Introduction/Motivation/Overview

Ultrafast Laser Physics Ursula Keller / Lukas Gallmann ETH Zurich, Physics Department, Switzerland www.ulp.ethz.ch Introduction/Motivation/Overview Ultrafast Laser Physics ETH Zurich

Ultrafast Laser Physics (ULP) Master Program, Physics, Core Course ETH Zurich, HS 2017 (14 Weeks) Lecture: Wednesday 10:45-11:30, HIT F11.1 Thursday 8:45-10:30, HIT H 51 PD Dr. Lukas Gallmann, ETH Zurich, HPT E7.1, 044/633 37 09, gallmann@phys.ethz.ch www.ulp.ethz.ch Lecture based on material by Prof. Dr. Ursula Keller Exercises: Wednesday 12:45 14:30, HIT F13 & F31.1 (start Sept. 27th) Dr. Chris Phillips, HPT E7.2, cphillips@phys.ethz.ch

Downloads on web http://www.ulp.ethz.ch/education/lectures/ultrafast-laser-physics.html

Week 1 (20./21. Sep.): Introduction/motivation/overview and linear pulse propagation Week 2 (27./28. Sep.): Linear pulse propagation and dispersion compensation Week 3 (4./5. Oct.): Dispersion compensation and nonlinear pulse propagation Week 4 (11./12. Oct.): Nonlinear pulse propagation Week 5 (18./19. Oct.): Chi(2)-nonlinearities with ultrashort pulses Week 6 (25./26. Oct.): Relaxation oscillations and Q-switching Week 7 (1./2. Nov.): Q-switching and active modelocking Week 8 (8./9. Nov.): Passive modelocking Outline Week 9 (15./16. Nov.): Passive modelocking and pulse duration measurements Week 10 (22./23. Nov.): Pulse duration measurements and noise Week 11 (29./30. Nov.): Pump-probe measurements, frequency combs and carrier-envelope offset phase Week 12 (6./7. Dec.): Frequency combs and carrier-envelope offset phase, high-harmonic generation and attosecond science Week 13 (13./14. Dec.): High-harmonic generation and attosecond science Week 14 (20./21. Dec.): Ultrafast THz science and hot topics

Ultrafast Laser Physics: Introduction Part I How do you generate short pulses?

Time-/Length Scale Length 1am Time 1as 1 femtosecond = 1 fs = 10 15 s = 0.000000000000001 s 1 attosecond = 1 as = 10 18 s G. Steinmeyer, D. H. Sutter, L. Gallmann, N. Matuschek, U. Keller Science 286, 1507, 1999

Measurements on short time scales Mechanical shutter triggered mechanical shutter 1/1000s = 1 ms time resolution 12 cameras over 50 feet 1878, Palo Alto Stock Ranch Is there an unsupported transit in gallop? Eadweard Muybridge (1830-1904)

Photography with 1 µs exposure time Electronics triggered flash lamps <µs time resolution (around 1935) Harold E. Edgerton, MIT 1903-1990

Laser Light amplification by stimulated emission of radiation The first laser: 1960 stimulated emission (1916 Einstein) coherent optical amplification inside a laser resonator Theodore H. Maiman

Active Modelocking

Passive Modelocking

Mode locking by forcing all modes in a laser to operate phase-locked, noise is turned into ideal ultrashort pulses τ 1 Δν ~ I (ω) I (t) ~ I (ω) I (t) +π +π ~ φ (ω) 0 -π axial modes in laser not phaselocked noise φ (t) ~ φ (ω) 0 -π φ (t) axial modes in laser phaselocked ultrashort pulse inverse proportional to phaselocked spectrum

Ultrashort pulse generation with modelocking A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8, 174, 1966 200 ns/div 50 ns/div Nd:glass first passively modelocked laser Q-switched modelocked 1960 1970 1980 1990 2000 Year Flashlamp-pumped solid-state lasers Diode-pumped solid-state lasers (first demonstration 1963)

Ultrashort pulse generation with modelocking FWHM pulse width (sec) 10 ps 1 ps 100 fs 10 fs 1 fs 1960 Science 286, 1507, 1999 1970 dye laser 27 fs with 10 mw 1980 Year compressed 1990 Ti:sapphire laser 5.5 fs with 200 mw 2000 1960 1970 1980 1990 2000 Year A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8, 174, 1966 Nd:glass first passively modelocked laser Q-switched modelocked 200 ns/div 50 ns/div E. P. Ippen, C. V. Shank et al, Appl. Phys. Lett. 21, 348, 1972 J. A. Valdmanis et al., Opt. Lett. 10, 131, 1985 (27 fs) R. L. Fork et al., Opt. Lett. 12, 483, 1987 (6 fs)

