GaAs quantum wells are simultaneously illuminated with near-infrared ͑NIR͒ radiation at frequency nir and intense far-infrared ͑FIR͒ radiation from a free-electron laser at fir . Magnetic fields up to 9 T are applied. Strong and narrow sidebands are observed at sideband ϭ nir Ϯ2 fir . The intensity of the sidebands is enhanced when either sideband or nir is near the onset of NIR absorption in the quantum well, or when fir is near the free-electron cyclotron frequency. We attribute these sidebands to four-wave mixing of NIR and FIR photons whose energies differ by more than a factor of 100.

We have developed a rapid scanning terahertz (THz) spectrometer based on a synchronized two-fiber-laser system. When the system is set to the asynchronous optical sampling mode, THz spectra extending to 3 THz can be acquired within 1 μs at a signal-to-noise ratio of the electric field of better than 20. Signal averaging results in a dynamic range of more than 60 dB, and frequency components of more than 4 THz can be detected. When the lasers are set to the same repetition rate, electronically controlled optical sampling at a rate of 2.5 kHz is demonstrated, making the system versatile for different spectroscopic applications. Finally, we compare the THz emission spectra of a photoconductive switch that is pumped at 780 nm and a nonlinear DAST crystal excited at 1550 nm. We find that the spectral range of the spectrometer is significantly enhanced at higher frequencies, while the dynamic range remains constant.

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We present time‐resolved Gd−Gd electron paramagnetic resonance (TiGGER) at 240 GHz for tracking inter‐residue distances during a protein's mechanical cycle in the solution state. TiGGER makes use of Gd‐sTPATCN spin labels, whose favorable qualities include a spin‐7/2 EPR‐active center, short linker, narrow intrinsic linewidth, and virtually no anisotropy at high fields (8.6 T) when compared to nitroxide spin labels. Using TiGGER, we determined that upon light activation, the C‐terminus and N‐terminus of AsLOV2 separate in less than 1 s and relax back to equilibrium with a time constant of approximately 60 s. TiGGER revealed that the light‐activated long‐range mechanical motion is slowed in the Q513A variant of AsLOV2 and is correlated to the similarly slowed relaxation of the optically excited chromophore as described in recent literature. TiGGER has the potential to valuably complement existing methods for the study of triggered functional dynamics in proteins.

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