Optical Stark Effect Research Articles

Autler-Townes splitting and linear dichroism in colloidal CdSe nanoplatelets driven by near-infrared pulses.

Coherent interaction between light fields and matter has led to many exotic physical phenomena, one of which is the Autler-Townes splitting originating from the optical Stark effect of a three-level system. It has been well documented for atoms, molecules, and low-dimensional, epitaxial-grown semiconductors but rarely for solution-processed samples. Here, we report on Autler-Townes splitting observed in CdSe nanoplatelets. The strong intersubband transition of these nanoplatelets situated in the near-infrared allows us to coherently drive it with femtosecond near-infrared pulses, and meanwhile, their strong interband transition in the visible records the resultant spectral changes. The splitting is most prominent with cross-polarized, linear pump-probe pulses, consistent with the orthogonal polarizations of interband and intersubband transition dipoles. When the nanoplatelets are driven from tilted incident angles, we observe anomalous spectral lineshapes arising from a competition between interband and intersubband drivings, which are nevertheless quantitatively captured by our dressed-state model simulation.

Laser-Induced Transparency in CdSe Nanoplatelets under Conditions of the Optical Stark Effect

Laser-Induced Transparency in CdSe Nanoplatelets under Conditions of the Optical Stark Effect

Valley-selective manipulation of moiré excitons through optical Stark effect

Abstract Semiconductor moiré superlattices provide great platforms for exploring exotic collective excitations. Optical Stark effect, a shift of the electronic transition in the presence of light field, provides an ultrafast and coherent method of manipulating matter states, which, however, has not been demonstrated in moiré materials. Here, we report the valley-selective optical Stark effect of moiré excitons in the WSe2/WS2 superlattice by using transient reflection spectroscopy. Prominent valley-selective energy shifts up to 7.8 meV have been observed for moiré excitons, corresponding to pseudo-magnetic fields as large as 34 T. Our results provide a route to coherently manipulate exotic states in moiré superlattices.

Cavity Floquet engineering

Floquet engineering is a promising tool to manipulate quantum systems coherently. A well-known example is the optical Stark effect, which has been used for optical trapping of atoms and breaking time-reversal symmetry in solids. However, as a coherent nonlinear optical effect, Floquet engineering typically requires high field intensities obtained in ultrafast pulses, severely limiting its use. Here, we demonstrate using cavity engineering of the vacuum modes to achieve orders-of-magnitude enhancement of the effective Floquet field, enabling Floquet effects at an extremely low fluence of 450 photons/μm2. At higher fluences, the cavity-enhanced Floquet effects lead to 50 meV spin and valley splitting of WSe2 excitons, corresponding to an enormous time-reversal breaking, non-Maxwellian magnetic field of over 200 T. Utilizing such an optically controlled effective magnetic field, we demonstrate an ultrafast, picojoule chirality XOR gate. These results suggest that cavity-enhanced Floquet engineering may enable the creation of steady-state or quasi-equilibrium Floquet bands, strongly non-perturbative modifications of materials beyond the reach of other means, and application of Floquet engineering to a wide range of materials and applications.

Dynamically Tunable Circularly Polarized Selectivity in Plasmon-Enhanced Halide Perovskite Nanocrystal Glasses.

Ultrafast spin manipulation for optical spin-logic applications requires material systems with strong spin-selective light-matter interactions. The optical Stark effect can realize spin-selective light-matter interactions by breaking the degeneracy of spin-selective transitions with an external electric field. Halide perovskites have large exciton binding energies, which enable a room-temperature optical Stark effect. However, halide perovskites are prone to degradation when interacting with light and polar solvents, limiting further integration with nanophotonic structures. We demonstrate a hybrid material system consisting of CsPbBr3 nanocrystal glass weakly coupled to resonant plasmonic silver nanoparticles, showing ultrafast tunable spin-based polarization selectivity at room temperature. We performed circularly polarized pump-probe characterizations to investigate the optical Stark effect in this material system, which resulted in a maximum energy shift of ∼3.67 meV (detuning energy of 0.11 eV and pump intensity of 0.62 GW/cm2). We show that halide perovskite nanocrystal glasses have excellent resistance to heat and moisture, which may be favorable for integration with nanophotonic structures for further engineering polarization states, energy tuning, and coherence time.

The Impact of Partial Carrier Confinement on Stimulated Emission in Strongly Confined Perovskite Nanocrystals.

