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Author: Thamarasee Jeewandara, Phys.org

Ultrafast imaging plays an important role in the physical and chemical studies of the femtosecond dynamics of heterogeneous samples. This method is based on the understanding of the phenomenon caused by ultrashort laser pump pulses, and then uses ultrashort probe pulses. The emergence of very successful ultrafast imaging technologies with extremely high frame rates is based on wavelength or spatial frequency coding. In a new report from Light: Science & Applications, Chen Xie, Remi Meyer, and a group of scientists in China and France used a pump-induced micrograft method to provide detailed in situ characterization of weak probe pulses. The method is non-destructive and fast to perform, so in-situ probe diagnosis can be repeated to calibrate experimental conditions. This technology will enable imaging that was previously impossible to achieve in ultrafast scientific fields at the micrometer and nanometer scales.

The concept of laser-matter interaction in ultrafast physics and chemistry is based on imaging with high spatial resolution and high temporal resolution. In this work, Xie and Meyer et al. Describes a high-sensitivity in-situ diagnosis of weak detection pulses to solve the problem of ultra-fast imaging with high spatial resolution. The team first derives the diffraction signal and demonstrates the optical device, and then demonstrates its function in any polarization configuration. Then, they retrieved the absolute pump detection delay through experiments, and used a visualization tool to solve the problem of removing the tilt of the pulse front. To set up the experiment, they used a spatial light modulator to form a two-wave interference field in the dielectric sample from a single pump beam to ensure synchronization between the two pump waves. In the experimental setup, the team used a titanium sapphire chirped pulse amplifier laser source to provide 50 femtosecond pulses with a center wavelength of 790 nm, and performed all measurements by integrating the 50-shot signal at a repetition rate of 1 KHz.

A Kerr-based transient grating suitable for all pump-probe polarization combinations

In this work, Xie and Meyer et al. Demonstrated how to generate a pump-induced micro-grating from the electron Kerr effect (a phenomenon in which the refractive index of a material changes due to an applied electric field) to provide detailed in-situ characterization of weak detection pulses. The scientists verified the measured diffraction signal and proved the validity of the measurement for all combinations of input pump and probe polarization. They first reported the verification of the technology, and then the optimization of the probe pulse. Then, they optimized the duration of the probe pulse to characterize the two polarizations, and showed that the method is very useful for detecting the spectral phase difference in the optical path of the pump and probe beams.

Space limitations for synchronization

During the experiment, Xie and Meyer et al. The synchronization standard of pump and probe pulse is defined to achieve precise focus position in the sample, and the interaction area between pump and probe is positioned to tens of microns. The powerful positioning of the experiment allowed them to retrieve the effect of group velocity differences on pump-probe synchronization. Probe pulses can produce pulse leading edge tilt, which limits ultrafast imaging experiments. To solve this problem, Xie and Meyer et al. By using two completely parallel prisms, an aberration-free prism compressor is used, although the parallelism may deviate by several milliradians experimentally. This deviation has a great influence on the detection pulse. Therefore, the team used transient gratings to provide direct visualization of the tilt of the pulse front, and then effectively resolved it by accurately adjusting the parallelism between the compressor prisms. This work shows a good agreement between experiment and simulation. The transient grating diagnosis introduced in this work helps to accurately eliminate the tilt of the pulse front, even if the deviation angle of the prism compressor changes slightly.

In this way, Chen Xie, Remi Meyer and colleagues devised an extremely localized in-situ diagnostic method to allow characterization and synchronization of weak detection pulses with higher intensity pumps. Diagnosis has a high degree of flexibility for different pump-probe crossing geometries to characterize probe pulses. This technique is also applicable to various pulse durations, even in the presence of spherical aberration, and is widely applicable to most ultrafast imaging and pump probe experiments. The results have multiple applications and can be used to determine transient phenomena on the micrometer scale and to understand laser-matter interactions in condensed matter. Further explore high-resolution, terahertz-driven atom probe tomography. More information: Xie C., Meyer R., etc. In situ diagnosis of femtosecond laser detection pulses for high-resolution ultrafast imaging, light: Science and Application doi.org/10.1038/s41377-021-00562-1

Wrinkler T. et al. Excite laser amplification in the dielectric. , Natural Physics, doi.org/10.1038/nphys4265

The leading edge of ultrashort pulse generation: pushing the limits of linear and nonlinear optics. Science, 10.1126/science.286.5444.1507 Journal information: Nature Physics, Light: Science & Applications, Science

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