MECHANICAL ENGINEERING SUMMER RESEARCH PROGRAN

Microscale ruling for the manufacture of biomimetic optics

Preston Hamby1, John Troutman 1, Joseph Owen1, Daniel Barnhardt1, Matthew A. Davies1, and Thomas J. Suleski2

1Department of Mechanical Engineering and Engineering Science, University of North Carolina at Charlotte

2Department of Physics and Optical Science, University of North Carolina at Charlotte

BACKGROUND: Optics in nature have structures over multiple scales that perform optical functions ranging from focusing light to anti-reflective coatings to polarization sensitivity. The manufacture of such optics presents significant research challenges since it requires the production of structures with a size scale less than the wavelength of visible light on curved surfaces that may be many millimeters in dimension. In this project a “zero-stiffness” passive ruling device is designed and tested. The device has the potential for producing sub-wavelength structures on optical surfaces that may be spherical, aspherical or even freeform in nature. Applications include infrared imaging, surveillance, and consumer electronics.

METHODS: Several device concepts such as flexures and inclined planes were examined. A design consisting of a balanceable rotational inertia on an air bearing support was chosen by down-selection. The air bearing used had radial and axial stiffnesses of 2.3 N-m/mrad and 17 N/micron respectively. The air bearing error motions of ±1 micro-inch at 40 psi with flight height of 3.7 micron. A single crystal diamond tool with a so-called “dead-sharp” tip (less than 100 nm nose radius) was mounted on the center inertia and balanced using precision screws. The system was mounted on a Moore Nanotechnology 350 FG diamond machine tool for testing. By introducing a small imbalance using a precision 3/16”-254TPI Fine Adjustment screw, a force as low as 0.5 milli-Newtons was introduced at the tool tip. The tool was brought into contact with a newly diamond turned naval brass 464 surface and moved along the surface using the machine axes to generate mico-scale features.

RESULTS: The device was mounted on the machine. A round nosed diamond tool was first used to diamond machine an optical quality test flat. The force level on the device was set at four different levels: 0.25 mN, 0.5 mN, 1 mN and 10 mN. The tip of the device was brought into contact with the workpiece at each force level. The tip of the device was them moved over the workpiece to nominally make linear structures separated by: 0.1 µm, 0.5 µm, 1 µm and 10 µm. Results of the tests were measured in the SEM.

CONCLUSIONS: A zero-stiffness ruling device can be used to draw sub-wavelength features on an optical surface. However, the following challenges remain: (1) damping tool oscillations; (2) calibration the force level and depth of the features produced; (3) testing the device for cutting features on curved optical surfaces.