We present a detailed investigation on the effect of functional group modulation at the edges of carbon quantum dots (CQDs) on the fluorescence from the CQDs. The CQDs attached by N, S, and P elements are synthesized via pyrolysis of a mixture of citric acid and NH3H2O, H2SO4, and H3PO4, respectively. Thus, part of –COOH at the edges of CQDs can be converted into –C=O and functional groups such as –NH2, –SO2, –HSO3, and –H2PO4 can connect to the carbon bonds. We find that the formation of the N/S/P-CQDs can reduce the amount of –COOH that attaches to the edges of sp2-conjugated π-domains located at centers of these CQDs. This effect can result in the reduction of the non-radiative recombination for electronic transition in these CQDs. As a result, the quantum yield (QY) for fluorescence from the CQDs can be efficiently enhanced. We demonstrate experimentally that the QYs for N/S/P-CQDs can reach up to 18.7%, 29.7%, and 10.3%, respectively, in comparison to 9% for these without functional group modulation. This work can provide a practical experimental approach in improving the optical properties of fluorescent CQDs.

Monolayer (ML) hexagonal boron nitride (hBN) is an important material in making, e.g., deep ultraviolet optoelectronic and power devices and van der Waals heterojunctions in combination with other two-dimensional (2D) electronic systems such as graphene and ML MoS ${}_2$ . In this work, we present a comparative study of the basic optoelectronic properties of low resistance ML hBN placed on different substrates such as SiO ${}_2$ /Si, quartz, PET, and sapphire. The measurement is carried out by using terahertz (THz) time-domain spectroscopy (TDS) in a temperature regime from 80 to 280 K. We find that the real and imaginary parts of the optical conductivity obtained experimentally for low resistance ML hBN on different substrates can fit well to the Drude–Smith formula. Thus, we are able to determine optically the key sample and material parameters (e.g., the electronic relaxation time or mobility, the carrier density, the electronic localization factor, etc.) of ML hBN. The effect of temperature on these parameters is also examined and analyzed. The results obtained from this study enable us to suggest the appropriate substrate for ML hBN based electronic and optoelectronic devices. This work is relevant to the application to a newly developed 2D electronic system as advanced electronic and optoelectronic materials.

By using the effective-index method (EIM) and the finite element methods (FEM), a surface plasmon polariton (SPP) waveguide structured by a dielectric ridge placed on monolayer black phosphorus is proposed and analyzed in the infrared spectral region. It is found that the strong anisotropic dispersion of the black phosphorus (BP) gives rise to direction-dependent waveguide modes in a dielectric-loaded black phosphorus waveguide (DLBPW). The effective mode index, propagation loss and cutoff wavelength of higher order modes are investigated along the armchair (AC) and the zigzag (ZZ) directions of the black phosphorus. Moreover, the propagation characteristics of single-mode are investigated for different widths of the dielectric ridge and different polarization directions of the black phosphorus. Via tuning the carrier density, the electromagnetic field propagation features can be effectively modified. Also, the coupling effect by adding more dielectric bridges can tune the propagation properties. These results will lead to great applications in black phosphorus-based optical integrated devices.

When a p- or s -polarized wave illuminates an optical system, strong inhibition of reflection can be observed at specific incident angles. Because this phenomenon does not strictly meet the definition of a classical Brewster effect, it is known as a generalized Brewster effect (GBE). Based on the fact that the graphene conductivity in the terahertz (THz) range can be dynamically modulated by optical pumping, we propose a new way to tune THz GBE in a graphene/substrate system via modulation light. A theoretical framework for the THz responses in the system tuned by modulation light has been established. We demonstrate the potential of graphene/dielectric structure as an optically tunable platform for GBE. Furthermore, we find that the tunable GBE predicted here can be classified into two categories according to physical mechanisms. One is due to the destructive interference which can be changed through tuning the modulation light. The other comes from the inhibition of the waves radiated by induced dipoles in their oscillation direction, which is dependent on the modulation light. The results obtained here can be extended to GBE systems based on other two-dimensional materials and be applied to the realization of new types of graphene-based devices.

As a three-dimensional (3D) analog of graphene, the cadmium arsenide $\left({\mathrm{Cd}}_3{\mathrm{As}}_2\right)$ based topological Dirac semimetal has attracted enormous research interest recently. In this work, the temperature-dependent terahertz (THz) properties of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ prepared epitaxially are investigated by using THz time-domain spectroscopy. In the regime of $1.5\sim 300$ K, we obtain the optical conductivity of ${\mathrm{Cd}}_3{\mathrm{As}}_2$ and observe a remarkable phonon absorption peak in the range of $1.7\sim 1.8$ THz. More importantly, it is demonstrated that the effects of weak electronic confinement and optical-phonon-limited behavior of carriers are dominated below and above $\sim 60$ K, respectively. The weak electronic confinement can be attributed to the diffusive restoring current in polycrystalline grains of ${\mathrm{Cd}}_3{\mathrm{As}}_2$. Based on the Drude-Lorentz formula and the modified Drude-Smith formula, the temperature dependencies of the characteristic parameters are extracted to describe the carrier and phonon behaviors. These results not only can provide detailed knowledge of the THz response of epitaxially grown ${\mathrm{Cd}}_3{\mathrm{As}}_2$ depending on temperature but also may find applications in optoelectronic devices based on 3D Dirac semimetals.