Coherent Light Pulses Enable Deterministic Excitation of Hidden Valleys in Monolayer TMDs

Controlling the flow of electrons through materials based on their ‘valley’, a quantum property related to momentum, is central to the emerging field of valleytronics, but accessing all available valleys has proven remarkably difficult. Youngjae Kim from the School of Physics, Korea Institute for Advanced Study, and colleagues now demonstrate a method for selectively exciting a single valley in atomically thin materials, overcoming a fundamental limitation in this field. The team theoretically shows that by carefully combining different types of laser light, they can precisely target and polarize electrons into a specific valley, completely separate from those typically used in valleytronics. This all-optical technique achieves exceptionally high polarization with unprecedented speed, spanning from terahertz to petahertz timescales, and opens up the possibility of creating more complex and powerful electronic devices based on multi-state valley information processing.

Valleytronics and Spin Control in 2D Materials

This body of research explores the fascinating world of two-dimensional materials, such as molybdenum disulfide and tungsten diselenide, and how they interact with light. A central theme is “valleytronics”, a field aiming to harness the “valley index” of electrons as a means of storing and processing information, analogous to how “spintronics” uses electron spin. Researchers are investigating how to control these valleys using light to create novel electronic and optical devices. A key focus is manipulating the spin and valley polarization of electrons within these materials using light, employing techniques like optical pumping and exploring how light can drive materials out of equilibrium.

Investigations also extend to utilizing terahertz and infrared light to manipulate valley states and generate electrical currents, examining non-linear optical effects including high harmonic generation and the Franz-Keldysh effect. Researchers are also developing theoretical models to understand the electronic structure and optical properties of these materials, helping explain phenomena like Landau-Zener tunneling. This theoretical work is crucial for designing and optimizing new devices, representing a rapidly growing field with the potential to revolutionize electronics and optoelectronics, opening up possibilities for valley-based devices, improved spintronics, quantum computing architectures, high-speed optoelectronics, and novel sensors.

Coherent Light Excites Targeted Material Valleys

Researchers have developed a new method to selectively excite specific valleys within the electronic structure of two-dimensional materials, overcoming limitations of conventional valleytronics. This approach utilizes a carefully orchestrated combination of light pulses to directly and deterministically excite a single, targeted valley, unlocking previously inaccessible degrees of freedom for information processing. The core of this methodology lies in coherently combining a circularly polarized light pulse with a linearly polarized pulse, creating engineered quantum pathways. This combination allows researchers to precisely steer electrons into a desired valley, effectively isolating it from all others.

Detailed simulations, based on a three-band tight-binding model, reveal how the combined light pulses adiabatically guide electrons into the targeted valley, ensuring high fidelity and minimizing unwanted excitations. Importantly, this method isn’t limited to a single valley; each of the sixfold degenerate valleys can be individually addressed by tailoring the properties of the light pulses, specifically the helicity of the circular polarization and the angle of the linear polarization. This versatility extends to an exceptionally broad range of timescales, from terahertz to petahertz, enabling femtosecond-scale control over valley polarization and promising significant advancements in valleytronic optoelectronics.

Sixfold Valley Control with Shaped Light Pulses

Researchers have achieved a breakthrough in controlling the flow of electrons in two-dimensional materials, specifically targeting previously inaccessible “valleys” within their electronic structure. This research demonstrates a method for selectively exciting and controlling sixfold degenerate valleys, significantly expanding the potential for information processing. The team developed an all-optical technique that uses precisely shaped light pulses to steer electrons into a targeted valley with near-perfect accuracy. This is achieved by combining a low-frequency driver pulse with a resonant, circularly polarized pump pulse, creating a coherent three-step process.

Initially, the driver pulse shifts the momentum of electrons without causing any transitions, subsequently the pump pulse selectively excites electrons into the desired valley, and finally the driver pulse smoothly guides these excited electrons to their final state, resulting in complete valley polarization. This level of control is remarkable, enabling the creation of highly defined electron states on femtosecond timescales. The method achieves near-unity valley polarization and operates across an exceptionally broad range of frequencies, offering a versatile platform for multi-state valley information processing and unlocking previously inaccessible degrees of freedom in quantum materials.

Selective Valley Control with Polarized Light

This research introduces a new method for controlling the flow of electrons in two-dimensional materials, specifically focusing on previously inaccessible valleys within their electronic structure. By combining circularly and linearly polarized light pulses, researchers engineered a pathway to populate a single valley with near-perfect fidelity, achieving robust control across an exceptionally broad range of ultrafast timescales. The significance of this work lies in its potential to move beyond conventional binary valleytronics, which relies on just two valleys for information storage. This new technique unlocks access to a sixfold degenerate valley, effectively increasing the potential states for encoding information and paving the way for more complex and efficient valley-based devices. Future work will likely focus on translating these theoretical findings into practical devices and exploring the full potential of multi-level valley information processing.