Open Access
Add to BibSonomyAbstract
We have investigated the effect of the dynamics of crater size on the poly(dimethylsiloxane) (PDMS) surface morphology in fs-laser micro-processing. PDMS surface was processed with varying both inter-pulse interval and inter-spot distance between successive laser pulses. With keeping the interval of 5 ms crater shape is round even if the spot is overlapped in space. But decreasing the interval to 0.02 ms the shape of the crater is no longer round. Decreasing the inter-distance between the craters results in roughened surface morphology even at time intervals of 5 ms. Temporal dependence of single-shot fs-laser induced crater size was measured as a function of time delay. Within 0.1 ms after pulse irradiation with a fluence of 4.8 J/cm2 on PDMS surface the crater size has reached to its maximum values and then decreased with a time constant of about 0.3 ms. The surface morphology after fs-laser pulse irradiation is strongly dependent on not only inter-spot distance between successive laser pulse but also their inter-pulse intervals. By proposing a theoretical model on their dynamic features, we will try to explain the current observation in quantitatively.
© 2015 Optical Society of America
1. Introduction
The interaction of laser and soft tissue, which is a type of polymeric elastomer, may be generalized as a dynamic process when the biomechanical properties might be perturbed by the laser beam during the time-course of the processing. Highly focused fs-laser energy is absorbed by the target elastomer including soft-tissues like cornea and skins, resulting in the ablation through nonlinear absorption [1,2]. Nonlinear ionization processes like multiphoton ionization and avalanche ionization lead to a drastic increase of free electron density. This solid plasma of free electrons and ionized molecules rapidly expands to the surrounding atoms and molecules, leading to a thermo-mechanical effect out of the breakdown volume. This process results in cavitation bubbles, which consists of a vapor-filled cavity and an outwardly propagating shock wave. The abrupt expansion of the cavitation bubble causes a disruption of the elastomers. Associated shockwaves and heat dissipation, however, may result in collateral tissue damage [2]. Lowering the femtosecond laser pulse energy allows to decreasing the amount of energy deposited into tissue, the shock wave amplitude, and the cavitation bubble size [3]. Despite the low total energies required, femtosecond laser ablation can lead to significant effects on micro-processing of a target material under processing conditions like high pulse repetition rate and overlapped portion. These cumulative effects may often occur at energies below the threshold for photo-disruption. If the time between the laser pulses is relatively shorter than the time it takes to diffuse heat out of the vicinity of the focal volume, thermal energy could be accumulated near the focal region [4]. Although the energy of each laser pulse is small, at the high repetition rates of fs-laser a significant amount of heat can build up at the focal point. While the changes caused by single pulse femtosecond laser can be negligible, continuous irradiation of laser pulses on the same position can lead to a significant alteration of the material nature that can eventually induce either useful or harmful alterations of target materials [5,6].
Poly(dimethylsiloxane) (PDMS) has several characteristics including high optical transparency from near infrared to ultra-violet (UV) region, chemical and thermal resistance and mechanical elasticity for nanocasting as well as bioassays. We have already demonstrated the formation of superhydrophobic PDMS surface by exposing the surface to ultrafast laser pulses [7]. The observed high contact angle and low sliding angle of water droplet could be explained in terms of fs-laser induced formation of much roughened PDMS surface in nano- and microscales, of which topography fairly well imitate a Lotus leaf. Ultrafast laser induced surface modification is also known to have superior spatial resolution with a minimal thermal and mechanical damage. The method to create rather complex topographic patterns directly on PDMS surface with a high spatial resolution would be an important tool for both fundamental research and biomimetic reproduction. PDMS used in this work is an elastomeric material that has been widely used in ophthalmic applications due to its excellent optical properties, biomechanical strength and biocompatibility. These properties have made PDMS an attractive candidate for an artificial cornea [8].
