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3D Photonic Crystal Fabrication through Holographic Lithography

Ying-Chieh (Christy) Chen, Graduate Student in Materials Science and Engineering

BS Materials Science and Engineering Department, National Tsing Hua University, Taiwan

Holographic lithography has attracted great interest among the various methods of 3D photonic crystals fabrication.  It is versatile for constructing crystals with different symmetry and basis. The fabrication method is to generate 3D periodic interference patterns with coherent laser beams, then use photoresist to record the patterns (Fig.1). The resulting crystal is large-area, defect free. The size of the crystal can be easily scaled up without increasing processing time (Fig.2).     

           

 

Fig.1 Fabrication process

Fig.2 Holographic photonic crystal

To obtain a photonic band gap, it is necessary to have high enough refractive index contrast.  Therefore the polymer-air crystal created by this process often serves as a template and inverted into high dielectric constant materials such as silicon, which can be created by chemical vapor deposition (CVD). 

Incorporating controlled defects into photonic band gap materials can lead to many attractive applications such as optical cavities and waveguides.  Although the holographic technique cannot inherently create defects, incorporating two-photon polymerization technique allowed us to do so. In two-photon polymerization, excitation only occurs at the focal point when two photons are absorbed simultaneously, allowing arbitrary 3D writing by polymerization.  This can be done by either writing into the same photoresist or by infiltration of a triacrylate photoresist system. (Fig. 3 & Fig. 4)

Fig.3 SEM image of two-photon polymerized features in a holographic photonic crystal

Fig.4  Fluorescent image of photo-initiated proton distribution of a photoresist film with 3D periodic holographic pattern and aperiodic two-photon feature. The left image is the cross-sectional (x-z) slice and the right images are the horizontal (y-z) slices.

 

We are interested in fabricating functional holographically based photonic structures and studying their optical properties and comparing to prediction. Currently we have two laser systems for holography; one has emission at 532nm and the other at 351nm. I have been focusing on the 532nm system.

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