Nonlinear optical characterizations and applications are reported from zero dimensional nanoparticles, one dimensional fibers, two dimensional (2D) graphenes then to the traditional bulk nonlinear crystals. Various nonlinear optical processes such as nonlinear absorption, supercontinuum generation and cascading optical parametric processes are studied. For all these phenomena, the artificial microstructure and size effect normally play important roles. For example, metallic nanoparticles in a dielectric matrix may give strong surface plasmon resonance (SPR) absorption. O. Sánchez-Dena et al. embedded Au nanoparticles in sapphire by using ion implantation. They observed a negative nonlinear absorption increasing with size, and size independent positive nonlinear refraction excited by 532 nm and 26 ps pulses [1]. Although photonic crystal fiber (PCF) has been a kind of popular optical element, the flexible structure design and material selection give PCF many freedoms toward the desired properties. Lead-bismuthgallium-oxide glass (PBG-08) is selected for PCF fabrication. Due to its high refractive index, high nonlinearity and high transmittance, the PBG-08 glass-based PCFs are found to be excellent media for broad supercontinuum generation from 900 to 2400 nm [2].

Graphene is the first 2D material widely used to obtain mode-locked operation in fiber and bulk lasers due to its broadband saturable absorption (SA). Recently, a new class of Dirac-materials called topological insulators (TI) has been strongly investigated including antimony telluride. It could be mechanically exfoliated then directly deposited onto a fiber connector tip inside an erbium-doped fiber modelocked laser cavity for picosecond pulse generation [3]. Francesco Bonaccorso and Zhipei Sun further presented an overview of graphene and 2D crystals based ultrafast lasers in their invited paper. The low cost solution processing of the raw materials were suggested, which offers a simple and cost-effective pathway to fabricate 2D crystal-based saturable absorbers for industrial-scale applications [4].

Optical parametric frequency conversion is the most historic nonlinear optical effect, people are still pursuing new nonlinear crystals operating in UV band with long-term operation potential. Loïc Deyra et al. reported a UV laser through the third harmonic generation in a new non-hygroscopic borate crystal Ca₅(BO₃)₃F (CBF). The measured nonlinear coefficient is close to the one of Type-II LBO. If larger crystals could be grown, the CBF even may have the potential to replace LBO by the authors [5]. In addition, a non-planar ring cavity is proposed and characterized with enhanced beam quality in high-pulse-energy optical parametric oscillators (OPO). The OPO is based on a ZnGeP2 crystal pumped by a Ho³⁺:LiLuF₄ MOPA system at 2.05 μm [6]. Although this is mainly a device level work, it still could be a nice supplement to other materials-orientated work so that this feature issue cover fully from materials preparation, characterization, to device applications.

Although the published papers in this issue are limited, we still could feel the trend of nonlinear optical materials research from them. Development and applications of new materials are always key subjects, while low dimensional materials, especially graphene and other 2D materials grow much faster than before. For bulk nonlinear materials, there are still some rooms as long as the related device performance and reliability could be improved.

We are so pleased to introduce this second feature issue on nonlinear optics. Just like the first one in 2011, we may see from it that NLO still possesses exuberant vitality with many exciting new progresses. We appreciate the outstanding work of all contributors to this special issue. We encourage researchers to keep contributing new theoretical and experimental results in this exciting field to Optical Materials Express.