1. Polymorphism control with optical tweezers

The light-matter interactions depending on the laser polarization mode in optical trapping-induced crystallization is intriguing. When optical trapping is applied to a glycine solution, two different polymorphs can be selectively observed, depending on the power and polarization mode of the trapping laser. The laser power and polarization play a critical role in determining the concentration and arrangement of solutes at the laser focus, thus influencing crystal polymorphism. Interestingly, it has been discovered that the control over polymorphism by the laser is more pronounced when the concentration of the sample solution is lower.

2. Control of crystal morphology with optical tweezers

The dynamic evolution of potassium chloride (KCl) crystal morphology is demonstrated using surface optical tweezers. By focusing laser irradiation at the air/solution interface, crystallization is triggered, and real-time observations reveal the dynamic changes in crystal morphology. At the early stage of crystallization, three different crystal morphologies, namely needle, rectangle, and cubic, are observed. With increasing laser power, there is a higher probability of generating cubic crystals, particularly when irradiated with linear polarization. Under linearly polarized laser irradiation, the morphology evolves in a stepwise manner from needle to rectangle, and eventually to cubic. However, when circularly polarized laser irradiation is applied, the morphology only evolves from needle to rectangle, without transitioning into the cubic form. This is because the rectangle crystal dissolves while rotating due to the circular polarization.

3. Enantioselectivity in chiral crystallization by plasmonic manipulation

By employing intricately crafted triangular trimeric gold nanostructures, plasmonic manipulation has achieved a remarkable crystal enantiomeric excess of sodium chlorate (NaClO₃) exceeding 50%. The pursuit of stronger asymmetric interactions between light and matter has been instrumental in attaining such high enantiomeric excess. In this study, the well-designed gold nanostructures were immersed in a saturated NaClO₃ D₂O solution and subjected to irradiation from a 1064 nm continuous-wave laser with linear, left-hand, and right-hand circular polarizations. Within a matter of seconds from the initiation of irradiation, an achiral metastable crystal formed at the focal point of the laser. Subsequent irradiation induced a polymorphic transition to a chiral crystal. Importantly, the chirality of the crystal proved to be sensitive to the handedness of circular polarization, enabling efficient enantioselectivity.

4. Laser-induced enantioselectivity in polymorphic Transition

This study explores the use of femtosecond laser pulses to induce enantioselectivity in the polymorphic transition process of sodium chlorate (NaClO₃) crystals. The researchers utilized a dry achiral crystal in a metastable state as the starting material and demonstrated that the laser pulses triggered the transition from the achiral phase to the chiral phase. The experiments showed that the laser fluence had a significant effect on the initiation of the polymorphic transition. Furthermore, the researchers achieved enantioselective control of the resulting chiral crystals by using circularly polarized (CP) laser light. The experiments revealed a significant chiral bias in crystal formation, with the handedness of CP light influencing the enantiomorph formed. The study also discussed the mechanism behind the enantioselectivity induced by laser pulses and suggested that the asymmetrical perturbations caused by the laser may play a role. This research provides insights into the manipulation of chirality in crystallization processes and has implications for the development of drugs with desired handedness.

5. Aggregation-induced emission enhancement with optical tweezers

Spatiotemporal control of aggregation-induced emission enhancement (AIEE) of a protonated tetra-phenylethene derivative is achieved using optical tweezers. Initially, a single submicrometer-sized aggregate is confined by laser irradiation, resulting in minimal detectable fluorescence. Continuous irradiation of the aggregate leads to sudden and rapid growth, accompanied by bright yellow fluorescence emission. The fluorescence intensity at the peak wavelength of 540 nm is significantly enhanced during the growth process, indicating the activation of AIEE by optical tweezers. Interestingly, the activation of AIEE can be switched on or off by alternating the laser power, demonstrating that optical tweezers can increase the local concentration and overcome the electrostatic repulsion between the protonated molecules.

6. Enhancing supramolecular assembly: optical trapping unleashes new possibilities for chiral photochemistry

This study demonstrates the application of optical trapping using a focused laser beam to enhance supramolecular assembly. By employing this method, thermodynamic limitations are overcome, and a higher-order 2:4 complex that was previously inaccessible through the dimerization of 1:2 complexes of γ-cyclodextrin with two 2-anthracenecarboxylic acids (ACs) is achieved Additionally, the use of optical trapping facilitates the chiral photocyclodimerization of ACs, leading to changes in regio- and enantio-selectivities. The combination of optical trapping and supramolecular photochirogenesis also allows for capturing photochemical snapshots of aggregation and disaggregation behavior, tracking the fate of mixed aggregates formed at the laser focus both during and after optical trapping. These observations reveal that disaggregation requires more than 24 hours to complete and demonstrate that dimerization of 1:2 complexes is restricted in densely packed aggregates but promotes in relaxed aggregates once laser irradiation ceases.

7. Amyloid fibril formation of proteins with optical tweezers

Protein amyloids have garnered significant attention due to their association with severe diseases and their potential as future materials with favorable mechanical and optical properties. Optical tweezers have been effectively utilized to create a single spherical assembly of amyloid fibrils derived from the cytochrome c domain-swapped dimer. The confirmation of amyloid fibrillar structures in the prepared aggregates is achieved through transmission electron microscopy. This marks the first successful demonstration of locally collecting proteins in a solution using optical tweezers, enabling the artificial production of amyloid fibers at specific positions and times.

8. Laser fabrication of nanoparticles

When organic solids suspended in a solution are exposed to laser pulses, a highly excited electronic state is formed at the surface layer of the organic solid. This excited state quickly relaxes back to the ground state through efficient non-radiative processes. During the temporal duration of the laser beam, this process leads to efficient photo-thermal conversion, causing significant lattice motion and resulting in fragmentation from the surface. Colloidal solutions of organic nanoparticles can be obtained by collecting the fragments using poor solvents. The size, morphology, and polymorph of these organic nanoparticles can be controlled by adjusting the input laser power and wavelength. For instance, when femtosecond laser pulses are used as a light source, quinacridone nanoparticles (red pigment) with an average diameter of 13 nm can be obtained. This represents the smallest organic nanoparticles produced using a top-down method.