X-ray yield measurements from targets coated with wavelength-scale spheres are compared with measurements from polished targets. Evidence for a hotter resonant electron temperature due to field enhancements from Mie resonances in the spheres is investigated

Churina, I. V.

Ditmire, T.

Donnelly, Thomas D.

Landry, J.

Sumeruk, H. A.

Symes, D. R.

English

Keck Graduate Institute

Claremont Colleges
Scholarship @ Claremont
All HMC Faculty Publications and Research HMC Faculty Scholarship
5-1-2005
X-Ray Generation from Metal Targets Coated with
Wavelength-Scale Spheres
D. R. Symes
University of Texas at Austin
H. A. Sumeruk
University of Texas at Austin
I. V. Churina
University of Texas at Austin
Thomas D. Donnelly
Harvey Mudd College
J. Landry
University of Texas at Austin
See next page for additional authors
This Conference Proceeding is brought to you for free and open access by the HMC Faculty Scholarship at Scholarship @ Claremont. It has been
accepted for inclusion in All HMC Faculty Publications and Research by an authorized administrator of Scholarship @ Claremont. For more
information, please contact scholarship@cuc.claremont.edu.
Recommended Citation
D. R. Symes, H.A. Sumeruk, I.V. Churina, A. Belolipetski, Stefan Kneip, J. Landry, T.D. Donnelly, T. Ditmire, "X-Ray Generation from
Metal Targets Coated with Wavelength-Scale Spheres," CLEO/QELS (Baltimore, Maryland, 2005). doi: 10.1109/
QELS.2005.1549203
Authors
D. R. Symes, H. A. Sumeruk, I. V. Churina, Thomas D. Donnelly, J. Landry, and T. Ditmire
This conference proceeding is available at Scholarship @ Claremont: http://scholarship.claremont.edu/hmc_fac_pub/738
JThD3 2005 Quantum Electronics and Laser Science Conference (QELS)
X-Ray Generation from Metal Targets Coated with
Wavelength-Scale Spheres
D. R. Symes, H. A. Sumeruk, I. V. Churina, T. D. Donnelly, J. Landry and T. Ditmire
Fusion Research Center, Department ofPhysics, University of Texas atAustin, I University Station C1510, Austin, Texas, 78712, U.S.A.
dansymesJnaiL.utexas.edu
Abstract: X-ray yield measurements from targets coated with wavelength-scale spheres are
compared with measurements from polished targets. Evidence for a hotter resonant electron
temperature due to field enhancements from Mie resonances in the spheres is investigated.
© 2004 Optical Society of America
OCIS codes: (320.7120) Ultrafast phenomena; (350.5400) Plasmas
1. Introduction
A major application ofplasmas produced by high-intensity laser pulses is the generation of short bursts of x-rays.
For efficient hard x-ray (>few keV) production the acceleration of hot electrons from resonant absorption processes
should be optimized. Recent experiments have demonstrated enhancements in x-ray yield achieved by using
structured surfaces such as gratings [1], velvets [2], and nanoparticles [3].
In this paper, we investigate hard x-ray (>100 keV) generation from copper targets coated with a partially
covering layer of microspheres. When a spherical object is illuminated with light of a wavelength, X, similar to the
diameter, d, ofthe object, a complicated electric field pattem arises due to interference between the incident and
scattered waves. The interaction of an intense laser with such a target can generate a spherical plasma with non-
uniform heating across its surface [4, 5]. For example, Fig. 1 displays the electric field pattem of a 1tm spherical
plasma irradiated with 0.8 pm laser light. In particular, when the size parameter m = idlX 1, the system can
