23 research outputs found
The origin of exciton mass in a frustrated Mott insulator NaIrO
We use a three-pulse ultrafast optical spectroscopy to study the relaxation
processes in a frustrated Mott insulator NaIrO. By being able to
independently produce the out-of-equilibrium bound states (excitons) of
doublons and holons with the first pulse and suppress the underlying
antiferromagnetic order with the second one, we were able to elucidate the
relaxation mechanism of quasiparticles in this system. By observing the
difference in the exciton dynamics in the magnetically ordered and disordered
phases we found that the mass of this quasiparticle is mostly determined by its
interaction with the surrounding spins
Observation of intervalley biexcitonic optical Stark effect in monolayer WS2
Coherent optical dressing of quantum materials offers technological
advantages to control their electronic properties, such as the electronic
valley degree of freedom in monolayer transition metal dichalcogenides (TMDs).
Here, we observe a new type of optical Stark effect in monolayer WS2, one that
is mediated by intervalley biexcitons under the blue-detuned driving with
circularly polarized light. We found that such helical optical driving not only
induces an exciton energy downshift at the excitation valley, but also causes
an anomalous energy upshift at the opposite valley, which is normally forbidden
by the exciton selection rules but now made accessible through the intervalley
biexcitons. These findings reveal the critical, but hitherto neglected, role of
biexcitons to couple the two seemingly independent valleys, and to enhance the
optical control in valleytronics
Valley-selective optical Stark effect in monolayer WS2
Breaking space-time symmetries in two-dimensional crystals (2D) can
dramatically influence their macroscopic electronic properties. Monolayer
transition-metal dichalcogenides (TMDs) are prime examples where the
intrinsically broken crystal inversion symmetry permits the generation of
valley-selective electron populations, even though the two valleys are
energetically degenerate, locked by time-reversal symmetry. Lifting the valley
degeneracy in these materials is of great interest because it would allow for
valley-specific band engineering and offer additional control in valleytronic
applications. While applying a magnetic field should in principle accomplish
this task, experiments to date have observed no valley-selective energy level
shifts in fields accessible in the laboratory. Here we show the first direct
evidence of lifted valley degeneracy in the monolayer TMD WS2. By applying
intense circularly polarized light, which breaks time-reversal symmetry, we
demonstrate that the exciton level in each valley can be selectively tuned by
as much as 18 meV via the optical Stark effect. These results offer a novel way
to control valley degree of freedom, and may provide a means to realize new
valley-selective Floquet topological phases in 2D TMDs
Preliminary investigation of non-invasive blood pressure estimation using speckle contrast optical spectroscopy
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Observation of exciton redshift-blueshift crossover in monolayer WS2
We report a rare atom-like interaction between excitons in monolayer WS2,
measured using ultrafast absorption spectroscopy. At increasing excitation
density, the exciton resonance energy exhibits a pronounced redshift followed
by an anomalous blueshift. Using both material-realistic computation and
phenomenological modeling, we attribute this observation to plasma effects and
an attraction-repulsion crossover of the exciton-exciton interaction that
mimics the Lennard-Jones potential between atoms. Our experiment demonstrates a
strong analogy between excitons and atoms with respect to inter-particle
interaction, which holds promise to pursue the predicted liquid and crystalline
phases of excitons in two-dimensional materials
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Light-induced charge density wave in LaTe3
When electrons in a solid are excited with light, they can alter the free
energy landscape and access phases of matter that are beyond reach in thermal
equilibrium. This accessibility becomes of vast importance in the presence of
phase competition, when one state of matter is preferred over another by only a
small energy scale that, in principle, is surmountable by light. Here, we study
a layered compound, LaTe, where a small in-plane (a-c plane) lattice
anisotropy results in a unidirectional charge density wave (CDW) along the
c-axis. Using ultrafast electron diffraction, we find that after
photoexcitation, the CDW along the c-axis is weakened and subsequently, a
different competing CDW along the a-axis emerges. The timescales characterizing
the relaxation of this new CDW and the reestablishment of the original CDW are
nearly identical, which points towards a strong competition between the two
orders. The new density wave represents a transient non-equilibrium phase of
matter with no equilibrium counterpart, and this study thus provides a
framework for unleashing similar states of matter that are "trapped" under
equilibrium conditions
Evidence for topological defects in a photoinduced phase transition
Upon excitation with an intense ultrafast laser pulse, a symmetry-broken
ground state can undergo a non-equilibrium phase transition through pathways
dissimilar from those in thermal equilibrium. Determining the mechanism
underlying these photo-induced phase transitions (PIPTs) has been a
long-standing issue in the study of condensed matter systems. To this end, we
investigate the light-induced melting of a unidirectional charge density wave
(CDW) material, LaTe. Using a suite of time-resolved probes, we
independently track the amplitude and phase dynamics of the CDW. We find that a
quick (1ps) recovery of the CDW amplitude is followed by a slower
reestablishment of phase coherence. This longer timescale is dictated by the
presence of topological defects: long-range order (LRO) is inhibited and is
only restored when the defects annihilate. Our results provide a framework for
understanding other PIPTs by identifying the generation of defects as a
governing mechanism