Polarization induced control of optical trap potentials in binary liquids

Precise Nanoparticle Manipulation Using Femtosecond Laser Trapping

Optical tweezers use strongly focused light for trapping, characterizing, and manipulating objects in the microscopic and nanoscopic regimes. However, fully understanding optical trapping at the nanoscale remains a significant challenge. This holds importance because the nanoscale is the frontier for numerous promising advancements, ranging from enhancing single-molecule investigations in biology to developing hybrid devices for nanoelectronics and photonics and exploring fundamental quantum phenomena in opto-mechanics. We report an experimental and theoretical study of nanoparticles of various sizes, showing the advantages of the immense peak power of ultrashort laser pulses over conventional optical tweezers. We also demonstrate highly stable trapping of nanoparticles for extended durations at low average laser power using femtosecond lasers.

Nanoparticle Assembling Dynamics Induced by Pulsed Optical Force

Femtosecond (fs) laser trapping dynamics is summarized for silica, hydrophobically modified silica, and polystyrene nanoparticles (NPs) in aqueous solution, highlighting their distinct optical trapping dynamics under CW laser. Mutually repulsive silica nanoparticles are tightly confined under fs laser compared to CW laser trapping and, upon increasing laser power, they are ejected from the focus as an assembly. Hydrophobically modified silica and polystyrene (PS) NPs are sequentially ejected just like a stream or ablated, giving bubbles. The ejection and bubbling take place with the direction perpendicular to laser polarization and its direction is randomly switched from one to the other. These characteristic features are interpreted from the viewpoint of single assembly formation of NPs at an asymmetric position in the optical potential. Temporal change in optical forces map is prepared for a single PS NP by calculating scattering, gradient, and temporal forces. The relative contribution of the forces changes with the volume increase of the assembly and, when the pushing force along the trapping pulse propagation overcome the gradient in the focal plane, the assembly undergoes the ejection. Further fs multiphoton absorption is induced for the larger assembly leading to bubble generation. The assembling, ejection, and bubbling dynamics of NPs are characteristic features of pulsed optical force and are considered as a new platform for developing new material fabrication method.

Correlated flickering of erythrocytes membrane observed with dual time resolved membrane fluctuation spectroscopy under different D-glucose concentrations

A correlated human red blood cell membrane fluctuation dependent on D-glucose concentration was found with dual time resolved membrane fluctuation spectroscopy (D-TRMFS). This new technique is a modified version of the dual optical tweezers method that has been adapted to measure the mechanical properties of red blood cells (RBCs) at distant membrane points simultaneously, enabling correlation analysis. Mechanical parameters under different D-glucose concentrations were obtained from direct membrane flickering measurements, complemented with membrane fluidity measurements using Laurdan Generalized Polarization (GP) Microscopy. Our results show an increase in the fluctuation amplitude of the lipid bilayer, and a decline in tension value, bending modulus and fluidity as D-glucose concentration increases. Metabolic mechanisms are proposed as explanations for the results.

Anomalously Large Assembly Formation of Polystyrene Nanoparticles by Optical Trapping at the Solution Surface

We demonstrated the optical trapping-induced formation of a single large disc-like assembly (∼50 μm in diameter) of polystyrene (PS) nanoparticles (NPs) (100 nm in diameter) at a solution surface. Different from the conventional trapping behavior in solution, the assembly grows from the focus to the outside along the surface and contains needle structures expanding radially in all directions. Upon switching off the trapping laser, the assembly disperses and needle structures disappear, while the highly concentrated domain of the NPs is left for a while. The single assembly is quickly restored by switching on the laser again, where the needle structures are also reproduced but in a different way. When a single 10 μm PS microparticle (MP) is trapped in the NP solution, a single disc-like assembly containing needle structures is similarly prepared outside the MP. Based on backscattering imaging and tracking analyses of the MP at the solution surface, it is proposed that scattering and propagation of the trapping laser from the central part of the NP assembly or the MP lead to this new phenomenon.