1. Laboratoire de Physique des Interfaces et des Couches Minces (LPICM) CNRS, Ecole Polytechnique 91128 Palaiseau France Gennaro Picardi Polarization properties of oblique incidence Tip enhanced Raman spectroscopy

2. Experimental set-up Raman (HORIBA JY)SPM (PSIA) Optical coupling Piezos x, y Piezo z Microscope Laser Grating Notch filter Detector Half-wave plate Analyzer Confocal Raman Feedback control Oblique back-scattering configuration Side illumination of the tip

3. STM Au tip etching 1-2 min Electrochemical etching of a gold wire (0.125 mm) in a solution of 1:1 ethanol and conc. HCl (37%). Applied Voltage: ~2.4 V 7-8 min Break of the tip (circuit closes) R tip = 20-30 nm L. Billot, L. Berguiga, M.L. de la Chapelle, Y. Gilbert and R. Bachelot; Eur. Phys. J. Appl. Phys. 31 (2005) 139 B. Pettinger, B. Ren, G. Picardi, R. Schuster and G. Ertl; Rev. Sci. Instrum. 75 (4) (2004) 837 Anodic oxidation (passivation) AuOH e-e- Au dissolution H +, 2 Cl - Disproportion Cl - ½ e -, Cl - AuCl 4 - AuCl 2 - + H 2 0 Au deposition H 2 0, Cl - Au*-Cl - Au*-H 2 0 Oscillatory electrodissolution of gold Passive region Active region Au electrodissolution in HCl is diffusion limited Z.L. Li,T.H. Wu, Z.J. Niu, W. Huang, H.D. Nie ; Electrochem. Comm. 6 (2004) 44 X. Wang, Z. Liu, H. Zhang, X. Wang, Z. Xie, D. Wu, B. Ren and Z. Tian; Appl. Phys. Lett. 91 (2007) 101105

4. Brilliant Cresyl Blue on Au (111) Au tip (oblique incidence) 585 cm -1 exc 633 nm 20 s D1 filter STM-TERS on dyes G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Europ. Phys. J. Appl. Phys. in press (2007)

5. TERS for structured materials nano-characterization W.X. Sun and Z.X. Shen Near-field scanning Raman microscopy using apertureless probes J. Raman spectrosc. 2003; 34: 668-676 Micro-Raman Spectroscopy provide mapping of stresses in Si structures ( well defined Raman shift due to strain ) 3700 nm Near-field Raman mapping at 521 cm -1 (  Si-Si ) (using aperture-less probe) 380 nm 300 nm

6. Introducing polarized TERS (p) - pol no analyzer analyzer at 90° « … polarization of the ligth scattered by the particle will differ from the polarization of the incident light. The light partly depolarized by the particle then inelastically scatters with the optical phonon in Si thereby producing allowed Raman signal. The allowed Si Raman signal should be associated with a local area around the particle. » V. Poborchii, T. Tada and T. Kanayama, Jpn. J. Appl. Phys. 44 (2005) (p) - pol

7. Half-wave plate Analyzer Sample incident light- tip interaction scattered light- tip interaction Incident light Scattered light Polarization control may become an important parameter in the TERS experiment I. The tip modifies the polarization state of the incident and scattered radiation. II. The far field signal can be reduced by using an analyzer. Polarization control in TERS

8. R j : Raman tensor of j-phonon e i : incident polarization state (polarizer P) e s : scattered polarization state (analyzer A) The scattered intensity depends on the polarization states e i, e s as well as on the sample orientation S Raman intensity Sample orientation S Analyzer Polarizer Calculation of the scattered intensity in the far field

9. Far field: experimental verification Calculation of the scattered intensity in the far field The sample orientation (or azimuth) S modulates the scattered intensity. (100) c-Si (111) c-Si P || A P ┴ A Sample orientation S (deg) Intensity (arb. un.)

