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Abstract
Time-resolved, single-shot measurements are performed to determine the reflectance, transmittance, and absorptance in ultrafast laser interaction with polypropylene for a wide range of laser pulse energies. An ellipsoidal mirror is used to collect the majority of the reflected light, enabling the detection of plasma emission starting at about 40 ns after the incident pulse. The measured transmittance is explained by a model that takes into account different effective absorption channels, and the non-linear absorption coefficient is estimated, which suggests that the non-linear absorption originates from the two-step or two-photon absorption through overtone. The results are useful for selecting laser parameters in the processing of polymeric materials.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast lasers have been used widely for high precision material processing in fundamental research and industrial applications since 1980s [1,2]. High-power ultrashort laser pulses are used in various types of material processing including laser drilling [3], cutting [4], surface hardening [5], polishing [6,7], cleaning [8] and micro/nano machining [9,10]. When an ultrafast laser pulse is focused onto a target material with the energy density (i.e., fluence) exceeding the damage threshold of that material, vaporization and eventually the formation of plasma (consisting of atoms, positive and negative ions, molecules, nanoparticles, clusters and agglomerates [11]) can form resulting in luminous light emission [11,12]. Laser-produced plasmas (LPPs) have great advantage in a variety of applications including laser-induced breakdown spectroscopy [13], high-order harmonic generation [14], attosecond pulse generation [15], nanoparticle generation [16] and material analysis [17]. However, LPPs might deteriorate laser processing quality and efficiency (especially for polymers, which typically have a low melting point) by absorbing and reflecting laser energy as the beam passes through the plasma, thereby reducing the amount of laser energy reaching the surface of material [18]. Therefore, it is essential to understand the dynamics of ultrafast laser interaction with polymeric materials for improving processing quality. In addition, optical properties such as reflectance, transmittance and absorptance are crucial in selecting laser-processing parameters, and such properties become fluence (intensity) dependent for high pulse energies.
The optical properties and dynamics of laser - material interaction depend on laser wavelength, pulse duration, intensity, temporal and spatial pulse shape, pulse repetition rate, ambient environment and material properties [19]. The characteristics varies with time and space rapidly, so optical properties and LPPs are transient effects that are not easy to measure and detect. To understand the dynamics of ultrafast laser material interaction, time-resolved measurements are essential. Researchers have used various methods for such measurements, including shadowgraph, holography, interferometry, beam deflection and probe beam diagnostics [20]. Most commonly used approaches are pump–probe beam diagnostics and shadowgraphy. Pump-probe experiments are complicated involving multiple beampaths and the event reconstruction requires delay scan, which can be time-consuming [21]. In case of the optical shadowgraphy, the duration of the probe beam is required to be short to image a fast event with a suitable detector (such as a CCD camera) with approximate magnification [22]. Although Z-scan is a simple and effective technique to measure the nonlinear refraction and absorption coefficients, it requires scanning the sample which is also time consuming [23,24]. To analyze the elementary composition of a target, researchers are using laser-induced breakdown spectroscopy (LIBS) technology, which can reveal the physical processes that lead to the formation of high-temperature plasma induced by a short pulse [25]. During the plasma cooling period, light emits from the plasma cloud as the electrons of the atoms and ions at the excited states fall down to the ground state. The ellipsoidal mirror-based method can also be used to detect such events if the sample is irradiated with sufficient fluence (intensity) to create plasma cloud.
In our previous study, it was found that heat accumulation and plasma or surface vaporization due to high repetition rate deteriorates the processing quality of polypropylene (PP) [26], an important material in many industrial applications, such as packaging for consumers products, plastic parts for various industries (including automotive industry), special devices like hinges and fabrics [27]. In this study, time-resolved measurements of the interaction between ultrafast lasers with PP are performed with an ellipsoidal reflector. The absorptance is determined by analyzing the transmitted and reflected signals detected by photodiodes and a GHz-bandwidth oscilloscope. The dynamics of ultrafast laser interaction with polypropylene is studied experimentally in single-shot configuration. The experimental results are explained with a model that accounts for different effective absorption channels and the nonlinear absorption coefficient is estimated, which suggests that the nonlinear absorption originates from two-step or two-photon (where two photons are simultaneously absorbed) absorption through overtones [28].
2. Experimental set up
The interaction of a Yb:KGW (Pharos, Light Conversion) laser, delivering ultrashort laser pulses of pulse length 170 fs and center wavelength 1030 nm, with white opaque PP sheets was studied. Each sheet was 300 µm thick containing small amounts of several impurities Si, C, Li, Na, Al, K, and SiO2 that exhibited characteristic peaks in the secondary ion mass spectroscopic spectrum. The experimental setup for time-resolved measurements is shown in Fig. 1. In this system, the femtosecond laser beam, whose diameter is enlarged to ∼10 mm by a beam expander and pulse energy adjusted by a variable neutral density filter (attenuator, in Fig. 1), is focused onto the sample with a lens of 250 mm focal length. A beam splitter is used to reflect 6.5% of the beam, which is used as a reference beam to determine the incident laser pulse energy.
