In 1961, the frequency-doubling effect was firstly observed by using ruby beam through quartz crystal. In 1962, the theory of light-wave mixing was established, which announced the birth of non-linear optics, and numerous organic non-linear optical (NLO) materials were designed accordingly. Under the effect of external light field, electric field or strain field, the frequency, phase and amplitude of organic non-linear optical (NLO) materials will change, which can modify the properties of refractive index, light absorption, light scattering and the others. For example, the amplification of weak optical signal is realized by using the mixing phenomenon, and the optical recording and operation function are realized by using the nonlinear response.

Applications:

The application of organic non-linear optical (NLO) materials mainly includes the following two aspects.

• Conversion of light wave frequency field: By using double frequency, sum frequency, difference frequency, mixing frequency and other ways, the conversion of light wave frequency can be realized, which is beneficial to broaden the range of laser wavelengths and develop new laser light sources.
• Optical signal processing field: Organic non-linear optical (NLO) materials can be used in optical signal processing, such as control, switch, deflection, amplification, calculation, storage, etc.

Classification:

According to the composition, organic non-linear optical (NLO) materials can be divided into the following categories.

• Organic small molecule non-linear optical (NLO) materials: Organic small molecule non-linear optical (NLO) materials generally include urea and its derivatives, nitrobenzene derivatives, azo compounds, aromatic ketones compounds and the others.

Figure 1. An example of organic small molecule non-linear optical (NLO) material.

• Polymer non-linear optical (NLO) materials: Polymer non-linear optical (NLO) materials mainly include polyacetylene, polydiacetylene and the others. Taking the advantages of large π electron conjugation system, large nonlinear optical coefficient, fast response speed and low permittivity, polymer non-linear optical (NLO) materials have attracted much attention. Furthermore, polymer non-linear optical (NLO) materials are considered to be the most promising materials because of their good chemical stability and strong structural flexibility.

Figure 2. An example of polymer non-linear optical (NLO) material.

• Organic/inorganic composite non-linear optical (NLO) materials: Inorganic non-linear optical (NLO) materials and organic non-linear optical (NLO) materials have many advantages respectively, but also have their own disadvantages. Therefore, based on the synergistic effect, the preparation of organic/inorganic composite non-linear optical (NLO) materials has become a hot topic.

Figure 3. An example of organic/inorganic composite non-linear optical (NLO) material.

• Metallic organic non-linear optical (NLO) materials: The study of metallic non-linear optical (NLO) materials began in 1986, which is a relatively new direction in the study of organic non-linear optical (NLO) materials. Metallic organic non-linear optical (NLO) materials that have the common advantages of organic non-linear optical materials and inorganic non-linear optical materials mostly include ferrocene derivatives, pyridine carbonyl complexes and the others.

Figure 4. An example of metallic organic non-linear optical (NLO) material.

References:

1. Yan Liu, Gao Li, Xing Li. Cation-Dependent Nonlinear Optical Behavior in an Octupolar 3D Anionic Metal–Organic Open Framework[J]. Angewandte Chemie International Edition, 2007.
2. Jiang L, Lu F, Gao Y, et al. Nonlinear optical properties of an ultrathin film containing porphyrin and poly (phenylenevinylene) units[J]. Thin Solid Films, 2006.