Valley Pseudospin Research Articles

Electron–hole collisions in an atomically thin semiconductor

Strong-field biasing of a solid with intense lightwaves leads to simultaneous interband excitation and intraband acceleration of electron–hole pairs. These coupled dynamics result in high-harmonic emission from the bulk solid. For a controlled acceleration of quasiparticles with well-defined initial conditions, we prepare coherent electron–hole pairs by a resonant near - infrared pulse before a strong multi-terahertz field accelerates these entities. The ballistic dynamics manifests itself as high-order sidebands to the near-infrared excitation spectrum. This mechanism allows for the implementation of a quasiparticle collider in order to study those entities in close analogy to conventional collision experiments. Accelerating electrons and holes in a monolayer of a transition metal dichalcogenide extends this scheme to internal quantum degrees of freedom. We show how a strong lightwave can transport electron–hole pairs from one valley to the other faster than one oscillation of the carrier wave, effectively switching the valley pseudospin on a sub-cycle scale. This scheme paves the way to ultimately fast valleytronics.

Mirror-symmetry induced topological valley transport along programmable boundaries in a hexagonal sonic crystal

Valley states, labeling the frequency extrema in momentum space, carry a new degree of freedom (valley pseudospin) for topological transport of sound in sonic crystals. Recently, the field of valley acoustics has become a hotspot due to its potentials in developing various topological-insulator-based devices. In most previous works, topological valley transport is implemented at the interfaces of two connected artificial crystals. With respect to the interface, the mirror symmetry of crystal structures supports either even-mode or odd-mode valley states. In this work, we propose a physical insight of transforming one hexagonal crystal into a virtual lattice by utilizing the mirror operation of rigid or soft boundaries, which greatly reduces the dimension of the acoustic structure and provides a possible way to implement the programmable routing of topological propagation. We investigate two cases that the rigid and soft boundaries are introduced either at the edge or inside a single hexagonal crystal. Our results clearly demonstrate the high-transmission valley transport along the folded boundaries, where reflection or scattering is prohibited at the sharp bending or corners due to topological protection. Three functional devices are exemplified, which are single-crystal-based topological delay-line filter, delay-line switcher and beam splitter. Our work reveals the inherent relation between the field symmetries of valley states and structural symmetries of sonic crystals. Programmable routing of topological sound transport through boundary engineering provides a platform for developing integrated and versatile topological-insulator-based devices.

Valleytronic properties of monolayer WSe2 in external magnetic field

Valley pseudospin, a novel quantum degree of freedom, is expected to show valley Zeeman effect in analogy to real spin in magnetic field B. By performing first-principles calculations, we studied the magnetic effect on valley pseudospin in monolayer WSe2. With the application of B, the time reversal symmetry is broken. Our calculation shows that the valley energy degeneracy is broken and the valley Zeeman splitting varies linearly with B, agreeing well with the experiments. It is found that the valley Zeeman splitting is contributed mainly from the atomic orbital magnetic moment of W atom, but the valley contribution is still appreciable. The Berry curvatures of the two inequivalent valleys of monolayer WSe2 are opposite and their change induced by B also depends linearly on B. The calculated circular dichroism and dielectric function reveal that the optical valley-dependent selection rule is preserved and the original single peak in polarization-resolved photoluminescence spectrum will be split into two peaks after the application of B. Our studies demonstrate the possibility of magnetic manipulation of the valley pseudospin.

Dark-exciton valley dynamics in transition metal dichalcogenide alloy monolayers

We report a comprehensive theory to describe exciton and biexciton valley dynamics in monolayer Mo1−xWxSe2 alloys. To probe the impact of different excitonic channels, including bright and dark excitons, intravalley biexcitons, intervalley scattering between bright excitons, as well as bright biexcitons, we have performed a systematic study from the simplest system to the most complex one. In contrast to the binary WSe2 monolayer with weak photoluminescence (PL) and high valley polarization at low temperatures and the MoSe2, that presents high PL intensity, but low valley polarization, our results demonstrate that it is possible to set up a ternary alloy with intermediate W-concentration that holds simultaneously a considerably robust light emission and an efficient optical orientation of the valley pseudospin. We find the critical value of W-concentration, xc, that turns alloys from bright to darkish. The dependence of the PL intensity on temperature shows three regimes: while bright monolayer alloys display a usual temperature dependence in which the intensity decreases with rising temperature, the darkish alloys exhibit the opposite behavior, and the alloys with x around xc show a non-monotonic temperature response. Remarkably, we observe that the biexciton enhances significantly the stability of the exciton emission against fluctuations of W-concentration for bright alloys. Our findings pave the way for developing high-performance valleytronic and photo-emitting devices.

