afosr young investigators program: …understanding intense laser interactions with solid density...

AFRL-AFOSR-VA-TR-2017-0009

AFOSR Young Investigators Program: Understanding Intense Laser Interactions with Solid Density Plasma

Alexander ThomasUNIVERSITY OF MICHIGAN503 THOMPSON STANN ARBOR, MI 48109-1340

01/04/2017Final Report

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1/5/2017https://livelink.ebs.afrl.af.mil/livelink/llisapi.dll

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To: [email protected]: Final Progress Statement to Dr. Jason Marshall

Contract/Grant Title: AFOSR Young Investigator Program: Understanding intense laser interactions with solid density plasmaContract/Grant #: FA9550-12-1-0310 Reporting Period: 1 Sep 2012 to 31 Aug 2016

Annual accomplishments (200 words max): .

We have performed time resolved measurements of the interaction of an ultrafast laser with thin solid density foil using wakefield accelerated electrons. Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have enormous benefits for ultrafast science. These novel sources promise to become indispensable tools for the investigation of structural dynamics on the femtosecond time scale, with spatial resolution on the atomic scale. We have demonstrated for the first time the use of laser-wakefield-accelerated electron bunches for time-resolved electron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranes pumped by an ultrafast laser pulse. In our proof-of-concept study, we resolve the Unco

rrecte

d proof

www.nature.com/scientificreports/

3Scientific RepoRts | 6:36224 | DOI: 10.1038/srep36224

To demonstrate that this set-up is not limited by the electron source but the magnetic-deflection diagnos-tic, we also placed the target closer to the source at d= 1.9 cm. By doing so, the resolution can be significantly enhanced. The lower panel of Fig. 2c, which was taken with d=1.9 cm, shows the temporal window is only about 15 ps because the electron bunch duration is shorter at this point but the upper limit on the resolution is now hundreds of femtoseconds.

Lattice dynamics of the silicon nano-membranes. The quality of the single-crystal silicon membranesand their thickness of only 30 nm (see Methods) allowed us to obtain high-quality diffraction images. Figure 3a shows a diffraction pattern of the [001] oriented Si sample. The energy spread of the electron beam leads to radial broadening of the Bragg peaks, which becomes more apparent at higher diffraction orders. Sending the pump beam causes photo-induced changes to the diffraction pattern which are easily measured by probing the sample at 300 ps after the arrival of the pump pulse. Figure 3b was obtained by subtracting the pump diffraction pattern from the unpumped pattern. The relative changes of the central undiffracted (0-order) spot and (220) Bragg peaksare shown in Fig. 3c. The dynamics associated with the four (220) diffraction peaks showed dissimilar trends,featuring an increase of diffraction efficiency for some (220) spots, such as spot 2 which increases by more than 10% in Fig. 3(b,c). Similar results on Si membranes were observed in ref. 35.

We may draw a few conclusions from Fig. 3. First, because some of the peaks increase in brightness, the photo-induced response cannot be explained simply by lattice heating and the Debye-Waller effect, which always reduce the intensity of all the Bragg peaks. Second, our observation cannot be explained by a change of thestructure factor, i.e., the positions of atoms in the unit cell. Indeed, it is readily seen that the structure factor forthe (220) silicon peaks is maximum, i.e., each atom in the unit cell contributes to the maximum constructive interference.

Reference 35 suggests that the increase of the Bragg peak intensity might originate from thermoelastic defor-mation of the crystal. In this scenario, electrons are first excited to high-energy states and then subsequently relax to the bottom of the conduction band while emitting phonons and causing ultrafast heating of the Si lattice on picosecond time scales36. The key point is that the heating is non-uniform (as it follows the transverse distribu-tion of the pump pulse), which in conjunction with the fixed boundary conditions imposed by the support grid structure, results in a thermal strain as well as surface bulging35. These effects could effectively alter the angles of the (220) planes, leading to changes in the diffracted intensity. In particular, this scenario is compatible with our experimental results as it can account for increasing Bragg peaks: a (220) peak which is initially slightly off Bragg could fully satisfy the Bragg condition after photo-excitation, explaining the increase in signal.

We now investigate the ultrafast dynamics by streaking the Bragg peaks. Results from two data sets are pre-sented in Fig. 4 for ultrafast dynamics of the undiffracted (0-order) and one of the (220)-order peaks. The pump

solenoid magnetic lens

LPA driver:10 mJ, 35 fs800 nm, 0.5 kHz

capillarygas jet

aperture

400 nm pump

slit

phosphor screen

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diffraction pattern without streaking

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Si filmSiSi filfilmm

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Figure 1. Experimental layout of streaked time-resolved electron diffraction. A 15-mJ Ti:Sapphire laser system is used for generating both the electron probe and the optical pump on the sample. Approximately 10 mJ of the 800 nm laser pulse energy is focussed into an argon gas jet produced by a 100µm capillary nozzle for generating bursts of electrons. The remaining fraction of the beam is frequency doubled to 400 nm and delivered to the sample for optical excitation at an absorbed fluence of 1–2 mJ/cm2. The electron beam is filtered by a 280µm aperture before entering a solenoidal magnetic lens. A 30 nm thick single-crystal silicon sample is placed at d=13.5 cm from the electron source. An optical micrograph of the array of si nano-membranes and supporting grid is displayed. The diffracted electrons enter a horizontal slit before they are spatially dispersed onto a detector screen via a pair of dipole magnets.


