DEMONSTRATION OF HIGH REPETITION RATE SOFT X-RAY LASERS AT WAVELENGTHS NEAR 30 NM IN NE-LIKE IONS

David AlessiColorado State UniversityElectrical and Computer Engineering Dept.December 8, 2006

Masters Thesis:

Outline• Introduction

– Extreme Ultraviolet and Soft X-Ray Light– Comparison of Soft X-Ray Laser Sources– Collisional Lasers– Ne-Like vs. Ni-Like– Previous Work of Collisional Lasers near 30 nm

• Experimental Configuration: – Grazing Incidence Pumping Geometry– Ti:Sapphire Pump Laser

• Simulation of the 32.6 nm Ne-Like Ti Laser• Experimental Method• Results

– On Axis Spectra– 5Hz Operation– Laser Gain Measurement– Pump Beam Grazing Incidence Angle– Optimization of Pre-Pulse Sequence– Dependence on Time Delay– Pump Laser Pulsewidth– Spatial Coherence Measurement

• Summary

Extreme ultraviolet (EUV) radiation and soft x-ray (SXR) light

Span ~0.1nm to ~100 nm• Relatively unexploited region of the spectrum

– Commercial sources not yet available• Can be used for a wide range of applications

– Short wavelength allows: nanoscale imaging and metrology, nanopatterning, dense plasma diagnostics

– High photon energy allows: atomic/molecular spectroscopy

Figure from: D. T. Attwood, Soft X-Rays and Extreme Ultraviolet Radiation: Principles and Applications(Cambridge University Press, Cambridge, England, 1999)

Comparison of Soft X-Ray Sources• Coherent SXR radiation from 3rd gen

synchrotron– Large facilities– High repetition rate– Wavelength tunable– Low energy per pulse– Requires GeV electrons

• Laser pumped collisional SXR lasers– Table-top size– 10 Hz repetition rate– Not wavelength tunable– High energy per pulse– High peak spectral brightness

• Higher Harmonic Generation– Table-top size– High repetition rate– Wavelength tunable– Low energy per pulse– Low temporal coherence

Advanced Light Source (LBL)

Table-top Soft X-Ray Laser (CSU)

Figure from Wikipedia

Collisional LasersCollisional Lasers

Singly ionized Ar ion, Kr ion lasers in the visible spectral region

Highly ionized (8-25 times) in the EUV/SXR spectral region

Plasma requirements:Te ~ 5 eVNe ~ 1x1014 cm-3

Laser created plasma

Discharge created plasma

2ZEh ∝∆=ν

Ar+

Ar

35eV

514 nm laser

e

e

Ar+

Ar

35eV

514 nm laser

e

e

Cd+20

>5000 eV

13.2 nm laser

e

Ionize 20 times

Ne x Te increases by 108-109

Te ~ 300 - 1000 eVNe ~ 2x10 20 cm-3

Ne-Like vs. Ni-Like

• Collisional lasing has been demonstrated in both Ne-like and Ni-like ions

• Ni-like ions are more efficient

• Ne-like ions provide access to lasers near 30 nm

• Solar corona studies

– HeII and FeXI-XVI lines

– EUV normal incidence spectrometer (NASA SOLAR-B mission)

• Wavelength specific spectroscopy

• Future applications that may require it

Why 30 nm ?

Previous Work of 30 nm Collisional Lasers

• 1990 – Ne-like Ti – First demonstration of 32.6 nm Ne-like Ti laser– Using GDL laser at LLE (Rochester)

• Single 200 J laser pulse (∆t = 650 ps)– Then confirmed at LLNL with NOVA laser

• Single 550 J laser pulse (∆t = 600 ps)

• 1993 – Ne-like Ti and Cr – First demonstration of 28.6 nm Ne-like Cr laser– Done with NOVA laser

• 6 J pre-pulse (∆t = 600 ps)• 1100 J pump pulse (∆t = 600 ps)

• 1995 – Ne-like V – First demonstration of 30.4 nm Ne-like V laser– Done with Asterix IV laser at Max Planck Institute

