Millisecond-level electrically switchable metalens for adaptive rotational depth mapping and diffraction-limited imaging

Millisecond-level electrically switchable metalens for adaptive rotational depth mapping and diffraction-limited imaging

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Yeseul Kim¹, Jihae Lee², Won-Sik Kim², Hyeonsu Heo¹, Dongmin Jeon¹, Beomha Yang³, Xiaotong Li¹, Harit Keawmuang¹, Shiqi Hu¹^,⁹, Young-Ki Kim², Trevon Badloe⁴^,⁵^,^*, Junsuk Rho¹^,²^,⁶^,⁷^,⁸^,^*

Opto-Electronic Advances | 2026, 9(2) : 250216-1 - 250216-11

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Opto-Electronic Advances | 2026, 9(2): 250216-1-250216-11

• Article •

Millisecond-level electrically switchable metalens for adaptive rotational depth mapping and diffraction-limited imaging

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Affiliations

¹Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

²Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

³Department of Quantum Information Science, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

⁴Department of Electronics and Information Engineering, Korea University, Sejong 30019, Republic of Korea

⁵Division of Smart Energy Convergence Engineering, Korea University, Sejong 30019, Republic of Korea

⁶Department of Electrical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

⁷POSCO-POSTECH-RIST Convergence Research Center for Flat Optics and Metaphotonics, Pohang 37673, Republic of Korea

⁸National Institute of Nanomaterials Technology (NINT), Pohang 37673 Republic of Korea

⁹Platform for Real-world Innovation in Smart Manufacturing and AI, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea

About Author:

^†

These authors contributed equally to this work.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.R. conceived the idea and initiated the project. Y.K. designed the metalens, conducted simulations, and obtained experimental measurements. J.L. conducted all nanofabrication processes. H.H. helped to conduct real-object imaging. D.J., B.Y., X.L., H.K., and S.H. collaboratively deliberated on the concept and idea. W.-S.K. and Y.-K.K. constructed the liquid-crystal cell and performed material characterization. J.R. and T.B. guided the entire work.

Published: 2026-02-12 doi: 10.29026/oea.2026.250216

Outline

Abstract

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We demonstrate an electrically tunable dual-mode metalens capable of polarization-sensitive focal control, combining high-resolution imaging and depth-sensing functionalities into a single compact device. By integrating hydrogenated amorphous silicon (a-Si:H) meta-atoms with a liquid crystal (LC) modulator, the proposed metasurface independently manipulates left- and right-circularly polarized (LCP/RCP) incident light, generating a rotating double-helix focal distribution for LCP and an extended depth-of-focus (DOF) for RCP illumination. The meta-atoms were rigorously optimized using propagation and geometric phases, enabling precise phase control and high transmittance at a wavelength of 635 nm. Experimental characterization confirmed near-diffraction-limited lateral and axial resolutions, closely aligning with theoretical predictions. The integrated LC cell facilitates milliseconds polarizationswitching between depth-sensitive double-helix and high-resolution DOF imaging modes. We further verified depth-extraction capabilities by analyzing rotation angles from dual-image focal spots under mixed-polarization illumination. Depth-resolved imaging of a rubber-tree leaf, a skeletal-muscle cross-section, and a live planarian retrieved color-coded depths, demonstrating the effectiveness on complex biological tissues. This polarization-driven, electrically tunable metalens thus provides a versatile and effective optical platform suitable for advanced applications in biomedical imaging, three-dimensional sensing, adaptive optics, and compact imaging systems.

Key words

chiral-dependent 3D imaging retrieval / chiral-dependent 3D imaging / spin dependent / electrically tunable / metasurface

Cite this Article

Yeseul Kim, Jihae Lee, Won-Sik Kim, Hyeonsu Heo, Dongmin Jeon, Beomha Yang, Xiaotong Li, Harit Keawmuang, Shiqi Hu, Young-Ki Kim, Trevon Badloe, Junsuk Rho. Millisecond-level electrically switchable metalens for adaptive rotational depth mapping and diffraction-limited imaging[J]. Opto-Electronic Advances, 2026 , 9 (2) : 250216-1 -250216-11 . DOI: 10.29026/oea.2026.250216

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1　Introduction

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Metasurfaces, composed of planar arrays of subwavelength structures known as meta-atoms, have emerged as powerful optical elements capable of precisely manipulating the amplitude, phase, and polarization of incident light¹⁻⁴. By engineering the geometry and arrangement of individual meta-atoms, metasurfaces can achieve complex wavefront transformations with ultra-thin profiles, presenting a compact and efficient alternative to conventional bulky optical components⁵⁻⁹. In particular, metalenses, which are metasurface-based lenses, have garnered considerable attention due to their ability to focus or image light with near diffraction-limited performance while significantly reducing device volume and weight compared to traditional refractive optics¹⁰⁻¹⁵.

