Spintronic terahertz emitters (STEs) have great potential for use in THz time-domain spectroscopy and its exploitation in scientific and industrial applications. While they typically emit broadband single-cycle THz pulses, there is interest in narrowband THz sources, with applications across a broad range of research areas. Here we show that any spintronic terahertz (THz) emitter can be driven to generate narrowband THz pulses, with continuous tuning of the emission frequency and bandwidth by employing temporal shaping of the drive laser pulses to manipulate the ultrafast demagnetization dynamics. Using a regenerative amplifier laser system with 50 fs transform-limited pulses chirped to 6 ps together with a chirped-pulse beating scheme, we demonstrate narrowband THz generation over a frequency range from 0.4 to 2.3 THz, in addition to bandwidths down to 40 GHz using 12 ps chirped pulses. These results highlight how STEs can provide opportunities for narrowband applications over the entire THz spectral range.
The recent emergence of spintronic terahertz (THz) emitters has provided a low-cost source of high field-strength, broadband THz radiation, exploiting the laser-induced electron spin properties in magnetic multi-layers to produce gap-free emission covering 1-30 THz and electric fields up to 300 kV/cm. An interesting property of spintronic emitters is that they have been shown to generate THz radiation polarized perpendicular to the applied magnetic field and independent of the pump laser polarization. We exploit this magnetic field-dependent emission process to create arbitrary terahertz polarization profiles. We show that by applying a specific magnetic field pattern to the source, it is possible to generate a quadrupole-like THz polarization profile. Measurements of the THz electric field at the focus of the beam revealed a polarity flip in the transverse profile of the quadrupole-like mode with a resulting strong, on-axis longitudinal component of 17.7 kV/cm.
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