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Abstract
Gallium phosphide (GaP) is a widely used and promising semiconductor material for photonics devices and we suppose the ultrafast laser can be a competitive tool for GaP processing. We used an 800 nm centered femtosecond (fs) laser with a pulse duration of 50 fs to irradiate the GaP crystal. The ablation threshold was first determined, and then the ultrafast dynamics including plasma expansion, shockwave formation and propagation, and spectral evolution were acquired and analyzed. The evolution of ejected plasma in the initial stage changed from cylindrical to planar propagation with the augment of laser fluence. The study on the propagation properties of shockwaves showed that the energy of propelling shockwaves accounted for 12% to 18% of the laser pulse energy at all fluences above the ablation threshold. A prominent plasma splitting was observed at a fluence slightly higher than the threshold, and a phenomenon that the plasma protruded out of the shockwaves was also found. Finally, the transient temperature and density of electron at different fluences were calculated. The temperature difference between the plasma and the shockwave proved the heating effect of the plasma during ablation.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Gallium phosphide (GaP) is an artificial semiconductor compound with a band-gap wavelength of ∼550 nm [1]. It has showed a great prospect as an integrated nonlinear photonics material by fabricating it into different structures [2,3]. Cambiasso et al. [1] and Remesh et al. [4] fabricated the single GaP nano-disk and dimmers on GaP matrix using e-beam lithography followed by etching steps, realized the highest second-harmonic-generation (SHG) efficiency in the visible wavelength and obtained a large fluorescence enhancement. Wilson et al. fabricated the GaP-on-SiO2 strip waveguides by direct wafer bonding and e-beam lithography, showing a wide prospect for GaP-on-insulator platform [2]. Insero et al. made an orientation-patterned GaP crystal by molecular beam epitaxy, lithography and reactive ion-etched, and used difference-frequency-generation (DFG) on it to produce a 5.85 μm coherent mid-infrared radiation [5].
Ultrashort pulse laser has been used to fabricate microstructures on GaP [6–9]. It features strong nonlinear effect on materials and little thermal damage, especially its advantages in processing transparent materials [10]. Some studies focused on the generation mechanisms of fs laser-induced ripples [11–13]. Li et al. investigated the ablation threshold of GaP using high-repetition fs pulses [9]. Sei et al. used the IR fs laser double-pulse induced the periodic nanostructures inside GaP wafer, and the induced nanostructures had a high electrical conductivities [14]. The micro-fabrication of GaP using fs laser strongly relates to the ablation threshold and fluences for a certain laser. Herein, a deep understanding of the plasma dynamics at different laser parameters is of great importance for fs laser processing [15]. The life cycles of a laser-induced plasma spans over several orders of magnitude in time, and its fundamental properties change many orders during its lifetime. The occurrence of plasma involves complex processes of ionization, heating, melting, vaporization, ejection of species, shockwaves, light emission, etc. Considering the transient nature of the plasma, visually time-resolved diagnostics play a vital role in understanding the physical properties and the optimization for various applications. It was reviewed by Harilal et al. that different methods including shadowgraphy, imaging, and spectroscopy were required to probe the property of laser-induced plasma [16].
In this study, we investigated the ablation and plasma dynamics of the GaP using an 800-nm centered fs laser. The ablation threshold was first determined and the surface morphology was characterized under different laser fluences. The transient properties of plasma and shockwave were observed using a pump-probe method. The plasma imaging was acquired using a fast-gated camera. Laser-induced breakdown spectroscopy (LIBS) was used to reveal the dynamics of electron temperature and density after laser irradiation. The evolution of the shockwave and phase transformations during the ablation was explored and discussed. The plasma splitting and the plasma surpassing out of the shockwave was found to be strongly relevant to the fluence-influenced mechanisms of phase transformations.
