It is shown that the laser induced ultrafast demagnetization of ferromagnetic films results in the emission of a terahertz electromagnetic pulse. This emission has been detected from Ni films using free-space electro-optic sampling. The radiated electric field E(t) is explained by Maxwell equations (radiation from a time dependent magnetic dipole), and is expected to be proportional to the second time derivative of the magnetization d2M/dt2, as measured in the far field. This technique opens appealing perspectives in the context of measuring and understanding the ultrafast spin dynamics as well as the interaction of electrons (both charge and spin) with electromagnetic fields.
Expressions are derived for the intensity of nuclear responses appearing in the frequency dispersed optical heterodyne detected (OHD) birefringence and dichroism of nonresonant materials. The dispersed birefringence of chloroform is reported and the detuning dependence of the two intramolecular modes (260 cm−1 and 370 cm−1) are compared with theory. These probe frequency dependent birefringence responses are pumped by a 40 fs pulse and probed with either a 40 fs transform limited Gaussian pulse (FWHH∼400 cm−1) or a one-sided exponential pulse (FWHH∼36 cm−1, Lorentzian). Excellent agreement with theory is found. Due to the CARS and CSRS resonances inherent to these responses, the relative magnitude of different nuclear responses which contribute to the total response of a system can be selectively enhanced in the dispersed pump–probe response when the probe spectrum is narrower than the difference between the relevant mode frequencies. It is shown how this two-dimensional (time and frequency) P(3) technique can be used as a measure of the extent of inhomogeneous broadening contributing to the impulsively excited low frequency intermolecular density of states by the appropriate choice of detection frequencies and pulse shape.
ZnTe͑110͒ is widely used as a source of terahertz radiation generated by optical rectification. However, when ZnTe͑110͒ is excited with 800 nm light, optical rectification is not the only process which can occur. Specifically, second harmonic generation and two-photon absorption are also possibilities. In addition, free carriers generated by two-photon absorption can absorb terahertz radiation, further reducing the efficiency of optical rectification. We have used terahertz emission spectroscopy to study these effects by analyzing the dependence of the terahertz waveform on excitation fluence. At high excitation fluences, the overall efficiency is reduced and the trailing edge of the waveform is attenuated. A simple model reproduces the measured behavior.
The influence of elliptically and circularly polarized excitation on terahertz emission from unbiased bulk GaAs at normal incidence and room temperature is reported. Illumination of GaAs above the bandgap produces both spin-polarized electrons and shift currents. The induced currents are monitored via terahertz emission spectroscopy. The terahertz emission amplitude is compared to theoretical calculations as a function of excitation beam ellipticity. Exciting slightly above the bandgap ͑800 nm at room temperature͒ with elliptical polarization generates shift currents that deviate substantially from theoretical predictions. On the other hand, exciting either below the bandgap ͑835 nm at 77 K͒ to produce optical rectification or far above the bandgap ͑400 nm at room temperature͒ to produce shift currents generates emission in agreement with theoretical calculations. Spin-polarized electrons created by elliptically polarized excitation are the source of the observed discrepancy.
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