Broadband, ultrashort terahertz pulses are commonly used to characterize picosecond changes to the optical conductivity of semiconductors and the vibrational and rotational dynamics in solids and gasses. In this talk, I will discuss mechanisms for generating broadband, picosecond terahertz (THz) pulses from optical femtosecond laser sources, including optical rectification in a nonlinear single crystal, carrier acceleration in photoconductive switches, the pondermotive acceleration of electrons in plasma, and via ultrafast demagnetization in ferromagnetic samples. In a photoconductive switch, an above band gap femtosecond laser pulse is injected into a semiconductor which produces photoexcited carriers that are accelerated by an applied electrical bias to emit a THz pulse. In contrast to a photoconductive switch, which relies on an ultrafast injection and acceleration of electronic charges in the semiconductor, we have recently demonstrated THz emission due to ultrafast changes to a single crystal iron sample's magnetization after excitation by an intense femtosecond pump pulse (Optics Letters, vol. 29, pg. 1805). I will show that the emitted THz pulse is a direct measure of the time-changing magnetic field inside the sample, in contrast to earlier all-optical studies of ultrafast demagnetization which demonstrate ambiguous results due to well known experimental artifacts. Finally, I will present the recent results of our terahertz time domain spectroscopy of iron and compare these results both to the terahertz emission spectroscopy study and to a variety of other materials.


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