Despite these advantages, conventional metasurfaces typically operate under fixed optical conditions, such as a predefined wavelength, polarization, or incidence angle. To address this limitation, significant efforts have been made to realize dynamically tunable metasurfaces¹⁶⁻²⁰. Various modulation strategies have been investigated, including mechanical deformation, thermal tuning^21,22, phase-change materials²³⁻²⁵, micro-electromechanical systems²⁶, and electro-optic effects^27,28. Among these methods, liquid crystal (LC)-based modulation stands out as especially advantageous for visible and near-infrared applications, owing to its low optical losses, relatively fast response times on the order of milliseconds, and convenient voltage-driven control mechanisms (Comparison of reported tunable metalenses are shown in Table S1 in Supplementary Section 1)²⁹⁻³³.

While recent work has shown that rotational point-spread functions generated by static metasurfaces can be used for depth sensing, these devices lack in-situ tunability and cannot seamlessly switch between high-resolution imaging and depth-encoded operation³⁴⁻³⁷. Addressing this gap requires a single platform that combines multifunctionality, depth retrieval, and high-resolution imaging, with rapid electronic control.

Here, we propose an electrically tunable metalens that leverages hydrogenated amorphous silicon (a-Si:H) meta-atoms combined with an integrated LC cell for rapid and precise polarization control. The metalens incorporates both propagation and geometric phase profiles, enabling independent manipulation of left-circularly polarized (LCP) and right-circularly polarized (RCP) incident beams. Specifically, under LCP illumination, the metalens produces a double-helix-shaped focal pattern, while under RCP illumination, it provides an extended depth-of-focus (DOF). This polarization-dependent behavior allows the metalens to simultaneously achieve high-resolution imaging and extensive depth sensing within a single optical component.

Moreover, when the incident beam contains both polarization states simultaneously, distinct and rotated focal spots emerge, encoding depth information directly into the optical field. By analyzing rotation angles, we demonstrate precise depth retrieval, thereby transforming the metalens into a versatile depth-sensing and imaging platform. Finally, we validate the metalens on biologically relevant specimens including a rubber-tree leaf, a skeletal-muscle cross-section, and a live planarian. Accurate, color-coded depth maps spanning roughly 90 µm are retrieved, demonstrating the device's practical utility for complex, scattering tissues. The use of an electrically driven LC cell enables rapid switching between these polarization states, highlighting its potential for real-time, dynamic imaging and adaptive optics applications.

Overall, the presented electrically tunable metalens represents a compact and multifunctional optical solution, merging polarization-sensitive high-resolution imaging capabilities with advanced depth-sensing features. Its versatile, LC-based reconfigurability holds significant promise for diverse applications, including biomedical imaging, optical communications, three-dimensional displays, and advanced microscopy systems.

2　Theory and design

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2.1　Designing the dual-mode metalens

The dual-mode metalens was designed to enable polarization-sensitive focal patterns, aiming to overcome the conventional trade-off between imaging depth range and resolution. By carefully engineering two distinct polarization-dependent phase profiles, the metalens can generate a double-helix focal distribution under LCP beam and an extended DOF under RCP beam, as illustrated in Fig. 1(a) and 1(d), respectively.

