Tin disulfide (SnS2) quantum dots (QDs) have been used in the fields of sensors. However, the reported SnS2 QDs were fabricated by the wet chemical method which was complicated. In this paper, we synthesized SnS2 QDs by a facile ultrasonic probe sonication process. The average size of SnS2 QDs was 3 nm which was observed in the transmission electron microscope (TEM) images. Two vibrational modes of SnS2 QDs were observed at 203 cm-1 (Eg) and 310 cm-1 (A1g) in the Raman spectrum. There was an absorption peak at 320 nm in the UV-Vis spectrum. Excitation wavelength dependent photoluminescence (PL) was measured. The maximum PL intensity of SnS2 QDs was observed at 450 nm under the excitation wavelength of 370 nm. This indicates that the SnS2 QDs have potential applications in optical devices.
Graphene is a two-dimensional carbon material which has been extensively studied for its applications in electronic devices due to its fast carrier kinetics. However, the weak photon absorption of graphene limits its application in photodetectors. Transition metal chalcogenides (TMDCs) quantum dots (QDs) have been used to modify the graphene properties since these QDs have abundant active edge sites and specific optoelectronic properties. In this study, we synthesized SnSe2 QDs by the process of sonication and laser ablation. The average size of SnSe2 QDs was characterized by the transmission electron microscopy (TEM). We demonstrated a sensitive ultraviolet (UV) photodetector based on graphene and SnSe2 QDs on a polyethylene terephthalate (PET) substrate. The responsibility of the device was up to 1830 AW-1 when the irradiation density was 155.2 μW/cm2 . The rising time τ𝒓 was 0.26 s. The device showed good stability even after bending 100 times. SnSe2 QDs enhanced the light absorption and the creation of photocarriers which could extend the applications of graphene in flexible optoelectronic devices.
Both the strong mode confinement and the low propagation loss are longed for designing highly integrated terahertz (THz) devices, but they are difficult to be achieved at the same time. Here, a graphene-coated nanowire with a dropshaped cross section (GNDCS) is proposed with the long-range propagation and strong confinement. We found this waveguide can support two kinds of graphene surface plasmon polaritons (GSPPs), outside-dominant and insidedominant modes, with distinctly different energy distributions. Interestingly, both modes can achieve low-loss propagation with strong mode confinement. In particular, the outside-dominant mode can attain an extremely long propagation length (1mm) and the inside-dominant mode has a very high energy utilization rate. These excellent characteristics make the waveguide very useful in the nanophotonics, bio-photonics and highly integrated THz circuits.
For a good THz waveguide, both low propagating loss and small mode width are usually very important. However, the high ohmic loss of metals and the high absorption loss of dielectric materials result in that it still remains a challenge to obtain the two capabilities at the same time. In this paper, planar dielectric-gap-metal (DGM) waveguides are presented to guide THz wave. According to the dispersion equations of the waveguides, we calculate their mode characteristics by numerical calculation, and we find that THz wave can propagate in the waveguides with low loss and simultaneously subwavelength mode width. When compared with the parallel-plate waveguide, the mode losses of the DGM waveguide can be 1-3 orders of magnitude lower, but the mode widths do not increase. The combination of low propagating loss and subwavelength mode width makes the DGM waveguides particularly useful for many THz applications such as sensing, communication, and imaging.
To enhance continuous wave terahertz (CW-THz) scanning images contrast and denoising, a method based on wavelet transform and Retinex theory was proposed. In this paper, the factors affecting the quality of CW-THz images were analysed. Second, an approach of combination of the discrete wavelet transform (DWT) and a designed nonlinear function in wavelet domain for the purpose of contrast enhancing was applied. Then, we combine the Retinex algorithm for further contrast enhancement. To evaluate the effectiveness of the proposed method in qualitative and quantitative, it was compared with the adaptive histogram equalization method, the homomorphic filtering method and the SSR(Single-Scale-Retinex) method. Experimental results demonstrated that the presented algorithm can effectively enhance the contrast of CW-THZ image and obtain better visual effect.
A theory for treating terahertz (THz) surface wave propagation on conical metal wire waveguides is presented. The
expression for describing surface wave propagation on the conical wire waveguide is obtained based on the Sommerfeld
surface wave model. According to the theory, the surface wave propagation on the conical copper wire waveguide is
investigated by the numerical calculation, and the results obtained agree well with that of experimental measurement by
Y. B. Ji et al. Furthermore, both the surface wave propagation properties of the conical copper wire waveguides versus
the wire diameter and versus the THz wave frequency are studied by the numerical calculation. The work presented provides a theoretical basis for describing surface wave propagation on the conical THz metal wire waveguide and is quite useful for designing this kind of waveguides.