Semiconductor lead halide perovskites are excellent candidates for realizing low threshold light amplification due to their tunable and highly efficient luminescence, ease of processing, and strong light-matter interactions. However, most studies on optical gain have addressed bulk films, nanowires, or nanocrystals that exhibit little or no size quantization. Here, we show by means of a multitude of optical spectroscopy methods that small CsPbBr3 nanocrystals (NCs) exhibit a progressive red shift of the band-edge transition upon addition of electron-hole pairs, at least one carrier of which occupies a 2-fold degenerate, delocalized state in agreement with strong confinement. We demonstrate that this combination results in a threshold for biexciton gain, well below the limit of one electron-hole pair on average per NC. On the other hand, both the luminescent lifetime and the optical Stark effect of 4.7 nm CsPbBr3 NCs indicate that the oscillator strength of the band-edge transition is considerably smaller than expected from the band-edge absorption. We assign this discrepancy to a mixed confinement regime, with one delocalized and one localized charge carrier, and show that the concomitant reduction of the oscillator strength for stimulated emission accounts for the surprisingly small material gain observed in small NCs. The conclusion of mixed confinement aligns with studies reporting small and large polarons for holes and electrons in lead halide perovskite nanocrystals, respectively, and creates opportunities for understanding multiexciton photophysics in confined perovskite materials.

Floquet Engineering of Excitons in Two-Dimensional Halide Perovskites via Biexciton States.

Layered two-dimensional halide perovskites (2DHPs) exhibit exciting non-equilibrium properties that allow the manipulation of energy levels through coherent light-matter interactions. Under the Floquet picture, novel quantum states manifest through the optical Stark effect (OSE) following intense subresonant photoexcitation. Nevertheless, a detailed understanding of the influence of strong many-body interactions between excitons on the OSE in 2DHPs remains unclear. Herein, we uncover the crucial role of biexcitons in photon-dressed states and demonstrate precise optical control of the excitonic states via the biexcitonic OSE in 2DHPs. With fine step tuning of the driven energy, we fully parametrize the evolution of exciton resonance modulation. The biexcitonic OSE enables Floquet engineering of the exciton resonance with either a blue-shift or a red-shift of the energy levels. Our findings shed new light on the intricate nature of coherent light-matter interactions in 2DHPs and extend the degree of freedom for ultrafast coherent optical control over excitonic states.

Exciton–polaritons in a 2D hybrid organic–inorganic perovskite microcavity with the presence of optical Stark effect

This study investigates the properties of exciton–polaritons in a two-dimensional (2D) hybrid organic–inorganic perovskite microcavity in the presence of optical Stark effect. Through both steady and dynamic state analyses, strong coupling between excitons of perovskite and cavity photons is revealed, indicating the formation of polaritons in the perovskite microcavity. Besides, it is found that an external optical Stark pulse can induce energy shifts of excitons proportional to the pulse intensity, which modifies the dispersion characteristics of the polaritons.

Signatures of the Optical Stark Effect on Entangled Photon Pairs from Resonantly Pumped Quantum Dots

Signatures of the Optical Stark Effect on Entangled Photon Pairs from Resonantly Pumped Quantum Dots

Optical stark effect on CdSe nanoplatelets with mid-infrared excitation for large amplitude ultrafast modulation

The optical Stark effect is a universal response of the electronic structure to incident light. In semiconductors, particularly nanomaterials, the optical Stark effect achieved with sub-band gap photons can drive large, narrowband, and potentially ultrafast changes in the absorption or reflection at the band gap through excitation of virtual excitons. Rapid optical modulation using the optical Stark effect is ultimately constrained, however, by the generation of long-lived excitons through multiphoton absorption. This work compares the modulation achievable using the optical Stark effect on CdSe nanoplatelets with several different pump photon energies, from the visible to mid-infrared. Despite expected lower efficiencies for spectrally-remote pump energies, infrared pump pulses can ultimately drive larger sub-picosecond optical Stark shifts of virtual excitons without creation of real excitons. The CdSe nanoplatelets show subpicosecond shifts of the lowest excitonic resonance of up to 22 meV, resulting in change in absorption as large as 0.32 OD (49% increase in transmission), with a long-lived offset from real excitons less than 1% of the peak signal.

