Hideo Kishida

Gigantic optical nonlinearity in one-dimensional Mott-Hubbard insulators

The realization of all-optical switching, modulating and computing devices is an important goal in modern optical technology. Nonlinear optical materials with large third-order nonlinear susceptibilities (χ(3)) are indispensable for such devices, because the magnitude of this quantity dominates the device performance. A key strategy in the development of new materials with large nonlinear susceptibilities is the exploration of quasi-one-dimensional systems, or ‘quantum wires’—the quantum confinement of electron–hole motion in one-dimensional space can enhance χ (3). Two types of chemically synthesized quantum wires have been extensively studied: the band insulators of silicon polymers, and Peierls insulators of π-conjugated polymers and platinum halides. In these systems, χ(3) values of 10-12 to 10-7 e.s.u. (electrostatic system of units) have been reported. Here we demonstrate an anomalous enhancement of the third-order nonlinear susceptibility in a different category of quantum wires: one-dimensional Mott insulators of 3 d transition-metal oxides and halides. By analysing the electroreflectance spectra of these compounds, we measure χ(3) values in the range 10-8 to 10-5 e.s.u. The anomalous enhancement results from a large dipole moment between the lowest two excited states of these systems.

A black body absorber from vertically aligned single-walled carbon nanotubes

Among all known materials, we found that a forest of vertically aligned single-walled carbon nanotubes behaves most similarly to a black body, a theoretical material that absorbs all incident light. A requirement for an object to behave as a black body is to perfectly absorb light of all wavelengths. This important feature has not been observed for real materials because materials intrinsically have specific absorption bands because of their structure and composition. We found a material that can absorb light almost perfectly across a very wide spectral range (0.2–200 μm). We attribute this black body behavior to stem from the sparseness and imperfect alignment of the vertical single-walled carbon nanotubes.

π-Conjugated Multidonor/Acceptor Arrays of Fullerene−Cobaltadithiolene−Tetrathiafulvalene: From Synthesis and Structure to Electronic Interactions

The synthesis, structure, photoelectrochemical behavior, and nonlinear optical (NLO) properties of a symmetric acceptor-acceptor-donor-acceptor-acceptor array, C(60)-Co-TTF-Co-C(60), have been described. The precursors, namely, cobalt dicarbonyl complexes Co(C(60)Ar(5))(CO)(2) were synthesized from the penta(organo)[60]fullerenes, C(60)Ar(5)H, as starting materials. In the next step, two cobalt-fullerene complexes were connected to a tetrathiafulvalene (TTF) tetrathiolate bridge to obtain the C(60)-Co-TTF-Co-C(60) array. In addition, the monomeric compounds, Co(C(60)Ar(5))(S(2)C(2)R(2)) (R = CO(2)Me and CN) and Co(C(60)Ar(5))(S(2)C(2)S(2) C = CS(2)C(2)R(2)) were synthesized as references. The C(60)-Co-TTF-Co-C(60) array exhibits very strong transitions in the near-infrared region (lambda(max) = 1,100 nm, epsilon = 30 000 M(-1) x cm(-1)) due to a ligand-to-metal-charge-transfer (LMCT) transition and six reversible electron transfer processes. In the crystal, a fullerene/TTF-layered packing structure is evident. Femtosecond flash photolysis revealed that photoexcitation of the array results in a charge separated state involving the strongly interacting cobaltadithiolene and TTF constituents which electronically relax via a resonance effect that extends all throughout the acceptor parts of the C(60)-Co-TTF-Co-C(60) array. The third-order NLO measurement of the array gave the magnitude of the third-order nonlinear susceptibility, |chi((3))|, values to be 9.28 x 10(-12) esu, suggesting the pi-conjugation of donors and acceptors in the array.

Third-order optical nonlinearity in regio-controlled polythiophene films

We have investigated the third-order optical nonlinearity of poly(3-hexylthiophenes) with various head-to-tail coupling ratios (r), using the third-harmonic generation method. An increase in r leads to a reduction in optical gap energy (Eg) and an increase in the third-order nonlinear susceptibility (χ(3)). For r=0 to 0.80, χ(3) is scaled by Eg as χ(3)∝Eg−6.7, while for r∼1, χ(3) is considerably enhanced beyond this scaling law. We discuss how the behavior of χ(3) is based upon the conjugation-length dependence of the transition dipole moments.

