In this paper, for the first time, a new generation of ultrafast reprogrammable multi-mission bias encoded metasurface is proposed for dynamic terahertz wavefront engineering by employing VO2 reversible and fast monoclinic to tetragonal phase transition. The multi-functionality of our designed VO2 based coding metasurface (VBCM) was guaranteed by elaborately designed meta-atom comprising three-patterned VO2 thin films whose operational statuses can be dynamically tuned among four states of “00”-“11” by merely changing the biasing voltage controlled by an external Field-programmable gate array platform. Capitalizing on such meta-atom design and by driving VBCM with different spiral-like and spiral-parabola-like coding sequences, single vortex beam and focused vortex beam with interchangeable orbital angular momentum modes were satisfactorily generated respectively. Additionally, by adopting superposition theorem and convolution operation, symmetric/asymmetric multiple beams and arbitrarily-oriented multiple vortex beams in pre-demined directions with different topological charges are realized. Several illustrative examples successfully have clarified that the proposed VBCM is a promising candidate for solving crucial terahertz challenges such as high data rate wireless communication where ultrafast switching between several missions is required.

/pdf/ultrafast-reprogrammable-multifunctional-vanadium-dioxide-3mijqnmkq5.pdf

Ultrafast reprogrammable multifunctional vanadium-dioxide-assisted metasurface for dynamic THz wavefront engineering.

A switchable and tunable terahertz (THz) metamaterial based on photosensitive silicon and Vanadium dioxide (VO2) was proposed. By using a finite-difference time-domain (FDTD) method, the transmission and reflective properties of the metamaterial were investigated theoretically. The results imply that the metamaterial can realize a dual electromagnetically induced transparency (EIT) or two narrow-band absorptions depending on the temperature of the VO2. Additionally, the magnitude of the EIT and two narrow-band absorptions can be tuned by varying the conductivity of photosensitive silicon (PSi) via pumping light. Correspondingly, the slow-light effect accompanying the EIT can also be adjusted. 

Switchable and Tunable Terahertz Metamaterial Based on Vanadium Dioxide and Photosensitive Silicon

Graphene field-effect-transistor (GFET) based terahertz (THz) modulators usually possess an unfulfilling modulation depth (MD) of 15% ∼ 20%. In this work we developed a flexible GFET based THz modulator, where the graphene monolayer is coated with an organic high-K dielectric as the screening layer and an ion-gel layer as the gate. With this exquisite composite modulating structure, the new device possesses a significantly enhanced modulation depth (MD) up to 70% over a broad frequency band, an extremely low insert loss (IL) of 1.3 dB, and unexpected good structural and properties stability. The large intrinsic MD, low IL, as well as its flexibility, render this performance enhanced modulator versatile in fabrication of novel THz devices, such as multi-level modulator, for nonplanar or wearable applications.

https://iopscience.iop.org/article/10.1088/2053-1591/aadeca/pdf

Flexible terahertz modulators based on graphene FET with organic high-k dielectric layer

We demonstrate a photo-induced high modulation depth terahertz (THz) modulator based on a VO x /Si/VO x hybrid structure. The thickness of the embedded high resistance silicon substrate is 510 µm and two layers of VO x films are 100 nm, respectively. Compared with other THz modulators based on the phase transition of vanadium oxide which were fabricated by complex chemical vapor deposition and sputtering processes, the VO x film in this device is fabricated using a simple spin coating process of VO x solution. The sample is characterized using a THz time-domain-spectroscopy system. In the test experiment, the insulator–metal-transition of the VO x is induced by two oblique incident pump lasers with a central wavelength of 650 nm from two sides. The experimental results show that it can be optically modulated in a wide frequency range from 0.2 to 2.0 THz, and the maximum modulation depth of 93% is realized when the optical power is 120 mW. Film theory and Drude theory are introduced to interpret the mechanism of modulation. This modulator has broadband, a simple fabrication process and very low optical power, which are important for broadband THz communication and all-optical THz systems.

https://iopscience.iop.org/article/10.1088/1361-6463/ab046f/pdf

Photo-induced high modulation depth terahertz modulator based on VO x ─Si─VO x hybrid structure

In this paper, we design a polarization-independent and broadband microwave metamaterial absorber (MMA) based on three-dimensional structure. The simulated results show that the proposed absorber h...

A polarization-independent and broadband microwave metamaterial absorber based on three-dimensional structure

Applying the photoexcitation characteristics of vanadium dioxide (VO2), a dynamic resonant terahertz (THz) modulation with the combination of a VO2 film and a metamaterial was suggested to realize THz wave active manipulation. The designed metamaterial with structured copper rings arrays can realize a passband from 0.776 to 1.045 THz. When insulator–metal phase transition in VO2 thin film, which is deposited on the other surface of the metamaterial substrate, is induced by optical pumping, the metamaterial/VO2 film hybrid structure behaves as an absorber with absorption rates of 90% at 0.88 THz and the transmission energy decrease to less than 3%. Therefore, about 78% modulation depth and more than 250 GHz modulation bandwidth have been reached under the photoinducing. The simulation results illustrate the promise of using phase transition materials for efficient broadband fast response modulators for THz waves.

Study on a terahertz modulator based on metamaterial with photoinduced vanadium dioxide film


 The chemical states of nuclides affect their physical and chemical properties including thermal conductivity, melting point, adsorption and desorption behavior, diffusion process, and chemical reactivity. It is an important issue in nuclear energy systems and spent fuel reprocessing. In this review, we summarize the theoretical calculations and experimental measurements to determine the chemical states of nuclides in nuclear energy systems. Software such as FactSage, HSC Chemistry and The Geochemist’s Workbench are generally used to determine the macroscopic-scale thermodynamic parameters such as thermodynamically stable phases. Quantum chemistry calculation software such as Gaussian 03 is employed for microscopic first-principles calculations to elucidate chemical reaction channels, microstructures, bonding characteristics, and rate constants. The experimental methods to determine chemical states of nuclides include X-ray, spectroscopy and chemical extraction.

Summary of Methods for Studying the Chemical States of Nuclides In Nuclear Energy Systems

A study on the clearance of metal frames of waste filters from ventilation system in nuclear power plant

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Study on a terahertz modulator based on metamaterial with photoinduced vanadium dioxide film

Summary of Methods for Studying the Chemical States of Nuclides In Nuclear Energy Systems

A study on the clearance of metal frames of waste filters from ventilation system in nuclear power plant