Post on 28-Jun-2020
Ch07_00VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
EXTREME ULTRAVIOLET ANDSOFT X-RAY LASERS
Chapter 7
Hot dense plasmalasing medium
Visible laser pump
d
λ
λθ
Ch07_F01VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
The Processes of Absorption, SpontaneousEmission, and Stimulated Emission
u
u
u
Absorption
Spontaneousemission
Stimulatedemission
Ch07_F02VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
The Lasing Process Begins withAmplified Spontaneous Emission (ASE)
Gain medium of invertedpopulation density
Amplifiedspontaneous
emission
Spontaneousemission
• both directions equally likely
Ch07_F03VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Equilibrium and Non-EquilibriumEnergy Distribution
Equilibrium energydistribution
Non-equilibrium invertedenergy distribution
Boltzmanndistribution
Ene
rgy
leve
l
Ene
rgy
leve
l
Population density Population density
Lasing requires an inverted population density (more atoms in the upper state than in the lower state.
g
u
g
u
Ch07_04VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Three-level Lasing Between anUpper State u and a Lower State
u
Pump Fastradiativedecay
Long livedstate
g
λλ
Lower state
Upper state
1
0Time
Pro
babi
lity
Osc
illat
ion
ampl
itude
Time
Radiative Decay Involves An Atom Oscillating Between Two Stationary States at the Frequency if = (Ei – Ef) /
Ch07_Ch01_F09VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Ch07_F06VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
EUV/SXR Lasers are High GainWith Minimal Cavity Optics
λIR
d
Refraction in anelectron plasmawith n = 1 –
~ 10–4L
EUV/Soft X-ray Laser
Visible Light Laser
λIR pump
λEUVλEUV
θ
Longitudinalmode selector
Transversemode selector
Gain medium+ flash lampMirror Mirror ~ 10–6
TEMOO
∆λλ
∆λλ
λvis
Spatial and temporalcoherence addressedin chapter 8.
12
nenc
Ch07_EarlySuccess.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Early Successes With Lasingin the 4-46 nm Wavelength Region
• Recombination lasing in rapidly cooled H-like carbon• Collisionally pumped lasing in Ne-like Se• Extension to shorter wavelengths with collisionally pumped Ni-like Ta, W, . . .• Saturation of Ni-like Ag, In, Sn, Sm, . . . with refraction compensated plasmas
• Table-top lasing in the EUV with high spatial coherence and mW average power• Compact lasing with fsec transient gain techniques
More recently:
Ch07_F05VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Stimulated n = 3 to n = 2 Emissionfor a Hydrogen-Like Ion
n = ∞
n = 2
n = 1
n = 3n = 4
Bindingenergy
(7.1)
Ch07_Tb7.1.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Transitions in Single Electron, Hydrogen-Like Carbon Ions (Z = 6)
(7.1)
• ω Z2
• τ τH/Z4
Ch07_F08_9VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Recombination Lasing With H-like Carbon Ions
EUV EUV
10.6 mlaser-heatingpulse
Expanding carbon plasma of length L
Magnet coils
Carbon disk target
B
B
Carbonblade
Expandingplasma
Laser-heating pulse
Carbondisk target
Diagnosticwindow
L/r ~ 10–2
r ~ mm12
