THz meets X-rays: Ultrafast X-ray Experiments Using Terahertz Excitation Matthias C. Hoffmann, LCLS Laser Science & Technology Division SLAC National Accelerator Laboratory, Menlo Park, CA, 94025
Overview Time-resolved X-ray experiments Introduction to LCLS Strong THz sources at LCLS Typical user experiments
What to learn from ultrafast X-ray experiments X-rays probe structure crystalline structure (we can reconstruct lattice structure from diffraction patterns) electron structure t (spin and orbital ordering superlattices) imaging of magnetic domains
Time-resolved X-ray measurements Ultrafast X-ray probe pulses can be used for time-resolved measurements of structural change in materials Requirements: short x-ray pulses (<100 fs) high x-ray flux (photons per pulse) synchronized pump lasers 4
Introduction to LCLS World s first femtosecond hard X-ray laser SACLA (Spring8) 2012 1 km linear accelerator ( linac ) 100 m undulator ~1 mj pulse energy (10 12 photons per pulse) coming soon: PAL (Korea) EuropeanXFEL Swiss-FEL 5 100 fs pulse duration 250 ev 12 kev photon energy Synchronized fs-lasers for pump-probe p p 5
LCLS introduction Movie 6
Physics in the THz range 1TH THz 300 μm 4.11 mev 30 cm -1 Lattice vibrations carrier dynamics in semiconductors Collective excitations in correlated materials ultrafast magnetism superconducting gap Molecular rotations and alignment (gas phase)...
why do we need strong THz pulses? control of low energy degrees of freedom in complex matter pump below optical transitions no parasitic effects from optical pump laser pulses observe field effects directly in time domain (ferroelectricity, magnetism, photoelectron streaking) 300 kv/cm ~ 0.1 T (peak) 3 MV/cm ~ 1 T (peak) spin waves Josephson phase in superconductors ferroelectric switching
why laser-generated THz at LCLS? beam based THz sources available but hard to implement transport and temporal overlap (THz generated after xrays!) laser generated THz pulses can be implemented with existing laser infrastructure range of THz frequencies and complex pulse shapes available can be delivered into complex X-ray setups (diffraction experiments, vacuum chambers) timing jitter of 100 fs comparable to THz pulse durations
LCLS Hutches Lasers in LCLS Science 10
LCLS hutches with pump-probe experiments AMO Maximum soft x-ray intensities SXR Gaseous samples, nanocrystals Monochromatic soft x-rays for K & L edge resonances XPP Materials research Hard x-rays Structural probe 11
LCLS NEH Laser systems AMO SXR XPP Coherent Oscillator 100 fs stretcher Wavelength 800 nm Femtolasers Oscillator Coherent Legend Elite (50 fs) Homebuilt 4 pass MPA Pulsewidth Energy Rep Rate ~35 fs < t < 150 fs 3 4 mj, 20 35 mj 120 Hz Lasers in LCLS Science Mike Minitti 12
AMO Evolution Chiller Shutter Enclosure TFP WP3 TFP TFP WP2 WP1 Transport Entrance Enclosure Stretched 240 Hz Up & Over Beam Transport Uncompressed Beam Vacuum Relay Imaging Tube Evolution Chillers 100fs Stretcher Beam Enclosure Compressed Output SSA-F Evolution HE Evolution HE i-pass Amplifier Multi Shutter Enclosure 100fs Stretcher Evolution HE Evolution HE Multi-Pass Amplifier SXR Evolution Chiller XPP Evolution Chillers 100fs Stretcher Evolution HE Evolution HE ass Amplifier Multi-Pa Shutter & Periscope Enclosure Beam Enclosure Shutter Enclosure Beam Enclosure Shutter Enclosure Evolution 15 Control Rack 51 Control Rack 53 Control Rack 52 Control Rack 54 AMO Laser SXR Laser XPP Laser AMO Table Layout SXR Table Layout XPP Table Layout
overview of intense laser-based THz sources Optical rectification usually results in single cycle pulses Tunable narrowband generation using difference frequency processes (DFG) Large gap between 5 and 15 THz because of strong intrinsic material absorption in most known crystals J. Phys. D, 44 083001 (2011)
Laser based THz sources available today Technique Frequency range (THz) Pump wavelength Energy (μj) type Peak field 0.2-3 800 nm 10-30 single-cycle 1 MV/cm Tilted pulse front LiNbO 3 Optical rectification in 0.5-3.5 800 nm 1.5 single-cycle <300 kv/cm ZnTe OR in Organic crystals 0.5-10 1300-1500 nm 1-10? quasi single-cycle >500 kv/cm Plasma sources 0.5-40 800+400 nm 1 single-cycle <1 MV/cm (?) DFG in GaSe 15-50 OPA 10-100 multi-cycle Up to 100 MV/cm CTR (LCLS, FLASH) 0.1-30 Beam-based 140 single-cycle 20MV/cm (?) Undulator (FLASH) 1-30 Beam-based 20 (?) multi-cycle l? R&D need: 5-15 THz sources with high intensity
THz single-cycle pulses from LiNbO 3 Present state-of the art at LCLS, LN tilted pulse front, room temperature : Single cycle pulse Peak spectrum <1 THz Single-cycle pulse, Peak spectrum <1 THz >10 μj THz pulse energies at 15 mj, 800 nm fs-laser pump, 120 Hz repetition rate >500 kv/cm field strength (1 MV/cm demonstrated through better focusing) >50 μj has been demonstrated at room temperature by other groups
