Microstructures for optical networks and solar light trapping

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.

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.

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.

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!

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.