Nonlinear optics is concerned with the optical properties of matter subjected to intense electromagnetic fields. For nonlinearity to manifest itself, the external field should not be negligible compared to the internal fields of atoms and molecules of which the matter consists. Lasers are capable of generating external fields sufficiently intense for nonlinearity to occur. Actually, The invention of lasers in 19605 was closely followed by the discovery of several nonlinear optical phenomena, such as second-harmonic generation (SHG) first demonstrated in crystalline quartz.
Second-harmonic generation (also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interacting with a nonlinear material are “combined”, and generate a new photon with twice the energy of the initial photons (equivalently, twice the frequency and half the wavelength), that conserves the coherence of the excitation. SHG has proven to be a versatile tool for characterizing surfaces, interfaces, and monolayers, and has progressively become central to a broad range of applications in optical telecommunications, data storage, and signal processing.
The design of materials exhibiting large SHG responses has been a continuous source of research in the past 30 years. Among others, organic compounds have attracted particular interest due to advantages such as tailored synthesis and easy processing, as well as the possibility of realizing flexible devices with large and fast responses. The potential use of nonlinear optical systems as sensors or active components in optoelectronic and photonic devices such as logic gates or high-density optical memories has motivated many recent works.
Most systems reported to date are based on dipolar π-conjugated dyes incorporating strong electron-donor and acceptor groups connected by a π-conjugated bridge. Symmetry can also be exploited to modulate the magnitude of the nonlinear optical responses.
The quest for new nonlinear optical materials usually relies on complementary experimental and theoretical approaches. Quantum chemical calculations are highly helpful to understand the factors governing the magnitude and character of molecular hyperpolarizabilities, and to disentangle the effects intrinsically linked to the molecular structure from those arising from the environment or the experimental setup. Owing to its computational efficiency, (time-dependent) Density Functional Theory (DFT) has rapidly become the method of choice for investigating real-life molecules, for which calculations with correlated wave function methods often remain prohibitive.
Most theoretical reports assumed a rigid picture of the investigated systems, the nonlinear optical responses being computed solely at the most stable geometry of the chromophores. Yet, recent developments combining classical molecular dynamics simulations and DFT calculations have evidenced the significant role of structural fluctuations, which may induce broad distributions of nonlinear optical responses, and even generate them in some instances.
Now, a team of researchers reviews 1 some recent case studies in which theoretical simulations have highlighted these effects. The discussion specifically focuses on the simulation of the second-order nonlinear optical properties that can be measured experimentally. The selected examples include organic chromophores, photochromic systems, and ionic complexes in the liquid phase, for which the effects of explicit solvation, concentration, and chromophore aggregation are emphasized, as well as large flexible systems such as peptide chains and pyrimidine-based helical polymers, in which the relative variations of the responses were shown to be several times larger than their average values.
Importantly, the impact of geometrical fluctuations is also illustrated for supramolecular architectures with the examples of nanoparticles formed by organic dipolar dyes in water solution, whose soft nature allows for large shape variations translating into huge fluctuations in time of their nonlinear optical response, and of self-assembled monolayers based on indolino-oxazolidine or azobenzene switches, in which the geometrical distortions of the photochromic molecules, as well as their orientational and positional disorder within the monolayers, highly impact their nonlinear optical response and contrast upon switching.
The authors conclude that future improvements should be oriented toward the development of fully integrated computational solutions, with the ultimate (and still out of reach) scheme being ab initio molecular dynamics including on-the-fly calculations of the optical properties. This approach would also enable to take into account spatial correlations between molecules, an effect important for liquids as well as for concentrated solutions.
Author: César Tomé López is a science writer and the editor of Mapping Ignorance
Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research papers