Various structures are also formed on the surfaces of the ceramics (ALO3), on copper surface, Another im) 
and on PMMA. In the case of ceramic due to the high band gap energy fourth harmonic near or abo 
Nd:YAG laser beam is chosen with multi-pulses (15) to make the structure, as shown in size can be 
figure 7. resolution iI 
The structure, made by 15 pulses, is 1.4pm in depth and width, period 3.5um; its aspect Figures 9 ai 
ratio has reached 1:1, which is significant for efficient modification of surface PMMA. It 
properties such as anti-friction, or modification of sound coupling. 
HER 
i 
Figure 7 linear structured ceramic(ALO3). EA 
The insert is 3D description of WLI. i 
#8! 
He 
i 
[4 2] 
For PMMA, fourth harmonic(266nm) is also needed to make the structure, due to the 
transparency of the material especially for longer wavelengths, which corresponds to a 
very small optical absorption coefficient. For comparison the optical absorption coefficient 
of PMMA is around 10° at wavelength 266 nm, while it is in the order of 10° for silicon 
and metals. The induced structure, as analysed by WLI, is of 20nm depth. Here one can 
also see that despite the great difference of optical absorption coefficients up to several 
orders of magnitude, the weakest absorbing material is still workable for the structuring. Figure 9 
Some other polymers with larger absorption coefficient are also structured to produce one Surface mz 
or two dimensional patternst!'] the 1-pulse 
3.2 Dependence of the feature size on the energy density 
Quantitatively figure 8 shows the dependence of feature sizes on the energy density for c-Si. 
When the energy density is changed from 0.6 to 1.2 Jen? the depth changed from 3.5 to 30 
nm; whereas the width changes at first from 1,8 to 3.1pum, and than remains constant at 
3.1m. Under certain energy level this proportionality is easy to understand: more energy, 
more molten material, more thermal diffusion and therefore deeper and wider craters. 
40 # Depth(nm) 
N 0) 8 Widih(200nm) 
hn 
| 
20° r 
nol I F 
= oo Me - Figure 11 
060 0,70 0,80 084 099 107 1,14 1,22 a AbO;3 
laser energy density(J/cm2) transforms 
produced 
Figure 8 Feature size changed with the increasing incident energy density in c-Si. All the 
experiments are done with one shot of the laser pulse.. 
34