Hong-Bo Sun

Finer features for functional microdevices

Micromachines can be created with higher resolution using two-photon absorption.

Three-dimensional photonic crystal structures achieved with two-photon-absorption photopolymerization of resin

Three-dimensional photonic crystal structures were fabricated with laser microfabrication techniques through two-photon-absorption photopolymerization of resin. Significant band-gap effects in the infrared wavelength region were observed from “layer-by-layer” structures.

Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots

Carbon nanodots (C-dots) synthesized by electrochemical ablation and small molecule carbonization, as well as graphene quantum dots (GQDs) fabricated by solvothermally cutting graphene oxide, are three kinds of typical green fluorescence carbon nanomaterials. Insight into the photoluminescence origin in these fluorescent carbon nanomaterials is one of the important matters of current debates. Here, a common origin of green luminescence in these C-dots and GQDs is unraveled by ultrafast spectroscopy. According to the change of surface functional groups during surface chemical reduction experiments, which are also accompanied by obvious emission-type transform, these common green luminescence emission centers that emerge in these C-dots and GQDs synthesized by bottom-up and top-down methods are unambiguously assigned to special edge states consisting of several carbon atoms on the edge of carbon backbone and functional groups with C═O (carbonyl and carboxyl groups). Our findings further suggest that the competition among various emission centers (bright edge states) and traps dominates the optical properties of these fluorescent carbon nanomaterials.

Recent developments in superhydrophobic surfaces with unique structural and functional properties

The surface wettability control of solid materials has been considered as an essential aspect of surface chemistry. In the past decade, superhydrophobic surfaces have revealed a cornucopia of novel structural and functional properties, exhibiting considerable importance in both fundamental research and practical applications. In this review, we summarize the recent developments of superhydrophobic surfaces with unique structural and functional properties. Both the fabricative methods and the working performance of superhydrophobic surfaces with multidisciplinary functionalities including self-cleaning, icephobicity, anti-corrosion, drag reduction, transparency, anti-reflection, structural color, droplet transportation, anisotropy, oil–water separation, water supporting force, superamphiphobicity and responsive switching, have been discussed briefly. Finally, the current challenges and future prospects of this dynamic field are discussed based on our own opinion.

Curvature‐Driven Reversible In Situ Switching Between Pinned and Roll‐Down Superhydrophobic States for Water Droplet Transportation

Artifi cial superhydrophobic surfaces [ 1–10 ] with water contact angles (CAs) greater than 150 ° have been intensively investigated due to their unique “anti-water” property that could be utilized in a wide range of applications. [ 11–13 ] Recent development of intelligent devices, such as microfl uidic switches and biomedicine transporters, makes strong demands on surface wettability control, therefore, responsive surfaces have become a signifi cant issue for superhydrophobic studies. Up to now, various smart surfaces have been successfully developed as reversible switches for wettability control through a micronanostructured surface on a responsive material. [ 14–25 ] These unique tunings of surface wettability greatly contributed to refi ned control of surface wettability. With the thorough understanding of superhydrophobic phenomenon, superhydrophobic surfaces have been classifi ed into fi ve states [ 26 ] according to the details of CA hysteresis, which have been well verifi ed on different samples based on experimental results. [ 1 , 8 , 27–29 ]

Multiple-spot parallel processing for laser micronanofabrication

A tightly focused femtosecond laser has been established as a unique tool for micronanostructure fabrication due to its intrinsic three-dimensional processing. In this letter, we utilize a microlens array to produce multiple spots for parallel fabrication, giving rise to a revolutionary augmentation for our previously developed single-beam two-photon photopolymerization technology [S. Kawata, H.-B. Sun, T. Tanaka, and K. Takada, Nature (London) 412, 697 (2001)]. Two- and three-dimensional multiple structures, such as microletter set and self-standing microspring array, are demonstrated as examples of mass production. More than 200 spot simultaneous fabrication has been realized by optimizing the exposure condition for the photopolymerizable resin, i.e., a two-order increase of yield efficiency. Potential applications of this technique are discussed.