In this report, we have investigated PDMS surface changes by exposing to ultrafast laser pulses. We supposed that the shock generated by fs-laser irradiation in PDMS might initiate a detonation through the generation of scission forces on the molecules comprising the solid PDMS, breaking chemical bonds, creating a distribution of free radicals, and supplying the energy required to initiate the formation of nano- and microstructures, resulting in much roughened PDMS surface. Even if a direct interband transition cannot be induced in PDMS by linear absorption at the laser wavelength, a multi-photon absorption process upon focusing rather high laser energy into PDMS may result in high-strain-rate disturbance if there is considerable density of defects inside the materials. In fact, the irradiation of fs-laser scarcely induces irregular structure inside laser spot where only single pulse of fs laser irradiated, while in the overlapped area between successive laser spots much roughened surface was observed in this study. We have studied the effects of the dynamics in ablation crater size on PDMS surface morphology in single-shot fs-laser pulse processing by adjusting two key experimental parameters: the laser repetition rate and distance between neighbor craters. Based on our experimental data, we proposed a theoretical model on their dynamic features to assess the correlation between the height of crater and inter-distance of the craters as well as roughness as a function of time-interval between laser pulses. Moreover, we will further discuss about the effects of the dynamics in crater size of cornea on the issues related on light scattering, which is frequently invoked as a major complication after LASIK (Laser-Assisted In Situ Keratomileusis) operation.
2. Experiments
Schematic diagram for the laser processing is shown in Fig. 1. The regenerative amplified femtosecond laser system (Light Conversion, PHAROS) produces 30 μJ/pulse at the wavelength of 1030 nm with a repetition rate of 200 kHz. The pulse duration of the laser is about 250 fs. The repetition rate can be changed by pulse picker combined with home-made digital pulse divider [9]. The laser energy could be attenuated with variable neutral density filter (Sigma Koki, VND). The laser beams are focused on the PDMS surface by using an objective lens (X5). The laser beam spot size at PDMS surface was determined to be about 17 μm in diameter. The laser polarization is set to be perpendicular to the processing direction by a variable wave plate (Newfocus). We have measured the beam profile of the laser at sample position without the focusing lens (Gentech, USA) and presented in Fig. 1(b). The laser beam has Gaussian distribution with the diameters in major and minor axis of about 4.1 mm and 3.9 mm, respectively.
Fig. 1 (a) Schematic diagram for laser ablation of PDMS substrate. Femtosecond laser beam is delivered and focused on PDMS surface through objective lens. The sample surface was illuminated with a white-light emitting diode (LED). During and after laser exposure the optical images of PDMS surface were captured by a fast camera. (b) Beam profile of the laser at sample position without a focusing lens. The image size is 11.3 mm x 6.0 mm.
The solid PDMS sheet was obtained by using a 10:1 mixture by weight of PDMS base/curing agent which was degassed under vacuum and cured at 25 °C for 24 h. PDMS sample is mounted on XY translation stage with a maximum speed of 1000 mm/sec. We have synchronized the digital pulse divider with the stage to irradiate the laser pulse only at the constant sample moving speed. The optical images under laser irradiation are measured by a high-speed charge-coupled device (CCD, FASTCAM, Photron, Japan) and fed into personal computer for further analysis. We have also characterized the surface of processed PDMS with atomic force microscopy (AFM, Agilent PicoPlus). Optical images were analyzed by using the freely available software ImageJ 1.46 from http://imagej.nih.gov/ij developed by Wayen Rasband, National Institute of Mental Health, USA. We have further confirmed the accuracy of our estimation by comparing with the crater topography measured by AFM.
3. Results and discussion
The surface optical image trains were captured with a fast CCD camera during laser irradiation. Figure 2(a) shows a series of crater images as a function of time delay after single-shot laser pulse with a fluence of 4.8 J/cm2 on PDMS surface. The crater diameter has reached to its maximum value of about 21 μm within less than 0.1 ms. After 1 ms, the crater size is eventually invariant of about 12.5 μm in diameter. Figure 2(b) exhibits AFM topographic images of the crater for reference. The temporal profiles of the crater diameter shown in Fig. 2(c) could be fitted with single exponential function with a time constant of about 0.3 ms. The result clearly shows the dependence of the dynamics of craters formed on PDMS surface with a sub-millisecond time constant under ambient condition. With increasing the laser fluence of about 8.8 J/cm2, the dynamics of the size of the crater is little bit more complicated. The increment of crater size has an apparent rise-time constant of about 0.1 ms. Two different relaxation time constants of about 0.25 ms and 2.0 ms need to fit the temporal profiles for the decrement of the crater size. The crater size after fully restoring is about 16.4 μm in diameter.