Two classes of singularities and novel topology in a specially designed synthetic photonic crystals.

Zak phase and topological protected edge state are usually studied in one-dimensional (1D) photonic systems with spatial inversion symmetry (SIS). In this work, we find that specific classes of 1D structure without SIS can be mapped to a system with SIS and also exhibit novel topology, which manifest as phase cut lines (PCLs) in our specially designed synthetic photonic crystals (SPCs). Zak phase defined in SIS is extended to depict the topology of PCLs after redefinition, and a topological protected edge state is also achieved in our 1D structure without SIS. In our SPCs, the relationship between Chern numbers in two dimensions (2D) and the extended Zak phases of 1D PCLs is given, which are bound by the first type singularities. Higher Chern numbers and multi-chiral edge states are achieved utilizing the concept of synthetic dimensions. The effective Hamiltonian is given, based on which we find that the band edges of each PCL play a role analogous to the valley pseudospin, and our SPC is actually a new type of valley photonic crystal that is usually studied in graphene-like honeycomb lattice. The chiral valley edge transport is also demonstrated. In higher dimensions, the shift of the first type singularity in expanded parameter space will lead to the Weyl point topological transition, which we proposed in our previous work. In this paper, we find a second type of singularity that manifests as a singular surface in our expanded parameter space. The shift of the singular surface will lead to the nodal line topological transition. We find the states on the singular surface possess extremely high robustness against certain randomness, based on which a topological wave filter is constructed.

Guiding robust valley-dependent edge states by surface acoustic waves

Recently, the concept of valley pseudospin, labeling quantum states of energy extrema in momentum space, has attracted enormous attention because of its potential as a new type of information carrier. Here, we present surface acoustic wave (SAW) waveguides which utilize and transport valley pseudospins in two-dimensional SAW phononic crystals. In addition to a direct visualization of the valley-dependent modes excited from the corresponding chiral sources, the backscattering suppression of SAW valley-dependent edge modes transport is observed in sharply curved interfaces. By means of band structure engineering, elastic wave energy in the SAW waveguides can be transported with remarkable robustness, which is very promising for new generations of integrated solid-state phononic circuits with great versatility.

Lightwave control of the valley pseudospin in a monolayer of tungsten diselenide

As conventional electronic is approaching its ultimate limits, tremendous efforts have been taken to explore novel concepts of ultrafast quantum control. Lightwave electronics - the foundation of attosecond science - has opened a spectacular perspective by utilizing the oscillating carrier wave of an intense light pulse to control the translational motion of the electron’s charge faster than a single cycle of light [1-7]. Despite their promising potential as future information carriers [8,10], the internal quantum attributes such as spins and valley pseudospins have not been switchable at optical clock rates. Here we demonstrate a novel subcycle control scheme of the electron’s pseudospin in a monolayer of tungsten diselenide using strong mid-infrared lightwaves [9]. Our work opens the door towards systematic valleytronic protocols at optical clock rates.

Valley coherent exciton-polaritons in a monolayer semiconductor

Two-dimensional transition metal dichalcogenides (TMDs) provide a unique possibility to generate and read-out excitonic valley coherence using linearly polarized light, opening the way to valley information transfer between distant systems. However, these excitons have short lifetimes (ps) and efficiently lose their valley coherence via the electron-hole exchange interaction. Here, we show that control of these processes can be gained by embedding a monolayer of WSe2 in an optical microcavity, forming part-light-part-matter exciton-polaritons. We demonstrate optical initialization of valley coherent polariton populations, exhibiting luminescence with a linear polarization degree up to 3 times higher than displayed by bare excitons. We utilize an external magnetic field alongside selective exciton-cavity-mode detuning to control the polariton valley pseudospin vector rotation, which reaches 45° at B = 8 T. This work provides unique insight into the decoherence mechanisms in TMDs and demonstrates the potential for engineering the valley pseudospin dynamics in monolayer semiconductors embedded in photonic structures.

