A novel experimental method is presented that uses electroabsorption effects above the band gap of semiconductors to measure electric fields on subpicosecond time scales. Broadband laser pulses are required in order to spectrally integrate over electric field-induced Franz-Keldysh oscillations, without the need for spectral resolution. For GaAs we show that the field-induced transmittance change of the broadband laser pulses depends quadratically on the amplitude of the electric field. The versatility of the method is demonstrated by femtosecond time resolved and spatially resolved field measurements in GaAs photoconductive switches.

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