Ultrafast Manipulation of Valley Pseudospin (1606.06806v1)

Abstract: The coherent manipulation of spin and pseudospin underlies existing and emerging quantum technologies, including NMR, quantum communication, and quantum computation. Valley polarization, associated with the occupancy of degenerate, but quantum mechanically distinct valleys in momentum space, closely resembles spin polarization and has been proposed as a pseudospin carrier for the future quantum electronics. Valley exciton polarization has been created in the transition metal dichalcogenide (TMDC) monolayers using excitation by circularly polarized light and has been detected both optically and electrically. In addition, the existence of coherence in the valley pseudospin has been identified experimentally. The manipulation of such valley coherence has, however, remained out of reach. Here we demonstrate an all-optical control of the valley coherence by means of the pseudomagnetic field associated with the optical Stark effect. Using below-bandgap circularly polarized light, we experimentally rotate the valley exciton pseudospin in monolayer WSe2 on the femtosecond time scale. Both the direction and speed of the rotation can be optically manipulated by tuning the dynamic phase of excitons in opposite valleys. This study completes the generation-manipulation-detection paradigm for valley pseudospin, enabling the platform of excitons in 2D materials for the control of this novel degree of freedom in solids.

• The paper demonstrates all-optical ultrafast manipulation of valley pseudospin using the optical Stark effect in monolayer WSe2.
• It achieves a 2 meV valley splitting and a 0.48 THz pseudospin beat frequency, confirming theoretical predictions.
• The study sets a complete generation-manipulation-detection paradigm for valleytronics, paving the way for advanced quantum devices.

Ultrafast Manipulation of Valley Pseudospin

The research paper in question presents a significant advancement in the field of valley pseudospin manipulation using ultrafast optical techniques. Conducted by Ziliang Ye, Dezheng Sun, and Tony F. Heinz, the paper explores the coherent control of valley pseudospins within transition metal dichalcogenide (TMDC) monolayers, specifically focusing on monolayer WSe$_2$.

Valley pseudospin refers to the quantum degree of freedom associated with the occupancy of electrons in distinct valleys in the momentum space, analogous to electronic spin. This property becomes particularly significant in monolayer TMDCs, which exhibit a nontrivial Berry phase due to broken inversion symmetry. The valleys, labeled K and K', can be selectively excited and controlled using polarized light, making them promising candidates for future quantum technologies such as quantum computing and communication.

Prior to this paper, the generation and detection of valley pseudospin had been achieved through optical and electrical methods. However, manipulating valley coherence remained a challenge. This paper tackles that gap effectively by employing the optical Stark effect as a means of all-optical control. The researchers utilize circularly polarized light below the bandgap to manipulate valley exciton pseudospin on an ultrafast timescale, demonstrating controlled rotation in the equatorial plane of the Bloch sphere representation of pseudospin.

The experimental setup involves first creating a coherent superposition state between K and K' valley excitons using linearly polarized light. Subsequent application of a circularly polarized control pulse induces a pseudomagnetic field, allowing for the breaking of time-reversal symmetry and resulting in the rotation of valley pseudospin. Notably, the direction and speed of pseudospin rotation are optically tunable by adjusting the dynamic phase difference induced between the valley excitons.

Key results include the successful demonstration of valley energy splitting of approximately 2 meV, as confirmed through transient reflectivity measurements. The induced Stark shift generated a pseudospin beat frequency of approximately 0.48 THz, leading to an observed pseudospin rotation angle of roughly 0.1π, aligning well with theoretical predictions. The dynamics of valley coherence were further probed by examining the photoluminescence polarization, with results illustrating controllable pseudospin rotations up to 44° depending on the helicity of the control pulses.

The implications of this paper are noteworthy. It outputs a complete generation-manipulation-detection paradigm for valley pseudospin, thereby opening new avenues for exciton-based quantum devices in two-dimensional materials. Valley pseudospin not only introduces a novel degree of freedom but also enhances the functionality and scope of TMDCs in quantum electronics and optoelectronics. Potential future extensions of this work include leveraging valley pseudospin control in quantum-confined excitonic states, which offer extended lifetimes and might enable more complex quantum operations within TMDCs.

Overall, this work signifies a substantial step forward in the optical control of quantum states in solid states and sets the stage for further advancements in valleytronics and related fields. The paper's methodology and findings provide vital insights into ultrafast valley pseudospin manipulation, paving the way for more sophisticated quantum applications utilizing two-dimensional materials.