1 First Comparison of Electric Field Induced Second Harmonic of NIR Femtosecond Laser Pulses in Reflection and Transmission Generated from Si/SiO 2 Interfaces of a Silicon Membrane G.P. Nyamuda 1,* , E.G. Rohwer 1 , C.M. Steenkamp 1 , and H. Stafast 2,3 1 Laser Research Institute, Physics Department, Stellenbosch University, P. Bag X1, Matieland 7602, South Africa 2 Institute of Photonic Technology, P. O. Box 100239, 07702 Jena, Germany 3 Faculty of Physics and Astronomy, University of Jena, Max-Wien-Platz 1, 07743 Jena, Germany For the first time electric field induced second harmonic (EFISH) generation of femtosecond (fs) laser pulses (λ = 800 nm, τ = 75 ± 5 fs, rep. rate = 80 MHz, E pulse ≤ 10 nJ) is observed in transmission through a thin free-standing silicon (Si) membrane of 10 μm thickness and compared to the well-known EFISH results in reflection by use of the z-scan technique. EFISH in reflection and transmission unequivocally originate from the front and rear Si/SiO 2 interfaces, respectively, with SiO 2 being the natural oxide on the Si surfaces. Frequency conversion is enhanced by photoinduced electric fields across the Si/SiO 2 interfaces caused by charge carrier injection from Si into the oxide. The z-scan results and time dependent measurements allow comparison of the EFISH signal amplitudes and time constants detected in transmission and reflection, demonstrating the need for further investigation. 1 Introduction Optical second harmonic generation (SHG) is well known for its atomic scale surface and interface sensitivity if applied to centrosymmetric materials like silicon (Si), for which SHG is dipole forbidden in the bulk. It is contactless, non-intrusive with in situ abilities of monitoring electronic defects during UV irradiation [1], chemical contamination [2], interfacial roughness * Corresponding authors: nyamuda@optronics.zeiss.com Tel: +27 72 951 3962 Fax: +27 12 674 0312 cmsteen@sun.ac.za Tel: +27 21 808 3374 Fax: +27 21 808 3385 egr@sun.ac.za Tel: +27 21 808 3380 Fax: +27 21 808 3385 herbert.stafast@ipht-jena.de Tel: +49 (0) 3641 206 400 Fax: +49 (0) 3641 206 499