Switchable Assembly and Guidance of Colloidal Particles on an All-Dielectric One-Dimensional Photonic Crystal

Optical assembly of multi-particle arrays by opto-hydrodynamic binding of microparticles close to a one-dimensional chain of magnetic microparticles

Two-dimensional materials possess a large number of interesting and important properties. Various methods have been developed to assemble two-dimensional aggregates. Assembly of colloidal particles can be achieved with laser-heating-induced thermal convective flow. In this paper, an opto-hydrodynamic binding method is proposed to assemble colloidal particles dispersed in a solution into multilayer structures. First, we use polystyrene (PS) microspheres to study the feasibility and characteristics of the assembly method. PS microspheres and monodispersed magnetic silica microspheres (SLEs) are dispersed in a solution to form a binary mixture system. Under the action of an external uniform magnetic field, SLEs in the solution form chains. An SLE chain is heated by a laser beam. Due to the photothermal effect, the SLE chain is heated to produce a thermal gradient, resulting in thermal convection. The thermal convection drives the PS beads to move toward the heated SLE chain and finally stably assemble into multilayer aggregates on both sides of the SLE chain. The laser power affects the speed and result of the assembly. When the laser power is constant, the degree of constraint of the PS microbeads in different layers is also different. At the same time, this method can also assemble the biological cells, and the spacing of different layers of cells can be changed by changing the electrolyte concentration of the solution. Our work provides an approach to assembling colloidal particles and cells, which has a potential application in the analysis of the collective dynamics of microparticles and microbes.

Heat-Mediated Optical Manipulation

Progress in optical manipulation has stimulated remarkable advances in a wide range of fields, including materials science, robotics, medical engineering, and nanotechnology. This Review focuses on an emerging class of optical manipulation techniques, termed heat-mediated optical manipulation. In comparison to conventional optical tweezers that rely on a tightly focused laser beam to trap objects, heat-mediated optical manipulation techniques exploit tailorable optothermo-matter interactions and rich mass transport dynamics to enable versatile control of matter of various compositions, shapes, and sizes. In addition to conventional tweezing, more distinct manipulation modes, including optothermal pulling, nudging, rotating, swimming, oscillating, and walking, have been demonstrated to enhance the functionalities using simple and low-power optics. We start with an introduction to basic physics involved in heat-mediated optical manipulation, highlighting major working mechanisms underpinning a variety of manipulation techniques. Next, we categorize the heat-mediated optical manipulation techniques based on different working mechanisms and discuss working modes, capabilities, and applications for each technique. We conclude this Review with our outlook on current challenges and future opportunities in this rapidly evolving field of heat-mediated optical manipulation.

Stable directional emission in active optical waveguides shielding external environmental influences

The skillful confinement of light brought by the composite waveguide structure has shown great possibilities in the development of photonic devices. It has greatly expanded the application range of an on-chip system in dark-field imaging and confined the laser when containing an active medium. Here we experimentally proved a stable directional emission in an active waveguide composed of metal and photonic crystal, which is almost completely unaffected by the external environment and different from the common local light field that is seriously affected by the structure. When the refractive index of samples on the surface layer changes, it can ensure the constant emission intensity of the internal mode, while still retaining the external environmental sensitivity of the surface mode. It can also be used for imaging and sensing as a functional slide. This research of chip-based directional emission is very promising for various applications including quantitative detection of biological imaging, coupled emission intensity sensing, portable imaging equipment, and tunable micro lasers.

Rapid Growth of Monolayer MoSe2 Films for Large-Area Electronics

The large-scale growth of semiconducting thin films on insulating substrates enables batch fabrication of atomically thin electronic and optoelectronic devices and circuits without film transfer. Here an efficient method to achieve rapid growth of large-area monolayer MoSe2 films based on spin coating of Mo precursor and assisted by NaCl is reported. Uniform monolayer MoSe2 films up to a few inches in size are obtained within a short growth time of 5 min. The as-grown monolayer MoSe2 films are of high quality with large grain size (up to 120 μm). Arrays of field-effect transistors are fabricated from the MoSe2 films through a photolithographic process; the devices exhibit high carrier mobility of ≈27.6 cm2 V-1 s-1 and on/off ratios of ≈105. The findings provide insight into the batch production of uniform thin transition metal dichalcogenide films and promote their large-scale applications.

Liquid Optothermoelectrics: Fundamentals and Applications

Liquid thermoelectricity describes the redistribution of ions in an electrolytic solution under the influence of temperature gradients, which leads to the formation of electric fields. The thermoelectric field is effective in driving the thermophoretic migration of charged colloidal particles for versatile manipulation. However, traditional macroscopic thermoelectric fields are not suitable for particle manipulations at high spatial resolution. Inspired by optical tweezers and relevant optical manipulation techniques, we employ laser interaction with light-absorbing nanostructures to achieve subtle heat management on the micro- and nanoscales. The resulting thermoelectric fields are exploited to develop new optical technologies, leading to a research field known as liquid optothermoelectrics. This Invited Feature Article highlights our recent works on advancing fundamentals, technologies, and applications of optothermoelectrics in colloidal solutions. The effects of light irradiation, substrates, electrolytes, and particles on the optothermoelectric manipulations of colloidal particles along with their theoretical limitations are discussed in detail. Our optothermoelectric technologies with the versatile capabilities of trapping, manipulating, and pulling colloidal particles at low optical power are finding applications in microswimmers and nanoscience. With its intricate interfacial processes and tremendous technological promise, optothermoelectrics in colloidal solutions will remain relevant for the foreseeable future.

Switchable Assembly and Guidance of Colloidal Particles on an All-Dielectric One-Dimensional Photonic Crystal

Dielectric multilayer photonic-band-gap structures, called one-dimensional photonic crystals (1DPCs), have drawn considerable attention in the fields of physics, chemistry, and biophotonics. Here, experimental results verify the feasibility of a 1DPC working as a substrate for switchable manipulations of colloidal microparticles. The optically induced thermal convective force on a 1DPC can assemble colloidal particles that are dispersed in a water solution, while the photonic scattering force on the same 1DPC caused by propagating evanescent waves can guide these particles. Additionally, in the 1DPC, one internal mode can be excited that has seldom been noticed previously. This mode shows an ability to assemble particles over large areas even when the incident power is low. The assembly and guidance of colloidal particles on the 1DPC are switchable just through tuning the polarization and angle of the incident laser beam. Numerical simulations are carried out, which are consistent with these experimental observations.