Fig. 2 (a) A series of optical images as a function of the delay time after exposing single-shot laser pulse on PDMS surface. The surface optical images were captured by a fast camera with a frame rate of 10,000 per second just after laser irradiation. (b) AFM topography of the crater formed with a laser fluence of 4.8 J/cm2. (c) The crater diameter as a function of delay time at two different laser fluences of 4.8 J/cm2 (black circles) and 8.8 J/cm2 (red squares). For the results with 4.8 J/cm2, the experimental data can be fitted by single exponential function with a time constant of 0.3 ms. For the crater size formed with 8.8 J/cm2, however, in addition to a rise component with a time constant of 0.1 ms two different relaxation time constants of about 0.25 ms and 2.0 ms need to fit the temporal profiles.
This observed dynamical features of the crater size should affect the overall ablation process if the laser spot is transiently overlapped with the next spot. To investigate the correlation between the surface morphology and inter-pulse interval (Δt) and inter-spot distance (dspot-spot), topographic images of processed PDMS were characterized with atomic force microscopy (AFM, Agilent PicoPlus) and shown in Fig. 3. It should be noted that the direction of the crater formation is from left to right. In order to simplify the temporal behavior of the crater size, we have fixed the laser fluence as constant to be 4.8 J/cm2 during all the following experiment. Moreover, cross-sectional plot at the center of the craters was inserted below each AFM image to make it easier to view at a glance. First, the inter-pulse interval was kept to be 5 ms i.e., the repetition rate of 0.2 kHz, and the inter-spot distance was varied from 15 μm to 10 μm by changing the stage speed from 3.0 mm/sec to 2.0 mm/sec [Fig. 3(a)]. If the inter-distance is 15 μm, each crater is completely separated and the shape is almost circular. When the inter-distance is 10 μm, which is significantly less than fully relaxed crater diameter of 12.5 μm [Fig. 2(c)], we could find the apparent overlapped portion of craters. Meanwhile, the eventual shape of craters is still circular. Second, the inter-pulse interval is decreased further to 0.2 ms and 0.02 ms as shown in Figs. 3(b) and 3(c), respectively. Then, we have systematically varied the inter-spot distance from 15 μm to 10 μm. It is of great interest to note that decrease in the interval (Δt) with keeping the inter-distance constant values of 15 μm results in apparent increase in the overlapped portion between the next laser spots. Further, the shape of the craters is no longer circular, i.e., the left side of the crater looks like circular, but the right side is significantly altered to giving asymmetric feature. When the inter-distance of the spot is 12 μm and 10 μm, the alteration of the crater shape and the overlapped portion is much more strongly dependent on the inter-pulse intervals.
Fig. 3 AFM topography images PDMS surface irradiated with fs-laser pulses. The laser fluence was kept to be 4.8 J/cm2. The craters were formed with inter-pulse intervals of Δt = 5.0 ms (a), Δt = 0.2 ms (b), and Δt = 0.02 ms (c). For each intervals, the inter-spot distance between the successive laser spots was also varied (dspot-spot = 15 μm, 12 μm, and 10 μm). The direction of laser polarization is presented by both sides of arrow bar.
In order to get a more quantitative understandings, we first define the height difference (H(Δt)) at a specific intervals of Δt as the depth of the overlapped portion from the intact PDMS surface depicted as a red line shown in the inset of Fig. 3 and present the results in Fig. 4(a).
Fig. 4 Plot of height difference H(Δt) as a function of inter-pulse interval (a) and overlapped length (L(Δt)) (b) with varying the inter-spot distance ( = dspot-spot) of 10 μm (filled red circle), 12 μm (open green circle), 15 μm (filled black square), and 20 μm (open blue square). The laser fluence is kept to be 4.8 J/cm2.
For reference, the negative values of H(Δt) observed under the inter-spot distance of 20 μm, for example, is due to usual bump formation, which induce local elevation of the surface. The height difference is strongly dependent on both the inter-spot distance and inter-pulse intervals. It is of great interest to note that the height difference as a function of time intervals is very similar to the temporal profiles of crater size shown in Fig. 2(c). The height difference observed at the intervals longer than about 1 ms is almost invariant. In addition, its temporal dependence of the decrement looks like that for the dynamics of the crater diameter. These observations allow us to suppose that the height difference is strongly related to the dynamics of the crater size during the course of laser pulse irradiation.
