The aim of this experiment is to examine the early plasma evolution induced by ultra short laser pulses with high temporal and spatial resolutions. This is achieved by setting up an optical system where one ultrashort laser pulse is split into a pump pulse and a probe pulse. As a second step.
The pump pulse and probe pulse are synchronized, providing the delay time zero for the optical delay device. The sample and stage are then prepared in order to achieve a high flatness of the sample top surface and to make the sample top surface parallel with the probe pulse. Next ablation is performed with the focal points slightly above and slightly below the target.
The pump pulse ablates the target and generates the early plasma while the prop pulse propagates through the plasma region and detects the non-uniformity of electron number density. Ultimately, the results show plasma expansion at successive delay times based on the measured shadow graph.Images. Animations are also generated using the calculated results to illustrate the plasma formation and evolution with a very high resolution.
This method can help answer key questions in the Outshot lead field, such as the effect of the focal point position, and the early plasma evolution on laser ablation. The implication of this technique extends towards development and effective use of ultra shore ledgers because the viewers, how much energy is absorbed by the early plasma during the ultra laser duration and can be used for the amplification of the plasma field for various diagnostics and material processing work. The optical system set up for pump probe, shadow graft measurement has a half wave plate and a polarizer following the laser output to adjust the laser pulse energy.
A beam splitter after the polarizer splits the laser pulse into two pulses, pump pulse, and probe pulse. Four reflecting mirrors and a manual translation stage are used to construct an optical delay device for the pump pulse. Another four reflecting mirrors guide the pump pulse to reach the target surface vertically.
A second harmonic generator transforms the laser pulse wavelength from 800 nanometers to 400 nanometers. Then a harmonic separator transmits the 800 nanometer pulse and reflects the 400 nanometer pulse. A beam reducer is next.
Another optical delay device for the probe. Pulse is constructed using four reflecting mirrors and a pair of focal lenses adjust the size and convergence of the probe pulse. An iris ring adjusts the area of the probe pulse and ensures that the probe pulse passes the target surface horizontally and intersects with the pump pulse next to objective lenses and several filters to generate the image of the plasma region to be received by the intensified charged coupled device camera.
Finally, the computer, the laser, the ICCD camera and its controller are connected using BNC cables or USB cables, adjust the delay time of the camera controller until the camera captures an image of the probe. Pulse effectively synchronizing the probe pulse and the camera pump probe synchronization is performed by a beam splitter at the intersection of the pump pulse and the probe pulse and two photo diodes receive the two pulses. These two photo diodes should have the same distance away from the beam splitter.
Then the beam splitter is removed using oscilloscope to receive the signals of these two photo diodes. Move the delay stage on the pump pulse beam path until the profiles of the pump pulse and the probe pulse overlap with each other on the oscilloscope screen. An accuracy of 20 picoseconds is achieved ing to the temporal resolution of the oscilloscope following synchronization.
Remove the two photodiodes. Adjust the delay stage on the pump pulse beam path until the air breakdown region can just be observed on the ICCD screen. The time when the formation of air breakdown can be detected instead of a uniform background is determined as delay time zero.
A critical part of this technique is to ensure that the target does not block any portion of the plasma image. This can be accomplished by preparing the sample and the stage in order to achieve a high flatness of the sample top surface, and to make the sample top surface parallel with the probe pass. Begin stage preparation with the setup of a lab jack and two manual linear stages.
In order, move the sample with three degrees of freedom, user dial indicator and high precision shims To achieve a high flatness of the stages, the height difference should be between one micron per distance of 25.4 millimeters. Next, use a milling machine to prepare a sample by cutting a square piece out of a copper sheet with a thickness of 0.8 millimeters. Use a polishing machine to polish a narrow side of the copper piece until the surface roughness is below 0.5 microns.
Fix the copper piece on the top manual stage with the polished narrow side face up. Move the target by one manual stage while monitoring its position via the ICCD camera, such that any tilt can be adjusted by inserting high precision SHIs below the target. Repeat this process with the other manual stage.
Finally, drill a dozen holes on the target while varying the position of the focal lens by a third high accuracy manual stage. The focal point location corresponds to the position of the focal lens where the smallest hole is drilled. The actual measurement should be done in the dark.
However, for filming purposes, some light was used to perform ablation. First, move the focal lens up to a distance of about 50 microns away from the focal point. Then move the delay stage on the prop pulse beam path with an interval of 0.3 millimeters to capture the image.
Every two picoseconds until 10 picoseconds is reached, or with an interval of three millimeters to capture the image every 20 picoseconds until 480 picoseconds is reached. Repeat this process several times for repeatability and accuracy. As a final step, move the focal lens down to a distance of about 50 microns away from the focal point and capture the images.
Again, the measured shadow graph images that result from copper plasma expansion at successive delay times are shown. These images correspond to the focal point being slightly above and slightly below the target surface. Here, the longitudinal and radial expansion positions are plotted for when the focal point is slightly above and slightly below the target surface.
The longitudinal expansions in these two cases are significantly different in the first 100 picoseconds for the first case, the early plasma within 100 picoseconds 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 picoseconds. However, the longitudinal expansions in the following 400 picoseconds and the radial expansions are similar. Here, a simulation model is used to investigate the mechanism of early plasma evolution through simulated electron density when the focal point is slightly above the target surface.
Time zero is defined as the time when the laser pulse peak reaches the target surface. The same model is used to explore when the focal point is slightly below the target surface. The simulated early plasma evolution processes agree well with the measured results.
In both of these cases, the formation of the early plasma within one picosecond is also predicted for the first case using the simulation model, the early plasma is found to have an air breakdown region and a copper plasma region. The air breakdown is first caused by multiphoton 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 copper plasma front and is caused by impact ionization owing to the free electrons ejected from the copper target After its development. This technique paved the way for researchers in the field of art shot laser to explore the early plasma evolution in the ablation process. After watching this video, you should have a good understanding of how to use shadowgraph for measuring plasmas.
