Photonics


Waveguides

Optical waveguides represent an integral part of many microphotonic devices ranging from optical amplifiers, optical switches, and ring resonators, to interferometers [1]. Two types of waveguiding structures have been fabricated using p-beam writing. The first type involves direct fabrication of a high refractive index core followed by coating with a lower refractive index cladding layer. This procedure has been used to make both single and multimode waveguides in polymers [2]. Waveguides fabricated in this way exhibit low transmission loss (estimated at 0.19 dB/cm at 632.8 nm) because of the smooth sidewalls and low edge roughness of the p-beam written structures [3]. The sidewall smoothness of these waveguides has been measured to be ~4 nm [4]. The second fabrication technique relies on the property that MeV protons traveling into a sample will lose most of their energy and cause most damage in the sample at the end of range. This beam-induced damage causes a volume change at the end of range, resulting in increased density and increased refractive index [5]. In order to achieve weak guiding of light, a refractive index contrast of about 10-4-10-3 is usually sufficient, depending on the material being used. The p-beam writing technique has also recently been applied to the fabrication of buried channel waveguides in phosphate glass that has been co-doped with Er3+ and Yb3+ to produce optical amplifiers with a maximum net gain of 1.72 dB/cm measured at a wavelength of 1.534 μm [6].

Lens arrays

P-beam writing has the flexibility to fabricate microlens arrays using the thermal reflow process [7] without the need for a high aspect ratio mask. Firstly, a layer of photoresist is spin coated onto a transparent substrate (e.g. a glass slide or coverslip) and patterned using p-beam writing. The exposed regions are then removed to leave behind an array of cylinders with diameters corresponding to that of the required final lens. The cylindrical structures are then heated to a temperature well above the glass transition temperature of the polymer but below the disintegration temperature. During the heating process, the polymer flows and the surface tension forms a hemispherical surface on the substrate. This process has been used to fabricate microlens arrays in PMMA [2]. The p-beam writing technique has also been used in conjunction with Ni electroplating to fabricate microlens array molds and stamps. These stamps have been used to replicate microlens arrays in polymers such as polydimethylsiloxane (PDMS) and polycarbonate [8].

Gratings

The ability of p-beam writing to produce vertical and smooth sidewalls in resist materials, coupled with a low line-edge roughness, makes it an ideal technique for many microphotonic devices. Gratings have been fabricated in various thickness of PMMA [2]. The metallic seed layer in these samples assists the adhesion of the PMMA to the Si substrate, although for the production of metallic gratings this layer can also act as a seed layer for electroplating. These exhibit extremely low line-edge roughness, indicating that higher density lines and spaces are possible with the p-beam writing technique.

Colloidal crystal templates

In 1987, Yablonovitch [9] and John [10] showed that electromagnetic radiation interacting with a periodic dielectric structure with a lattice constant of the order of the wavelength of the radiation exhibits behavior analogous to electrons in a crystalline material. This discovery has led to a surge of interest in the development of new microoptical systems, since the ability to control light in three dimensions will lead to numerous applications in the field of optoelectronics and microphotonics. These photonic crystals can be used to make waveguides with 90º bends, enhance the emission from light-emitting diodes [11], and fabricate optical fibers that have air as a core material [12]. One way of fabricating photonic crystals uses the crystallization of a monodisperse colloidal system to create a three-dimensional lattice of micro- and nanospheres. Because p-beam writing can write precise and accurate three-dimensional structures with vertical walls and low edge roughness, it has been shown to be a useful tool for the fabrication of polymeric templates for directed self-assembly [2]. The precision of the high aspect ratio template structures that can be machined using p-beam writing can be used to support more layers than are otherwise possible with standard colloidal crystallization techniques.

References:


1. Prieto, F., et al., Nanotechnology (2003) 14, 907
2. Bettiol, A. A., et al., Nucl. Instr. Meth. Phys. Res. B (2005) 231, 364
3. Sum, T. C., et al., Appl. Phys. Lett. (2003) 83, 1707
4. Sum, T. C., et al., Appl. Phys. Lett. (2004) 85, 1398
5. Roberts, A., and von Bibra, M. L., J. Lightwave. Technol. (1996) 14, 2554
6. Liu, K., et al., Appl. Phys. Lett. (2004) 84, 684
7. Borrelli, N., Microoptics Technology: Fabrication and Applications of Lens Arrays
and Devices. Marcel Dekker Inc., New York, (1999)
8. Dutta, R. K., et al., Nucl. Instr. Meth. Phys. Rev. B (2007), in press
9. Yablonovitch, E., Phys. Rev. Lett. (1987) 58, 2059
10. John, S., Phys. Rev. Lett. (1987) 58, 2486
11. Wierer, J. J., et al., Appl. Phys. Lett. (2004) 84, 3885
12. Russell, P., Science (2003) 299, 358