• 430 J (pre-pulse and main pulse energy) (∆t = 450ps)

NOVA Laser

Previous Work of 30 nm Collisional Lasers

• 1997 – Ne-like Ti– Transient excitation of collisional SXR laser– In quasi-steady state, upper level population is distributed by collisions– Transient uses preferential excitation of upper level on short time scale– Resulted in much higher gain than quasi-steady-state– At Max Born Institute

• 4-6 J pre-pulse (∆t = 1.5 ns)• 1.5 J pump pulse (∆t = 0.7 ps)

• 1997 – Ne-like Ti– Saturation of the 32.6 nm line – Table-top COMET laser at LLNL

• 6 J pre-pulse (∆t = 800 ps)• 6 J pump pulse (∆t = 1 ps)• 1 shot every 3 min

• 2005 (this work) – Ne-like Ti, V, Cr– First demonstration of these lasers with grazing incidence pumping– Saturation of 32.6 nm (Ti) line and 30.4 nm (V) line – Table-top CPA Ti:Sapphire laser at CSU

• 350 mJ pre-pulse (120 ps)• 1 J pump pulse (1 ps)• 5Hz repetition rate

[D. Alessi et al. Opt. Express 13, 2093 (2005)]

Grazing Incidence Pumping

Grazing Incidence Pumping

NCritical

AbsorptionRegion

PumpPulse

Pre-Pulse

Ne GainRegion• Normal incidence pre-pulse

– 350 mJ, 120 ps• Plasma expansion

– Few hundred ps• Pump pulse at grazing incidence wrt

target– 1J, 8 ps

• Selects density of plasma to deposit laser energy

c

o

nn

=θsin

( )mcme

emn o

c µλωε

2

321

2

2 /1011.1 ×≈=

no = density at perigree

θ

Ti:Sapphire Pump Laser

• Mode locked oscillator – 5 nJ/pulse (∆t = 50 fs)

• Stretcher• 8 pass 1st stage

– 2 mJ/pulse (∆t = 120 ps)• 5 pass 2nd stage

– 150 mJ/pulse• 3 pass 3rd stage

– 2 J/pulse (uncompressed)• 1200 l/mm grating compressor

350 mJ, 120 ps

1 J, 8 ps FigureFrom Y. Wang

Simulation of 32.6 nm Ne-like Ti laserPre-pulse: 300 mJ 120 ps time delay = 0Pump pulse: 1 J 8 ps time delay = 400 ps

1.5D Hydrodynamic/Atomic model with radiation transport code and post processor ray tracing developed by Mark Berrill, Colorado State University

Simulation of 32.6 nm Ne-like Ti laser

go = 86 cm-1

G×L = 19.2E(4mm) = 5 µJ

3p1S0→3s1P1 transition

Simulated Energy vs. Plasma Length

Experimental Method• Pre-pulse

– 350 mJ for Ti– 500 mJ for V and Cr– 30 µm × 4.1 mm FWHM line

focus– 120 ps FWHM pulsewidth

• Pump Pulse– 1 J for Ti– 0.9 J for V and Cr– 30 µm × 4.1 mm FWHM line

focus– 8 ps FWHM pulsewidth

• Grazing incidence grating spectrometer

– Hitachi VLS grating• Laser attenuation

– Wire meshes– Al filter

On Axis SpectraSpectra of 4 mm long plasmas showing:

1. Lasing in Ne-like 3p1S0→3s1P1 : 32.6 nm (Ti), 30.4 nm (V), 28.6 nm (Cr)

2. Lasing in Ne-like 3d1P1→3p1P1 : 30.1 nm (Ti)

Ne-like Ti Energy Levels

3p 1So

3d 1P1

2p 1So

3s 1P1

3p 1P1

3s

3d

30.1 nm

32.6 nm

Ti+12 ground state

2.33 nm

Figure similar to that by Nilsen et al. Phys. Rev. A 55, 3271 (1997)

