A research team led by Associate Professor Longqing CONG from the Department of Electronic and Electrical Engineering at Southern University of Science and Technology (SUSTech) has reported an all-optically driven hybrid metasurface that can operate as either a spectral or a spatial light modulator in the terahertz (THz) regime, enabling THz computational spectral reconstruction and single-pixel imaging. The approach leverages optically pumped ultrafast relaxation dynamics to generate a series of low-correlation encoded transmission spectra that form a modulation/measurement matrix, and computational algorithms are used to reconstruct unknown spectra and images. The scheme is compatible with mature THz time-domain spectroscopy (THz-TDS) systems while requiring only intensity readout at the detection end, thereby avoiding mechanical delay lines and coherent-detection optics and offering a pathway toward compact, ultrafast, and integrable THz photonic devices. The work, titled “All-optical hybrid metasurfaces for ultrafast computational spectrometer and single-pixel imaging,” was published in Nature Communications.
Background
THz technologies have strong potential for chemical sensing, biomedical diagnostics, and security screening. However, existing THz spectroscopy and imaging platforms typically rely on (i) mechanical scanning (e.g., delay-line or grating scanning) to obtain temporal or spatial information and (ii) coherent detection to retrieve the electric-field amplitude and phase for high-precision spectral measurements. These requirements often lead to bulky systems, speed limitations imposed by mechanical motion or electronic bandwidth, stringent alignment and stability constraints, and difficulties in engineering integration, which hinder on-site deployment, portable testing, and on-chip integration. To address these bottlenecks, the study proposes a hybrid metasurface spatial light modulator based on bound states in the continuum (BIC) physics, achieving ultrafast all-optical control and enabling reconstruction from low-correlation intensity-only measurements.
Figure 1. Comparison between a conventional THz time-domain spectroscopy (THz-TDS) system and a computational spectrometer/single-pixel image (THz-CS/SPI).
Key Highlights
The hybrid metasurface unit cell combines a metallic resonator with a silicon epitaxial structure. Band folding brings multiple BIC modes into the same spectral window, and symmetry breaking converts BICs into quasi-BICs (q-BICs), producing a dense set of high-Q resonances across 0.30-0.55 THz. By engineering the silicon distribution, different q-BIC modes can be flexibly tuned, allowing transient conductivity changes in silicon to generate a series of low-correlation encoded spectra and enabling “multi-channel, low-correlation” measurement matrices for subsequent computational spectroscopy and single-pixel imaging.
Figure 2. Design concept of a broadband THz metasurface modulator.
Under optical pumping, the relaxation dynamics of the hybrid metasurface produce “nanosecond-scale, low-correlation” dynamic encoding. Optical-pump THz-probe (OPTP) measurements show that the relative THz-field change ΔE/E evolves with delay time τ: photo-carriers reach a peak within ~30 ps after pumping and then relax back toward the initial state over a ~1.9 ns timescale. Ten spectral frames are selected during relaxation to form a measurement matrix, and the spectral correlation is characterized via autocorrelation; the frequency resolution for spectral reconstruction (quantified by the autocorrelation FWHM) reaches ~0.024 THz.
Figure 3. Optical pump modulation experiments of the hybrid metasurface and spectral correlation characterization.
Figure 4 presents the conceptual framework of a THz computational spectrometer: optical pumping activates the metasurface to generate a time-sequenced multi-frame encoding matrix that modulates an unknown spectrum, while a single-pixel intensity detector records the corresponding intensity sequence; regularized reconstruction algorithms are then used to recover the spectrum. Reconstructions of unknown Gaussian spectra with different bandwidths reproduce spectral profiles well, achieving ~0.03 THz resolution. A dual-peak spectrum with ~0.05 THz peak separation can also be resolved, and reconstruction quality improves with a larger number of measurements.
Figure 4. Conceptual framework of a THz computational spectrometer based on all-optical modulation in a hybrid metasurface.
Figure 5 shows a conceptual demonstration of single-pixel imaging: the hybrid metasurface is scaled and assembled into a pixelated array. For a 3×3 pixel array operating at 0.45 THz, a pump beam simultaneously activates nine pixel elements; representative encoding frames are selected to build the measurement matrix. A single-pixel detector acquires the corresponding intensity vector, and L1-regularized reconstruction recovers binary patterns such as “T/H/Z.” Reconstruction error remains close to zero at compression ratios above 66.7%, and the method shows a degree of robustness to noise.
Figure 5. Concept demonstration of THz single-pixel imaging based on a pixelated hybrid metasurface.
Summary and Outlook
This work is expected to promote compact, ultrafast THz spectroscopy and imaging devices toward integration and practical applications, providing new system-level solutions for chemical detection, biomedicine, and security screening.
Zijian LIAO (PhD student, SUSTech) is the first author, and Longqing CONG is the sole corresponding author. Zhanqiang XUE, Junxing FAN, Guizhen XU, and Hongyang XING also made important contributions.
Proofread ByNoah Crockett, Yifei REN
Photo ByYan QIU
