Research Papers

Ultrafast Laser Induced Structural Modification of Fused Silica—Part I: Feature Formation Mechanisms

[+] Author and Article Information
Siniša Vukelić, Panjawat Kongsuwan, Y. Lawrence Yao

Department of Mechanical Engineering, Columbia University, New York, NY 10027

J. Manuf. Sci. Eng 132(6), 061012 (Dec 17, 2010) (8 pages) doi:10.1115/1.4002767 History: Received July 27, 2010; Revised August 08, 2010; Published December 17, 2010; Online December 17, 2010

Nonlinear absorption of femtosecond-laser pulses enables the induction of structural changes in the interior of bulk transparent materials without affecting their surface. This property can be exploited for transmission welding of transparent dielectrics, three dimensional optical data storages, and waveguides. In the present study, femtosecond-laser pulses were tightly focused within the interior of bulk fused silica specimen. Localized plasma was formed, initiating rearrangement of the network structure. Features were generated through employment of single pulses as well as pulse trains using various processing conditions. The change in material properties were studied through employment of differential interference contrast optical microscopy and atomic force microscopy. The morphology of the altered material as well as the nature of the physical mechanisms (thermal, explosive plasma expansion, or in-between) responsible for the alteration of material properties as a function of process parameters is discussed.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 1

Schematic diagram of ionization induced by femtosecond-laser irradiation: photoionization as function of Keldysh adiabatic parameter γ: (a) multiphoton ionization, (b) tunneling ionization, and (c) avalanche ionization

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Figure 2

Schematic illustration experimental setup. The shadowed plane (cross section) shows that the laser beam is focused to the interior of the fused silica sample. Laser beam scanning direction is along y axis.

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Figure 3

DIC optical microscope side view (y-z plane) of features created via single femtosecond-laser pulses in fused silica sample. Pulse energy is 30 μJ.

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Figure 4

Comparison between numerical and experimental observations of focal and affected volume longitudinal and lateral radius ratio

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Figure 5

Optical microscopy of cross section view (x-z plane) of femtosecond-laser-irradiated fused silica at different energy levels (from 2 μJ up to 35 μJ) and scanning speeds (from 0.04mm/s up to 1mm/s for each energy level). Scanning is along the y direction, perpendicular to the cross section x-z plane. In order to improve the image quality, the sample was diced and polished after experiment. The rectangle region of 30 μJ pulse energy is magnified in Fig. 6.

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Figure 6

Optical microscopy of cross section view (x-z plane) of femtosecond-laser-irradiated fused silica sample: (a) energy level of 30 μJ with various scanning speeds, (b) feedrate 1 mm/s and different energy levels; both extracted from Fig. 5

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Figure 7

Experimental measurement of ratio between lateral and longitudinal radii of affected volume. Error bars represent standard deviation.

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Figure 8

DIC optical microscope side view (y-z plane) of femtosecond-laser-irradiated fused silica sample at energy level of 30 μJ with three different scanning speeds: (a) with scanning speed of 1 mm/s, (b) with scanning speed of 0.5 mm/s, and (c) with scanning speed of 0.04 mm/s. Images are taken using (a) 100× and ((b) and (c)) 40× oil immersed objective lenses.

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Figure 9

AFM cross section (x-z plane) topography of laser-irradiated fused silica sample. The five lines (from left to right) shown in the figure correspond to three different experimental regimes, scanning speeds from left to right 0.6, 0.7, 0.8, 0.9, and 1.0 mm/s with the same 30 μJ pulse energy.

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Figure 10

Maximum depth of the cavities found via AFM topography of the cross section. Voids are created by the 30 μJ pulse energy at different feedrates. The comparison is qualitative and similar trend is observed at various cross section.





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