Nano Woodpile Structure via Two Photon Absorption Polymerization

Nano Woodpile Structure via Two Photon Absorption

Polymerization

Boyoung Kang , Jeong Weon Wu (a) , Kwang-Sup Lee (b) , Bum Ku Rhee (c)

a: Division of nano sciences, Ewha Womans Univ., 11-1 Daehyun-dong, Sodaemun-gu, Seoul, Korea,

b : Dept. of Polymer Science and Engineering, Hannam Univ., Korea.

c: Dept. of physics, Sogang Univ., Korea.

bobo@https://www.360docs.net/doc/915484644.html,

Abstract Woodpile structures for three dimensional photonic crystal were fabricated via two photon

polymerization. Resins with high two-photon crosssection undergoes a polymerization and forms the

woodpile structure upon irradiation of a 780nm Ti:Sapphire femtosecond laser. The bandgap of woodpile

depends on the parameters of woodpile, which can be varied by nano-positioning system.

1. Introduction

The woodpile structure is an attractive 3D photonic crystal possessing the photonic band gap coming from its 3D periodicity. The woodpile structures have layers of one-dimensional rods with a stacking sequence that repeats itself every four layers with a repeat distance of c. Each layer consists of the parallel rod whose width is w with a distance d between them. The orientations of the axes are rotated by 90° and the rods are shifted relative to each other by d/2 between adjacent layers. Especially when (c/d) 2 =2, the lattice can be derived from a face-centred-cubic unit cell with a basis of two rods [1]. And these parameters of woodpile structure change the character of photonic bandgap of woodpile structures.

In this paper, the woodpile structures were fabricated by two-photon absorption (TPA) polymerization that allows a direct-writing inside the polymeric materials as well as 3D sub-diffractive manufacturing [2].

2. Experiment

For the TPA process, the fine-positioning devices such as the piezoelectric transducer (P.Z.T.) and picomotor (New Focus) were adopted to control the position of focal point in the resin which responds to the high photon-flux density near the vicinity of the focal spot. The infrared light source was a 780 nm mode-locked Ti:sapphire laser with pulse width of 80 fs operating at the repetition rate of 82 MHz. The beam intensity and exposure time was controlled by use of a ND filter set and an electric shutter, respectively. To achieve the required spatial resolution, we adopted an oil immersed objective lens with a magnification of ×100 and N.A. = 1.25.

The nano positioning system was composed of the “focus-positioning” and “resin-positioning,” which identify the position of the focal point inside the polymer and to achieve high resolution nano woodpile structures [3, 4].

Figure 1 (a) The FE-SEM image of woodpile structure with d = 1.0μm and c = 1.2μm. (b) The calculated photonic bandgap.

The TPA photopolymer resin with a two-photon cross section of δ = 4.7 × 10-48 cm 4sec/photon at 780 nm and a peak wavelength of linear absorption at 423 nm undergoes two-photon absorption and polymerization upon irradiation with 780nm from a Ti:Sapphire femtosecond laser.

3. Result

The laser power of the incident beam at the resin was fixed at 5 mW. The woodpile structures were formed in the two photon resin upon irradiation. The field emission scanning electron microscope (FE-SEM, JEOL 6460) image of experimental result is shown in figure 1 (a). The distance between adjacent rods is d = 1.0μm, the rod width is w = 300nm and the repeat distance c = 1.2μm. Figure 1(b) shows the calculated photonic bandgap when refractive index n = 1.52 and number of total layers N = 8 by plane-wave method. In this case, the gap center wavelength is located at 1.651 μm.

5. Conclusions

Nano woodpile structures, important in photonic device applications, were fabiracted by a TPA photo-polymerization. The high resolution 3D structures are built up inside the IR transparent resins through a TPA polymerization. The geometric parameters of woodpile structures determined the center wavelength and the width of bandgap. Using the mechanical nano positing systems which identify the focal position, various woodpile structures with photonic band gap can be manufactured.

4. References

[1] Y . Lin, J. G . Fleming, D. L. Hetherington, B. K. Smith, R. Biswas, K. M. Ho, M. M. Sigalas, W. Zubrzycki, S. R. Kurtz, and J. Bur, “A three-dimensional photonic crystal operating at infrared wavelengths”, Nature, 394, 251 (1998).

[2] H.B. Sun, S. Matsuo and H. Misawa, “Three-dimensional photonic crystal structures achieved with twophoton-absorption photopolymerization of resin,” Appl. Phys. Lett. 74, 786 (1999).

[3] Bo Young Kang, Jeong Weon Wu, Kwang-Sub Lee, Bum Ku Rhee, and S. H. Han, “Fabrication of 3D nano-structures by using two-photon absorption polymerization,” J. Korean Phys. Soc. 45, 1154 (2004).

[4] Bo Young Kang, Jeong Weon Wu, Kwang-Sub Lee, Bum Ku Rhee, “Fabrication of nano woodpile structure”, in Proc. of SPIE (The International Society For Optical Engineering, 2006), 63520O-1.