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Dynamic manipulation of terahertz (THz) beams plays an important role in THz application systems. The PB metasurface provides an effective scheme for space separation and deflection of the spin beam. However, mirror symmetry locking of the conjugated spin states severely limits the versatility of the device. In this work, we demonstrate a liquid crystal (LC) cascaded bilayer metasurface that includes an LC layer, anisotropic metasurface, and PB metasurface. By controlling anisotropy and polarization conversion effects, dynamic spin asymmetric transmission is realized. Meanwhile, two different dynamic energy distribution processes are realized between the L and R state with the corresponding deflection side. The results show that the device achieves a large angular spatial dispersion within the frequency-angle scanning range of ±35° to ±75° corresponding to the broadband range of 0.6–1.1 THz. Moreover, it achieves a spin beam spatial separation with a maximum proportion of energy distribution greater than 26 dB, and the active modulation rate in the energy distribution process reaches 98 %. This work provides a dynamic THz spin conversion and efficient large-angle beam scanning, with important potentials in wavelength/polarization division multiplexing and frequency-scanning antenna for large-capacity THz wireless communication, radar, and imaging systems.

Nonlocal engineering, as an extension of the local design of the metasurface, provides a new scheme to precisely tune the electromagnetic response through diffraction of the constitutive supercells. It has shown great potential in large-gradient wavefront shaping, but just limited to one-dimensional or rotation-symmetric two-dimensional applications. In this study, we extend the nonlocal method to design large-gradient metasurfaces without rotation symmetry, through the optimization of the metagratings and their deformation along the isophase contour path. The effectiveness of the method is verified by the three-foci metalenses with NA of 0.89 and NA of 0.97. Consistent simulation and experimental results are obtained at 140 GHz. Obvious advances are observed in this method especially when the spatial phase gradient is increased. • A high-NA metalens with multiple foci is proposed based on the deformed nonlocal metagratings. • The nonlocal diffraction optimization of each supercell is done for TE and TM polarization. • The 1-D supercells are deformed accroding to the isophase profile. • Three-foci metalens with NA of 0.97 shows much better preformance using the nonlocal deformed mmethod. [ABSTRACT FROM AUTHOR]

Active broadband terahertz (THz) phase difference and its dispersion engineering based on the combination of dielectric metasurface and dual-frequency liquid crystal (DFLC) has been demonstrated, which can realize broadband achromatic wave plates and active manipulation by tuning the alternating (AC) frequency of electric field from 1 to 90 kHz. The artificial anisotropy of the dielectric metasurface is introduced to offset the tunable natural birefringence of the DFLC, so that the effective tuning range of the composite metadevice has been significantly improved, across from 0 to 1.0π, thus realizing the conversion from linear polarization (LP) light to arbitrarily polarized light. Moreover, a negative-dispersion phase difference of the dielectric metasurface can be achieved in the range of 0.69–1.4 THz by designing proper geometric parameters, which can be compensated with the positive-dispersion phase difference of DFLC to realize an achromatic device with broadband zero dispersion. The results show that the device can be tuned from OFF state without polarization conversion to a half-wave plate in 0.97–1.3 THz with the polarization conversion ratio (PCR) of over 97%, or to a quarter-wave plate in 0.67–1.3 THz with the PCR of over 96% by changing the AC frequency of the electric field. The active AC frequency tuning characteristics, anisotropic enhancement and broadband dispersion compensation show its great potential in THz application systems. [ABSTRACT FROM AUTHOR]

• Taking advantage of the large flexibility of dielectric metasurfaces in tailoring the spectral responses, the spatial modulation required for single-pixel imaging is changed from a time-resolved to a frequency-resolved manner. • Combined with the mature terahertz time-domain spectroscopy (THz-TDS) system, one-time scan of the optical delay line gives all the detected signals corresponding to different modulation patterns. • Complex modulation patterns containing both amplitude and phase information help to achieve high-fidelity image reconstruction through simple matrix operation. Single-pixel imaging is a very attractive technique at the terahertz band due to the lack of mature array detectors, over which the spatial light modulator is the most crucial component. Metasurfaces are usually introduced into the modulator to enhance the modulation depth of active materials. Here we take advantage of the flexible spectral response of pixelated metasurface resonators to encode different spatial masks into different frequency windows, without active materials and external stimuli. The measurements are obtained in parallel as opposed to in sequence in the time-domain spectroscopy system through a single terahertz signal. Images with 2 × 2 pixels as proof of the concept are reconstructed with high fidelity and plenty of choices of the frequency windows. As the measurement time does not increase with the number of pixels, this scheme can be generalized for imaging with higher resolution and increased number of pixels for practical applications in noninvasive inspection and detection. [ABSTRACT FROM AUTHOR]