Under LCP illumination, the required phase mask (

$ {\varphi }_{{\mathrm{LCP}}} $

) was composed by combining two separate components shown in Fig. 1(b): a hyperbolic lens phase profile (

$ {\varphi }_{f0} $

) and a two-fold self-rotating phase profile (

$ {\varphi }_{{\mathrm{rot}}} $

). The hyperbolic lens phase (

$ {\varphi }_{f0} $

) is described by the following equation^38,39:

$ {\varphi }_{f0}\left(x, y}\right)=\frac{2{\mathrm{\pi}} }{\lambda }\cdot \left({f}_{0}-\sqrt{{x}^{2}+{y}^{2}+{f}_{0}^{2}}\right)\;, $

(1)

where λ denotes the target wavelength of the incident light (635 nm in this study),

$ {f}_{0} $

denotes the focal length (= 2 mm), and x and y are the Cartesian coordinates on the metasurface plane. The metasurface had a diameter of 500 µm, corresponding to a numerical aperture of 0.13.

The two-fold self-rotating phase profile (

$ {\varphi }_{{\mathrm{rot}}} $

) is expressed by⁴⁰⁻⁴²:

$ {\varphi }_{{\mathrm{rot}}}\left({r},{\theta }\right)=k\cdot {\mathrm{round}}\left(a\cdot {r}^{b}\right)\cdot \left(\theta +\frac{\mathrm{\pi }}{2}\right)\;, $

(2)

where

$ k $

is controlled parameter for the lobe number for the splitting patterns, r represents the radial coordinate (

$ r=\sqrt{{x}^{2}+{y}^{2}} $

$ \theta $

indicates the azimuth angle

$\left( \theta ={{\tan}}^{-1}\left(\dfrac{y}{x}\right)\right) $

, a regulates the tail of the rotating beam, and b modified the overall phase pattern. A detailed parametric sweep was performed to identify optimal values for a and b, carefully evaluating the intensity at circular points, circularity, number of circles, Euler number, and rotation angle. The optimal values were found to be a = 0.65 and b = 1, with full parametric analysis and optimization criteria detailed in Supplementary Section 2. Additionally, simulated maximum intensity distributions and rotation angle characteristics over the range z = 1.9 mm to z = 2.1 mm using these optimal parameters are provided in Supplementary Section 3. Normalized intensity distributions around the designed focal plane (from z = 1.92 mm to z = 2.08 mm) under LCP illumination clearly illustrate the rotating double-helix pattern (Fig. 1(c)).

For RCP illumination, the phase mask (

$ {\varphi }_{{\mathrm{RCP}}} $

) was constructed by summing 20 distinct pairs of elements in Fig. 1(e), each pair containing a hyperbolic lens phase with varying focal lengths and corresponding random binary masks^43,44. The hyperbolic lens phases (

$ {\varphi }_{f1} $

${\varphi }_{f2} $

...,

$ {\varphi }_{f20} $

) were defined by:

$ {\varphi }_{fn}\left(x,\mathrm{ }{y}\right)=\frac{2{\mathrm{\pi}} }{\lambda }\cdot \left({f}_{n}-\sqrt{{x}^{2}+{y}^{2}+{f}_{n}^{2}}\right), n = 1, 2, \cdots, 20\;. $

(3)

The focal lengths (

$ {f}_{n} $

) were systematically calculated starting from a central focal length

$ {f}_{0} $

, using carefully selected parameters: the total 20 focal points with 10 µm inter-focal spacing. This allowed precise axial positioning of the resulting focal points.

The corresponding random binary masks (

$ {\varphi }_{{\mathrm{r}}1} $

$ {\varphi }_{{\mathrm{r}}2} $

, ... ,

$ {\varphi }_{{\mathrm{r}}20} $

) were generated such that at each pixel position only one mask had a value of 1 and the others were set to 0. This selection process was mathematically expressed as:

$ \begin{split} &{\varphi }_{fn}\left(x,{y}\right)=\left\{\begin{array}{l}1,{\mathrm{if}}\; {\mathrm{mask}}\;k\;{\mathrm{is}}\; {\mathrm{assigned}}\; {\mathrm{to}}\; {\mathrm{pixel}} (x,y)\\ 0,{\mathrm{otherwise}}\end{array}\right.. \\&\quad\quad\quad\quad\quad k = 1, 2, \cdots, 20\end{split} $

(4)

This structured combination of phase elements resulted in a consistent and extended depth-of-focus for RCP illumination. Normalized intensity distributions under RCP illumination within the axial range from z = 1.92 mm to z = 2.08 mm (Fig. 1(f)) demonstrate a single, narrow full-width-half-maximum (FWHM) PSF consistently maintained along the extended focal region. Uniformity of the RCP extended-DOF was verified numerically by sweeping the detection plane from 1.90 mm to 2.10 mm in 10 µm steps. (See Supplementary Section 9 for the x-z map, lateral line profiles, and focal-plane PSFs.) This sophisticated design strategy provides distinct and stable focal modes selectively activated by controlling the polarization state of the incident beam. This design approach provides a unique capability where LCP illumination creates a rotating double-helix PSF pattern near the focal plane, while RCP illumination produces an extended DOF with a consistently narrow PSF.

