Time-resolved spectroscopy

Transcription

1 Time-resolved spectroscopy Chih-Wei Luo ( 羅志偉 ) Department of Electrophysics, National Chiao Tung University, Taiwan Ultrafast Dynamics Lab

2 Outline 1. Introduction of pulses. Spectroscopic methods for studying ultrafast dynamics 3. Some examples in time-resolved spectroscopy

3 Introduction of pulses What is the ultrashort pulse? ~10-6 s ~10-9 s ~10-1 s ~10-15 s

4 Introduction of pulses The shortest laser pulse fs Opt. Lett. 1, 483 (1987) fs Opt. Lett., 10 (1997) 4.5 fs Opt. Lett., 5 (1997) 3.9 fs Opt. Lett. 7, 306 (00) Baltuška, Fuji, Kobayashi 3.8 fs Phys. Rev. Lett. 88, (00) as Nature 47, 817 (004) as Science 314, 443 (006) as Science 30, 1614 (008) as Opt. Lett. 37, 3891 (013) Visible extreme ultraviolet Shorter pulse with shorter wavelength!!

5 Introduction of pulses Timescales 10 fs light pulse Computer clock cycle Camera flash 1 minute One month Age of pyramids Human existence Age of universe femtosecond 1 picosecond Time (seconds) a pulse : 1 minute ~ 1 minute : age of universe

6 Introduction of pulses Ultrafast camera!! femtosecond laser

7 Introduction of pulses The possibility for nuclear fusion! Short pulse = intense peak power 100 mj, 100 fs = 1 TW φ = 10 μm (10 10 V/cm) Pump Verdi Long pulse, high energy Pump Evolution Seed Mira Short pulse, low energy Amplifier Legend Short pulse, high energy

8 Introduction of pulses USA National Ignition Facility Output power ~ 300 TW

9 Introduction of pulses

10 Introduction of pulses Free electron laser - Japan

11 Outline 1. Introduction of pulses. Spectroscopic methods for studying ultrafast dynamics 3. Some examples in time-resolved spectroscopy

12 Section Outline.1 Pump-probe methods General principle Time-resolved absorption in the UV-visible range Time-resolved absorption in the IR range. Time-resolved Emission spectroscopy: Electronic methods Broad-bandwidth photodetectors The streak camera Single-photon counting.3 Time-resolved Emission spectroscopy: Optical methods The Kerr shutter Up-conversion method

13 -1 Pump-probe methods General Principles

14 -1 Pump-probe methods General Principles Space time Who is the first one to use this idea? a(t) n(t) pump pulses 13 ns time probe pulses t (delay) time

15 -1 Pump-probe methods Typical pump-probe system Ar+ laser, all lines Ti:sapphire laser 75MHz Chamber λ/ P AOM Delay stage AOM prism prism Monitor CCD P λ/ M RF F D PD Lock-in amp. Computer

16 -1 Pump-probe methods Time-resolved absorption in the UV-visible spectrum Beer-Lambert law I εν N ( t )l ( ν, Δt) = I ( ν) 0 10 Where ε ν is the absorption coefficient of the sample at frequency ν, N(Δt) is the population absorbing at time t at frequency ν, l is the length of sample excited. The measured optical density (OD) N ( t) = ( Δt) I ( ν) ( ν, Δt) 0 OD ν, = log = εν N( t)l I N 0 ln OD Δ e t - τ ( t) = ln N ( 0) ε ν Δt l τ

17 -1 Pump-probe methods Time-resolved absorption in the UV-visible spectrum Detection systems with lock-in technique (SNR~10 6 ) n(t) pump pulses time Typical noise spectrum probe pulses 13 ns time t (delay) AO 87KHz at pump pulses probe pulses (from sample) I 0 (t) ΔI (t) 0.01 ms time detector & lock-in 87KHz ΔI(t) / I 0 (t) = ΔR(t) / R(t) n(t) R R pump closed I = I pump pump R open closed r I r I ( I ) pump ( I ) pump r r open r closed = R R i I i I I = = i ( I ) pump r I 0 pump open pump closed pump closed closed Where ( I ) pump ( ) pump i = I i close and ( I r ) pump I 0 closed open

18 -1 Pump-probe methods Time-resolved absorption in the UV-visible spectrum Experimental tricks to avoid artifacts Polarization of the pump and probe (1) For liquid (isotropic) probe pump Pump//probe relaxation of transition moment Pump probe relaxation of transition moment + reorientation of transition moment () For solid materials Pump//probe larger coherent peak during the pulse duration. Pump probe smaller coherent peak during the pulse duration. This coherent effect is more serious for shorter pulse, smaller angle between pump and probe beam. pump probe pump probe

19 -1 Pump-probe methods Time-resolved absorption in the UV-visible spectrum Experimental tricks to avoid artifacts Probe and GVD the zero delay for each wavelength is different. lose time resolution a(t) n(t) pump pulses 13 ns time probe pulses Zero delay time

20 - Time-resolved emission spectroscopy: electronic methods Broad-bandwidth photodetectors Time resolution ~ a few ps, limited by the bandwidth of the electronic system. The sensitivity of these system is limited. The streak camera Time resolution ~ sub-ps. The main limitation is the dynamics range of single-shot measurement. Obtain the emission spectrum simultaneously with the associated dynamics at each wavelength.

