Extreme Ultraviolet and Soft X-ray Lasersattwood/sxr2009/... · Ch07_F21_22VG.ai Professor David...

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


The Processes of Absorption, SpontaneousEmission, and Stimulated Emission

u

u

u

Absorption

Spontaneousemission

Stimulatedemission

Ch07_F02VG.ai


The Lasing Process Begins withAmplified Spontaneous Emission (ASE)

Gain medium of invertedpopulation density

Amplifiedspontaneous

emission

Spontaneousemission

• both directions equally likely

Ch07_F03VG.ai


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


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


Ch07_F06VG.ai


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


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


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


Transitions in Single Electron, Hydrogen-Like Carbon Ions (Z = 6)

(7.1)

• ω Z2

• τ τH/Z4

Ch07_F08_9VG.ai


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


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

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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


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


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


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


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


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


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


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


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


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


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


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


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


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


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%


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


Extreme Ultraviolet and Soft X-ray Lasersattwood/sxr2009/... · Ch07_F21_22VG.ai Professor David...

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