Next generation XUV source for time-resolved nano-ARPES and PEEM

• Project/Area Number: 22K18270
• Research Category: Grant-in-Aid for Challenging Research (Pioneering)
• Allocation Type: Multi-year Fund
• Review Section: Medium-sized Section 13:Condensed matter physics and related fields
• Research Institution: Okinawa Institute of Science and Technology Graduate University
• Principal Investigator: Dani Keshav 沖縄科学技術大学院大学, フェムト秒分光法ユニット, 准教授 (80630946)
• Project Period (FY): 2022-06-30 – 2025-03-31
• Project Status: Granted (Fiscal Year 2022)
• Budget Amount: ¥25,740,000 (Direct Cost: ¥19,800,000、Indirect Cost: ¥5,940,000); Fiscal Year 2024: ¥8,450,000 (Direct Cost: ¥6,500,000、Indirect Cost: ¥1,950,000); Fiscal Year 2023: ¥8,710,000 (Direct Cost: ¥6,700,000、Indirect Cost: ¥2,010,000); Fiscal Year 2022: ¥8,580,000 (Direct Cost: ¥6,600,000、Indirect Cost: ¥1,980,000)
• Keywords: Extreme UV light source / High order / harmonic generation / XUV-PEEM / Nano-APRES / Twisted 2D materials / Moire pattern / Extreme Ultraviolet / Higher Harmonic / Angle resolved / photoemission / electron microscopy

Outline of Research at the Start

Creating new sources of light with non-standard wavelengths can provide powerful tools for science and technology. The XUV region of the electromagnetic spectrum creates new opportunities in photoemission spectroscopy like the powerful technique of Angle Resolved Photoemission Spectroscopy, and industrial fields like semiconductor fabrication. We propose to build a novel table-top source of XUV radiation that will provide significantly higher flux densities, to further push the boundaries of ARPES techniques and to study the newly emerging two-dimensional semiconductor heterostructures.

Outline of Annual Research Achievements

Nanoscale spatial resolution imaging and nano-ARPES with the XUV photon source with high enough photon flux has been the goal of this Kakenhi proposal. Such a resolution allows the complete investigation of the nanoscale variations in the electronic structure of a material. In this fiscal year, we have finished setting up a new XUV beamline, which provides a flux density of 4 x1018 photons/sec/cm2 that is two-order in magnitude higher than what we have previously done. We have coupled this beamline to an electron microscope. In our initial test runs, we demonstrated that we are capable of doing imaging with XUV photon reaching a spatial resolution better than 100nm and we can also perform APRES with few micron spatial resolution. We have also started a study on twisted 2D materials, preliminary result shows that we can image and resolve different ferroelectric domains in the moire structure.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

Objectives of this research include (A) development of a XUV light source with photon flux density orders of magnitude higher than what is currently available; (B) to utilize this new light source to do time resolved PEEM and nano-ARPES measurement, and (C) to explore moire structure and their excitons in 2D heterostructure.
In this fiscal year, we have progressed rather smoothly, given that we are doing quite well in every aspect. Below is a summary of what we have achieved so far.
(A) We have finished building a new XUV beamline in Jan 2023. We have achieved a spot size of 32 x 7 um on the sample and reaching a flux density up to 4 x1018 photons/sec/cm2, which is two orders better than our previous light source (1x 1016 photons/sec/cm2) and we are just one order short from what we are aiming for.
(B) We have coupled the beamline to our electron microscope, we have succeeded in doing XUV-PEEM reaching spatial resolution better than 100nm, and we could already perform micro-APRES on samples within our setup with few micron meter resolution.
(C) We have started the study of twisted 2D materials. As preliminary results, we have successfully visualized the ferroelectric domains in a moire superlattice.

Strategy for Future Research Activity

In this year, we will continue our work towards the three objectives, (A) to further optimize the flux density of our XUV light source, we should potentially gain another order of magnitude in flux density by (1) increasing the repetition rate of the light source, (2) by improving the phase matching condition and laser parameters like changing gas jet diameter, pump wavelength, pulse duration and gas pressure, and (3) to further reduce the spot size, by alignment using a wave front sensor.
(B) With a brighter light source, we could perform XUV-PEEM and do TR-XUV-PEEM measurement at higher efficiency. With higher flux, we would also investigate the possibility of performing high resolution nano-ARPES.
(C) Our preliminary result from the 2D moire material is very promising. This year we will continue our work to investigate the domain and domain wall structure in these moire materials, we also planned to image the localized excitons that forms in the moire super lattice using TR-XUV-PEEM and TR-ARPES techniques.

Research Products

• [Int'l Joint Research] Stanford University(米国)