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Extending the optical control capabilities of metasurfaces for broader functionalities has recently attracted extensive attention. Simultaneously achieving amplitude and phase control is an effective route as it allows rebuilding the full information of the field. However, related previous studies mostly rely on anisotropic meta-atoms, which restrict the available incident polarizations. Here, a polarization-insensitive amplitude and phase control method is proposed and experimentally demonstrated in the terahertz regime, which is actualized by introducing interference effect in reflective-type meta-molecules composed of isotropic meta-atoms. Two kinds of functional meta-mirror devices, i.e., multi-order meta-gratings and multi-focal meta-lenses, are designed and characterized, where the results verify this method very well. This proposed method further enriches the routes to control amplitude and phase and may also find broad applications in realizing flexible wavefront control devices with complex functionalities.

As one kind of special beams, a terahertz (THz) cylindrical vector Bessel beam processes centrosymmetric polarization distribution and nondiffractive propagation properties. Such a vector Bessel beam is potential in various practical applications ranging from THz communication and electron acceleration to sensing and imaging. Here, we propose and experimentally demonstrate a method to directly emit a broadband THz cylindrical vector Bessel (CVB) beam using nonlinear effects. By photo-exciting an indium tin oxide film coated axicon with a circularly polarized femtosecond laser pulse, a high-quality CVB beam is obtained in a broadband THz frequency range. The proposed method is universal and low-cost, opening a simple avenue for the emission of broadband THz specialty beams.

Terahertz (THz) single-pixel near-field imaging, producing images by introducing a scene with a series of spatially resolved subwavelength patterns while recording the correlated intensity on a single-element detector, is a promising technology for imaging applications. Spatial light modulator (SLM) is one of the key devices in THz single-pixel near-field imaging. Combined with a Digital Micromirror Device to shape the optical pump beam, a photo-induced tunable high-efficiency, ultra-thin, and fast THz SLM is presented by integrating designed subwavelength metallic grating into silicon on sapphire. In the experiments, the SLM is demonstrated with an over 60% THz peak amplitude modulation depth and broad bandwidth under a relative low pump fluence (80 μJ/cm2). The measurements, simulations, and calculations agree well with each other. Meanwhile, sub-nanosecond decay time extracted by fitting the |ΔT| curve suggests a fast response device. A two-dimensional grating is also explored, and the polarization-independent feature makes it easier to use. Finally, an imaging demonstration is conducted to verify the usability of the designed device. These results demonstrate the feasibility of realizing super-resolution and even real-time imaging simultaneously.

We present a transmissive all-dielectric terahertz (THz) metamaterial half-wave plate with a double-working-layer structure. One layer works as a half-wave plate to enable polarization conversion of the incident THz wave, and the other layer functions as an antireflection layer to improve the transmission. The device is made of pure silicon only and can realize a high-performance polarization conversion at the designed THz frequency. Numerical simulations have been performed to show how the polarization properties of the THz wave can be adjusted by the structural parameters of the metamaterial. With appropriate structural parameters, the transmission for cross-polarization can reach 90%, and the polarization conversion rate can reach almost 100% at the designed operation frequency of 1 THz in simulation. Several samples have been fabricated and characterized, and the experimental results show a cross-polarized transmission of about 80% and a polarization conversion rate of almost 100% and agree well with the simulations.

We present experimental results of actively controlled anisotropic near-field coupling in a metamaterial composed of two concentric in-plane split ring resonators and a closed-ring resonator functioning at terahertz (THz) frequencies. Transparency windows are observed due to the coupling effect between two dipole-like modes for both polarizations of the THz field, but at different frequencies. By properly incorporating silicon islands into the metamaterial unit cells, the transparency windows can be optically controlled with a modulation depth reaching 68% and 62% for horizontal and vertical polarizations, respectively. The results are of value for designing active THz anisotropic devices.