2. Experimental setup
The whole experimental setup is shown in Fig. 1. A regeneratively amplified Ti:sapphire laser with a central wavelength of 800 nm and an FWHM (full width at half maximum) of 50 fs was used as the irradiation source. The output laser beam was divided into two beams, i.e., a pump beam and a probe beam. The pump beam propagated to an objective lens (5×, NA = 0.15, Olympus Inc.) to irradiate the sample in normal direction. The pulse energy was varied by a combination of half-wave plate and a Glan-Thompson prism. A top imaging module (the yellow path shown in Fig. 1) was established to monitor the irradiation process. The illumination beam from an LED was guided onto the target and the reflected light was collected by a CCD to image the sample surface. The wedges with an angle of 5° are used as beamsplitters for eliminating the influence of the light reflection on the backside, which may cause ghost shadow on the CCD. A dichroic mirror is used to reflect the pump beam but allow the LED illumination light passing through. The probe beam was guided to a 1D mechanical delay stage. By multiple reflecting the beam using retroreflectors mounted on the stage (the multiple reflection of the probe beam is not shown in scale in Fig. 1), we can realize a maximum delay of 15.6 ns. The probe beam was then frequency doubled by passing through a phase-matching BBO (β-BaB2SO4) crystal (CASTECH Inc.). The energy of the probe beam is varied for highly clear imaging but no damaging the CCD. The probe beam propagated through the sample in parallel to its upper surface, and a CCD with a bandpass filter (400 ± 10 nm) facing to the incident probe beam was used to receive the shockwave shadowgraph. The zero delay between the pump and the probe beam was determined by gradually monitoring the shadowgraphs of the laser-ionized air without the sample. Since the BBO crystal is phase matched, the pulse duration of the probe beam is around 50 fs and this is the time resolution of the pump-probe setup. Subject to the length of the 1D stage, for a delay longer than the maximum value of 15.6 ns, another method was used to capture the shockwave evolution. A CW (continuous wave) laser with a wavelength of 473 nm act as the “probe” beam, which passed through a bandpass filter (470 ± 10 nm) and finally arrived into an intensified CCD (ICCD, iStar 334, Andor Inc.) carrying the information of the shockwave. By adapting the gate delay and exposure time of the ICCD, the evolution of shockwave at a longer time (>15.6 ns) can be acquired. Since the evolution of the shockwave becomes slower, the exposure time of the ICCD is set to be 2 ns which hence is the time resolution of this method. The time-resolved emissive plasma was directly captured by an ICCD combined with a lens tube, so the expansion of the plasma can be intuitively demonstrated.
Fig. 1. Experimental setup for laser ablation and multi-scale observations (M, mirror; BS, beam splitter; ½λ, half-wave plate; GP, Glan-Thompson prism; BBO, β-BaB2SO4 crystal; ICCD, Intensified CCD).
To acquire the precise electron density and temperature evolution of the plasma during ejection, the emissive spectra of the plasma were obtained by a co-linear “4f system”, the 5× objective lens and another lens could collect the emitted light to the entrance of a fiber bundle, which consisted of 19 fiber cores with a diameter of 200 μm, then the light could be guided to the spectrograph (Shamrock 750, Andor Inc.) and received by the ICCD. The time-resolved spectra were acquired for the plasma analysis.
The GaP sample (<110 > crystal orientation, 10 × 10 × 1 mm3 bulk) was placed on a precise 3D stage with a resolution of 1 μm in three directions, which could guarantee the sample surface at the laser focal plane and hence a fresh spot was always irradiated. The fs laser, CCD and ICCD are synchronized using a digital delay generator (DG645, SRS Inc.) To avoid the influence of the air breakdown, the sample surface was positioned 5 μm higher than the focal plane. Craters were ablated at different laser fluences. The surface morphologies were characterized by a SEM (Hitachi SU8220), and the crater depth was measured employing a laser confocal microscope (Olympus LEXT OLS5100).
