Recent years have seen a rapid expansion of research into nanophotonics based on surface plasmon–polaritons. These electromagnetic waves propagate along metal–dielectric interfaces and can be guided by metallic nanostructures beyond the diffraction limit. This remarkable capability has unique prospects for the design of highly integrated photonic signal-processing systems, nanoresolution optical imaging techniques and sensors. This Review summarizes the basic principles and major achievements of plasmon guiding, and details the current state-of-the-art in subwavelength plasmonic waveguides, passive and active nanoplasmonic components for the generation, manipulation and detection of radiation, and configurations for the nanofocusing of light. Potential future developments and applications of nanophotonic devices and circuits are also discussed, such as in optical signals processing, nanoscale optical devices and near-field microscopy with nanoscale resolution.

Plasmonics beyond the diffraction limit

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Photoinduced motions in azo-containing polymers.

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

Convergent Dendrons and Dendrimers: from Synthesis to Applications

This letter describes the electrical control of structure and lasing in the photonic bandgaps of cholesteric liquid crystals (CLCs). Photoexcitation of dye-doped CLC cells with a linearly polarized laser gives rise to laser emission at the edge(s) of the chiral photonic band gap. Applying voltages to the optically pumped CLC cells enables reversible switching of the laser action as a result of the structural changes in the chiral photonic band gap.

Electrical control of the structure and lasing in chiral photonic band-gap liquid crystals

Intermolecular coupling enhancement of the molecular hyperpolarizability in multichromophoric dipolar dendrons

This letter describes the phototunable photonic bandgap of cholesteric liquid crystal (CLC) doped with a fluorescent dye to generate the distributed feedback effect. Photoirradiation of the dye-doped CLC cell under UV light at 254 nm resulted in continuous changes in the chiral photonic bandgap from 550 to 720 nm due to photolysis reaction of the cholesteryl iodide embedded in the CLC host. We showed that the laser oscillation wavelength could be controlled in a wide wavelength range from 610 to 700 nm by photoassisted adjustment of CLC photonic bandgaps.

Phototunable photonic bandgap in a chiral liquid crystal laser device

Electro-optic side-chain polymers have been synthesized by the post-functionalization of methacrylate isocyanate polymers with novel phenyl vinylene thiophene vinylene bridge (FTC) nonlinear optical chromophores. For this application, FTC-based chromophores were modified in their electronic donor structure, exhibiting much larger molecular hyperpolarizabilities compared with the benchmark FTC. Of these new chromophores, absorption spectra, hyper-Rayleigh scattering experiment, and thermal analysis were carried out to confirm availability as effective nonlinear optical units for electro-optic side-chain polymers. The electro-optic coefficients (r33) of obtained polymers were investigated in the process of in situ poling by monitoring the temperature, current flow, poling field, and electro-optic signal. Compared with the nonsubstituted analogue, benxyloxy modified FTC chromophore significantly achieved higher nonlinear optical property, exhibiting molecular hyperpolarizability at 1.9 μm of 4600 × 10−30 esu and an r33 value of 150 pm/V at the wavelength of 1.31 μm. Synthesized electro-optic polymers showed high glass transition temperature (Tg), so that the temporal stability examination exhibited >78% of the electro-optic intensity remaining at 85 °C over 500 h. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Nonlinear optical side-chain polymers post-functionalized with high-β chromophores exhibiting large electro-optic property

We prepared high-gain media for laser emission by using a conventional laser-dye (DCM) and dendrimer. A dendrimer can encapsulate the laser dye, and therefore increase its concentration up to 9.0 mM with limited self-aggregation and intermolecular quenching. The optical confinement is attributed to gain guiding under optical excitation. The laser spectrum clearly indicated periodic resonant modes though the laser feedback was caused in an unusual way. Above the lasing threshold, the spectral linewidth became less than 0.1 nm. The laser-dye-doped dendrimer showed a particular ability to decrease the lasing threshold intensity by increasing the dye concentration.

Akira Otomo

Papers

Electrical control of the structure and lasing in chiral photonic band-gap liquid crystals

Intermolecular coupling enhancement of the molecular hyperpolarizability in multichromophoric dipolar dendrons

Phototunable photonic bandgap in a chiral liquid crystal laser device

Nonlinear optical side-chain polymers post-functionalized with high-β chromophores exhibiting large electro-optic property

Laser emission from high-gain media of dye-doped dendrimer