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Micro/Nano Lithography

Microstructures for optical networks and solar light trapping

Novel microstructures enhance light-matter interactions to provide new functions, including wavelength-division multiplexing for optical networks and solar-light absorption.
15 December 2006, SPIE Newsroom. DOI: 10.1117/2.1200612.0535

Although breakthroughs in materials science may take decades, they enable completely new devices and functions. Microstructures of composite materials (MCMs), which are different arrangements of existing materials on a microscale, have unique properties that rival those of the most desirable naturally available materials. For example, diffractive optical elements (DOEs) can be used for three-dimensional interconnects,1 information encryption,2 materials science,3 and biology.4

Microstructures can dramatically change the interaction of light and matter. One example is photonic bandgap structures, in which repetitive arrangements of different materials, on a wavelength-size scale, generate barriers for light propagating in certain directions. Here we explore simple designs to enhance device functionalities.

Figure 1 shows a design that is both a wavelength-selective polarizer (WSP) and a wavelength-division multiplexer (WDM).The structure consists of two layers of gratings. The total thickness of the gratings is merely 230nm, which is about one order of magnitude thinner than thin-film filters (TFFs). The angle of incidence can be widely tuned, even to the surface-normal direction, which is impossible with TFFs.

Figure 1. Green, blue, and red curves show the simulated transmission of the wavelength-division multiplexer (WDM)/wavelength-selective polarizer (WSP) for p, s, and random polarization, respectively. The inset shows a cross-section of a single spatial period of the structure, in which brown stands for silicon, while blue and turquoise colors are silicon dioxide. The period is 740nm, and each grating layer is only 90nm thick.

In contrast, waveguide interference filters require multiple stages of waveguides and occupy substantial physical space for the same function. The MCM is a good solution for the ‘last mile’ of access networks, e.g., for homes and offices (Figure 2 of Ref. 5), where low-cost, compact WDM and WSP are required for the transmitting lasers at every bidirectional access point. The dream of unlimited bandwidth and new services over transparent, hair-thin, glass fibers will be realized only when the economics of these components and their packaging are resolved.

Figure 2. The upper part shows cross-sectional (left) and aerial (right) scanning electron microscope images of a transmission-only polarizer (TOPOL). The lower graph shows maximum-reflectance spectra in the visible wavelength range, as patterned (RIE), filled with water and a cover glass (H2O), and filled with optical index-matching liquid from Nye Gel (OCF452).

Another example exploits the anisotropic properties of nanoscale metallic gratings. Figures Figure 2 and Figure 3 represent a transmission-only polarizer (TOPOL).5,6 The reflection and transmission for transverse-electric (TE) and transverse-magnetic (TM) polarization are independently controlled with nanoscale features (width×height = 50 × 180nm) ofaluminum, fully imbedded in oxides.

Figure 3. The extinction ratios in transmission are shown for the TOPOL in Figure 2. A close-up cross-sectional image of the structure is shown at the lower right.

For clean-energy applications, light-trapping or solar cells are good candidate solutions. The strikingly simple examples in Figures Figure 4 and Figure 5 demonstrate the versatile functions that MCMs allow. Tiny silicon wires (width×height = 15×200nm), surrounded by silicon dioxide, absorb over 90% of solar light energy around 400–500nm. Similar structures work in the ultraviolet region of the spectrum. The simple, common materials used just flow through one's toes anywhere when walking on the beach!

Figure 4. Tho top panel shows a schematic view of an absorbing polarizer for solar light applications, consisting of a silicon grating embedded in silicon dioxide. The period is about 150nm, and the height is about 200nm. The bottom panel shows the unusual scaling properties of the transmitted extinction ratio as a function of the grating period ΔD,X and wavelength.

Figure 5. The nominal performance parameters for the light-trapping structure of Figure 4 are read on the left axis, while the extinction ratio (green line) is read on the right axis.

Besides being thinner, other advantages of MCMs include relaxed angular sensitivity. They may be implemented very effectively through nanoimprint lithography (NIL). NIL has unprecedented molecular-size resolution and achieves sub-20nm multilevel alignment. The single-step, three-dimensional patterning available with NIL hardly exists in competing technologies. The major challenges for NIL are surface damage, defect control, and infrastructures for the master templates. All of these issues are diminishing, and the technology is emerging as a prevailing manufacture platform in niche markets.

In short, good designs for microstructures of composite materials and economical fabrication platforms promise unique solutions to a wide range of challenging applications in communication and energy.

We thank Xiaoming Liu, Xu (Kelvin) Zhang, Paul Sciortino Jr., Lei Chen, and Anguel Nikolov for discussions. Michiko Harumoto (SEI) and Zhengping Fu (USTC) provided some references. Encouragement and support by Barry Weinbaum, Maria Light, Nada O'Brien, and Bruce Nonnemaker are appreciated.

Xuegong Deng, Jian Wang, Feng Liu
NanoOpto Corporation
Somerset, NJ

Xuegong Deng is a development manager at NanoOpto. A member of SPIE and OSA, he holds PhDs in physics and electrical engineering.

1. X. Deng, R. T. Chen, Generic three-dimensional wavelength routers based on cross connects of multilayer diffractive elements, Optoelectronic Interconnects VIII, Proc. SPIE 4292, pp. 17-24, 2001.
2. H. T. Chang, W. C. Lu, C. J. Kuo, Multiple-phase retrieval for optical security systems by use of random-phase encoding, Appl. Opt. 41, no. 23 pp. 4825-4834, 2002.
3. M. B. Sinclair, M. A. Butler, S. H. Kravitz, W. J. Zubrzycki, A. J. Ricco, Synthetic infrared spectra, Opt. Lett. 22, no. 13 pp. 1036-1038, 1997.
4. Y. Zhao, Q. Zhan, Y. Zhang, Y.-P. Li, Creation of a three-dimensional optical chain for controllable particle delivery, Opt. Lett. 30, no. 8 pp. 848-850, 2005.
5. X. Deng, J. Wang, F. Liu, Wideband antireflective polarizers based on integrated diffractive multilayer microstructures, Opt. Lett. 31, no. 3 pp. 344-346, 2006.
6. X. Deng, J. Wang, X. Liu, Q. Wu, F. Liu, Planarized multilayer composite microstructures for optical function integration, in Frontiers in Optics, Laser Science, Optical Fabrication and Testing and Organic Photonics and Electronics, Washington, DC, 2006. OFMD3OSA
7. X. Deng, J. Wang, Q. Wu, F. Liu, Microstructures for enhanced light-matter interactions: optical network devices and solar light trapping,  Proc. SPIE 6368, pp. 63680R, 2006.