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We present a modular quasi-optical pulse slicer designed for use at terahertz (THz) frequencies. Given a quasi-cw input, the two outputs of a module are (1) a pulse with programmable duration and (2) its complement. The quasi-optical design incorporates a laser-driven silicon switch at Brewster's angle to the incoming THz beam, which limits undesired reflections before the switch is activated such that THz power is only transmitted when the switch is “on.” An “off” switch ensures that no power is leaked after the pulse and that the switching profile is sharp. The slicer's small footprint (0.048×0.072×0.162 m3) and small insertion loss (1.2 dB at 320 GHz) as well as high switching efficiency (∼70%) allows modules to be stacked to create multiple pulses. The output channel that is not used for experiments can be used for concurrent analysis of beam parameters. Stacking modular assemblies will enable more complex sequences of kW-level pulses than are currently achievable for applications including free-electron-laser or gyrotron-powered pulsed electron spin resonance at high magnetic fields.

 

Here, we present a rapidly prototyped, cost-efficient, and 3D printed quasi-optical sample holder for improving the signal-to-noise ratio (SNR) in modern, resonator-free, and high-field electron paramagnetic resonance (HFEPR) spectrometers. Such spectrometers typically operate in induction mode: The detected EPR (“cross-polar”) signal is polarized orthogonal to the incident (“co-polar”) radiation. The sample holder makes use of an adjustable sample positioner that allows for optimizing the sample position to maximize the 240-gigahertz magnetic fieldB₁and a rooftop mirror that allows for small rotations of the microwave polarization to maximize the cross-polar signal and minimize the co-polar background. When optimally tuned, the sample holder was able to improve co-polar isolation by ≳20 decibels, which is proven beneficial for maximizing the SNR in rapid-scan, pulsed, and continuous-wave EPR experiments. In rapid-scan mode, the improved SNR enabled the recording of entire EPR spectra of a narrow-line radical in millisecond time scales, which, in turn, enabled real-time monitoring of a sample’s evolving line shape.

 

High-order sideband generation (HSG), as an analog of the interband processes in high-harmonic generation (HHG) in solids, is a nonperturbative nonlinear optical phenomenon in semiconductors that are simultaneously driven by a relatively weak near-infrared (NIR) laser and a sufficiently strong terahertz (THz) field. We derive an explicit formula for sideband polarization vectors in a prototypical two-band model based on the saddle-point method. Our formula connects the sideband amplitudes with the laser-field parameters, electronic structures, and nonequilibrium dephasing rates in a highly nontrivial manner. Our results indicate the possibility of extracting information on band structures and dephasing rates from high-order sideband generation experiments with simple algebraic calculations. We also expect our approach to be useful on the quantitative understanding of the interband HHG. 

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.

 

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.

 

Electron paramagnetic resonance (EPR) is a powerful tool for research in chemistry, biology, physics and materials science, which can benefit significantly from moving to frequencies above 100 GHz. In pulsed EPR spectrometers driven by powerful sub-THz oscillators, such as the free electron laser (FEL)-powered EPR spectrometer at UCSB, control of the duration, power and relative phases of the pulses in a sequence must be performed at the frequency and power level of the oscillator. Here we report on the implementation of an all-quasioptical four-step phase cycling procedure carried out directly at the kW power level of the 240 GHz pulses used in the FEL-powered EPR spectrometer. Phase shifts are introduced by modifying the optical path length of a 240 GHz pulse with precision-machined dielectric plates in a procedure we call phase cycling with optomechanical phase shifters (POPS), while numerical receiver phase cycling is implemented in post-processing. The POPS scheme was successfully used to reduce experimental dead times, enabling pulsed EPR of fast-relaxing spin systems such as gadolinium complexes at temperatures above 190 K. Coherence transfer pathway selection with POPS was used to perform spin echo relaxation experiments to measure the phase memory time of P1 centers in diamond in the presence of a strong unwanted FID signal in the background. The large excitation bandwidth of FEL-EPR, together with phase cycling, enabled the quantitative measurement of instantaneous electron spectral diffusion, from which the P1 center concentration was estimated to within 10%. Finally, phase cycling enabled saturation-recovery measurements of T 1 in a trityl-water solution at room temperature – the first FEL-EPR measurement of electron T 1 .