Co2 laser: 10.6 µm, 5 1012 W/cm2, 70 nsIonization of C+5 = 490 eVPlasma: κT = 100-200 eV, 5 1018 e/cm3
Recombination of C+6 and e–
cascading down excitation states (n)
Courtesy of S. Suckewer et al. (1985) Princeton University
0 1000500Channel (pixel)
O+5 15.0 nm
O+517.3 nm
C+5 18.2nm
C+418.6nm
C+4
18.6nm
Axialspectrum
Transversespectrum
C+5 18.2 nm(3 → 2)
O+5 17.3 nm
Ch07_F12VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Laser Produced Plasma for Ne-likeLasing in Selenium (Z = 34)
• Double-sided irradiation of a thin Se foil• 2.4 TW, 2ω, 450 ps, 7 1013 W/cm2
• 200 µm 1.1 cm focal spot• About 20% ions Ne-like• Ionization of Fl-like Se+23 is 1036 eV (Ne-like is 2540 eV)
Courtesy of D. Matthews et al. (1985)Lawrence Livermore National Laboratory
Two time-gatedspectrometers
Ne-like seleniumions in a laser-
produced plasma
High power0.527 mlaser light
Time-resolvedtransmission
grating spectrometer(λ/∆λ ~ 200; ∆τ ~ 20 ps)
(λ/∆λ ~ 2,000; ∆τ ~ 0.5 ns)
Alignment mirror
77
Ch07_F14VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Exponential Gain ObservedWith Increasing Plasma Length
15 16 17 18 19 20 21Wavelength, nm
Rel
ativ
e in
tens
ity
22 23
1086420
30
200
150
100
50
0
20
10
0
0 2 4 6Target length, mm
Inte
grat
ed li
ne in
tens
ity,
J/S
r
8 10 12
20.64 nm20.98 nm
6070
5040302010
0
G = 5/cm
Courtesy of D. Matthews et al. (LLNL)
• Gain in axial direction only• Use of on and off-axis time gated spectrometers• 3p → 3s lasing at 20.64 nm and 20.98 nm in Ne-like Se+24
4.6-mm foil
10.1-mm foil
22.4-mm foil
Ch07_F13VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Population Inversion in Neon-like Selenium
• Laser produced plasma conditions optimized for dominant Ne-like ionization stage• Collisionally pumped to a 3p excited state• Fast radiative depopulation of the lower 3s state• 3p → 3s lasing at 20.64 nm and 20.98 nm based on selective depopulation
Simplified Energy Level Diagram for Se+24
Lasing transitions
Fastradiativedecay
Electroncollisional excitation(~1.5 keV)
1s22s22p53p(J = 2)
1s22s22p53s(J = 1)
1s22s22p6
20.98 nm(59.10 eV)
20.64 nm(60.07 eV)
Courtesy of M. Rosen et al. (1985), LLNL.
Ch07_F15_16VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Extension to Nickel-like Ions Permits Lasingat Shorter Wavelengths Without the Need for Higher Electron Temperature
Lasing transitionat 4.483 nm
Fast radiative
decay
Electroncollisional excitation(~1.1 keV)
3d94d(J = 0)
3d94p(J = 1)
3d10 groundstate (J = 0)
0.3
0.2
0.1
0
03.0 4.0 5.0 6.0
Wavelength (nm)
Inte
nsity
7.0 8.0 9.010.0
• Ni-like Ta (Z = 73, 28 e–, +45)• 1s22s22p63s23p63d10 ground state (28 e–)
Courtesy of B. MacGowen et al. (1987), LLNL.
4.483 nm(276 eV)
2.5 cm Ta foil
1.7 cm Ta foil
Lasing in Ni-like Ta+45
Ch07_F17VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Nickel-like Lasing Across the K-edge of Neutral Carbon
3.0 4.0 5.0 6.0Wavelength (nm)
Inte
nsity
• W at 4.318 nm• Ta at 4.483 nm• C-K at 4.36 nm
The water window is defined by theK-absorption edgesof neutral carbonand oxygen.
Courtesy of B. MacGowen et al. (1987), LLNL.
