From quantum mechanics to auto-mechanics

1. Frontiers in Spectroscopy. Ohio State University, March 2004 Nonlinear Spectroscopy: From quantum mechanics to auto-mechanics Paul Ewart

2. Lecture Outline • Lecture 1: Linear and Nonlinear Optics Nonlinear spectroscopic techniques Lasers for nonlinear spectroscopy • Lecture 2: Basic theory of wave mixing Coherent signal generation Spectral simulation • Lecture 3: Spectroscopy and diagnostics High resolution spectroscopy Combustion diagnostics

3. Combustion Diagnostics • Measurement required of: • Flows (including 2 phases): Velocity, particle size etc. • Thermodynamic parameters: Temperature, Pressure, Density etc • Chemical properties: major and minor species, reaction rates etc • Measurement challenges: • High temperature and pressure • Steep temperature and density gradients • Non-invasive probes • Scattering and luminous environments • Restricted optical access • Low concentrations of key species – ppm. • Space and time resolution • etc. ! Nonlinear Solution: Laser Spectroscopic Techniques ^

4. Nonlinear spectroscopy • Coherent signal generation • Time and space resolution • Sensitive to trace quantities • High signal-to-noise ratio • Doppler-free spectral response • Species and state selective • Microscopic (molecular) and Macroscopic parameter measurements

5. Four Wave Mixing techniques

6. CARS Coherent Anti-Stokes Raman Scattering

7. Coherent Anti-Stokes Raman Scattering, CARS: Narrowband

8. Coherent Anti-Stokes Raman Scattering, CARS: Broadband or Multiplex Time resolved spectra and temperatures

9. DFWM Degenerate Four Wave Mixing

10. DFWM in oxy-acetylene flame

11. DFWM in flames: OH spectra

12. Doppler-free DFWM spectra of OH in methane/air flame

13. Boltzman plot for temperature determination

14. Multiplex DFWM spectroscopy in flames 1 3 2 • Broad laser spectrum overlaps • molecular resonances • 2. Broadband FWM spectrum • recorded on CCD camera • 3. Theoretical spectrum fitted to • find temperature. • C2 spectrum in oxy-acetylene flame

15. Broadband DFWM Spectroscopy of C2 in oxy-acetylene flame

16. LITGS: Laser Induced Thermal Gratings

17. Density Perturbation in LITGS • Acoustic gratings interfering… • Speed of sound Temperature (2) Temperature grating… Decay by diffusion Pressure

18. LITGS Laser induced Thermal Grating Spectroscopy of OH in high pressure flame • 5 nsec pulses at 308 nm • excite Thermal Grating in OH • cw Argon ion laser at 488 nm • probes Thermal Grating • Scattered LITGS signal records dynamics of grating up to 40 bar • Signal intensity limited by intensity of cw probe laser 1 Watt

19. LITGS in OH in high pressure CH4/air flame

20. Lasers for Nonlinear Spectroscopy Multiplex Spectroscopy • Broad, variable bandwidth • Frequency tunable • No mode structure High Resolution Spectroscopy • High power – pulsed • Narrow linewidth – Single longitudinal mode, SLM • Wide SLM tuneability ~nm • UV, visible and IR wavelengths

21. Fluctuation in relative intensity or phase of modes leads to fluctuation • In relative intensity of scattered molecular spectrum i.e. noise • Noise limits precision of fitting theoretical to experimental spectrum

22. Lasers and mode structure • Conventional lasers impose mode structure by standing wave resonator • Modeless laser uses travelling wave with no resonant cavity – hence no modes • Noise limited only by quantum fluctuations

23. Temperature measurement in firing si engine using broadband CARS with modeless laser • Pump laser: Frequency doubled • single mode Nd:YAG • Stokes laser: Modeless laser • Low noise gives precise fit to • theoretical CARS spectrum – • precision of 3 – 5% resolves • cycle-by-cycle variations

24. Multiplex CARS Spectroscopy of H2 in CVD Plasma using Modeless Laser Plasma off. Room Temperature, 300 K. Single-shot spectrum

25. Multiplex CARS Spectroscopy of H2 in CVD Plasmausing Modeless Laser Plasma on. Temperature, 2340 K. Single-shot spectrum S.D. ~ 7%

26. Candidate laser systems for high resolution nonlinear spectroscopy • Pulse amplified cw dye lasers • Pulsed Optical Parametric Oscillators • Diode seeded Alexandrite lasers • Diode seeded Modeless lasers

27. The Diode-Seeded Modeless Laser • No cavity mode matching required: robust and stable seeding. • Linewidth of diode laser (< 2MHz): output is transform limited. • Tuning determined by SLM diode laser: tuning range ~ 10nm. • Dye selected to suit diode output: 630 - 850 nm. • SIMPLE • NARROWBAND • WIDE TUNEABILITY • MODULAR DESIGN

28. High power DSML system

29. Experimental Area SLM Nd:YAG Pump Laser Seeded Modeless Laser

30. Spectrum of STL output Fabry-Perot interferogram Bandwidth of output: 165 MHz

31. The Diode-Seeded Modeless Laser, DSML • High power – pulsed • Narrow linewidth – Single longitudinal mode SLM • Wide SLM tunability • UV, visible and IR wavelengths 30 mJ, 5 ns pulse: 6MW SLM linewidth: 165 MHz, 0.006 cm-1 10 nm SLM tuning range 315 – 425 nm, 635 ± 5 nm (650 / 670 / 690 etc) 2.4 – 4.2 mm by DFG

32. High Resolution DFWM Spectroscopy In a low pressure flame Pressure broadening Power broadening OH A-X (0,0) system

33. DFWM:Experimental Layout

34. Low-Pressure Burner

35. DFWM Pressure Broadening in OH methane/oxygen flame