21 - Time-resolved emission spectroscopy: electronic methods Single-photon counting For high-repetition-rate laser source. Time resolution is limited by the pulse duration and by the response function of the electronic devices. An accurate measurement of the response function allows sophisticated deconvolution procedures to reach time resolutions of the order of ten ps.

22 -3 Time-resolved emission spectroscopy: optical methods The Kerr shutter Kerr cell: the ability of isotropic materials (CS or glass) to become anisotropic under the action of an applied electric field (optical Kerr effect). Time resolution: depends on the opening pulse duration and on the relaxation time of the anisotropy. Experimental tricks (1) Respective polarizations of probe and pump pulses (45 ). () Leakage through polarizers P1 and P. (3) Spectral dispersion of the transmission function. (4) Parasitic light from the opening beam. (5) Spectral limitation. (6) Time resolution. (7) The excited volume should be as small as possible to avoid spatial dispersion. (8) The sample and Kerr cell should be as thin as possible and one should reduce the aperture of optical collection.

23 -3 Time-resolved emission spectroscopy: optical methods Up-conversion methods Is well suited to low-energy laser pulses with a high repetition rate. The spectral range of this detection technique is wider in the infrared region than that of photocathodes. Time resolution: <100 fs

24 -3 Time-resolved emission spectroscopy: optical methods Up-conversion methods Frequency mixing in the nonlinear crystal Phase-matching conditions ω Σ = ωs + ωl and Σ k = k + k S L 1 λ Σ 1 1 = + λ λ S L n eff ( θ, λ ) n ( λ ) n ( λ ) λ Σ Σ = o λ S S + o λ L L Calculate the phase-matching angle θ between the propagation direction and the optical axis (n e <n o ) n θ 1 λσ eff sin θ cos θ = + n e n o ( θ ) λ ( ) ( ) Σ λσ cos λ Σ λ Σ ( n ) ( ) ( ) ( ) o neff λ Σ λσ ne no 1 = λ L n o λ n Σ e K L Optical axis θ K Σ λ S n o K S λσ n o

25 Outline 1. Introduction of pulses. Spectroscopic methods for studying ultrafast dynamics 3. Some examples in time-resolved spectroscopy

26 Section Outline 3.1 Optical pump-probe 3. Optical pump Mid-IR probe 3.3 Optical pump X-ray probe 物理雙月刊,010 六月號

27 3-1 Optical pump-probe Electron-phonon coupling in metals Standard scattering rate formulas - Two temperature model dt 3 R = a T e e + b T Ce( Te ) = - λ ω ( Te-T ) dt πkb dt 3 C = λ ω ( Te-T ) dt πk B C e is the electronic specific heat, C l is the bosonic specific heat, λ is the electron-boson coupling constant ω is the second moment of the boson spectrum At high temperature τ = πk B T 3 λ ω P. B. Allen, Phys. Rev. Lett. 59, 1460 (1987). S. D. Brorson, et al., Phys. Rev. Lett. 64, 17 (1990).

28 3-1 Optical pump-probe The explanation for the sign change in ΔR/R from the energy band point of view. smearing effect Copper (Cu) G. L. Eesley, Phys. Rev. Lett. 51, 140 (1983).

29 3-1 Optical pump-probe High-T c superconductor YBa Cu 3 O 7 Identify how many relaxation processes occur by the slope in semilogarithmic scale. C. W. Luo, Dissertation, National Chiao Tung University, Taiwan (003).

30 3-1 Optical pump-probe Multiferroics HoMnO 3 R/R (arb. units) T=90K T=0K T=180K T=140K T=100K T=80K T=71K T=67K T=60K R/R (arb.units) d-d excitation by photon relaxation Oscillation 1 λ = 800 nm T = 170 K Delay Time (ps) Delay time (ps) H. C. Shih, T. H. Lin, C. W. Luo, et al., PRB 80, 0447 (009)

31 3-1 Optical pump-probe Multiferroics HoMnO 3 Charge transfer from e g to a 1g by pump pulses Normalized amplitude of R/R R/R T=90 K Delay time (ps) T 0 =140 K 815nm Mn 3 + Pump energy 3d levels Room temperature T=90K T=140K Pump energy :1.5 ev Low temperature Pump energy d d 3z r ( x y ),( xy) E Temperature (K) Observed the blueshift of energy gap! Woo Seok Choi, et al PRB 78, (008)

32 3-1 Optical pump-probe Multiferroics HoMnO 3 Charge transfer from e g to a 1g by pump pulses Normalized amplitude of R/R T 0 =117 K 815nm 800nm Mn 3 + 3d levels Room temperature Pump energy T=90K T=140K T=117K Pump energy :1.55 ev Low temperature Pump energy d d 3z r ( x y ),( xy) E Temperature (K) Observed the blueshift of energy gap!

