Processing‐Dependent Microstructure of AgCl–CsAgCl₂ Eutectic Photonic Crystals

Abstract

with large refractive index contrasts and varied structural motifs have been successfully fabricated from a wide range of materials. [7,8] However, top-down (e.g., lithographic) formation of large volumes of photonic crystals is a challenge. Selforganization techniques, such as eutectic solidification, have been shown as a possible path to forming large volumes of photonic crystals. [9][10][11][12][13][14] Among possible motifs provided by eutectic solidification, the regular microstructures of lamellar and rod eutectics have direct resemblance to 1D and 2D photonic crystals, respectively, where the phase-separated components provide the required contrast in the refractive index to exhibit a unique optical response. [6] The components of eutectic materials can be chosen from metals, semiconductors, polymers, organics, ceramics, or salts; thus providing metal, dielectric, or even metallodielectric composites with which to synthesize (or to act as templates for) photonic crystals. [11,13,[15][16][17][18][19][20][21][22][23] Recent examples from literature have demonstrated the formation of photonic crystals and other optically interesting structures (for applications like diffraction gratings, phase-separated scintillators with light guiding, and absorption-induced transparency) in directionally solidified chloride-based molten salt eutectics, such as AgCl-KCl, [16,18,22] NaCl-CsI, [23,24] CuI-KCl, [15] and KCl-LiF. [25] The eutectic solidification-based synthesis route is particularly simple if the eutectics have a low melting temperature and low surface energy, are devoid of any corroding components, like fluorides, and do not require controlled atmospheres during fabrication. However, even without these ideal factors, eutectic solidification is a quite well-established industrial process, and many challenging chemistries can be directionally solidified.The binary salt eutectic AgCl-CsAgCl 2 has the advantageous properties of a eutectic temperature (258 °C) and surface energy (135 mJ m −2 ) at its eutectic temperature lower than most other eutectic salt systems, but it has received only minimal attention. [26][27][28] Here, we show that when directionally solidified, AgCl-CsAgCl 2 has a tendency to form either a rod structure or lamellar structure depending on the directional solidification draw rates. While not unprecedented, as some binary metal eutectics, e.g., Al-Al 4 Ca, [29] Au-Co, [30] Cd-Sn, [31] Ni-W, [32] Ag-Cu, [33] and Al-Cu, [34] have been known to show transitions Directional solidification of a eutectic melt allows control over the resultant eutectic microstructure, which in turn impacts both the mechanical and optical properties of the material. These self-organized phase-separated eutectic materials can be tuned to have periodicities from tens of micrometers down to nanometers. Furthermore, the two phases possess differences in their refractive index leading to interesting optical properties that can be tailored within the visible to infrared wavelength regime. It is found the binary salt eutecti...

Template‐Directed Solidification of Eutectic Optical Materials

“…[94,105] At wavelengths above the periodicity of the lamellae, the alternating lamellar layers could be utilized as wire-grid polarizers as demonstrated for InSb-Sb and Ga-In eutectic systems in the mid-IR to sub-THz wavelength regime. As exhibited in salt eutectics, AgCl-KCl, [93] and AgCl-CsAgCl 2 , [108] the photonic bandgap can be in the visible to near-IR regime. As exhibited in salt eutectics, AgCl-KCl, [93] and AgCl-CsAgCl 2 , [108] the photonic bandgap can be in the visible to near-IR regime.…”

“…[106,107] These lamellar structures resemble 1D photonic crystals, and because they consist of alternating layers of materials with different dielectric constants, they exhibit a photonic bandgap. [109] 2D photonic crystals with bandgaps in the visible to near-IR wavelength range are produced when the eutectic organizes into a rod structure (as in AgCl-CsAgCl 2 , [108] CaF 2 -MgO, [98] NaCl-LiF, [110] and MgF 2 -MgO [111] eutectic). Similar eutectic structures have also been used as broadband planar optical waveplates (as in calcia stabilized zirconia, CaZrO 3 , eutectic).…”

“…One approach is known AgCl-KCl [94] Wire-grid polarizer InSb-Sb Mid-IR [106] Ga-In Sub-THz [107] Photonic bandgap crystals AgCl-KCl Visible to near-IR [93] AgCl-CsAgCl 2 [108] Planar optical waveguide Calcia-stabilized zirconia CaZrO 3 Visible to mid-IR [109] Fiber optic waveguide CaF 2 -MgO Visible to near-IR [98] NaCl-LiF [110] Fiber optic faceplate MgF 2 -MgO Visible to near-IR [111] Phase-separated scintillators with light guiding CsI-NaCl, CsI-RbF Visible to near-IR [112] RbI-NaCl, CsBr-NaCl There are two approaches for treating interfaces mathematically.…”

“…[106,107] These lamellar structures resemble 1D photonic crystals, and because they consist of alternating layers of materials with different dielectric constants, they exhibit a photonic bandgap. As exhibited in salt eutectics, AgCl-KCl, [93] and AgCl-CsAgCl 2 , [108] the photonic bandgap can be in the visible to near-IR regime. Similar eutectic structures have also been used as broadband planar optical waveplates (as in calcia stabilized zirconia, CaZrO 3 , eutectic).…”

“…Similar eutectic structures have also been used as broadband planar optical waveplates (as in calcia stabilized zirconia, CaZrO 3 , eutectic). [109] 2D photonic crystals with bandgaps in the visible to near-IR wavelength range are produced when the eutectic organizes into a rod structure (as in AgCl-CsAgCl 2 , [108] CaF 2 -MgO, [98] NaCl-LiF, [110] and MgF 2 -MgO [111] eutectic). The rod structures of some of the alkali halide binary salt eutectics have also been demonstrated for light guiding in phase-separated scintillators (one of the two phases is selectively doped with metallic or rare-earth elements).…”