On-chip valley topological materials for elastic wave manipulation.

Valley topological materials, in which electrons possess valley pseudospin, have attracted a growing interest recently. The additional valley degree of freedom offers a great potential for its use in information encoding and processing. The valley pseudospin and valley edge transport have been investigated in photonic and phononic crystals for electromagnetic and acoustic waves, respectively. In this work, by using a micromanufacturing technology, valley topological materials are fabricated on silicon chips, which allows the observation of gyral valley states and valley edge transport for elastic waves. The edge states protected by the valley topology are robust against the bending and weak randomness of the channel between distinct valley Hall phases. At the channel intersection, a counterintuitive partition of the valley edge states manifests for elastic waves, in which the partition ratio can be freely adjusted. These results may enable the creation of on-chip high-performance micro-ultrasonic materials and devices.

Broadband nonlinear optical resonance and all-optical switching of liquid phase exfoliated tungsten diselenide

As a kind of two-dimensional transition metal dichalcogenide material, tungsten diselenide (WSe2) has attracted increasing attention, owing to its gapped electronic structure, relatively high carrier mobility, and valley pseudospin, all of which show its valuable nonlinear optical properties. There are few studies on the nonlinear optical properties of WSe2 and correlation with its electronic structure. In this paper, the effects of spatial self-phase modulation (SSPM) and distortion influence of WSe2 ethanol suspensions are systematically studied, namely, the nonlinear refractive index and third-order nonlinear optical effect. We obtained the WSe2 dispersions SSPM distortion formation mechanism, and through it, we calculated the nonlinear refractive index n2, nonlinear susceptibility χ(3), and their wavelength dependence under the excitation of 457 nm, 532 nm, and 671 nm lasers. Moreover, by use of its strong and broadband nonlinear optical response, all-optical switching of two different laser beams due to spatial cross-phase modulation has been realized experimentally. Our results are useful for future optical devices, such as all-optical switching and all-optical information conversion.

Robust quantum valley Hall effect for vortices in an interacting bosonic quantum fluid

Topologically protected pseudospin transport, analogous to the quantum spin Hall effect, cannot be strictly implemented for photons and in general bosons because of the lack of symmetry-protected pseudospins. Here we show that the required protection can be provided by the real-space topological excitation of an interacting quantum fluid: a quantum vortex. We consider a Bose-Einstein condensate at the Γ point of the Brillouin zone of a quantum valley Hall system based on two staggered honeycomb lattices. We demonstrate the existence of a coupling between the vortex winding and the valley of the bulk Bloch band. This leads to chiral vortex propagation on each side of the zigzag interface between two regions of inverted staggering. The topological protection provided by the vortex winding prevents valley pseudospin mixing and resonant backscattering, allowing a truly topologically protected valley pseudospin transport.

Slow and robust plate acoustic waveguiding with valley-dependent pseudospins

In this letter, we present an elastic slow-wave waveguide, which utilizes and transports valley pseudospins in a two-dimensional plate phononic crystal (PnC). Both straight and bent waveguides are made for demonstrating the backscattering suppression of elastic edge-state transport due to valley-dependent pseudospins. By band structure engineering, elastic waves in a plate waveguide can be transported at ultralow velocities with considerable robustness, which is very promising for the generation of new solid-state phononic circuits with great versatility.

Generation, transport and detection of valley-locked spin photocurrent in WSe2-graphene-Bi2Se3 heterostructures.

