Fully Controllable Structural Phase Transition in Thermomechanical Molecular Crystals with a Very Small Thermal Hysteresis.

The construction of a practical crystalline molecular machine faces two challenges: to realize a collective molecular movement, and to amplify this movement into a precisely controlled mechanical response in real time and space. Thermosalient single crystals display cooperative molecular movements that are converted to strong macroscopic mechanical responses or shape deformations during temperature-induced structural phase transitions. However, these collective molecular movements are hard to control once initiated, and often feature thermal hystereses that are larger than 10 °C, which greatly hamper their practical applications. Here, it is demonstrated that the phase boundaries of the thermomechanical molecular crystal based on a fluorenone derivative 4-DBpFO can be used to finely control its structural phase transition. When this phase transition is triggered at two opposite crystal faces, it is accompanied by two parallel phase boundaries that can be temperature controlled to move forward, backward, or to halt, benefitting from the stored elastic energy between the parallel boundaries. Moreover, the thermal hysteresis is greatly decreased to 2-3 °C, which allows for circular heating/cooling cycles that can produce a continuous work output.

Halide Perovskites for Nonlinear Optics.

Halide perovskites provide an ideal platform for engineering highly promising semiconductor materials for a wide range of applications in optoelectronic devices, such as photovoltaics, light-emitting diodes, photodetectors, and lasers. More recently, increasing research efforts have been directed toward the nonlinear optical properties of halide perovskites because of their unique chemical and electronic properties, which are of crucial importance for advancing their applications in next-generation photonic devices. Here, the current state of the art in the field of nonlinear optics (NLO) in halide perovskite materials is reviewed. Halide perovskites are categorized into hybrid organic/inorganic and pure inorganic ones, and their second-, third-, and higher-order NLO properties are summarized. The performance of halide perovskite materials in NLO devices such as upconversion lasers and ultrafast laser modulators is analyzed. Several potential perspectives and research directions of these promising materials for nonlinear optics are presented.

Robust thermoelastic microactuator based on an organic molecular crystal.

Mechanically responsive molecular crystals that reversibly change shape triggered by external stimuli are invaluable for the design of actuators for soft robotics, artificial muscles and microfluidic devices. However, their strong deformations usually lead to their destruction. We report a fluorenone derivative (4-DBpFO) showing a strong shear deformation upon heating due to a structural phase transition which is reproducible after more than hundred heating/cooling cycles. Molecular dynamic simulations show that the transition occurs through a nucleation-and-growth mechanism, triggered by thermally induced rotations of the phenyl rings, leading to a rearrangement of the molecular configuration. The applicability as actuator is demonstrated by displacing a micron-sized glass bead over a large distance, delivering a kinetic energy of more than 65 pJ, corresponding to a work density of 270 J kg-1. This material can serve as a prototype structure to direct the development of new types of robust molecular actuators.

Enhanced Second Harmonic Generation from Ferroelectric HfO2-Based Hybrid Metasurfaces.

Integrated nonlinear metasurfaces leading to high-efficiency optical second harmonic generation (SHG) are highly desirable for optical sensing, imaging, and quantum photonic systems. Compared to traditional metal-only metasurfaces, their hybrid counterparts, where a noncentrosymmetric nonlinear photonic material is incorporated in the near-field of a metasurface, can significantly boost SHG efficiency. However, it is difficult to integrate such devices on-chip due to material incompatibilities, thickness scaling challenges, and the narrow band gaps of nonlinear optical materials. Here, we demonstrate significantly enhanced SHG in on-chip integrated metasurfaces by using nanometer thin films of ferroelectric Y:HfO2. This material has the merit of CMOS compatibility, ultraviolet transparency up to 250 nm, and significant scalability down to sub-10 nm when deposited on silicon. We observe a 20-fold magnitude enhancement of the SHG intensity from the hybrid metasurface compared to a bare ferroelectric HfO2 thin film. Moreover, a 3-fold SHG enhancement is observed from the hybrid metasurface compared to a control structure using nonferroelectric HfO2, demonstrating a major contribution to the SHG signal from ferroelectric Y:HfO2. The effective second-order nonlinear optical coefficient χ(2) of Y:HfO2 is determined to be 6.0 ± 0.5 pm/V, which is comparable to other complex nonlinear photonic oxide materials. Our work provides a general pathway to build an efficient on-chip nanophotonic nonlinear light source for SHG using ferroelectric HfO2 thin films.

