Ultrafast Optical Switching by using Nanocrystals of a Halogen‐Bridged Nickel‐Chain Compound Dispersed in an Optical Polymer
Ultrafast Optical Switching by using Nanocrystals of a Halogen‐Bridged Nickel‐Chain Compound Dispersed in an Optical Polymer
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DOI：
10.1002/adma.200602811
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发表时间：
2007-09
影响因子：
29.4
通讯作者：
S. Tao;T. Miyagoe;A. Maeda;H. Matsuzaki;H. Ohtsu;M. Hasegawa;S. Takaishi;M. Yamashita;H. Okamoto
中科院分区：
文献类型：
--
作者：
S. Tao;T. Miyagoe;A. Maeda;H. Matsuzaki;H. Ohtsu;M. Hasegawa;S. Takaishi;M. Yamashita;H. Okamoto
随着光通信网络的发展，对具有太赫兹操作的超快光开关的需求正在上升。具有大的三阶非线性极化率χ（3）[1]和小的光激发态弛豫时间t1的非线性光学（NLO）材料是这些器件不可缺少的。最近，据报道，卤素桥接的Ni链化合物的1D Mott绝缘体表现出大的χ（3）[2]和小的t1。[3]对于将这些镍化合物应用于使用例如光波导的超快光学开关器件，薄膜的制造是最重要的问题。在这里，我们报告了一种用于制造高质量薄膜的方法，其中具有烷基链的Ni化合物的纳米晶体分散在光学聚合物PMMA（聚甲基丙烯酸甲酯）中。在这些薄膜中，太赫兹重复的光开关的双光子吸收（TPA）过程被证明。本方法为过渡金属化合物在光开关器件中的应用提供了一种新的策略。图1a显示了[Ni（chxn）2Br] Br 2（chxn=环己二胺）的晶体结构，[4]它是卤素桥接的Ni链化合物的代表。在该化合物中，Ni 3+和Br-离子沿b轴沿着交替排列。两个chxn分子中氨基上的4个N原子在垂直于B的平面上与Ni ~（3+）离子配位，产生强配位场，使Ni ~（3+）离子处于低自旋态，dz ~ 2轨道上存在未成对电子. Ni ~（3+）的dz ~ 2轨道和Br ~-的pz轨道形成一个纯一维电子态。由于Ni位上的大的电子-电子库仑排斥（U），Mott-Hubbard带隙在Ni 3d带中打开。如图1f所示，占据的Br p带位于Ni 3d上哈伯德（UH）带和下哈伯德（LH）带之间，因此从Br到Ni的电荷转移（CT）跃迁对应于光学带隙。[5]先前的电反射（ER）和三次谐波产生（THG）光谱研究表明，[Ni（chxn）2Br] Br 2在一维半导体中显示出最大的χ（3）;最大值的χ（3）（-ω; 0，0，ω）ε和ε χ（3）（-3ω; ω，ω，ω）ε分别为9× 10- 5esu（1 esu= 3.335641× 10-10 C）和4× 10-8 esu。[2，6，7]这些测量是在单晶上进行的（图1c），因为由于在有机溶剂中的低溶解度以及气相沉积的困难，薄膜样品的制造还没有成功。
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 χ (3)[1] and a small relaxation time t1 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 χ (3)[2] and small t1.[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) 2Br] Br2 (chxn= cyclohexanediamine),[4] which is representative of the halogen-bridged Ni-chain compounds. In this compound, the Ni3+ and Br–ions are arranged alternately along the b-axis. Four N atoms of the amino groups in two chxn molecules coordinate a Ni3+ ion in a plane normal to b and produce a strong ligand field, so that the Ni3+ ion is in a low-spin state and an unpaired electron exists in the dz2 orbital. The dz2 orbitals of Ni3+ and the pz 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 Ni3d 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) 2Br] Br2 shows the largest χ (3) among 1D semiconductors; the maximum values of Imχ (3)(–ω; 0, 0, ω) and χ (3)(–3ω; ω, ω, ω) 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.