A novel broadband and wide-angle 2-bit coding metasurface for radar cross section (RCS) reduction is proposed and characterized at terahertz (THz) frequencies. The ultrathin metasurface is composed of four digital elements based on a metallic double cross line structure. The reflection phase difference of neighboring elements is approximately 90° over a broadband THz frequency. The mechanism of RCS reduction is achieved by optimizing the coding element sequences, which redirects the electromagnetic energies to all directions in broad frequencies. An RCS reduction of less than −10 dB bandwidth from 0.7 THz to 1.3 THz is achieved in the experimental and numerical simulations. The simulation results also show that broadband RCS reduction can be achieved at an incident angle below 60° for TE and TM polarizations under flat and curve coding metasurfaces. These results open a new approach to flexibly control THz waves and may offer widespread applications for novel THz devices.

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Broadband and wide-angle RCS reduction using a 2-bit coding ultrathin metasurface at terahertz frequencies

Vanadium dioxide and tungsten-doped (W-doped) vanadium dioxide thin films deposited by aqueous Sol-Gel method were characterized with several different techniques (i.e. X-ray photoelectron spectroscope, atomic force microscope, X-ray diffraction), to determine their morphology and microstructure. Their metal-to-insulator (MIT) phase transition behavior in infrared spectral region (λ=4 μm) and terahertz (THz) spectral region (0.3–1.0 THz) were observed respectivele. The results demonstrate that the transmittance of W-doped VO 2 film at room temperature is visibly lower than that of undoped VO 2 film in both infrared and terahertz spectral region. The transition temperature of W-doped VO 2 film is also lower than that of undoped VO 2 film in the THz range. The MIT and structural phase transition (SPT) are observed during the phase transition of VO 2 and W-doped VO 2 , and an obvious change of peak position occurs in W-doped VO 2 film.

Study on Phase Transition Property of Tungsten-doped Vanadium Dioxide Thin Film at Terahertz Range

Molecular rotation-caused autocorrelation behaviors of thermal noise in water

Vanadium dioxide (VO2) has attracted much interest in material field due to its unique semiconductor-metal phase transition properties. And now, the problem of stability has been highlighted and concerned for the practical application in VO2. In order to establish a relatively complete guide of stability study on VO2 which would contribute to deeply and systematically researches, the article made a review of the stability study on VO2 from thermal excitation, photo-excitation and electro-excitation. Moreover, the key aspects for future research on the stability study of VO2 was proposed.

The Stability Study on Vanadium Dioxide

Design and fabrication of frequency-responsive terahertz transmittance modulator based on vanadium dioxide spherical films

The photo-induced insulator-metal transition for silicon-based VO 2 nanofilm is studied by THz time-domain spectroscopy (THz-TDS). Obvious variations of THz ray transmittance are observed before and after the CW laser beam exciting, and the conductivity of metallic-phased VO 2 film in the THz region is calculated in the thin film approximation. According to the measured results, the metallic-phased VO 2 film is characterized equivalently with Drude’s model, and complex conductivity, dielectric function and refractive index are acquired by the model. As an examination on the equivalent Drude model, numerical simulation based on the finite integral method in time domain is carried out. The results show that they are in good agreement with the experimental results. This work provides a reference for the study on phase transition of VO 2 nanofilm and its application in the THz region.

Photo-induced insulator-metal transition of silicon-based VO 2 nanofilm by THz time domain spectroscopy

In this study, we illustrate the effective medium theories in the designs of three-dimensional composite metamaterials of both negative permittivity and negative permeability. The proposed metamaterial consists of random coated spheres with sizes smaller compared to the wavelength embedded in a dielectric host. Simple design rules and formulas following the effective medium models are numerically and analytically presented. We demonstrate that the revised Maxwell—Garnett effective medium theory enables us to design three-dimensional composite metamaterials through the assembly of coated spheres which are random and much smaller than the wavelength of the light. The proposed approach allows for the precise control of the permittivity and the permeability and guides a facile, flexible, and versatile way for the fabrication of composite metamaterials.

