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 Si_1−xGe_x 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