In Fig. 5, the conceptual diagrams for the cross-sectional height variation was illustrated at inter-pulse delay time (t = ∆t) and infinite delay time (t = ∞). By proposing a theoretical model on their dynamic features, we have tried to explain the current observation in quantitatively. With a conceptual cross-sectional view depicted in Figs. 5(a) and 5(b), the overlapped length between two successive pulses at the arrival time of the next pulse, L(Δt), could be equated as following Eq. (1),
where d(∆t) and wlaser are the diameter of crater at a time ∆t and laser beam radius, respectively. dspot-spot is inter-spot distances between the craters. It should be noted that the value of L(∆t) should be positive since there is no chance for the interaction between the successive ablation craters if the overlapped area is not available. Supposing that the successive laser pulse ablates whole the portion of PDMS inside effective laser beam diameter, simple arithmetic procedure allow us to propose that the height difference should be linearly correlated with the overlapped length of the two laser pulses. If this is the case, one can make the following equations:The time dependence of crater diameter, d(∆t), could be determined as d(∆t) = 12.5 + 12 × exp(-∆t/0.30) by fitting the temporal profiles of the crater size shown in Fig. 2(c) with single-exponential function. The radius of laser beam diameter is about 17 μm. We have plotted the height difference, H(Δt), as a function of L(Δt) and presented in Fig. 4(b) for all the data observed with changing the inter-spot distance of 10 μm, 12 μm, 15 μm, and 20 μm. As expected, the height difference is almost constant if the values of L(∆t) is not positive. However, it should be noted that the height difference exhibits quite good linear dependence on the overlapped length, which is further dependent on the crater size dynamics.Fig. 5 Conceptual diagrams for the cross-sectional height variation of the craters at t = ∆t (a) and infinite time delay (b). The first pulse (yellow) formed a crater (solid-line inverted trapezoid) and then its size increased due to tensile forces after time = ∆t (dash-line inverted trapezoid). If the second pulse (red) remove the portion of PDMS inside L(∆t), overlapped length between the two successive pulses, the center portion between the two craters after infinite time (b) should be eventually affected by not only inter-pulse intervals (∆t) but also inter-spot distance, dspot-spot.
It is necessary to explain why the crater size is varying after fs-laser pulse irradiation. One can produce shock waves yielding peak pressures of about several GP when ultrafast laser pulses with a fluence of several J/cm2 is applied to ablate samples [10]. Meanwhile, the phenomena of the mechanical forces to provide material changes after ultrafast laser exposing can be explained by a two-stage process including an instantaneous uniaxial elastic compression, followed by a slower relaxation to a plastically deformed state exhibiting a hydrostatic pressure. The former should provide a laser peening to give changes in back surface just below the laser exposing area, while the latter might result in the shear forces on the samples near the laser focal position [11]. The crater on PDMS surface deformed after laser ablation should be restored to the steady state. Even if we have no any quantitative description for the time constant of 0.3 ms which is observed for the crater size upon irradiating the laser pulse with a fluence of 4.8 J/cm2, we can conjecture that the origin of the dynamics related on that time constant should be related on the modulus of elasticity for PDMS. Without complete knowledge about the material properties related on those transient phenomena, no clear statement on the time constant at this stage. It should be necessary to investigate the effect of the modulus of PDMS elasticity as well as applied shear forces on the dynamics of crater size in more detailed.
It should be notified that when the interval between the laser craters is shorter than 0.2 ms apparent nano-structures could be formed on the overlapped area as shown in Fig. 4. Furthermore, the surface morphology after fs-laser pulse irradiation is strongly dependent on not only inter-distance between successive laser pulse but also their time-intervals. This observation can be explained as followings. fs-laser irradiation scarcely induces irregular structure inside laser spot when only single pulse of fs-laser is irradiated. However, in the overlapped area between successive laser spots much roughened surface could be observed if the intervals are less than 0.2 ms. Multiphoton absorption process caused by delivering high laser energy into PDMS may result in high-strain-rate disturbance if considerable density of defect is transiently formed inside materials due to the former pulse. If this is the case, we have therefore proposed that the shock generated by fs-laser irradiation in PDMS initiates detonation through the generation of scission forces on the molecules comprising the solid PDMS, breaking chemical bonds, creating a distribution of free radicals, and supplying high kinetic energy required enough to initiate the formation of nano- and micro-structures [7].
