One-dimensional Photonic Crystals by Multiple-component AC Electrodeposition and Dealloying

One-dimensional photonic crystals (Fig. 1), such as Bragg reflectors, rugate filters, and microcavities, are interesting for both applied and fundamental research. The major challenge of 1-D photonic crystals is in fabrication of these structures with sufficient precision and good scalability. This is especially an outstanding obstacle in the emerging branch of optics based on metallic subwavelength photonic materials. Here the researchers propose an economical and versatile approach to construct 1-D photonic crystals by combining the two well-established techniques: multi-component AC electrodeposition and selective dealloying (Scheme 1). The key idea here is to deposit a binary metallic structure with a carefully designed compositional profile by electrodeposition and then selectively etch away one component by dealloying. In so doing, the periodic metal/dielectric (i.e., metal/air) interface is generated, which is crucial for the occurrence of photonic behaviors for surface plasmon materials.Multi-component AC electrodeposition has been widely used for growing high-resolution composition-modulated binary alloy films in an automated one-pot one-step manner. During this process, a single bath containing a mixture of the metal salts corresponding to the target film components is used, and a particular AC waveform is applied to precisely control the composition along the growth direction of the film in a periodic manner (Fig. 2). Consider a simple example. The deposition for a Cu-Ni multilayer with a composition varying between the two extremes – Cu and Ni – can be carried out with a voltage square wave including a copper deposition cycle of 0.17 V and a nickel deposition cycle of 1.19 V. The amount of charge passed during each copper or nickel deposition cycle can be controlled to give the corresponding layer thicknesses. Interestingly, the use of this convenient technique has been limited to systems of close packed metallic multilayers. The periodic interface between metals and dielectrics is thus missing in these metallic multilayers.In this regard, dealloying provides a potentially effective method for transforming the above multi-component metallic deposition into porous photonic structures. In the process of dealloying, one component can be selectively etched from a bimetallic alloy resulting in a porous metal structure. This is a widely used industrial process and is commonly applied to homogeneous alloy systems – the generation of porous systems of photonic crystals has not been explored by the dealloying method. It is by combining these two powerful tools of multi-component AC electrodeposition and dealloying that the researchers intend to fabricate 1-D photonic crystals with accurately modulated structures. In particular they intend to achieve the first demonstration of porous metallic rugate filters, whose continuously modulated composition profile together with the processing difficulty of metals have been posing a great challenge for the current fabrication techniques of photonic crystals.