A terahertz absorber with controllable and switchable bandwidth that is insensitive to polarization is of great interest. Here, we propose and demonstrate a metasurface absorber with switchable bandwidth based on a phase-change material of vanadium dioxide (VO ₂ ) and verify its performance by finite element method simulations. The metasurface absorber is composed of a hybrid cross fractal as a resonator separated from a gold ground plane by a polyimide spacer. Switching from narrowband to broadband absorber is achieved via connecting VO ₂ patches to the gold first-order cross fractal converting the resonator to a third-order cross fractal. In the insulator phase of VO ₂ , the main narrowband absorption occurs at the frequency of 6.05 THz with a 0.99 absorption and a full-width half-maximum (FWHM) of 0.35 THz. Upon insulator-to-metal transition of VO ₂ , the metasurface achieves a broadband absorption with FWHM of 6.17 THz. The simulations indicate that by controlling the partial phase transition of VO ₂ , we can tune the bandwidth and absorption level of the absorber. Moreover, the designed absorber is insensitive to polarization due to symmetry and works well for a very wide range of incident angles. In the metallic state of VO ₂ , the absorber has an absorption exceeding 0.5 in the 3.57–8.45 THz frequency range with incident angles up to 65°.

Although, quantum dots (QDs) of two-dimensional (2D) molybdenum disulfide (MoS₂) have shown great potential for various applications, such as sensing, catalysis, energy storage, and electronics. However, the lack of a simple, scalable, and inexpensive fabrication method for QDs is still a challenge. To overcome this challenge, a lot of attention has been given to the fabrication of QDs, and several fabrication strategies have been established. These exfoliation processes are mainly divided into two categories, the ‘top-down’ and ‘bottom-up’ methods. In this review, we have discussed different top-down exfoliation methods used for the fabrication of MoS₂ QDs and the advantages and limitations of these methods. A detailed description of the various properties of QDs is also presented.

We investigated the optical and the electrical properties of GaAs solar cells (SCs) by using photoreflectance (PR) spectroscopy and current-voltage (J-V) measurements. The electric fields (F _j ) in p-n junctions were evaluated through an analysis of the Franz-Keldysh oscillations (FKOs) of the PR spectra. From the excitation light intensity (I _ex ) dependence of the PR, we found that the photovoltaic effect resulted in a reduction of the F _j . The F _j was gradually reduced by up to 37% from the dark built-in electric field when the I _ex was increased (I _ex = 2.3 ∼ 181 mW/cm2). We evaluated the ideality factor (n) of the SC via PR spectroscopy, and the results matched the results of the J-V measurements well. From these results, we demonstrate that the F _j and the n of the SCs can be evaluated by using PR spectroscopy.