2.2　Meta-atom simulation and fabrication

The design of the meta-atom was essential for achieving the dual-mode metalens with differing focal responses for LCP and RCP light. Figure 2(a) illustrates that rectangular a-Si:H pillars were chosen for the meta-atoms owing to their elevated transmittance and a measured complex refractive index of 2.9529 + 0.0095i at 635 nm, as specified in Supplementary Section 4.

The designed meta-atoms had a fixed height H = 600 nm and periodicity P = 300 nm, fabricated on a fused silica substrate. Rigorous coupled-wave analysis (RCWA) was employed to perform parameter sweeps over the meta-atom dimensions along the x and y directions (

$ {L}_{x} $

and

$ {L}_{y} $

), varying from 50 nm to 250 nm. This design approach leveraged both propagation and geometric (Pancharatnam-Berry) phases, enabling independent manipulation of LCP and RCP light within a single metasurface.

Each rectangular meta-atom acts effectively as a half-wave plate, requiring a phase retardation difference

$ |{\phi }_{x}-{\phi }_{y}|\approx \mathrm{\pi } $

, where

$ {\phi }_{x} $

and

$ {\phi }_{y} $

are phase delays for x- and y-polarized incident light, respectively. For simplicity, the propagation phase

$ {\varphi }_{{\mathrm{prop}}}\left(x,y\right) $

was assumed equivalent to

$ {\phi }_{x} $

. By rotating each meta-atom at an angle

$ \alpha $

, a geometric phase

$ {\varphi }_{{\mathrm{geo}}}\left(x,y\right) $

equal to twice the rotation angle

$ 2\alpha $

, was imparted with opposite signs for LCP and RCP illumination.

To independently control LCP and RCP illumination responses, both propagation and geometric phases must be combined. Mathematically, the total phases for LCP and RCP illumination can be expressed as:

$ {\varphi }_{{\mathrm{LCP}}}\left(x,y\right)={\varphi }_{{\mathrm{prop}}}\left(x,y\right)+{\varphi }_{{\mathrm{geo}}}\left(x,y\right)\;, $

(5)

$ {\varphi }_{{\mathrm{RCP}}}\left(x,y\right)={\varphi }_{{\mathrm{prop}}}\left(x,y\right)-{\varphi }_{{\mathrm{geo}}}\left(x,y\right) \;. $

(6)

These equations yield:

$ {\varphi }_{{\mathrm{prop}}}\left(x,y\right)=\frac{{\varphi }_{{\mathrm{LCP}}}\left(x,y\right)+{\varphi }_{{\mathrm{RCP}}}\left(x,y\right)}{2}\;, $

(7)

$ {\varphi }_{{\mathrm{geo}}}\left(x,y\right)=\frac{{\varphi }_{{\mathrm{LCP}}}\left(x,y\right)-{\varphi }_{{\mathrm{RCP}}}\left(x,y\right)}{2}\;. $

(8)

The corresponding required propagation and geometric phases are shown in Fig. 2(b) and 2(c), extracted directly from the designed metalens profiles. The propagation phase was discretized into eight fundamental meta-atoms, carefully selected to uniformly span the 0 to

$ 2{\mathrm{\pi }}$

phase range. Figure 2(d) presents the transmittance and corresponding phase profiles of these eight selected fundamental meta-atoms, indicating high transmittance (~0.88 average) and good agreement with the desired linear phase distribution (dashed line).