Chiral Lead Halide Perovskite Nanowires for Second-Order Nonlinear Optics.

Hybrid organic/inorganic lead halide perovskites (LHPs) have recently emerged as extremely promising photonic materials. However, the exploration of their optical nonlinearities has been mainly focused on the third- and higher-order nonlinear optical (NLO) effects. Strong second-order NLO responses are hardly expected from ordinary LHPs due to their intrinsic centrosymmetric structures, but are highly desirable for advancing their applications in the next generation integrated photonic circuits. Here we demonstrate the fabrication of a novel noncentrosymmetric LHP material by introducing chiral amines as the organic component. The nanowires grown from this new LHP material crystallize in a noncentrosymmetric P1 space group and demonstrate highly efficient second harmonic generation (SHG) with high polarization ratios and chiroptical NLO effects. Such a chiral perovskite skeleton could provide a new platform for future engineering of optoelectronic functionalities of hybrid perovskite materials.

Controlling the Growth of Molecular Crystal Aggregates with Distinct Linear and Nonlinear Optical Properties.

Two novel donor-acceptor molecules, 2,7-diphenylbenzo[1,2-b:4,3-b']difuran-4,5-dicarbonitrile and 2,7-bis(4-methoxyphenyl)benzo[1,2-b:4,3-b']difuran-4,5-dicarbonitrile containing cyano group as the electron acceptor, were synthesized. Their single-crystal structures, molecular packing, and self-assembly behaviors were also investigated. By simple solvent evaporation techniques, these compounds self-assemble into various low-dimensional microstructures that demonstrate distinctive nonlinear optical properties depending on the orientations of their transition dipoles. This study highlights the importance of the transition dipole moment in the construction of low-dimensional molecular materials with highly efficient nonlinear optical properties.

Extended π-conjugated ruthenium zinc-porphyrin complexes with enhanced nonlinear-optical properties.

Zinc porphyrin dimers and trimers, in which the porphyrins are connected via a ruthenium metal core, exhibit large third-order nonlinear optical absorption coefficients and refractive indices. These properties are a result of the presence of multiple porphyrins, which lead to an extension of the π-conjugated system and the octupolar effect.

Organized chromophoric assemblies for nonlinear optical materials: towards (sub)wavelength scale architectures.

Photonic circuits are expected to greatly contribute to the next generation of integrated chips, as electronic integrated circuits become confronted with bottlenecks such as heat generation and bandwidth limitations. One of the main challenges for the state-of-the-art photonic circuits lies in the development of optical materials with high nonlinear optical (NLO) susceptibilities, in particular in the wavelength and subwavelength dimensions which are compatible with on-chip technologies. In this review, the varied approaches to micro-/nanosized NLO materials based on building blocks of bio- and biomimetic molecules, as well as synthetic D-π-A chromophores, have been categorized as supramolecular self-assemblies, molecular scaffolds, and external force directed assemblies. Such molecular and supramolecular NLO materials have intrinsic advantages, such as structural diversities, high NLO susceptibilities, and clear structure-property relationships. These "bottom-up" fabrication approaches are proposed to be combined with the "top-down" techniques such as lithography, etc., to generate multifunctionality by coupling light and matter on the (sub)wavelength scale.

Self-assembled organic microfibers for nonlinear optics.

While highly desired in integrated optical circuits, multiresponsive and tunable nonlinear optical (NLO) active 1D (sub)wavelength scale superstructures from organic materials are rarely reported due to the strong tendency of organic molecules to self-assembly in centrosymmetric modes. Here a solution-processed assembly approach is reported to generate non-centrosymmetric single-crystalline organic microfibers with a cumulative dipole moment for anisotropic combined second- and third-order NLO.

Structural transition in peptide nanotubes.

Phase transitions in organic and inorganic materials are well-studied classical phenomena, where a change in the crystal space group symmetry induces a wide variation of physical properties, permitted by the crystalline symmetry in each phase. Here we observe a conformational induced transition in bioinspired peptide nanotubes (PNTs). We found that the PNTs change their original molecular assembly from a linear peptide conformation to a cyclic one, followed by a change of the nanocrystalline structure from a noncentrosymmetric hexagonal space group to a centrosymmetric orthorhombic space group. The observed transition is irreversible and induces a profound variation in the PNTs properties, from the microscopic to the macroscopic level. In this context, we follow the unique changes in the molecular, morphological, piezoelectric, second harmonic generation, and wettability properties of the PNTs.