3. Results and discussion
3.1 Crater morphology and ablation threshold
Craters were ablated by single shot at different incident pulse energies of 10∼50 μJ with an increment of 10 μJ. Figure 2 shows the microstructures and the cross-sectional profiles of the ablated craters. The actual beam waist radius on the target and the ablation threshold of GaP were obtained through the diameter-square (D2) method [17] as shown in Fig. 3. The calculated beam waist radius on the sample surface is 13.57 μm. And the ablation threshold Fth is 2.27 J/cm2, which is in the same order as the single shot of picosecond laser (1.2 J/cm2, λ = 4.7 μm, τ = 1 ps) [9], as well as the five shots of nanosecond laser (5.4 J/cm2, λ = 1.06 μm, τ = 8 ns) [18]. However, the single shot threshold is much higher than that for high repetition fs laser irradiation with a central wavelength of 1040 nm, and this was explained by the mechanisms of the laser absorption induced by different laser wavelengths as well as the incubation effect caused by pulse repetition [9].
Fig. 2. Microstructures (a) and 2D depth imaging (b) of the ablated craters at different pulse energies by single shot; (c) the cross-sectional profiles of the craters depicted in (b).
It is intuitively seen in Fig. 2 that the diameter and the depth of the crater increases with the increase of the laser fluence. The splashing and re-solidification of effect of the ablated material become more obvious at higher laser fluences, which is due to the hydrodynamic explosion generated bubble cavitation and the redeposition of nanoparticles. During the pulse irradiation, the pulse energy is partly absorbed and transferred to the heating of the material, the temperature of irradiated area is increased higher than the melting/vaporing point. The material was then ejected to the surroundings and push the air forward. The ejected material has high temperature, and emits lights out. The peak laser fluences corresponding to the craters are 1.52Fth, 3.05Fth, 4.57Fth, 6.09Fth, and 7.61Fth at incident pulse energies from 10 to 50 μJ. The corresponding plasma dynamics at different peak fluences are probed and analyzed as follows.
3.2 Plasma expansion
The shadowgraphs of the laser generated plasma and shockwave were obtained by the pump-probe technique in a time ranges of 0∼15.6 ns (Fig. 4(a)) at different peak fluences. For all the fluences, strong material expansion from surface can be seen until 1.8 ns. It is seen that the outer edge of expanding material is darker than the inner part, which is inferred as compressed materials with a higher density than the inner part during the adiabatic expansion. Meanwhile, once the critical free electron density is reached, the transparent material becomes opaque, and the absorbed energy is mainly deposited in a very thin layer within a short period of time. That’s why we can see the outer ring is darker than the inner core.
Fig. 4. Time-resolved shockwave shadowgraphs captured by: (a) the side view pump-probe method; (b) the combination of a CW laser and an ICCD.
As time elapses, the outer edge of the plume become blurs and finally disappears due to the continuing phase transformations. The shockwave front becomes visible after 7.5 ns, also the ejected material in the shockwave can be observed. Due to the travel limit of motion stage, the largest delay was 15.6 ns using the pump-probe technique. For a longer delay, the evolution of the shockwave shown in Fig. 4(b) was observed using a time gated ICCD (gate width is 2 ns) and a CW laser. The shockwave before 50 ns cannot be obtained by this method due to the strong bremsstrahlung emission in the initial breakdown stage. The shape of the shockwave is cone-like in the first 100 ns and then gradually becomes round after then. The shockwave after 400 ns is not apparently intuitive.
The propagation of the laser-generated shockwave can be regarded as the generation from an instantaneous, massless point explosion. The relationship between the vertical radius R of shockwave and time-delay t can be acquired using the Sedov-Taylor equation [19,20].
The front edge of the plasma shown in Fig. 5(a) is the ejected plume before 1.8 ns, and we cannot categorize it as a spherical shockwave propagation in this time range. The corresponding fitting results show an increasing power exponent from 0.47 (β = 2.2) to 0.60 (β = 1.3) with the augment of the fluence. This indicates that the material expansion behavior changes from cylindrical to planar propagation at higher fluences. On the contrary, the shockwave fitting after 7.8 ns shown in Fig. 5(b) is perfectly fit to the spherical propagation (β = 3). By differentializing the fitting functions, the expansion speed of the plasma (shockwave) front is shown in Fig. 5(c) (Fig. 5(d)). Specifically, for a peak fluence of 7.61Fth, the expansion speed of the material can reach ∼70 Mach at 0.5 ns and then gradually decreases to ∼45 Mach. On the other hand, the early shockwave at 7.8 ns propagates at a speed of ∼10 Mach, then rapidly drops to less than 3 Mach in the first 100 ns, and its final speed is lower than 1 Mach.
