An experimental method to examine the early plasma evolution induced by ultrashort laser pulses is described. Using this method, high quality images of early plasma are obtained with high temporal and spatial resolutions. A novel integrated atomistic model is used to simulate and explain the mechanisms of early plasma.
Early plasma is generated owing to high intensity laser irradiation of target and the subsequent target material ionization. Its dynamics plays a significant role in laser-material interaction, especially in the air environment1-11.
Early plasma evolution has been captured through pump-probe shadowgraphy1-3 and interferometry1,4-7. However, the studied time frames and applied laser parameter ranges are limited. For example, direct examinations of plasma front locations and electron number densities within a delay time of 100 picosecond (ps) with respect to the laser pulse peak are still very few, especially for the ultrashort pulse of a duration around 100 femtosecond (fs) and a low power density around 1014 W/cm2. Early plasma generated under these conditions has only been captured recently with high temporal and spatial resolutions12. The detailed setup strategy and procedures of this high precision measurement will be illustrated in this paper. The rationale of the measurement is optical pump-probe shadowgraphy: one ultrashort laser pulse is split to a pump pulse and a probe pulse, while the delay time between them can be adjusted by changing their beam path lengths. The pump pulse ablates the target and generates the early plasma, and the probe pulse propagates through the plasma region and detects the non-uniformity of electron number density. In addition, animations are generated using the calculated results from the simulation model of Ref. 12 to illustrate the plasma formation and evolution with a very high resolution (0.04 ~ 1 ps).
Both the experimental method and the simulation method can be applied to a broad range of time frames and laser parameters. These methods can be used to examine the early plasma generated not only from metals, but also from semiconductors and insulators.
1. Optical System Setup (Fig. 1)
2. Pump-probe Synchronization
3. Sample and Stage Preparation
4. Ablation and Measurement
5. Representative Results
The measured shadowgraph images are shown in Fig. 2 and Fig. 3, for the focal point slightly above and below the target surface, respectively. The longitudinal and radial expansion positions are plotted in Fig. 4 and Fig. 5. The longitudinal expansions of these two cases in the first 100 ps are significantly different; however, their longitudinal expansions in the following 400 ps and their radial expansions are similar. For the first case, the early plasma within 100 ps has a one-dimensional expansion structure consisting of multiple layers. For the second case, the early plasma has a two-dimensional expansion structure that does not change very much within 100 ps.
The simulation model12 is used to investigate the mechanism of early plasma evolution. Time zero is defined as the time when the laser pulse peak reaches the target surface. The simulated early plasma evolution processes agree well with the measured results for both of these two cases, as shown in Fig. 6 and Fig. 7, respectively. The formation of the early plasma within 1 ps is also predicted for the first case using the simulation model and shown in Fig. 8. The early plasma is found to have an air breakdown region and a Cu plasma region. The air breakdown is first caused by multi-photon ionization and then followed by avalanche ionization. For the second case, however, the focal point is below the target surface and no separate air breakdown region is formed. Instead, air ionization occurs near the Cu plasma front and is caused by impact ionization owing to the free electrons ejected from the Cu target.
Figure 1. Schematic of the pump-probe shadowgraph measurement.
Figure 2. Cu plasma expansion at successive delay times with the focal point slightly above the surface. Laser wavelength: 800 nm; pulse duration: 100 fs; power density: 4.2 × 1014 W/cm2; target: Cu.
Figure 3. Cu plasma expansion at successive delay times with the focal point slightly below the surface. Laser wavelength: 800 nm; pulse duration: 100 fs; power density: 4.2 × 1014 W/cm2; target: Cu.
Figure 4. Plasma longitudinal and radial expansion positions at successive delay times with the focal point slightly above the surface. Laser wavelength: 800 nm; pulse duration: 100 fs; power density: 4.2 × 1014 W/cm2; target: Cu.
Figure 5. Plasma longitudinal and radial expansion positions at successive delay times with the focal point slightly below the surface. Laser wavelength: 800 nm; pulse duration: 100 fs; power density: 4.2 × 1014 W/cm2; target: Cu.
Figure 6. Animation of measured and calculated plasma expansion within a delay time of 70 ps with the focal point slightly above the surface. Laser wavelength: 800 nm; pulse duration: 100 fs; power density: 4.2 × 1014 W/cm2; target: Cu. Click here to view animation.
Figure 7. Animation of measured and calculated plasma expansion within a delay time of 70 ps with the focal point slightly below the surface. Laser wavelength: 800 nm; pulse duration: 100 fs; power density: 4.2 × 1014 W/cm2; target: Cu. Click here to view animation.
Figure 8. Animation of measured and calculated plasma expansion within a delay time of 1 ps with the focal point slightly above the surface. Laser wavelength: 800 nm; pulse duration: 100 fs; power density: 4.2 × 1014 W/cm2; target: Cu. Click here to view animation.
The measurement and simulation methods presented in this paper enable more accurate examinations of the early plasma dynamics and a better understanding of the ionization mechanisms for both air and Cu. High quality plasma structures are captured with a temporal resolution of 1 ps and a spatial resolution of 1 μm. This measurement has a high repeatability too. The critical procedure is to align the beam very well and prepare a target surface with a high flatness as well as a low roughness.
This approach can be applied to other target materials and various laser parameters. The only limitation of the pump-probe shadowgraph method is a too low electron number density variation.
The authors have nothing to disclose.
The authors wish to gratefully acknowledge the financial support provided for this study by the National Science Foundation (Grant No: CMMI-0653578, CBET-0853890).
Name of the equipment | Company | Catalogue number |
Laser | Spectra-Physics | SPTF-100F-1K-1P |
ICCD camera | Princeton Instruments | 7467-0028 |
Oscilloscope | Rigol | DS1302CA |
Photodiode | Newport | 818-BB30 |
Linear stage | Newport | 433 |
Dial indicator | Mitutoyo | ID-C112E |