E( t) = e Γt 2 e iω 0t. A( t) = e Γt 2, Γ Γ 1. Frontiers in ultrafast laser technology. Time and length scales. Example: Gaussian pulse.

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1 Time and length scales Frontiers in ultrafast laser technology Prof. Ursula Keller Department of Physics, Insitute of Quantum Electronics, ETH Zurich International Summer School New Frontiers in Optical Technology Tampere, Finland, Aug. 5-9, picosecond = 1 ps = s = s 1 femtosecond = 1 fs = 1 ps / 1000 = s = s 1 attosecond = 1 as = 1 fs / 1000 = s = s Time Fast processes are important in engineering, physics, chemistry and biology: We need to better understand, model and control atomic and molecular chemical reactions and energy transfer processes on an atomic and molecular level. 1as Ultrafast Laser Physics Lecture 1: Introduction ETH Zurich Length 1am Laser pulse pulse envelope A(t) Example: Gaussian pulse pulse envelope A(t) Einhüllende A(t) electric field E(t) t E( t) = e Γt 2 e iω 0t E(t) electric field E(t) A( t) = e Γt 2, Γ Γ 1 iγ 2 E( t) = A( t)e iω 0t τ p = 2 ln 2 Γ 1 ( ) dφ tot ( t) ω t dt = ω 0 + 2Γ 2 t 1

Chirped Gaussian Pulse E( t) = A( t)exp( iω 0 t) = exp( Γt 2 )exp( iω 0 t) Pulse envelope A(t) Electric Field E(t) Laser pulse E( t) = 1 2π E ( ω )e iωt dω E( t) = A( t)e iω 0t

tot ( t) ω 0 t + Γ 2 t 2 ω ( t) dφ tot ( t) = ω 0 + 2Γ 2 t dt 0 A(Δω) ~ Δω A ( Δω ) = E ( ω 0 + Δω ) Time and length scales electron motion vibrations rotations collisions in

Nobel prize in chemistry 10-9 s 10-6 s A. Zewail, 1999 Measurements with attosecond resolution isolates purely electronic dynamics on short time scales.

2 Chirped Gaussian Pulse E( t) = A( t)exp( iω 0 t) = exp( Γt 2 )exp( iω 0 t) Pulse envelope A(t) Electric Field E(t) Laser pulse E( t) = 1 2π E ( ω )e iωt dω E( t) = A( t)e iω 0t where A( t) = 1 2π A ( Δω )e iδωt dδω E( t) = 1 E ( ω 0 + Δω )e i ( ω 0 + Δω )t dδω 2π = 1 ~ E(ω) 2π eiω 0t A ( Δω )e iδωt dδω Γ Γ 1 iγ 2 Γ 2 = 0 Γ Γ 1 iγ 2 Γ ω 0 Δω 0 ω φ tot ( t) ω 0 t + Γ 2 t 2 ω ( t) dφ tot ( t) = ω 0 + 2Γ 2 t dt 0 A(Δω) ~ Δω A ( Δω ) = E ( ω 0 + Δω ) Time and length scales electron motion vibrations rotations collisions in air Scientific questions addressed by SLS Francis Crick (1962): if you want to understand function, study structure s attosecond s s femtosecond and picosecond Nobel prize in chemistry 10-9 s 10-6 s A. Zewail, 1999 Measurements with attosecond resolution isolates purely electronic dynamics on short time scales. We can study time-dependent electronic and atomic structure for the first time. Synchrotron Light Source (SLS) (PSI in Villigen) time slow 100 ps = fs length fine 20 nm 0.1 nm 2

Scientific questions addressed Francis Crick (1962) updated if you want to understand function, study time-dependent structure.

... with ultrafast science and technology Harold E.

and application How to access the fast time scales?

Δt following the gun shot. (photo showing balloon with a hole) How do I do this with ultrafast lasers?