Finally, it is of interest to discuss about the implication of current results on the performance of surface roughness during surgical application of fs-laser pulses. Femtosecond laser pulses have been used in the field of eye surgery, particularly corneal flapping procedure, providing the advantages of combined high precision and minimized collateral tissue damage [12,13]. Juhasz et al. developed an optical technique to cut this flap by taking advantage of an optical nonlinearity in fs-laser micro-ablation [14]. The technology has been applied widely, most notably in refractive surgery like LASIK (Laser-Assisted In Situ Keratomileusis) [15,16]. In this operation, the laser replaces mechanical tools to create a precise corneal flap preparing for the secondary excimer laser operation in order to correct the patient’s ocular refractive error [17,18]. While prominent side-effects after LASIK are relatively rare, fs-laser flap-creation results in creation abnormal roughened corneal surfaces, vision defects such as irregular astigmatism, interface haze, and transient light sensitivity syndrome caused by increased light scattering [19]. The challenge in femtosecond laser dissection is to optimize clinical operating conditions for maximal precision and minimal damage to surrounding corneal tissues [20,21]. This could contribute to more precise, reproducible and safe LASIK operation. The observation from the current work as well as proposed theoretical model unequivocally revealed that the height variation of polymeric elastomer surface, which is directly related to the surface roughness, has been affected by dynamical features of crater shape. In order to achieve better performance in fs-laser cornea flapping operation, it is quite necessary to design both inter-pulse interval (laser pulse repetition rate) and inter-spot distance (laser beam scanning speed) in addition to more detailed information about the dynamics of crater size formed in bio-elastomers like cornea.
4. Conclusion
We have performed systematic studies on the dynamics in crater size formed on PDMS surface with a single-shot fs-laser pulse processing. In order to assess the effect of the dynamics in the ablation crater size on the ultrafast laser surface processing of elastomeric materials, PDMS surface modified by varying the laser repetition rate and the distance between craters was carefully characterized. We have found that the height difference between the intact PDMS surface and overlapped area formed due to successive laser pulses on a material is affected by both time-interval and the distance between craters. We have quantitatively explained the linear dependence of the height difference on time dependent interaction length between the successive laser spot by proposing a simple theoretical model.
Acknowledgment
This work was financially supported by the KRISS program (15011058).
References and links
1. J. G. Fujimoto, W. Z. Lin, E. P. Ippen, C. A. Puliafito, and R. F. Steinert, “Time-resolved studies of Nd:YAG laser-induced breakdown. Plasma formation, acoustic wave generation, and cavitation,” Invest. Ophthalmol. Vis. Sci. 26(12), 1771–1777 (1985). [PubMed]
2. T. Juhasz, G. A. Kastis, C. Suárez, Z. Bor, and W. E. Bron, “Time-resolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in corneal tissue and water,” Lasers Surg. Med. 19(1), 23–31 (1996). [CrossRef] [PubMed]
3. F. H. Loesel, A. Tien, S. Backus, H. Kapteyn, M. Murnane, R. M. Kurtz, S. Sayegh, and T. Juhasz, “Effect of reduction of laser pulse width from 100 ps to 20 fs on the plasma-mediated ablation of hard and soft tissue,” Proc. SPIE 3565, 116–123 (1999). [CrossRef]
4. C. B. Schaffer, A. Brodeur, J. F. García, and E. Mazur, “Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy,” Opt. Lett. 26(2), 93–95 (2001). [CrossRef] [PubMed]
5. C. B. Schaffer and E. Mazur, “Micromachining using ultrashort pulses from a laser oscillator,” Opt. Photonics News 12(4), 20–23 (2001). [CrossRef]
6. A. Vogel, J. Noack, G. Huttman, and G. Paltauf, “Mechanisms of femtosecond laser nanosurgery of cells and tissues,” Appl. Phys. B-Lasers Opt. 81(8), 1015–1047 (2005). [CrossRef]
7. T. O. Yoon, H. J. Shin, S. C. Jeoung, and Y.-I. Park, “Formation of superhydrophobic poly(dimethysiloxane) by ultrafast laser-induced surface modification,” Opt. Express 16(17), 12715–12725 (2008). [CrossRef] [PubMed]