5Hz Operation

• Sequential laser shot intensities• Obtained by moving target at constant speed• STD = 17% of mean• Average laser energy = 530 nJ• Average Power = 2.6 µW

32.6 nm Ne-like Ti

Laser Gain Measurement32.6 nm Ne-like Ti30.4 nm Ne-like V

go = 72 cm-1

G×L = 21.7El,max = 590 nJ

go = 57 cm-1

G×L = 18.4El,max = 790 nJ

( )( )( ) ( )( ) 2/10

2/30

01

LG

LG

oavLeG

eII −=

( ) LgIILG o

s

av =+ 20 Solid line is fit of experimental data to expression of ASE laser taking into accountthe saturation. [Tallents et al. AIP Conf. Proc. 641, 291 (2002)]

Pump Beam Grazing Incidence Angle

17º → ne = 1.5×1020 cm-3

20º → ne = 2.0×1020 cm-3

23º → ne = 2.6×1020 cm-3

The density of the laserHeated region is controlledBy the angle

Optimization of Pre-Pulse Sequence

• Add an additional pre-pulse arriving 5 ns before main pre-pulse

• Diverting 5% into this pulse can increase the output laser intensity by a factor of 5

~5x

32.6 nm Ne-like Ti laser

30.1 nm vs. 32.6 nm Lasers in Ne-like Ti

Spectra at 420 ps delay

Spectra at 620 ps delay

Delay between the main pre-pulse and the pump pulse

Dependence on Time Delay

Ne-like V

32.6 nm Ne-Like Ti laser Dependence on Pump Laser Pulsewidth

2 ps 4 ps

6 ps 9 ps

•Longer pulse durationreduces transient effect

•Shorter pulse durationreduces ionization

Spatial Coherence Measurement

Measurement made in collaboration with Y. Liu, University of California Berkeley

Young’s Double Slit Experiment• 4mm × 5 µm slits• 30 µm, 50 µm, 75 µm separation• placed where the beam is 500

µm in diameter• Measure fringe visibility for each

slit separation

Spatial Coherence Measurement

Modulation with 30 µm slit separation CCD image

72.016.1116.01

minmax

minmax =+−

=+−

=IIIIV

Imin

Visibility:

Modulation with 50 µm slit separation CCD image

64.022.1122.01

minmax

minmax =+−

=+−

=IIIIV

Imin

Spatial Coherence Measurement

Modulation with 75 µm slit separation CCD image

Spatial Coherence Measurement

26.059.1159.01

minmax

minmax =+−

=+−

=IIIIV

Imin

Fit to gaussian source• Rc = 47 µm

Recall beam size• Rb = 250 µm

Laser is only moderately coherent

Spatial Coherence Measurement

The spatial coherence can be improved significantly by seeding the amplifier with a high harmonic generated pulse. [Y. Wang PRL 97, 123901 (2006)]

Summary

• Ne-like soft x-ray lasers near 30 nm were demonstrated with grazing incidence pumping geometry using a 1 J table-top pump laser– 32.6 nm Ne-like Ti laser

• Saturated operation demonstrated (GxL = 18)• 2.6 µW average power while operating at 5Hz repetition rate• 790 nJ best shot energy• Strong lasing over a wide range of parameters• Moderate spatial coherence• Strong lasing also demonstrated at 30.4 nm in Ne-like V, 30.1 nm in

Ne-like Ti, and 28.6 nm in Ne-like Cr

Acknowledgments

• Advisor– Dr. Jorge Rocca

• Committee Members– Dr. Carmen Menoni– Dr. Chiao-Yao She (physics)

• This work would not be possible without: Yong Wang, Miguel Larotonda, Brad Luther, Mark Berrill, Mario Marconi, Ann Dummer, Dinesh Patel

• Fellow ERC students who have helped me at one point: Jorge Filevich, Mike Grisham, Fernando Brizuela, Scott Heinbuch, Brendan Reagan, Georgiy Vaschenko, Dale Martz, Federico Furch, Jonathan Grava, and Courtney Brewer

• National Science Foundation

Questions