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silicon lattice dynamics on a picosecond time scale by deflecting the momentum-time correlated electrons in the diffraction peaks with a static magnetic field to obtain the time-dependent diffraction efficiency. Further improvements may lead to femtosecond temporal resolution, with negligible pump-probe jitter being possible with future laser-wakefield-accelerator ultrafast-electron-diffraction schemes. [2]

Archival publications (published) during reporting period:

1) C. Zulick, A. Raymond, A. McKelvey, V. Chvykov, A. Maksimchuk, A. G. R. Thomas, L. Willingale, V. Yanovsky, and K. Krushelnick, Target surface area effects on hot electron dynamics from high intensity laser-plasma interactions, New J Phys. 18, 063020 (2016).

2) Z.-H. He, B. Beaurepaire, J. A. Nees, G. Gallé, S. A. Scott, J. R. Sánchez Pérez, M. G. Lagally, K. Krushelnick, A. G. R. Thomas & J. Faure, Capturing Structural Dynamics in Crystalline Silicon Using Chirped Electrons from a Laser Wakefield Accelerator Scientific Reports (Accepted 2016).

Changes in research objectives, if any: None

Change in AFOSR program manager, if any:

Extensions granted or milestones slipped, if any:

Include any new discoveries, inventions, or patentdisclosures during this reporting period (if none, report none): None


AFOSR Deliverables Submission Survey

Response ID:7175 Data

1.

Report Type

Final Report

Primary Contact EmailContact email if there is a problem with the report.

[email protected]

Primary Contact Phone NumberContact phone number if there is a problem with the report

734 764-2729

Organization / Institution name

University of Michigan

Grant/Contract TitleThe full title of the funded effort.

Understanding intense laser interactions with solid density plasma

Grant/Contract NumberAFOSR assigned control number. It must begin with "FA9550" or "F49620" or "FA2386".

FA9550-12-1-0310

Principal Investigator NameThe full name of the principal investigator on the grant or contract.

Alexander Thomas

Program OfficerThe AFOSR Program Officer currently assigned to the award

Jason Marshall

Reporting Period Start Date

09/01/2012

Reporting Period End Date

08/31/2016

Abstract

We have performed time resolved measurements of the interaction of an ultrafast laser with thin soliddensity foil using wakefield accelerated electrons. Recent progress in laser wakefield acceleration has ledto the emergence of a new generation of electron and X-ray sources that may have enormous benefits forultrafast science. These novel sources promise to become indispensable tools for the investigation ofstructural dynamics on the femtosecond time scale, with spatial resolution on the atomic scale. We havedemonstrated for the first time the use of laser-wakefield-accelerated electron bunches for time-resolvedelectron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranespumped by an ultrafast laser pulse. In our proof-of-concept study, we resolve the silicon lattice dynamics ona picosecond time scale by deflecting the momentum-time correlated electrons in the diffraction peaks witha static magnetic field to obtain the time-dependent diffraction efficiency. Further improvements may lead tofemtosecond temporal resolution, with negligible pump-probe jitter being possible with future laser-wakefield-accelerator ultrafast-electron-diffraction schemes.

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Archival Publications (published) during reporting period:

1) C. Zulick, A. Raymond, A. McKelvey, V. Chvykov, A. Maksimchuk, A. G. R. Thomas, L. Willingale, V.Yanovsky, and K. Krushelnick, Target surface area effects on hot electron dynamics from high intensitylaser-plasma interactions, New J Phys. 18, 063020 (2016).2) Z.-H. He, B. Beaurepaire, J. A. Nees, G. Gallé, S. A. Scott, J. R. Sánchez Pérez, M. G. Lagally, K.Krushelnick, A. G. R. Thomas & J. Faure, Capturing Structural Dynamics in Crystalline Silicon UsingChirped Electrons from a Laser Wakefield Accelerator Scientific Reports (Accepted 2016).

New discoveries, inventions, or patent disclosures:Do you have any discoveries, inventions, or patent disclosures to report for this period?

No

Please describe and include any notable dates

Do you plan to pursue a claim for personal or organizational intellectual property?

Changes in research objectives (if any):

N/A

Change in AFOSR Program Officer, if any:

N/A

Extensions granted or milestones slipped, if any:

This years funding was an extension on the project from last year. It enabled us to carry out the work onprobe-probe electron diffraction measurements of ultrafast laser-solid target interactions that has beenaccepted for publication.

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