Ultrafast valley-selective coherent optical manipulation with excitons in WSe2 and MoS2 monolayers

Increasing the speed limits of conventional electronics requires innovative approaches to manipulate other quantum properties of electrons besides their charge. An alternative approach utilizes the valley degree of freedom in low-dimensional semiconductors. Here we demonstrate that the valley degeneracy of exciton energies in transition metal dichalcogenide monolayers may be lifted by coherent optical interactions on timescales corresponding to few tens of femtoseconds. The optical Stark and Bloch-Siegert effects generated by strong nonresonant circularly-polarized light induce valley-selective blue shifts of exciton quantum levels by more than 30 meV. We show these phenomena by studying the two most intensive exciton resonances in transiton metal dichalcogenide monolayers and compare the results to a theoretical model, which properly includes the Coulomb interaction and exciton dispersion. These results open the door for ultrafast valleytronics working at multiterahertz frequencies.

Reexamination of the Giant Oscillator Strength Effect in CdSe Nanoplatelets

CdSe nanoplatelets have large extinction coefficients and, at low temperatures, very short photoluminescence lifetimes. To explain these observations, the giant oscillator strength (GOST) effect has been hypothesized to exist in such semiconductor nanoplatelets. In principle, suppression of phonon scattering can increase the area of coherent exciton motion up to the full size of the nanoplatelet, increasing oscillator strength proportional to its size. Yet at high temperatures, the measured area of exciton motion as estimated from various photophysical methods is much smaller than the size of nanoplatelets and insensitive to nanoplatelet size. To examine the emergence of this discrepancy, this work uses temperature-dependent measurements of steady-state absorption, transient optical bleaching, and the optical Stark effect. Although the excitonic oscillator strength (and size) does increase somewhat at reduced temperature, a large difference remains between the area of coherent exciton motion and the entire nanoplatelet area. These measurements indicate that the area of coherent exciton center-of-mass motion does not increase sufficiently to explain observed rapid radiative lifetimes in CdSe nanoplatelets at 3 K. Instead, the data are consistent with localization of excitons to areas much smaller than whole NPLs.

The Interaction of 2D Materials With Circularly Polarized Light.

2D materials (2DMs), due to spin-valley locking degree of freedom, exhibit strongly bound exciton and chiral optical selection rules and become promising material candidates for optoelectronic and spin/valleytronic devices. Over the last decade, the manifesting of 2D materials by circularly polarized lights expedites tremendous fascinating phenomena, such as valley/exciton Hall effect, Moiré exciton, optical Stark effect, circular dichroism, circularly polarized photoluminescence, and spintronic property. In this review, recent advance in the interaction of circularly polarized light with 2D materials covering from graphene, black phosphorous, transition metal dichalcogenides, van der Waals heterostructures as well as small proportion of quasi-2D perovskites and topological materials, is overviewed. The confronted challenges and theoretical and experimental opportunities are also discussed, attempting to accelerate the prosperity of chiral light-2DMs interactions.

Programmable Polariton Topological Insulators All-Optically Controlled by the Stark Effect.

Efficiently and flexibly manipulating unidirectional edge states is key to developing topological insulators as functional devices. In this work, we propose an all-optical method that utilizes the valley-selective optical Stark effect to realize programmable topological insulators. We pattern a two-dimensional honeycomb structure in an exciton-polariton platform resulting from a strong light-matter coupling in a monolayer transition metal dichalcogenide. The optical Stark effect is induced to generate a pseudo magnetic field, combined with spin-orbit coupling to form the topological one-way edge states of the polariton. On account of the ultrafast switching speed and precisely spatial controllability of the optical Stark effect, two applications, i.e., ports ratio tunable polariton splitter and programmable polariton router, were demonstrated, showing designable and rewritable functionality of all-optically controllable polariton topological insulators. This study paves the way to robustly and intelligently control/form polaritonic and spintronic devices for future classical and quantum information processing and application.