Three photodiodes (PD 1-3, Thorlabs, FDS100) are used in the experiment, and data acquisition is through a 1-GHz oscilloscope (Tektronics, MDO3104). The PP sheet is placed on the top of a fixture, which is used to hold both the PP sample and PD 3. The PP sheet is irradiated with a single laser pulse and the reflection signal of the irradiated pulse is captured by an ellipsoidal reflector (E180, Optiforms) with silver coating. We choose to use an ellipsoidal reflector because it enables to collect most of the reflected light [29,30] and to have easy access to the sample. In this system, the sample is placed at lens 2’s focus, which coincides with the internal (first) focal point of the ellipsoidal reflector. The surface normal of the sample is tilted with an angle of 17° with respect to the laser beam to reduce light backscattering through the entrance hole of the ellipsoidal reflector, while both specular and diffusive reflections are collected by this ellipsoidal reflector and detected by PD 2, which is located near the external (second) focal point of the ellipsoidal mirror. To position PD 2, the ellipsoidal mirror is held with a fixture that is mounted on a 3D stage. By adjusting the 3D stage, the light reflected by the ellipsoidal mirror is focused on PD 2 at low intensity. Then, the signal from PD 2 is maximized by tuning the position of PD 2. The active area for the photodiode is 3.6 mm × 3.6 mm. PD 3 is placed at the back of the PP sheet to detect the transmission signal. A fixture attached to a 3D stage is used to move sample to ensure a fresh surface for each laser shot. The rise/fall time, tr, of this photodiode (FDS100) is 2.12 ns, which is calculated using the load resistance Rload = 40 Ω and junction capacitance Cj = 24 pF. The impulse response time is measured as 3.23 ns, which is the FWHM of the shortest pulse measured by the oscilloscope for this photodiode. Compared to other techniques such as pump-probe and Z-scan, our setup does not require scanning of delay or sample position, which is time-consuming. The presented setup can be modified to collect spectral data by replacing PD 2 with a spectrometer.
3. Experimental results
Figure 2(a) shows the output signal recorded by the oscilloscope for incident, reflected and plasma, and transmitted beam by PD 1, PD 2 and PD 3, respectively. The signal is obtained when a pulse with energy of 20 µJ and peak fluence of 3.76 J/cm2 irradiates the sample. As mentioned before, 6.5% of the incident light is obtained as the reference beam and 86% reaches at the sample. To avoid the saturation in PD 2 the reflection signal is attenuated by 10 times by using a ND filter in front of PD 2. The shaded regions are the integrated area at FWHM for each of the signals and the integrated area for each signal is shown in the inset. The temporal behavior of the PD 2 signal exhibits features different from the incident and transmission signal. It can be seen from Fig. 2(a) that PD 2 outputs two peaks, a large (“main”) peak between t = 0 and 40 ns, and a small peak centered around t = 90 ns. Here, the first peak corresponds to the reflection of the laser pulse itself and second peak is attributed to plasma emission [11] due to the laser-material interaction as the emission is not observed below the damage threshold. Although our experimental setup cannot detect plasma emission in the UV wavelength due to the silver coating of the ellipsoidal mirror and the responsivity of the silicon photodiodes, the measurement of reflectance is not influenced by the plasma because only the first peak is used in the analysis, while the second peak, which is attributed to plasma emission, is ignored. However, events that occur shorter than ns might not be detected due to slow response of photodiodes (ns response time). Figure 2(b) shows the PD 2 signal as a function of pulse energies. It can be seen from Fig. 2(c) that the signal of the second peak increases with a higher pulse energy due to increased plasma formation. The second peak signals are absent for peak fluence < 0.56 J/cm2, which corresponds to 3 µJ pulse energy, where the damage threshold is found to be 0.15 J/cm2, which corresponds to pulse energy of 800 nJ.
Fig. 2. PD signals observed on the oscilloscope (a) Incident, reflected and transmitted signal for 20-µJ pulse energy. The signals are scaled by 6.5%, 10%, and 100%, respectively. Shaded region for each of the signal represents the FWHM integrated area. Inset shows the linearity of the PDs (only PD 1 is shown here). (b) Signal detected by PD 2 for different pulse energies. (c) Second peaks with different pulse energies.
Figure 3 shows the total reflectance, R (specular and diffusive), transmittance, T and absorptance, A as a function of peak fluence and peak intensity. In calculating R and T, integrated areas at FWHM of the PD traces for reflection and transmission are used with proper scaling based on their attenuation. Then, the integrated areas for reflection and transmission are divided by the corresponding integrated area of the incident signals to get R and T, respectively. In determining R, only the first peak of the PD 2 signal is considered. Absorbance, A is calculated as A = 1 – T – R. It is found that the reflectance of the sample remains constant (R = 0.6) for peak fluence <0.3 J/cm2, because the material surface does not undergo any change. The reflectance begins to decrease quickly after the damage threshold peak fluence 0.15 J/cm2 is reached, and continues to decrease beyond the threshold of plasma formation, 0.56 J/cm2 (discussed in Fig. 2). Since the damage doesn’t occur below the damage threshold fluence, the transmittance in Fig. 3 and the absorptance in Fig. 3 remain constant until the damage threshold peak fluence is reached. This regime is called the “linear absorption region”, which is marked with shade in Fig. 3. As the peak fluence increases, the reflectance drops because of non-linear absorption, which will be explained in details in the next section.