The periodic-metamaterial-based perfect absorber has been studied broadly. Conversely, if the unit cell in the metamaterial-based absorber is arranged aperiodically (aperiodic-metamaterial-based absorber), how does it perform? Inspired by this, here we present a systematic study of the aperiodic-metamaterial-based absorber. By investigating the response of metamaterial absorbers based on periodic, Fibonacci, Thue-Morse, and quasicrystal lattices, we found that aperiodic-metamaterial-based absorbers could display similar absorption behaviors as the periodic one in one hand. However, their absorption behaviors show different tendency depending on the thicknesses of the spacer. Further studies on the angle and polarization dependence of the absorption behavior are also presented.

Seeking effective terahertz functional devices has always aroused extensive attention. Of particular interest is the terahertz beam splitter. Here, we have proposed, designed, manufactured, and tested a broadband non-polarizing terahertz beam splitter with a variable split ratio based on an all-dielectric metasurface. The metasurface was created by patterning a dielectric surface of the N-step phase gradient and etching to a few hundred micrometers. The conversion efficiency as high as 81% under the normal incidence at 0.7 THz was achieved. Meanwhile, such a splitter works well over a broad frequency range. The split ratio of the proposed design can be continuously tuned by simply shifting the metasurface, and the angle of emergences can also be easily adjusted by choosing the step of phase gradients. The proposed design is non-polarizing, and its performance is kept under different polarizations.

We experimentally demonstrate a dynamic process of effective Parity-Time (PT)-symmetry breaking through eigen-polarization variation in a hybridized metasurface. The metasurface consists of two coupled resonators made from metal and type II superconductor niobium nitride, respectively. By varying the temperature from 2 to 13 K, the hybridized system is able to pass through the effective PT-symmetry phase transition point, providing a feasible way to investigate the dynamic PT-symmetry transition.

Controlling the propagation of surface plasmon polaritons is essential in developing highly integrated photonic devices. By using near-field scanning terahertz microscopy, we experimentally demonstrate that polarization-controlled tunable surface plasmons (SPs) could be directionally excited on a metal surface with carved columns of aperture resonators under special arrangement. The experimental results reveal that terahertz SPs could be unidirectionally launched in opposite directions owning to destructive and constructive interferences on the two sides with circularly polarized incident waves of opposite handedness. Meanwhile, the linearly polarized wave is able to excite the terahertz SPs along either side of the structures. The presented results would be useful to implement functional terahertz plasmonic devices.

Using the terahertz time-domain spectroscopy, we demonstrate the spin reorientation of a canted antiferromagnetic YFeO3 single crystal, by evaluating the temperature and magnetic field dependence of resonant frequency and amplitude for the quasi-ferromagnetic (FM) and quasi-antiferromagnetic modes (AFM), a deeper insight into the dynamics of spin reorientation in rare-earth orthoferrites is established. Due to the absence of 4f-electrons in Y ion, the spin reorientation of Fe sublattices can only be induced by the applied magnetic field, rather than temperature. In agreement with the theoretical predication, the frequency of FM mode decreases with magnetic field. In addition, an obvious step of spin reorientation phase transition occurs with a relatively large applied magnetic field of 4 T. By comparison with the family members of RFeO3 (R = Y3+ or rare-earth ions), our results suggest that the chosen of R would tailor the dynamical rotation properties of Fe ions, leading to the designable spin switching ...

Polarization conveys valuable information for electromagnetic signal processing exhibiting tremendous potential in developing application driven photonic devices. Manipulation of polarization state of an electromagnetic wave has drawn a lot of research interests in many different fields, especially in the terahertz regime. Here, we propose a unique approach to efficiently rotate the linear polarization of terahertz wave in a broadband configuration with tri-layer metasurfaces. We experimentally observe a nearly perfect orthogonal polarization conversion with an ultrahigh efficiency, demonstrating a ultrathin terahetz rotator. The Fabry-Perot cavity effect in the tri-layer metasurfaces is attributed to the underlying mechanism of high transmittance and polarization rotation.