Current oscillation originating from negative differential resistance in one-dimensional halogen-bridged nickel compounds

We demonstrate current oscillation phenomena using the negative differential resistance in a one-dimensional halogen-bridged nickel compound, [Ni(chxn)2Br]Br2 (chxn=cyclohexanediamine). By attaching external resistors and a capacitor to a [Ni(chxn)2Br]Br2 sample, we obtain stable current oscillation at 90 K. The oscillation and its period are explained by a simple model.

Phase retrieval in nonlinear optical spectroscopy by the maximum-entropy method: an application to the |χ (3) | spectra of polysilane

A method of phase retrieval from the experimental modulus |χ(3)| spectra of third-order nonlinear optical susceptibility is presented on the basis of the maximum-entropy model. This method enables one to derive the real and the imaginary parts of χ(3) without using the nonlinear Kramers–Kronig calculation, which is usually hazardous in nonlinear optical spectroscopy because of the limited range of measurements. Theoretical and experimental modulus spectra of polysilane are analyzed. The results from the experimental modulus are compared with the Kramers–Kronig calculations as well as with the measured phase values. It is also shown how the method can be optimized to reduce the distortions that result from noise in the calculated spectra.

Optoelectronic conversion by polarization current, triggered by space charges at organic-based interfaces

We report that a highly efficient optoelectronic conversion can be achieved by photogenerated space charges, which usually damp photocurrent. Theoretical analysis of metal-organic insulator-organic semiconductor-metal photocells indicates the generation of a large transient current that is triggered by photogenerated space charges and governed by the dielectric properties of the insulator layer. We experimentally demonstrated this mechanism with model photocells, revealing that the quantum efficiency can be dramatically increased by increasing the dielectric constant of the insulator.

Optical spectra of silicon oligomers

Optical absorption spectra have been measured for finite-chain analogs of linear polysilane, silicon oligomers CH 3 [Si(CH 3 ) 2 ] n CH 3 , with controlled chain length n (=2 to 16). The intense lowest electronic absorption peak and its higher-lying side bands, which correspond to the one-dimensional exciton series in the infinite chain, shift to higher energy with decrease of the chain length because of the confinement of the excited states. The oscillator strength of the main absorption peak increases with the chain length, while the linewidth of the main peak drastically decreases, especially in the region n =2 to 6. These finite size effects of the electronic (excitonic) absorption are argued in terms of spatial extension of the excited states, motional narrowing and electron correlation effect.

VISIBLE LUMINESCENCE FROM BRANCHED SILICON POLYMERS

Luminescence spectra and their variation with temperature have been investigated for solid films of poly(methylphenylsilane) and its branched analogs. Introduction of branching points to the polymers enhances the broad luminescence band in the visible region while suppressing the sharp UV luminescence band due to the resonant recombination of the exciton. The intensity of the visible luminescence is observed to keep on increasing with decreasing temperature in contrast with the nearly temperature‐independent intensity of the resonant luminescence. The behavior is interpreted in terms of phonon‐assisted tunneling between luminescence centers and nonradiative ones.

Optical spectra of Si/Ge‐network copolymers: [Si(C6H13)]1−x[Ge(C6H13)]x

Optical properties of silicon‐ and germanium‐based network copolymers, [Si(C6H13)]1−x‐ [Ge(C6H13) ]x, have been investigated for various Si/Ge compositions (x). The backbone Si/Ge atoms are coordinated by the three other Si/Ge backbone atoms in the polymers. These Si/Ge copolymers show the absorption spectra which are typical of amorphous semiconductors. Their color continuously traverses the visible range with x, which is due to the change of the band gap (Tauc gap) energy as well as to the presence of localized subgap states. The Si/Ge copolymer shows a broad band of luminescence in the visible region. The luminescence band maximum was observed to shift by ∼0.5 eV with change of the Si/Ge ratio.