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Owing to its intense localized near-field and the versatile tunable capability, graphene surface plasmons(GSPs) provide an excellent platform to develop novel terahertz radiation source. Compared to the previous studies on terahertz radiation sources with complete grapheme sheet, graphene metasurface (such as periodical ribbon arrays) provide a new method by varying graphene ribbon width to tune output performance. In this paper, a terahertz radiation source based on the excitation of GSPs on the periodical graphene ribbon arrays by electron beam is proposed and studied both theoretically and numerically. First, the general dispersion relation of SSP mode on the graphene ribbon arrays which is deposited on a dielectric medium with a conducting ground metal plate is obtained by a modal expansion method. Besides, a terahertz radiation source on the graphene-dielectric-metal structure based on the interaction between GSPs and injected electron beam is modeled and implemented by particle-in-cell simulation. A tunable terahertz radiation source via different ribbon width is proposed and the output with different biased voltage is also analyzed. The presented studies based on gradient graphene ribbon arrays can provide a new way to develop tunable terahertz radiation source.

A novel highly birefringent photonic crystal fiber (PCF) based on an octagonal structures with isosceles triangular-latticed cladding is proposed. Using a full-vector finite element method (FEM) simulator (COMSOL Multiphysics), we study the modal birefringence of the fundamental modes with different parameters in such PCFs. In addition, the proposed octagonal PCF is compared with hexagonal PCF with the same air-filling fraction (f=0.41811), and found that the property such as birefringence and confinement loss of octagonal PCF is relatively better than that of hexagonal PCF.

We have presented a novel design of PCF in order to control the dispersion and the dispersion slope of index-guiding PCFs. This analysis includes a thorough description of the dependence of the fiber geometrical dispersion on the structural parameters of a PCF, which allows us to design PCF with both ultra-flattened and nearly-zero dispersion in a wide wavelength range. Numerical results show that a PCF with dispersion of 0±0.75ps/(nm-km) over 380nm bandwidth from 1.39μm to 1.77μm can be designed. Moreover, a small effective mode area is possible so that we can get high nonlinearity. PCFs with these novel properties have potential uses in communication systems, optical parametric amplification and so on. The software COMSOL based on finite element method with anisotropic perfectly matched layers is used to analyze the dispersion properties and effective mode area in a PCF with two-ring air holes.