Fig. 5. The radius of (a) the expanding plasma and (b) the shockwave as a function of time; the corresponding velocity of (c) the expanding plasma and (d) the shockwave with time.
The shockwave energy can be obtained by comparing the fitting parameters in Fig. 5(b) to Eq. (1). All the ESW at each fluence was calculated using this method, and the results are shown in Fig. 6(a). It is found that ESW gradually increases with the increase of laser fluences. We define the “shockwave energy ratio” as ESW/E0, where E0 is the irradiation laser pulse energy. It can be seen in Fig. 6(a) that from the min to max pulse energy, the ratio is in a range of 12%-18%. It decreases first then increases with the fluence. This phenomenon may be related to the complex processes of laser absorption and the phase transformation. Based on the shockwave energy ESW, the pressure (P) and temperature (T) of the air near the shockwave can be calculated as [21]
Fig. 6. (a) The shockwave energy and its ratio as a function of the laser fluence, (b) the pressure and temperature of the air near the shockwave.
3.3 Plasma plume emission
Figure 7 shows the evolution of the plasma evolution, and the shockwave front derived from shockwave images was depicted in white dashed line. Each image was normalized to its highest intensity for a high contrast. In accordance with the setting during shockwave imaging, the gate width of ICCD was also set to be 2 ns for plasma imaging. A strong bremsstrahlung emission can be seen at the initial 25 ns, and the shockwave edge was not detected or depicted at this time. At 50 ns, a slow-moving component with a stronger emission is adjacent to the target whereas a fast-moving component far from the surface with weaker emission is observed. These two parts of plume evolves and finally cools down along with time. The gap between these two components remains apparent until 150 ns. Then the whole plasma emission intensity tends to be the same, and after 400 ns the emission basically disappears.
The splitting of plasma plume was previously reported by several studies, and the main reason was the velocity differences for different species in the plume [22,23]. The species differences in the two components are strongly related to the mechanisms of phase transformations, i.e., Coulomb explosion [22] and thermal melting/evaporation [24]. The Coulomb explosion occurs at laser fluences close to the ablation threshold. The fs laser excited electrons are ejected from the bulk material, generating highly charged atoms at the surface and a strong Coulomb repulsion is created between the ions to repel the ions out of the target [25]. This is regarded as a non-thermal “gentle” ablation and a smooth ablation crater in nanometer scale can be formed [22]. In thermal melting/vaporization, the electron-phonon collisions increase the local temperature of the irradiated area above the melting/vaporization point so that the created high-temperature plasma adiabatically expands and spurts out of the target surface, which is called “strong” ablation [24]. It was investigated using time-of-flight (TOF) that the velocities of the ions with similar momenta in the case of Coulomb explosion were significantly higher than the ions with similar kinetic energy due to the thermal process [26]. The coexistence of the two ablation mechanisms results in a first component of faster energetic electrons and higher-charged ions ejected away from the target followed by the hydrodynamic hot expanding plasma consisting of mostly neutral atoms, electrons, and lower-charged ions.
It is very interesting to note that the fast-moving component is higher than the edge of shockwave in all the fluences except the highest one of 7.61Fth. This is different from our previous study that the fs laser-induced material ejects into the shockwave and generates a small protrusion at the shockwave edge through local heating of the air [27]. In this study, there is no protrusion occurs at the edge but the emissive species escape out of the wrapping of the shockwave. This may attribute to the domination of Coulomb explosion. It has been known the shockwave originates from the heat out of the hot plasma and its formation was regarded as an adiabatic expansion from a point, which has been evidenced in Fig. 5(b) with a spherical propagation. The Coulomb explosion is regarded as a non-thermal process and repelled species from the target surface are faster than those generated by thermal melting/evaporation. Meanwhile, the velocity of the shockwave in the early time is close to the speed of the ejecting thermalized plume. Therefore, the fast-moving component has a higher speed and surpasses the shockwave. The fast-moving component emits light out and cools down with time, so that the shockwave can wrap the whole plasma. On the contrary, for high fluences that thermal melting/evaporation dominates, the shockwave may obtain enough energy to expand faster than the created species by Coulomb explosion, and shockwave surpassing is not obvious.
