Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

A review on electrospinning for membrane fabrication: Challenges and applications

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminesce...

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

A device for optical switches and logic gates is proposed in two-dimensional photonic crystals based on self-collimated beams. The main structure of the device is a line-defect-induced 3 dB splitter. Operating principle, as revealed by both theoretical calculation and finite-difference time-domain simulation, is based on the interference of reflected and transmitted self-collimated beams. This device is potentially applicable for photonic integrated circuits.

Optical switches and logic gates based on self-collimated beams in two-dimensional photonic crystals

We report flexible and elastic enough nanofibers with diameters down to 60 nm and lengths up to 500 mm, fabricated by one-step drawing process from molten poly(trimethylene terephthalate) (PTT), exhibiting high surface smoothness and length uniformity. A series of ultracompact devices (such as optical beam splitters, couplers, rings, resonators, and tweezer/scissor-shaped structures) and nanophotonic device arrays have been assembled by the PTT nanofibers. Quantitative studies demonstrate that the PTT nanofibers/nanofiber devices exhibit good guiding properties with low optical loss from visible to near infrared region. The results suggest that the PTT nanophotonic fibers/wires would be promising candidates in constructing miniaturized photonic devices and ultracompact photonic integrated circuits (PICs), and a one-step drawing as an alternative to the standard optical bench technique.

Nanofibers drawing and nanodevices assembly in poly(trimethylene terephthalate).

Optical methods to manipulate and detect nanoscale objects are highly desired in both nanomaterials and molecular biology fields. Optical tweezers have been used to manipulate objects that range in size from a few hundred nanometres to several micrometres. The emergence of near-field methods that overcome the diffraction limit has enabled the manipulation of objects below 100 nm. A highly free manipulation with signal-enhanced real-time detection, however, remains a challenge for single sub-100-nm nanoparticles or biomolecules. Here we show an approach that uses a photonic nanojet to perform the manipulation and detection of single sub-100-nm objects. With the photonic nanojet generated by a dielectric microlens bound to an optical fibre probe, three-dimensional manipulations were achieved for a single 85-nm fluorescent polystyrene nanoparticle as well as for a plasmid DNA molecule. Backscattering and fluorescent signals were detected with the enhancement factors up to ∼103 and ∼30, respectively. The demonstrated approach provides a potentially powerful tool for nanostructure assembly, biosensing and single-biomolecule studies. By using photonic nanojets, a team in China has succeeded in simultaneously manipulating and detecting single nanoparticles and biomolecules. Optical tweezers are well known for their ability to manipulate small objects down to several hundred nanometres, but optical manipulation techniques capable of shifting even smaller objects are highly desired. Now, Baojun Li at Jinan University in Guangzhou and co-workers have used tiny light beams known as photonic nanojets to move a 85-nm polystyrene nanoparticle and a plasmid DNA molecule. Their technique can also simultaneously detect the particles due to the large signal enhancement generated by the nanojets. The nanojets were obtained from the near-field generated by a spherical polystyrene microlens attached to an optical fibre. Unlike other near-field techniques, this one does not require complex nanofabrication processes or bulky optical elements.

Manipulation and detection of single nanoparticles and biomolecules by a photonic nanojet

We report a growth of p-CuO nanowire arrays with a simple thermal oxidation and a fabrication of nanowire-based heterojunctions by coating the p-CuO nanowire arrays in an n-ZnO layer through a thermal decomposition method. Their optoelectronic properties and photovoltaic performance were investigated. Compared with the conductance in the dark, a 154% increase in photoconductance was obtained under a white light illumination of 100 mW/cm2. The heterojunctions exhibit an obvious photocurrent increment of 0.264 mA under the illumination of 141 mW/cm2. After annealing the heterojunctions at 100°C for 25 min, a larger open-circuit voltage of 0.37 V was obtained, the short-circuit current density increase to 0.63 mA/cm2 from original 0.49 mA/cm2. The overall power conversion efficiency is 0.1%.

ZnO-coated CuO nanowire arrays: fabrications, optoelectronic properties, and photovoltaic applications.

In advanced nanoscience, there is a strong desire to trap and detect nanoscale objects with high-throughput, single-nanoparticle resolution and high selectivity Although emerging optical methods have enabled the selective trapping and detection of multiple micrometer-sized objects, it remains a great challenge to extend this functionality to the nanoscale Here, we report an approach to trap and detect nanoparticles and subwavelength cells at low optical power using a parallel photonic nanojet array produced by assembling microlenses on an optical fiber probe Benefiting from the subwavelength confinement of the photonic nanojets, tens to hundreds of nanotraps were formed in three dimensions Backscattering signals were detected in real time with single-nanoparticle resolution and enhancement factors of 103–104 Selective trapping of nanoparticles and cells from a particle mixture or human blood solution was demonstrated using the nanojet array The developed nanojet array is potentially a powerful tool 

Baojun Li

Papers

Optical switches and logic gates based on self-collimated beams in two-dimensional photonic crystals

Nanofibers drawing and nanodevices assembly in poly(trimethylene terephthalate).

Manipulation and detection of single nanoparticles and biomolecules by a photonic nanojet

ZnO-coated CuO nanowire arrays: fabrications, optoelectronic properties, and photovoltaic applications.

Trapping and Detection of Nanoparticles and Cells Using a Parallel Photonic Nanojet Array.