The quasi-phase-matching(QPM)technique has drawn increasing attention due to its promising applications in areas such as nonlinear frequency conversion for generating new laser light sources.In this paper,we will briefly review the main achievements in this field.We give a brief introduction of the invention of QPM theory,followed by the QPM-material fabrication techniques.When combing QPM with the solid-state laser techniques,various laser light sources,such as single-wavelength visible lasers and ultraviolet lasers,red–green–blue three-fundamentalcolor lasers,optical parametric oscillators in different temporal scales,and passive mode-locking lasers based on cascaded second-order nonlinearity,have been presented.The QPM technique has been extended to quantum optics recently,and prospects for the studies are bright.
High-power cw green laser radiation is generated by intra-cavity frequency doubling of a diode-pumped Nd:GdVO_(4) laser with a MgO-doped periodically-poled LiNbO_(3)(MgO:PPLN)crystal at room temperature.An average power of 2.4 W at 0.53μm is obtained under the pump 15 W at 808 nm,corresponding to an overall optical-to-optical conversion efficiency of 16%.The M^(2) factor of the green beam is 3.90 and 1.34 for the horizontal and vertical direction,respectively.In addition,the power fluctuation is measured to be about±5%.
Anisotropic localization of Dirac fermions in graphene along both the x and y axes was studied using the transfermatrix method. The two-parameter scaled behavior around the Dirac points was observed along the x axis with off-diagonal disorder. In contrast, the electronic state along the y axis with armchair edges was delocalized, which can be described well by single parameter scaling theory. This implies that the breakdown of the single-parameter scaling is related to the zigzag edge along the x axis. Furthermore, dimerization induced by the substrate suppresses the two-parameter scaling behavior along the x axis and preserves the delocalized state along the y axis. Our results also demonstrate anisotropic localization in graphene with diagonal disorder that can be tuned by dimerization.