http://iopscience.iop.org/article/10.1088/1674-1056/21/11/117802/pdf

New assembly route for three-dimensional metamaterials obtained through effective medium theory

Recently, terahertz radiation has been a branch of cutting-edge science and technology involving many fields such as public security, military defense and national economy. In the past, far-field measurements were widely carried out based on terahertz time-domain spectroscopy. But the spatial resolution is limited by far-field diffraction effect. In order to break diffraction limit and gain sub-wavelength spatial resolution in terahertz frequency region, a series of near-field detection methods came into being, such as confocal microscopy, using an aperture, guided mode, scattering, direct detection in the near-field, etc. Each method has its own advantages and disadvantages. Using the photoconductive-antenna tip is one of the direct detection methods and it delivers the possibility of near-field measurements of terahertz waves. In this method, the photoconductive-antenna tip is a tapered photoconductive tip probe. So it can be close enough to the sample surface and receive the near-field signal on the basis of principle of photoconductivity. In this way, high spatial resolution can be gained. In this article, we introduce our recent progress of near-field and far- field scanning terahertz spectroscopy system with photoconductive-antenna in detail. Firstly, we analyze and summarize the near-field detection methods that have been developed in these years. And then, using the femtosecond laser whose center wavelength is 800 nm and the photoconductive-antenna tip detector coupled with fiber, we construct fiber near-field/ far-field scanning terahertz spectroscopy (N/F-STS). The frequency bandwidth is in a range from 0.2 THz to 1.5 THz and the terahertz spot is circular and uniform indicated by performance test. Also the amplitude and phase of the terahertz field are recorded simultaneously. It has the ability to perform three-dimension scan in various experiment conditions conveniently. Finally, we introduce the real applications in our laboratory. N/F-STS can be used to scan spatial electric distribution in three dimensions and test the spectral properties in terahertz range like other traditional far-field methods. Nevertheless, the most importantly, N/F-STS is used to scan the terahertz near-field of samples, such as terahertz surface plasmon polaritons, etc. The presented method thus is useful in some application areas, such as metamaterials, graphene, surface plasmons, waveguide transmission, near-field imaging, biological test, and chip inspection.

Near-field and far-field scanning terahertz spectroscopy based on photoconductive microprobe

This study uses time domain terahertz radar system to discuss systematic imaging studies on the scaled models based on the improved Back-Projection(BP) algorithm. We image the scaled models with different shapes and are able to distinguish spatial gaps as small as 6 mm. TheTheoretical calculation predicts that the lateral resolution and the axial resolution can be as high as 0.125 mm. Center enhancement and background rings caused by the algorithm in the imaging results are also qualitatively analyzed and are proposed methods to overcome this problem.

Terahertz Radar Imaging Based on Time-domain Spectroscopy

The far-infrared optical properties of melamine and its deuterated isotope,melamine-d_6 were experimentally and theoretically investigated in the frequency range from 0.2 to 3.0 THz.Under the room temperature and dry air nitrogen conditions,three absorption bands were observed at 2.0,2.3 and 2.6 THz in the melamine sample by use of terahertz time-domain spectroscopy.Whereas,in the melamine-d_6 sample,the observed absorption bands shift towards lower frequencies and the relative intensity of the absorption bands reduces.Numerical simulation results based on the Parameterized Model number 3(PM3) were compared with the experimental data and the observed vibration spectra were assigned according to the PM3 calculations.The absorption bands of the measured melamine samples at terahertz frequencies are highly correlated with the intermolecular hydrogen bond stretching andπ-πstacking vibration.Also,the red shift of the absorption bands is due to hydrogen/deuterium substitution.

Tian Zhen

Papers

Photo-induced insulator-metal transition of silicon-based VO 2 nanofilm by THz time domain spectroscopy

New assembly route for three-dimensional metamaterials obtained through effective medium theory

Near-field and far-field scanning terahertz spectroscopy based on photoconductive microprobe

Terahertz Radar Imaging Based on Time-domain Spectroscopy

Terahertz spectral properties of melamine and its deuterated isotope,melamine-d_6