experience a Mie resonance in which the electric fields are greatly enhanced. By depositing wavelength-length scale
spheres across the surface ofa metal substrate, we aim to exploit the Mie resonance to increase the temperature of
the resonantly heated electrons which are responsible for the generation of hard x-rays.
(a) b) 2.2
1.0
+ +
1 1
-2 X + - b + +I
Fig. 1. Calculated electric field plot for an ionized, micron-diameter plasma sphere for laser polarization (a) parallel and
(b) perpendicular to the page. The laser propagation direction is +z. Note the enhancement in the field visible in (a).
2. Target preparation
We prepared the targets using commercially available polystyrene spheres of diameter 0.26, 0.5, 1.0, 1.5 and 2.9 pm
with a size distribution of3%, diluted with water to a concentration of =1:1000, and applied to polished copper
substrates. The solution was made more dilute for the smaller spheres to improve the consistency of the surface. We
observed the target surfaces using a scanning electron microscope (SEM) to ascertain the uniformity of the coating
and to check the depth of the sphere layers. SEM images of copper coated with a 2.9 pm sphere solution, shown in
Fig. 2, at magnification of (a) lOOx and (b) 5000x demonstrate that the spheres form a monolayer over a large area.
1549
JThD3
(a) (b)
Fig. 2. Scanning electron microscope images of a copper substrate coated with 2.9pm spheres at (a) lOOx and (b) 5000x
magnification. Note the uniformity of coverage in (a) and the size consistency shown in (a).
3. Experiment
We performed the experiment using the THOR laser system at the University of Texas which provided 35 fs pulses
at a wavelength of 800 nm. The pre-compression energy was set at =68 mJ yielding a focal intensity 4lx1018 Wcm-2.
This limit was required to avoid saturation of the x-ray detectors.
X-ray emission from the plasma was observed using six filtered Nal scintillation detectors arranged around the
target chamber. The filters were composed of 125 mm Al (450 keV), 12 mm Cu (162 keV), 32 mm Cu (500 keV),
43 mm Cu (600 keV), 24 mm Pb (700 keV) and 44 mm Pb (900 keV), where the bracketed values are the lI/e cutoff
energies of each filter after accounting for any extra material between the detector and the target.
Ion emission from the front surface of the target was measured using a charge collector (Faraday cup) at a
distance of 0.20 m behind a mesh biased at -300V to reduce the emission of secondary electrons from the detector
surface. A magnetic field of 1 kG was placed =1 cm in front of the collector to deflect electrons.
4. Results
We irradiated polished copper targets at laser incidence angles between 00 and 450 from the target normal to
ascertain the optimum angle for hard x-ray production under our conditions. The maximum x-ray emission occurred
for an incident angles =1 0°-20° suggesting that the dominant mechanism for hot electron generation was resonance
absorption. The signal levels reduced towards 450 indicating that vacuum heating was inefficient. This was probably
due to the formation of a preplasma caused by the interaction of a low level prepulse.
X-ray yields and ion spectra from polished copper targets were then directly compared to sphere coated
copper targets at incidence angles of 00, 100, 240 and 45°. Initial results using 2.9 .tm spheres are shown in Figs. 3
and 4. Fig. 3 shows the x-ray yield through the 32 mm Cu filter corresponding to an integrated signal of photon
energies above z500 keV. The sphere-coated targets display a clear enhancement in yield at all four angles of
