A femtosecond absorption spectrometer tunable from 50 000 to 800 cm−1: Nonlinear optics and pump/probe geometries

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Abstract
Solid state Ti:sapphire lasers were constructed and combined with nonlinear optics to produce a 100 fs resolution absorption spectrometer with tunability from 50 000 to 800 cm−1 and <1 m o.d./s sensitivity. Tunable light generation and array detection technology are presented, including a discussion of the suitability of different infrared nonlinear optical materials. A systematic approach to stable infrared pulse generation is provided. Critical parameters of the pump/probe beam geometry are enumerated, revealing detrimental nonlinear contributions to the observed pump‐induced change in probe absorbance. These effects include pump‐induced probe phase modulation and pump‐induced refractive index lenses. Measurements and simulations of the unwanted features in various system geometries reveal their systematic control and elimination.