Visualization of propagating fields Tilted pulse front excitation in LiNbO 3 with 100 fs pump laser Air 1.5 Solution of coupled phononpolariton equations in LN 2D finite-element simulation, y(mm m) 1.0 total simulation time 15 ps Shown is E z 0.5 LN 05 0.5 x (mm) 10 1.0
optical rectification in DAST 4-N,N-dimethylamino-4'-N'-methyl 4 N stilbazolium tosylate organic crystal engineered to have extremely large nonlinear coefficient collinear phase matching has been used for 2 decades but hard to grow, easy to damage pump at >1.1 micron to minimize damage and optimize phase matching
preliminary test results at LCLS pump at 1300 nm with 1.3 mj 4 mm sample diameter Used in three experiments so far about 5 μj of THz pulse energy >500kV/cm delivered at sample damage of crystal at relatively low fluences
THz-pump/X-ray probe experiments at LCLS
Typical user experiments Typical user experiment: Most experiments require vacuum chamber, optics inside chamber necessary to achieve tight focus /x ray detector /sample THz pulse generated outside vacuum chamber THz pulse characterized on-target using electro-optical sampling (EOS) EOS also establishes spatial and temporal overlap of THz pulse with NIR probe Use NIR probe to find coarse temporal overlap with XUV Block probe and perform THz-pump/X-ray probe experiment 21
Examples 1. Temporal X-ray pulse characterization 2. Resonant Excitation of Phonons 3. Field induced modulation of magnetic order in TbMnO3 4. THz induced Insulator-metal Transition in VO2 22
Example 1:Temporal x-ray pulse characterization with THz fields x-ray pulse photoelectron spectrum X-ray generates photoelectons from a noble gas vector potential of THz pulse causes kinetic energy shift in photoelectron spectrum at zero crossing of vector potential, spectrum is broadened mapping between electron energy and time -> pulse duration measurement (analog to streak camera)
photoelectron streaking at LCLS LCLS experiment L417 AMO hutch Tilted pulse front in LiNbO 3 Measure x-ray pulse duration and time of arrival Requires ~500 fs streaking ramp Moderate to high electric fields (>200 kv/cm) A.Cavalieri, Univ. Hamburg
THz Spectral Shifting & Broadening Away from vector potential zero-crossing, streaking power (gradient) decreases widest feature occurs at zero-crossing, with negligible shift narrowest feature occurs at vector potential extrema, with maximum shift
Pulse duration measurement ~ 200 fs FWHM
Example 2: Phonon pumping - CMR Manganites La 1/2 Sr 3/2 MnO 4 (LSMO) shows complex phase diagram with orbital and magnetic order. Superlattice peaks can be probed with soft X-rays 650 ev M. Först et al., Physical Review B 84, 241104 (2011). 33
Phonon pumping of LSMO Mid-IR pump (15 micron/20 THz) is used to selectively target phonon mode coupled to this order TOPAS-HE and DFG in GaSe Different dynamics for orbital and magnetic order and very different behavior from 800 nm excitation. M. Först et al., Physical Review B 84, 241104 (2011). 34
Example 3: Few-cycle THz excitation and soft-x-ray probe of multiferroics
Excitation of multiferroics Excitation ti with few-cycle THz pulses Generated with DAST 2 mj 1.5 micron pump pulses with TOPAS HE Broadband spectrum centered at 2 THz Up to 350 kv/cm delivered at sample 36
TbMnO3 LCLS collaboration PSI: U.Staub (co-pi) S.-W. Huang J. Johnson C. Vicario Ch. Hauri S. Gruebel P. Beaud L. Patthey SLAC: M. Hoffmann S. de Jong J. Turner W. Schlotter G. Dakovski M. Minitti Spin structure response (resonant XRD) ETHZ: T. Kubacka L. Huber S. L. Johnson (co-pi) Stanford: W.-S. Lee R. G. Moore LBNL: Y. -D. Chuang Driving electric field Time (ps) Funding: NCCR MUST, ETHZ, SwissFEL
Example 4: THz-induced insulator metal transition in VO 2 VO2 undergoes insulator-metal transition at ~335K Can also be switched by optical laser pulses and by electrical gating Can we switch with THz pulses and what is the mechanism? 38
Actual Experimental Geometry Sample Temperatures: RT, 360K, 331K CSPAD Detector XAS/XES Photon Energies: 5470 5550 ev and 6500 ev 50 50 FEL Repetition Rate: 120 Hz FEL Incidence Angle: 49-50 THz LN source 15 uj Ge Analyzer Crystal CSPAD Detector XRD
Summary and Outlook Ultrafast X-ray experiments give valuable insights into dynamics of matter THz pulses can control materials with sub-bandgap excitation Laser based THz pump sources are important to enhance our possibilities to control complex systems Development need for narrowband tunable sources 5-15 THz Exciting times ahead with more FELs coming on-line 42
Thank you!