Direct laser writing (DLW) has been shown to render 3D polymeric optical components, including lenses, beam expanders, and mirrors, with submicrometer precision. However, these printed structures are limited to the refractive index and dispersive properties of the photopolymer. Here, we present the subsurface controllable refractive index via beam exposure (SCRIBE) method, a lithographic approach that enables the tuning of the refractive index over a range of greater than 0.3 by performing DLW inside photoresist-filled nanoporous silicon and silica scaffolds. Adjusting the laser exposure during printing enables 3D submicron control of the polymer infilling and thus the refractive index and chromatic dispersion. Combining SCRIBE’s unprecedented index range and 3D writing accuracy has realized the world’s smallest (15 µm diameter) spherical Luneburg lens operating at visible wavelengths. SCRIBE’s ability to tune the chromatic dispersion alongside the refractive index was leveraged to render achromatic doublets in a single printing step, eliminating the need for multiple photoresins and writing sequences. SCRIBE also has the potential to form multicomponent optics by cascading optical elements within a scaffold. As a demonstration, stacked focusing structures that generate photonic nanojets were fabricated inside porous silicon. Finally, an all-pass ring resonator was coupled to a subsurface 3D waveguide. The measured quality factor of 4600 at 1550 nm suggests the possibility of compact photonic systems with optical interconnects that traverse multiple planes. SCRIBE is uniquely suited for constructing such photonic integrated circuits due to its ability to integrate multiple optical components, including lenses and waveguides, without additional printed supports.
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...
Interference lithography is a flexible technique for creating 3D periodic nano‐ and microstructures that can be used to make a wide variety of crystal lattices, but as it is found, some lattices require index‐matched substrates to eliminate reflections at the photoresist–substrate interface. In this study a tunable‐refractive index quarter wavelength‐thickness polystyrene/poly(vinyl methyl ether) homopolymer blend backside antireflection coating, which alleviates this issue, is presented. The coating's refractive index can be tuned from 1.47 to 1.6, drastically reducing reflections at the photoresist–substrate interface for substrates with refractive indices as low as 1.35 for normal incidence and even lower for angled illumination. By injecting the light through the substrate and applying the antireflection layer to the top of the photoresist, interference lithography can even be performed on high refractive index substrates, such as indium tin oxide (ITO)‐coated glass. Fabrication of hexagonal, face‐centered cubic, and simple cubic lattices in SU‐8 photoresist (refractive index of 1.59) is demonstrated using 532 nm laser light on nonindex matched substrates including ITO‐coated glass and borosilicate glass, and the effects of reflection interference on the photonic bandstructure are investigated.
Lack of good ways to incorporate functional features and materials into 3D architectures has impeded progress in 3D photonic crystals (PhCs). Utilizing a modified transfer printing strategy, this study demonstrates the introduction of functional external materials of a diversity of form‐factors into the interior of holographically defined 3D PhCs. PhCs containing solid SU‐8 features, layers of porous silicon (PSi) or emissive LaF3:Nd3+ nanocrystals, and silica colloids are formed. For the LaF3:Nd3+ layer, both enhancement (≈50%) and suppression (≈25%) of the spontaneous emission (λ ≈ 1.32 μm) could be realized by modifying the position of the photonic crystal stop band relative to the rare earth emission. Finite‐difference time‐domain simulations suggest the observed spontaneous emission modification is a result of Bragg mirror‐like reflection, while the measured enhancement is likely caused by the spontaneous emission coupling to a defect mode. Via electrodeposition, this study demonstrates structural inversion of a low refractive index photonic crystal (photoresist‐based) to a high (Cu2O) refractive index contrast photonic crystal in the presence of an embedded defect, providing an opportunity to enhance the light‐matter interactions using a materials system with transparency in the visible.
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