Intense high-frequency laser-field control of spin-orbit coupling in GaInAs/AlInAs quantum wells: A laser dressing effect

The Rashba spin-orbit (SO) coupling can flexibly be controlled via an external gate, while the Dresselhaus term, which mainly depends on quantum confinement, is in general immune to electrical control. Here we theoretically report optical manipulation of the SO coupling by resorting to an intense high-frequency laser field, which ``dresses'' the confining potential for electrons as a result of optical stark effect and enables a flexible and simultaneous control of the Rashba and Dresselhaus couplings. Focusing on ordinary GaInAs/AlInAs quantum wells with two occupied subbands subject to both laser and gate fields, we perform a self-consistent Poisson-Schr\"odinger calculation in the Hartree approximation to determine electro-optical control of the intrasubband (intersubband) Rashba ${\ensuremath{\alpha}}_{\ensuremath{\nu}}$ ($\ensuremath{\eta}$) and Dresselhaus ${\ensuremath{\beta}}_{\ensuremath{\nu}}$ ($\mathrm{\ensuremath{\Gamma}}$) SO terms with $\ensuremath{\nu}=1,2$. Under the impact of laser field, we find that the Rashba terms of the two subbands ${\ensuremath{\alpha}}_{1}$ and ${\ensuremath{\alpha}}_{2}$ may remain locked to equal strength in a broad gate range, providing a means for unified manipulation of the two-subband Rashba couplings. Further, as the laser field varies, we observe that ${\ensuremath{\alpha}}_{1}$ and ${\ensuremath{\alpha}}_{2}$ may have either the same or opposite signs, or even ${\ensuremath{\alpha}}_{2}$ vanishes while ${\ensuremath{\alpha}}_{1}$ is finite, greatly fascinating for selective SO control of distinct subbands. For the Dresselhaus coupling, we disclose two distinct scenarios depending on the interplay of the well width and the laser field strength, and reveal that ${\ensuremath{\beta}}_{2}$ may decrease rapidly when the laser field strengthens, even though ${\ensuremath{\beta}}_{1}$ remains essentially constant. Regarding the intersubband Rashba ($\ensuremath{\eta}$) and Dresselhaus ($\mathrm{\ensuremath{\Gamma}}$) terms, which mainly depend on the overlap and parity of the wave functions of the two subbands, they have relatively weak dependence on the laser field. Moreover, the combined effect of intra- and intersubband SO terms may lead to crossings and avoided crossings of the energy dispersion of multiband spin branches and may even trigger the spin polarization of an originally spin degenerate (unpolarized) band, tunable by the laser field. Our results should stimulate experiments probing the laser field mediated multiband SO control and further enables its spintronic applications.

Photon‐dressed biexciton‐mediated anomalous optical Stark effect in CsPbBr3 nanocrystals

Intense optical pumping below bandgap creates photon‐dressed states, which interact with and result in the shift of electronic levels, known as optical Stark effect (OSE). The OSE persists only for pump pulse duration implying the coherent response. Previous researches in semiconductors have shown that coherent OSE is evident only at cryogenic temperatures. Only recently lead halide perovskites (LHPs) were found to possess room temperature OSE. However, despite strong exciton–exciton interactions known in LHPs, OSE is described in non‐interacting framework that treats an exciton like isolated two‐level systems. The present work demonstrates anomalous OSE in interacting regime at room temperature in CsPbBr3 nanocrystals having exciton resonance (E0) at 2.40 eV using circularly polarized transient absorption spectroscopy. The results show that photon dressing for same pump–probe polarization (σ+σ+) lead to observation of blueshift of exciton resonance across all pump detuning energies (Ω = E0 − ħω; ħω is pump photon energy). On the contrary, under opposite pump–probe polarization (σ+σ−), the strong exciton−biexciton interaction leads to a coherent red shift and splitting of the exciton resonance as a function of the drive photon frequency. The hallmark observations under σ+σ− result from large biexciton binding energy of ~71 meV and exciton−biexciton transition dipole moment of ~25 Debye. Further, we demonstrate unusual crossover from linear to nonlinear dependence of the OSE as a function of the drive photon frequency. Our systematic study reveals crucial information on unexplored many body interacting regimes of coherent manipulation in LHPs, important for exotic quantum phase applications like quantum information processing and ultrafast switching.

Excitonic Bloch–Siegert shift in CsPbI3 perovskite quantum dots

Coherent interaction between matter and light field induces both optical Stark effect and Bloch–Siegert shift. Observing the latter has been historically challenging, because it is weak and is often accompanied by a much stronger Stark shift. Herein, by controlling the light helicity, we can largely restrict these two effects to different spin-transitions in CsPbI3 perovskite quantum dots, achieving room-temperature Bloch–Siegert shift as strong as 4 meV with near-infrared pulses. The ratio between the Bloch–Siegert and optical Stark shifts is however systematically higher than the prediction by the non-interacting, quasi-particle model. With a model that explicitly accounts for excitonic effects, we quantitatively reproduce the experimental observations. This model depicts a unified physical picture of the optical Stark effect, biexcitonic optical Stark effect and Bloch–Siegert shift in low-dimensional materials displaying strong many-body interactions, forming the basis for the implementation of these effects to information processing, optical modulation and Floquet engineering.

