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.

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 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.

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.

Active terahertz beam deflection and nonreciprocal spin chirality selection based on magneto-optical P-B metasurface with stacked-graphene layers.

Multifunctional, high-efficiency, and active manipulation devices are significant for terahertz (THz) technology and application. In this Letter, a stacked-graphene meta-atom (SGM) structure is investigated, which is composed of periodically patterned graphene in the 2D plane and stacked graphene-dielectric layers perpendicularly to the plane. This structure not only has strong THz artificial anisotropy but also enhances the cyclotron resonance response of graphene to a THz wave under an external magnetic field (EMF). Based on these two characteristics, the SGM can realize dynamic conversion between two functions for the manipulation of THz spin chiral states under different EMFs: from the reciprocal spin-flip without EMF to nonreciprocal spin-selection with EMF. Furthermore, a Pancharatnam-Berry (P-B) metasurface composed of the SGMs with different discrete orientation angles has been designed, which achieves active conversion between THz spin chiral beam deflection and the nonreciprocal one-way transmission for two conjugated spin beams, dynamically manipulated by both the biased voltage and EMF. The spin-select isolation is 42.3 dB with a transmission efficiency of over 70% at 1.38 THz. This manipulation mechanism of the spin beam and related devices has great potential in future THz communication, dynamical imaging, and radar scanning systems.

Silicon bowtie structure based adjustable nonrigid all-nonmetal metamaterial terahertz filter.

An all-nonmetal metamaterial (ANM) terahertz device with a silicon bowtie structure has been developed, which has comparable efficiency to that of its metallic counterparts, and better compatibility with modern semiconductor fabrication processes. Moreover, a highly tunable ANM with the same structure was successfully fabricated through integration with a flexible substrate, which demonstrated large tunability over a wide frequency range. Such a device can be used in terahertz systems for numerous applications, and is a promising substitute for conventional metal-based structures.

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.

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.

Magnetically active terahertz wavefront control and superchiral field in a magneto-optical Pancharatnam-Berry metasurface.

Nowadays, the manipulation of the chiral light field is highly desired to characterize chiral substances more effectively, since the chiral responses of most molecules are generally weak. Terahertz (THz) waves are related to the vibration-rotational energy levels of chiral molecules, so it is significant to actively control and enhance the chirality of THz field. Here, we propose a metal/magneto-optical (MO) hybrid Pancharatnam-Berry (PB) phase structure, which can serve as tunable broadband half-wave plate and control the conversion of THz chiral states with the highest efficiency of over 80%. Based on this active PB element, MO PB metasurfaces are proposed to manipulate THz chiral states as different behaviors: beam deflector and scanning, Bessel beam, and vortex beam. Due to the magnetic-tunablibity, these proposed MO PB metasurfaces can be turned from an "OFF" to "ON" state by changing the external magnetic field. We further investigate the near-field optical chirality and the chirality enhancement factors in far field of the chiral Bessel beam and vortex beam, achieving the superchiral field with the highest chiral enhancement factor of 40 for 0th Bessel beam. These active, high efficiency and broadband chiral PB metasurfaces have promising applications for manipulation the THz chiral light and chiroptical spectroscopic techniques.

Multi-band terahertz linear polarization converter based on carbon nanotube integrated metamaterial.

Herein, we fabricated and investigated the carbon nanotube (CNT) integrated metamaterial for orthogonal polarization control in the THz regime, which is composed of a sandwiched CNT layer with the adjacent metal gratings in the sub-wavelength integration. Under the mechanism of multilayer polarization selection and multiple reflections in CNT constructed micro-cavity, the perfect orthogonal polarization conversion is achieved and the transmittance spectrum presents multi-band peaks and valleys, which coincide with the theoretical Fabry-Perot resonance. Besides, by controlling the layer number and orientations of the middle CNT, the active modulation of the amplitude and phase in compound metamaterials are realized. Based on the simulation of CNT in the grating model, it obtains a good agreement with the experimental results, and the simulated electric field distribution also confirmed the inner polarization conversion mechanism. This work combines nanomaterials with optical microstructures and successfully applies them to the THz polarization control, which will bring new ideas for design novel THz devices.

Terahertz magneto-optical response of bismuth-gadolinium-substituted rare-earth garnet film.

We report the magneto-optical Faraday response of bismuth-gadolinium-substituted rare-earth iron garnet at terahertz frequencies ranging from 100 GHz to 1.2 THz. The maximum transmittance of ±45° component is about 60% near the frequency point of 0.63 THz. When the external magnetic field change from -100 mT to +100 mT, the Faraday rotation angle is between -6° and +7.5°. The overall change of ellipticity is relatively small. The maximum value of the Verdet constant is about 260 °/mm/T at 0.1 THz and then gradually decreases to 80 °/mm/T at 1.2 THz. Within the considered frequency range, the thick film exhibits magnetically tunable, non-reciprocal characters and a strong magneto-optical effect within a small external magnetic field at room temperature, which will be widely used for the terahertz isolators, circulators, nonreciprocal phase shifters, and magneto-optical modulators.