incidence of the laser beam, even where resonance absorption is expected to be optimized in the polished copper
target. This suggests that the presence of the spheres not only modifies the target surface allowing resonant
processes at off-resonant angles but also increases the magnitude of the driving fields through an interference effect
(Mie resonance) as predicted. We note that the size parameter for these spheres is relatively large, m=1 1, so the
effect for smaller spheres should be even more significant.
1550
JThD3
50OkeV filter
60 * Polished Cu
0) 0 3F1m spheres
.40:
20
0 10 20 30 40 50
Angle (degrees)
Fig. 3. Hard x-ray yields versus angle of incidence of the laser beam for polished copper targets and 2.9 pm sphere coated copper targets.
Time-of-flight (TOF) traces from polished copper and 2.9 pan sphere-coated copper, shown in Fig. 4, for an incident
angle of45° differed considerably. The earliest time we could resolve after the photon and hot electron burst was
=1.12ps. This TOF corresponds to protons of energy =200 eV, below which protons would be deflected by the 1 kG
magnetic field. The maximum visible energy of 4I0 keV (assuming the ion mass of copper) is too low considering
the much higher electron energies implied by the x-ray measurements. This suggests that we only see the colder ions
from a distribution driven by two electron temperatures [6]. It is likely that the TOF trace from the sphere targets is
predominantly carbon and oxygen whereas the signal from the metal may contain all three species (since
contaminants will be present [7]). However, when we irradiated copper coated with a thin plastic film (13 ptm), no
ion signal was visible implying that the entire trace shown is due to copper ions. This would imply a significant shift
in the thermal temperature ofthe plasma between the two cases and an energy spectrum calculated assuming that all
the signal comes from copper ions is shown in the inset.
-10060 'a
4 10
.0~~~~~~~~~~~~~~~~~~~~~~~~~~1.
40).
Time of flight (Ls)
Fig. 4. Time-of-flight traces forpolished copper targets and 2.9 m sphere coated copper targets. (Inset) Kinetic energy spectra assuming Cu ions.
[1] 5. P. Gordan, T. Donnelly, A. Sullivan, H. Hamster, and R. W. Falcone, "X rays from microstructured targets heated by femtosecond lasers,"
Op. Lett. 19,484(1994).
[2] G. Kulesar, D. AlMawlawi, F. W. Budnik, P. R. Herman, M. Moskovits, L. Zhao, and R. S. Marjoribanks, "Intense Picosecond X-Ray Pulses
from Laser Plasmas by Use ofNanostructured "Velvet" Targets,"' Phys. Rev. Lett. 84, 5149(2000).
[3] P. P. Rajeev, P. Taneja, P. Ayyub, A. S. Sandhu, and G. Ravindra Kumar, "Metal Nanoplasmas as Bright Sources of Hard X-Ray Pulses,"
Phys. Rev. Lett. 90, 115002 (2003).
[4] T. D. Donnelly, M. Rust, I Weiner, M. Allen, R. A. Smith, C. A. Steinke, S. Wilks, J. Zweiback, T. E. Cowan, and T. Ditmire, "Hard X-Ray
and Hot Electron Production from Intense Laser Irradiation of Wavelength-Scale Particles," J. Phys. B 34, L3 13 (2001).
[5] D. R.
-Symnes, A. J. Comley, and R. A. Smi-th, "Fast-Ion Production from Short-Pulse Irradiation of Ethanol Microdroplets," Phys. Rev. Lett.
93, 145004 (2004).
[6] A. A. Andreev and J. Limpouch, "Ion Acceleration in Short-Pulse Laser-Target Interactions," J. Plasma Physics 62, 179 (1999).
[7] E. L. Clark, K. Krushelnick, M. Zepf, F. N. Beg, M. Tatarakis, A. Machacek, M. I. K. Santala. I. Watts. P. A. Norreys, and A. E. Dangor,
"Energetic Heavy-Ion and Proton Generation from Ultraintense Laser-Plasma Interactions with Solids," Phys. Rev. Lett. 85, 1654 (2000).
1551


