Spectroscopy of the instantaneous all-optical switching nonlinearity of thin films  F.P. Strohkendl, R.J. Larsen, L.R. Dalton, University of Southern California  Z.K. Kafafi Naval Research Laboratory

“All-optical switching” “Linear” optical properties are intensity dependent : I 2  cos 2  NL    I 1  sin 2  NL    Nonlinear phase shift: Hope for - fast all-optical switching -”Packet switching”

Heating Electronic excitation Nuclear movement/alignment Distortion of electronic wave function “instantaneous, electronic nonlinearity” Short-pulse experiments favor “instantaneous” electronic nonlinearity Mechanisms contributing to n 2

Phase-mismatched DFWM: Amplitude and phase of   [F. P. Strohkendl et al., J. Opt. Soc. Am. B 14, 92 (1997)] Diffraction Thin Film: Raman-Nath, Thick Substrate: Bragg Signal addition     DFWM is directly related to all-optical switching para- meters

Four-wave mixing signals under absorptive / non-absorptive conditions Non-absorptiveAbsorptive, 2500 cm -1 Femtosecond pulses under absorptive conditions are NOT sufficient to measure instantaneous response

“Instantaneous,” electronic   Internal transition energies are revealed through multi-photon resonances – THG :         , 1-, 2-, and 3-photon resonances – DFWM:         , only 1-, and 2-photon resonances

Example: DFWM vs. Third Harmonic Generation in C 60 Third Harmonic Generation (THG) and DFWM spectra are unrelated. Two-photon resonance observed with DFWM remains undetected by THG.

Exploration of the instantaneous nonlinearity under absorptive conditions Why? The all-optical switching behavior is determined once all one- and two-photon resonances are known ! Problem: DFWM signal under resonant conditions is dominated by (non-instantaneous) sample excitation ! Solution: Two-color FWM.

Nearly degenerate four-wave mixing (nDFWM) suppresses non-instantaneous response Example Amplitude of excited state population rating is reduced due to moving grating by

nDFWM Geometry 3-beam signals at A, B, and E Measure thin film at B, and thick fused silica reference at A Fused Silica: n 2 = 3.0 x cm 2 /W

Comparison of nDFWM/DFWM in C 60 DFWM signal under absorptive conditions is unrelated to instantaneous response

Exploring the Absorption Spectrum? Full UV-Vis SpectrumExplored Region Absorption Spectrum of C 60

Conclusion 1- and 2-photon resonances determine the “instantaneous” all-optical switching parameters (n 2,  2 ) of a material. DFWM / nDFWM are uniquely suited to explore the unknown 2-photon spectrum. Used nDFWM successfully to suppress non instantaneous response (  < cm -1 ). Implemented new method for two-color femtosecond-pulse generation.

Tunable femtosecond light source: µm

Automated femtosecond optical parametric generator/amplifier µm

Dual color generation for nDFWM in a single femtosecond Optical Parametric Amplifier