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.
In this report, a required aperiodically poled structure is designed, where two quasi-phase-matched conditions of optical
parametric oscillator and frequency difference are required to be satisfied simultaneously. By numerical simulation, we
analyze the effects of crystal structure, the cavity parameter of optical parametric oscillator, and terahertz absorption
coefficient on terahertz generation. And, the terahertz wave is generated via cascaded processes, which is of great
interest in fundamental and applied sciences, and is a great demand for many applications in spectroscopy, sensing,
communication, medial diagnoses and biomedical imaging. In addition, the design method for poled ferroelectric crystal
is universal and applicable to many other fields, particularly in those processes where multiple quasi-phase-matched
conditions are required to be satisfied simultaneously.