The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

4. Survey of Novel Multiphoton Active Materials 1257 4.1. Multiphoton Absorbing Systems 1257 4.2. Organic Molecules 1257 4.3. Organic Liquids and Liquid Crystals 1259 4.4. Conjugated Polymers 1259 4.4.1. Polydiacetylenes 1261 4.4.2. Polyphenylenevinylenes (PPVs) 1261 4.4.3. Polythiophenes 1263 4.4.4. Other Conjugated Polymers 1265 4.4.5. Dendrimers 1265 4.4.6. Hyperbranched Polymers 1267 4.5. Fullerenes 1267 4.6. Coordination and Organometallic Compounds 1271 4.6.1. Metal Dithiolenes 1271 4.6.2. Pyridine-Based Multidentate Ligands 1272 4.6.3. Other Transition-Metal Complexes 1273 4.6.4. Lanthanide Complexes 1275 4.6.5. Ferrocene Derivatives 1275 4.6.6. Alkynylruthenium Complexes 1279 4.6.7. Platinum Acetylides 1279 4.7. Porphyrins and Metallophophyrins 1279 4.8. Nanoparticles 1281 4.9. Biomolecules and Derivatives 1282 5. Nonlinear Optical Characterizations of Multiphoton Active Materials 1282

Multiphoton Absorbing Materials : Molecular Designs, Characterizations, and Applications

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

3.2. Thienothiophenes 1216 3.2.1. Thieno[3,4-b]thiophene Analogues 1216 3.2.2. Thieno[3,2-b]thiophene Analogues 1217 3.2.3. Thieno[2,3-b]thiophene Analogues 1218 3.3. , ′-Bridged Bithiophenes 1219 3.3.1. Dithienothiophene (DTT) Analogues 1220 3.3.2. Dithieno[3,2-b:2′3′-d]thiophene-4,4-dioxides 1221 3.3.3. Dithienosilole (DTS) Analogues 1221 3.3.4. Cyclopentadithiophene (CPDT) Analogues 1221 3.3.5. Nitrogen and Phosphor Atom Bridged Bithiophenes 1222

Functional oligothiophenes: molecular design for multidimensional nanoarchitectures and their applications.

While organic electronics is mostly dominated by light-emitting diodes, photovoltaic cells and transistors, optoelectronics properties peculiar to organic semiconductors make them interesting candidates for the development of innovative and disruptive applications also in the field of light signal detection. In fact, organic-based photoactive media combine effective light absorption in the region of the spectrum from ultraviolet to near-infrared with good photogeneration yield and low-temperature processability over large areas and on virtually every substrate, which might enable innovative optoelectronic systems to be targeted for instance in the field of imaging, optical communications or biomedical sensing. In this review, after a brief resume of photogeneration basics and of devices operation mechanisms, we offer a broad overview of recent progress in the field, focusing on photodiodes and phototransistors. As to the former device category, very interesting values for figures of merit such as photoconversion efficiency, speed and minimum detectable signal level have been attained, and even though the simultaneous optimization of all these relevant parameters is demonstrated in a limited number of papers, real applications are within reach for this technology, as it is testified by the increasing number of realizations going beyond the single-device level and tackling more complex optoelectronic systems. As to phototransistors, a more recent subject of study in the framework of organic electronics, despite a broad distribution in the reported performances, best photoresponsivities outperform amorphous silicon-based devices. This suggests that organic phototransistors have a large potential to be used in a variety of optoelectronic peculiar applications, such as a photo-sensor, opto-isolator, image sensor, optically controlled phase shifter, and opto-electronic switch and memory.

Organic light detectors: photodiodes and phototransistors.

Novel two-photon absorption (TPA) chromophores are synthesized; they have dithienothiophene (DTT) as a π-center which is attached by electron donor (D) or electron acceptor (A) at each end through a π-conjugation. Their TPA cross section, measured using nanosecond- as well as femtosecond-pulsed laser, exhibits one of the largest values known so far. This is attributable mainly to the unique electronic properties of DTT.