X-Ray Generation from Metal Targets Coated with Wavelength-Scale Spheres

Scholarship@Claremont

Claremont CollegesScholarship @ ClaremontAll HMC Faculty Publications and Research HMC Faculty Scholarship5-1-2005X-Ray Generation from Metal Targets Coated withWavelength-Scale SpheresD. R. SymesUniversity of Texas at AustinH. A. SumerukUniversity of Texas at AustinI. V. ChurinaUniversity of Texas at AustinThomas D. DonnellyHarvey Mudd CollegeJ. LandryUniversity of Texas at AustinSee next page for additional authorsThis Conference Proceeding is brought to you for free and open access by the HMC Faculty Scholarship at Scholarship @ Claremont. It has beenaccepted for inclusion in All HMC Faculty Publications and Research by an authorized administrator of Scholarship @ Claremont. For moreinformation, please contact scholarship@cuc.claremont.edu.Recommended CitationD. R. Symes, H.A. Sumeruk, I.V. Churina, A. Belolipetski, Stefan Kneip, J. Landry, T.D. Donnelly, T. Ditmire, "X-Ray Generation fromMetal Targets Coated with Wavelength-Scale Spheres," CLEO/QELS (Baltimore, Maryland, 2005). doi: 10.1109/QELS.2005.1549203AuthorsD. R. Symes, H. A. Sumeruk, I. V. Churina, Thomas D. Donnelly, J. Landry, and T. DitmireThis conference proceeding is available at Scholarship @ Claremont: http://scholarship.claremont.edu/hmc_fac_pub/738JThD3 2005 Quantum Electronics and Laser Science Conference (QELS)X-Ray Generation from Metal Targets Coated withWavelength-Scale SpheresD. R. Symes, H. A. Sumeruk, I. V. Churina, T. D. Donnelly, J. Landry and T. DitmireFusion Research Center, Department ofPhysics, University of Texas atAustin, I University Station C1510, Austin, Texas, 78712, U.S.A.dansymesJnaiL.utexas.eduAbstract: X-ray yield measurements from targets coated with wavelength-scale spheres arecompared with measurements from polished targets. Evidence for a hotter resonant electrontemperature due to field enhancements from Mie resonances in the spheres is investigated.© 2004 Optical Society of AmericaOCIS codes: (320.7120) Ultrafast phenomena; (350.5400) Plasmas1. IntroductionA major application ofplasmas produced by high-intensity laser pulses is the generation of short bursts of x-rays.For efficient hard x-ray (>few keV) production the acceleration of hot electrons from resonant absorption processesshould be optimized. Recent experiments have demonstrated enhancements in x-ray yield achieved by usingstructured surfaces such as gratings [1], velvets [2], and nanoparticles [3].In this paper, we investigate hard x-ray (>100 keV) generation from copper targets coated with a partiallycovering layer of microspheres. When a spherical object is illuminated with light of a wavelength, X, similar to thediameter, d, ofthe object, a complicated electric field pattem arises due to interference between the incident andscattered waves. The interaction of an intense laser with such a target can generate a spherical plasma with non-uniform heating across its surface [4, 5]. For example, Fig. 1 displays the electric field pattem of a 1tm sphericalplasma irradiated with 0.8 pm laser light. In particular, when the size parameter m = idlX 1, the system canexperience a Mie resonance in which the electric fields are greatly enhanced. By depositing wavelength-length scalespheres across the surface ofa metal substrate, we aim to exploit the Mie resonance to increase the temperature ofthe resonantly heated electrons which are responsible for the generation of hard x-rays.(a) b) 2.21.0+ +1 1-2 X + - b + +IFig. 1. Calculated electric field plot for an ionized, micron-diameter plasma sphere for laser polarization (a) parallel and(b) perpendicular to the page. The laser propagation direction is +z. Note the enhancement in the field visible in (a).2. Target preparationWe prepared the targets using commercially available polystyrene spheres of diameter 0.26, 0.5, 1.0, 1.5 and 2.9 pmwith a size distribution of3%, diluted with water to a concentration of =1:1000, and applied to polished coppersubstrates. The solution was made more dilute for the smaller spheres to improve the consistency of the surface. Weobserved the target surfaces using a scanning electron microscope (SEM) to ascertain the uniformity of the coatingand to check the depth of the sphere layers. SEM images of copper coated with a 2.9 pm sphere solution, shown inFig. 2, at magnification of (a) lOOx and (b) 5000x demonstrate that the spheres form a monolayer over a large area.1549JThD3(a) (b)Fig. 2. Scanning electron microscope images of a copper substrate coated with 2.9pm spheres at (a) lOOx and (b) 5000xmagnification. Note the uniformity of coverage in (a) and the size consistency shown in (a).3. ExperimentWe performed the experiment using the THOR laser system at the University of Texas which provided 35 fs pulsesat a wavelength of 800 nm. The pre-compression energy was set at =68 mJ yielding a focal intensity 4lx1018 Wcm-2.This limit was required to avoid saturation of the x-ray detectors.X-ray emission from the plasma was observed using six filtered Nal scintillation detectors arranged around thetarget chamber. The filters were composed of 125 mm Al (450 keV), 12 mm Cu (162 keV), 32 mm Cu (500 keV),43 mm