Figure 5.
(a) Experimental THz-E-field amplitudes of the Fe (2 nm)/Pt (3 nm) emitters grown at 300 °C on MgO and Al2O3 in the time domain and (b) their corresponding spectra. The emitter grown on MgO exhibits a stronger dispersion compared to the one grown on sapphire as the MgO itself has multiple absorption lines above a frequency of 3 THz (see methods, Fig. 11) while the group and phase velocities of THz waves in Al2O3 are mostly frequency independent up to 10 THz. (c) Spectra of the Fe (2 nm)/Pt (3 nm) grown on MgO where the Pt layers have been grown at room temperature (RT) or 300 °C, respectively. (d) Spectra of two thicker samples, Fe(12 nm)/Pt(6 nm) grown on MgO with different crystal quality of the Pt layer. In both (c and d) the epitaxial samples exhibit singificantly larger THz-E-field amplitudes. The insets show the spectral width of the corresponding samples. The large change in the amplitude is present in all measurement geometries (detected pulse from the Pt side, with and without the lens), demonstrating the intrinsic origin of the effect. The presented data are obtained from the substrate side using an hyperhemispherical silicon lens attached directly to the substrate.