Yonggang Zhang’s research while affiliated with Anhui Science and Technology University and other places

In this study, a series of temperature-controlled terahertz (THz) wave modulators based on a combination of magnetron-sputtered strontium titanate (STO) thin films and metasurfaces have been proposed. As the sputtering temperature increased from 25 to 600°C, the STO thin films exhibited an evolution from amorphous to polycrystalline states and thus different dielectric responses were observed in the THz band. Differences in the performance of the THz wave modulators such as differences in the variation trends of the amplitude and frequency of the resonance peaks as a function of the heating temperature were observed in the THz transmission spectra. The variation in the permittivity as a function of heating also exhibited different trends for the as-deposited STO films and fabricated modulators when the sputtering temperature was increased. Although a maximum amplitude modulation depth of 29.8% was obtained for the modulator with STO sputtered at 25°C, the frequency-shift exhibited nearly no regular variation on heating. On the contrary, amplitude as well as frequency modulations were achieved for the THz wave when the sputtering temperature of STO for the modulator exceeded 200°C. This study presents a high-temperature sputtering method for fabricating STO temperature-controlled modulators that can provide simultaneous modulation of amplitude and frequency of THz waves.

Vortex beams carrying orbital angular momentum (OAM) exhibit excellent potential for applications in fields such as sixth-generation communication, quantum information processing, and imaging technologies. However, the realization of multi-mode and full-space continuous precision modulation of vortex beams still faces significant challenges. In this study, a terahertz full-space coding metasurface based on vanadium dioxide (VO2) is proposed, combined with a more accurate generalized coding strategy, convolution, and generalized superposition methods to achieve continuous and precise modulation of vortex beams. First, the phase state of VO2 is dynamically adjusted by temperature changes, enabling the coding metasurface to switch between transmission and reflection operating modes. Second, a precise generalized coding strategy is developed to support the generation of coding sequences for arbitrary Ni values by logically discretizing the size Ni of the super-subunit. Meanwhile, by convolving different coding sequences, continuous and precise modulation of different modes of vortex beams in the range of 0 to 360° is realized. Finally, the deflection vortex beams carrying different OAM are effectively superimposed using the generalized superposition method. This innovative coding strategy significantly improves the ability of continuous and precise modulation of the vortex beam wavefront, laying a solid foundation for future breakthroughs in intelligent communication and high-resolution imaging technologies.

The sensitivity of phase-change materials to low-energy photons has enabled the development of tunable terahertz (THz) generators for vortex beams. In this study, a composite unit structure based on Dirac semimetal (DSM), vanadium dioxide ( ${{\rm VO}_2}$ V O 2 ), and polyimide is proposed, with 360° phase coverage achieved by rotating the angle of the DSM structure. In addition, the switching of ${{\rm VO}_2}$ V O 2 between medium and metal is controlled by changing its temperature, resulting in a metasurface orbital angular momentum (OAM) vortex beam to switch between the transmission of a five-layer structure and the reflection of a three-layer structure. The metasurface, operating in a five-layer structured transmission mode, is designed as a vortex beam generator with topological charges of ${-}{1}$ − 1 and ${+}{2}$ + 2 , and realized a tunable vortex beam generator operating frequency by changing the Fermi energy level of the DSM. The metasurface, operating in a three-layer structured reflection mode, is designed as a broadband tunable vortex beam generator with topological charges of ${-}{1}$ − 1 and ${+}{2}$ + 2 . The switchable OAM modes generated by the vortex beam generator are realized by changing the Fermi energy level of the DSM, and selective incidence of left circularly polarized and right circularly polarized waves is realized in the THz band. This switchable vortex beam generator based on DSM and ${{\rm VO}_2}$ V O 2 has potential applications in wireless communication systems in the THz range.

Arbitrarily designed resonant-based metasurfaces are particularly attractive and present a unique platform for biosensing applications owing to their ability to confine light to nanoscale regions and their spectral selectivity. In this study, we experimentally demonstrate a metasurface sensor based on terahertz fingerprint spectroscopy that enables the specific recognition of trace samples. The results of simulations and experiments show that this metasurface sensor detects glycine, L-arginine, and L-threonine, respectively, with different resonance coupling. The frequency shift of the resonance peak of the metasurface sensor was the largest when the resonance peak matched the fingerprint peak of the sample, with a maximum of 123 GHz for detecting L-threonine. Therefore, combining the frequency shift of the resonance peaks with the fingerprint spectrum of the sample can achieve specific recognition of the sample. This study provides new ideas for specific recognition of samples using metasurface sensors in biomedicine, food safety, and other fields.