The calculated phase difference Δ

$\phi $

$ {L}_{x} $

and

$ {L}_{y} $

vary from 50 nm to 250 nm is shown in Fig. 2(e), highlighting a robust near

$ {\mathrm{\pi}} $

phase difference. Positions of the selected fundamental meta-atoms are marked with red '×' symbols. Detailed geometrical parameters

$ {L}_{x} $

and

$ {L}_{y} $

for each meta-atom are summarized in Supplementary Section 5. Also, Supplementary Section 6 contains further data on transmittance and phase delay as

$ {L}_{x} $

and

$ {L}_{y} $

are varied.

Fabrication of the designed metalens followed standard electron-beam lithography and reactive-ion etching processes. Initially, a 600 nm-thick a-Si:H film was deposited onto a silica substrate using plasma-enhanced chemical vapor deposition, with specific gas flows of SiN₄ (10 sccm) and H₂ (75 sccm) at 300 °C and 40 mTorr. Subsequently, a positive-tone resist (ZEP520A) was spin-coated and patterned using an electron-beam lithography system (Elionix ELS-7800, 100 pA). A 60 nm-thick Cr layer was evaporated as an etching mask, followed by lift-off. The pattern was transferred onto the a-Si:H layer by inductively coupled reactive-ion etching (ICP-RIE) using the Cr mask. Residual Cr was removed with an etchant solution (ETCR-400, APCT). Scanning electron microscopy (SEM) from Hitachi (Regulus 8100, 7 kV) and optical microscopy (Olympus MX63, 20× magnification) confirmed successful fabrication and structural integrity of the metalens. Optical microscopy images (Fig. 2(f)) and SEM images (Fig. 2(g–h)) confirm the structural fidelity and precision of the fabricated meta-atoms. The high-resolution SEM images clearly show the uniformity and accuracy achieved in the fabrication process, essential for realizing the targeted optical performance.

3　Results and discussion

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3.1　Characterization of the polarization-sensitive focusing properties of the metalens

To evaluate the focusing properties of the electrically-tunable polarization-sensitive metalens, experimental characterizations were performed under varying polarization conditions. Figure 3(a) illustrates the experimental setup, consisting of a 635 nm laser source whose beam was expanded via a custom beam expander. The polarization state of the incident beam was precisely controlled by employing a LC cell in combination with a linear polarizer (LP), as detailed in Fig. 3(b). Specifically, applying a voltage of 1.4 V to the LC cell resulted in LCP incident beam. Increasing the voltage to 1.9 V converted the incident polarization to RCP. A function generator (SIGLENT, Agilent 33220A) facilitated rapid voltage modulation, enabling quick switching between LCP and RCP states. The fully integrated system, wherein the metalens and LC cell were assembled onto a transparent substrate, is shown in Fig. S6 in Supplementary Section 7.

The LC modulator fabrication involved initially applying polyimide (Nissan Chemical, Korea) onto glass substrates. The polyimide was spin-coated at speeds of 1000 rpm for 10 s followed by 2500 rpm for 30 s, resulting in a measured thickness of approximately 182 nm. Subsequently, the coated plates were heated at 230 °C for 60 min and rubbed to achieve uniform liquid crystal alignment. Two glass plates were combined with a precise 10 µm spacer using UV glue (Norland Optical Adhesive 65). The resulting sandwich structure was then filled with 5CB (4-Cyano-4'-pentylbiphenyl) liquid crystals. The final LC cell was attached to the metalens substrate using UV adhesive and electrically connected to the function generator for effective voltage modulation.

The focusing performance was characterized using an optical imaging system composed of a 50× objective lens, tube lens (TL), quarter-wave plate (QWP), LP, and a CCD camera, mounted on a motorized z-axis translation stage to facilitate axial scanning. Detailed information of optical components and image-processing latency analysis for real-time feasibility is explained in Supplementary Section 8. Under LCP illumination (Fig. 3(c–f)), axial intensity distributions, generated by stacking xz-plane intensities at y = 0 (from z = 1.8 mm to z = 2.2 mm), showed the expected double-helix focusing behavior (Fig. 3(c)). To quantify spatial resolution, the point-spread function (PSF) was analyzed in both lateral and axial directions. The axial resolution, determined from line scanning along the z-axis through a single PSF, yielded a FWHM of approximately 74.4 µm (Fig. 3(d)). Focal-plane intensity profiles at three axial positions (z₁ = 1.92 mm, z₂ = 2.00 mm, z₃ = 2.08 mm) are presented in Fig. 3(e). Analysis of the lateral resolution, based on intensity profiles connecting two PSFs, indicated an FWHM of roughly 3.6 µm at position z₂ (Fig. 3(f)).

