Free-space electro-optics sampling of mid-infrared pulses

Despite the rapid advance of terahertz optoelectronics in the recent decade, the frequency region above 10 THz has yet been accessible for coherent detection. Whereas Katzenllenbogen et al. reported a frequency response of 6 THz from their photoconducting dipole antenna ~PDA!, the record was recently pushed to 7 THz with a much broader 3-dB bandwidth in a GaP-based free-space electro-optic sampling ~FS-EOS! system. However, because of the Reststrahlen region near 11 THz, a GaP sensor is unlikely to reach 10 THz. This is generally the case for most electro-optic ~EO! sensor materials since the frequency of the fundamental TO phonon resonance in GaP is one of the highest known of many semiconductor EO crystals. The possibility of extending FS-EOS into the midinfrared region is nevertheless favored by the following factors. First, most semiconductor EO materials are transparent in the frequency region between phonon resonance ~far-IR! and the electronic resonance ~near-IR!; Second, it is reasonable to assume an instantaneous nonlinear response and a constant electro-optic coefficient across the mid-infrared region for covalent semiconductor EO crystals, where the dominant nonlinearity is electronic in origin; And the last, 10 fs lasers, which now proliferates rapidly, can readily provide a sampling bandwidth ~FWHM! of as high as 44 THz. The generation of broadband mid-infrared radiation through optical rectification was recently demonstrated by Bonvalet et al. In their experiment, an incoherent detector ~HgCdTe! and interferometric technique were employed to characterize the mid-infrared pulses. The spectrum of the generated radiation, as they noted, spanned beyond the detection bandwidth of the HgCdTe detector. Furthermore, due to the dispersion of GaAs, which has an index of n(5 mm) 53.2978 and n(25 mm)53.058, the spectral phase of the mid-infrared pulse in fact cannot be assumed constant even for their 100-mm-thick GaAs emitter. Incoherent detector based linear correlation technique was therefore unable to provide an accurate characterization of the ultra-broadband radiation. FS-EOS, as a coherent detection technique, becomes attractive when combined with an ultra-broadband mid-infrared source. In this letter, we report the use of electro-optic sensors for coherent characterization of mid-infrared terahertz beams with a bandwidth up to 37 THz. This represents a significant improvement over the previous record of 7 THz, and also