CPM dye laser Ring laser Colliding pulse modelocked (CPM) dye laser: Gain: Rhodamine 6G Saturable absorber: DODCI Center wavelength: 620 nm Typical pulse duration: >27 fs J. A. Valdmanis et al., Opt. Lett. 10, 131, 1985 Typical average power: a few 10 mw

High power diode arrays metal contact proton implant p-gaas (cap) p-gaalas GaAlAs (active layer) n-gaalas n-gaas (substrate) metal contact cleaved facet individual emitter 0.6 Flashlamp Emission Diode Laser Emission Absorption (OD) 0.4 0.2 Nd:YAG Absorption Spectral Radiance 0.0 300 400 500 600 700 800 900 Wavelength (nm)

Diode-pumped solid-state lasers Longitudinal pumping laser rod diode laser focussing optics AR coating: HR - laser λ HT - diode λ output coupler Transversal pumping laser rod HR mirror AR diode laser AR output coupler

Ultrashort pulse generation with modelocking FWHM pulse width (sec) 10 ps 1 ps 100 fs 10 fs 1 fs 1960 Science 286, 1507, 1999 1970 dye laser 27 fs with 10 mw 1980 Year compressed 1990 Ti:sapphire laser 5.5 fs with 200 mw 2000 1960 1970 1980 1990 2000 Year A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8, 174, 1966 KLM 200 ns/div 50 ns/div Kerr lens modelocking (KLM): D. E. Spence, P. N. Kean, W. Sibbett, Opt. Lett. 16, 42, 1991 Nd:glass first passively modelocked laser Q-switched modelocked

World record pulse duration at ETH Tages Anzeiger 7./8. Juni 1997

How does such a short pulse look like? λ/c = 2.7 fs @800 nm 1.5 µm speed of light

Carrier-Envelope Offset (CEO) Phase H.R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, U. Keller Appl. Phys. B 69, 327 (1999) F. W. Helbing, G. Steinmeyer, U. Keller IEEE J. of Sel. Top. In Quantum Electron. 9, 1030, 2003 Mode-locked pulse train Pulse envelope A(t) T R CEO phase Δφ 0 f CEO = Δϕ 0 2πT R t Electric field: λ/c = 2.7 fs @800 nm E( t) = A( t)exp( iω c t + iϕ 0 (t)) CEO phase controlled in laser oscillator

Kerr Lens Modelocking (KLM) First Demonstration: D. E. Spence, P. N. Kean, W. Sibbett, Optics Lett. 16, 42, 1991 Explanation: U. Keller et al., Optics Lett. 16, 1022, 1991 Advantages of KLM very fast thus shortest pulses very broadband thus broader tunability Disadvantages of KLM not self-starting critical cavity adjustments (operated close to the stability limit) saturable absorber coupled to cavity design (limited application)

Ultrashort pulse generation with modelocking A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8, 174, 1966 Q-switching instabilities continued to be a problem until 1992 Nd:glass first passively modelocked laser Q-switched modelocked 200 ns/div 50 ns/div KLM SESAM First passively modelocked (diode-pumped) solid-state laser without Q-switching U. Keller et al. Opt. Lett. 17, 505, 1992 IEEE JSTQE 2, 435, 1996 Nature 424, 831, 2003 1960 1970 1980 1990 2000 Year Flashlamp-pumped solid-state lasers Diode-pumped solid-state lasers (first demonstration 1963)

SESAM technology ultrafast lasers for industrial application U. Keller et al. Opt. Lett. 17, 505, 1992 IEEE JSTQE 2, 435, 1996 Progress in Optics 46, 1-115, 2004 Nature 424, 831, 2003 SESAM solved Q-switching problem for diode-pumped solid-state lasers Gain Output coupler SESAM SEmiconductor Saturable Absorber Mirror self-starting, stable, and reliable modelocking of diode-pumped ultrafast solid-state lasers

What is special about ultrafast solid-state lasers? FWHM pulse width (sec) 10 ps 1 ps 100 fs 10 fs 1 fs 1960 1970 dye laser 27 fs with 10 mw 1980 Year compressed 1990 Ti:sapphire laser 5.5 fs with 200 mw 2000 Mode-locked pulse train Pulse envelope A(t) Electric field: λ/c = 2.7 fs @800 nm E t ( ) = A t ( ) ( )exp iω c t + iϕ 0 (t) T R CEO phase Δφ 0 f CEO = Δϕ 0 2πT R t World record results in ultrashort pulse generation: two-optical-cycle regime ( 5 fs) using KLM and chirped mirrors, Science 286, 1507, 1999 Carrier envelope offset (CEO) control and stabilization using frequency combs: Appl. Phys. B 69, 327, 1999 and IEEE JSTQE 9, 1030, 2003 f 1 = f CEO + nf rep

What is special about ultrafast solid-state lasers? U. Keller et al. Opt. Lett. 17, 505, 1992 IEEE JSTQE 2, 435, 1996 Progress in Optics 46, 1-115, 2004 Nature 424, 831, 2003 Gain SESAM SEmiconductor Saturable Absorber Mirror self-starting, stable, and reliable modelocking of diode-pumped ultrafast solid-state lasers Output coupler Solved Q-switching problem for passively modelocked diode-pumped solid-state lasers (after more than 25 years) - important for real-world applications Pushed pulse energy of diode-pumped ultrafast solid-state lasers by four orders of magnitude: More than 80 µj at MHz repetition rate and 1 ps pulses (and 275 W [600 fs] of average output power 2.4 kw intra-cavity power) Pushed pulse repetition rate of diode-pumped solid-state lasers by more than two orders of magnitude: 160 GHz at 1 µm and 100 GHz at 1.5 µm Passively modelocked vertical external-cavity surface-emitting laser (VECSEL): for high pulse repetition rates, power scaling to 6.4 W, full wafer-scale integration Invited review paper in Physics Reports 429, pp. 67-120, 2006 MIXSEL - a new class of ultrafast semiconductor lasers

Ultrafast Laser Physics: Introduction Part II Applications of ultrashort laser pulses

Applications of ultrafast lasers Good time resolution (short pulses) Measurements of fast processes High pulse repetition rates Optical communication, clocking High peak intensity at moderate energies Nonlinear optics Precise material processing Broad optical spectrum Metrology (frequency comb), OCT,

Pump-Probe Measurement Δt 2Δz = c Δt Δz = 1 µm Δt 2 3.3 fs

Semiconductor saturable absorber Δt

Measure fast electronics J. A. Valdmanis, G. A. Mourou, IEEE JQE 22, 69, 1986 K. J. Weingarten et al., IEEE JQE 24, 198, 1988

Pump-Probe Measurement Δt 2Δz = c Δt Δz = 1 µm Δt 2 3.3 fs Semiconductor dynamics Optoelectronics, high-speed electronics, terahertz radiation, electro-optic sampling Chemical reaction dynamics Femtochemistry (Nobel prize in chemistry 1999: Ahmed Zewail) Biology Ultrafast processes in photosynthesis, vision, Ultrafast near-field, nonlinear and confocal microscopy

LIDAR and Optical coherence tomography (OCT) incoming light retarded reflected light reflected intensity penetration depth u Optical Radar (LIDAR - LIght Detection And Ranging) u Resolution better for shorter pulses u Resolution of cell structures possible Science, 254, 1178, 1991

Optical coherence tomography (OCT) 30 fs pulse duration -> 10 µm axial resolution 10 fs pulse duration -> 3 µm axial resolution Resolution dependence on pulse duration (Prof. J. G. Fujimoto, MIT)

Control of chemical reactions Prof. Gerber, Universität Würzburg

Passive modelocking Gain Output coupler Short cavity length = high pulse repetition rate Saturable Absorber (Light modulator) Pulse repetition rate is given by the cavity round trip time. 1 GHz: cavity round trip time 1 ns and a cavity length 15 cm. 1 THz: cavity round trip time 1 ps and a cavity length 150 µm. No high speed electronics needed.

Applications of Multi-GHz Ultrafast Laser Sources Telecom Optical Clocking Biomedical applications Frequency comb

Quasi-Monolithic Cavity Setup L. Krainer et al., Electron. Lett. 35, 1160, 1999 (29 GHz) APL 77, 2104, 2000 (up to 59 GHz), Electron. Lett. 36, 1846, 2000 (77 GHz) IEEE J. Quant. Electron. 38, 1331, 2002 (10 to 160 GHz) 4 Crystal lengths: 440 µm - 2.3 mm (FSR ~ 160-29 GHz) Nd:YVO 4 doping: 3 % (90 µm absoption length)

All-optical 77-GHz pulse generation at 1.5 µm Autocorr. signal (a.u.) 1.0 0.5 3.2 ps 0.0-20 -10 0 10 Time delay (ps) 77 GHz at 1538.8 nm (standing wave cavity length 1.9 mm) 3.2 ps with average output power of 10 mw, TBP 0.37 (nearly transform limited) S. C. Zeller et al, Electron. Lett., vol. 43, pp. 32, 2007 Spectral intensity (db) 0-10 -20-30 -40-50 -60 1532 1534 1536 1538 Wavelength (nm) (b) 1540

All-optical 100-GHz pulse generation at 1.5 µm Autocorrelation Optical spectrum Photo of actual setup Pulse repetition rate: 101 GHz Max. output-power: 35 mw Optical bandwidth: 2.6 nm Pulse width: 1.6 ps (1.7x time-bandwidth limit) A. E. H. Oehler et al., Opt. Express 16, 21930, 2008

Comparison of Ultrafast GHz Lasers Vertical external cavity surface emitting laser (VECSEL) or semiconductor disk laser VECSELs or MIXSELs Modelocked VECSEL MIXSEL Review article: U. Keller and A. C. Tropper, Physics Reports, vol. 429, Nr. 2, pp. 67-120, 2006

Short-pulse and high-power GHz-lasers Ultrafast surface emitting semiconductor lasers (VECSELs, MIXSELs) produce short pulses with high average power at GHz pulse repetition rates Review article: B. W. Tilma et al., Light: Science & Applications 4, e310 (2015), doi:10.1038/lsa.2015.83

Summary: key enabling technology is QD-SESAM We confirmed the optimization guideline for QD-SESAM with low F sat - ΔR increases with ML coverage (dot density) and N QD-layers - Post-growth annealing à significantly reduces F sat à significantly reduces slow component amplitude D. H. C. J. Maas et al., Opt. Express 16, 18646 (2008) For the first time: antiresonant MIXSEL demonstrated ( right-side-up ) à allows power scaling with up-side-down structure First OP-VECSEL (Kuznetsov et al.) First VECSEL-SESAM modelocking First 1:1 modelocking with resonant QD-SESAM First MIXSEL with resonant design First 1:1 modelocking with anti-resonant QD-SESAM First antiresonant MIXSEL 1997 Power scaling of 2000 VECSEL-SESAM 2005 2007 modelocking: 20 W cw TEM 00 2.1 W picosecond pulses 2009 nano-tera.ch

Optical amplifier Laser oscillator pulse energy: typically nanojoule level ( 1 nj) pulse repetition rate: typically 100 MHz Laser amplifier pulse energy: mj to J pulse repetition rate: Hz to 1 khz (10 khz) P av = E p f rep E p = 10 nj 1 mj ( 10 5 ) f rep = 100 MHz 1 khz ( 10 5 ) P av = 1 W 1 W

Chirped pulse amplification (CPA) - G. Mourou fs, nj pulses > 25 fs, mj to J pulses TW to PW peak terawatt - petawatt 10 12 W to 10 15 W

Chirped pulse amplification (CPA) fs, nj pulses > 25 fs, mj to J pulses TW to PW peak terawatt - petawatt 10 12 W to 10 15 W Commercial Ti:sapphire-based CPAs with peak powers of 10 to 100 GW Femtolasers: 5 mj to 10 mj, 25 fs, 1 khz, CEO phase stabilized (ETH, group of Prof. Keller ) Pulse compression towards about two cycle pulse durations: 5 fs at 800 nm, a few 100 µj hollow core fiber compressor filament compressor National Facilities (LBL, LLNL, Rutherford Appleton, MPQ): moving towards PW systems

Short Pulse - High Intensity: 1 TW at ETH (2010) Same weight F 1 Terawatt = 10 12 W = 1 TW A Same energy Intensity I I p Intensity I p A I Intensity II = 1000 x KKW KKW Leibstadt 1.03 Gigawatt Time t Time t Time t = femtoseconds Laser system (ULP@ETH): 5 mj energy, 5 fs pulse duration gives a peak power 1TW

Nd:glass single-shot lasers η <1%, 1-2k /J, 2-15 shots/day NIF/LLNL 192 beams,1.8mj/3ω/20ns OMEGA/Rochester 60 beams, 40 kj/3ω GEKKO XII/Osaka 12 beams, 10 kj/1-2ns/2ω VULCAN/Rutherford 1 beam, 2.6 KJ/1ω PHELIX/Darmstadt 1 beam 1 kj/1ω/5ns LULI2000/Palaiseau 2 beams 2 kj/1ω/1.5 ns KrF η ~1%, 30 shots per day NIKE/NRL 5 kj/4 ns Iodine η <1%, 1.5 k /J, 20 shots/day Selection of Facilities in Operation High-Energy Lasers PALS/Prague 1 beam, 400 J/3ω/400 ps ISKRA4/Arzamas 8 beams, 2 kj/1ω, 500 J/2ω ISKRA5/Arzamas 12 beams, 30 kj/1ω, 0.25 ns High-Power High-Intensity Lasers Nd:glass 1053 nm, flash lamp-pumped, 400 fs-1 ps, 5-10 shots per day TRIDENT/Los Alamos 100J/500fs/200TW, K~10 11 OMEGA EP/Rochester 4 x250j/1ps 1PW VULCAN/Rutherford 500J/500fs/1 PW PHELIX/GSI 250J/500fs 500 TW PETAL/Le Barp 3kJ/1ps/3 PW GEKKO/Osaka 4x2.5kJ/10 ps 1 PW Yb:glass 1035 nm, diode-pumped POLARIS/Jena15J/150fs/100TW/0.1Hz Ti:sapphire 800nm, laser-pumped, up to 250 TW at 10 Hz commercially available for 1M /100TW Callisto/LLNL 10J/100fs/<1mHz 100 TW ASTRA Gemini/Rutherf. 2x15J/30fs/1min per shot 1 PW ATLAS/MPQ 3J/30fs/10 Hz 100 TW YETI /Jena 2J/60fs/10 Hz 30 TW HERCULES/U Michigan 9J/28fs/0.1 Hz 300 TW JAEA/Kyoto 28J/33fs/ss 0.85 PW LOA/Palaiseau 30fs/2J/10Hz 70 TW

Route to Exawatt Lasers ATLAS, POLARIS ELI = Extreme Light Infrastructure (European Project) - https://eli-laser.eu/

ELI = Extreme Light Infrastructure (European Project) - https://www.eli-laser.eu/ Hungary Attosecond science http://www.eli-alps.hu/ Czech Republic Secondary sources https://www.eli-beams.eu/ Romania Nuclear physics http://www.eli-np.ro/

Frontier in high intensity laser sources High-Power High-Intensity Lasers 100 TW to 100 PW in 5 fs to 1ps On-target intensities 10 18-10 25 W/cm² Nd:glass 1.06 µm (single shot + reprated), Ti:sapphire 800 nm, Ytterbium:glass 1035 nm, OPCPA and Mixed systems Physics Ultra-relativistic plasma physics GeV electron + ion bunches, sub-gev protons X-rays for physics, chemistry, biomedicine, homeland security MeV γ-rays for micro-neutron beams Fast ignition Atto- and zeptosecond physics QED in strong fields

Example: ELI Beamlines facility in Prague http://www.eli-beams.eu/

High energy and high (MHz) pulse repetition rates T. Südmeyer et al., Nature Photonics 2, 599, 2008

High average power lasers DP-SSL: diode-pumped solid-state lasers Nat. Photonics 2, 559, 2008 First time >10 µj pulse energy from a SESAM modelocked Yb:YAG thin disk laser: Opt. Express 16, 6397, 2008 and CLEO Europe June 2007 80 µj with a 3-MHz extended cavity, 242 W of average power, 1.07 ps Saraceno et al., Opt. Lett. 39, 9 (2014) 275 W of output power, 580 fs, 16 MHz: Saraceno et al., Opt. Express 20, 23535 (2012)

High average power lasers - moving towards 100 µj 100 µj 5 MHz 500 W average power P av = E p f rep

Application for high-power modelocked lasers: material processing Long Pulse Short Pulse

Material processing with long pulses

Material processing with short pulses High precision

Medical Applications: material processing of the eye creation of corneal flap for LASIK procedure ultrafast pulse cornea hole pattern eye from Gerard A. Mourou, University of Michigan, USA

VUV and EUV generation via HHG HHG: high harmonic generation (1988 first experiments) Laser, w IR Focused laser intensity 10 13 10 15 Wcm -2 HHG Strength plateau cutoff 800 nm 5 fs 30 fs Gas target (= nonlinear medium), eg. atoms HHG Harmonic order 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) Laser, w IR Focused laser intensity 10 13 10 15 Wcm -2 Strength plateau cutoff 800 nm 5 fs 30 fs Gas target (= nonlinear medium), eg. atoms Harmonic order