Under RCP illumination (Fig. 3(g–j)), the metalens displayed a significantly extended depth-of-focus. Axial intensity distributions from identical scanning conditions (from z = 1.8 mm to z = 2.2 mm) confirmed the extended focal region (Fig. 3(g)). Axial line scans through the PSF provided an axial resolution estimate with an FWHM of approximately 148.6 µm (Fig. 3(h)). Focal-plane intensity distributions at representative axial positions are shown in Fig. 3(i). The lateral resolution was evaluated at an orientation comparable to the LCP illumination condition, yielding an FWHM of about 2.5 µm at position z₂, as detailed by the line scan in Fig. 3(j). For a fair comparison, all lateral profiles in Fig. 3(f) and 3(j) are now normalized to the same reference, the peak intensity at z₂, recorded under identical illumination and exposure.

The experimental results confirm that the electrically-tunable polarization-sensitive metalens attains near diffraction-limited resolution along its optimized axes, closely aligning with theoretical predictions, while deliberately incorporating particular axial and lateral resolution attributes to enhance advanced depth-sensing and imaging capabilities. Simulated data corresponding to Fig. 3(c–j) are provided in Fig. S8 in Supplementary information of Section 9, demonstrating agreement between experimental and simulated results. The discrepancies between the simulation and experimental data are attributed to fabrication defects and measurement errors. The slight intensity imbalance between the two PSF lobes under LCP illumination observed at z₁ and z₃ is attributed mainly to fabrication deviations that round and occasionally merge the designed rectangular posts, with a smaller contribution from residual alignment error during axial scanning. A side-by-side comparison of PSFs from a sub-optimal and the best device, highlighting the impact of fabrication quality on lobe balance and rotational fidelity, is shown in Supplementary Section 10.

3.2　Depth imaging characterization using the electrically tunable metalens

To evaluate the depth-sensing capability of the metalens with extended focal characteristics, imaging experiments were conducted as illustrated in Fig. 4(a). The experimental setup consisted of a 4f imaging system, where a 10× objective lens and the designed metalens were employed. The axial displacement of the focal plane (Δz) relative to the metalens was precisely controlled using a translation stage to adjust the distance between the objective lens and the customized imaging target. Due to the magnification of the 10× objective lens, the image projected onto the metalens was reduced by a factor of ten compared to the original target dimensions. To validate the imaging performance accurately, we used a specially fabricated custom-designed target.

The method for extracting depth information using the electrically tunable dual-mode metalens is depicted in Fig. 4(b). Images captured under LCP illumination exhibit two distinct and rotated target shapes, whereas images obtained under RCP illumination yield a single image with superior spatial resolution. Due to intrinsic fabrication imperfections and PSF distortions, images acquired under LCP illumination required correction. This correction step involved retaining the original rotation angles derived from LCP mode images and applying shape correction using the higher-resolution images captured under RCP illumination. The subsequent image processing facilitated accurate separation and refinement of the target shapes.

Following image processing, rotation angles (

$ \beta $

) were systematically extracted. These rotation angles provided essential depth information (Δz) due to their direct correlation with axial displacement. By matching extracted rotation angles to known Δz values, precise depth estimations of the imaging targets were achieved.

Experimental results confirming this approach are demonstrated in Fig. 4(c). The X-shaped negative resolution target measures 136.7 µm in both length and width for evaluating depth imaging which are shown in Fig. 4(a). Three distinct depth positions (Δz₁ = 30.5 µm, Δz₂ = 0 µm, and Δz₃ = −48.3 µm) were tested. Initially overlapping images obtained from LCP illumination were successfully separated and processed to yield clear angle-extracted images. These angles were subsequently matched with corresponding depths. Finally, depth information extracted from these angles was color-mapped onto the high-resolution images obtained under RCP illumination. The true stage displacements were Δz₁ = 30.0 µm, Δz₂ = 0 µm, and Δz₃ = − 50 µm, yielding absolute errors of 0.5 µm, 0 µm, and 1.7 µm, respectively. The resulting mean absolute error is 0.73 µm, or roughly 1.5% of the 80 µm full measurement span, corresponding to an axial sensitivity of ~1 µm. The resultant images shown in Fig. 4(a) highlight the robustness of the imaging approach, demonstrating consistent image quality and clear depth differentiation even across varying focal depths.

