Biomedical Engineering Reference
In-Depth Information
Figure 10.15
(a,c,d) SEM images of a processed surface and a twisted
nanoneedle fabricated by an OV with
j
=−
2 (clockwise). (b,d,f) SEM
images of a processed surface and a twisted nanoneedle fabricated by an
OV with
j
2 (counter-clockwise). (c,d) The 25
◦
views and (e,f) top views
of the nanoneedle. The focusing lens had an NA of 0.08. Reprinted with
permission from Ref. [17]. Copyright 2012 American Chemical Society.
=
These results indicate that the twisting direction (chirality) of
the chiral nanoneedle is selectively determined by the helicity of
the OV pulse. We noticed that as the magnitude of
j
increased, the
spiral frequency of the nanoneedle (defined as the winding number
per 1
μ
m) increased. We must also mention that ultrashort (i.e.,
femtosecond) OV pulses cannot provide any chiral nanoneedles.
The tip curvature of the chiral nanoneedle was also inversely
proportionaltotheNAoftheobjectivelens,anditwasminimizedto
nearly 36 nm (Fig. 10.16), which was less than 1/25th of the laser
wavelength (1064 nm) at NA
=
0.18. The nanoneedle still had a
twisted conical surface, andits height was measured to 7.5
μ
m.
The chiral nanoneedle may have been oxided, because all of
the experiments were performed at atmospheric pressure and
room temperature. We investigated the electrical properties of the
nanoneedle by using two 50
μ
m diameter tungsten probes with an
internalresistanceofnearly1.0
.Oneprobecontactedwiththetop
of the nanoneedle, while the other was tightly pressed against the
substratesurfacesoastominimizethecontactresistance.Asshown
in Fig. 10.17, the current between the nanoneedle and the substrate
was found to be directly proportional to the supplied voltage. The
