Two-photon absorption coefficient measurements based on widely tunable femtosecond pulses from parametric generation

We discuss an experimental arrangement for fast and reliable measurements of the two-photon absorption coefficient beta and its wavelength dispersion. Widely tunable pulses, with sufficient power (and therefore intensity) to detect the nonlinear absorption in samples with a small beta L (L is the sample thickness), are provided by travelling-wave parametric generation. The short time duration of the pulses minimizes the accumulation of carriers excited by two-photon absorption (TPA), or by an eventual linear mechanism, which could otherwise produce an appreciable additional absorption which would be difficult to take account of. To produce a relative calibration of the set-up, we suggest the use of second-order crystals which, when phase-matched for second harmonic generation, mimic a two-photon absorber. We give examples of measurements on semiconductors and on organic thin films, and we report the TPA spectra of CdTe and CdSe semiconductor nanocrystals in a glass matrix.