3.3　Depth-resolved imaging of biological specimens

To demonstrate that the dual-mode metalens can recover depth information from real, complex specimens, we imaged three representative biological samples: a rubber-tree leaf, a transverse section of skeletal muscle, and a live planarian. All measurements employed the same optical setup described in Fig. 4(a), and only the specimen was exchanged between acquisitions. A 10× bright-field objective provided the reference image for each sample (left-hand column in Fig. 5; black scale bar = 250 µm), after which LCP and RCP mode images were captured using the LC-integrated metalens exactly as outlined in Fig. 4. For completeness, we note that differences in sharpness relative to the 10× objective arise from intrinsic NA/PSF and throughput considerations; a quantitative analysis with simulated focal-plane PSFs is provided in Supplementary Seciton 11. Detailed discussion for scalability of the double-helix mode at higher NA and through-focus simulation comparing the RCP extended-depth-of-focus design with a standard NA = 0.13 lens are also included in Supplementary Section 11.

For every field of view, we recorded a pair of raw metalens images: a double-helix LCP mode image and a high-definition RCP frame. After the LCP/RCP mode images separation step, the LCP mode image was split into two images and its shape was corrected with the corresponding RCP intensity map. After yielding the rotation angle β, which was converted to an axial offset Δz via the calibration curve established in Fig. 4(b). The resulting depth values (+42.1 µm, 0 µm, and −46.9 µm for the three examples) are reported alongside the extracted angles. Finally, each high-resolution RCP image was color-coded according to Δz using the hue scale shown at the right of Fig. 5 (white scale bar = 2 µm).

These results confirm that the metalens can recover depth over a ~90 µm range in heterogeneous, light-scattering tissues while preserving sub-cellular lateral detail. Because the LC modulator switches polarization in milliseconds, the same device can rapidly alternate between depth-encoded and conventional high-resolution imaging modes, making it attractive for live-cell microscopy, histological inspection, and compact endoscopic probes. Supplementary Section 7 describes the analysis of thermal robustness and tuning speed.

4　Conclusions

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In this study, we have successfully demonstrated an electrically tunable dual-mode metalens capable of generating polarization-sensitive focal patterns, effectively addressing the longstanding trade-off between depth-of-focus and imaging resolution. By precisely engineering phase profiles for LCP and RCP illumination, the metalens produces distinctive optical responses: a rotating double-helix PSF under LCP illumination and an extended DOF with a narrow PSF under RCP illumination. This dual functionality enables the metalens to simultaneously achieve wide depth sensing range and high lateral resolution, significantly enhancing its applicability in advanced imaging systems.

The meta-atom structures, fabricated from low-loss a-Si:H, were meticulously designed using RCWA, enabling precise manipulation of propagation and geometric phases. Experimental characterization validated near-diffraction-limited performance with close alignment to theoretical predictions. High-resolution SEM and optical microscopy imaging confirmed excellent fabrication fidelity.

Comprehensive experimental assessments demonstrated that the metalens provided robust imaging performance under varied polarization conditions, highlighting its capability for rapid polarization switching via an integrated LC modulator. The depth-sensing experiments further verified that the engineered focal profiles facilitated reliable extraction of depth information through the rotation angles of dual image patterns under LCP illumination, subsequently refined by high-resolution RCP imagery.

Overall, the electrically tunable dual-mode metalens represents a promising optical platform, merging depth-sensitive imaging and high-resolution capabilities into a compact, integrated device. Its proven performance on real bio-samples, combined with rapid electronic control, paves the way for applications in biomedical imaging, 3-D microscopy, optical sensing, and adaptive optics^32,45,46.

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Year 2026 volume 9 Issue 2

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doi: 10.29026/oea.2026.250216

Receive Date：2025-08-20
Online Date：2026-10-19
Published：2026-02-12

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Received：2025-08-20
Accepted：2025-12-02

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