Two-photon photoconductivity in SiC photodiodes and its application to autocorrelation measurements of femtosecond optical pulses

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By measuring the output signal of a SiC photodiode under short-pulse illumination with photon energies below the band gap, it was found that the response is determined by a second-order process, i.e., two-photon induced photoconductivity. This nonlinearity is suitable for realizing autocorrelation measurements of short laser pulses in the visible region (420–760 nm), where the nonlinear element and the photodetector are integrated in a single device. Laser pulses of 90-fs duration have successfully been measured with this technique. PACS: 42.60.By; 42.65.-k; 42.79.Fm Laser pulses as short as several femtoseconds can now be generated in many laboratories all over the world [1]. Since electronic devices and even the fastest streak cameras are too slow to measure the temporal evolution of these pulses, there has been an extensive search for appropriate techniques over the past decades. All of these methods rely on autocorrelation or crosscorrelation of the pulse electric field by using various nonlinear effects. The most commonly applied Fig. 1. The laser pulses stem from a dye laser system, pumped by XeCl excimer laser, that delivers pulses at 497 nm with a temporal duration of about 500 fs. The pulses were directed into a single-mode fiber and subsequently compressed by a SF14 prism compressor down to about 90 fs. The pulses were subsequently analysed by a background-free intensity autocorrelator using second-harmonic generation or a SiC photodiode are second-harmonic generation [2] and the optical Kerr effect [3]. Recently it has been demonstrated that two-photon conductivity in photodetectors [4] or even commercial photodiodes [5] can serve as the nonlinear process in an autocorrelation measurement using a Ti:sapphire laser. The wavelength region where these devices are applicable (in the case of second-order processes) is restricted to λg < λ < 2λg, where λg is the wavelength that corresponds to the energy gap of the semiconductor material. In order to realize such a setup for shorter and shorter wavelengths, semiconductor materials with higher bandgaps have to be used. A promising candidate for the visible region, i.e. from about 760 nm down to 420 nm, is silicon carbide SiC, which has a band gap of the order of 3.1 eV. The exact band-gap energy depends on the crystal structure of SiC and can vary between 2 eV and 3.4 eV. Using a commercially available SiC photodiode (Laser Components) a multiple-shot autocorrelator was realized and pulses with a full width at half maximum (FWHM) as short as 90 fs were measured at a wavelength of 497 nm. Figure 1 shows the experimental setup. The femtosecond laser pulses stem from a dye laser system pumped by a XeCl excimer laser, with a temporal duration of about 500 fs and