Quantum optoelectronic devices capable of isolating a target degree of freedom (DoF) from other DoFs have allowed for new applications in modern information technology. Many works on solid-state spintronics have focused on methods to disentangle the spin DoF from the charge DoF1, yet many related issues remain unresolved. Although the recent advent of atomically thin transition metal dichalcogenides (TMDs) has enabled the use of valley pseudospin as an alternative DoF2,3, it is nontrivial to separate the spin DoF from the valley DoF since the time-reversal valley DoF is intrinsically locked with the spin DoF4. Here, we demonstrate lateral TMD-graphene-topological insulator hetero-devices with the possibility of such a DoF-selective measurement. We generate the valley-locked spin DoF via a circular photogalvanic effect in an electric-double-layer WSe2 transistor. The valley-locked spin photocarriers then diffuse in a submicrometre-long graphene layer, and the spin DoF is measured separately in the topological insulator via non-local electrical detection using the characteristic spin-momentum locking. Operating at room temperature, our integrated devices exhibit a non-local spin polarization degree of higher than 0.5, providing the potential for coupled opto-spin-valleytronic applications that independently exploit the valley and spin DoFs.

Valley-Selective Response of Nanostructures Coupled to 2D Transition-Metal Dichalcogenides

Monolayer (1L) transition-metal dichalcogenides (TMDCs) are attractive materials for several optoelectronic applications because of their strong excitonic resonances and valley-selective response. Valley excitons in 1L-TMDCs are formed at opposite points of the Brillouin zone boundary, giving rise to a valley degree of freedom that can be treated as a pseudospin, and may be used as a platform for information transport and processing. However, short valley depolarization times and relatively short exciton lifetimes at room temperature prevent using valley pseudospins in on-chip integrated valley devices. Recently, it was demonstrated how coupling these materials to optical nanoantennas and metasurfaces can overcome this obstacle. Here, we review the state-of-the-art advances in valley-selective directional emission and exciton sorting in 1L-TMDC mediated by nanostructures and nanoantennas. We briefly discuss the optical properties of 1L-TMDCs paying special attention to their photoluminescence/absorption spectra, dynamics of valley depolarization, and the valley Hall effect. Then, we review recent works on nanostructures for valley-selective directional emission from 1L-TMDCs.

Evidence for line width and carrier screening effects on excitonic valley relaxation in 2D semiconductors

Monolayers of transition metal dichalcogenides (TMDC) have recently emerged as excellent platforms for exploiting new physics and applications relying on electronic valley degrees of freedom in two-dimensional (2D) systems. Here, we demonstrate that Coulomb screening by 2D carriers plays a critical role in excitonic valley pseudospin relaxation processes in naturally carrier-doped WSe2 monolayers (1L-WSe2). The exciton valley relaxation times were examined using polarization- and time-resolved photoluminescence spectroscopy at temperatures ranging from 10 to 160 K. We show that the temperature-dependent exciton valley relaxation times in 1L-WSe2 under various exciton and carrier densities can be understood using a unified framework of intervalley exciton scattering via momentum-dependent long-range electron–hole exchange interactions screened by 2D carriers that depend on the carrier density and the exciton linewidth. Moreover, the developed framework was successfully applied to engineer the valley polarization of excitons in 1L-WSe2. These findings may facilitate the development of TMDC-based opto-valleytronic devices.

Valley Manipulation by Optically Tuning the Magnetic Proximity Effect in WSe2/CrI3 Heterostructures.

Monolayer valley semiconductors, such as tungsten diselenide (WSe2), possess valley pseudospin degrees of freedom that are optically addressable but degenerate in energy. Lifting the energy degeneracy by breaking time-reversal symmetry is vital for valley manipulation. This has been realized by directly applying magnetic fields or via pseudomagnetic fields generated by intense circularly polarized optical pulses. However, sweeping large magnetic fields is impractical for devices, and the pseudomagnetic fields are only effective in the presence of ultrafast laser pulses. The recent rise of two-dimensional (2D) magnets unlocks new approaches to controlling valley physics via van der Waals heterostructure engineering. Here, we demonstrate the wide continuous tuning of the valley polarization and valley Zeeman splitting with small changes in the laser-excitation power in heterostructures formed by monolayer WSe2 and 2D magnetic chromium triiodide (CrI3). The valley manipulation is realized via the optical control of the CrI3 magnetization, which tunes the magnetic exchange field over a range of 20 T. Our results reveal a convenient new path toward the optical control of valley pseudospins and van der Waals magnetic heterostructures.

