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A bifunctional broadband absorber in the terahertz band based on patterned bulk Dirac semimetal (BDS) and strontium titanate (STO) is proposed. The properties of the absorber are investigated using the finite-difference time-domain (FDTD) method. The results show that the width of absorption can be modulated from 0.59 THz to 0.7 THz when the Fermi energy of the BDS is independently shifted from 40 meV to 50 meV. By tuning the temperature from 250 K to 400K, the center frequency of the broadband absorption spectrum can be changed from 1.311 THz to 1.505 THz, and the absorption bandwidth broadens from 0.66 THz to 0.81 THz. In addition, the simulation results show that the absorber is insensitive to electromagnetic wave polarization, and can still maintain a stable broadband absorption effect when the oblique incidence is within 40° for TE and TM modes. Based on the impedance matching theory, the physical mechanism of the broadband absorption is analyzed theoretically. This work can provide an alternative way to design high-performance multifunctional tunable terahertz devices.

Accelerator scientists have high demands on photocathodes possessing high quantum efficiency (QE) and long operational lifetime. p-GaN, as a new photocathode type, has recently gained more and more interest because of its ability to form a negative electron affinity (NEA) surface. Being activated with a thin layer of cesium, p-GaN:Cs photocathodes promise higher QE and better stability than the known photocathodes. In our study, p-GaN samples grown on sapphire or silicon were wet chemically cleaned and transferred into an ultra-high vacuum (UHV) chamber, where they underwent a subsequent thermal cleaning. The cleaned p-GaN samples were activated with cesium to obtain p-GaN:Cs photocathodes, and their performance was monitored with respect to their quality, especially their QE and storage lifetime. The surface topography and morphology were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) spectroscopy. We have shown that p-GaN could be efficiently reactivated with cesium several times. This paper systematically compares the influence of wet chemical cleaning as well as thermal cleaning at various temperatures on the QE, storage lifetime and surface morphology of p-GaN. As expected, the cleaning strongly influences the cathodes’ quality. We show that high QE and long storage lifetime are achievable at lower cleaning temperatures in our UHV chamber.

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