We demonstrate the design, characterization, and interference-theory interpretation of a terahertz triple-band metamaterial absorber (MA). The experiments show that the fabricated MA has three distinctive absorption peaks at 0.5, 1.03, and 1.71 THz with absorption rates of 96.4%, 96.3%, and 96.7%, respectively. We use the multi-reflection interference theory to investigate the physical insight of the proposed triple-band terahertz MA, which provides a design guideline for MA of such type. The theoretical predictions of the interference model have excellent agreements with experimental results. The designed multiband absorber is easy to manufacture and insensitive to incident polarizations with high absorption, which is favorable for various applications.

We observe the excitation and tuning of electromagnetically induced transparency (EIT) by the interference between different excitation pathways of the dark mode in a planar terahertz metamaterial. The EIT unit cell consists of a cut wire as the bright resonator and a pair of split ring resonators (SRRs) as the dark element. The dark mode resonance is excited by both the electric and magnetic fields when the SRR pair translates along the wire without altering the lateral distance between the resonators. The electric and magnetic pathways of exciting the dark mode allows for a giant amplitude modulation of the EIT resonance.

We investigate resonant transmission of planar asymmetric metamaterials made from double split-ring resonators. As the symmetry of the unit cell resonator is broken by displacing the two gaps away from the center in opposite directions, a giant amplitude modulation is observed at the fundamental inductive-capacitive resonance due to strong polarization conversion. The modulation is nearly absent when the gaps are moved together in the same direction. This effect persists in metamaterials with different structural designs. These asymmetric metamaterials may open up new avenues toward the control of terahertz waves and the development of modulator and polarizer based terahertz devices.

We characterize the behavior of split ring resonators made up of high transition temperature yttrium barium copper oxide superconductor using terahertz time-domain spectroscopy measurements and numerical simulations. The superconductor metamaterial is found to show a remarkable change in the transmission spectra at the fundamental inductive-capacitive resonance as the temperature dips below the critical transition temperature. This resonance switching effect is normally absent in traditional metamaterials made up of regular metals. The temperature-dependent resonance behavior of the superconducting metamaterial would lead to development of low loss terahertz switches at cryogenic temperatures.

We report the coupling between surface plasmons and dipole-localized surface plasmons in a composite hole-patch structure at terahertz frequencies. The coupling is found to be changed by increasing the inner patch size, which causes prominent resonance frequency shift in the enhanced transmission. The experimental results show good agreement with numerical simulation. The study clarifies well the nature of coupling between surface plasmons and dipole-localized surface plasmons and is thus of help to identify the role of surface plasmons in the enhanced transmission observed in subwavelength metallic structures.

The resonance of split ring resonators (SRRs) is known to shift upon the addition of a dielectric overlayer, a feature useful for practical applications. Here, we demonstrate that the frequency shift is enlarged by increasing the SRR height, thereby potentially enhancing sensitivity and tunability. We fabricated SRRs resonating at terahertz frequencies using a focused proton beam. This resulted in SRRs nearly 10 μm high, with smooth and vertical sidewalls. Terahertz time domain spectroscopy was used for characterization. Upon applying a dielectric overlayer (ϵ=2.7), a resonance located at 640 GHz shifted by nearly 120 GHz. Simulations also indicate a widening frequency shift as SRR height increases.

We present reflection properties of terahertz pulses in periodic arrays of metallic rectangles of subwavelength dimensions. Resonant reflection is characterized by terahertz time-domain spectroscopy and is attributed to the contributions from the dipole localized surface plasmons of isolated metallic rectangles, their interactions between the rectangles, and the nonresonant direct reflection. A number of factors, including the polarization of the terahertz electric field, the shape of the metal structures, the dielectric function of the substrate, and the lattice constant of the arrays are found to influence terahertz reflection properties. The measured resonant reflectance is well fit by numerical simulations.

