Flexible terahertz phase shifter for optically controlled polydimethylsiloxane-vanadium dioxide composite film.

In the terahertz (THz) band, modulation research has become a focal point, with precise control of the phase shift of THz waves playing a pivotal role. In this study, we investigate the optical control of THz phase shift modulation in a polydimethylsiloxane (PDMS)-vanadium dioxide (VO2) flexible material using THz time-domain spectroscopy. Under the influence of an 808-nm continuous wave (CW) laser with power densities ranging from 0 to 2.74 W/cm2, the PDMS-VO2 flexible material exhibits significant phase shift modulation in the frequency range of 0.2 to 1.0 THz. The maximum optical-pumping phase shift reaches 0.27π rad at 1.0 THz in a composite material with a VO2 mass fraction of 5% and a thickness of 360 µm, and the amplitude transmittance from 0.2 THz to 1.0 THz exceeds 70%. Furthermore, the composite material exhibits good stability under at least 640 switching cycle times, as confirmed through repeatability tests. The proposed composite devices offer a new approach for more flexible phase shift modulation owing to the flexibility of the composite material and the non-contact and precise modulation of light control. Additionally, the stress-adjustable characteristics of flexible materials make them highly suitable for use in wearable THz modulators, highlighting their significant application potential.

Flexible broadband metamaterial absorber in long-wave infrared with visible transparency fabricated by laser direct writing.

Absorption of the long-wave infrared from human beings and the surroundings is a key step to infrared imaging and sensing. Here we demonstrate a flexible and transparent broadband infrared absorber using the photoresist-assisted metamaterials fabricated by one-step laser direct writing. The photoresist is patterned by the laser as an insulator layer as well as a mask to build the complementary bilayer metamaterials without lithography. The average absorptivity is 94.5% from 8 to 14 µm in experiment due to the broadband destructive interference of the reflected beam explained by the Fabry-Perot cavity model. The proposed absorber is applicable to various substrates with additional merits of polarization insensitivity and large angle tolerance, which offers a promising solution for thermal detection and management.

Polarization-independent bound state in the continuum without the help of rotational symmetry.

Recently, research about bound states in the continuum (BICs) has become more and more attractive. Nanostructures with rotational symmetry are usually utilized to realize polarization-independent quasi-BIC resonances. Here, we propose a new, to the best of our knowledge, scheme for a polarization-independent quasi-BIC without the help of rotational symmetry. With the rotation of the polarization direction of the incident light, a quasi-BIC resonance can be consistently observed in a dielectric cubic tetramer metasurface without rotational symmetry. Based on far-field multipolar decomposition and near-field electromagnetic distributions, it is found that different multipoles exhibit different dependences on the polarization direction, and the switch between electric and magnetic quadrupoles results in polarization-independent quasi-BIC resonance. Our findings provide an alternative scheme to design polarization-independent devices and promote wider potential applications.

THz enantiomers of drugs recognized by the polarization enhancement of gold nanoparticles on an asymmetric metasurface.

Chirality plays an important role in biological processes, and enantiomers often possess similar physical properties and different physiological functions. Thus, chiral detection of enantiomers has become a hot topic in recent years, and methods to enhance chiral molecular recognition are in urgent demand. In this work, a polarization detection method was used for different chiral drugs based on a specially designed metasurface composed of asymmetric double-opened rings and the surface enhancement effect of gold nanoparticles (GNPs). The experiment results show that the frequency shifts caused by the nearfield interaction of the metasurface and biomolecules have been significantly improved by GNPs, and both the limit of detection and detection precision of the metasurface can reach the 10-5 g ml-1 level. Moreover, the polarization sensing characterized by right circular polarization (RCP), the polarization elliptical angle (PEA), and the polarization rotation angle (PRA) shows that the enantiomers of three drugs can be distinguished, especially using the PEA spectrum; the maximum difference between enantiomers is over 30° with a precision of 6.6 × 10-7 g mL-1. Our THz polarization sensing and the GNP enhancement method inspire an efficient strategy for the highly sensitive detection of enantiomers.

Terahertz polarization and chirality modulation induced by asymmetry inversion combining chiral metasurface and liquid crystal anisotropy.

We experimentally demonstrate a dynamic terahertz (THz) chiral device based on a composite structure of anisotropic liquid crystals (LCs) sandwiched between a bilayer metasurface. The device supports the symmetric mode and antisymmetric mode under the incidence of left- and right-circular polarized waves, respectively. The different coupling strengths of the two modes reflect the chirality of the device, and the anisotropy of the LCs can change the coupling strength of the modes, which brings tunability to the chirality of the device. The experimental results show that the circular dichroism of the device can be dynamically controlled from 28 dB to -32 dB (i.e., inversion regulation) at approximately 0.47 THz and from -32 dB to 1 dB (i.e., switching regulation) at approximately 0.97 THz. Moreover, the polarization state of the output wave is also tunable. Such flexible and dynamic manipulation of THz chirality and polarization might build an alternative pathway for complex THz chirality control, high-sensitivity THz chirality detection, and THz chiral sensing.

