Large-Area Subwavelength Hole Arrays (25039)
A novel high throughput method for fabricating free-standing hole array films with precise dimensions and spacing has been developed by Northwestern researchers. Enhanced transmission and standing wave patterns are generated by coupling light to the subwavelength hole arrays using surface plasmon resonances. The tailorable combination of hole materials, shapes and polarized radiation afford a wide range of spectroscopic outputs with potential nanophotonic and sensing applications.
ADVANTAGES: A simple, high throughput, parallel, process to generate large-area films (> 1 in2) of subwavelength hole arrays versus alternative serial focused ion beam milling procedures. Multi-layered, shaped hole array films of noble metals, magnetic and dielectric materials can be fabricated with exquisite dimensional and thickness control. These features permit varied spectral tuning for applications in microscopy, sensors, magneto-optic data storage, and solar cells.
SUMMARY: Hole arrays are generally produced by focused ion beam (FIB) milling, a serial, low throughput process. Free-standing suspended films can be fabricated by FIB and reactive ion etching, but generation of multi-layered films has been limited to a few metals. No technique has been developed for producing optical quality hole arrays in a parallel fashion from multiple materials and in areas larger than hundreds of square microns.
The present process employs a combination of phase-shifting photolithography, wet-chemical etching, and electron-beam deposition to generate single or multiple material films of desired thickness. Removal of the substrate affords defect free, large-area (> 1 in2), hole array films on glass(Figure 1). Single metal or multiple material layered structures including 50, 100 nm thick Au films with 250 nm holes, 50/50 nm thick Au/Ni films, 40/20/40 nm Au/Ni/Au films, and 20/70/20 nm Au/SiOx/Au films have been fabricated The process enables construction of anisotropic hole arrays of varying size, shape, configuration, curvature and pitch.
The optical quality of these films has been demonstrated by transmission near-field scanning optical microscopy (NSOM). Under 633 nm illumination, a 100‑nm thick Au film produced enhanced transmission at the holes, generated by the localized surface plasmon resonance with the holes and standing wave patterns between adjacent holes. Use of polarized light exhibits dramatic changes in emission properties as hole material, shape and configuration are altered (Figure 2). Process variables permit fine control of the hole array spectral response. Nanoholes and nanoparticles appear to provide complementary structures, which should be facilitated by the present invention. This technology provides a flexible strategy to create subwavelength films whose properties are tunable for applications in microcopy, sensors, magneto-optic data storage, and solar cells.
Teri Odom, Joel Henzie, Eun-Soo Kwak and Min Hyung Lee
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