3 Scientific questions addressed Francis Crick (1962) updated if you want to understand function, study time-dependent structure... how a structure evolves in time during a process (e.g. chemical reaction) Flash photography with 1-µs time exposure How do we do this?... with ultrafast science and technology Harold E. Edgerton MIT, USA # New tools that allow us to clearly see a process in real-time, always bring real progress both in theory and application How to access the fast time scales? Example: Fast process: balloon explosion Fast process started with a bullet Explosion observed with flash photography after a time delay Δt following the gun shot. (photo showing balloon with a hole) How do I do this with ultrafast lasers? first pulse bullet How to access the fast time scales? Example: Fast process: balloon explosion Fast process started with a bullet Explosion observed with flash photography after a time delay Δt following the gun shot. (photo showing balloon with a hole) How do I do this with ultrafast lasers? second pulse flash Δt first pulse bullet Δz = 1 µm Δt fs 3

$Δt 1 Δt 2 Δt 3 Δt Structure on atomic scale: flash photo replaced by diffraction patterns Δt second pulse$ flash Δt first pulse bullet Δt Δz = 1 µm Δt 2 3.3 fs Δz = 1 µm Δt 2 3.

flash Δt first pulse bullet Δt Δz = 1 µm Δt 2 3.3 fs Δz = 1 µm Δt 2 3.

$understand function, study time-dependent structure One approach: diffraction We want to follow the$ $movements of atoms One approach: diffraction We want to follow the movements of atoms Atoms are small$ 000 000 000 000 001 s Atoms are small 000 000 000 000 001 s chemical reactions magnetic properties

000 000 000 000 001 s Atoms are small 000 000 000 000 001 s chemical reactions magnetic properties

4 How to access the fast time scales? How to access the fast time scales? Δt 1 Δt 2 Δt 3 Δt Structure on atomic scale: flash photo replaced by diffraction patterns Δt second pulse flash Δt first pulse bullet Δt Δz = 1 µm Δt fs Δz = 1 µm Δt fs Scientific questions addressed Francis Crick (1962) updated if you want to understand function, study time-dependent structure Scientific questions addressed Francis Crick (1962) updated if you want to understand function, study time-dependent structure One approach: diffraction We want to follow the movements of atoms One approach: diffraction We want to follow the movements of atoms Atoms are small (<nm) and move very fast (fs) 1 nm = m 1 fs = s Atoms are small (<nm) and move very fast (fs) 1 nm = m 1 fs = s chemical reactions magnetic properties nanostructures biomolecules PSI, SwissFEL chemical reactions magnetic properties nanostructures biomolecules SwissFEL (PSI in Villigen) Time fast fs Space ultrafine 20 nm 0.1 nm 4

Tools and Techniques in a University Lab Ultrafast

regime with femtosecond pulses Ultrafast lasers fast

attohand: 1 revolution per fs High pulse repetition

clocking High peak intensity Nonlinear Optics

fusion) Broad optical spectrum Frequency metrology

tomography (OCT) Attoclock ultra-ultrafast 1 as at

ultrafast 100 as fine > 10 nm (> 1 nm) Applications

laser pulses) Measurements of fast processes Good

fast processes High pulse repetition fusion) High

Broad optical spectrum Frequency metrology

repetition clocking Good fast processes Future

ablation) long pulse short pulse Precision

quality Functional surfaces Inner glass micro

5 Tools and Techniques in a University Lab Ultrafast lasers in the UV, visible, IR, mid-ir, THz spectral regime with femtosecond pulses Ultrafast lasers fast 5 fs rough >100 nm Applications of ultrafast lasers attohand: 1 revolution per fs High pulse repetition rates Optical communication Interconnect Optical clocking High peak intensity Nonlinear Optics Precise material processing New energy souces (Laser fusion) Broad optical spectrum Frequency metrology (frequency comb) Optical clocks Optical coherence tomography (OCT) Attoclock ultra-ultrafast 1 as at ETH single atom or molecule Attoline at ETH: ultrafast 100 as fine > 10 nm (> 1 nm) Applications of ultrafast lasers Good time resolution (short laser pulses) Measurements of fast processes Good time resolution (short laser pulses) Measurements of fast processes High pulse repetition rates Optical communication Interconnect Optical clocking High peak intensity Nonlinear Optics Precise material processing New energy souces (Laser fusion) High peak intensity Nonlinear Optics Precise material processing New energy souces (Laser fusion) Broad optical spectrum Frequency metrology (frequency comb) Optical clocks Optical coherence tomography (OCT) Broad optical spectrum Frequency metrology (frequency comb) Optical clocks Optical coherence tomography (OCT) Optical interconnects in servers, data centers, supercomputers... Applications of ultrafast lasers High pulse repetition rates Optical communication Interconnect Optical clocking Good time resolution (short laser pulses) Measurements of fast processes Future Application: Precise material processing (cold ablation) long pulse short pulse Precision Intra-board & intra-chip interconnects High surface quality Functional surfaces Inner glass micro marking 3D printing... 5