Federated Semi-Supervised Learning for Medical Image Segmentation via Pseudo-Label Denoising.

Distributed big data and digital healthcare technologies have great potential to promote medical services, but challenges arise when it comes to learning predictive model from diverse and complex e-health datasets. Federated Learning (FL), as a collaborative machine learning technique, aims to address the challenges by learning a joint predictive model across multi-site clients, especially for distributed medical institutions or hospitals. However, most existing FL methods assume that clients possess fully labeled data for training, which is often not the case in e-health datasets due to high labeling costs or expertise requirement. Therefore, this work proposes a novel and feasible approach to learn a Federated Semi-Supervised Learning (FSSL) model from distributed medical image domains, where a federated pseudo-labeling strategy for unlabeled clients is developed based on the embedded knowledge learned from labeled clients. This greatly mitigates the annotation deficiency at unlabeled clients and leads to a cost-effective and efficient medical image analysis tool. We demonstrated the effectiveness of our method by achieving significant improvements compared to the state-of-the-art in both fundus image and prostate MRI segmentation tasks, resulting in the highest Dice scores of 89.23% and 91.95% respectively even with only a few labeled clients participating in model training. This reveals the superiority of our method for practical deployment, ultimately facilitating the wider use of FL in healthcare and leading to better patient outcomes.

Active terahertz beam deflection based on a phase gradient metasurface with liquid crystal-enhanced cavity mode conversion.

Active manipulation of terahertz (THz) beam deflection and intensity is highly desired for possible applications in wireless communication, radar, and remote sensing. Here, by integrating the phase-gradient metasurfaces and tunable liquid crystal materials, we demonstrate an active THz beam deflection device based on polarization mode conversion. The resonant modes in the photonic cavity formed by the double-layer metasurface and the tunable anisotropic liquid crystal material in the cavity not only improve the polarization conversion efficiency of the device, but also actively regulate the resonance matching conditions. As a consequence, a beam deflection of 47.5° with 50% diffraction intensity at 0.69 THz is achieved in the x-to-y polarization conversion mode, and this beam can be actively modulated with an ultrahigh modulation depth of 99.6% by rotating the anisotropic optical axis of liquid crystals. Moreover, the proposed device can also work as the deflection of 32.5° in the y-to-x polarization conversion mode at 0.94 THz with a maximum diffraction intensity of 38% and an intensity modulation depth of 97.8%. This work provides a new approach based on liquid crystal photonic devices for wavefront manipulation and active modulation for THz waves.

Chiral enantiomer recognition of amino acids enhanced by terahertz spin beam separation based on a Pancharatnam-Berry metasurface.

The highly sensitive detection and identification of chiral biochemical substances have attracted extensive attention. Terahertz (THz) spectroscopy and sensing technology have obvious advantages in non-contact and label-free biochemical detection, but the THz chiral spectral response of chiral biochemical substances is too weak to realize highly sensitive chiral enantiomer recognition. Herein, we propose a method of spin beam deflection and separation by using a Pancharatnam-Berry (PB) metasurface to enhance the THz chirality response of chiral amino acids, realizing the identification of chiral enantiomers of the same kind of amino acid. The conjugate spin transmittances and circular dichroism (CD) spectra of d- and l-tyrosine samples on the PB metasurface were measured by an angle-resolved THz time-domain polarization spectroscopy system, and their CD values reached 16.4° and -11.6° at a deflection angle of ±33°, respectively, which were enhanced by about 9.3 and 11.9 times compared with the maximum CD values of the sample without the metasurface. Therefore, this THz chiral sensing method based on a PB metasurface has great potential in highly sensitive chirality identification and enhancement for chiral substances.

Coaxial dual-beam wavefront shaping using nonlocal diffractive metasurfaces in terahertz frequencies.

Metasurfaces for wavefront shaping rely on local phase modulation in subwavelength unit cells, which show limited degree of freedom in dealing with complex and multiple beam transformation. Here, we assign multiple beams into different diffraction orders coaxially located along the same direction, whose wavefronts are tailored by optimizing the diffraction coefficients in two orders and two polarization states of a supercell. By evenly splitting the energy into two orders and adjusting the zeroth-order diffraction phase, a Bessel beam and a vortex beam are simultaneously generated in the near field and far field along a coaxial direction. The effectiveness of the method is validated by the excellent agreement between the simulation and experimental characterization of the two beams.

