Metallic and dielectric photonic crystals with chiral elements by combined nanoimprint and reversal lithography in SU-8 Bing-Rui Lu a , Jing Wan a , Zhen Shu a , Shen-Qi Xie a , Yifang Chen b , Ejaz Huq b , Xin-Ping Qu a , Ran Liu a, * a State key lab of Asic and system, Department of Microelectronics, Fudan University, No. 220, Handan Road, Shanghai 200433, China b Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK article info Article history: Received 29 September 2008 Received in revised form 12 January 2009 Accepted 13 January 2009 Available online 20 January 2009 Keywords: Reversal lithography Nanophotoic chiral crystals Optical chirality abstract We report a novel nanoprocess combining nanoimprint lithography and conventional lithography to fab- ricate metallic and dielectric nanophotonic crystals with chiral elements in SU-8. The previously devel- oped nanoimprint process was modified for much smaller feature size. Four different types of nanophotonic crystals with different materials in both large and small dimensions are fabricated. The new proposed reversal lithography is used to fabricate one type among the above mentioned four. The success of reversal lithography provides a solution for near-field lithography to achieve nanosize struc- tures with simple conventional lithography. Optical measurements of the laser polarization state from the fabricated photonic crystals indicate an optical chirality which distinguishes the chiral elements from other normal symmetric structures. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction With their unique properties, metallic and dielectric nanopho- tonic crystals have attracted increasing attention in photonics, plasmonics and their applications in optics, opto-electronics, com- munications, sensing and imaging technologies [1,2]. Meanwhile, such nanophotonic crystals with embedded arrays of nanoscale perforated chiral elements in thin films show special optical activ- ities in either diffracted or sub-wavelength regions [3]. Various nano-fabrication techniques are utilized to build two dimensional (2D) nanophotonic crystals, such as electron beam lithography (EBL), laser holographic lithography, multi-exposure interference lithography and self-assembly related techniques [4–7]. Nanoim- print lithography stands out to be an effective and economical method to achieve both flexibility and reliability in the mass pro- duction of 2D nanophotonic crystals. In this work, we have modified the previously developed nano- imprint process [8] for much smaller featured nanophotonic chiral structures with period of 600 nm and line width of 150 nm on SU-8, and combined it with other conventional processes for both metallic and non-metallic structures. And we proposed a combined nanoimprint and reversal lithography to fabricate nanosize struc- tures with metallic and dielectric bi-layer materials. Using the developed process, photonic crystals with chiral ele- ments for both infrared and visible light frequencies have been successfully fabricated. The optic chirality of the fabricated chiral structures is also investigated. Comparison between asymmetrical chiral structure and symmetrical cross structure is made, which confirms that the unique shape of the chiral element is the reason for the polarization state change of the incident light. This research opens up a promising application of chiral structures in nanophotonics. 2. Fabrication details In this work, four types of nanophotonic chiral crystals with dif- ferent materials in multiple layers were prepared. As shown in Fig. 1, the four types of structures all started with a direct nanoim- print of the chiral templates into SU-8 resist on a Pyrex substrate and then followed by other processes to achieve different struc- tures sequentially. 2.1. Nanoimprint process modification for smaller feature size For the nanoimprint process, we have fabricated smaller fea- tured nanophotonic chiral structures for application in the visible light frequencies with the sub-micron period of 600 nm. The small nanoimprint templates with height of 350 nm and line width of 145 nm are fabricated by the state-of-the-art EBL followed by a high resolution reactive ion etch (RIE). For this structure, the nano- imprint process is improved from the one developed previously in [8]. In addition to the optimized imprint condition in [8], which is a temperature of 120 °C and a pressure of 50 bar, a new 2 mm layer of PDMS is used as a buffer layer to achieve uniform pressure between the template and the substrate. Meanwhile, The UV 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.021 * Corresponding author. Tel./fax: +86 21 55664548. E-mail address: rliu@fudan.edu.cn (R. Liu). Microelectronic Engineering 86 (2009) 619–621 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee