Optical rectification of ultra-short laser pulses is an attractive technique for efficient generation of terahertz pulses. An analytical expression of the optical-to-terahertz conversion efficiency in optical rectification by the use of the method of tilted-pulse-front pumping has been derived and used to simulate the maximum optical-to-terahertz conversion efficiency. The variations of the conversion efficiency of optical-to-terahertz with the length of crystal, the intensity and the pulse duration of the pumping ultra-short laser have been investigated, both in numerical and analytical solutions. In addition, the absorption of terahertz wave in LiNbO<sub>3</sub> has also been taken into account. The results show that, the optical-to-terahertz conversion efficiency increases with the intensity of pump pulse and the length of crystal, whereas decreases with the increasing of the pulse duration for the case of the relatively large pulses duration.
Based on the theory of split ring resonators (SRRs) and the rigorous theory of electromagnetic field, the spatial
distributions of the electric field and electric energy density in split ring resonators with different structures such as
square, circle and triangle have been analyzed. The variation of electric energy density with different terahertz (THz)
frequencies has also been investigated. It can be shown from our numerical simulation results that the electric field
enhancement effect occurs in the split ring resonators with square, circle and triangle structures and the electric field near
the opening is obviously stronger than that in other regions, especially in the gap of the split ring resonators. The
maximum of the electric energy density appears in the opening of the SRRs and the electric field is obviously stronger
near the resonance frequency of the SRRs. The results obtained in this paper have important significance on the design of
the split ring resonators and the function of resonant magnetic response of left hand materials (LHMs).