• Inverse design of the metasurface with large geometry complexity is done by a convolution neural network given multiple target optical responses. • The metasurface unit cell is composed of 10 × 10 pixels for possible generation of diversified patterns to meet various optical requirements. • Metasurfaces for holographic imaging are validated at different frequencies and orthogonal polarization for multifunctional manipulation. Learning based on deep neural network brings new opportunities for intelligent design of metasurfaces as well as exploration of powerful optical functionalities. Inverse design of the metasurface geometry given the target optical response is a challenging ill-conditioned problem, which is usually solved by simplifying the input response and reducing the number of output geometry neurons. In this study, a convolutional neural network is employed to inverse design metasurface unit cells with nonintuitive generic patterns described by 100 binary pixels according to a group of target spectra including the real and imaginary transmission curves in TE and TM polarization. After careful tuning of the hyperparameters and comparison of the classification/regression output model, the network is used to design unit cells with user-defined phase delay at different frequencies and different polarization states. Two metasurfaces for holographic imaging at 0.8 THz and 1.2 THz are designed from the same network with the later one showing different images in orthogonal polarization states, which are validated by full-wave simulation. The large degree of freedom in the input and output of the neural network opens enormous searching space to meet various requirement for multifunctional wave manipulation. [ABSTRACT FROM AUTHOR]

Inverse design through topology and dimension optimization can significantly improve the performance of metasurfaces as compared to the traditional point-by-point designs. However, it is very challenging to optimize large-area 3D gradient metasurfaces due to heavy electromagnetic simulation and slow converging speed. In this work, we propose a quick and efficient design approach of 3D large-area high-NA metalenses by simple rotation and translation of the 2D cylindrical lens, where the later can be optimized using basic searching algorithms. The simulation and experimental results together show the effectiveness of this approach, which has great potential to be generalized for fast optimization of other types of wavefront shaping metasurfaces with spatial symmetry. • Quick and efficient design of 3D large-area high-NA metalenses from 2D metalenses. • Rotation and translation are two ways for 2D-to-3D extension. • 2D metalenses are optimized using gradient descent algorithms. • Experimental results using 3D printed samples validate the effectiveness of this approach at terahertz frequencies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The combination of graphene and a metasurface holds great promise for dynamic manipulation of the electromagnetic wave from low terahertz to mid-infrared. The optical response of graphene is significantly enhanced by the highly-localized fields in the meta-atoms, and the characteristics of meta-atoms can in turn be modulated in a large dynamic range through electrical doping of graphene. Graphene metasurfaces are initially focused on intensity modulation as modulators and tunable absorbers. In this paper, we review the recent progress of graphene metasurfaces for active control of the phase and the polarization. The related applications involve, but are not limited to lenses with tunable intensity or focal length, dynamic beam scanning, wave plates with tunable frequency, switchable polarizers, and real-time generation of an arbitrary polarization state, all by tuning the gate voltage of graphene. The review is concluded with a discussion of the existing challenges and the personal perspective of future directions. [ABSTRACT FROM AUTHOR]

Abstract Broadband terahertz (THz) phase shift engineering and zero-dispersion waveplates based on dielectric metasurface have been investigated, of which structure is a periodical rectangular scattering units on silicon substrates. By designing proper geometric parameters of metasurface structure, the value, dispersion and bandwidth of the phase shift curves can be effectively manipulated. Based on this, the broadband half waveplate (HWP) and quarter waveplate (QWP) have been designed and fabricated. The experimental results show that the HWP can work in the broad range of 0.7–1.35 THz with the polarization conversion ratio (PCR) of close to 100% and the transmission of over 70%. And the QWP can operate in the range of 0.7 ∼ 0.85THz with the PCR of over 90% and the transmission of over 70%. The method of phase shift engineering based on dielectric metasurfaces and these broadband zero-dispersion waveplates have great potential in promoting the performance of THz application systems. [ABSTRACT FROM AUTHOR]

A terahertz (THz) magneto-optical (MO) polarization rotator in reflective geometry was proposed based on metasurface structure. Combining MO material and microstructure brings new effects: (1) MO makes the classical structure of metasurface absorber becomes a polarization rotator; (2) Due to the resonance of the metasurface, Kerr rotation effect is significantly enhanced, of which rotation angle is over 90°and 5.3 times larger than the bare ferrite film. Moreover, the device can maintain an excellent performance when incident angle varies from 0 to 75 degrees. This work has broadly potentials for THz isolator, MO modulator, and polarization convertor in THz applications. [ABSTRACT FROM AUTHOR]