Abstract
Because of their high peak intensity, femtosecond laser pulses have been exploited extensively to generate reflected SH radiation with unprecedented efficiency from weakly nonlinear interfaces such as Si(001).1 In this paper, we show that the broad bandwidth and coherence of such pulses also enable a powerful technique for measuring the spectrum and phase of the SH radiation across the bandwidth of a 15 fs Ti:Sapphire laser pulse without tuning the laser. Such pulses have spectral bandwidths large enough to cover features such as the E, critical point resonance in Si thus allowing essentially real-time parallel acquisition of SH spectra via a spectrometer on the detection side. As an example, Fig. l(a) shows SH spectra of Si1−xGex alloys, each acquired in ~ 1s, clearly showing the redshift of the E1 resonance with increasing Ge content. Fig. 1(b) shows real-time monitoring of the E1 resonance of silicon during a heating and cooling cycle. When such SH signals are combined with a SH reference pulse, the spectral phase of the SH signal can also be measured across this entire range through Fourier analysis of frequency domain interferograms.2 Although SH phases have previously been measured for single frequencies,3 to our knowledge this is the first application of frequency domain interferometry (FDI) to the measurement of broadband spectral phase of the nonlinear susceptibility.
© 1999 Optical Society of America
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