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Manipulating terahertz (THz) polarization in an efficient and broadband manner is of great significance to facilitating THz applications, including communications, imaging, defense, and homeland security. In this work, a dispersion compensation scheme is proposed for high‐efficiency and ultra‐broadband THz polarization manipulation using an anisotropic dielectric‐metal hybrid metadevice. The operating bandwidth is broadened by dispersion compensation, where the dielectric grating provides an artificial birefringence with a phase dispersion of positive slope and the metallic grating provides a phase dispersion of negative slope. Experimental results show that the device achieves two ultra‐broadband dispersion compensation, corresponding to the achromatic quarter‐wave plate in the lower frequency band (QWP: 0.5–2.0 THz, PCR >0.95) and the achromatic half‐wave plate in the higher frequency band (HWP: 1.0–2.1 THz, PCR >0.9), respectively. Theoretically, the bandwidth of QWP can be further improved to 2.6 THz by optimizing the grating dispersion, which brings huge application space to cover most of the THz radiation. This hybrid metadevice configuration offers a versatile platform for engineering electromagnetic waves, and the strategy of phase compensation can be generalized to extend the bandwidth of the metadevice in imaging and communications. [ABSTRACT FROM AUTHOR]

Broadband terahertz (THz) chirality and wavefront manipulation play important roles in wireless communication, imaging, and radar systems. In this work, by integrating liquid crystal (LC) into a dielectric metasurface, the gradient phase distribution from the Si metasurface and the tunable birefringent phase shift from LC are engineered to perform an active spin‐decoupled beam steering with the strong spin asymmetric transmission. By using geometric symmetry analysis, the relationships between the symmetry breaking and the optical chirality of the metadevice are revealed under different LC orientations, and both the simulations and experiments further verify the dependence of this chirality on spin‐decoupled beam steering with different spin states. The results show that with the orientation changes of the LC by driving the different magnetic fields, the circular dichroism of the device at the 0° deflection angle is ≈100% dynamically modulated and even flipped in the broadband range of 0.8–1.3 THz. With these changes in THz chirality, a tunable spin‐decoupled beam steering occurs in the range of 25°‐45°. This work shows that tunable LC anisotropy leads to more complex symmetry breaking in this structure, and results in more functions than conventional spin‐decoupled wavefront manipulations, which expands the scope of THz chiral metadevice and its applications. [ABSTRACT FROM AUTHOR]

Controlling terahertz (THz) waves, especially the polarization state and deflection angle, is important in THz application systems. But most of conventional THz metasurfaces have limitations in efficiency and working bandwidth. Although Pancharatnam-Berry (P-B) metasurfaces shows excellent manipulation for the circular polarized (CP) waves (or called photonic spin states) without phase dispersion, there is still challenge in active controlling. Here, we developed a magnetically tunable gyrotropic P-B metasurface by combining the gyroelectric semiconductor InSb and the P-B elements, which obtain two important effects: One is the active photonic spin Hall effect, which controls the conversion between two CP states and the deflection in space. A broadband working frequency of 1.02∼1.7 THz is obtained with the sweeping deflection angle from 36.6° to 83.5° and the highest efficiency of over 70%. The other is photonic spin filter effect, which reflects one CP state but absorb the other orthogonal CP state, realizing nonreciprocal absorption with the isolation of 24 dB. This active, high efficiency and broadband P-B metasurface opens a new way in manipulation of the THz wave propagation and polarization states in both space and frequency domains. [ABSTRACT FROM AUTHOR]

Coupled dielectric-metal gratings are investigated for broadband terahertz (THz) wave polarization conversion and asymmetric transmission by the experiments and numerical simulations, which are composed of the subwavelength Si grating and metallic wire grating layers. The dielectric grating layer with a large artificial birefringence and low dispersion is employed as a phase engineered waveplate, and the metal wire grating arranged with a 45° angle to the dielectric grating is utilized as a high-efficiency polarizer. Due to the subwavelength integration, this coupled grating presents a local resonance coupling mechanism between dielectric and metal gratings, which greatly enhances the polarization rotation and expands the bandwidth, not a simple combination with dielectric and metallic gratings. The results demonstrate that a broadband asymmetric transmission with an extinction ratio of 30dB from 0.2 to 1.2 THz is achieved and the highest transmission of 90% can be obtained. It provides a simple way towards practical applications for THz artificial dispersion materials, polarization control and asymmetric transmission. [ABSTRACT FROM AUTHOR]