Rapid sub-diffraction-limit laser micro/nanoprocessing in a threshold material system

We report on sub-diffraction-limit (SDL) micro/nanofabrication via two-photon-absorption (TPA) photopolymerization. SDL spatial resolution was found possible in a threshold system, where materials responded to light excitation with a pronounced threshold behavior. The diffraction limit in this case became just a measure of focal spot size, but did not put any actual restraint to voxel dimensions. Experimentally, lateral spatial resolution down to 120 nm was realized by using high numerical-aperture optics. In addition, we proposed a profile scanning method, by which the fabrication efficiency was significantly increased. The TPA processing time, in the example given here, was reduced by 90%.

Arbitrary-lattice photonic crystals created by multiphoton microfabrication

We used voxels of an intensely modified refractive index generated by multiphoton absorption at the focus of femtosecond laser pulses in Ge-doped silica as photonic atoms to build photonic lattices. The voxels were spatially organized in the same way as atoms arrayed in actual crystals, and a Bragg-like diffraction from the photonic atoms was evidenced by a photonic bandgap (PBG) effect. Postfabrication annealing was found to be essential for reducing random scattering and therefore enhancing PBG. This technique has an intrinsic capability of individually addressing single atoms. Therefore the introduction of defect structures was much facilitated, making the technique quite appealing for photonic research and applications.

Improved spatial resolution and surface roughness in photopolymerization-based laser nanowriting

The fundamental issues on the smallest possible processing accuracy and the best feasible surface smoothness in pinpoint polymerization-based laser fabrication were experimentally investigated. The lateral spatial resolution is improved from the previously reported value, 120nm, to around 100nm by intentionally introducing radical quenchers in the resin. The roughness measured from 10μm×10μm surface areas were averaged to 4–11nm, which is found slightly affected by the laser pulse energy but independent on the scanning pitch when it is smaller than a critical value. The surface quality of this level could fully satisfy the requirement of various photonic elements and devices.

Ferrofluids for Fabrication of Remotely Controllable Micro‐Nanomachines by Two‐Photon Polymerization

Miniaturized smart machines with micro-nanometer sized moving parts have now been utilized for on-site, in vivo sensing, monitoring, analysis and treatment in narrow enclosure, harsh environment, and even inside human body. [ 1–10 ] Despite the fact that a vast majority of currently available micromachines are produced with silicon by lithography, represented by Si: MEMS (silicon microelectromechanical systems) technique, a recent trend of the fi eld resorts to polymers. [ 11 ] As a designable three-dimensional micro-nanoprocessing method, two-photon photopolymerization (TPP) of photopolymers provides a novel route for fabricating micro-nanomechines with higher spatial resolution and smaller size. [ 12–14 ] However, introduction of driven force to these tiny devices for precise micro-manipulation constitutes the main problem for the advanced applications of these micro-nanomachines, for example, remote control is indispensable for intelligent micromachine that may be placed in blood vessels for health care. It is therefore of great importance to fi nd novel fabricative and driven techniques for making functional micronanomachines with precise motion control. Magnetic force drive technique which has been widely applied in macroscopical instruments would be an ideal method for remote control due to its simple, mind, safe and non-contact properties. [ 15–18 ] However, up to now, magnetic force is still not properly applied to remote control of micro-nanomachines for accomplishing desired task. Possible reason for this gap would be the lack of nanotechnology of appending magnetic components to existing micro-nanomachines, or to photopolymer precursors for laser fabrication. Generally, magnetic components such as nanoparticles which usually behave inorganic properties are really diffi cult to be introduced into polymeric carriers in a highly dispersed, largely doped fashion without signifi cant phase separation. [ 19 ] If magnetic nanomaterials can be homogeneously, largely and stably embedded in photopolymerizable resin through a simple method, magnetic force controllable micro-nanomachines would be fabricated accordingly.