A vacuum-ultraviolet laser with submicrometer spot for spatially resolved photoemission spectroscopy

January 27, 2021

The rapid development of two-dimensional quantum materials, such as twisted bilayer graphene, monolayer copper superconductors, and quantum spin Hall materials, has demonstrated both important scientific implications and promising application potential. To characterize the electronic structure of these materials/devices, angle-resolved photoemission spectroscopy (ARPES) is commonly used to measure the energy and momentum of electrons photoemitted from samples illuminated by X-ray or vacuum ultraviolet (VUV) light sources. Although the X-ray-based spatially resolved ARPES has the highest spatial resolution (~100 nm) benefitting from the relatively short wavelength, its energy resolution is typically mediocre (>10 meV), which makes it difficult to visualize the fine details of the electronic structure in many novel quantum materials. Complementary to X-ray light sources, VUV laser-based light sources can offer much better energy resolution (~0.2 meV), deeper depth of detection and lower cost (compared to synchrotron light sources). However, the longer wavelength of the VUV light source also deteriorates its spatial resolution (typically several micrometres to date), making it insufficient for characterizing small-size flake samples or spatially inhomogeneous (e.g., magnetic, electronic or composite domain) materials.

In a new paper published in Light Science & Application, Mao and his co-workers have developed a 177 nm VUV laser system for scanning photoemission microscopy with a focal spot of <1 μm at a long focal length (~45 mm) by using a spherical-aberration-free zone plate. Based on this microscopy, they also built an off-axis fluorescence detection platform that exhibits superior capability to conventional laser systems in revealing subtle features of materials.

Compared with the current DUV laser source with spatial resolution used for ARPES in the world, the 177 nm VUV laser source could help the ARPES measurement cover a larger momentum space and has the better energy resolution, but there are still many challenges and difficulties to make it have excellent spatial resolution:

"First, severe spherical aberration exists in a high-NA refraction lens. Second, only very limited materials can be used in optics for correcting the spherical aberration due to the strong absorption at VUV frequencies. Third, it is practically difficult to check the quality (collimation, uniformity and efficient diameter) of the incident beam and the alignment among optical elements, as the VUV beam is invisible and all optics have to be placed in vacuum or a sealed chamber filled with inert gas."

This VUV laser focusing system contains five functional parts: a 355 nm laser, a second-harmonic generation stage, a beam shaping stage, a polarization adjustment part and a focusing element of the flat lens.

"To avoid the spherical aberration, we introduce planar diffractive lenses that can realize tight focusing of light by fine tuning of the interference from multiple beams" they added.

"This VUV laser system has ultra-long focal length (~45 mm), sub-micron spatial resolution (~760 nm), ultra-high energy resolution (~0.3 meV) and ultra-high brightness (~355 MWm-2). It can be directly applied to scientific research instruments such as photoemission electron microscopy (PEEM), angle-resolved photoelectron spectrometer (ARPES) and deep ultraviolet laser Raman spectrometer. At present, this system has been connected with the ARPES at ShanghaiTech University revealing the fine energy band features of various new quantum materials such as quasi-one-dimensional topological superconductors TaSe3, magnetic topological insulators (MnBi2Te4)(Bi2Te3)m family, etc.. " the scientists summarized.

Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

Related Energy Articles from Brightsurf:

Energy System 2050: solutions for the energy transition
To contribute to global climate protection, Germany has to rapidly and comprehensively minimize the use of fossil energy sources and to transform the energy system accordingly.

Cellular energy audit reveals energy producers and consumers
Researchers at Gladstone Institutes have performed a massive and detailed cellular energy audit; they analyzed every gene in the human genome to identify those that drive energy production or energy consumption.

First measurement of electron energy distributions, could enable sustainable energy technologies
To answer a question crucial to technologies such as energy conversion, a team of researchers at the University of Michigan, Purdue University and the University of Liverpool in the UK have figured out a way to measure how many 'hot charge carriers' -- for example, electrons with extra energy -- are present in a metal nanostructure.

Mandatory building energy audits alone do not overcome barriers to energy efficiency
A pioneering law may be insufficient to incentivize significant energy use reductions in residential and office buildings, a new study finds.

Scientists: Estonia has the most energy efficient new nearly zero energy buildings
A recent study carried out by an international group of building scientists showed that Estonia is among the countries with the most energy efficient buildings in Europe.

Mapping the energy transport mechanism of chalcogenide perovskite for solar energy use
Researchers from Lehigh University have, for the first time, revealed first-hand knowledge about the fundamental energy carrier properties of chalcogenide perovskite CaZrSe3, important for potential solar energy use.

Harvesting energy from walking human body Lightweight smart materials-based energy harvester develop
A research team led by Professor Wei-Hsin Liao from the Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong (CUHK) has developed a lightweight smart materials-based energy harvester for scavenging energy from human motion, generating inexhaustible and sustainable power supply just from walking.

How much energy do we really need?
Two fundamental goals of humanity are to eradicate poverty and reduce climate change, and it is critical that the world knows whether achieving these goals will involve trade-offs.

New discipline proposed: Macro-energy systems -- the science of the energy transition
In a perspective published in Joule on Aug. 14, a group of researchers led by Stanford University propose a new academic discipline, 'macro-energy systems,' as the science of the energy transition.

How much energy storage costs must fall to reach renewable energy's full potential
The cost of energy storage will be critical in determining how much renewable energy can contribute to the decarbonization of electricity.

Read More: Energy News and Energy Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.