Biomedical Engineering Reference
In-Depth Information
helicity, referred to as a spin,
s
, angular momentum. Consequently,
circularly polarized optical vortices carry a helicity, known as the
total angular momentum,
j
(
=
l
+
s
), given by the vector sum of
the orbital andspin angular momenta.
If the helicity (wavefront, polarization or both) of the vortices is
e
cientlytransferredtothemelted(orvaporized)metal,themelted
(or vaporized) metal will rotate azimuthally about the annular
intensityprofileoftheOV.Resultingspiralstructureswillbeformed.
In Section 10.2.1, we mentioned that OV laser ablation produces
metal nanoneedles. However, we investigated the morphology
of the ablated target only by using a confocal laser-scanning
microscope, and we did not completely determine the structure of
the nanoneedles on the nanoscale. Also, we did not investigate the
performance of OV laser ablation by changing the sign of the OV
helicity, either. In this section, we report on how to determine the
chirality of nanoscale metal structures by the OV helicity. We also
mention that the laser energy used in the present experiments was
controlled in the range of 0.075-0.3 mJ, which is less than one-sixth
that (2 mJ) used in the experiments mentioned in Section 10.2.2.
At an energy level above 1 mJ, dense plasma (vaporized material)
produced by the leading edge of the OV pulse shielded the rest of
the optical pulse, resulting in an insu
cient transfer of the angular
momentum to the melted material. And thus, only low-energy pulse
deposition permitted chiral nanoneedles to form.
To reverse the helicity of the OV, the SPP and the quarter wave
plate were inverted. Four vortex pulses with a
φ
∼
65
μ
m annular
spot onto the target were overlaid. The ablated target was observed
byascanningelectronmicroscope,SEM(JEOL,JSM-6010LA),witha
spatial resolution of 8nm at 3kV.
A target surface ablated by an OV pulse with
j
=−
2 is shown
in Fig. 10.15a. A needle was formed at the center of the ablated
zone with a smooth outline, and it typically had a tip curvature of
nearly 70 nm and a height of nearly 10
μ
m. The conical surface of
the magnified needle (see Fig. 10.15c,d) was twisted azimuthally
in the clockwise direction. In contrast, a needle fabricated by the
irradiation of an OV with
j
=
2 was twisted azimuthally in the
counter-clockwise direction. We call these twisted needles chiral
nanoneedles (seeFig. 10.15e,f).
