Biomedical Engineering Reference
In-Depth Information
by the total angular momentum) of the optical vortex (OV) can
be directly transferred to an irradiated metal sample through an
ablation process, forming twisted nanoneedles (which we term
chiral nanoneedles) [15-17], andthe twisteddirection (chilarity) of
these nanoneedles can be controlled merely by changing the sign of
theOVhelicity.WefurtherfoundthatthehelicitiesoftheOVcanalso
provide smooth processed surfaces with little debris. The ablation
process based on helicity transfer of the optical vortices is termed
OV laser ablation.
ChiralnanostructuresfabricatedbyOVlaserablationwillenable
us to produce many new material structures, such as planar chiral
metamaterials[18,19]andplasmonicnanostructures[20,21].They
might also have potential to distinguish the chirality and optical
activity of molecules and chemical composites on ananoscale [22].
In this chapter, we review OV laser ablation, including the
formationofchiralmetalnanoneedles.Thechiralmetalnanoneedles
have a twisted conical surface, and their minimum tip curvature
is measured to be less than 40 nm, which is less than 1/25th of
the laser wavelength (1064 nm). We also briefly address a recent
progress concerning OV laser technologies based on fiber laser
architecture. Over 10 W picosecond (or nanosecond) vortex-pulse
generation is possible using a stressed fiber amplifier with a large
mode-area activecore.
10.2 Radiation Force of the Light
When light is incident on particles, they become trapped by optical
radiation forces including gradient and scattering forces [23, 24]. If
the particles are su
ciently smaller than the wavelength of light,
they can be treated as a dipole. Therefore, these optical radiation
forces can be considered as interactions of the electric field,
E
(
r
,
t
),
of light with the dipole (Rayleigh approximation). The gradient and
scattering forces depend in a large part on the polarizability,
α
,of
the particles. The
α
of nano-scale dielectric spheres (particles) is
expressed by
ε
0
m
2
−
1
n
2
a
3
α
=
4
π
+
2
,
(10.1)
m
2