Lightwave valleytronics in a monolayer of tungsten diselenide.

As conventional electronics is approaching its ultimate limits1, nanoscience has urgently sought for novel fast control concepts of electrons at the fundamental quantum level2. Lightwave electronics3 – the foundation of attosecond science4 – utilizes the oscillating carrier wave of intense light pulses to control the translational motion of the electron’s charge faster than a single cycle of light5–15. Despite being particularly promising information carriers, the internal quantum attributes of spin16 and valley pseudospin17–19 have not been switchable on the subcycle scale20–21. Here we demonstrate lightwave-driven changes of the valley pseudospin and introduce distinct signatures in the optical read out. Photogenerated electron–hole pairs in a monolayer of tungsten diselenide are accelerated and collided by a strong lightwave. The emergence of high odd-order sidebands and anomalous changes in their polarization direction directly attest to the ultrafast pseudospin dynamics. Quantitative computations combining density-functional theory with a non-perturbative quantum many-body approach assign the polarization of the sidebands to a lightwave-induced change of the valley pseudospin and confirm that the process is coherent and adiabatic. Our work opens the door to systematic valleytronic logic at optical clock rates.

Topological valley-chiral edge states of Lamb waves in elastic thin plates

We investigate the nontrivial topology of the band structure of Lamb waves in a thin phononic crystal plate. When inversion symmetry is broken, a valley pseudospin degree of freedom is formed around K and K′ valleys for the A0 Lamb mode, which is decoupled from the S0 and SH0 modes in the low-frequency regime. Chiral edge states are explicitly demonstrated, which are immune to defects and exhibit unidirectional transport behaviors when intervalley scattering is weak. The quantum valley Hall effect is thus simulated in a simple way in the context of Lamb waves.

Valley-selective optical Stark effect probed by Kerr rotation

The ability to monitor and control distinct states is at the heart of emerging quantum technologies. The valley pseudospin in transition metal dichalcogenide (TMDC) monolayers is a promising degree of freedom for such control, with the optical Stark effect allowing for valley-selective manipulation of energy levels in WS$_2$ and WSe$_2$ using ultrafast optical pulses. Despite these advances, understanding of valley-sensitive optical Stark shifts in TMDCs has been limited by reflectance-based detection methods where the signal is small and prone to background effects. More sensitive polarization-based spectroscopy is required to better probe ultrafast Stark shifts for all-optical manipulation of valley energy levels. Here, we show time-resolved Kerr rotation to be a more sensitive probe of the valley-selective optical Stark effect in monolayer TMDCs. Compared to the established time-resolved reflectance methods, Kerr rotation is less sensitive to background effects. Kerr rotation provides a five-fold improvement in the signal-to-noise ratio of the Stark effect optical signal and a more precise estimate of the energy shift. This increased sensitivity allows for observation of an optical Stark shift in monolayer MoS$_2$ that exhibits both valley- and energy-selectivity, demonstrating the promise of this method for investigating this effect in other layered materials and heterostructures.

Spontaneous valley splitting and valley pseudospin field effect transistors of monolayer VAgP2Se6.

Valleytronics has attracted much attention due to its potential applications in the information processing industry. Creation of permanent valley polarization (PVP), i.e. unbalanced occupation at different valleys, is a vital requirement for practical devices in valleytronics. However, the development of an appropriate material with PVP remains a main challenge. Here we used first-principles calculations to predict that the spin-orbit coupling and magnetic ordering allow spontaneous valley Zeeman-type splitting in the pristine monolayer of VAgP2Se6. After suitable doping of VAgP2Se6, the Zeeman-type valley splitting results in a PVP, similar to the effect of spin polarization in spintronics. The VAgP2Se6 monolayer has nonequivalent valleys which can emit or absorb circularly polarized photons with opposite chirality. It thus shows great potential to be used as a photonic chirality filter and a circularly polarized light source. We then designed a valley pseudospin field effect transistor (VPFET) based on the monolayer VAgP2Se6, akin to the spin field effect transistors. In contrast to the current common transistors, VPFETs carry information of not only the electrons but also the valley pseudospins, far beyond common transistors.