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Scientists Innovate On-site in situ High-Pressure Ultrafast Pump-Probe Spectroscopy Instrument

Date:30-11-2021 Print

Time-resolved ultrafast spectroscopy has been widely employed in condensed matter physics (and other sciences) due to its unique advantages, such as ultrahigh temporal resolution, capability of detecting excited-states above the Fermi surface, and generation of coherent boson excitations. It has been extensively used to investigate various condensed matter physics, including high-temperature superconductivity, complex phase transition, coupling between different degrees of freedom, coherent control, laser-induced novel quantum states and hidden states, etc. High pressure technique has also been widely employed in condensed matter physics (and other scientific areas), owing to its unique capability of tuning the electronic band structure by directly modifying the lattice constants. In recent years, efforts have been devoted to combining the two areas together, driven by the rich and deep fundamental scientific demands aforementioned to probe ultrafast dynamics under high pressure [Chin. Phys. Lett. (Express Letter) 37, 047801 (2020)].

Challenges mainly come from the data reliability. Because ultrafast dynamics investigations are very delicate experiments and high pressure also causes rich complex physical effects, maintaining reliable data is very crucial for correct understanding of the mechanism, and also for the initiation and development of such a cross-field direction—high pressure ultrafast dynamics. Conventionally, when tuning and calibrating the high pressure, the diamond anvil cell (with a sample enclosed) is often taken outside of the light path and latter put back. During such processes, sample motion or rotation relative to the light beam can be introduced. Such repositioning artifacts introduced is worse for a two-incident-beam experiments (e.g. pump-probe ultrafast spectroscopy) under high pressure conditions. In practice, the a few worldwide preliminary attempts mostly report on the lifetimes of the quasiparticles, rather than providing precise information of the amplitudes, which presents obstacles in fully understanding the ultrafast dynamics and mechanisms. For example, one of the key signatures of the quantum properties of superconducting phase transition, charge density wave ordering formation and topological phase transition is that there is an energy gap opening or closing. This variation in energy gap directly corresponds to the phonon-bottleneck effect in the excited state ultrafast spectroscopy experiments. Experimentally, verification of the phonon bottleneck effect needs full information on both the amplitude and the lifetime. Lifetime information alone is not adequate to identify it. Therefore, simultaneously obtaining reliable amplitude and lifetime is critical for the development of the cross-field high-pressure ultrafast dynamics.

Two pivotal requirements for the instrument innovation thus follow: (1) Technology aspect—innovate a reliable and accurate on-site in situ high-pressure ultrafast pump-probe spectroscopy instrument. (2) Standard aspect—initiate a standard description for such investigations and experiments. Colleagues are invited to explicitly state whether their experiments are on-site in situ or not, so that the reported experimental data are comparable with each other without systematic errors. Following the standard description, the reliability and accuracy of the data in the community are greatly enhanced and potential artifacts or even misleading understanding will be prominently reduced.

Recently, Professor Jimin Zhao and his postdoc Yanling Wu, phd student Jiazila Hasaien and Zhenyun Tian at Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences collaborate with Professor Yang Ding and his phd student Xia Yin at Center for High Pressure Science and Technology Advanced Research to successfully develop an on-site in situ high pressure ultrafast pump-probe spectroscopy instrument at room temperature (TOC Figure and Figure 1). Its construction has achieved essential breakthroughs: (1) In technical aspect, they realized the on-site in situ technology. Both the sample and the diamond anvil cell are fixed in the light path during the whole ultrafast pump-probe experiment, especially including tuning and calibrating the high pressure, to avoid the repositioning fluctuation (Figure 2). The sample motion and rotation are removed to the maximum possible extent (within the fluctuation range using the CCD monitoring and fine-adjustment system). Thus, unnecessary artifacts are avoided during the experiment and the precision and reliability of the experimental data are maximized. (2) In standard aspect, they proposed an on-site in situ standard description. Comparison between the results under on-site in situ and off-site in situ conditions is shown in Figure 3. By explicitly describing whether the DAC is removed out and put back into the light path, the comparability, compatibility, and consistency of the experimental data can be greatly improved within the academic community (Figure 3). With this standard description, unnecessary systematic comparison error and interpretation deviation is avoided to the maximum extent (note that in synchrotron beamline experiments, precise kinematic mounts are routinely employed, and thus, the experimental conditions are also guaranteed to be equivalent to on-site in situ conditions). In summary, based on the above two aspects of instrument innovation and breakthrough, the research team obtained at ambient temperature both reliable amplitudes and lifetimes of the ultrafast dynamics, thus providing rich and comprehensive information. Their work provides a reliable means for obtaining the high pressure ultrafast dynamics of quantum materials and further understanding the complex phase transition and excited state dynamics induced or modified by high pressure.

This study entitled "On-site in situ high-pressure ultrafast pump-probe spectroscopy instrument" was published on Review of Scientific Instruments.

The study was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences Interdisciplinary Innovation Team, the International Partnership Program of CAS, the Strategic Priority Research Program of CAS, the Beijing Natural Science Foundation.

Fig. 1. Schematic experimental setup of the on-site in situ high-pressure time-resolved ultrafast pump-probe spectroscopy instrument. (Adapted from Rev. Sci. Instrum. 92, 113002 (2021).)

Fig. 2. Drawbacks of sample motion or rotation on obtaining reliable ultrafast pump-probe data. (Adapted from Rev. Sci. Instrum. 92, 113002 (2021).)

Fig. 3. Comparison of the experimental data of relative differential reflectivity ΔR/R between on-site in site and off-site in situ conditions. (Adapted from Rev. Sci. Instrum. 92, 113002 (2021).)

Institute of Physics
ZHAO Jimin
Key word:
Ultrafast dynamics; High pressure; On-site in situ; Standard description; Repositioning fluctuation.

Scientists innovate an on-site in situ high-pressure time-resolved ultrafast pump-probe spectroscopy instrument. Both the DAC and the sample are fixed within the light path without motion and rotation throughout the whole ultrafast spectroscopy experiment, including tuning and calibrating the pressure. Conventional possible artifacts are greatly reduced by using this on-site in situ innovative instrument. The instrument and the proposed standard description facilitate the high-pressure ultrafast science investigation to develop into a promising new area, which enables the exploration of nonequilibrium excited quantum states' dynamics in the high-pressure regime.