Good time resolution (short laser pulses) Measurements of fast processes High pulse repetition rates Optical communication Interconnect Optical clocking Applications of ultrafast lasers OCT: Incoming

Optical communication Interconnect Optical clocking Applications of ultrafast lasers Optical clock (>100 THz): time is defined by a certain number of oscillations within a given time interval: the

001 Hz: Sand clock 1 Hz: Mechanical clock 30 fs Pulsdauer -> 10 µm axiale Auflösung High peak intensity Nonlinear Optics Precise material processing New energy souces (Laser fusion) Broad optical

Fujimoto, MIT OCT image of retina (around fovea) Early detection of retinal detachment High peak intensity Nonlinear Optics Precise material processing New energy souces (Laser fusion) Broad optical

(short laser pulses) Measurements of fast processes High pulse repetition rates Optical communication Interconnect Optical clocking Applications of ultrafast lasers High peak intensity Nonlinear

THz): time is defined by a certain number of oscillations per fixed time interval: higher frequency more accurate clock optical frequency: too high for counting frequency comb of ultrafast laser:

6 Good time resolution (short laser pulses) Measurements of fast processes High pulse repetition rates Optical communication Interconnect Optical clocking Applications of ultrafast lasers OCT: Incoming pulse Device under test (DUT) delayed reflected pulse backscattered intensity penetration depth Good time resolution (short laser pulses) Measurements of fast processes High pulse repetition rates Optical communication Interconnect Optical clocking Applications of ultrafast lasers Optical clock (>100 THz): time is defined by a certain number of oscillations within a given time interval: the higher the frequency the more accurately the time interval is defined Hz: Sand clock 1 Hz: Mechanical clock 30 fs Pulsdauer -> 10 µm axiale Auflösung High peak intensity Nonlinear Optics Precise material processing New energy souces (Laser fusion) Broad optical spectrum Frequency metrology (frequency comb) Optical clocks Optical coherence tomography (OCT) 10 fs Pulsdauer -> 3 µm axiale Auflösung Prof. J. G. Fujimoto, MIT OCT image of retina (around fovea) Early detection of retinal detachment High peak intensity Nonlinear Optics Precise material processing New energy souces (Laser fusion) Broad optical spectrum Frequency metrology (frequency comb) Optical clocks Optical coherence tomography (OCT) 10 MHz: Quartz oscillator Piezoelectric crystal resonator 9 GHz: Atomic clock GPS Good time resolution (short laser pulses) Measurements of fast processes High pulse repetition rates Optical communication Interconnect Optical clocking Applications of ultrafast lasers High peak intensity Nonlinear Optics Precise material processing New energy souces (Laser fusion) Broad optical spectrum Frequency metrology (frequency comb) Optical clocks Optical coherence tomography (OCT) Optical clock (>100 THz): time is defined by a certain number of oscillations per fixed time interval: higher frequency more accurate clock optical frequency: too high for counting frequency comb of ultrafast laser: detector measures the difference in frequency = beating signal beating signal measures the distance of unknown frequency to the lines in the frequency comb optical frequency becomes measurable Potential measurements: are fundamental constants constant? Frequency comb Detector Unknown frequency Detector 1 ps 1 fs Femtoseconds to Attoseconds ps and fs time domain (today) short laser pulses directly out of laser oscillators frontier in near infrared spectral regime (nonlinear optics to extend spectrum) industrial applications emerging (more established in ps domain) 80 as P. B. Corkum and Z. Chang, The attosecond revolution, OPN Oct. 2008, p as time domain VUV/XUV regime need intense fs pulses highly nonlinear process: high harmonic generation 6