33 3-1 Optical pump-probe Multiferroics HoMnO 3 Normalized amplitude of R/R Energy gap E dd (ev) Temperature (K) T AFM Temperature (K) Slope nm 800nm 785nm 770nm 755nm 740nm Temperature (K) Mn 3 + 3d Room temperature χ (emu/oe) Pump energy 1.x x x x x10-6.0x10-6 levels T=90K T=63K T Ho ZFC 100 Oe H//c-axis T SR Pump energy :1.68 ev Low temperature Pump energy d E d z r ( x /χ (Oe/emu) Temperature (K) Curie-Weiss Law Temperature (K) y ),( xy)

34 3-1 Optical pump-probe Multiferroics HoMnO 3 Normalized amplitude of R/R Energy gap E dd (ev) nm 800nm 785nm 770nm 755nm 740nm Temperature (K) T AFM Temperature (K) Slope Temperature (K) Mn 3 + 3d Room temperature Pump energy levels T=90K T=63K T=63K Pump energy :1.68 ev Low temperature Pump energy Pump energy d d d 3z r ( x ( x y ),( xy) y ),( xy) Extra-blueshift comes from AFM ordering!! E

35 3-1 Optical pump-probe Multiferroics HoMnO 3 Demagnetization dynamics τ m τ m T=90K T=180K T=75K R/R (arb. units) 75K Delay time (ps) 90K 180K Delay time (ps) T e T l T s 800nm 785nm 770nm 755nm 740nm Temperature (K) τ m

36 3-1 Optical pump-probe Multiferroics HoMnO 3 Oscillation component Strain Pulse Model τ osc ( λprobe / υ sound n sin θ )

37 3-1 Optical pump-probe Multiferroics HoMnO 3 Oscillation component H. C. Shih, T. H. Lin, C. W. Luo, et al., New J. Phys. 13, (011)

38 3-1 Optical pump-probe Topological insulator Bi Se Bi Se 3 (Bridgeman) R/R [x10-4 ] (arb. units) Sample: Bi Se 3 R/R [x10-3 ] (arb. units) R/R [x10-5 ] (arb. units) Fit curve Delay time (ps) Delay time (ps) Delay time (ps) Intensity [x10-14 ] (arb. units) 1 0 FFT Wavenumber (cm -1 )

39 3-1 Optical pump-probe Topological insulator

40 3-1 Optical pump-probe Topological insulator Bi Bi Se Bi Se 3 A 1g 1 A 1g 1 Time delay (ps) V. Chis et al., Phys. Rev. B 86, (01). M. Hase, et al., Appl. Phys. Lett (1996)

41 3-1 Optical pump-probe Topological insulator Bi Bi Se Bi Se 3 A 1g 1 QL chain Å A 1g 1 A 1g 1 ΔL ~ 0.38% Δω=-4.11 cm Å.987 Å Δω=f(ΔL) ΔL ~ -.6% Δω=13.03 cm -1 Time delay (ps) Bi-Bi bond Å H. Lind, et al., Phys. Rev. B 7, (005)

42 3- Optical pump Mid-IR probe Topological insulator Bi Se 3 Bi Se 3 #3

43 3- Optical pump Mid-IR probe Topological insulator Bi Se 3 Spectral density (morm.) Wavelength (μm) (a) Photon energy (mev) Central Wavelength Pump Beam Spot Size (FWHM) 485 μm Probe Beam 800 nm 8 ~ 14 μm 39 μm Pulse Width ~ 100 fs ~ 500 fs

44 3- Optical pump Mid-IR probe Topological insulator Bi Se 3 Estimate the shift of absorption peak by First moment = We can obtain the energy loss rate near Dirac point is ~ 1 mev/ps. ( R / R) E photon ( R / R) de de photon photon Energy loss rate = 15 (mev) / (ps)

45 3- Optical pump Mid-IR probe Topological insulator Bi Se 3 Energy- and time-resolved pump probe spectroscopy

46 3-3 Optical pump X-ray probe Coherent Femtosecond Motion in Laser-Excited Bismuth Bismuth unit cell (Peierls distorted) Optical Pump X-ray probe setup S. L. Johnson, et al., Phys. Rev. Lett. 100, (008)

47 3-3 Optical pump X-ray probe Coherent Femtosecond Motion in Laser-Excited Bismuth Dependence of the dynamics of the diffracted intensity on absorbed laser fluence with α=0.45 : 0.56 mj/cm (blue squares), 1.10 mj/cm (red circles), and.4 mj/cm (purple triangles). X-ray: 7.1 kev Time resolution: ~195 fs S. L. Johnson, et al., Phys. Rev. Lett. 100, (008)

48 3-3 Optical pump X-ray probe Ultrafast inter-ionic charge transfer of transition-metal complexes [Fe(bpy) 3 ] + (PF 6- ) B. Freyer, et al., J. Chem. Phys. 138, (013)

49 3-3 Optical pump X-ray probe Ultrafast inter-ionic charge transfer of transition-metal complexes charge redistribution B. Freyer, et al., J. Chem. Phys. 138, (013)