1. Summer Research Experience for Undergraduates 2015
Department of Mechanical Engineering and Engineering Science
Introduction:
Microscale Ruling for the Manufacture of Biomimetic Optics
R. Preston Hamby, Dr. Matt Davies
Goal: Creating a device which can apply sub-wavelength ruling lines on any freeform optical surface.
Background as to why?
Ruling lines on a sub-wavelength scale have special properties only achieved at this scale.
When observing optics in natures such as eyes of Moth’s or Fly’s, Ruling lines have been found which gives the eye enhanced visual prowess ranging from
focusing light to anti-reflective coatings to polarization sensitivity. As seen in Figure 1 below you can see the lines of the Moth’s eye which gives it these
enhanced abilities.
Results and Conclusions:
Acknowledgements:
References:
1. Liu, M., Shultz, J., Owen, J., Davies, M. & Suleski, T. Moth’s eye anti-reflection gratings on germanium freeform surfaces. 91920L–91920L–10 (2014). doi:10.1117/12.2061421
2. Davies, M., Owen, J., Troutman, J., Barnhardt, D.,
3. Evans, C. Precision Engineering: an Evolutionary View. 136-137, 142-144 (Cranfield Press, 1991)
4. Yan, J., Maekawa, K., Tamaki, J. & Kuriyagawa, T. Micro grooving on single-crystal germanium for infrared Fresnel lenses. J. Micromech. Microeng. 15, 1925 (2005).
5. Clapham, P.B. & Hutley, M. C. Reduction of lens reflection by the moth eye principle. Nature (London) 287-282 doi:10.1038/244281a0
Figure 1 SEM images of multiscale moth’s eye structure
First thing to do after having a good understanding of the research already done is break down to take into its components.
Physics of the goal: 1. apply 2mili-Newton of force to the surface while applying a larger force to the control arm. 2. Constant force against freeform surfaces requires balanceable control apparatus.
Several device concepts were considered to achieved this such as flexures, inclined planes, and electro magnetic fields. After much design consideration the design consisting of a balanceable
rotational inertia on an air bearing support was chosen by down-selection. The key factors were the physics behind this in order to apply such a small force. This design utilized gravity as the force due
to the fact that it is the only force constant and never changing, while the air bushing was chosen to do its zero friction rotation not to impede the devices rotation. Gravity presented a problem due to
it being to strong, the next consideration was how to apply a small force to a surface while applying a large force to the system….. Simple using leverage or the reverse of leverage. The designed a
device seen in Figure 4 to use leverage against ourselves allowing a change of 0.5mili-Newtons of force.
The device was machined and able to be mounted on a Smithy. Balancing was achieved via alteration of adjustment of screws.
Function of air bushing was not within tolerance and has yet been allowed to test. Force level on the device was to be calibrated at four different levels: 0.25 mN, 0.5 mN, 1 mN and 10 mN.
Due to the air bushing not being to specifications other arrangements had to be made to rest with a dead sharp cutting tool. This will be done on the nanotechnology 350FG diamond machine.
Results of the final test will be measured with SEM.
A zero-stiffness ruling device has now been made.
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.
Methodology
Background to the Project Creation
The manufacture of optics with these characteristics 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. No one around the world has to date been able to accomplish this task that I have undertaken. Reaching to achieve this task evolved into
this project of a “zero-stiffness” passive ruling device that was to be designed and tested. The device created 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. In principle the utilization of natures ability to enhance optics the applications are limitless.
How
With no one successfully achieving this a large problem arises on the how. The plan involved utilized the Effective Medium Theory in combination with an anti-reflective coating and the application
of sub-wavelength structures. Anti-reflective coating are currently used in practice for optical surfaces to reduce relativity. My task was to design and test a device to create these sub-wavelength
structures. As seen below in Figure 2 is a representation of the Effective Medium Theory.
Figure 2. Geometry of
subwavelength gratings
and effective medium
approximation.
Figure 4. Evolution of physics into working Device
Specifics:
Air bushing at 40 psi:
radial stiffnesses - 2.3 N-m/mrad
axial stiffnesses - 17 N/micron
Motion error of ±1 micro-inch
Fly Height of 3.7 micron.
For testing: The tool apparatus was intended to be 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 to be induced.
The tool would then be 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. Thus this would lead to the creation of sub-wavelength ruling lines on the optical surface.
Tool:
Single crystal diamond tool
Dead-Sharp tip (less than 100nm nose radius)
Figure 5: Air bushing and 2 thrust bearing for free rotational
motion in combination with zero friction lateral motion
Figure 6. Moore Nanotech 350FG
at UNC Charlotte (a) external
view (b) fix-axis view
Figure 3. Examples of geranium free
form optics fabricated at UNC
Charlotte though diamond milling;
(a) Lens array; (b) cubic Alvarez lens
surface with integrated alignment
seats.
Figure 8. Completed and
completed assembly of project
As of current the created device was able to
successfully produce lines in a surface of
Producing the pictures seen to the right in
Figure 9. The lines were able to be created
with significant control of the balanced.
Figure 9. The completed ruling lines on the chosen surface.
Figure 7. All components
Needed for device.