Time Integrated Investigation of the Nonlinear Kerr Coefficient
Most of the nonlinear phenomena described in Sect. 2 are well understood if the oscillation period of the exciting field clearly exceeds the time constant of the induced dynamics Buckland, Boyd Opt Lett 21:1117, 1996, [1 ].
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Time Integrated Investigation of the Nonlinear Kerr Coefficient
Most of the nonlinear phenomena described in Sect. 2 are well understood if the oscillation period of the exciting field clearly exceeds the time constant of the induced dynamics [1]. As laser pulses with femtosecond duration are in most cases shorter than the oscillation period of acoustic and typical optical phonons in solids [2], the established values of the nonlinear Kerr coefficient n 2 (Sect. 2.1) loose validity [3, 4]. To estimate the magnitude of the optical Kerr effect in the interaction of the few cycle pulses used in this study with dielectrics, n 2 is determined with two different time integrating techniques. On the one hand the well-established Z-scan method is employed [5]. On the other hand a concept based on the detection of the intensity dependent reflectivity is developed. It is particularly suitable for broadband few-cycle laser pulses.
3.1 Z-Scan A well-proven concept for the determination of the nonlinear Kerr coefficient is the Z-scan technique introduced by M. Sheik-Bahae in 1990 [5]. It makes use of the spatial implications of the intensity dependent refractive index described in Sect. 2.2. The basic setup is depicted in Fig. 3.1: The investigated target is positioned in the focused laser beam of few-cycle VIS/NIR pulses (appendix A.1). The Rayleigh length clearly exceeds the sample thickness by several orders of magnitude. The transmitted beam is split into two arms: in the open aperture arm its power is recorded. In the small aperture arm a pinhole is positioned in the far field of the laser beam and the transmitted power detected on a powermeter. When the sample is moved stepwise along the optical axis towards the focus, the intensity it is exposed to grows. The transverse Gaussian intensity profile of the beam inhomogeneously modifies the optical material density and introduces a nonlinear lens (Sect. 2.2). With increasing peak intensity the focal length of this lens decreases and the self focusing of the beam becomes © Springer International Publishing Switzerland 2016 A.M. Sommer, Ultrafast Strong Field Dynamics in Dielectrics, Springer Theses, DOI 10.1007/978-3-319-41207-8_3
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3 Time Integrated Investigation of the Nonlinear Kerr Coefficient
Fig. 3.1 Schematic setup of the Z-scan to determine the nonlinear Kerr coefficient n 2
Fig. 3.2 The impact of self focusing on the effective focal length. Panel a when the target is positioned in front of the geometrical focus the additional Kerr lens decreases the effective focal length and the divergence of the transmitted beam grows. Panel b when the sample is located behind the geometrical focus self focusing decreases the beam divergence
more pronounced. While the target is situated in front of the geometrical focus the additional nonlinear lens decreases the effective focal length and rises the divergence of the transmitted beam Fig. 3.2a. Therefore less power is transmitted through the pinhole in the small aperture arm. As soon as the target is located behind the
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