1. Introduction to Laser Spectroscopic Techniques for Condensed Matter

2. Coherent Radiation Sources for Small Laboratories CW: Tunability: IR  Visible Linewidth: 1 Hz Intensity:  W  10W Pulsed: Tunabality: THz  Soft X-ray Pulse width: ns, ps, fs Linewidth: Transform limited Intensity: up to TW for fs pulses Quasi-cw: ~100 MHz mode-locked pulses Tunability: IR  Visible Frequency Comb: Periodic fs pulses Spectral components with~100 MHz spacing and <KHz linewidth Stable carrier/envelope phase relation

3. Light Scattering 22 11

4. 11 11 22 11 Raman ScatteringRayleigh Scattering

5. Estimates of Signal Strength For a 1-mW beam focused to a spot of 10x10x10  m 3,

6. Luminescence/Fluorescence Luminescence >> Rayleigh Scattering (Single Molecule Detection)

7. Fluorescence Quenching M Excitation energy transferred to neighbors through interactions Fluorescence quenching of molecules on metals Flourescence resonance energy transfer (FRET) between molecules

8. Labeling Spectroscopy Elimination or reduction of inhomogeneous broadening Selective detection or probe of species in an ensemble Studies of interactions and energy transfer between neighbors Examples: Carbon nanotubes Semiconductor nanoparticles

9. Resonant Light Scattering & Hot Luminescence 22 11 Resonant enhancement of scattering:

10. Excitation Spectrum Resonant enhancement in yields the excitation spectrum.

11. Spectral Hole Burning  S hole width ~ homogeneous linewidth = 1/dephasing time hole recovery time ~ population relaxation time hole broadening with time ~ excitation transfer to neighbors (spectral diffusion)

12. Multi-Resonance Spectroscopy (Multi-Dimensional Spectroscopy)  20  10  es  det  ex  det  10  20  10  20  10  20 Transient multi-dimensional spectroscopy yields dynamic information about coupling and correlation.

13. Coherent Transient Multi-Resonance Spectroscopy  10  20  10 t S 11 11 22 22 Each pulse establishes coherent mixing of states: Pulse a sets up coherence between 0 and 1; pulse b enhances or reduces the coherence; pulse c sets up coherence between 0 and 2 following population change. Fourier transform of S(t, t a, t b, t c ) yields 3D spectrum: Fourier transform of S(t, ,T) yields 2D spectrum S(  t,  ,T) reflecting population transfer after time T. 00 2 2 1 1

14. Example: Photosynthesis with Bacteriochlorophyll Protein

15. Multi-Photon Excitation 22 11 Multi-photon absorption Multi-photon-excited Luminescence

16. Hyper-Light Scattering 11 11 1’1’ 1’1’ 22 22

17. Stimulated Raman Spectroscopy 22 11

18. Estimate of Detection Limit Consider a 100-fs input pulse at  1 with 0.1  J/pulse, focused to a 10  m spot: I ~ 10 12 W/cm 2 as compared to typical

19. Experimentals fsec Laser Continuum Generation Spectral Filtering Broadband Probe Narrowband Pump Sample Detector pump probe McCamant et al, Rev. Sci. Inst. 75, 4971(2004)

20. Probing Vibrations in Excited Electronic Configurations

21. Merits: Background elimination Fluorescence rejection (making resonant Raman studies possible) Fast data collection (pump intensity, ~1 GW/cm 2 ; path length, ~3 mm; single-pulse SNR ~ 2) Ultrafast time resolution (~ 100 fs) Reasonable spectral resolution (< 10 cm -1 ) Capable of probing transient vibrational modes in the excited electronic configurations

22. Example: Light-Induced cis- trans Isomerization of Retinal in Rhodopsin Kukura et al, Science 310, 1006 (2005)

23. Stimulated Raman Scattering as a Third-Order NLO Effect Stokes output interferes with Stokes input

24. Coherent Anti-Stokes Raman Spectroscopy (CARS) Anti-Stokes Output

25. Four-Wave Mixing Spectroscopy Independent Variables: Frequencies, wave vectors, polarizations, intensities Capable of coherent excitations of three sequential resonances

26. Quantum Beats (resulting from pulse excitation) Fourier transform  Spectrum

27. Impulsive Excitations 100-fs pulse  ~ 300 cm -1 spectral width fs pulse sets up coherent excitations in a set of excited states. Signal appears as oscillation on the fs time scale. Fourier transform of the time-dependent signal yields the spectrum.

28. Time-Resolved Impulsive Raman Spectroscopy

29. Weiner et al JOSA B8, 1264 (1991) Impulsive Stimulated Raman Spectroscopy: Perylene Crystal

30. Time-Resolved Impulsive Spectroscopy Molecular vibrations Phonons Plasmons Phonon-plasmons Magnons (spin waves) Other elementary excitations

31. Impulsive Spin Excitations in DyFeO 3 Detection by Faraday Rotation Kimel et al, Nature 435, 655(2005)

32. Optical field induces magnetization?

33. Optical Rectification & Opical-Field-Induced Magnetization Time-averaged Free Energy: Induced dc polarization: Induced dc magnetization:

34. Optical-Field-Induced Magnetization H eff ~ 10 4 gauss (fs pulse of magnetic field)

35. |+m’> |-m’> |-m> |+m> Physical Interpretation (AC Stark Effect) 00  m  m’

36. Optical Rectification THz Generation and Detection Radiation efficiency depends on frequency Pulse distortion fs pulse THz field E(t) fs pulse EO output EO output detects E(t)

37. Example: THz pulse generation from ZnTe Nahata et al, Appl. Phys. Lett. 69, 14(1996)

39. Hertz = Hurts

40. THz Spectroscopy E in (t) E T (t) Fourier Transform gives

41. Shan et al, PRL 90, 247401 (2003) Example: THz spectral response of sapphir

42. Optical Modification of Materials Resonant excitation Nonresonant field effect Coherent optical modification  <1| <2| <1+| <1  | <2  | <2+|

43. Example: coherent control of two-photon absorption 11 22 mm <1| <2| <3| <d| <1| 11 22 <3| <2  | <2+|

44. Ultrafast X-Ray Diffraction ( Melting of a Crystal Lattice) Delayed Probe Pulse Diffracted Pulse pump

45. Other Topics Second-order nonlinear optics processes to probe ferroelectrics. SHG and SFG surface spectroscopy Nonlinear optical microsopy Nonlinear optical probing of nanostructures

46. Feedback Control can yield  < 10 -3 rad. Frequency Comb

47. Characteristics of Frequency Comb Well defined optical field: E(t)=A(t)cos  t Femtosecond periodic pulses: A(t)=A(t+T) Thousands of equally spaced spectral lines with KHz linewidth