Nanotechnology 24 (2013) 115306
M Stach et al
Figure 2. (a) The simulated TM and TE transmittances with different WGP structure pitches. PEN filtered all the UV transmittances below
380 nm. The non-linear changes of the TE and TM transmittances implied tunable polarization extinction ratios. (b) The calculated ER. The
subscript represents the pitch of the WGP in nm. The average ER increased then decreased as the pitch decreased.
Figure 3. The schematic plot of the micropositioning system. The
Figure 4. The schematic plot of the spectrophotometer setup. The
clamps held the WGP at its two ends, producing strain by moving
incident light was filtered by a commercial Glan­Thompson
the clamps in the x-direction. The clamps were designed with proper
polarizer, producing a linearly polarized TM wave for the WGP. If
structures to prevent slip or angle shifting between the WGP and the
the nanowires of a perfect WGP are parallel to the TM, the detector
clamp during straining.
should receive zero transmittance. Conversely, if the nanowires of a
perfect WGP are perpendicular to the TM wave, the detector should
receive complete transmittance. By precisely rotating the WGP
Ti and 25 nm Al. A lift-off process was done in acetone with
between 0 and 90, the detector can determine its polarization
ultrasonic vibration to produce the final WGP structure on
characteristics.
PEN.
x-direction (
4. Tuning the WGP by strain
x) and -2.6% in the y-direction (y). If the
PEN is strained over 59%, it breaks. The modification of the
patterns and the optical properties of the WGP were based
Polymers such as PEN start to deform under a relatively
on this working principle because only after the elastic region
small force. The proposed tuning method takes advantage
does PEN enter its plastic region, where it does not completely
of this phenomenon, and an external loading force in the
return to its original size after release.
x-direction causes structural lengthening in the x-direction and
shrinking in the y-direction (figure 1). Because of the strain
A homemade micropositioning system with a stepwise
property of the PEN and the WGP, it is possible to advance
movement precision of 1 µm was used to apply a strain on the
the post-lithography pattern modification and modulate the
WGP. To achieve the maximum controllability and uniformity
optical properties of the WGP.
of strain applied to the substrate, the machine direction (MD)
The working principle of the straining process identifies
of the PEN was aligned with the metal nanowires and the
three different working regions: elastic, plastic and break-
loading force (figure 3). The maximum uniaxial tensile force
down. The sample fabricated by the LIL process served as
generated by this system was 8 N and the corresponding
benchmark of 0% strain. If the applied strain is lower than the
stepwise x and y values of the system were ±0.04% and
material's elastic strain, the substrate retreats to its original
±0.02%, respectively. The subscript represents the loading
state and no changes in polarization characteristics occur.
direction, and figure 3 shows a schematic plot of the system
The elastic strain for PEN before release is +4.5% in the
setup.
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