Ultrafast Optical Switching by using Nanocrystals of a Halogen‐Bridged Nickel‐Chain Compound Dispersed in an Optical Polymer

Abstract

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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