8. F. A. Duck, “Physical properties of Tissue,” in A Comprehensive Reference Book (Academic, 1990).
9. S. C. Jeoung, H.-S. Lee, J. S. Yahng, H. K. Lee, H. Y. Moon, K. J. Kim, D. G. Lee, D. H. Park, Y. S. Yu, and S. J. Ji, “Micro-structuring of CIGS thin-film coated on Mo back contact by ultrafast laser ‘rail-roading’ patterning,” Opt. Express 19(18), 16730–16738 (2011). [CrossRef] [PubMed]
10. J. S. Wittenberg, M. G. Merkle, and A. P. Alivisatos, “Wurtzite to rocksalt phase transformation of cadmium selenide nanocrystals via laser-induced shock waves: transition from single to multiple nucleation,” Phys. Rev. Lett. 103(12), 125701 (2009). [CrossRef] [PubMed]
11. B. Wu, S. Tao, and S. Lei, “Numerical modeling of laser shock peening with femtosecond laser pulses and comparisons to experiments,” Appl. Surf. Sci. 256(13), 4376–4382 (2010). [CrossRef]
12. I. Ratkay-Traub, T. Juhasz, C. Horvath, C. Suarez, K. Kiss, I. Ferincz, and R. Kurtz, “Ultra-short pulse (femtosecond) laser surgery: initial use in LASIK flap creation,” Ophthalmol. Clin. North Am. 14(2), 347–355 (2001). [PubMed]
13. P. Kim, G. L. Sutton, and D. S. Rootman, “Applications of the femtosecond laser in corneal refractive surgery,” Curr. Opin. Ophthalmol. 22(4), 238–244 (2011). [CrossRef] [PubMed]
14. P. S. Tsai, B. Friedman, A. I. Ifarraguerri, B. D. Thompson, V. Lev-Ram, C. B. Schaffer, Q. Xiong, R. Y. Tsien, J. A. Squier, and D. Kleinfeld, “All-optical histology using ultrashort laser pulses,” Neuron 39(1), 27–41 (2003). [CrossRef] [PubMed]
15. S. MacRae, “Thin-flap femtosecond LASIK,” J. Refract. Surg. 26(7), 469–470 (2010). [PubMed]
16. P. S. Binder, “Femtosecond applications for anterior segment surgery,” Eye Contact Lens 36(5), 282–285 (2010). [CrossRef] [PubMed]
17. G. Sutton and C. Hodge, “Accuracy and precision of LASIK flap thickness using the IntraLase femtosecond laser in 1000 consecutive cases,” J. Refract. Surg. 24(8), 802–806 (2008). [PubMed]
18. G. M. Kezirian and K. G. Stonecipher, “Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis,” J. Cataract Refract. Surg. 30(4), 804–811 (2004). [CrossRef] [PubMed]
19. G. D. Kymionis, G. A. Kontadakis, I. Naoumidi, V. P. Kankariya, S. Panagopoulou, A. Manousaki, M. A. Grentzelos, and I. G. Pallikaris, “Comparative study of stromal bed of LASIK flaps created with femtosecond lasers (IntraLase FS150, WaveLight FS200) and mechanical microkeratome,” Br. J. Ophthalmol. 98(1), 133–137 (2014). [CrossRef] [PubMed]
20. M. Han, G. Giese, L. Zickler, H. Sun, and J. Bille, “Mini-invasive corneal surgery and imaging with femtosecond lasers,” Opt. Express 12(18), 4275–4281 (2004). [CrossRef] [PubMed]
21. K. G. Stonecipher, J. G. Dishler, T. S. Ignacio, and P. S. Binder, “Transient light sensitivity after femtosecond laser flap creation: Clinical findings and management,” J. Cataract Refract. Surg. 32(1), 91–94 (2006). [CrossRef] [PubMed]
