Radiation pressure, or radiation force, is produced by the electromagnetic interaction between light and a small particle, which is polarized by light. This interaction is the driving force behind the optical trapping of small particles. Tools based on optical trapping and manipulation are called “optical tweezers.” Since the first successful experimental demonstration of optical trapping, the related science and technology have undergone enormous growth. Currently, optical tweezers are a versatile tool and have reached practical usefulness; several optical tweezers systems are now commercially available.
The basic setup of conventional optical tweezers is rather simple. A single continuous-wave light beam from a solid-state laser is introduced into an optical microscope and is focused by an objective lens with a high numerical aperture to generate radiation force on a small particle at the focal point. For a scanning trapping system, a Galvano-mirror system is built into the optical setup. Features of the optically trapped particle are monitored in situ with a video camera equipped with a microscope. In some setups, the operator can manipulate and analyze the trapped particle by micro-spectroscopy (fluorescence, absorption, and Raman scattering).
Despite the simplicity of this scheme, optical tweezers have become a powerful tool mainly in the life and medical sciences, in which small biological materials such as living cells, bacteria, and microsphere-linked DNA are optically manipulated. With the recent rapid progress that has taken place in these research fields, an urgent need has arisen to manipulate relatively smaller bioparticles such as fragmented DNA, enzymes, oligopeptides, and so on. Likewise, in the field of chemistry, the manipulation of molecules and catalytic nanoparticles would be a valuable technique. However, for conventional optical tweezers, it is rather difficult to perform stable optical trapping of small nanoparticles, which have sizes on the order of nanometers, because the radiation pressure decreases as the size of the particle get smaller, and the pressure becomes too small to overcome the thermal fluctuation.
Professor Zheng and his research group have developed a new technique to build colloidal matter in a versatile manner with a light-controlled temperature field. This new technique offers a superior feature set compared to traditional optical tweezer manipulation, including applicability to diverse colloidal sizes (from sub-wavelength scale to micron-scale) and materials (polymeric, dielectric, and metallic colloids), tunable bonding strength and length, versatile colloidal configurations from one-dimensional to three-dimensional, and low-power operation (100 to 1,000 times lower than optical tweezers). The colloidal building technique provides a new optical platform to build diverse colloidal matter, and is useful in many applications such as optical matter, flat optics, photovoltaics, and bio-sensing.
Existing technologies to build colloidal matter rely on specific functionalization of the colloidal particles, including the surface charges, magnetic performance, surface roughness, or region-selectively modified with biological molecules (e.g., DNA), which features rigorous assembly rule and limited geometry control. For comparison, the colloidal building technique shows general applicability to colloidal particles with diverse size, material, and surface chemistry, all without specific functionalization. This building technique differs from conventional manipulation techniques like optical tweezers, which require tightly focused laser beams of a high-power intensity (10 to 100 mW/µm2) and are challenging to manipulate nanoscale particles with. Meanwhile, this invention utilizes extremely low optical power (~0.1 mW/µm2), and is capable of manipulating colloidal particles with sizes ranging from the micron- to nano-scale.
This invention has been developed to solve current challenges in the construction of colloidal matter, which currently is limited to specific types of colloidal particles and geometry control. The colloidal building technique enables the construction of diverse colloidal particles into versatile geometries with a high degree of control. Use of this technique will greatly expand the possible level of structural complexity achievable in the colloidal matter, which will enable the creation of novel colloidal structures for applications in optical components, energy, sensors, and others.