In situexperimental studies have been key in uncovering the often elusive pathways of nucleation and crystal growth. In the field of material science and medicine this offers the prospect of controlling crystallization processes to fight disease or tailor materials towards specific applications. To further advance this quest there is a need for flexible techniques mapping the different stages of crystallization with maximal sensitivity. This article reviews the benefits of nonlinear optical techniques to take on this challenge. We provide a perspective on various nucleation and crystal growth studies that were carried out by nonlinear optical probing techniques. A theoretical background is established, different relevant nonlinear optical phenomena are defined, and optical setups that have been used by various authors are summarized. A primary focus is demonstrating the benefits of nonlinear optical techniques for thein situstudy of crystallization. These benefits include low detection limits, complementary information by combining second- and third-order techniques, as well as relatively simple bench-top setups. Through microscopy, high contrast imaging of concomitant formations can moreover be achieved. The discussion outlines several studies involving ionic compounds, noble metal nanoparticles, polymers, metal organic frameworks and pharmaceutical compounds. Finally, we discuss future evolutions in nonlinear optical probing that are expected to further advance the field.@en

We studied the effects of two types of ultrasonic waves, shear waves and longitudinal waves, using two nonlinear optical techniques, second-harmonic generation and hyper-Rayleigh scattering. Since shear waves hardly propagate in liquids, their influence on molecules at the interface between a surface and a liquid was studied using second-harmonic generation. Longitudinal waves propagate easily in solution, thus we used hyper-Rayleigh scattering to probe the ultrasonic effects on chromophores in solution. While we did not find shear waves to alter the second-harmonic generation from chromophores at the liquid/surface interface, the longitudinal waves caused effects comparable to our earlier observations. Longitudinal ultrasound caused a strong intensity modulation of the nonlinear optical signal according to a wave-pattern.@en

Recent developments in nonlinear optical light scattering techniques have opened a window into morphological and structural characteristics for a variety of supramolecular systems. However, for the study of dynamic processes, the current way of measuring is often too slow. Here we present an alternative measurement scheme suitable for following dynamic processes. Fast acquisition times are achieved through Fourier imaging, allowing simultaneous detection at multiple scattering angles for different polarization combinations. This allows us to follow the crystal growth of the metal organic framework ZIF-8 in solution. The angle dependence of the signal provides insight into the growth mechanism by probing the evolution of size, shape and concentration, while polarization analysis yields structural information in terms of point group symmetry. Our findings highlight the potential of dynamic angle-resolved harmonic light scattering to probe crystal growth processes, assembly–disassembly of biological systems, adsorption, transport through membranes and myriad other applications.@en

p-Nitroaniline presents the typical motif of a second-order nonlinear optically (NLO) active molecule. However, because of its crystallization in an antiparallel and hence centrosymmetric structure, the NLO activity is lost. In this contribution, the p-nitroaniline motif was built successfully into the MIL-53 metal-organic framework. More precisely, MIL-53 was synthesized with 2-amino-5-nitroterephthalate as organic linker, with Al3+, Ga3+, or In3+ as inorganic cation. The Al and Ga structures are polar, as confirmed by second-harmonic generation microscopy, yielding stable NLO materials. Indeed, they contain a 22-36% surplus of the dipolar 2-amino-5-nitro-terephthalate oriented in a parallel fashion. The indium compound was shown to be less crystalline and centrosymmetric. Ab initio modeling of the second-order NLO response shows that the Al and Ga materials show a response comparable to typical inorganic commercial NLO materials such as KDP. As a hybrid material, capable of low-temperature synthesis and processing and the ultrafast NLO responses associated with organic materials, this material can potentially provide an interesting venue for applications with respect to traditional inorganic NLO materials.@en

The surface-induced polymerization of a chromophore-functionalized monomer was probed in situ for the first time using a nonlinear optical technique, second-harmonic generation. During the first hours of the polymerization reaction, dramatic changes in the tilt angle of the chromophore-functionalized side groups were observed. Following evaluation of the nonlinear optical data with those obtained from atomic force microscopy and ultraviolet-visible, we conclude that second-harmonic generation efficiently probes the polymerization reaction and the conformational changes of the surface-grafted polymer. With polymerization time, the conformation of the surface-tethered polymer changes from a conformation with the polymer backbone and its side groups flat on the surface, i.e., a “pancake” conformation, to a conformation where the polymer backbone is stretched away combined with tilted side groups or an enlarged tilt angle distribution, i.e., a “brush-type” conformation.@en