configurations, the transmit power remained constant to ensure a fair comparison between communication links with and without the metasurface. On the receiving side, the Mi-wave F-band standard gain horn captures the reflected RF signal and down-converts to the baseband signal through the mixer. It was thus sent to the Agilent UXR0404AP real-time oscilloscope for channel analysis. The communication performance was analyzed using the Agilent PathWave Vector Signal Analysis software (89 600 VSA).
## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work was supported in part by the Research Grants Council of the Hong Kong SAR, China (Project Nos. CityU 11210621, C1009-22GF, and AoE/E-101/23-N), the Shenzhen-HongKong-Macau Science and Technology Project (Category C) under project SGDX 20230821100459006, and the support of the XLIM's Platinum platform, with funding from Nouvelle Aquitaine region council and the European Union under the PILIM program.

## Conflict of Interest

The authors declare no conflict of interest.

## Author Contributions

H.W. and A.C. suggested the designs, planned and supervised the entire study, and led the project, in consultation with C.D. and L.H. Y.T. and J.D. conceived the idea of this work, carried out the analytical modeling and numerical simulations. A.C. fabricated the prototype and performed the theoretical analysis. Y.T. and J.D. carried out experimental measurements and data analysis. Y.T. contributed to the writing of the paper. All authors discussed the theoretical aspects and numerical simulations, interpreted the results, and reviewed the manuscript.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

communication, metasurface, phase change material, sensing, terahertz

Received: June 13, 2025

Revised: August 14, 2025

Published online:

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