Wavelength and Stacking Order Dependent Exciton Dynamics in Bulk ReS2

AbstractExciton dynamics in ReS2 are rich and complicated due to its unique lattice and band structures. With femtosecond pump‐probe spectroscopy, the wavelength‐dependent exciton dynamics in bulk ReS2 are studied at room temperature, and the effects from polarization and stacking order are also considered. Signatures from both bright and dark excitons are observed, where the lifetime at bright exciton levels is only several picoseconds, while it is about tens of picoseconds at dark exciton levels. The extremely long up‐/down‐going trend at selective wavelengths is attributed to the transition between different dark exciton levels. Two types of coherent dynamics are observed when the laser wavelength is scanned through different exciton levels: coherent optical stark effect and exciton‐exciton coherent interaction, both occurring within the laser pulse width. Exciton dynamics show a similar trend for the sample with AA stacking order, regardless of polarization. For AB stacking, dark excitons for Exciton II and III are missing when probe pulse polarization is perpendicular to b‐axis, which is attributed to the polarization and stacking‐order dependent band structure. Studying the rich exciton dynamics in bulk ReS2 at room temperature not only has scientific significance, but also facilitates the development of high‐performance excitonic devices.

Exciton-driven renormalization of quasiparticle band structure in monolayer MoS2

Optical excitation serves as a powerful approach to control the electronic structure of layered Van der Waals materials via many-body screening effects, induced by photoexcited free carriers, or via light-driven coherence, such as optical Stark and Bloch-Siegert effects. Although theoretical work has also pointed to an exotic mechanism of renormalizing band structure via excitonic correlations in bound electron-hole pairs (excitons), experimental observation of such exciton-driven band renormalization and the full extent of their implications is still lacking, largely due to the limitations of optical probes and the impact of screening effects. Here, by using extreme-ultraviolet time-resolved angle-resolved photoemission spectroscopy together with excitonic many-body theoretical calculations, we directly unmask the band renormalization effects driven by excitonic correlations in a monolayer semiconductor. We revealed a surprising bandgap opening, increased by 40 meV, and a simultaneous enhancement of band effective mass. Our findings unmask the novel exciton-driven mechanism towards the band engineering in photoexcited semiconducting materials, opening a new playground to manipulate the transient energy states in layered quantum materials via optical controls of excitonic many-body correlations.

Strengthened spin–valley polarization and negative magnetoresistance based on an asymmetrical double-ferromagnetic WSe2 junction

The spin–valley transport and tunneling magnetoresistance (TMR) are studied in an off-resonant circularly polarized light (CPL)-modulated asymmetrical double-ferromagnetic (FM) WSe 2 junction. Based on the results, we propose a device that can simultaneously operate as spin–valley filter and exhibit giant magnetoresistance in a single system. For the antiparallel configuration, the broken spin degeneracy and optical Stark effect in the first FM barrier produce 100% spin (valley)-polarized particles. Subsequently, the spin valve effect from the second FM barrier gives rise to a 100% spin (valley)-selective polarization in the strengthened polarized energy windows of the first CPL. Furthermore, increasing the asymmetrical magnetization of such a junction is found to substantially enhance the TMR, leading to a positive 100% TMR. The TMR can be tuned from −100% to 100% and is sensitively dependent on the chirality of the second CPL due to the coupling between the spin valve effect and the optical Stark effect. Our findings indicate that double-FM barriers combined with off-resonant CPL modulation can be an excellent option to improve the spin–valley polarization and magnetoresistance of transition metal dichalcogenide-based structures. • A model was proposed to improve the magnetoresistance response as well as obtain two well-defined spin–valley polarization states. • The structure gives rise to a 100% spin (valley)-selective polarization in strengthened polarized energy windows of light. • Increasing the asymmetrical magnetization of this junction is found to substantially enhance the tunnel magnetoresistance to 100%. • The magnetoresistance can be tuned from −100% to 100% and is sensitively dependent on the chirality of light.