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

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

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

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)

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

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

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

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

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

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

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

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

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

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

Raman scattering: a classical approach

Raman scattering: a classical approach

Raman scattering: a classical approach

Raman scattering: a classical approach

Raman scattering: a classical approach

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

Surface enhancement 2 1

Surface enhancement 2 1 E0

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

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

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

Surface enhancement + + - - + + - - - + + - - + - + - + - + E0

Surface enhancement

Surface enhancement

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

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

Femtosecond laser-structured substrates

Femtosecond laser-structured substrates From: M. Shen, C.H. Crouch, J.E. Carey, and E. Mazur, Appl. Phys. Lett., 85, 5694-5696 (2004)

Femtosecond laser-structured substrates

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

Experimental procedure Benzenethiol self-assembled monolayer (SAM)

Experimental procedure 20x, 0.25NA Objective

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

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

Film thickness dependence

Excitation profile

Signal uniformity

Signal uniformity

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

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.

Few molecule spectroscopy

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

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

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.

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.

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

Experimental procedure 20x, 0.25NA Objective

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

Few molecule spectroscopy Contaminants present on surfaces after fabrication

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