High-repetition-rate femtosecond laser micromachining
of poly(methyl methacrylate)
Yiming Luo (罗义鸣), Wei Jia (贾 威)*, Youjian Song (宋有建), Bowen Liu (刘博文),
Minglie Hu (胡明烈), Lu Chai (柴 路), and Chingyue Wang (王清月)
Ultrafast Laser Laboratory, School of Precision Instruments and Optoelectronics Engineering, Key Laboratory
of Optoelectronic Information Technology (Ministry of Education), Tianjin University, Tianjin 300072, China
*Corresponding author: jiaw@tju.edu.cn
Received March 12, 2015; accepted May 7, 2015; posted online June 10, 2015
Investigations are performed to explore high-repetition-rate femtosecond laser ablation effects on the physical
and chemical properties of poly(methyl methacrylate) (PMMA). A scanning electron microscopy (SEM) is used
to characterize the morphology change in the laser-ablated regions. The infrared and Raman spectroscopy
reveals that the fundamental structure of the PMMA is altered after laser ablation. We demonstrate the
cumulative heating is much greater during high-repetition-rate femtosecond laser ablation, supporting a
photothermal depolymerization mechanism during the ablation process.
OCIS codes: 140.7090, 140.3390.
doi: 10.3788/COL201513.070003.
Poly(methyl methacrylate) (PMMA) has been used in
many technological fields due to its excellent optical trans-
parency, light weight, and good mechanical properties.
Recently, femtosecond laser micromachining has at-
tracted great attention for microdevice fabrication, as it
provides excellent materials with high precision and few
heat-affected zones
[1–3]
. The mechanism of the femtosecond
laser ablation of polymers has been thoroughly studied in
the past decades
[4,5]
. Suriano et al. examined the ablation of
PMMA with a Ti:sapphire femtosecond laser, and the
chemical functional groups of PMMA were found to re-
main unaffected, suggesting a thermal depolymerization
process
[6]
. Morikawa et al. investigated the 800 nm femto-
second laser processing of PMMA, and observed the
breaking of bonds and the formation of defects
[7]
. Lee et al.
reported that the chemical bond was less decomposed
during infrared (IR) femtose cond laser micromachining,
suggesting the optical-thermal effect was more significant
than the optical-chemical effect
[8]
. In most cases, chirped
pulse amplification technology was used to generate fem-
tosecond lasers with pulse repetition rates ranging from a
few kilohertz up to 100 kHz, and the corresponding
processing speeds were too low for industrial applications,
due to their weak average power and low repetition
rates
[9,10]
. On the other hand, high-repetition-rate pulse
lasers could improve the processing speeds, but could
also cause heat to accumulate in the surrounding
material. Thus, there were very few studies on polymer
micromachining with a high-repetition-rate femtosecond
laser
[11]
.
In this study, we applied high-repetition-rate, ultra-
short pulse lasers for the precise micromachining of
PMMA. The morphological character was observed
with a scanning electron microscope (SEM). To better
understand the process of chemical modifica tion, an IR
spectrometer and a micro-Raman spectrometer were used.
The laser source was a homemade large-mode-area
photonic crystal fiber amplifier, which delivered pulses
with a duration of 115 fs (FWHM) and had a center wave-
length at 1040 nm and a repetition rate of 57 MHz. The
laser power was continuously adjusted with a combination
of a half-wave plate and a polarization beam splitter. The
laser beam was tightly focused by a long working distance
objective lens with a numerical aperture of 0.40. In order
to void the cross section of any contaminants, the sample
was linearly scratched with a sharp knife on the back sur-
face before laser ablation. Then, the sample was mounted
on a precision three-dimensional translation stage at a
position with respect to the incident laser beam by a com-
puter, and the reflected laser beam from the sample was
monitored using a CCD camera. An electronic shutter was
placed in the incident beam path for the exact adjustment
of the ablation time. The direction of the laser scanning
was perpendicular to the scratched line. After laser abla-
tion on the front surface, the sample was fractured so
we could observe the cross-section profile. Therefore,
the fracture was always perpendicular to the grooves.
The features of the craters and grooves ablated with
the femtosecond laser were determined by field emission
scanning electron microscopy (Nova NanoSEM 430,
FEI, USA). The specular reflection Fourier transform IR
(FTIR) spectra of PMMA both before and after the laser
ablation were acquired using a Bio-Rad FTS 6000 FTIR
spectrometer. Raman spectroscopy, measured with an In-
Via-Reflex system, was carried out to obtain further
insight into chemical changes induced by the femtosecond
laser micromachining of the PMMA.
Figure
1 illustrates the morphology evolution of the
laser-irradiated areas produced by a laser power of 1.3 W
on a PMMA plate. Figure
1(a) shows the swelling of
the focal spot of the sample after 5 ms of laser exposure.
A volcanic crater encircled by a ruptured dome-like
COL 13(7), 070003(2015) CHINESE OPTICS LETTERS July 10, 2015
1671-7694/2015/070003(4) 070003-1 © 2015 Chinese Optics Letters
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