By Michael Zuerch

In the last decade, quantum materials have garnered much attention due to their unique properties resulting from interactions between charge, spin, and lattice degrees of freedom at the molecular level in addition to topological effects that can create unique properties at material surfaces. These new materials have the potential to revolutionize fields such as clean energy production, energy storage, quantum computation, and
communications. 

Understanding the underlying phenomena is critical for the design of future quantum materials and their integration into complex devices. Traditionally, researchers have focused on modifying these materials’ properties using static methods such as strain, temperature, applied electric and magnetic fields, chemical composition, or advanced geometric assembly. Now researchers are starting to study time-dependent properties in quantum materials by using light pulses to dynamically engineer material properties with response times down to the femtosecond timescale often referred to as “ultrafast timescale”. Such dynamical control is particularly important in applications that require fast switching of a material property such as in memory and computation. 

Largely unexplored are such questions as, how tiny energy fluctuations in a system with coupled degrees of freedom can lead to drastic changes of macroscopic properties; how exotic ground or excited states emerge from changes in crystal symmetry or electronic topology; and how quantum materials behave in realistic environments for integration into real-world applications. An example of this is a mysterious phase of matter called strange-metal phase, which exhibits transport phenomena without well-defined quasiparticles. While we know this phase exists and has been observed in many quantum materials, it does not follow known concepts of transport and even a theoretical description is absent. Sparked by curiosity to understand the underlaying physical phenomena, scientists are currently working to describe and measure this phase from a spectroscopic or symmetry point of view. 

Ultrafast techniques such as photoemission, scattering, and optical spectroscopies have become powerful tools for studying time-dependent properties in quantum materials. However, conventional time-resolved methods 

lack sensitivities for example to separate contributions by atomic elements, buried interfaces, symmetry, and topology on relevant timescales. These shortcomings call for the development of more advanced methods straddling the interface between materials
science, physics, and chemistry. 

The group of Assistant Professor Michael Zuerch is exploring this interesting new space of non-equilibrium phenomena in quantum materials by the development and application of novel types of spectroscopies in pursuit of unlocking the mysteries of quantum materials. In a recent review published in Nature Reviews Materials, three ultrafast spectroscopy methods are discussed, including Attosecond Transient Absorption Spectroscopy (ATAS); Solid-state High Harmonic Generation Spectroscopy (sHHG); and Extreme-ultraviolet Second Harmonic Generation Spectroscopy (XUV-SHG). These techniques have only begun to be applied to the study of quantum materials in the past few years. Zuerch has established the Ultrafast Materials Chemistry Laboratory operated by his group in the basement of Giauque Hall. 

About these new spectroscopy techniques

ATAS is a technique that creates non-equilibrium conditions in a material using a femtosecond optical pulse and probes them using a time-delayed extreme-ultraviolet attosecond pulse. The extremely short probe pulse allows for studying the fastest carrier interactions within the material, while the use of extreme ultraviolet light allows the resolution of dynamics in a material from the viewports of the different atomic elements. 

In contrast, sHHG spectroscopy uses a femtosecond mid-infrared laser pulse to periodically accelerate electrons within a material.
The visible light emission from these accelerated electrons encodes information about the energy landscape and interactions between electrons in the material. sHHG provides information about crystal symmetry and dynamic symmetry on ultrafast timescales. Further sHHG is highly relevant for studying quantum materials in operando as all involved wavelengths readily transmit through layers of glass and liquids.

Extreme ultraviolet second-harmonic generation (XUV-SHG) spectroscopy uses two photons in the extreme ultraviolet to emit a photon at twice the frequency, allowing the study of functional properties resulting from symmetry breaking. It’s useful for studying dynamics at buried interfaces and surfaces with femtosecond resolution, which is highly relevant in layered quantum materials. XUV-SHG can also be used to study ion dynamics at interfaces, which is valuable for developing new solid-state electrolyte materials for batteries.

Quantum materials remain one of the most active and exciting areas of research in materials science, physics, and chemistry. Emerging ultrafast techniques we are currently actively developing at the College of Chemistry offer unique capabilities to address some of the most challenging problems in this field and open up exciting possibilities that could lead to new breakthroughs in materials chemistry and technology.