4d → 4p lasing
AuW
Ta
Yb
Ch06_RefracIndxPlas.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Refractive Index of a Plasma
(6.113a)
(6.114a)
(6.114b)
For ω > ωp there is a real propagating wave with phase velocity
The refractive index of the plasma is
or equivalently
Ch07_RefracComp.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Refraction Compensating Double-Targets Used to Achieve Saturation of Lasing at 13.99 nm in Ni-like Ag
• Prepulse allows plasma expansion, decreases transverse electron density gradients and increases plasma volume• Double targets permit refraction compensation (opposite turning angles) and double the gain length.• Time delay of one target irradiation enhances gain in one direction• Saturated 4d → 4p lasing at 13.99 nm in Ni-like Ag+19 (Z = 47, 28e–)
Nd laser: 1.05 µm, 2 1013 W/cm2, 75 psTarget: Ag stripes, 200 µm 25 mmPlasma: 700 eV, 6 1020 e/cm3
43 57 µm, 1.5 3.5 mr
Courtesy of J. Zhang et al. (1997)Rutherford Appleton Laboratory
13.99 nm
+60 ps
AgAg
Ch07_F19_20VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Saturation of Nickel-like Lasingin Ag, In, Sn, and Sm
Si L-edge
7
6
5
4
3
2
1
00 15 20
Wavelength (nm)
Inte
nsity
(arb
. uni
ts)
25
8
1×108
1×107
1×106
1×105
1×104
1×103
1×102
1×101
1×106
1×105
1×104
1×103
1×102
1×101
1×100
Target length (mm)
G = 8/cm
G = 8.8/cm
G = 9.5/cmG = 12.5/cm
Lase
r out
put (
arb.
uni
ts)
0 5 10 15 20 25 30 4035
1×107
1×106
1×105
1×104
1×103
1×102
1×1010 5 10 15 20 25 30 4035
1×106
1×105
1×104
1×103
1×102
1×101
1×100
Refraction compensatingdouble targets
Note: Wavelengths quoted are calculated by J. Scofield and B. MacGowan
Courtesy of J. Zhang et al. (1977)Appleton Rutherford Laboratory
Courtesy of J.Y. Lin et al. (1999)Appleton Rutherford Laboratory
Ni-like Ag4d → 4p13.99 nm
Ni-like Ag13.99 nm
Ni-like In12.59 nm
Ni-like Sn11.98 nm
Ni-like Sm7.355 nm
Ch07_F21_22VG.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
High Average Power, High SpatialCoherence Table-Top Laser at 46.86 nm
5
3
1
4
2
0
46 47 48 4946.5 47.5Wavelength (nm)
Inte
nsity
(rel
. uni
ts)
48.5
6 cmAr+8
3p→3s(J = 0 to 1)
160
120
0
40
80
12 cmAr+8
3p→3s(J = 0 to 1)
Al-like Ar 3d-3pNe-like Ar
2.5
1.5
0.5
2
1
0
3 cm
Al-like ArMg-like ArNe-like Ar
3d-3pNe-like Ar
Mg-like Ar
Courtesy of J. Rocca (1995), Colorado State University
• Discharge plasma• Fast, high current pulse compresses (J B) and heats plasma• 70 eV, 5 1018 e/cm3
300 µmD 36 cm long (1:1000)• Ne-like Ar+8, 3p → 3s at 46.86 nm• P = 3.5 mW, spatially coherent
EUV
Ch07_SpatialCoh.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Spatial Coherence of 46.86 nm Laser
EUVCCDarray
Pinholemask
Capillarydischargesoft x-raylaser
Courtesy of Y. Liu, UC Berkeley, and J. Rocca, Colorado State University (2001)
1 512 1024 1 51227 cm capillary 36 cm capillary18 cm capillary
Nor
mal
ized
inte
nsity
1024 1 512 1024
1
0.75
0.50
0.25
0
60 S(60.8)
Cl(52.9)
Ar(46.9)
Ca(38.3)
Ti(32.6)
Cr(28.5)Fe(25.5)
Ni(23.1)Ge(19.6)
Sn(12.0)In(12.6)
Cd(13.2)
Pd(14.7)Mo(18.8)
Nb(20.4)Zr(22)
Y(24.1)Sr(26.6)
Kr(33.7)
Ag(13.9)
50
40
30
20
10
6 8 10 12 14 16Ionic charge
Ne-like
Ni-like
Wav
elen
gth
(nm
)
18 20 22 24
Scaling to 13 nm Requires Excitationof Ni-Like Cd Ions (Cd + 20)
ScalingNeAndNiLikeLasers.ai
Professor David AttwoodAST 210/EECS 213Univ. California, Berkeley
Generation of laser radiation at 13 nm requires significantly hotter and denser plasma columns
0 100 200 300 400
1E+16
1E+17
1E+18
1E+19
1E+20
1E+21
1E+22
1E+23
Electron Temperature (eV)
Elec
tron
Den
sity
(cm
)-3
EUV lasers10-20 nm
EUV lasers40-65 nm
EUV lithography sources
Plasmawaveguides
AbsorptionRegion
NePumpBeam
8 ps
120 ps
Pumping configurations for collisional EUV lasers in laser-createdplasmas: Grazing incidence increases energy deposition efficiency*
• Two pulse sequence at normal incidence
• Grazing incidence
* R. Keenan, et al. Phys. Rev. Lett , 94, 103901 (2005). B.M. Luther et al. Optics Lett, 30, 165 (2005)
NCritical
GainRegion
AbsorptionRegion
Ne
PumpBeam
120 ps
8 ps
c
e
N
N=!