Femtosecond Domain: Passive Modelocking 1 femtosecond = 1 fs = 10 15 s 1 attosecond = 1 as = 10 18 s Pulse generation directly from

fs 30 fs Focused laser intensity 10 13 10 15 Wcm -2 Gas target (= nonlinear medium), eg.

University Press 2000 Simple model to explain HHG HHG: high harmonic generation (1988 first experiments, Anne L Huillier) Laser, ω IR

experiments) Laser, ω IR Focused laser intensity 10 13 10 15 Wcm -2 Strength plateau cutoff 800 nm 5 fs 30 fs Gas target (= nonlinear

7 Femtosecond Domain: Passive Modelocking 1 femtosecond = 1 fs = s 1 attosecond = 1 as = s Pulse generation directly from laser oscillators: milliseconds to femtosecond regime Fundamental limit: one optical cycle Attosecond pulse generation: need to move up in frequency Time 1as# λ c = 2.7 nm VUV and EUV generation via HHG HHG: high harmonic generation (1988 first experiments, Anne L Huillier) Laser, ω IR 800 nm 5 fs 30 fs Focused laser intensity Wcm -2 Gas target (= nonlinear medium), eg. atoms HHG HHG Strength plateau Harmonic order cutoff Length 1am# David Attwood, Soft X-rays and extreme ultraviolet radiation, Cambridge University Press 2000 Simple model to explain HHG HHG: high harmonic generation (1988 first experiments, Anne L Huillier) Laser, ω IR Focused laser intensity Wcm -2 Strength plateau cutoff Attosecond pulses using HHG HHG: high harmonic generation (1988 first experiments) Laser, ω IR Focused laser intensity Wcm -2 Strength plateau cutoff 800 nm 5 fs 30 fs Gas target (= nonlinear medium), eg. atoms Harmonic order 800 nm 5 fs 30 fs Gas target (= nonlinear medium), eg. atoms Harmonic order Attosecond pulse generation: τ 1 Δν analogy to modelocking equally spaced frequency comb at 2ω IR ~ I (ω) +π I (t) two times within one optical cycle 0 every 1.4 fs a pulse ~ φ (ω) -π φ (t) 7

Techniques for attosecond pulses Long IR pulse attosecond pulse train (APT) many recollisions t 1 to 2 cycle IR pulse single attosecond pulses + + intensity intensity spectral comb broad continuum

dye laser 27 fs with 10 mw compressed Ti:sapphire laser 5.

Lett. 16, 42, 1991 50 ns/div# KLM mechanism explained for the first time: U. Keller et al., Opt. Lett. 16, 1022, 1991# A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8, 174, 1966 Nd:glass first passively modelocked laser Q-switched modelocked!

8 Techniques for attosecond pulses Long IR pulse attosecond pulse train (APT) many recollisions t 1 to 2 cycle IR pulse single attosecond pulses + + intensity intensity spectral comb broad continuum pulse train single pulse just 1 recollision t ω ω! FWHM pulse width (sec) 10 ps 1 ps 100 fs 10 fs 1 fs Ultrashort pulse generation with modelocking Science 286, 1507, 1999! dye laser 27 fs with 10 mw compressed Ti:sapphire laser 5.5 fs with 200 mw # Year Year# Kerr lens modelocking (KLM): 200 ns/div# discovered - initially not understood ( magic modelocking ) D. E. Spence, P. N. Kean, W. Sibbett, Opt. Lett. 16, 42, ns/div# KLM mechanism explained for the first time: U. Keller et al., Opt. Lett. 16, 1022, 1991# A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8, 174, 1966 Nd:glass first passively modelocked laser Q-switched modelocked! KLM! compressed Chirped pulse amplification (CPA) - G. Mourou fs, nj pulses 30 fs Cascaded filament compressor 0.66 mj 5.7 fs, 0.38 mj (core part), 57% transmission C. P. Hauri et al., Appl. Phys. B. 79, 673 (2004) 5.1 fs, 0.18 mj (core part), 27% transmission A. Guandalini et al., J. Phys. B. 39, S257 (2006) 10 fs 4.9 fs, 0.12 mj (core part), 18% transmission A. Zair et al, Opt. Express 15, 5394 (2007) For the shortest pulse duration spatial aperture needs to be smaller A B B > 25 fs, mj to J pulses TW to PW peak terawatt - petawatt W to W 4.9 fs A Double pulses: Conical emission due to the plasma 8