Terahertz polarization sensing for protein concentration and a crystallization process on a reflective metasurface.

Terahertz (THz) waves have attracted much attention in the field of biosensing due to advantages including non-destructiveness, being label-free, and high-sensitivity detection. Here we have experimentally demonstrated a THz polarization sensing method based on reflective metasurface sensors for detecting concentrations of protein solutions and their crystallization process. The protein with varying concentrations has been detected by five different polarization parameters, which show different spectral responses and sensing sensitivities. The sensing accuracy can reach the order of ng/mm2. Furthermore, the crystallization process of the protein sample from the dissolved state to the crystalline has been dynamically measured by polarization sensing, of which the highest sensitivity can reach 0.67 °/%. Therefore, this new sensing platform can have broad development prospects in the trace matter detection of the biological sample.

Recent Progress of Terahertz Spatial Light Modulators: Materials, Principles and Applications.

Terahertz (THz) technology offers unparalleled opportunities in a wide variety of applications, ranging from imaging and spectroscopy to communications and quality control, where lack of efficient modulation devices poses a major bottleneck. Spatial modulation allows for dynamically encoding various spatial information into the THz wavefront by electrical or optical control. It plays a key role in single-pixel imaging, beam scanning and wavefront shaping. Although mature techniques from the microwave and optical band are not readily applicable when scaled to the THz band, the rise of metasurfaces and the advance of new materials do inspire new possibilities. In this review, we summarize the recent progress of THz spatial light modulators from the perspective of functional materials and analyze their modulation principles, specifications, applications and possible challenges. We envision new advances of this technique in the near future to promote THz applications in different fields.

Terahertz tight-focused Bessel beam generation and point-to-point focusing based on nonlocal diffraction engineering.

Metasurfaces transform the wavefront by spatially varying the amplitude or phase of the incoming beam. Instead of encoding such variation by subwavelength unit cells, it is achievable over diffraction engineering of supercell structures, which outperforms the unit-cell method when the spatial gradient is large. In addition to tight focusing, here we apply this method to achieve plane wave-to-Bessel beam transformation and point-to-point focusing at terahertz frequencies. The Bessel beam has a small beam waist (0.57λ) and long depth of focus (9.1λ) for subwavelength-resolution imaging over a long distance. The point-to-point focusing changes the divergence angle from 16° to 70°. Both devices are validated by numerical simulations and experimental results with good agreement.

Directional control of the off-normal scattering from a single nanodisk by superposed linearly and radially polarized beams.

A scheme to dynamically control the off-axis directional scattering from a silicon nanodisk is proposed, which is based on focused fields formed by the coherent superposition of radially and linearly polarized beams. When the phase condition of the generalized Kerker conditions is satisfied at a specified wavelength, the amplitude requirement for the off-axis directional scattering along a required direction can be fulfilled by tuning the magnitude ratio of the two focused beams. Therefore, directional control of the off-axis scattering in the meridional plane is achieved without the manipulation of the working wavelength. Our findings provide new possibilities of future potential applications of all-dielectric nanoantennas.

Ultrathin freestanding terahertz vector beam generators with free phase modulation.

Simultaneous control of phase and polarization offers a large degree of freedom to tailor the beam properties, for instance, enabling generation of structured beams such as vector beams and vector vortex beams. Here, we propose an ultrathin freestanding metasurface operating at the terahertz frequency for efficient generation of vector vortex beam with an arbitrarily defined topological charge from linearly polarized excitation. The metasurface is composed of bilayer metallic patterns separated by a thin quartz slab, with one layer determining the transmission polarization and the other controlling the transmission phase. The tightly cascaded two layers form a Fabry-Perot cavity to maximize the efficiency of the polarization and phase control. Two metasurfaces for generation of radially polarized vector beam with uniform phase and vortex phase are fabricated and tested at 0.14 THz. The experimental results successfully demonstrate the generation of high-quality vector beams with the desired phase. In the experiment, the ultrathin and freestanding properties allow the metasurface to be easily combined with other components, which shows great potential for the development of various compact terahertz systems.

Sharper photonic nanojets generated by microspheres under higher-order radially polarized beam illumination.

Photonic nanojets (PNJs) generated from a single microsphere illuminated by higher-order radially polarized (RP) beams are investigated. The effects of the size parameters of higher-order RP beams, the refractive index, and radius of the dielectric microsphere on the full width at half-maximum and peak intensity of the PNJ are numerically discussed and qualitatively interpreted. The results show that the minimal width of the PNJ can be obtained by optimally adjusting the size parameter. The PNJ beam waist becomes gradually narrower with increasing the radial mode number. As compared to the case of plane wave illumination, sharper PNJs are more easily generated when irradiated by a higher-order RP beam, even for microspheres with lower refractive indices or larger radii. Our findings can promote potential applications of PNJs in a variety of fields including super-resolution microscopy, nanolithography, and optical data storage.

