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As the development of optical communication networks progresses, the demand for ultrafast optical switching with terahertz operation is rising. Nonlinear optical (NLO) materials with large third-order nonlinear susceptibility v (3) [1] and a small relaxation time t 1 of the photoexcited states are indispensable to these devices. Recently, it has been reported that 1D Mott insulators of halogen-bridged Ni-chain compounds exhibit large v (3) [2] and small t 1 . [3] For the application of these Ni compounds to ultrafast optical switching devices using, for example, optical waveguides, fabrication of a thin film is a most important issue. Here, we report a method for the fabrication of high-quality thin films, in which nanocrystals of a Ni compound with alkyl chains are dispersed in an optical polymer, PMMA (poly(methyl methacrylate)). In these films, terahertz repetition of optical switching by two-photon absorption (TPA) processes is demonstrated. The present approach represents a new strategy for the application of transitionmetal compounds to optical switching devices. Figure 1a shows the crystal structure of [Ni(chxn) 2 Br]Br 2 (chxn=cyclohexanediamine), [4] which is representative of the halogen-bridged Ni-chain compounds. In this compound, the Ni 3+ and Br -ions are arranged alternately along the b-axis.Four N atoms of the amino groups in two chxn molecules coordinate a Ni 3+ ion in a plane normal to b and produce a strong ligand field, so that the Ni 3+ ion is in a low-spin state and an unpaired electron exists in the d z 2 orbital. The d z 2 orbitals of Ni 3+ and the p z orbitals of Br -form a purely 1D electronic state. Due to the large electron-electron Coulomb repulsion (U) on the Ni site, a Mott-Hubbard gap is opened in the Ni 3d-band. As shown in Figure 1f, the occupied Br p-band is located between the Ni 3d upper-Hubbard (UH) band and the lower-Hubbard (LH) band, so that the chargetransfer (CT) transition from Br to Ni corresponds to the optical gap.[5]Previous electroreflectance (ER) and third-harmonic generation (THG) spectroscopic studies have revealed that [Ni(chxn) 2 Br]Br 2 shows the largest v (3) among 1D semiconductors; the maximum values of Imv(-x;0,0,x) and v (3) (-3 x;x,x,x) were 9 × 10 -5 esu (1 esu = 3.335641 × 10 -10 C) and 4 × 10 -8 esu, respectively. [2,6,7] These measurements were performed on single crystals (Fig. 1c) because the fabrication of film samples has not been successful, owing to the low solubility in organic solvents as well as the difficulty of vapor deposition. One way of fabricating an optical thin film in such cases is to disperse nanocrystals in optically transparent polymers. This method has been employed for the fabrication of good optical films of single-wall carbon nanotubes, using gelatin, polyvinyl alcohol, and carboxymethylcellulose as matrices.[8-10] Before we can apply this method to a Ni compound, its affinity to organic solvents should be improved. An effective method for this is to introduce alkyl chains. Indeed, in halogen-bridged mixed-valence plati...

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Variable-energy positron annihilation spectroscopy (VEPAS) has been extensively applied to study defects in near-surface regions and buried interfaces, but there is an inherent limit for depth resolution due to broadening of the positron implantation profile. In order to overcome this limit and obtain optimum depth resolution, iterative chemical etching of the sample surface and VEPAS measurement are employed. This etch-and-measure technique is described in detail and the capabilities are illustrated by investigating the depth profile of defects in Si after B and P implantations with 2×1014/cm2 at 100 keV followed by annealing. Defect tails can be accurately examined and the extracted defect profile is proven to extend beyond the implanted ion range predicted by the Monte Carlo code TRIM. This behavior is more remarkable for P ion implantation than B, and the mass difference of the implanted ions is strongly related to it. No significant difference is recognized in the annealing behavior between B and P implantations. After annealing at 300 °C, the defect profile is hardly changed, but the ratio of the characteristic Doppler broadening, S, a parameter for defects, to that for the bulk Si rises by 0.01, indicating that divacancies, V2, are transformed into V4. Annealing at more than 500 °C causes diffusion of the defects toward the surface and positron traps are annealed out at 800 °C. It is proved that this resolution-enhanced VEPAS can eliminate some discrepancies in defect profiles extracted by conventional means.