9 Carrier-Envelope Offset (CEO) Phase (CEP) H.R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, U. Keller Appl. Phys. B 69, 327 (1999) Mode-locked Pulse Train CEO Phase or CEP Why not XUV-XUV attosecond pump-probe? Cross-sections for XUV-XUV two-photon interactions are very small Current attosecond sources provide low photon flux (low energy AND low repetition rate) Further work on attosecond sources required U. Keller, IEEE Photonics Journal 2, 225 (2010) (invited review) t Femtosecond domain: nj pulses at 100 MHz 100 mw average power Attosecond domain: nj pulses at 1 khz 1 µw average power Streaking techniques instead of pump-probe attosecond resolved measurements strongly signal-to-noise limited use phase sensitive techniques instead: energy streaking: mapping time to energy with linear polarization angular streaking: mapping time to momentum with circular pol. attosecond pulse synchronized with strong infrared field strong infrared field can be used for streaking Attoclock: Maps Time to Angle Rotating infrared electric field vector of laser pulse streaks electron mapping angle to moment of ionization energy streaking: mapping time to energy (linear polarized streaking field) [1] R. Kienberger et al., Science, 207, 1144 (2002) [2] R. Kienberger et al., Nature, 427, 817 (2006) [3] E. Goulielmakis et al., Science, 305, 1267 (2004) Nature Physics, vol. 7, May 2011 Time zero: What do we measure? What do we want to know? Is given by a reference such as an additional attosecond pulse or is self referenced The final electron momentum after the short pulse Tunneling delay time 3 9

to 100 ev pulse energy of attosecond pulses: < nj pulse duration: 100 as Higher pulse repetition rate: MHz Power scaling: Advanced concepts for heat removal Optimize surface to volume ratio

oscillator domain low energy fiber The femtosecond average power revolution Courtesy of A.

10 Laser-based HHG What is next? amplifier domain low pulse repetition rate HHG and attosecond science Intense ultrafast Ti:sapphire CPA ( 800 nm) pulse repetition rate: 1 khz (moving towards 10 khz) pulse energy center: up to 100 ev pulse energy of attosecond pulses: < nj pulse duration: 100 as Higher pulse repetition rate: MHz Power scaling: Advanced concepts for heat removal Optimize surface to volume ratio thin disk laser focusing optic thin disk 100 µm cooling laser beam slab type laser y z HR-mirror slab focusing optic HR-mirror out coupling mirror fiber laser focusing optic HR-Spiegel Laser oscillator domain low energy fiber The femtosecond average power revolution Courtesy of A. Tünnermann: Systems with < 1ps pulse duration The femtosecond average power revolution Femtosecond fiber amlifier system (Tünnermann): Courtesy of A. Tünnermann: Systems with < 1ps pulse duration 640 fs pulse duration 830 W average power 12 MW peak power 78 MHz repetition rate Yb:KYW oscillator 200 fs, 78 MHz, 1042 nm, 150 mw ISO Output Öffner type stretcher compressor 2- stage preamplifier Main amplifier ISO Fraunhofer IOF Courtesy of A. Tünnermann 10

The femtosecond average power revolution Oscillator versus CPA-fiber amplifier Courtesy of A.

