Strong terahertz chiral manipulation plays a vital role in polarization imaging, chiral spectroscopy, and multichannel communications. Here, a composite structure based on the double-layer metasurface sandwiched with the ultrathin liquid crystal layer is proposed to realize a strong terahertz chiral manipulation. On account of breaking mirror symmetry in the propagation direction and the local field enhancement effect in the cavity, the composite structure possesses a strong terahertz chiral response. Moreover, this terahertz chiral response can achieve a large tuning range by changing the optical axis of the liquid crystal: the experimental co-polarization circular dichroism can achieve a large controllable range of −31dB to 30dB at 0.772 THz and a chirality control range from 11dB to −22dB at 0.759 THz. This composite device demonstrates rich characteristics for the feasible manipulation of THz chiral response, which has greatly promoted the development and practical application of THz polarization and chiral control devices.
Graphene films are characterized by high electrical conductivity and high optical transmittance. In this paper, we mixed graphene solution and polyvinyl alcohol (PVA) solution to make different concentrations of graphene solution. The electrical conductivity, optical transmittance, and phase shift of multilayer graphene films were measured in the THz range by coating graphene solution on the substrate. According to experimental results, the following conclusions are drawn: the film with a larger proportion of graphene has a smaller square resistance, that is, a better conductivity, a smaller transmittance, and a phase shift in the terahertz range under the same thickness. Subsequently, the solution made of 1:1.75 PVA and graphene solution was coated on the outside of the glass sheets containing E7 and HTD liquid crystals, respectively. The distance between the two glass sheets was 800um, which was the thickness of the liquid crystals. Then, a magnetic field is added along one side of the glass sheet (defined as the X direction). Through experiments, it is found that when the voltage is added to 100V (130V), the orientation of E7 (or HTD) liquid crystal can completely change the direction of the electric field. And in the case of the only magnetic field, E7 (or HTD) liquid crystal produces 0.5π phase difference corresponding frequency is 0.8THz (0.6THz), π phase difference corresponding frequency is 1.4THz (1.0THz).
Wideband metasurfaces usually have increased structure complexity as compared to their narrowband counterparts, by integrating multiple in-plane or cascaded resonant structures. We propose a dielectric grating structure as a quarter-wave plate with alleviated structure complexity and wide bandwidth at terahertz frequencies, by simply enlarging the period from subwavelength to superwavelength scale. From the modal theory, the superwavelength grating supports more than one waveguide modes. The superposition of the fundamental mode and higher-order mode with diverse dispersion characteristics is used to engineer the spectral response and to achieve a wideband linear-to-circular polarization conversion, which is validated by experimental study.
Phase and polarization are the basic parameters of electromagnetic wave, which can not only carry useful electromagnetic information, but also manipulate the propagation and states of light. With the development of terahertz (THz) technology and its application system, high-performance THz phase control devices are urgently needed. Our recent research work on THz phase control devices based on liquid crystals integrating with artificial microstructures or nanomaterials was reviewed in this paper. The proposed dielectric metasurface can realize a polarization-dependent electromagnetically induced transparency effect with a large artificial birefringence and a negative-dispersion phase difference with large artificial anisotropy. The double-layer graphene gratings can achieve a switchable function to switch between linear-to-linear and linear-to-circular polarization states. On this basis, we presented to combine the artificial microstructures with liquid crystals for controllable THz phase shifters and broadband wave plates. Moreover, by using the strong interaction between liquid crystals and the electrically or magnetically sensitive nanomaterials, we proposed a series of high-efficiency control THz phase devices. This work has greatly promoted the development and practical application of THz phase and polarization control devices.
Terahertz waves have a wide range of applications in the field of sensing because of their good coherence, high signal-noise ratio, low radiation energy, and non-ionization. In addition, THz sensing also has the advantages of real-time, non-contact, and label-free, so it has important application in biological sensing, especially in the sensing of active biological substances. But it also has some defects, such as low sensitivity, strong water absorption, limited detection information, and poor applicability. Herein, we propose a THz time-domain polarization spectroscopy (TDPS) sensing system. The metal metasurface structures are used as the sensor, and the transmission or reflection sensing method is used to detect the PVA thin film, tumor cell and amino acid solution respectively. The experimental results show that: compared with the traditional resonant sensing method, the quality (Q) factor and figure of merit (FoM) of polarization sensing method are improved by at least 4~5 times, so the sensing sensitivity is significantly improved. For tumor cell sensing, its minimum detection accuracy has reached 103 cells/ml. For amino acid solution sensing, the minimum detection accuracy is 10-5 g/ml magnitude. In addition, using the chiral metasurface structure as sensor, this method can also identify the difference between the chiral enantiomer solutions of amino acids.
As the developing of the Terahertz (THz) optics, the THz functional devices is highly desired. However, most conventional materials and devices are limited with the efficiency, bandwidth and tunability. Thus, the THz magneto-optical (MO) materials play an important role in the THz systems due to its unique gyrotropic material properties and the magnetic tunability. Here, we review the magneto-optical materials, especially the unique gyrotropic semiconductor |InSb, in THz band. Based on these MO materials, MO devices are introduced, including magneto-metasurfaces, magneto-plasmonics and magnetic photonic crystals. On the basis of the unique properties of InSb, we proposed several nonreciprocal THz devices. Due to the nonreciprocity and magneto-tunability, the acitve wavefront manipulation and nonreciprocal transmission are realized. These de- vices have strong potentials in morden terahertz application systems and provide effective way of designing terahertz functional devices.
