Name: investigation of early plasma evolution induced by ultrashort laser pulses
Uploaded: 2012-07-02T08:02:00
Duration: 11 min 20 s
Description: Watch this Scientific Journal Video about Investigation of Early Plasma Evolution Induced by Ultrashort Laser Pulses at JoVE.com

The authors wish to gratefully acknowledge the financial support provided for this study by the National Science Foundation (Grant No: CMMI-0653578, CBET-0853890).

1. Optical System Setup (Fig. 1)
<ol>
 <li>Set up a half-wave plate and a polarizer following the laser output to adjust the laser pulse energy.</li>
 <li>Set up a beam splitter after the polarizer to split the laser pulse to two pulses: pump pulse and probe pulse.</li>
 <li>Use four reflecting mirrors and a manual translational stage to construct an optical delay device for the pump pulse.</li>
 <li>Use another four reflecting mirrors to guide the pump pulse to reach the target surface vertically.</li>
 <li>Set up a second harmonic generator (SHG) to transform the laser pulse wavelength from 800 nm to 400 nm.</li>
 <li>Use a harmonic separator to transmit the 800-nm pulse and reflect the 400-nm pulse.</li>
 <li>Set up a beam reducer and a pair of focally lenses to adjust the size and convergence of the probe pulse.</li>
 <li>Set up another optical delay device, as mentioned in Step 1.3, for the probe pulse.</li>
 <li>Use an Iris ring to adjust the area of the probe pulse and make sure the probe pulse pass the target surface horizontally and intersect with the pump pulse.</li>
 <li>Set up two objective lenses and several filters to generate the image of the plasma region to be received by the intensified charge-coupled device (ICCD) camera.</li>
 <li>Connect the computer, the laser, the ICCD camera and its controller using BNC cables or USB cables.</li>
 <li>Adjust the delay time of the camera controller till the camera captures an image of the probe pulse. Thus, the probe pulse and the camera are synchronized.</li>
</ol>
2. Pump-probe Synchronization
<ol>
 <li>Place a beam splitter at the intersection of the pump pulse and the probe pulse, and set up two photodiodes to receive these two pulses. These two photodiodes should have a same distance away from the beam splitter.</li>
 <li>Use an oscilloscope to receive the signals of these two photodiodes, and move the delay stage on the pump pulse beam path till the profiles of the pump pulse and the probe pulse overlap with each other on the oscilloscope screen. An accuracy of 20 ps is achieved owing to the temporal resolution of the oscilloscope.</li>
 <li>Remove the beam splitter and the two photodiodes as mentioned in Step 2.1.</li>
 <li>Adjust the delay stage on the pump pulse beam path till the air breakdown region could just be observed on the ICCD screen. The time when the formation of air breakdown could be detected instead of a uniform background is determined as delay time zero.</li>
</ol>
3. Sample and Stage Preparation
<ol>
 <li>Set up a lab-jack and two manual linear stages in order to move the sample with three degrees of freedom.</li>
 <li>Use a dial indicator and high precision shims to achieve a high flatness of the stages. The height difference should be within 1 &mu;m per a distance of 25.4 mm.</li>
 <li>Cut a square piece (30 mm &times; 30 mm) out of a Cu sheet with a thickness of 0.8 mm using a milling machine.</li>
 <li>Polish a narrow side (30 mm &times; 0.8 mm) of the Cu piece until the surface roughness is below 0.5 &mu;m.</li>
 <li>Fix the Cu piece on the top manual stage with the polished narrow face up.</li>
 <li>Move the target by one manual stage as mentioned in Step 3.1) while monitor its position via the ICCD camera such that any tilt could be adjusted by inserting high precision shims below the target.</li>
 <li>Repeat Step 3.6 with the other manual stage.</li>
 <li>Drill a dozen of holes on the target while vary 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.</li>
</ol>
4. Ablation and Measurement
<ol>
 <li>Move the focal lens up to a distance of about 50 &mu;m away from the focal point.</li>
 <li>Move the delay stage on the probe pulse beam path with an interval of 0.3 mm to capture the image every 2 ps till 10 ps, or with an interval of 3 mm to capture the image every 20 ps till 480 ps.</li>
 <li>Repeat Step 4.2 for several times for repeatability and accuracy.</li>
 <li>Move the focal lens down to a distance of about 50 &mu;m away from the focal point, and repeat Step 4.3.</li>
</ol>
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.
<img alt="Figure 1" src="/files/ftp_upload/4033/4033fig1.jpg" /> 
Figure 1. Schematic of the pump-probe shadowgraph measurement.
<img alt="Figure 2" src="/files/ftp_upload/4033/4033fig2.jpg" /> 
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 &times; 1014 W/cm2; target: Cu.
<img alt="Figure 3" src="/files/ftp_upload/4033/4033fig3.jpg" /> 
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 &times; 1014 W/cm2; target: Cu.
<img alt="Figure 4" src="/files/ftp_upload/4033/4033fig4.jpg" /> 
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 &times; 1014 W/cm2; target: Cu.
<img alt="Figure 5" src="/files/ftp_upload/4033/4033fig5.jpg" /> 
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 &times; 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 &times; 1014 W/cm2; target: Cu. <a href="https://www.jove.com/files/ftp_upload/4033/4033movie6.avi" target="_blank">Click here to view animation</a>.
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 &times; 1014 W/cm2; target: Cu. <a href="https://www.jove.com/files/ftp_upload/4033/4033movie7.avi" target="_blank">Click here to view animation</a>.
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 &times; 1014 W/cm2; target: Cu. <a href="https://www.jove.com/files/ftp_upload/4033/4033movie8.avi" target="_blank">Click here to view animation</a>.