10. The tip-enhancement tensor A describes the field enhancing and polarization properties of the tip a and b : tip-dependent TERS parameters The tip tensor A transforms the sample Raman tensor R to an effective scattering tensor R’ Far field (tip withdrawn): R (R : Raman polarizability tensor) Near field (light-tip intaction): R’ = A T R A (A : tip-enhancement tensor) Total field (tip in contact): Far field + Near field (the tip-enhancement tensor) Calculation of the scattered intensity in the near field R. Ossikovski, Q. Nguyen and G. Picardi, Phys. Rev. B 75, 045412 (2007)

11. NH 4 F etched Si (111) NC-AFM STM (b) Etch-pit initiation by dissolved oxygen on terraces of Si (111) C.P. Wade and C.E.D. Chidsey Appl. Phys. Lett. 71 (12) 1997 U t = -1.5 V I t = 50 pA Pt/Ir tip U t = -1.0 V I t = 100 pA Au tip 750 nm Optical microscope 10  m

12. TERS on Si(111) with polarization control (I) Raman intensity of the 1 st order Si phonon peak ( 521 cm -1 ) tip down tip up a : b = 1.6 : 1 Analyzer fixed at 90° a : b = 5.5 : 1  = 20°  = 71° ( s ) ( p ) ( s ) Tip #1Tip #2 G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Appl. Spectr. 61 (12), 2007

13. Oblique incidence: p - polarization or s - polarization ? (with p-pol ) (with s-pol ) Higher intensity with incident (p) polarization but strong TERS also with incident (s) light BCB on Au (111) No analyzer Tip #2 G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Appl. Spectr. 61 (12), 2007

14. Influence of the incident polarization in TERS ( I ) ( p ) – polarization (in-plane) of the incident electric field EFE up to 35 EFE up to 10 ( s ) – polarization (out-of-plane) of the incident electric field ‘ Understanding TERS ’ A. L. Demming, F. Festy and D. Richards J. Chem. Phys. 122, 1847 (2005) ‘ Finite Element simulations of TERS ’ A. Downes, D. Salter and A. Elfick J. Phys. Chem. B 110, 6692 (2006)

15. Influence of the incident polarization in TERS ( II ) 1. Overall higher field enhancement with p – polarized excitation, but field enhancement also with s – polarized ligth. Near-field Raman spectra of SWCNT measured under p- and s- polarization conditions. Polarization measuraments in TERS applied to SWCNT Y. Saito, H. Hayazawa, H. Kataura, T. Murakami, T. Tsukagoshi, Y. Inouye and S. Kawata Chem. Phys. Lett. 410, 136 (2005) 2 bis. 2. With p – polarized light illumination also enhancement of the field component in the substrate plane (i.e. out of the plane of incidence). With s – polarized light illumination also enhancement of the field component normal to the substrate plane (i.e. in the plane of incidence). Cross polarization effect (depolarized enhancement) 3. Imaging should be (?) different. S. Foteinopoulou, J. P. Vigneron and C. Vandenbem Opt. Expr. 15, 4253 (2007) ‘Under p - pol or s - pol light illumination the charge is differently concentrated.’

16. Far field artifacts when TERS probing bulk samples ‘ The (tip in) contact signal may include a component that is unrelated to the plasmon resonance enhancement due to reflection and scattering from the tip … leading to additional unlocalized Raman signal.’ ‘ … significant enhancement of Raman scattering from silicon substrates can be achieved without a field enhancement effect by plasmon resonces. Pure laser deflection and near field scattering cause similar effects which are difficult to distinguish.’ C. Georgi, M. Hecker and E. Zschech, Appl. Phys. Lett. 90, 171102 (2007). N. Lee, R. D. Hartschuh, D. Methani, …and A. P. Sokolov, J. Raman Spectrosc. 38, 789 (2007).

17. ACKNOWLEDGEMENTS Quang Nguyen Razvigor Ossikovski Bernard Drevillon (LPICM director)