interaction of intense ultrashort vuv pulses with
TRANSCRIPT
INTERACTION OF INTENSE ULTRASHORT VUV PULSES WITH DIFFERENT SOLIDS
RESULTS FROM THE TESLA TEST FACILITY FEL PHASE I
Jacek Krzywinski
Institute of Physics, Polish Academy of SciencesInstitute of Physics, Polish Academy of Sciencesand and DeutschesDeutsches ElektronenElektronen--Synchrotron DESYSynchrotron DESY
Outline:
� Motivation� Experimental� Damage and surface modification � short introduction� Results� Conclusions
Why XUV/x-ray ablation should be investigated?
1. estimating and minimizing damages to surfaces of heavily loaded XUV/x-ray optical elements developed and used for guiding and focusing of short-wavelength laser beams
2. durability assessments of materials suggested for the first walls of ICF reactors and optical elements exposed to intense XUV/x-ray radiation in a laser-plasma interaction chamber
3. diffraction-limited nanostructuring of solid surfaces for fabrication of microelectronic and micromechanical elementsand devices
4. determination of radiation field characteristics: imaging of spatial energy distribution in a focused beam ablatively imprinted on the irradiated material and determination of pulse energy content
Absorption of visible and VUV radiation
Au
nm
Si
µm
SiO2
km
VIS
For a first time it was possible to excite surfaces of metals, semiconductors, insulators in the very similar way: linear absorption, similar absorption coefficient
and concentrate on the following question:How different materials respond to a similar excitation?
VUV 90 nm Au Si SiO2
9 nm 10 nm 8.5 nm
Layout of the experiment
Sample holder X,Y,Z,ϕ
FEL XUV laser beam
Focusingmirror
TOF ions / electrons Video microscope
XUV detector
Evaporation cell
CCD
Metals:Au, Al, Cu
SemiconductorsSi, a-C
InsulatorsAl2O3 SiO2 YAG
MgF2 BaF2Organic compounds
PMMA, PTFE
Photon Beam Parameters
Samples
Wavelength 86-98 nmPulse length 50 fsPulse energy 1-0 uJMin. Spot size 10 umMax. Intensity ~1014 W/cm2
absorption & electron excitation, nonthermal meltingfemtoseconds
photoelectronshot electrons
hν
electron difussion & thermalization,
thermal melting, evaporation
picoseconds
heat difiussion, ablation front propagation
nanoseconds
Heat diffiusion by phonons,Pressure wave propagationnanoseconds, 1000 nm
Hot electrons diffusion phononspicoseconds, 100 nm
absorption + thermalization of electrons, femtoseconds, 10nm
hot electrons
phonons
Example of theoreticalpredictions: Au
hν = 14 eVλ= 86 nm
Photon beam
Boltzman equation
Two temperature model
Hydronynamical code: XUV � Ablator, Lagrangian finite-difference method, Mie-Grüneisen equation of state (EOS) based on the code developed by T. Anderson:. PhD thesis, University of California at Berkeley, 1996 and modified by V. Letal, L. Juha, A. M. Bittner, J. Krzywinski, J. R. Sobierajski)
ε
E2
energy
time
Ef=5.5 Ev
10 fs
Au
( ) ( ) ( )][][ εεε nCnCdt
dnfeee −− +=
E3
E4
collision integrals
Thermalization of electron gas in metalse-e and e-ph scattering (R.Sobierajski)
Time constant τ depends on energy deposition
For energy deposition ~ 1 eV/ atom τ~ 5 fs
Boltzman equation
Electron distributioncan be described by
temperature for t > 5fs
fluency ~ 0.005 J/cm2
fluency ~ 3 J/cm2
fluency ~ 0.4 J/cm2fluency ~ 0.05 J/cm2
50 µm
50 µm
50 µm50 µm
200 400 600 800 10000
1000
2000
3000In
tens
ity (a
.u)
Raman shift (cm-1)
Amorphization Micro Raman spectra
It is interesting to notice, (that except for the Au-10 and the PMMA) the ΦII values oscillate around 0.03 J/cm2 This number is in the order of the fluency Fcoh at which the absorbed energy per atom is equal to the cohesion energyEcoh: Fcoh= Ecoh ·n /αwhere n is theconcentration and α is theabsorption coefficient.
For typical values: Ecoh~3 eV, n~5·1022, and α~106 cm-1 Fcoh~0.025 J/cm2
corresponds to the measured value.
Table 1: Surface modification thresholds
0.010.01PMMA
0.020.02Ce:YAG
0.030.03SiO2
0.040.005Si
0.030.01a-C-40
0.010.01Au-15
0.020.02Au
ΦII[J/cm2]
ΦI [J/cm2]Sample
I
ΦI is the fluency at which we notice the change of the refractive indexΦII is the fluency at which we notice thedeformation of the surface detected by AFM.