Dirac semimetal (DSM) coding metasurfaces enable switchable beam control in the terahertz (THz) communication field, thereby providing additional options for the regulation of electromagnetic waves. This study proposes a new structure based on a DSM THz coding metasurface. By adjusting the DSM size, a coding metasurface containing eight units is constructed to realize 3-bit coding. Simultaneously, by adjusting the DSM Fermi level (E F ), the relative phase delay of adjacent units is controlled, resulting in a switchable coding metasurface. Using specific coding arrangements, multi-functions, such as beam deflection, beam splitting, vortex beams, and vortex beams with adjustable deflection angles can be controlled. Furthermore, a reduce radar cross-section (by approximately 13 dB at an operating frequency of 1.05 THz), is achieved. This study provides novel concepts and methods for metamaterials in the fields of communication and radar.

In this study, a hybrid amorphous strontium titanate (STO) and terahertz metasurface were studied. Because of the excellent physical properties of amorphous STO, such as its dielectric properties and high transmittance in the terahertz region, it plays a core role in realizing a novel terahertz (THz) temperature sensor with high performance in the temperature range of 500–608 K. A blue shift of the absorption peaks appeared for the THz wave as the temperature increased, which confirmed the temperature-sensing function. The physical mechanisms underlying this phenomenon were also investigated. After optimization, the best THz temperature sensor with a sensitivity of 2.08 GHz/K was obtained, in which the thickness of the amorphous strontium titanate film was approximately 0.36 µm. This study provides a new opportunity for amorphous STO materials to be applied in THz sensors and demonstrates the realization of amorphous STO-based THz temperature sensors with high performance, low cost, and simple processes.

A broadband and narrowband switchable terahertz (THz) absorber based on a bulk Dirac semimetal (BDS) and strontium titanate (STO) is proposed. Narrowband and broadband absorption can be switched by adjusting the Fermi level of the BDS. When the Fermi level of the BDS is 100 meV, the device is an absorber with three narrowband absorption peaks. The frequencies are 0.44, 0.86, and 1.96 THz, respectively, when the temperature of STO is 250 K. By adjusting the temperature of STO from 250 to 500 K, the blue shifts of the frequencies are approximately 0.14, 0.32, and 0.60 THz, respectively. The sensitivities of the three absorption peaks are 0.56, 1.27, and 2.38 GHz/K, respectively. When the Fermi level of the BDS is adjusted from 100 to 30 meV, the device can be switched to a broadband absorber with a bandwidth of 0.70 THz. By adjusting the temperature of STO from 250 to 500 K, the central frequency shifts from 1.40 to 1.79 THz, and the bandwidth broadens from 0.70 to 0.96 THz. The sensitivity of the central frequency is 1.57 GHz/K. The absorber also has a wide range of potential applications in multifunctional tunable devices, such as temperature sensors, stealth equipment, and filters.

The combined application of metasurface and terahertz (THz) time-domain spectroscopy techniques has received considerable attention in the fields of sensing and detection. However, to detect trace samples, the THz wave must still be enhanced locally using certain methods to improve the detection sensitivity. In this study, we proposed and experimentally demonstrated a fano resonance metasurface-based silver nanoparticles (FaMs-AgNPs) sensor. AgNPs can enhance the sensitivity of the sensor by generating charge accumulation and inducing localized electric field enhancement through the tip effect, thereby enhancing the interaction between the THz waves and analytes. We investigated the effects of four different contents of AgNPs, 10 µl, 20 µl, 30 µl and 40 µl, on the detection of acetamiprid. At 30 µl of AgNPs, the amplitude change of the FaMs-AgNPs sensor was more pronounced and the sensitivity was higher, which could detect acetamiprid solutions as low as 100 pg/ml. The FaMs-AgNPs sensor has the advantages of a simple structure, easy processing, and excellent sensing performance, and has a great potential application value in the field of THz trace detection and other fields.