Direct 3D Patterning of TiO₂ Using Femtosecond Laser Pulses

Abstract

Two-photon polymerization of photosensitive materials irradiated by femtosecond laser pulses is now considered as an enabling technology for the fabrication of 3D structures, especially photonic crystals and photonic crystal templates. [1][2][3][4][5][6][7] Depending on the topology and dielectric constant contrast of photonic crystals, their optical properties can be tailored in a desired manner. Since 1987, when the concept of 3D photonic crystal was introduced by Yablonovitch [8] and John, [9] photonic crystals have been a subject of intensive research. In spite of this, the fabrication of photonic crystals with a full 3D (or omni-directional) bandgap in the visible range is still a challenging task. To realize photonic crystals with a full photonic bandgap, 3D structuring of high refractive index materials is required. The high refractive index (ca. 2.4) and high transparency in the visible spectrum makes TiO 2 a very promising photonic material. For many applications, it is desirable to have a simple technique for patterning this material in twoand even three dimensions. Conventionally, TiO 2 is patterned by sputtering it onto a pre-patterned organic resist followed by lift-off. When thick films of TiO 2 or complicated features are desired, the lift-off process does not work well. For 3D fabrication, the most attractive option so far has been to fabricate templates which were later infiltrated with a high refractive index material, followed by the removal of the original template structure. [5,7,10,11] Another approach is to use inorganic-organic photosensitive materials (ormocers) for the fabrication of photonic crystals. [12] In this case, there exists a possibility to skip the replication steps and to fabricate 3D inorganic structures directly. However, the inorganic content in these materials is not very high and the attempts to fabricate in three dimensions have resulted in porous structures. In order to eliminate the issue of making templates and their subsequent infiltration with a high refractive index material, a direct 3D fabrication of structures appears to be an attractive option. As 2 S 3 , a chalcogenide glass, was directly patterned in three dimensions by exploiting its photo-induced metastability.[6] On the other hand, examples of direct patterning of oxides are nonexistent. In this paper, we will demonstrate a direct 3D patterning of TiO 2 structures using femtosecond laser pulses. To enable such fabrication, a photosensitive solgel-based spin-coatable TiO 2 resist was developed. The experimental setup used for the patterning is shown in Figure 1 and is similar to that used before. [4,12] The TiO 2 resist is transparent (refractive index = 1.68) to the femtosecond laser radiation of 780 nm wavelength, and therefore, allows focusing of laser pulses tightly into the material volume. In the focal area, where the intensity of femtosecond laser pulses is high enough to initiate multiphoton processes, bond-breaking in the TiO 2 resist makes the irradiated regions insoluble in organic solvents such...

TiO2 microstructures by inversion of macroporous silicon using atomic layer deposition

“…Meanwhile, a broad range of techniques has been established for the fabrication of ordered porous materials [6][7][8][9][10][11][12]. However, the controlled growth of uniform structures with reproducible pore shapes is still a challenging task, especially for three-dimensional (3D) geometries.…”

“…However, the controlled growth of uniform structures with reproducible pore shapes is still a challenging task, especially for three-dimensional (3D) geometries. Recently, new approaches towards 3D structures were presented, e.g., direct laser writing by two-photon polymerization [8,9], holographic lithography [10], and microsphere self-assembly [11]. Although these methods are capable to provide very nice templates, they require high technological effort for shrinkage compensation [12] and are not easily scalable.…”

Thermally Stable Organic–Inorganic Hybrid Photoresists for Fabrication of Photonic Band Gap Structures with Direct Laser Writing

“…Although several inorganic-organic hybrid photoresists have recently been developed for microfabrication using multiphoton polymerization, [32][33][34] all of these contain significant amounts of organic functional groups that can lead to shrinkage or cracks at high temperatures. For example, while woodpile photonic crystals have been prepared using a TiO 2 -based resist, [35] the resulting structures also suffered significant contraction upon heating. In contrast, we utilize an inorganic-organic hybrid photoresist with low organic content and high thermal stability.…”

Two-photon controlled sol–gel condensation for the microfabrication of silica based microstructures. The role of photoacids and photobases

“…4 Later on, those strategies were extended to Zr-Si based oxide materials, 5 SiCN nanostructures, 6 or TiO 2 patterning. 7,8 In all these examples, the control of the microstructure was performed via twophoton induced polymerization reactions and the polymeric organic network was used as template for the structuration. The inorganic reaction was separately produced via for instance hydrolysis of metal-organic precursors.…”