The influence of a dielectric thin film on resonant transmission of terahertz pulses in a plasmonic array of subwavelength holes of asymmetric dielectric-metal interfaces is presented. A giant tuning of up to 0.80 THz at the surface plasmon metal-air [±1,0] mode at 1.95 THz and a further enhanced transmission at the metal-Si [±1,0] mode at 0.5 THz are achieved by modifying the film thickness. The experimental results are characterized by numerical simulation based on finite element method and angle-dependent measurements. The sensitive nature in the plasmonic hole array of asymmetric interfaces is promising in applications of biochemical sensing and tunable integrated plasmonic devices.

Transmission spectra of terahertz pulses through periodic array of subwavelength holes exhibit a characteristic evolution with various hole widths. The peak absolute transmittance approaches a maximum value at a critical hole width, while linewidth broadening and blueshift of peak frequency are observed with increasing hole width. Such characteristic evolution is attributed to the coupling between discrete resonant excitation of surface plasmons and continuum nonresonant transmission through the holes; this agrees well with the numerical analysis based on the Fano model and the measured angle-resolved transmission band structures.

We report far-infrared dielectric properties of powder form ferroelectric SrTiO3. Terahertz time-domain spectroscopy (THz-TDS) measurement reveals that the low-frequency dielectric response of SrTiO3 is a consequence of the lowest transverse optical (TO) soft mode TO1 at 2.70 THz (90.0 1/cm), which is directly verified by Raman spectroscopy. This result provides a better understanding of the relation of low-frequency dielectric function with the optical phonon soft mode for ferroelectric materials. Combining THz-TDS with Raman spectra, the overall low-frequency optical phonon response of SrTiO3 is presented in an extended spectral range from 6.7 1/cm to 1000.0 1/cm., Comment: 14 pages; 4 figures

The low-frequency optical and dielectric properties of ZnO tetrapod structures prepared by thermophysical method were studied by terahertz time-domain spectroscopy. The power absorption, refractive index, and the complex dielectric function were measured in the frequency range from 0.2to3.5THz. Based on a simple effective medium theory, the low-frequency dielectric properties of ZnO tetrapods were found to be associated with the transverse optical E1 phonon mode, which is consistent with that observed in bulk single-crystal ZnO.

resistivity single-crystal ZnO. 7 ZnO is considered as a promising material for terahertz optics because of a number of advantages: Ease in fabrication, wide band gap, rather high mobility and resistivity, and transparency in a broad frequency range. The experimental result indicated that the high breakdown electric field enables ZnO to be an intriguing semiconductor in high-power terahertz generation. It is essential to explore the detailed optical and dielectric properties of ZnO in the broad terahertz frequency region. In this letter, the far-infrared optical properties and the complex dielectric function of high-resistivity single-crystal ZnO are experimentally characterized by use of terahertz time-domain spectroscopy THz-TDS. The measured refractive index, power absorption, and complex dielectric function are compared with the pseudo-harmonic model. In the scope of our frequency range from 0.3 to 3.5 THz, the experimental data agree well with theoretical fitting. The measured power absorption, which increased approximately quadratically with frequency, was found to be dominated by the transverse optical TO E1 phonon at frequency TO/2 12.42 THz. The ZnO sample is an undoped 5 mm 5m m0.5mm-thick freestanding single crystal obtained from MTI Corporation, USA. The both-side polished sample has crystal orientation 0001 with c-axis perpendicular to the surface. Hydrothermal growth method was used to fabricate the wurtzite high-resistivity ZnO with purity higher than 99.99%. The ZnO crystal was characterized with a photoconductive switch-based THz-TDS system which has an 8-F configuration and an useable bandwidth extending from

The far-infrared terahertz(THz) spectra of ZrC, ZrN, ZrO2, and ZrF4 have been measured using terahertz time-domain spectroscopy. The characterization of the frequency-dependent power absorption, index of refraction and complex dielectric function in the frequency range 0.1–2.0THz are obtained. A standard Drude-Lorentz model gives a good fit on the transmission measurement results. We first observe the shift in the low-frequency vibrational spectra of different zirconium compounds. We analyze that the influence of the anion on the vibration modes causes the shift in low-frequency vibrational spectra of zirconic compounds.