1. Eric Diebold and Eric Mazur SEAS, Harvard University
Femtosecond laser-nanostructured substrates for surface-enhanced Raman scattering (SERS)
Eric Diebold and Eric Mazur
SEAS, Harvard University

2. Motivation Raman spectroscopy has applications in: Pharmaceuticals
Homeland Security
Forensics
Medical diagnostics
Analytical chemistry

3. However, Raman scattering cross sections are very small (~10-30 cm2)
Background
However, Raman scattering cross sections are very small (~10-30 cm2)

4. However, Raman scattering cross sections are very small (~10-30 cm2)
Background
However, Raman scattering cross sections are very small (~10-30 cm2)
Trace detection using Raman spectroscopy is insensitive; fluorescence is much preferred for its larger cross sections (~10-16 cm2)

5. Background
SERS promises to enable the use of Raman spectroscopy in a wide variety of new applications

6. Background
SERS promises to enable the use of Raman spectroscopy in a wide variety of new applications
However, a current dearth of inexpensive, reliable, high performance substrates is limiting the application of SERS

7. Outline Raman scattering Surface enhancement
Femtosecond laser-structured substrates
Experimental results
Conclusions

8. Outline Raman scattering Surface enhancement
Femtosecond laser-structured substrates
Experimental results
Conclusions

9. Raman scattering } Electronic states } Virtual states
Energy
} Vibrational states
Ground state

10. Raman scattering } Electronic states } Virtual states
Energy
} Vibrational states
Ground state
Stokes Raman

11. Raman scattering } Electronic states } Virtual states
Energy
} Vibrational states
Ground state
Stokes Raman

12. Raman scattering } Electronic states } Virtual states
Energy
} Vibrational states
Ground state
Stokes Raman
Anti-Stokes Raman

13. Raman scattering } Electronic states } Virtual states
Energy
} Vibrational states
Ground state
Stokes Raman
Anti-Stokes Raman

14. Raman scattering } Electronic states } Virtual states
Energy
} Vibrational states
Ground state
Stokes Raman
Anti-Stokes Raman

15. Raman scattering: a classical approach

16. Raman scattering: a classical approach

17. Raman scattering: a classical approach

18. Raman scattering: a classical approach

19. Raman scattering: a classical approach

20. Outline Surface enhancement Raman scattering
Femtosecond laser-structured substrates
Experimental results
Conclusions

21. Surface enhancement 2 1

22. Surface enhancement 2 1 E0

23. Surface enhancement 2 1 E0+ -
Near-field scattered electric field enhances polarization of molecules located near surface 2 1 E0

24. Surface enhancement + -
Near-field scattered electric field enhances polarization of molecules located near surface
Field from molecular polarization generates polarization of surface at Raman frequency E0

25. Surface enhancement + -
Near-field scattered electric field enhances polarization of molecules located near surface
Field from molecular polarization generates polarization of surface at Raman frequency
Surface polarization radiates Raman field into far field E0

26. Surface enhancement + + - - + + - - - + + - - + - + - + - + E0

27. Surface enhancement

28. Surface enhancement

29. Outline Raman scattering Surface enhancement
Femtosecond laser-structured substrates
Experimental results
Conclusions

30. Femtosecond laser-structured substrates
SHG crystal
ti:sapphire laser
 = 800 nm
300 J, 1 kHz, 100 fs
f = 25cm
xy translation
stages

31. Femtosecond laser-structured substrates

32. Femtosecond laser-structured substrates
From: M. Shen, C.H. Crouch, J.E. Carey, and E. Mazur, Appl. Phys. Lett., 85, (2004)

33. Femtosecond laser-structured substrates

34. Outline Raman scattering Surface enhancement
Femtosecond laser-structured substrates
Experimental results
Conclusions

35. Experimental procedure
Benzenethiol self-assembled monolayer (SAM)

36. Experimental procedure
20x, 0.25NA Objective

37. Experimental procedure
neat benzenethiol
20x, 0.25NA Objective
coverslip
500m spacer
glass slide

41. Enhancement factor calculation
EF (1000 cm-1 band)
EF (1572 cm-1 band)

42. Film thickness dependence

43. Excitation profile

44. Signal uniformity

45. Signal uniformity

46. Few molecule spectroscopy
Blinking in SERS spectrum: thermal diffusion of molecules in and out of “hot spots”.

47. Few molecule spectroscopy
Blinking in SERS spectrum: thermal diffusion of molecules in and out of “hot spots”.
Time resolved spectroscopy shows blinking effect of molecules on the substrate.

48. Few molecule spectroscopy

50. Outline Raman scattering Surface enhancement
Femtosecond laser-structured substrates
Experimental results
Conclusions

51. Conclusions
Femtosecond laser-nanostructured SERS substrates are easy, cheap to produce.

52. Conclusions
Femtosecond laser-nanostructured SERS substrates are easy, cheap to produce.
We have demonstrated SERS from femtosecond laser-nanostructured substrates; - enhancement factor, signal uniformity of substrates are very competitive in field.

53. Conclusions
Femtosecond laser-nanostructured SERS substrates are easy, cheap to produce.
We have demonstrated SERS from femtosecond laser-nanostructured substrates; - enhancement factor, signal uniformity of substrates are very competitive in field.
Quantitative single molecule spectroscopy should be possible using substrates.

55. Acknowledgements Mazur Group X. Sunney Xie (Harvard)
Nathan Mack (LANL)
Stephen Doorn (LANL)
NDSEG Fellowship
Army Research Office
Horiba Jobin-Yvon

58. Experimental procedure
20x, 0.25NA Objective

60. Few molecule spectroscopy
Contaminants present on surfaces after fabrication can be removed by 1mM HCl wash

61. Few molecule spectroscopy
Contaminants present on surfaces after fabrication

62. Experimental procedure
neat benzenethiol
20x, 0.25NA Objective
coverslip
500m spacer
glass slide