Terahertz ultrasensitive dual-core photonic crystal fiber microfluidic sensor for detecting high-absorption analytes.

In this paper, we propose an ultrasensitive microfluidic refractive index sensor for detecting high-absorption analyte. The sensor is based on heterogeneous dual-core photonic crystal fiber structure and operates in the terahertz (THz) regime. ZeonexE48R is chosen as the background material. The dual-core structure is composed of the gradient porous core and the microfluidic channel. Simulation results show that before infiltrating a liquid analyte, a highly stable modal birefringence can be obtained around 2.0×10-2 from 0.5 to 2 THz, along with the effective material losses of less than 0.182/cm for both the x- and y-polarization modes. After infiltration, this device can be employed for accurate refractive index (RI) sensing owing to the cross point selective coupling effect between the two core modes. This sensor offers RI sensitivities of 78.095 THz/RIU and 110.931 THz/RIU within dynamic measurement ranges from 1.41 to 1.429 for x-polarization mode and from 1.435 to 1.449 for y-polarization mode, respectively, and the measurable refractive index range reaches 0.0327. Our research gives a unique insight into the sensing mechanism for detecting high-absorption analyte in the THz band, which has broad application prospects for high-accuracy dynamic sensing in the fields of chemistry, biomedicine, and real-time environmental monitoring.

Intensity-tunable terahertz bandpass filters based on liquid crystal integrated metamaterials.

In this paper, we demonstrate an intensity-tunable THz bandpass filter by introducing liquid crystal (LC) integrated with asymmetric frequency selective surface (FSS) and subwavelength metal gratings. Here, the tunable THz filter is derived from the inner polarization state conversion in composited devices, and the incident linear polarization can be converted into 90° orthogonal components. By controlling the LC orientation under the applied electric field with the metamaterial electrodes, the polarization conversion process can be actively modulated; thus, the polarization-dependent and tunable THz bandpass filter is achieved. Based on the multilayer design and the inner Fabry-Perot-like resonance mechanism, the LC-integrated metamaterials filter presents better filtering performance than the single FSS filter, and the Q-value is improved from 7.7 to 13.8 at the working frequency. Our simulated work paves the way for the design of new and efficient THz filters.

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.

Terahertz magneto-optical effect of wafer-scale La: yttrium iron garnet single-crystal film with low loss and high permittivity.

The wafer-scale La:YIG single crystal thick films were fabricated on a three-inch gadolinium gallium garnet (GGG) substrate by liquid phase epitaxy method. The terahertz (THz) optical and magneto-optical properties of La:YIG film were demonstrated by THz time domain spectroscopy (THz-TDS). The results show that a high refractive index of approximately 4.09 and a low absorption coefficient of 10-50 cm-1 from 0.1 to 1.6 THz for this La:YIG film. Moreover, the THz Faraday rotation effect of La:YIG film was measured by the orthogonal polarization detection method in THz-TDS system, which can be actively manipulated by a weak longitudinal magnetic field of up to 0.155 T. With 5 samples stacked together, the Faraday rotation angle varies linearly from -15° to 15°, and the Verdet constant of La:YIG is about 100 °/mm/T within the saturation magnetization. This magneto-optical single crystal thick film with large area shows low loss, high permittivity and strong magneto-optical effect in the THz regime, which will be widely used in magneto-optical polarization conversion, nonreciprocal phase shifter and isolator for THz waves.

Temperature-dependent chirality of cholesteric liquid crystal for terahertz waves.

Recently, the terahertz (THz) chiral field control opens a new window to THz devices and their applications. In this Letter, the active manipulation for THz chiral states based on the cholesteric liquid crystal (CLC) has been demonstrated by THz time domain cross-polarization spectroscopy. The results show that the CLC has strong THz optical activity and circular dichroism (CD) effect, and the strongest THz CD of 22 dB and a polarization rotation angle of 88.4° occur around the phase transition temperature TS-N=250K. Rising to a room temperature of 300 K, the CLC turns from a chiral state to an isotropic state for THz waves with the phase transition processes of CLC molecules. Therefore, this CLC device can be performed as a thermally active THz circular polarizer, which brings potential applications in THz polarization imaging, broadband communication, and spectroscopy.

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.

Wideband sub-THz half-wave plate using 3D-printed low-index metagratings with superwavelength lattice.

High-index dielectric metasurfaces are rarely reported around 0.1-0.3 THz, as an extremely large etching depth is needed according to the millimeter-scale wavelength. In this work, we propose an easy solution to sub-THz wideband polarization control by utilizing 3D-printed low-index (n~1.5) metagratings. The metagrating with subwavelength lattice is shown as a very efficient half-wave plate (net polarization conversion of 87%) at 0.14 THz but showing noisy spectrum. The design with superwavelength lattice offers a smooth and wide bandwidth for linear polarization rotation. Study of the mechanism shows that the lattice size slightly above wavelength is a better choice for the low-index metadevice as it maintains high efficiency in the zero diffraction order and wide bandwidth due to the small mode dispersion. Such designs offer a feasible solution especially suitable for sub-THz polarization and phase control, complementary to the existing high-index dielectric and metallic metasurfaces.