New Class of Two-Photon-Absorbing Chromophores Based on Dithienothiophene

Polymeric nanocomposites have demonstrated great potential for the construction of physically flexible, large-area optoelectronic devices that can remain photoactive over a wide spectral bandwidth, depending on the customizable constituents. They bolster the prospect of developing ultrafast, sensitive, and superior optical counterparts of the corresponding electronic devices. With the successful integration of inorganic quantum dots (QDs) into conjugated polymeric matrices, efficient photodetection in different ranges of the electromagnetic spectrum has already been realized. This is enabled by the tunable optical absorption and emission of the QDs (by virtue of quantum size effects), and also by the fact that the QDs can often preserve their optoelectronic integrity within a host matrix. To generate excitons at a good quantum efficiency, the QDs should have an appreciable absorption cross section at the excitation wavelength. The photogenerated excitons should disintegrate into free charge carriers at higher rate than competitive exciton recombination. Subsequently, these charge carriers should be extracted from the photoconverter before relaxation. To realize efficient photon conversion, the rates of photogenerated-carrier separation, interfacial transfer across the different contacts, transport through the matrix, and subsequent collection at the electrodes must all be fast enough compared to exciton recombination. Therefore, it is imperative to provide an efficient transport of charges within the nanocomposite by incorporating compatible constituents that facilitate the transport process. In research on photoconducting devices that use conjugated polymers, electron-accepting materials such as C60 and singlewalled carbon nanotubes (SWNTs) have been utilized in some of the past studies. SWNTs are fascinating materials because of several peculiar physical properties. Envisioned as a rolledup graphene sheet capped with fullerene-like structures, a SWNT can behave as a metal or semiconductor as a function of the wrapping angle of the sheet and the diameter of the nanotube. In the “metallic” state, SWNTs are good ballistic conductors with a reported current density at least one order of magnitude higher than that of copper wires with the same diameter. Because of their good mechanical properties (high elastic modulus and high optical transparency), SWNTs have been used for constructing large-area transmissive films, making them an ideal component for electrical coupling in futuristic photonic devices. The aim of the current study is to investigate the photodetection efficiency of a polymeric nanocomposite containing IR-active PbSe QDs and conducting SWNTs that are chemically attached to each other by a novel procedure. PbSe QDs, endowed with a large Bohr radius, exhibit excellent quantum size effects and can be tuned to a precisely targeted absorption wavelength in the IR region. Therefore, a stable SWNT– PbSe conjugate (by chemical bonding of both components) is a promising candidate for the realization of efficient optoelectronic behavior. The large interfaces in the SWNT–PbSe conjugate offer a tremendous opportunity for efficient exciton dissociation after photogeneration in the PbSe QDs. Efficient dissociation and charge transfer depend on the differences in potential energy and electron affinity between the photoactive species (PbSe QDs) and the other components. As can be seen in Scheme 1, the band alignments with a higher electron affinity than SWNTs allow a nonactivated electron transfer from PbSe QDs to SWNTs. Because the ionization potential of the polymer (PVK: poly(vinyl carbazole)) lies closer to the value for a vacuum than that of the QDs, the transfer of photogenerated holes to the polymer also proceeds without C O M M U N IC A TI O N

/pdf/efficient-photodetection-at-ir-wavelengths-by-incorporation-51rcpte8ah.pdf

Efficient Photodetection at IR Wavelengths by Incorporation of PbSe–Carbon‐Nanotube Conjugates in a Polymeric Nanocomposite

We demonstrate a relative improvement in power conversion efficiency of polymer nanocomposite photovoltaic cells consisting of poly(3-hexylthiophene) (P3HT) functionalized CdSe nanocrystals. Thermal deprotection processing of the tert-buthoxycarbonyl moiety in the carbamate ligand surrounding the surface of CdSe nanocrystal significantly shortened the length of the ligand between nanocrystals and between the nanocrystal and the polymer matrix. The resulting device performance was investigated as a function of the composition ratio of P3HT/CdSe and the heating temperature. This simple and straightforward ligand deprotection strategy resulted in a significant increase in current density due to improvement of charge transport between the constituent materials.

Polymer nanocomposite photovoltaics utilizing CdSe nanocrystals capped with a thermally cleavable solubilizing ligand

This paper reports synthesis and semiempirical modeling of 1,4-bis(cyanostyryl)benzene (CSB)-based quadrupolar isomeric molecules (α- and β-CSB-TPs), designed to produce enhancement in fluorescence quantum yield and two-photon absorption cross-sections for the nanoaggregate form. Fluorescence yield together with high two-photon optical properties of the isomers have been fine-tuned by moving the cyano group from α to β position, which results in longer absorption-fluorescence wavelengths, and higher one- and two-photon absorptivities for the β-CSB-TP. In nonpolar toluene solution, both isomers exhibit strong one- and two-photon induced fluorescence. Both isomers followed a trend of strong solvatochromisms which was gradually on increasing solvent polarity. Aqueous dispersions of nanoparticles with diameters of ca. 150 nm have been prepared by aggregation of each isomer. The quenched fluorescence in polar media was greatly intensified followed by a 21 fold increase in TPA cross-sections. This helped achieve intense up-converted fluorescence by two-photon absorption of excited β-CSB-TP organic nanoparticles. Coaggregation-enhanced tunable fluorescence has also been demonstrated as a possible application of the isomeric α- and β-CSB-TP mixture.

Aggregation-enhanced two-photon absorption and up-converted fluorescence of quadrupolar 1,4-bis(cyanostyryl)benzene derivatives showing solvatochromic fluorescence

We report an approach to produce predefined patterns of quantum dots and multipod nanocrystals using optical lithography for direct writing of films for optoelectronic and electronic devices. To obtain photopatternability, the nanostructures (for example, CdSe, CdTe, and PbSe nanocrystals) were functionalized by incorporation of the functional ligand t-butoxycarbonyl (t-BOC) which has an acid-labile moiety. This change in the surface chemistry results in the ability to photopattern the semiconductor nanocrystals where desired for a number of optoelectronic device geometries. We demonstrate that the ultimate resolution (line width and spacing) of this technique is below 5 μm (the limit of our optical apparatus used for writing).

/pdf/robust-microstructures-using-uv-photopatternable-4s3edqyqjz.pdf

Kwang Sup Lee

Papers

New Class of Two-Photon-Absorbing Chromophores Based on Dithienothiophene

Efficient Photodetection at IR Wavelengths by Incorporation of PbSe–Carbon‐Nanotube Conjugates in a Polymeric Nanocomposite

Polymer nanocomposite photovoltaics utilizing CdSe nanocrystals capped with a thermally cleavable solubilizing ligand

Aggregation-enhanced two-photon absorption and up-converted fluorescence of quadrupolar 1,4-bis(cyanostyryl)benzene derivatives showing solvatochromic fluorescence

Robust microstructures using UV Photopatternable Semiconductor Nanocrystals