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

attoscience beamline ( attoline ): ETH-Keller Rabitt Trace HHG in Argon 450 as FWHM (average pulse in APT) 160 as transform limit (with 400 nm Al foil - flux reduced by 2)

attoscience beamline ( attoline ): ETH-Keller electron energy (ev) 30 25 20 15 10 0 1 a) b) c) 0 1 norm. intensity 205 as phase (rad) Streaking trace of an isolated attosecond pulse generated by polarization gating. 5-6 -4-2 0 2 4 6 delay (fs) -6-4 -2 0 2 4 6 delay (fs) time (as)

HHG and attosecond science Laser-based HHG 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 Laser-based HHG: a success story Initially: there was a new laser HHG is an unexpected experimental discovery M. Ferray, Anne L Huillier et al., J. Phys. B 21, L31,1988 P. B. Corkum and Z. Chang The attosecond revolution OPN Oct. 2008, p. 25-29

HHG and attosecond science Laser-based HHG 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 Laser-based HHG: a success story Challenges/Problems of laser based HHG: Moving towards hard X-ray? Low pulse repetition rates and low pulse energy!

Sand clock Best mechanical clock Accuracy of clocks Quartz oscillator clock GPS Atomic clock (PTB) 1 min/day 1s/10 years 1s/1'000'000 years 10-3 10-6 10-9 10-12 10-15 Oscillation Þ The higher the frequency oscillation frequency the more accurate the clock 0.001 Þ Optical Hz clocks1 can Hz have an oscillation 10 MHz frequency 10'000x 9 GHzhigher than atomic clocks Þ Potential for more accurate clocks

Femto-Clock Magnification: 100'000 x Magnification: 100'000 x u Spectrum of a fs modelocked laser consists of millions of fine lines u Lines have everywhere the same distance to each other u Femtosecond laser is for frequency the same as a ruler for length! Frequency ruler extremely precise! T. Udem, R. Holzwarth, T. W. Hänsch, Nature 416, 233, 2002

How can a femtosecond laser measure time? Frequency comb Detector Unknown frequency Detector u optical frequency too high for counting u detector measures the difference in frequency = beating signal u measures the distance of unknown frequency to the lines in the frequency comb u optical frequency becomes measurable u time is defined by a certain number of oscillations per fixed time interval

Superposition of two waves with two different frequencies f1 and f2 Measure beat note at (f2 f1)

Are physical constants really constant?

Applications of ultrafast lasers Good time resolution (short pulses) High pulse repetition rates High peak intensity at moderate energies Broad optical spectrum Ultrafast Lasers are based on basic science: materials, spectroscopy, nonlinear dynamics, and putting everything together! are the key enabling technology Many applications - industrial and research Nature 424, 831, 2003 Nobel prizes: femtochemistry (1999), metrology (2005)

Impact of femtosecond science Femtosecond chemistry: Observe chemical reactions in real time Nobel prize: Ahmed H. Zewail, chemistry 1999 Frequency metrology: frequency comb from fs lasers Nobel prize: Ted W. Hänsch and John J. Hall, physics 2005 optical clocks, are physical constants really constant?, find earth-like planets Ultrafast opto-electronics: ultrafast relaxation processes in semiconductors Next generation of telecom and datacom networks need ultrafast lasers Material processing: higher precision with shorter pulses, brittle material processing with less energy and shorter pulses, Medical application: laser cutting, laser diagnostics, Biomedical imaging: OCT, two-photon microscopy, sub-wavelength imaging Compact coherent white light generation (continuum generation) Now extend this to attoseconds ( no one ever went there before ) Now combine this with X-rays (19 Nobel prizes awarded to X-ray based structure analysis emphasizes its high impact)

Time-/Length Scale 1 femtosecond = 1 fs = 10 15 s = 0.000000000000001 s 1 attosecond = 1 as = 10 18 s

U. Keller Ultrafast solid-state lasers Landolt-Börnstein, Group VIII/1B1, Laser Physics and Applications. Subvolume B: Laser Systems. Part 1. Edited by G. Herziger, H. Weber, R. Poprawe, Springer- Verlag, Berlin, Heidelberg, New York, October, pp. 33-167, 2007 ISBN 978-3-540-26033-2 Further reading