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No conflicts of interest declared.

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 &mu;m. 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.

<table border="1">
<tbody>
 <tr>
 <td>Name of the equipment</td>
 <td>Company</td>
 <td>Catalogue number</td>
 </tr>
 <tr>
 <td>Laser</td>
 <td>Spectra-Physics</td>
 <td>SPTF-100F-1K-1P</td>
 </tr>
 <tr>
 <td>ICCD camera</td>
 <td>Princeton Instruments</td>
 <td>7467-0028</td>
 </tr>
 <tr>
 <td>Oscilloscope</td>
 <td>Rigol</td>
 <td>DS1302CA</td>
 </tr>
 <tr>
 <td>Photodiode</td>
 <td>Newport</td>
 <td>818-BB30</td>
 </tr>
 <tr>
 <td>Linear stage</td>
 <td>Newport</td>
 <td>433</td>
 </tr>
 <tr>
 <td>Dial indicator</td>
 <td>Mitutoyo</td>
 <td>ID-C112E</td>
 </tr>
 </tbody>
</table>

investigation of early plasma evolution induced by ultrashort laser pulses

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.

Watch this Scientific Journal Video about Investigation of Early Plasma Evolution Induced by Ultrashort Laser Pulses at JoVE.com

Investigation of Early Plasma Evolution Induced by Ultrashort Laser Pulses

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 ...

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We demonstrate the use of the Laser-induced forward transfer technique (LIFT) for the printing of high-viscosity Ag paste. This technique offers a simple, low temperature, robust process for non-lithographically printing microscale 2D and 3D structures.

Over the past decade, there has been much development of non-lithographic methods1-3 for printing metallic inks or other functional materials. Many of ...

An Atmospheric Pressure Plasma Setup to Investigate the Reactive Species Formation

Non-thermal atmospheric pressure (&#39;cold&#39;) plasmas have received increased attention in recent years due to their significant biomedical potential. The reactions of cold plasma with the surrounding atmosphere yield a variety of reactive species, which can define its effectiveness. While efficient development of cold plasma therapy requires kinetic models, model benchmarking needs empirical data. Experimental studies of the source of reactive species detected in aqueous solutions exposed to plasma are still scarce. Biomedical plasma is often operated with He or Ar feed gas, and a specific interest lies in investigation of the reactive species generated by plasma with various gas admixtures (O2, N2, air, H2O vapor, etc.) Such investigations are very complex due to difficulties in controlling the ambient atmosphere in contact with the plasma effluent. In this work, we addressed common issues of &#39;high&#39; voltage kHz frequency driven plasma jet experimental studies. A reactor was developed allowing the exclusion of ambient atmosphere from the plasma-liquid system. The system thus comprised the feed gas with admixtures and the components of the liquid sample. This controlled atmosphere allowed the investigation of the source of the reactive oxygen species induced in aqueous solutions by He-water vapor plasma. The use of isotopically labelled water allowed distinguishing between the species originating in the gas phase and those formed in the liquid. The plasma equipment was contained inside a Faraday cage to eliminate possible influence of any external field. The setup is versatile and can aid in further understanding the cold plasma-liquid interactions chemistry.

An experimental setup was created for the helium-operated kHz frequency plasma jet. The setup includes a cage for the plasma power supply and jet and an in-house built reactor to monitor plasma-induced reactive species without the interference of the ambient atmosphere.

Non-thermal atmospheric pressure (&#39;cold&#39;) plasmas have received increased attention in recent years due to their significant biomedical potential. ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering thermal wave fields that are disturbed by the defect. This effect is measured and used to locate the defect. We form the destructively interfering wave fields by using a modified projector. The original light engine of the projector is replaced with a fiber-coupled high-power diode laser. Its beam is shaped and aligned to the projector&#39;s spatial light modulator and optimized for optimal optical throughput and homogeneous projection by first characterizing the beam profile, and, second, correcting it mechanically and numerically. A high-performance infrared (IR) camera is set up according to the tight geometrical situation (including corrections of the geometrical image distortions) and the requirement to detect weak temperature oscillations at the sample surface. Data acquisition can be performed once a synchronization between the individual thermal wave field sources, the scanning stage, and the IR camera is established by using a dedicated experimental setup which needs to be tuned to the specific material being investigated. During data post-processing, the relevant information on the presence of a defect below the surface of the sample is extracted. It is retrieved from the oscillating part of the acquired thermal radiation coming from the so-called depletion line of the sample surface. The exact location of the defect is deduced from the analysis of the spatial-temporal shape of these oscillations in a final step. The method is reference-free and very sensitive to changes within the thermal wave field. So far, the method has been tested with steel samples but is applicable to different materials as well, in particular to temperature sensitive materials.

This method aims at locating vertical subsurface defects. Here, we couple a laser with a spatial light modulator and trigger its video input to heat a sample surface deterministically with two anti-phased modulated lines while acquiring highly resolved thermal images. The defect position is retrieved from evaluating thermal wave interference minima.

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering ...