Excitons In Quantum Research Articles

Quantum enhanced Josephson junction field-effect transistors for logic applications

Josephson junction field-effect transistors (JJFETs) have recently re-emerged as promising candidates for superconducting computing. For JJFETs to perform Boolean logic operations, the so-called gain factor αR must be larger than 1. In a conventional JJFET made with a classical channel material, due to a gradual dependence of superconducting critical current on the gate bias, αR is much smaller than 1. In this Letter, we propose a new device structure of quantum enhanced JJFETs in a zero-energy-gap InAs/GaSb heterostructure. We demonstrate that, due to an excitonic insulator quantum phase transition in this zero-gap heterostructure, the superconducting critical current displays a sharp transition as a function of gate bias, and the deduced gain factor αR ∼ 0.06 is more than 50 times that (∼0.001) reported in a classical JJFET. Further optimization may allow achieving a gain factor larger than 1 for logic applications.

(Invited) Resonant Exciton Transfer in Mixed-Dimensional Heterostructures for Overcoming Dimensional Restrictions in Optical Processes

Nanomaterials exhibit unique optical phenomena, in particular excitonic quantum processes occurring at room temperature. The low dimensionality, however, imposes strict requirements for conventional optical excitation, and an approach for bypassing such restrictions is desirable. Here we report on exciton transfer in carbon-nanotube/tungsten-diselenide heterostructures, where band alignment can be systematically varied. The mixed-dimensional heterostructures display a pronounced exciton reservoir effect where the longer-lifetime excitons within the two-dimensional semiconductor are funneled into carbon nanotubes through diffusion. This new excitation pathway presents several advantages, including larger absorption areas, broadband spectral response, and polarization-independent efficiency. When band alignment is resonant, we observe substantially more efficient excitation via tungsten diselenide compared to direct excitation of the nanotube. We further demonstrate simultaneous bright emission from an array of carbon nanotubes with varied chiralities and orientations. Our findings show the potential of mixed-dimensional heterostructures and band alignment engineering for energy harvesting and quantum applications through exciton manipulation.Parts of this study are supported by JSPS (KAKENHI JP22K14624, JP22K14625, JP21K14484, JP22F22350, JP22K14623, JP21H05233, JP23H00262, JP20H02558) and MEXT (ARIM JPMXP1222UT1135). Y.R.C. is supported by JSPS (International Research Fellow). N.F. and C.F.F. are supported by RIKEN Special Postdoctoral Researcher Program. We thank the Advanced Manufacturing Support Team at RIKEN for technical assistance.[1] N. Fang, D. Yamashita, S. Fujii, M. Maruyama, Y. Gao, Y. R. Chang, C. F. Fong, K. Otsuka, K. Nagashio, S. Okada, Y. K. Kato, "Resonant exciton transfer in mixed-dimensional heterostructures for overcoming dimensional restrictions in optical processes," Nature Commun., accepted (2023). Preprint at arXiv:2307.07124.

Size-Tunable Manganese-Doped Spheroidal CsPbCl3 Quantum Dots.

Manganese doping has been demonstrated as a versatile tool to tune the emission of CsPbCl3 nanocrystals (NCs). Although this has been demonstrated in nanocubes and nanoplatelets, strategies for doping Mn2+ in size-tunable, excitonic CsPbCl3 quantum dots (QDs) remain absent. In this work, we demonstrate the synthesis of size-tunable spheroidal CsPbCl3:Mn2+ QDs, which can be obtained by a water-hexane interfacial combined anion and cation exchange strategy starting from CsPbBr3 QDs. Interestingly, the QDs exhibit a fast 0.2 ms Mn2+ photoluminescence (PL) lifetime and an energy transfer (ET) time of approximately 100 ps from the excitonic state of the QD to the atomic state of the Mn2+ ion. The size dependence observation of the manganese PL efficiency and the slow ET rate suggest that Mn2+ mainly gets incorporated at the QD's surface, highlighting the importance of strategies chosen for the incorporation of Mn2+ into perovskite QDs.

Impact of functional groups in spacer cations on the properties of PEA-based 2D monolayer halide perovskites

Incorporating low-dimensionalization technologies effectively tackle the challenge of inadequate long-term stability in hybrid halide perovskites, however their wide bandgap and strong quantum well confinement remain substantial obstacle for various optoelectronic applications. Addressing these issues without compromising long-term stability has emerged as a pivotal focus in materials science, in particular exploring the effects of the functional groups within spacer cations. Our simulations reveal that the robust π-π stacking interactions involving PEA+ and the strong hydrogen bonding interactions between PEA+ and MX64− contribute to narrowing the electronic bandgap in 2D monolayer PEA2MX4 (e. g. 2D monolayer PEA2SnI4: 1.34 ​eV) for reasonable visible-light absorption while simultaneously ensuring their favorable long-term stability. Moreover, the delocalized orbitals and relatively high dielectric constants in PEA+, attributed to the conjugated benzene ring, has been observed to weaken the potential barrier, exciton binding effect and quantum well confinement in 2D monolayer PEA2MX4, thus facilitating photogenerated electron-hole separations and out-of-plane carrier transport. The impact of spacer cations on the optoelectronic and transport properties of 2D monolayer perovskites highlights the critical role of meticulously chosen and well-designed spacer cations, especially functional groups, in shaping their photophysical properties and ensuring long-term stability even under extremely operating conditions.

Population and Energy Transfer Dynamics in an Open Excitonic Quantum Battery.

In a previous study, we proposed an open quantum network model of a quantum battery (QB) that possesses dark states owing to its structural exchange symmetries. While in a dark state, the QB is capable of storing an exciton without any environment-induced population losses. However, when the structural exchange symmetry is broken, the QB begins to discharge the exciton towards its exit site. In this article, we start by demonstrating that this QB is not only loss-free with respect to exciton population during the storage phase, but also with respect to the QB energy. We then explore the exciton population and energy transfer dynamics of the QB during the discharge phase over a wide range of site energies, bath temperatures, and bath reorganization energies. Our results shed light on how to optimize the QB's population and energy transfer dynamics for different purposes.

Hidden excitonic quantum states with broken time-reversal symmetry

Hidden excitonic quantum states with broken time-reversal symmetry

Resonant exciton transfer in mixed-dimensional heterostructures for overcoming dimensional restrictions in optical processes

Nanomaterials exhibit unique optical phenomena, in particular excitonic quantum processes occurring at room temperature. The low dimensionality, however, imposes strict requirements for conventional optical excitation, and an approach for bypassing such restrictions is desirable. Here we report on exciton transfer in carbon-nanotube/tungsten-diselenide heterostructures, where band alignment can be systematically varied. The mixed-dimensional heterostructures display a pronounced exciton reservoir effect where the longer-lifetime excitons within the two-dimensional semiconductor are funneled into carbon nanotubes through diffusion. This new excitation pathway presents several advantages, including larger absorption areas, broadband spectral response, and polarization-independent efficiency. When band alignment is resonant, we observe substantially more efficient excitation via tungsten diselenide compared to direct excitation of the nanotube. We further demonstrate simultaneous bright emission from an array of carbon nanotubes with varied chiralities and orientations. Our findings show the potential of mixed-dimensional heterostructures and band alignment engineering for energy harvesting and quantum applications through exciton manipulation.

Entropic Ligand Mixing for Engineering 2D Layered Perovskite from Colloidal Monolayer Building Blocks

AbstractLayered 2D perovskites are solution‐processed quantum‐wells. Their effective band‐gap is determined via the inorganic perovskite layer thickness and exciton quantum confinement effects. Alternatively, by changing the organic moieties, one can tune the dielectric constant and distance between the monolayers modifying the excitonic interactions. In colloidal perovskites, a dynamic equilibrium exists between the free organic moieties in the solution and the surface of the nanocrystal. Colloidal synthesis is used to make single monolayer L2PbBr4 platelets and assemble these into layered 2D stacks. In the experiment, L is an alkylamine surface ligand whose length (4‐18 carbons) determines the interlayer distances between the quantum‐wells. The dynamic equilibrium of ligand mixtures in solution and perovskite surfaces leads to optimal mixing of the molecules. During the self‐assembly of monolayers, the distance between the inorganic layers is thus engineered. The interlayer distance is proportional to the average ligand mixture length. This results in controlled interactions between the 2D‐excitons, enabling red‐shifted absorption and emission and extended lifetimes for longer alkyl chains. Using entropic mixing of ligands for the engineering of 2D excitonic interactions is therefore demonstrated. Formation of layered 2D perovskites from colloidal building blocks allows intermixing of dissimilar materials opening possibilities for new heterostructures and junctions.

Many-Exciton Quantum Dynamics in a Ruddlesden-Popper Tin Iodide.

We present a study on the many-body exciton interactions in a Ruddlesden-Popper tin halide, namely, (PEA)2SnI4 (PEA = phenylethylammonium), using coherent two-dimensional electronic spectroscopy. The optical dephasing times of the third-order polarization observed in these systems are determined by exciton many-body interactions and lattice fluctuations. We investigate the excitation-induced dephasing (EID) and observe a significant reduction of the dephasing time with increasing excitation density as compared to its lead counterpart (PEA)2PbI4, which we have previously reported in a separate publication [J. Chem. Phys.2020, 153, 164706]. Surprisingly, we find that the EID interaction parameter is four orders of magnitude higher in (PEA)2SnI4 than that in (PEA)2PbI4. This increase in the EID rate may be due to exciton localization arising from a more statically disordered lattice in the tin derivative. This is supported by the observation of multiple closely spaced exciton states and the broadening of the linewidth with increasing population time (spectral diffusion), which suggests a static disordered structure relative to the highly dynamic lead-halide. Additionally, we find that the exciton nonlinear coherent lineshape shows evidence of a biexcitonic state with low binding energy (<10 meV) not observed in the lead system. We model the lineshapes based on a stochastic scattering theory that accounts for the interaction with a nonstationary population of dark background excitations. Our study provides evidence of differences in the exciton quantum dynamics between tin- and lead-based Ruddlesden-Popper metal halides (RPMHs) and links them to the exciton-exciton interaction strength and the static disorder aspect of the crystalline structure.

Full‐Landscape Condensation Phases for Long‐Lived Excitons in 2D Tellurium: Crystal‐Field Splitting and Finite‐Momentum Excitons

AbstractThe realization of high‐temperature exciton macroscopic quantum phases in materials still remains challenging due to strict constraints in thermal stabilities and excitonic lifetimes. In this work, by using first‐principles calculations, the exciton dispersions and macroscopic quantum phase transitions of 2D α‐ and β‐Te are investigated. The excitons with lowest eigen energy for both α‐ and β‐Te are dark restricted by the crystal‐field symmetries. The Bose–Einstein condensation (BEC) transition for α‐ and β‐Te can be realized at 165.4 and 32.8 K with low excitation power densities of 0.42 × 1012 and 1.0 × 1012 cm−2, respectively. Furthermore, the phase transitions from insulating free exciton (FE) gas to conducting electron–hole plasma (EHP) and electron–hole liquid (EHL), as well as that from BEC to superfluidity phases are also predicted. Finally, the authors investigate the microscopic dynamics for bright excitons of 2D tellurium and find that they reach thermal equilibrium at 1–50 fs and excitonic lifetimes can reach 1–40 ns, beneficial for experimental observation of quantum condensate states. The findings in this work not only demonstrate the excellent optoelectronic properties of 2D tellurium allotropes, but also provide a promising platform for experimental realization of high‐temperature excitonic macroscopic quantum condensates.

Biochemical reactions remote-sensitized by excitons propagating across macroscopic distances along biological fibers: Isomerization of 11-cis-retinal

Presently we report a second example of a biochemical reaction sensitized remotely by excitons (electronic excited states) efficiently transferred along intermediate filaments (IFs) over macroscopic distances. IFs are excellent conductors of energy in the form of excitons, with the efficiency of ca. 0.53 reported in vitro. Excitons were generated by visible light and propagated along Müller cell (MC) intermediate filaments, about 100 μm long. These experiments used a capillary matrix filled by MC IFs extracted from porcine retina. Excitons induced efficient isomerization of 11-cis- to all-trans-retinal, with the reaction quantum yield φcis/trans = 0.347 obtained for 11-cis-retinal concentration of 1.0 × 10−3 M (0.284 g/L) using 546 nm light at 6.87 mW/cm2 power density. Exciton quantum yield φexc at 546 nm was also measured in function of radiation power density, with nonlinearity indicative of biphotonic processes. This is the first case of biphotonic processes occurring at such low light intensities with steady-state illumination. The present results support quantum mechanism of high-contrast vision of vertebrate eyes.

Exciton quantum beats in GaAs/AlAs disk-shaped quantum dots

Using the renormalized wavefunction approach, we examined the excitonic quantum beat in a three-level system in disk-shaped quantum dots. The non-stationary electron wave function and the time-dependent exciton absorption intensity under the effect of an intensity pump laser that resonates with two excited electron states were also provided. Our numerical findings demonstrate that the time-dependent exciton absorption intensity takes the shape of a periodic oscillation when the quantum dot is subjected to a powerful resonant pump laser. This is clear evidence that quantum beats appears in the quantum disk. Additionally, the dot radius and the pump field detuning significantly affect the beat characteristics, such as frequency and amplitude.

Interaction-driven transport of dark excitons in 2D semiconductors with phonon-mediated optical readout

The growing field of quantum information technology requires propagation of information over long distances with efficient readout mechanisms. Excitonic quantum fluids have emerged as a powerful platform for this task due to their straightforward electro-optical conversion. In two-dimensional transition metal dichalcogenides, the coupling between spin and valley provides exciting opportunities for harnessing, manipulating, and storing bits of information. However, the large inhomogeneity of single layers cannot be overcome by the properties of bright excitons, hindering spin-valley transport. Nonetheless, the rich band structure supports dark excitonic states with strong binding energy and longer lifetime, ideally suited for long-range transport. Here we show that dark excitons can diffuse over several micrometers and prove that this repulsion-driven propagation is robust across non-uniform samples. The long-range propagation of dark states with an optical readout mediated by chiral phonons provides a new concept of excitonic devices for applications in both classical and quantum information technology.

Tunable phononic coupling in excitonic quantum emitters.

Engineering the coupling between fundamental quantum excitations is at the heart of quantum science and technologies. An outstanding case is the creation of quantum light sources in which coupling between single photons and phonons can be controlled and harnessed to enable quantum information transduction. Here we report the deterministic creation of quantum emitters featuring highly tunable coupling between excitons and phonons. The quantum emitters are formed in strain-induced quantum dots created in homobilayer WSe2. The colocalization of quantum-confined interlayer excitons and terahertz interlayer breathing-mode phonons, which directly modulates the exciton energy, leads to a uniquely strong phonon coupling to single-photon emission, with a Huang-Rhys factor reaching up to 6.3. The single-photon spectrum of interlayer exciton emission features a single-photon purity >83% and multiple phonon replicas, each heralding the creation of a phonon Fock state in the quantum emitter. Due to the vertical dipole moment of the interlayer exciton, the phonon-photon interaction is electrically tunable to be higher than the exciton and phonon decoherence rate, and hence promises to reach the strong-coupling regime. Our result demonstrates a solid-state quantum excitonic-optomechanical system at the atomic interface of the WSe2 bilayer that emits flying photonic qubits coupled with stationary phonons, which could be exploited for quantum transduction and interconnection.

Zero-field quantum beats and spin decoherence mechanisms in CsPbBr3 perovskite nanocrystals

Coherent optical manipulation of exciton states provides a fascinating approach for quantum gating and ultrafast switching. However, their coherence time for incumbent semiconductors is highly susceptible to thermal decoherence and inhomogeneous broadening effects. Here, we uncover zero-field exciton quantum beating and anomalous temperature dependence of the exciton spin lifetimes in CsPbBr3 perovskite nanocrystals (NCs) ensembles. The quantum beating between two exciton fine-structure splitting (FSS) levels enables coherent ultrafast optical control of the excitonic degree of freedom. From the anomalous temperature dependence, we identify and fully parametrize all the regimes of exciton spin depolarization, finding that approaching room temperature, it is dominated by a motional narrowing process governed by the exciton multilevel coherence. Importantly, our results present an unambiguous full physical picture of the complex interplay of the underlying spin decoherence mechanisms. These intrinsic exciton FSS states in perovskite NCs present fresh opportunities for spin-based photonic quantum technologies.

Perspectives and recent advances of two-dimensional III-nitrides: Material synthesis and emerging device applications

Both two-dimensional (2D) transitional metal dichalcogenides (TMDs) and III–V semiconductors have been considered as potential platforms for quantum technology. While 2D TMDs exhibit a large exciton binding energy, and their quantum properties can be tailored via heterostructure stacking, TMD technology is currently limited by the incompatibility with existing industrial processes. Conversely, III-nitrides have been widely used in light-emitting devices and power electronics but not leveraging excitonic quantum aspects. Recent demonstrations of 2D III-nitrides have introduced exciton binding energies rivaling TMDs, promising the possibility to achieve room-temperature quantum technologies also with III-nitrides. Here, we discuss recent advancements in the synthesis and characterizations of 2D III-nitrides with a focus on 2D free-standing structures and embedded ultrathin quantum wells. We overview the main obstacles in the material synthesis, vital solutions, and the exquisite optical properties of 2D III-nitrides that enable excitonic and quantum-light emitters.

Macroscopic transition metal dichalcogenides monolayers with uniformly high optical quality

The unique optical properties of transition metal dichalcogenide (TMD) monolayers have attracted significant attention for both photonics applications and fundamental studies of low-dimensional systems. TMD monolayers of high optical quality, however, have been limited to micron-sized flakes produced by low-throughput and labour-intensive processes, whereas large-area films are often affected by surface defects and large inhomogeneity. Here we report a rapid and reliable method to synthesize macroscopic-scale TMD monolayers of uniform, high optical quality. Using 1-dodecanol encapsulation combined with gold-tape-assisted exfoliation, we obtain monolayers with lateral size > 1 mm, exhibiting exciton energy, linewidth, and quantum yield uniform over the whole area and close to those of high-quality micron-sized flakes. We tentatively associate the role of the two molecular encapsulating layers as isolating the TMD from the substrate and passivating the chalcogen vacancies, respectively. We demonstrate the utility of our encapsulated monolayers by scalable integration with an array of photonic crystal cavities, creating polariton arrays with enhanced light-matter coupling strength. This work provides a pathway to achieving high-quality two-dimensional materials over large areas, enabling research and technology development beyond individual micron-sized devices.

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.

Crossover from strong to weak exciton confinement in thickness-controlled epitaxial PbI2 thin films

Spatially confined excitons undergo two distinct quantization effects depending on the confinement length, the enhanced binding energy under strong confinement, and the center-of-mass quantization under weak one. However, the transition between them has not been experimentally identified in two-dimensional (2D) materials due to the lack of thin films satisfying large-scale uniformity and atomic-level flatness in a wide thickness range. Here, we reveal the crossover in high-quality epitaxial thin films of a 2D semiconductor PbI2 grown by molecular beam epitaxy. The absorption spectra exhibit oscillatory structures manifesting the exciton center-of-mass quantization, and the quantization energies show an additional blue shift associated with the strong confinement effect below five-layer thickness (35 Å). The precise control of exciton quantum states will lead to the further development of optoelectronic functionalities of 2D materials.

High-lying valley-polarized trions in 2D semiconductors

Optoelectronic functionalities of monolayer transition-metal dichalcogenide (TMDC) semiconductors are characterized by the emergence of externally tunable, correlated many-body complexes arising from strong Coulomb interactions. However, the vast majority of such states susceptible to manipulation has been limited to the region in energy around the fundamental bandgap. We report the observation of tightly bound, valley-polarized, UV-emissive trions in monolayer TMDC transistors: quasiparticles composed of an electron from a high-lying conduction band with negative effective mass, a hole from the first valence band, and an additional charge from a band-edge state. These high-lying trions have markedly different optical selection rules compared to band-edge trions and show helicity opposite to that of the excitation. An electrical gate controls both the oscillator strength and the detuning of the excitonic transitions, and therefore the Rabi frequency of the strongly driven three-level system, enabling excitonic quantum interference to be switched on and off in a deterministic fashion.