3.4 Spectral emission of the GaP plasma
The time-resolved spectra of typical Ga element peaks in plasma at each fluence were measured. Figure 8 shows typical time-resolved spectra at the fluence of 7.61Fth. The gate width is 2 ns and the increasing step is 50 ns, respectively. The spectrum at 0 ns was reduced to 1/3 times to fit the whole graph. The information of Ga I peaks in the spectra is listed in Table 1. A clear downward trend can be observed, corresponding to the plasma plume imaging in Fig. 7.
Fig. 8. The time-resolved spectra of the generated plasma at a fluence of 7.61Fth. The gate width for each spectrum is 2 ns.
Table 1. The peaks information of Ga I emission in Fig. 8.
Assuming that the plasma is in local thermal equilibrium (LTE), the time-resolved plasma temperature (T) can be obtained by Boltzmann plot, described as [28,29]
In the study of LIBS, it is generally believed that Starks broadening (ΔλStark) and instrumental broadening (Δλinst) are the main causes of spectral line broadening. The Vogit profile was used to fit the measured spectrum, from which the measured FWHM (Δλmeasured) can be calculated, and the relationship between them can be given as [30]
By measuring the spectrum of standard mercury lamp with known wavelength (546.08 nm of Hg I), the instrument broadening is determined to be 0.16 nm, and there is an approximate linear relationship between Starks broadening and electron density (Ne) [31]
where ω is the electron impact parameter. The Ga I 417.20 nm line is used for calculation [32]. The time-resolved electron density at each fluence is obtained as shown in Fig. 9(b), and the inset also gives an example of calculating the Ne using the Vogit fitting profile. The LTE assumption is checked and ensured using McWhirte criterion during calculations. Similar to the plasma plume intensity, with the increase of laser fluence, the initial electron density increases, and the difference of electron density only remains before the delay of 100 ns. After then, with the expansion of plasma, the difference of electron density under each fluence is not significant. Furthermore, at all fluences, Ne decreased to a relatively stable value almost in the same time (100 ns), only at the maximum fluence (7.61Fth) in this study a slightly longer time (150 ns) was taken.4. Conclusions
The single shot ablation on GaP crystal by an 800 nm centered fs laser was thoroughly studied. The ablation threshold was first obtained using diameter square method, which was in the same order of picosecond and nanosecond laser using single or multiple shots. The plasma dynamics were probed by different optical methods. The side-view pump-probe technique and a combination of an ICCD and a CW laser were used to capture the evolution of plasma expansion and shockwave propagation after irradiation. In the first 1.8 ns, the electron density of the ejected plasma was increased by laser excitation and then decreased due to continuous expansion, corresponding to a color change of the layered plasma. Also, the Sedov-Taylor fitting results showed the plasma expansion gradually changed from a cylindrical to a planar propagation. On the other hand, a complete hemispherical propagation of shockwave was formed after a delay of 7.8 ns. By studying the percentage of the energy that propelled the shockwave in the laser pulse energy, we found that the ratio of the shockwave was in the range of 12%∼18%. Plasma splitting was also observed through emissive plasma imaging. By depicting the shockwave edge onto the plasma imaging, an interesting phenomenon that the ejected plume escapes out of the shockwave edge is found. This is related to the significant influence of the Coulomb explosion. The evolution of electron temperature and electron density was obtained by analyzing the laser-induced emission at different fluences. It is found the plasma temperature is higher than that of the shockwave, which is in accordance with the heat transfer from the plasma to the surrounding air. This study provides a variety of ultrafast information in a lifetime span after the irradiation of fs laser pulse, which will be of significance for ultrafast laser processing of GaP crystal and its various applications in the future.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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