An extraordinary optical transmission effect in a complementary metallic disk and hole array has been experimentally and numerically investigated in the terahertz (THz) regime, which shows a high transmission peak despite having zero effective aperture in the direction of light propagation. The results confirm that this effect is contributed not only by the surface plasmon excited on metallic disk and hole but also the resonance in the dielectric rod between them. Moreover, an optically pumped THz wave modulation with 40% amplitude modulation depth has been demonstrated in this device using a 532-nm laser with only 150 mW/mm2 power density. This novel plasmonic structure with high $Q$ transmission peak shows great promising applications in the THz modulation, filtering, and sensing. [ABSTRACT FROM PUBLISHER]

Terahertz (THz) silicon photonic column (PC) array has been designed and fabricated, and its resonance effect has been experimentally and numerically investigated. Using this resonance effect, the refractive index sensing for liquids has been demonstrated in this device by the theoretical model calculation, numerical simulation, and experimental measurement. Moreover, resonance dependence on different incident angles in this device has also been shown. These results confirm that this THz PC array can be used as a THz refractive index sensor with high sensitivity and robustness. This sensing strategy shows that the structured surface of this device is compatible for liquid monitoring and easy to couple and transmit THz waves, and therefore can be more easily implemented in liquid refractive index sensing. [ABSTRACT FROM PUBLISHER]

Abstract: The transmission and tuning properties of a cross-shaped plasmonic crystal based on periodic metal–semiconductor–metal (MSM) structures have been investigated in the terahertz (THz) regime. According to the mode analysis, we find that the different resonance modes in the plasmonic crystal show the different changes when this device is actively controlled by the carrier injection of the MSM structures. The longitudinal modes disappear, while the horizontal mode moves to a higher frequency. The former leads to an intensity modulation at 0.5THz and 1.1THz when the groove depth h =60μm, and the later leads to a band blue-shift from 1.325THz to 1.38THz. These results will be applied to THz modulation and tunable filtering. [Copyright &y& Elsevier]

Abstract: The gyromagnetic properties of ferrite materials and the nonreciprocal property of a silicon-ferrite photonic crystal cavity are investigated in the terahertz region. Through the structure optimization and analysis of defect mode coupling, we design a magnetically tunable circulator, of which central operating frequency can be tuned from 180 to 205GHz and the maximum isolation is 65.2dB. Moreover, the further study shows that the gyrotropy, dispersion, and ferromagnetic loss of ferrite materials under the different external magnetic fields greatly affect the transmission and isolation property of this device. This circulator is flexible to realize functions of controllable splitting, routing, filtering and isolation by changing the external magnetic field for the THz applications. [Copyright &y& Elsevier]

The dynamic tuning of terahertz (THz) electromagnetically induced transparency (EIT) offers significant potential in rapidly changing THz regulation fields, such as the next generation wireless communication, switching, and sensing. In this work, we demonstrate a THz EIT metamaterial by combining a circular and a rectangular split-ring resonator (CSRR and RSRR) based on the bright-bright mode coupling. The introduction of graphene into the EIT structure enables active tuning, and reveals two distinct regulation mechanisms in the metal-graphene hybrid EIT metamaterial: 1. the internal coupling effect diminishes with the increase of the Fermi level in the CSRR-graphene integration; 2. the surface electric field undergoes redistribution in the RSRR-graphene integration. These two dynamic regulation mechanisms are well confirmed by the simulated electric field distribution. As a result, the transmittance within the EIT window of the hybrid metamaterial can be precisely modulated, exhibiting a significant modulation depth of >85%. Notably, the integration with graphene in the RSRR allows for simultaneous modulation of resonances on either side of the EIT peaks, which expands more THz modulation channels. This work enriches the design and implementation methods for active THz EIT and shows potential applications in THz dynamic application scenarios. • A THz hybrid EIT metamaterial incorporates monolayer graphene is proposed. • This work achieves simultaneous modulation of the EIT-window and two adjacent bright modes. • The proposed structure has the advantages of active control, more modulation channels, and higher modulation depth. [ABSTRACT FROM AUTHOR]

