Photonic devices based on in-plane hetero photonic crystals
Photonic devices based on in-plane hetero photonic crystals
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DOI：
10.1126/science.1083066
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发表时间：
2003-06-06
期刊：
影响因子：
56.9
通讯作者：
Asano, T
中科院分区：
文献类型：
--
作者：
Song, BS;Noda, S;Asano, T
光子晶体（PC）是具有周期性折射率的光学材料，旨在阻挡某些波长（1-5）的光。人工缺陷，如线和/或点缺陷，可以引入到PC中，以允许光被操纵。通过利用人工缺陷之间的光耦合，有望实现尺寸为传统光学器件的1/1000至1/10，000的超小型光子器件。然而，具有令人信服的性能的设备尚未实现。这部分是由于PC的纳米纤维技术不成熟，但主要是由于缺乏通用的设计规则来获得宽波长范围的最佳性能。因为性能是由人工缺陷（5-7）之间的各种光学耦合的平衡决定的，改变仅仅一个点缺陷的谐振波长来调谐工作波长，平衡可能受到严重影响并且性能降低。我们证明了生产多个PC的平面阵列的能力，成比例的unitcell大小，平面内异质PC（IP-HPC），以解决上述问题，并制造一个超小通道添加-删除功能设备作为一个例子。异质界面对改善器件特性起着重要作用。为了理解这个概念的本质，让我们考虑PC的能带图，它是由晶格常数（6）归一化的频率f和波矢量k之间的色散关系。虽然晶格常数可以不同的PC，带图将是相同的，只要它们具有相同的晶胞结构。因此，除了工作波长由晶格常数的绝对值决定外，任何具有比例单胞结构的光子晶体的光学性质都可以是相同的。这保持了多个波长的人工缺陷之间的光耦合平衡：一旦具有特定晶胞尺寸（或晶格常数）的一个PC的结构被设计为实现最大性能，那么人们就可以期望通过简单地连接多个具有成比例晶胞尺寸的PC来实现多波长操作，同时保持最佳性能。图1A示出了基于IP-HPC的器件的示意性结构，其由具有不同晶格常数的七个PC组成。每个PC都有一个带有一排缺失空气孔的线缺陷波导和一个带有三个缺失空气孔的点缺陷腔（图1B）。通过波导传播的光被点缺陷捕获并发射到自由空间（反之亦然）。虽然相邻PC之间的晶格常数差异太小（1.25 nm），无法在视觉上区分异质界面（图1C），但光学性质仍然受到如此小的晶格常数差异的影响。电影S1展示了结果，图1D分别显示了通过波导的透射光谱和来自PC 1至PC6中的缺陷1至6的发射光谱。缺陷7的液滴光谱未在图1D中示出，因为其受到入射波导边缘处的散射光的影响。具有5至7 nm间隔的不同波长的光子成功地从每个PC顺序地落下。每个缺陷的Q因子高达3800，波长分辨率达到0.4nm。此外，Q因子对于所有缺陷几乎相等，因为Q因子是无量纲参数并且可以通过采用IP-HPC来使其相等。从缺陷1到4下降的光的强度几乎相等，这也是由于IP-HPC的效果。缺陷处的下降效率...
Photonic crystals (PCs) are optical materials of periodic refractive index, designed to block light of certain wavelengths (1–5). Artificial defects such as line-and/or pointdefects can be introduced into PCs to allow light to be manipulated. Ultrasmall photonic devices, with sizes 1/1000 to 1/10,000 of those of conventional optical devices, are expected to be realized by using optical coupling among the artificial defects. However, devices with convincing performance have yet to be realized. This is due, in part, to the immature nanofabrication technique of PCs, but mainly due to a lack of a general design rule to obtain optimum performance for a broad range of wavelengths. Because performance is determined by a balance of various optical couplings among artificial defects (5–7), changing the resonant wavelength of just one point-defect to tune the operational wavelength, the balance can be seriously affected and the performance reduced. We demonstrate the ability to produce in-plane arrays of multiple PCs with proportional unitcell sizes, in-plane hetero PCs (IP-HPC), to address the above issues and to fabricate an ultrasmall channel add-drop functional device as an example. It is also shown that the hetero interface plays an important role to improve the device characteristics. To understand the essence of the concept, let us consider a band diagram of PC, which is a dispersion relation between frequency f and wavevector k normalized by a lattice constant (6). Although lattice constants can be different for the PCs, the band diagrams will be the same whenever they have the same unit-cell structures. Therefore, the optical properties of any PCs with proportional unit-cell structures can be the same except for the operation wavelength, which is determined by the absolute value of lattice constant. This maintains the balance in optical coupling among artificial defects for multiple wavelengths: Once the structure of one PC with a specific unit-cell size (or lattice constant) has been designed to achieve maximum performance, one can then expect to achieve multiwavelength operation by simply connecting multiple PCs with proportional unit-cell sizes while maintaining the optimum performance. Figure 1A shows a schematic structure of the device based on IP-HPC, which is composed of seven PCs with different lattice constants. Each PC has a line-defect waveguide with a row of missing air holes and a point-defect cavity with three missing air holes (Fig. 1B) in a twodimensional PC slab with triangular-lattice structure. Light propagating through the waveguide is trapped by the point-defect and emitted to free space (or vice versa). Although the lattice constant difference between neighboring PCs is too small (1.25 nm) to visually distinguish the hetero interface (Fig. 1C), the optical properties are, nonetheless, sharply affected by such a small lattice-constant difference. Movie S1 demonstrates the result, and Fig. 1D shows spectra of transmission through the waveguide and emission from defects 1 to 6 in PC1 to PC6, respectively. The drop spectrum of defect 7 is not shown in Fig. 1D because it is affected by the scattered light at the incident waveguide edge. Photons with different wavelengths separated by 5 to 7 nm are successfully dropped from each PC sequentially. The Q factor of each defect was as large as 3800, and the wavelength resolution of 0.4 nm has been achieved. Moreover, the Q factors are almost equal for all defects, because the Q factor is a dimensionless parameter and can be made equal by employing IP-HPC. The intensities of light dropped from defects 1 to 4 are almost equal, which is also due to the effect of IP-HPC. The drop efficiencies at defects …