A review is given of recent theoretical and experimental studies on the liquid crystal (LC) infiltration of 3D photonic crystal (PC) structures so as to obtain tunable Bragg reflection and transmission characteristics. It is shown that large-pore and non-close-packed inverse opals formed by sintering, or by a multiple-layer conformal deposition technique, provide a simple and effective dielectric scaffold for liquid crystal infiltration. The dynamic optical properties are strongly dependent on the scaffold structure and the dielectric contrast between the scaffold and the LC. Experimental structures were fabricated using precise, conformal, low temperature atomic layer depositions of Al2O3 and TiO2 to create inverse opals and non-close-packed inverse opals, which were subsequently infiltrated with the nematic liquid crystals 5CB and MLC2048. The dependence of the visible/infrared reflectance and transmittance were investigated as functions of applied electric field amplitude and frequency for applications in optical modulation and switching.
We present experimental and theoretical investigations of tunable large-pore inverse opals fabricated by combining conformal films in patterned template structures with infiltrated liquid crystals. Ultra-conformal films allow opal templates to be inverted and used as scaffolding for fabricating a large-pore dielectric backbone that serves as a patterned template for electro-optic/non-linear or conventional materials. Additionally, theoretical results of tunable non-close-packed inverse opals fabricated by a multi-layer atomic layer deposition process and infiltrated with lead lanthanum zirconium titanate are presented. The structural properties of the device are defined by the template, while the dynamic properties are controlled independently by the choice of electro-optic/non-linear material. A variety of dielectric templates were modeled by choosing conformal coatings to define structures that exhibit either large Bragg peak tunability or width. The dynamic optical properties of the tuned large-pore and non-close-packed inverse opals are discussed and a model is presented for characterizing the controlled fabrication of optimized photonic crystal structures using multi-component conformal film deposition. Experimental measurements and modeling both indicate enhanced static and dynamic tunability to the photonic properties of infiltrated inverse templates compared to typical tunable opal-based inverse structures.
We report the controllable and tunable fabrication of structurally modified non-close-packed inverse shell opals using multi-layer atomic layer deposition and present a model and simulation algorithm to calculate the structural parameters critical to fabrication. This powerful, flexible and unique technique enables opal inversion, structural modification and backfilling and was applied to the fabrication of TiO2 non-close-packed inverse opals. Using successive conformal backfilling it was possible to tune the Bragg peak over 600 nm and enhance the Bragg peak width by >50%. Additionally, band structure calculations, using dielectric functions approximating the true network topology, were used to predict the optical properties during the fabrication process. 3D finite-difference-time-domain results predict experimentally achievable structures with a complete band gap as large as 7.2%. Additionally, the refractive index requirement was predicted to decrease from 3.3 in an 86% infiltrated inverse shell opal to 3.0 in an optimized non-close-packed inverse shell opal. It was also shown for these structures that the complete photonic band gap peak can be statically tuned by over 70% by increasing the backfilled thickness.
We report the fabrication of photonic crystal phosphors by atomic layer deposition and the subsequent removal of self-assembled opal templates. ZnS:Mn and TiO2 inverse opals as well as ZnS:Mn/TiO2 composite inverse opals were formed. Shifts in the G-L photonic band gap positions were confirmed by reflectivity and transmission measurements and were consistent with photonic band structure calculations. The peak positions confirm that filling terminates at ~86% of the pore volume in agreement the maximum possible filling fraction for the “shell” infiltration of an opal structure. For TiO2 depositions, SEM and AFM analysis reveals ultra-smooth highly conformal films. In addition, infiltration control to < 1 nm was achieved, making fine-tuning of PC properties possible. Significant changes were observed in the emission characteristics for composite ZnS:Mn/TiO2 photonic crystals. This work demonstrates that precisely controlling the placement of materials is possible by ALD, enabling the fabrication of “optimized” structures, including those which modify emission properties.
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