Giant Enhancement of Surface Second Harmonic Generation inBaTiO3due to Photorefractive Surface Wave Excitation

Abstract

We report observation of strongly enhanced surface SHG in BaTiO3 due to excitation of a photorefractive surface electromagnetic wave. Surface SH intensity may reach 10 −2 of the incident fundamental light intensity. Angular, crystal orientation, and polarization dependencies of this SHG are presented. Possible applications of this effect in nonlinear surface spectroscopy are discussed. PACS no.: 42.65. Ky; 42.65.Tg; 42.70.Nq. Linear surface electromagnetic waves (SEW) such as surface plasmons or surface polaritons [1] play a very important role in such surface optical phenomena as surface enhanced Raman scattering, surface second harmonic generation (SHG), etc. They are extremely useful in applications such as chemical and biological sensing since the electromagnetic field of a SEW is strongly enhanced near the interface. A linear SEW may be excited at the interface between media with opposite signs for their dielectric constants ǫ, such as at a metal-vacuum interface. Another example is the interface between a vacuum and a dielectric that has a sharp absorption line. Such a dielectric has ǫ(ω) < 0 for frequencies just above the absorption line. In both cases the SEW free propagation length in the visible range does not exceed a few micrometers because of high losses [1]. This limits the potential advantages of using SEW in surface enhanced nonlinear optical studies and sensor applications.Recently, a new kind of nonlinear SEW called a photorefractive surface wave has been predicted [2] and observed experimentally [3] in BaTiO 3 . This phenomenon is closely related to self-trapped optical beams (also known as spatial solitons) and self-bending beams observed in photorefractive crystals [4]. A photorefractive SEW occurs when a beam self-bent towards the positive direction of the optical axis (the poling direction) undergoes a cycle of deflections towards the face of the crystal and total internal reflections. The resulting nonlinear SEW is localized near the crystal-air interface with a penetration depth as small as 10 micrometers into the photorefractive crystal [2]. This leads to a strong enhancement of the optical field near the interface that is common for all SEWs. On the other hand, since BaTiO 3 is transparent in the visible range, the free propagation length of the photorefractive SEW along the surface is limited only by the size of the crystal. As a result, a very strong enhancement of all nonlinear surface optical phenomena (such as surface adsorbed molecular luminescence, Raman scattering, surface SHG, etc.) may be expected due to photorefractive SEW excitation. This effect may also be used in combination with further field enhancements produced by surface topographical defects or by the probe tip of a scanning probe microscope (the field enhancement by a probe tip is discussed in the context of a feasibility study of molecular resolution fluorescence near-field microscopy using multi-photon absorption by Kawata et al. [5]).In this letter we report the first observation of strongly enhanced surface ...

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