Highly boosted trace-amount terahertz vibrational absorption spectroscopy based on defect one-dimensional photonic crystal.

Although terahertz (THz) spectroscopy demonstrates great application prospects in the fields of fingerprint sensing and detection, traditional sensing schemes face unavoidable limitations in the analysis of trace-amount samples. In this Letter, a novel, to the best of our knowledge, absorption spectroscopy enhancement strategy based on a defect one-dimensional photonic crystal (1D-PC) structure is proposed to achieve strong wideband terahertz wave-matter interactions for trace-amount samples. Based on the Fabry-Pérot resonance effect, the local electric field in a thin-film sample can be boosted by changing the length of the photonic crystal defect cavity, so that the wideband signal of the sample fingerprint can be greatly enhanced. This method exhibits a great absorption enhancement factor, of about 55 times, in a wide terahertz frequency range, facilitating the identification of different samples, such as thin α-lactose films. The investigation reported in this Letter provides a new research idea for enhancing the wide terahertz absorption spectroscopy of trace samples.

Vanadium dioxide-assisted switchable multifunctional metamaterial structure.

A multifunctional design based on vanadium dioxide (VO2) metamaterial structure is proposed. Broadband absorption, linear-to-linear (LTL) polarization conversion, linear-to-circular (LTC) polarization conversion, and total reflection can be achieved based on the insulator-to-metal transition (IMT) of VO2. When the VO2 is in the metallic state, the multifunctional structure can be used as a broadband absorber. The results show that the absorption rate exceeds 90% in the frequency band of 2.17 - 4.94 THz, and the bandwidth ratio is 77.8%. When VO2 is in the insulator state, for the incident terahertz waves with a polarization angle of 45°, the structure works as a polarization converter. In this case, LTC polarization conversion can be obtained in the frequency band of 0.1 - 3.5 THz, and LTL polarization conversion also can be obtained in the frequency band of 3.5 - 6 THz, especially in the 3.755 - 4.856 THz band that the polarization conversion rate is over 90%. For the incident terahertz waves with a polarization angle of 0°, the metamaterial structure can be used as a total reflector. Additionally, impacts of geometrical parameters, incidence angle and polarization angle on the operating characteristics have also been investigated. The designed switchable multifunctional metasurfaces are promising for a wide range of applications in advanced terahertz research and smart applications.

Realization of absorption, filtering, and sensing in a single metamaterial structure combined with functional materials.

In this paper, a hybrid vanadium dioxide (VO2)-graphene-based bifunctional metamaterial is proposed. The realization of the different functions of perfect transmission and high absorption is based on the insulator-metal phase transition of VO2 material. The Fermi energy level of graphene can be treated to dynamically tune the absorption and transmission rates of the metamaterial structure. As a result, when VO2 is in the insulating state, the designed metamaterial can be used as a filter providing three adjustable passbands with center frequencies of 1.892 THz, 1.124 THz, and 0.94 THz, and the corresponding transmittances reach 93.11%, 98.62%, and 90.01%, respectively. The filter also shows good stopband characteristics and exhibits good sensing performance at the resonant frequencies of 1.992 THz and 2.276 THz. When VO2 is in metal state, the metamaterial structure acts as a double-band absorber, with three absorption peaks (>90%) in the range of 0.684 THz to 0.924 THz, 2.86 THz to 3.04 THz, and 3.28 THz to 3.372 THz, respectively. The designed structure is insensitive to the polarization of vertically incident terahertz waves and still maintains good absorption performances over a large range of incidence angles. Finally, the effects of geometric parameters on the absorption and transmission properties of the hybrid bifunctional metamaterials have also been discussed. The switchable metamaterial structures proposed in this paper provide great potential in terahertz application fields, such as filtering, smart sensing, switching, tunable absorbers, and so on.

Polymer negative curvature ring-core fiber for OAM modes guidance.

In this paper, a novel, to the best of our knowledge, polymer-based negative curvature ring-core fiber (NC-RCF) is proposed and investigated. The hollow-core NC-RCF is composed of TOPAS as background material. The inner and outer negative curvature structure layers are connected to the annular area, and the orbital angular momentum (OAM) modes can propagate in the annular core. In the frequency region of 1.0-1.5 THz, the designed NC-RCF can stably transmit 82 OAM modes. Investigation results indicate that the effective refractive index differences between the corresponding HE and EH modes are above 10-4. The confinement losses of EH or HE modes are smaller than 10-8 d B/m, and the dispersion variations are lower than 0.31 ps/THz/cm. Effective mode areas are larger than 5.14m m 2. Additionally, the highest mode purity of all vector modes is 99.78%. In addition, modal birefringence, also known as the walk-off length, has also been discussed. All these operation performances indicate that the designed NC-RCF make contributions to the optical communication systems.

One-dimensional terahertz dielectric gradient metasurface for broadband spoof surface plasmon polaritons couplers: publisher's note.

This publisher's note contains corrections to Opt. Lett.46, 290 (2021)OPLEDP0146-959210.1364/OL.412229.

One-dimensional terahertz dielectric gradient metasurface for broadband spoof surface plasmon polaritons couplers.

At present, most of the gradient metasurfaces used to construct surface plasmon polaritons (SPPs)/spoof SPPs (SSPs) couplers are usually compact metal antennas working under reflection and transmission. In reflection mode, meta-couplers link propagating waves and surface waves (SWs), and SWs will undergo significant scattering before coupling to an Eigen SPP in the target system. In transmission mode, metal meta-couplers will encounter complex multilayer designing at the microwave/terahertz region and metal absorption loss at optical frequencies. In this Letter, to the best of our knowledge, a novel design using dielectric gradient metasurfaces instead of metal metasurface couplers is proposed to excite broadband SSPs on the metal groove array. We demonstrate that the well-designed phase dielectric gradient metasurface converts the normal incident terahertz wave to the predetermined angle in the dielectric substrate and then excites the broadband SSPs with the transmission coupling between the dielectric meta-coupler and SSPs surface. This research may open up new avenues in simple and broadband plane dielectric meta-couplers for SSPs in ultra-thin and compact functional devices for versatile applications.

Tetraenol, a novel sesquiterpenoid from the relict plant Tetraena mongolica in China.

A novel furansesquiterpenoid, tetraenol, was isolated from a relict shrub plant, Tetraena mongolica, collected from the northern desert of the Ningxia Hui Autonomous Region. The structure of the new compound was elucidated on the basis of spectroscopic analysis.