Enhanced Terahertz Amplification Based on Photo-Excited Graphene-Dielectric Hybrid Metasurface.

Graphene under optical pump has been shown to be an attractive gain medium with negative dynamic conductivity at terahertz frequencies. However, the amplification over a monolayer graphene is very weak due to its one-atom thickness. In this paper, the proposed graphene-dielectric reflective metasurface effectively improved terahertz field localization and enhanced coherent amplification. The amplification coefficient of 35 was obtained at 3.38 THz at room temperature with an infrared pump intensity of 8 W/mm2. As pump intensity increased from 0 to 15 W/mm2, we observed a loss-gain-loss transition process, which was discussed in detail through coupled-mode theory. In addition, amplification at different frequencies was achieved by merely re-optimizing the geometric parameters of the dielectric resonators. This study offers an effective solution for enhancing terahertz radiation and developing terahertz lasers.

Terahertz birefringence anisotropy and relaxation effects in polymer-dispersed liquid crystal doped with gold nanoparticles.

Terahertz (THz) birefringence anisotropy of the polymer-dispersed liquid crystal (PDLC) doped with gold nanoparticles (Au NPs) is investigated by using terahertz time domain polarization spectroscopy. Controlled by the electric field, the change rate of refractive index for PDLC doped with Au NPs is 0.91% V-1 as the voltage increases, smaller than the pure PDLC, which indicates that the response of the PDLC doped with Au NPs to electric field is more uniform than that of pure PDLC. Therefore, the PDLC doped with Au NPs is more suitable for tunable phase shifters. Furthermore, we found that under the high-frequency alternating electric field, the anisotropic polarization effect of PDLC will disappear to this electric field, namely polarization relaxation phenomenon. However, the results show that the PDLC doped with Au NPs can respond to an electric field with higher alternating frequencies, and the relaxation frequency of PDLC with an Au NPs concentration of 0.2 wt% was improved over two times compared with the pure PDLC and four times higher than that of the precursor mixture without ultraviolet radiation. This work has the significance for the potential applications of tunable THz liquid crystal phase and polarization devices, providing a more uniform and faster relaxation response to the operating electric field.

Widely tunable polarization conversion in low-doped graphene-dielectric metasurfaces based on phase compensation.

We propose a tri-band half-wave plate in the reflection mode, composed of rectangular silicon bar arrays on a 10-layer graphene substrate. By merely varying the Fermi energy of graphene from 0 to 0.25 eV, the three frequency bands shift in step and merge to a continuous dynamic bandwidth from 0.88 to 1.81 terahertz (THz). In addition, it can also dynamically switch the reflected wave among cross-linear polarization, right-handed and left-handed circular polarization in 0.93-1.35 THz. We found that the large dynamic bandwidth originates from the tunable reflection phase from the graphene layers. As it no longer depends on the plasmonic resonance in graphene, the proposed hybrid metasurface offers an alternative solution for active THz polarization devices with low biasing voltages.

Nonreciprocal terahertz beam steering based on magneto-optic metagratings.

In this work, an active nonreciprocal THz beam steering has been proposed based on a transversely magnetized metal/InSb metagrating. The nonreciprocal dispersion relation and phase shift characteristics of the metal/InSb waveguide are investigated in details. A metagrating structure with gradient phase shift has been designed based on the metal/InSb waveguide. Under the external magnetic field (EMF), the THz beam can be changed among 0, +1st, and -1st order of the metagrating. Due to the nonreciprocity of the metal/InSb metagrating, the deflection angle can be controlled by changing the positive and negative directions of the EMF, to realize bilateral symmetric scanning from -67.8° to 67.8° with over 70% diffraction efficiency, and this device also exhibits the nonreciprocal one-way transmission as an isolator with the isolation of 13 dB. This low-loss, large deflection degree, nonreciprocal beam scanner has a great potential application in the THz regime.

Bi-functional polarization conversion in hybrid graphene-dielectric metasurfaces.

In this Letter, we propose a hybrid graphene-dielectric metasurface as a bi-functional polarization converter. It can switch between a reflective half-wave plate and a quarter-wave plate around 1 THz by merely applying external biasing voltage, without reoptimizing the dielectric structure. Switching of the two wave plates originates from distinct dispersion of the orthogonal eigenmodes with the chemical potential, which is further explained by the overlapping of graphene and the dielectric resonance modes. Compared with graphene-metallic metasurfaces, a combination of graphene with dielectric microstructures offers an alternative solution for active terahertz devices with high efficiency and large flexibility.