Cu (600 keV), 24 mm Pb (700 keV) and 44 mm Pb (900 keV), where the bracketed values are the lI/e cutoffenergies of each filter after accounting for any extra material between the detector and the target.Ion emission from the front surface of the target was measured using a charge collector (Faraday cup) at adistance of 0.20 m behind a mesh biased at -300V to reduce the emission of secondary electrons from the detectorsurface. A magnetic field of 1 kG was placed =1 cm in front of the collector to deflect electrons.4. ResultsWe irradiated polished copper targets at laser incidence angles between 00 and 450 from the target normal toascertain the optimum angle for hard x-ray production under our conditions. The maximum x-ray emission occurredfor an incident angles =1 0°-20° suggesting that the dominant mechanism for hot electron generation was resonanceabsorption. The signal levels reduced towards 450 indicating that vacuum heating was inefficient. This was probablydue to the formation of a preplasma caused by the interaction of a low level prepulse.X-ray yields and ion spectra from polished copper targets were then directly compared to sphere coatedcopper targets at incidence angles of 00, 100, 240 and 45°. Initial results using 2.9 .tm spheres are shown in Figs. 3and 4. Fig. 3 shows the x-ray yield through the 32 mm Cu filter corresponding to an integrated signal of photonenergies above z500 keV. The sphere-coated targets display a clear enhancement in yield at all four angles ofincidence of the laser beam, even where resonance absorption is expected to be optimized in the polished coppertarget. This suggests that the presence of the spheres not only modifies the target surface allowing resonantprocesses at off-resonant angles but also increases the magnitude of the driving fields through an interference effect(Mie resonance) as predicted. We note that the size parameter for these spheres is relatively large, m=1 1, so theeffect for smaller spheres should be even more significant.1550JThD350OkeV filter60 * Polished Cu0) 0 3F1m spheres.40:200 10 20 30 40 50Angle (degrees)Fig. 3. Hard x-ray yields versus angle of incidence of the laser beam for polished copper targets and 2.9 pm sphere coated copper targets.Time-of-flight (TOF) traces from polished copper and 2.9 pan sphere-coated copper, shown in Fig. 4, for an incidentangle of45° differed considerably. The earliest time we could resolve after the photon and hot electron burst was=1.12ps. This TOF corresponds to protons of energy =200 eV, below which protons would be deflected by the 1 kGmagnetic field. The maximum visible energy of 4I0 keV (assuming the ion mass of copper) is too low consideringthe much higher electron energies implied by the x-ray measurements. This suggests that we only see the colder ionsfrom a distribution driven by two electron temperatures [6]. It is likely that the TOF trace from the sphere targets ispredominantly carbon and oxygen whereas the signal from the metal may contain all three species (sincecontaminants will be present [7]). However, when we irradiated copper coated with a thin plastic film (13 ptm), noion signal was visible implying that the entire trace shown is due to copper ions. This would imply a significant shiftin the thermal temperature ofthe plasma between the two cases and an energy spectrum calculated assuming that allthe signal comes from copper ions is shown in the inset.-10060 'a4 10.0~~~~~~~~~~~~~~~~~~~~~~~~~~1.40).Time of flight (Ls)Fig. 4. Time-of-flight traces forpolished copper targets and 2.9 m sphere coated copper targets. (Inset) Kinetic energy spectra assuming Cu ions.[1] 5. P. Gordan, T. Donnelly, A. Sullivan, H. Hamster, and R. W. Falcone, "X rays from microstructured targets heated by femtosecond lasers,"Op. Lett. 19,484(1994).[2] G. Kulesar, D. AlMawlawi, F. W. Budnik, P. R. Herman, M. Moskovits, L. Zhao, and R. S. Marjoribanks, "Intense Picosecond X-Ray Pulsesfrom Laser Plasmas by Use ofNanostructured "Velvet" Targets,"' Phys. Rev. Lett. 84, 5149(2000).[3] P. P. Rajeev, P. Taneja, P. Ayyub, A. S. Sandhu, and G. Ravindra Kumar, "Metal Nanoplasmas as Bright Sources of Hard X-Ray Pulses,"Phys. Rev. Lett. 90, 115002 (2003).[4] T. D. Donnelly, M. Rust, I Weiner, M. Allen, R. A. Smith, C. A. Steinke, S. Wilks, J. Zweiback, T. E. Cowan, and T. Ditmire, "Hard X-Rayand Hot Electron Production from Intense Laser Irradiation of Wavelength-Scale Particles," J. Phys. B 34, L3 13 (2001).[5] D. R.-Symnes, A. J. Comley, and R. A. Smi-th, "Fast-Ion Production from Short-Pulse Irradiation of Ethanol Microdroplets," Phys. Rev. Lett.93, 145004 (2004).[6] A. A. Andreev and J. Limpouch, "Ion Acceleration in Short-Pulse Laser-Target Interactions," J. Plasma Physics 62, 179 (1999).[7] E. L. Clark, K. Krushelnick, M. Zepf, F. N. Beg, M. Tatarakis, A. Machacek, M. I. K. Santala. I. Watts. P. A. Norreys, and A. E. Dangor,"Energetic Heavy-Ion and Proton Generation from Ultraintense Laser-Plasma Interactions with Solids," Phys. Rev. Lett. 85, 1654 (2000).1551

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X-Ray Generation from Metal Targets Coated with Wavelength-Scale Spheres

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