Graphene has attracted widespread attention in dynamic optoelectronic devices due to its tunable electrical and optical properties. But different modulation capabilities of the graphene-based designs at different frequencies are less studied. We study the electrical tunability of transmissive metalenses based on graphene when working at three frequencies 0.3, 1.25, and 2 THz, respectively. The constitutive meta-atoms are composed of graphene patches and metallic gratings for efficient phase shift in the orthogonal polarization. Although the conductivity of graphene is tunable at all the frequencies, responses of meta-atoms show weak and strong dependence on the Fermi level at the low and high frequencies, respectively. Therefore, the focal length of the metalens is not electrically tunable at 0.3 THz. In contrast, the metalenses designed at 1.25 and 2 THz show electrically adjustable focal lengths, and the tuning range of the focal length increases with frequency. The research here provides clear guidance for the design of graphene metalenses with different electrical tunabilities for a variety of application scenarios.
A double-layer subwavelength dielectric grating has been designed, fabricated, and characterized for terahertz (THz) wave asymmetric transmission. The subwavelength grating resonance effect and the first-order diffraction effect of the two different grating structures lead to the different light paths of the forward and backward waves, and thus the one-way transmission can be realized. The transmission and resonance properties of this grating with different polarization and propagation directions have been demonstrated experimentally and theoretically, which show that this is a nonmagnetic one-way transmission device with an extinction ratio of over 15 dB and insertion loss of lower than 5 dB at 1.45 THz. We provide an effective way to nonmagnetic THz asymmetric transmission devices operating at room temperature without rotating or converting the polarization state of incident wave.
Our recent research work on artificial birefringence and broadband polarization converter in terahertz (THz) functional devices was reviewed in this paper, we proposed the subwavelength dielectric gradient grating structure with artificial high birefringence, broadband and low dispersion, and the dielectric metasurface with line-square compound lattice which can realize polarization dependent EIT effect with a large artificial birefringence effect. On the basis, we presented a compound metasurface and a coupled dielectric-metal grating for broadband THz wave polarization conversion and asymmetric transmission. Moreover, we introduced two-dimensional materials into THz polarization devices, and proposed a switchable quarter-wave plate based on graphene grating and a carbon nanotube attached subwavelength grating for broadband THz polarization conversion and dispersion control. This work has greatly promoted the development and practical application of THz polarization devices.
In this paper, we have reviewed our research on terahertz (THz) isolators and nonreciprocal transmission devices. In addition, we present a new kind of THz isolator based on asymmetry magneto-metasurface. A structured InSb layer coats on the silica substrate in which the numerical simulation shows that this metasurface has isolation over 60dB at 0.646 THz and a 10dB operating bandwidth of 13 GHz under an external magnetic field of 0.3T with an insertion loss less than 2.5dB. This kind of low-loss, high isolation, easy coupling THz magneto-metasurface isolator has broadly potentials for THz application systems. Importantly, we discuss and conclude the necessary conditions of forming THz nonreciprocal transmission in the magneto-material devices, which is strongly related with magneto-material and asymmetric transmission system.
A narrow-band filter with a broad tuning range is designed based on a magnetic field controlled ferrite defect in photonic crystal for a terahertz (THz) wave. The resonance defect modes of a ferrite defect in photonic crystal in the THz region are studied by using the finite difference time domain method. Detailed calculations on the shifts of the defect mode frequency and transmission properties reveal that the peak frequency of transmission spectra can continuously vary from 0.77 to 0.95 THz under the external magnetic field and the bandwidth of the filter is about 0.015 THz.
In this work, we present a terahertz (THz) isolator in metal parallel plate waveguide (PPWG). A magneto-optical film with 30μm thickness is coated on one side of the metal plate of PPWG with 100μm width, forming a metal- magnetoair- metal hybrid waveguide. Due to the non-reciprocity of magneto-optical medium and the asymmetry of the waveguide structure, this waveguide show a strong one-way transmission property. The numerical simulation shows that this THz isolator has a maximum isolation of 30dB and a 20dB operating bandwidth of 90GHz under a magnetic field of 0.3T, and its insertion loss is smaller than 0.5dB. Moreover, this operating frequency band can be widely tuned by changing the external magnetic field and temperature. This low-loss, high isolation, broadband tunable nonreciprocal THz waveguide has a great potential for THz application systems.
Our recent important progress in terahertz (THz) functional devices based on photonic crystals and plasmonics was reviewed in this paper, involving THz modulator, isolator and sensor. For THz modulators, we demonstrate the transmission and modulation properties of a state conversion photonic crystals and a metal-semiconductor-metal plasmonic waveguide based on theoretical and experimental investigations. We also show the nonreciprocal transmission and enhancement mechanism of the structured metal/magneto-optical plasmonic waveguide and plasmonic lens for THz isolator and circulator. Moreover, a real-time quantitative THz microfluidic sensing based on photonic crystal pillar array is introduced by experimental results. These THz functional subwavelength devices exhibit great promising potential in THz application systems.
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