Post on 10-Jan-2016
description
Modelling the sharp focusing of laser light
Voronezh, 2010
Sergey StafeevSamara State Aerospace University Image Processing System Institute
REC-14, Samara
Introduction Decreasing the focal spot size is critical in lithography,
optical memory and micromanipulation Sharp focusing is a reaching a minimal focal spot size
beyond the diffraction limits.
Recently plasmons with FWHM = 0.35λ [Opt.Lett. - 2009. - Vol.34, no.8. - P.1180-1182], FWHM = 0.4λ [Opt. Lett. - 2009. - vol.34, no12. - p.1867-1869] had been obtained.
In this research we used two types of axicons: refractive and diffractive which were illuminated by radially polarized light
Radial-FDTD FDTD = finite
difference time domain
This method involves the numerical solution of Maxwell's equations in cylindrical coordinate system
We used a modification for a radially polarized light (R-FDTD)
There are three equations with three components Er, Ez and Hφ
z
jiHjiH
t
jiEjiEji
nnnr
nr
)21
,21
()21
,21
(),21
(),21
(),
2
1(
2
1
0,2
1
0,10,0,
0
r
jiHirjiHir
irt
jiEjiEji
nnnz
nz
)21
,21
()21
()21
,21
()21
(
)(
1)21
,()21
,()2
1,(
2
1
0,2
1
0,10,0,
0
r
jiEjiE
z
jiEjiE
t
jiHjiH nz
nz
nr
nr
nn
)21
,()21
,1(),21
()1,21
()21
,21
()21
,21
( 0,0,0,0,2
1
0,2
1
0,
0
,0 ,00 ,0
rr
H EE
z t
,0 ,00 ,0
1 zz
rH EE
r r t
,0,0 ,00
r z HE E
z r t
(1)
(2)
tt
Refractive microaxiconFocusing of radially-polarized mode R-TEM01
(3)
using refractive (conical) microaxicon
Radial section of a conical glass (n=1.5) microaxicon of radius R =
7 µm and height h = 6 µm
The (absolute value of) radial component of the electric field strength of the mode R-TEM01
FWHM=0.30λ HMA=0.071λ2
2
2
exp)(rr
erE rr
Instantaneous distributions of the amplitude Er and Ez for diffraction of the R-TEM01 laser mode by the refractive
microaxicon
The Intensity and FWHM of the focal spot as a function of axicons height
Er Ez
Refractive microaxicon: 3D modelling
the intensity distribution in focal plane
the intensity distribution along axicon axis
Binary microaxicon
FWHM = 0.39λ HMA= 0.119λ2
binary axicon with step height 633nm, period 1.48um, index of refraction n =
1.5
the intensity distribution along axicon axis
the intensity distribution in focal plane (on the axicons surface)
Focusing of radially-polarized mode R-TEM01 using binary microaxicon
Manufacture and experiment Three diffractive binary axicons
of periods 4 µm, 6 µm, and 8 µm and height 500nm were fabricated on silica substrate (n=1.46) using a laser writing system CLWS-200 (bought with CRDF money) and plasmo-chemical etching.
Diameter of the focal spot in the near-zone (z<40um) varies from 3.5λ to 4.5λ with a 2-um period for the axicon with period 4um.
The minimal diameter of focal spot equal to 3.6λ (FWHM=1.2λ)
An oblique image of the central part of the binary axicon of period 8 µm,
produced with the Solver Pro microscope (bought with CRDF money).
The diameter of the light spot on the axis (in wavelengths) as a function of distance from
binary axicons with period 4µm
Diffraction pattern and radial section of the intensity distribution recorded with the CCD-camera from the axicons with period 4 µm at different distances: 5 µm and 2 µm (λ=532nm)
Conclusions We have numerical shown that when illuminating a
conical glass microaxicon of base radius 7 µm and height 6 µm by a radially polarized laser mode R-TEM01 of wavelength λ =1 µm, in the close proximity (20 nm apart) to the cone apex, we obtain a sharp focus of transverse diameter at half-intensity FWHM=0.30λ and axial spot size at half-intensity FWHMz=0.12λ. The focal spot area at half-intensity equals HMA=0.071λ2.
Three diffractive binary axicons of periods 4 µm, 6 µm, and 8 µm and height 500nm were fabricated on silica substrate (n=1.46) using a laser writing system CLWS-200 (bought with CRDF money) and plasmo-chemical etching.
Diameter of the focal spot in the near-zone (z<40um) varies from 3.5λ to 4.5λ with a 2-um period for the axicon with period 4um.
The results of numerical simulation agree with experiment