2 W, 10 MHz 350 fs, 1030 nm preamplifier AOM temporal scaling larger stretching factor fs fiber amplifier 80 MHz.

7 MW peak power 60 MHz repetition rate Courtesy of A.

femtosecond CPA-fiber amplifier always needs a fs oscillator Thin disk laser d<d 30 cm 141 W, Yb:Lu2O3 hopt = 40% spacial scaling increased mode-field-area output

preamplifier AOM 100 cm temporal scaling larger stretching factor 80 MHz...10 khz Isolator dielectric grating stretcher A. Giesen, et al., Appl. Phys.

11 The femtosecond average power revolution Oscillator versus CPA-fiber amplifier Courtesy of A. Tünnermann: Systems with < 1ps pulse duration Femtosecond SESAM modelocked Yb:Lu2O3 thin disk laser isolator fs-oscillator laser oscillator no amplifier! 2 W, 10 MHz 350 fs, 1030 nm preamplifier AOM temporal scaling larger stretching factor fs fiber amplifier 80 MHz...10 khz Isolator dielectric grating stretcher dielectric grating compressor Yb-doped power amplifier 885 fs pulse duration 103 W average power 1.7 MW peak power 60 MHz repetition rate Courtesy of A. Tünnermann Fraunhofer IOF Oscillator versus CPA-fiber amplifier High power fs oscillator: same complexity as low power fs oscillator Modelocked Thin Disk Lasers femtosecond CPA-fiber amplifier always needs a fs oscillator Thin disk laser d<d 30 cm 141 W, Yb:Lu2O3 hopt = 40% spacial scaling increased mode-field-area output Output coupler Highest average powers and highest energies of any ultrafast oscillator technology isolator fs-oscillator 2 W, 10 MHz 350 fs, 1030 nm SESAM Ep > 40 µj #1 preamplifier AOM 100 cm temporal scaling larger stretching factor 80 MHz...10 khz Isolator dielectric grating stretcher A. Giesen, et al., Appl. Phys. B 58, 365 (1994) + SESAM dielectric grating compressor Yb-doped power amplifier output Pav > 140 W #1, #2 Semiconductor saturable absorber mirror New Result: spacial scaling increased mode-field-area Pav = 270 W 583 fs d<d Yb:Lu2O3 thin disk laser head C. R. E. Baer et al., Optics Lett. 35, 2302, 2010 #1 U. Keller, et al., IEEE J. Sel. Top. Quant. 2, 435 (1996) #2 D. Bauer et al., Optics Express 20, (2012) C. R. E. Baer, et al., Optics Letters 35, (2010) 11

Results High average power lasers A. Giesen, et al., Appl.

4% P pump = 840 W M 2 < 1.05 f rep = 16.3 MHz τ P Δν = 0.

Saraceno et al., Opt. Express submitted Aug.

2012 First time >10 µj pulse energy from a SESAM

Express 16, 6397, 2008 and CLEO Europe June 2007 26 µj

12 Results High average power lasers A. Giesen, et al., Appl. Phys. B 58, 365 (1994) Result P avg = 275 W η opt = 32.4% P pump = 840 W M 2 < 1.05 f rep = 16.3 MHz τ P Δν = E P = 16.9 µj (ideal: 0.315) P pk = 25.6 MW C. J. Saraceno et al., Opt. Express submitted Aug Postdeadline Paper, Europhoton Conf., Aug First time >10 µj pulse energy from a SESAM modelocked Yb:YAG thin disk laser: Opt. Express 16, 6397, 2008 and CLEO Europe June µj with a multipass gain cavity and larger output coupling of 70% (Trumpf/Konstanz) Opt. Express 16, 20530, 2008 High average power lasers - moving towards 100 µj Shorter pulses? 100 µj 5 MHz 500 W average power P av = E p f rep 12

High average power ultrafast lasers

High average power ultrafast lasers C. J. Saraceno, F. Emaury, O. H. Heckl, C. R. E. Baer, M. Hoffmann, C. Schriber, M. Golling, and U. Keller Department of Physics, Institute for Quantum Electronics Zurich,