THz field strength larger than MV/cm generated in organic crystal

Transcription

1 SwissFEL Wir schaffen Wissen heute für morgen 1 2 C. Vicario 1, R. Clemens 1 and C. P. Hauri 1,2 THz field strength larger than MV/cm generated in organic crystal 10/16/12 Workshop on High Field THz science Pecs Hungary

2 SwissFEL Xray free electron laser project THz source 700 meters Parameters Hard X-ray Soft X-ray Electron beam energy 5.8 GeV 2.2 GeV Wavelength 1-7 Å 7-70 Å FEL pulse energy 0.15 mj 0.20 mj FEL peak power 2.8 GW 5.5 GW Pulse length (rms) 21 fs 20 fs Beam radius 26.1 µm 77.9 µm Seite 2

3 THz streak camera o Requirements: 10 fs resolution for free electron laser X-ray pulses generated (λ=1-70 Å, Δt<50 fs, E=150 μj) o Desired characteristics of the diagnostics o Shot-to-shot reconstruction of the Xray temporal characteristics o Not destructive for the X-ray o Arrival time monitor o THz source parameters o 1-5THz, Electric field: <0.5 MV/cm, Power: 0.3GW/cm 2, energy: 2 μj Experiment THz field THz field energy time FEL Nature Photonics 3, 523 (2009) Seite 3

4 Preparation THz pump X-ray probe experiment Synchronized THz pump to X-ray probe Reliable and versatile THz source (intensity, high-field, good focus, temporal & spectral properties) Applications O C THz E-field pulse Transition metal CHIMIA 65, No. 5, 323 (2011) o Initiation chemical reactions o ultrafast magnetic interaction More details at: o superconductivity o spinwave o multiferroic materials o charge density wave o magnetic switch o biochemistry Seite 4

5 Outlines o THz generation by optical rectification in organic nonlinear crystals o THz emitted by different organic salt crystals o DAST o OH1 o DSTMS o Multi-octave THz pulse covering the THz gap o Intense THz field inducing magnetization ultrafast dynamic o Conclusions Seite 5

6 Organic crystal for THz generation Highest THz generation efficiency for optical rectification Organic crystals: 1% IR to THz conversion efficiency Velocity matched THz generation Collinear pump and THz output Intrinsically CEP stable and controllable linear polarization Crystals larger than 1 cm can be now grown Organic material Low pass filter Crystal material differ in THz absorption IR pump IR +THz THz Seite 6

7 Crystal properties comparison Pump wavelength [µm] Peak absorption location [THz] Peak absorption strength [1/mm] OH1 DAST DSTMS Nonlinear suscept. [pm/v] OH [1] 36 [1] 570 [1] DAST [2] 45 [6] 490 [5] DSTMS [3] 23 [3] 490 LiNbO 3 (@T=300K) [4] GaP [1] Opt. Express, 16, , (2008) [2] J. Opt. Soc. Am. B, OSA, 23, , (2006) [3] J. Opt. Soc. Am. B, 25, ,(2008) [4] J. Opt. Soc. Am. B, 25, B6-B19, (2008) [5] Opt. Express, 14, , (2006) [6] Opt. Lett., 25, , (2000) Seite 7

8 Experimental setup High power IR pump from fs OPA (1.3 and1.5 µm) Generation in dry air Electro optical sampling with balanced detection Probe pulse => 0.1 mm GaP linear up to 5 THz Spot size measurement => knife edge/ pyroelectric 2D array Energy per pulse => Golay cell thermopile Pump 1.5 μm Seite 8

9 THz generation in DAST Parameters Crystal thickness: 0.18 mm Free aperture: 5 mm Pump energy: 850 µj Results Peak field: 940 kv/cm Central frequency: 2.5 THz Multioctave spanning spectrum EOS field trace Spectrum Hauri Appl. Phys. Lett. 99, (2011) 9

10 THz generation in DAST Results II THz pulse energy up to 45 uj (rms 1%) IR THz energy conversion 2% Tight focus FWHM 550 um Control of the field polarity FWHM 550 μm 10

11 Optical rectification in OH1 L=0.4 mm D=3 mm E p =0.4 MV/cm Frequencies <3 THz Efficiency <1.5 Energy 10 μj THz focus waist: 1 mm Damage > 200 GW/cm² Ruchert Opt. Lett. 37, 5 (2012) 11

12 Optical rectification in DSTMS EOS field trace Spectrum Crystal: 9 x 0.5 mm Peak field: 1.5 MV/cm Multioctave spanning spectrum ν 0 =2.6 THz THz focus waist: 0.3 mm Diffraction limit: 0.27 mm Energy per pulse 15 μj 12

13 Broadband THz generation and detection THz generation in OH1 120 μm thick Low pass filter based on TOPAS polymer + C nanoparticles Pulse length measurement and spectral reconstruction with THz interferometer (avoiding of the GaP spectral distortion and IR probe pulse length) Spectrum covering the THz gap between THz Amplitude [a.u.] Temporal time [ps] 13

14 Application of THz source THz-driven ultrafast magnetization dynamic in Cobalt film 14

15 Laser/THz driven ultrafast magnetization history Tc(Ni)=628 K Beaurepaire PRL (1996) 15

16 Three temperature model: laser vs THz C e,s,l : specific heat G e,s,l: interaction constant Beaurepaire PRL (1996) hν THz 0.008eV (λ 150 µm) 0.8 mj/cm mj/cm 2 T>T c : Phase transition T<T c : No phase transition 16

17 Experimental setup A Co sample Δφ(t) +H ext -H ext t Δτ B Ruchert et al, submitted 10/16/12 Workshop on High Field THz science Pecs Hungary 17

18 Experimental setup A Co sample Δφ(t) +H ext -H ext t Δτ B Ruchert et al, submitted 10/16/12 Workshop on High Field THz science Pecs Hungary 18

19 THz-induced magnetization dynamics THz field [T] MO polarization rotation [mrad] 19

20 Coherent magnetization dynamics B ext! Damping! M! Landau Lifshitz Gilbert (LLG) equation dm = γ ( M B ) dt ext γα M M M 0 ( ( B )) ext 20

21 Numerical results d M dt ( B ext ) γα M 0 = γ M M M ( B ext ) ( ) P. M. Derlet 21

22 Conclusions Organic crystals are well suited compact and efficient source E p =1.5 MV/cm, 45 μj per pulse and 2% conversion efficiency Diffraction limited focus Control of polarization (linear) Broadband spectrum covers the THz gap Magnetization dynamics in Co film induced by THz Sub-cycle magnetization and demagnetization THz stimulus imprinted in the magnetic response THz phase-sensitive response Off-resonant process No heat deposition/no phase transition Results approximated by LLG simplified model Test bed for more accurate models 22