Dual-frequency liquid crystals (DFLCs), the mixture of positive and negative liquid crystal (LC) molecules, exhibit unique alternating (AC) frequency dependent anisotropic properties. Carbon nanotube (CNT), as a novel nanomaterial with strong anisotropy, has attracted much attention in recent years. Herein, we investigate the tunable terahertz birefringence and phase shift characteristics of the DFLC doped with CNT (CNT-LC) by using the terahertz time-domain polarization spectroscopy. The results show that the CNT-LC (1.5 wt%) can reach 0.5π at 0.793 THz, which can be used as a tunable THz phase shifter that is 0.12π higher than the pure DFLCs. Furthermore, through measuring the output polarization state, it is confirmed that the dielectric anisotropy enhancement mechanism of CNT-LC originates from the surface interaction between CNTs and LC molecules. Therefore, a tunable quarter-wave plate with the active polarization conversion from linearly polarized (LP) to LP or LP to circularly polarized (CP) can be realized at 0.925 THz in the CNT-LC, while the pure DFLC cannot be achieved at the same frequency. The dielectric anisotropy enhancement of CNT-LC shows its utility in the improvement of various tunable terahertz LC phase shifter and wave plate. Image 1 [ABSTRACT FROM AUTHOR]

An active terahertz (THz) liquid crystal (LC) metamaterial has been experimentally investigated for THz wave modulation. Some interesting phenomena of resonance shifting, tunable electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) have been observed in the same device structure under different DC bias directions and different incident wave polarization directions by the THz time domain spectroscopy. Further theoretical studies indicate that these effects originate from interference and coupling between bright and dark mode components of elliptically polarized modes in the LC metamaterial, which are induced by the optical activity of LC alignment controllable by the electric field as well as the changes of LC refractive index. The LC layer is indeed a phase retarder and polarization converter that is controlled by the DC bias. The THz modulation depth of the analogs of EIT and EIA effects are 18.3 dB and 10.5 dB in their frequency band, respectively. Electrical control, large modulation depth and feasible integration of this LC device make it an ideal candidate for THz tunable filter, intensity modulator and spatial light modulator. [ABSTRACT FROM AUTHOR]

A phase transition photonic crystals (PhC) based silicon column arrays with VO 2 coating have been proposed and fabricated, which can be actively converted between PhC directional coupler and metallic PhC waveguide by the thermal means to form a switchable directional coupler. The bandgap and transmission properties of this device have been investigated by the numerical simulation. With the changes of temperature, this device exhibits three different modulation and coupling effects. The direct output port shows the two opposite intensity modulation processes, of which transmissions decrease from dielectric PhC coupling state to the loss state and then increase from the loss to the metallic PhC waveguide state; the coupling port realizes another intensity modulation from the coupling state to non-coupling state. This device can realize switching, modulation, actively splitting and routing functions in the future's THz applications. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we design a novel Y-junction power splitter which is optimized according to coupled mode theory. Two rods are added in the junction. One rod is a silicon rod and another one is a ferrite rod, the refractive index of which can be varied by adjusting the external magnetic field. So, it not only can split the power, but also controls the output energy of the two branches automatically. The transmission spectrum is calculated by finite difference time domain (FDTD) method, which shows that the output energy of the two branches is closely related to the frequency of the incident terahertz (THz) wave and the refractive index of the ferrite rod. The device is a good candidate for integrated optical circuit, the tuning rate of which is about a few microseconds. [Copyright &y& Elsevier]