Courtesy of J. RoccaColo. State U.
5-10 Hz Pump laser: 1 J Short Pulse Table-top Ti:Sapphire System
Courtesy of J. Rocca, Colo. State U.
Line Focus• Pre-pulse: 30µm FWHM,• Short-pulse: 30µm FWHM,
Configuration for 30 µm wide short pulse line focusat 14 to 26 degrees grazing incidence
Courtesy of J. Rocca, Colo. State U.
Simulation predicts high gain at 18.9 nm in Ni-like Mo,g ~100 cm-1 (Mark Berrill, CSU)
Pre-pulse: 330 mJ, 120 ps Heating pulse: 800 mJ, 8 ps heating pulse
Courtesy of J. Rocca, Colo. State U.
Fit with the expression from Tallents et. al.8th Int. Conf. XRL, AIP Vol. 641, 2002.
LgI
I2GL 0
s
av =+
( )[ ]( )[ ]1/2
3/2
0avGLexp GL
1GLexpII
!=
18 19 20 21 22 23
wavelength (nm)
1.5 mm
2 mm
2.5 mm
3 mm
4 mm
Saturated 18.9 nm Ni-like Molybdenum Laser
Long pulse: 340 mJ - Short pulse: 1 J
go = 65 cm-1
gxl=15.3
Courtesy of J. Rocca, Colo. State U.
Lasing observed at wavelengths as short as 10.9 nm
Gain saturatedoperationdemonstrated
Courtesy of J. RoccaColo. State U.
850 nJ/shot
Saturated laser operation at 13.9 nm in Nickel-like Ag
1 J short (8 ps) pulse excitation
g0=67 cm-1 ; gxl= 16.8
Courtesy of J. Rocca, Colo. State U.
g0=69 cm-1
G×L=17.6Inte
nsit
y (a
rb. U
nits
)
Length (mm)wavelength (nm)
Inte
nsit
y (a
. u.)
CC
D c
ha
nn
el
(x10
-2)
Gain-saturated Ni-like 13.2 nm Ni-like Cd laser
1 J short pulse – 23 degrees grazing incidence angle
Courtesy of J. Rocca, Colo. State U.
A Ni-like Cd at 13.2 nm provides a closer laser wavelengthmatch to Mo/Si lithography mirrors
At the wavelength of the Ni-like Cd laser the reflectivityof Mo-Si mirrors centered at λ=13.5 nm is ~ 55%
Courtesy of J. Rocca, Colo. State U.
Example: 13.9 nm Table-top EUV microscope with resolution better than 50 nm
100 nm lines, exposure time:20 seconds.
The new compact EUV lasers are enablingapplication testbeds
From 13.9 nm laser
Testpattern
Objective zoneplate
CCD
Grad. StudentFernando Brizuela
Courtesy of J. Rocca, Colo. State U.