**1. Introduction**

224 Recent Advances in Nanofabrication Techniques and Applications

Yanagisawa, M.; Tsuji, Y.; Yoshinaga, H.; Kono, N. & Hiratsuka, K. (2009). Evaluation of

Yanagisawa, M.; Tsuji, Y.; Yoshinaga, H.; Kouno, N. & Hiratsuka, K. (2011). Wafer-level

lithography*, Proc. SPIE,* 7970, pp. 797014, ISSN 0277-786X

2780, ISSN 1071-1023

nanoimprint lithography as a fabrication process of phase-shifted diffraction gratings of distributed feedback laser diodes*, J. Vac. Sci. Technol.,* B 27, pp. 2776-

fabrication of distributed feedback laser diodes by utilizing UV nanoimprint

Methods of microscale and nanoscale patterning can be applied to fabricate a variety of optical devices. Periodic layered structures are found in integrated optics, communication systems, spectroscopy, lasers, and in many other important optical systems. Diffractive optical elements and photonic crystals consist of fine periodic patterns affecting the spectrum, polarization, phase, and amplitude of light. Often, holographic interferometry, or direct electron-beam patterning, is used to define the periodic structure. As an alternative method, soft lithography is effective for fabricating and transferring periodic patterns and structures as reported in recent papers (Xia & Whitesides, 1998) (Xia et al., 1999) (Schmid & Michel, 2000) (Odom et al., 2002) (T.-W. Lee et al., 2005) (Tang et al., 2003) (Rogers & Nuzzo, 2005) (D.-H. Lee et al., 2007). Accordingly, using methods and processes associated with soft lithography, new narrow-band resonant optical filters fabricated in hybrimer compounds are presented.

In this work, a new material system for fabricating the resonant optical filters is employed. Hybrimers are typical organic-inorganic hybrid materials fabricated using a sol-gel process (Choi et al., 2005) (Kim et al., 2006) (Kim et al., 2005). Hybrimers have several advantageous properties, including high modulus, low surface tension, low shrinkage, and high etching resistance. In particular, they have excellent optical properties including high transparency (>90% in the visible region), controllable refractive indices, low optical loss (<0.2 dB/cm), low birefringence (~10-4), and low viscosity compared to common ultraviolet (UV)-curable polymers (Kim et al., 2005) (T.-H. Lee et al., 2006). These materials possess thermal stability beyond 300 ºC. The versatile properties of hybrimers offer new options for practical applications related to microoptical devices. In the case of the fluorinated hybrimer used in this work, an organoalkoxysilane precursor functionalized with a perfluoroalkyl chain is used in the sol-gel reaction to lower the surface tension of the final compound. Hybrimers qualify both as molds and as resists in nanoimprint lithography (Kim et al., 2006). Significantly, there is no additional chemical treatment needed to release the mold due to the presence of fluorine molecules in the hybrimer compound.

Guided-Mode Resonance Filters Fabricated with Soft Lithography 227

Fig. 1 summarizes the procedure for fabrication of the GMR device using a µTM method. The first step of the fabrication is making a master template, which has the requisite grating structure on the surface of a silicon wafer. The silicon grating structure can be made by photoresist spin-coating, holographic recording, photoresist development, and plasma etching. These conventional fabrication process steps using photolithography are described, for example, in (Priambodo et al., 2003). However, for the results reported here, two types of commercial holographic gratings (Newport Co.) are used as master templates. One has 556 nm grating period (1800 grooves/mm) and ~170 nm grating depth for developing GMR devices operating in the near-infrared region (~850 nm wavelength). Another grating has 1111-nm grating period (900 grooves/mm) and ~340 nm grating depth for the communication band (~1550 nm wavelength). These gratings

have sinusoidal profiles.

Fig. 1. Schematic fabrication process of a GMR device by TM

We present guided-mode resonance (GMR) filters fabricated by soft lithography with hybrimer materials. The term GMR refers to a rapid variation in the intensities of the electromagnetic fields in a periodic waveguide, or photonic crystal slab, as the wavelength or the angle of incidence of the excitation light varies around their resonance values. A resonance occurs when incident light is phase-matched to a leaky guided mode allowed by the waveguide-grating structure (Magnusson & Wang, 1992). Numerous potentially useful devices based on resonant waveguide modes have been theoretically predicted and experimentally verified (Magnusson & Wang, 1992) (Avrutsky & Sychugov, 1989) (Peng & Morris, 1996) (Ding & Magnusson, 2004) (K.J. Lee & Magnusson, 2011)(Sharon et al., 1996) (Brundrett et al., 1998) (Priambodo et al., 2003) (Liu et al., 1998) (K.J. Lee et al., 2008). However, these devices were designed to work with conventional materials and processes. Therefore, additionally, we provide example fabrication and characterization of GMR filters made by soft lithography. As these resonant elements are highly sensitive to parametric variations, it is important to develop methods for their reliable fabrication. Thus, we provide a fabrication process that is consistent and simple, employing an elastomeric mold and a UV-curable organic-inorganic hybrid material. A particular fabricated device exhibits measured spectra showing ~81% reflectance and ~8% transmittance at a resonance wavelength of 1538 nm. The filter's linewidth is ~4.5 nm, and the sideband reflectance is ~5%. Experimental and theoretical results are in good agreement. We conclude that soft lithography combined with hybrimer media is an advantageous methodology for fabricating resonant photonic devices.