Fig. 3. Reflectance (R), transmittance (T) and absorptance (A) as a function of laser fluence and intensity.
4. Model and discussion
Although ultrafast lasers are suited for processing polymeric material [20], understanding of several fundamental questions, including relative significance of different linear and nonlinear absorption processes, is still lacking. In the case of focusing femtosecond laser pulses into polymeric materials, a photon may not have enough energy to promote an electron from the valance band to the conduction band at low laser intensity. However, for wide band-gap material like polymers, absorption of laser is a mix of linear (single-photon) and nonlinear (multiphoton) processes. The equation describing the attenuation of the ultrafast laser pulse with intensity I(z) passing through the material undergoing single-photon and two steps absorption or two-photon absorption is given by
where ${\alpha _1}$ is the linear absorption coefficient and ${\alpha _2}$ is the third order nonlinear absorption coefficient. The transmittance of the ultrafast laser pulse through the sample can be found from the solution to the differential Eq. (1), which can be written asFigure 4(a) shows experimental data on transmittance, T, that can be classified into three regions. In Region I, T remains nearly constant up to an ablation threshold fluence (or intensity), and then T decreases as the fluence increases in Region II indicating nonlinear laser-matter interaction. In Region III, however, the data fluctuates but the average value exhibits nearly fluence-independent transmittance indicating linear absorption. The absorption spectrum in Fig. 5 reveals the degree of nonlinear absorption in Region II due to the presence of two absorption peaks at 1030 and 515 nm. While the peak at 1030 nm indicates the vibrational overtone absorption of the incident laser photons, the peak at 515 nm provides a mechanism for two-step or two-photon absorption through overtone [28]. To determine the linear absorption coefficient ${\alpha _1}$, the two-photon absorption coefficient ${\alpha _2}$ was set to zero in Eq. (2) to obtain the solution of single-photon absorption, $T = (1 - R){e^{ - {\alpha _1}L}}$. This equation was fitted to the experimental data in Region I using the values of R as a function of I0 from Fig. 3 and L = 300 µm, and ${\alpha _1}$ was found to be 5260 m−1. Using this value of ${\alpha _1}$, Eq. (2) was fitted to the experimental data in both Regions I and II for the nonlinear absorption coefficient, α2 = 1.57 × 10−12 mW−1. This value is consistent with previously found values for polymeric materials [31,32].
Fig. 4. (a) Fitting of the experimental results using Eq. (2). The laser intensity used in this experiment can be divided into three regions (Region I, II and III). Damage observed by (b) optical and (c) scanning electron microscopy (SEM) at corresponding intensities 9.4 (A), 94.1 (B), 378.3 (C) and 658.2 (D) PW/ m2 shown in (a).
Fig. 5. Absorptance, A of impure PP sample is determined from the transmittance, T and the specular reflection measurement, Rs of PP sample by using a spectrophotometer (Cary 500).
The reversal of nonlinear absorption to linear absorption in Region III, which begins at the peak intensity 470 PW/m2 in this study, may be attributed to air breakdown in front of the substrate surface at high intensities. Several studies [33–36] have reported the air breakdown threshold peak intensity in the range of 100 to 1000 PW/m2 for femtosecond laser with pulse duration 100–200 fs and wavelength 800 nm. The air plasma formed in front of the substrate due to this breakdown can reflect and absorb much of the incident laser energy. Therefore, the actual laser intensity at the substrate surface is reduced significantly, resulting in linear absorption in the substrate. Figures 4(b) and 4(c) show laser damages on the surfaces of the PP sheet at the fluences 0.15, 1.88, 7.52 and 13.16 J/cm2 for damages A, B, C and D, respectively. Damage A corresponds to the damage threshold fluence/intensity in Region I, whereas damage D corresponds to Region III. Damages B and C indicate that the damage size changes with fluencies/intensities in Region II, whereas the damage size was found to be nearly the same as the damage size D in Region III. This effect of intensity on damage size correlates with the measured transmittance very well since the transmittance decreases steadily in Region II but remains almost constant in Region III.
5. Conclusions
In summary, time-resolved measurements of reflectance, transmittance and absorbance are performed for laser intensity ranging from well below to well-above the damage threshold. An ellipsoidal mirror enables the collection of majority of the reflected light (>98.2%), while providing easy access to the sample compared to an integration sphere. We observe in the reflection signal a distinct “double-peak” feature, which is interpreted as light emission from laser-induced plasma. A model that includes linear and nonlinear absorption explains the experimental data. Accordingly, the laser intensity used for processing polymers can be divided into three regions, each with a dominant absorption channel. This study provides information for designing and optimizing laser processing systems for polymeric materials.
Acknowledgments
The authors thank Dr. Xinpeng Du and Boyang Zhou for their helpful discussion. The authors also thank Yingjie Chai for his support in SEM imaging.
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