It is interesting to notice, (that except for the Au-10 and the PMMA) the ΦII values oscillate around 0.03 J/cm2 This number is in the order of the fluency Fcoh at which the absorbed energy per atom is equal to the cohesion energy Ecoh: Fcoh= Ecoh ·n /αwhere n is the concentration and α is the absorption coefficient. For typical values: Ecoh~3 eV, n~5·1022, and α~106 cm-1 Fcoh~0.025 corresponds to the measured value.
a-C film on Si substrate
fluency ~ 0.05 J/cm2
Stress related damage
Multi-shot modification of silicon and quartz surfaces
0 10 20 30 40 50 600
50
100
150
200
250
300
350
X[µm]
Z[nm
]
0 100 200 300 400 500-160-140-120-100
-80-60-40-20
02040
Z [n
m]
X [µm]
Si
SiO2 Profiles of the craters
SiO2 ablation rate
-0,005
0,000
0,005
0,010
0,015
0,020
0,025
0,030
0,000 0,100 0,200 0,300 0,400 0,500 0,600
Fluence [J/cm^2]
Dep
th [u
m]
Serie1Serie2spalled [um]Wielom. (spalled [um])
40 shots22 shots
theory
Spallation depth were calculated with a help of the XUV ABLATOR hydrodynamic code
Printing interference pattern into surfaces of different insulators
PMMA
YAG
fluency ~ 3 J/cm2fluency ~ 0.1 J/cm2
Si02
Very „clean” ablation. Edges of the craters are sharp, no cracks, debris, columnstructures. It was easy to drill a nice holes trough the samples (SiO2, BaF2)Morphology of the craters’ interior hardly depends on the applied fluency.
Ion emission for conducting and insulating materials
0 20 40 60 80 100 120
0
0.2
0.4
0.6
0.8
1
Ion energy/charge [eV/e]
Nor
mal
ized
sig
nal [
a.u] Si+
O+ - Si+- O+
0 20 40 60 80 100 120
0
0.2
0.4
0.6
0.8
1
Ion energy/charge [eV/e]
Nor
mal
ized
sig
nal [
a.u] Si+
O+ - Si+- O+
0 200 400 600 800 1000
0
0.2
0.4
0.6
0.8
1
Ion energy/charge [eV/e]
Nor
mal
ized
sig
nal [
a.u] +1
+2+3+4
Only single ions states and low energetic ions (~50 eV) were detected for insulators for all irradiation conditions.
High charge states and energetic ions (~a few keV) were typical for conductors and semiconductors. Ion energu ~charge field emissionn
Si SiO2
LIPSS = spontaneously-grown periodical structures on the laser-illuminated surfaces
( spatial period of the structures is usually related to the laserlaser wavelength)
LIPSS in PMMA267 nm < Λ < 413 nm
10µmx10µm
Can we find LIPSS with a period correlated to the irradiation wavelength?
Does the wavelength always control the LIPSS period?
1. Can we find LIPSS with a period correlated to the irradiation wavelength?
Answer for the first question is : yes!, the period is 80 nm
Does the wavelength only control the LIPSS period?Clearly, in most cases, not!
120mmx120mm
µmx10µm10
LIPSS-II of the second kind induced at the bottom of craters ablated in PMMA at increasing fluency.
The spatial period increases with fluency.
Craters ablated before, behind, and exactly at the tight focus position. LIPSS-II in PMMA
267 nm < Λ < 413 nm
10µmx10µm
LIPSS-II in PMMA600 nm < Λ < 700 nm
70mmx70mm
4µmx4µm
80µmx80µm
LIPSS-II LIPSS-II in PMMA500 nm < Λ < 1000 nm
increasingfluence
Conlussion 1
� Measured damage threshold for all investigated materials was estimated to be between 10 - 40 mJ/cm2
� This value corresponds well to the fluency at which the absorbed energy per atom is equal to the cohesion energy.
� The result is surprising in case of bulk metals. Oneexpects that hot electrons should take most of the deposited energy from away the absorption volume and thus increase the threshold value.
Conclussion 2
� A distinct difference in the behavior of conducting materials and insulators was observed. The difference is dramatic while the absorption coefficients are similar for all the materials at the TT1 FEL wavelength.– For insulators the ablation is very „clean”. Edges of the
craters are sharp, no cracks, debris, column structures. Morphology of the craters’ interior hardly depends on the applied fluency. Ablation rate agrees with hydrodynamic simulations.
– In contrast, the irradiated silicon surface becomes very rough when the intensity exceeds the damage threshold.
– There was also a clear difference between insulators and conducting material with respect to ejected ions spectra. High charge states and energetic ions (~a few keV) were typical for conductors and semiconductors. Only single ions states and low energetic ions (~50 eV) were detected for insulators for all irradiation conditions.
Conclusion 3� LIPSS-I have been found at the rim of a few craters
created in a-C and PMMA by 98-nm and 86-nm radiation, respectively.
� LIPSS of the second kind (LIPSS-II) has also been found. LIPSS-II spatial periods depend more on laser intensity than wavelength and are significantly longer than the laser wavelength. Very common phenomenon.
Time resolved ablation /transmissionmesurements for TTF FEL2
� The VUV R can be rotated (in the theta �two theta mode) and can be set infront of the VUV T. Thisfeature allows calibration of both devices with respect to each other.
CCD
TOF
Microscope
XUV (in)
Probe
XUV (out)
XUV Transmission
XUV Reflection
Sample