Excitons In Semiconductors Research Articles

Exciton-Photonics: From Fundamental Science to Applications.

Semiconductors in all dimensionalities ranging from 0D quantum dots and molecules to 3D bulk crystals support bound electron-hole pair quasiparticles termed excitons. Over the past two decades, the emergence of a variety of low-dimensional semiconductors that support excitons combined with advances in nano-optics and photonics has burgeoned an advanced area of research that focuses on engineering, imaging, and modulating the coupling between excitons and photons, resulting in the formation of hybrid quasiparticles termed exciton-polaritons. This advanced area has the potential to bring about a paradigm shift in quantum optics, as well as classical optoelectronic devices. Here, we present a review on the coupling of light in excitonic semiconductors and previous investigations of the optical properties of these hybrid quasiparticles via both far-field and near-field imaging and spectroscopy techniques. Special emphasis is given to recent advances with critical evaluation of the bottlenecks that plague various materials toward practical device implementations including quantum light sources. Our review highlights a growing need for excitonic material development together with optical engineering and imaging techniques to harness the utility of excitons and their host materials for a variety of applications.

Bandgap control in two-dimensional semiconductors via coherent doping of plasmonic hot electrons

Bandgap control is of central importance for semiconductor technologies. The traditional means of control is to dope the lattice chemically, electrically or optically with charge carriers. Here, we demonstrate a widely tunable bandgap (renormalisation up to 550 meV at room-temperature) in two-dimensional (2D) semiconductors by coherently doping the lattice with plasmonic hot electrons. In particular, we integrate tungsten-disulfide (WS2) monolayers into a self-assembled plasmonic crystal, which enables coherent coupling between semiconductor excitons and plasmon resonances. Accompanying this process, the plasmon-induced hot electrons can repeatedly fill the WS2 conduction band, leading to population inversion and a significant reconstruction in band structures and exciton relaxations. Our findings provide an effective measure to engineer optical responses of 2D semiconductors, allowing flexibilities in design and optimisation of photonic and optoelectronic devices.

Inducing and Probing Localized Excitons in Atomically Thin Semiconductors via Tip‐Enhanced Cavity‐Spectroscopy

AbstractIn atomically thin semiconductors, localized exciton (XL) coupled to light provides a new class of optical sources for potential applications in quantum communication. However, in most studies, XL photoluminescence (PL) from crystal defects has mainly been observed in cryogenic conditions because of their sub‐wavelength emission region and low quantum yield at room temperature. Hybrid‐modality of cavity‐spectroscopy to induce and probe the XL emissions at the nanoscale in atomically thin semiconductors is presented. By placing a WSe2 monolayer on the two extremely sharp Au tips in a bowtie antenna with a radius of curvature of <1 nm, tensile strain of ≈0.3% is effectively induced in a <30 nm region to create robust XL states. The Au tip then approaches the strained crystal region to enhance the XL emissions and probe them with tip‐enhanced photoluminescence (TEPL) spectroscopy at room temperature. Through this triple‐sharp‐tips cavity‐spectroscopy with <15 nm spatial resolution, TEPL enhancement as high as ≈4.0 × 104 by the Purcell effect is achieved, and peak energy shifts of XL up to ≈40 meV are observed. This approach combining nano‐cavity and ‐spectroscopy provides a systematic way to induce and probe the radiative emission of localized excitons in 2D semiconductors offering new strategies for dynamic quantum nano‐optical devices.

Asymmetric Rydberg blockade of giant excitons in Cuprous Oxide

The ability to generate and control strong long-range interactions via highly excited electronic states has been the foundation for recent breakthroughs in a host of areas, from atomic and molecular physics to quantum optics and technology. Rydberg excitons provide a promising solid-state realization of such highly excited states, for which record-breaking orbital sizes of up to a micrometer have indeed been observed in cuprous oxide semiconductors. Here, we demonstrate the generation and control of strong exciton interactions in this material by optically producing two distinct quantum states of Rydberg excitons. This is made possible by two-color pump-probe experiments that allow for a detailed probing of the interactions. Our experiments reveal the emergence of strong spatial correlations and an inter-state Rydberg blockade that extends over remarkably large distances of several micrometers. The generated many-body states of semiconductor excitons exhibit universal properties that only depend on the shape of the interaction potential and yield clear evidence for its vastly extended-range and power-law character.

Exciton–Plasmon Coupling in 2D Semiconductors Accessed by Surface Acoustic Waves

We theoretically demonstrate the coupling between excitons in 2D semiconductors and surface plasmons in a thin metal film by means of a surface acoustic wave (SAW), proving that the generated exciton-plasmon polaritons (or plexcitons) are in the strong coupling regime. The strain field of the SAW creates a dynamic diffraction grating providing the momentum match for the surface plasmons, whereas the piezoelectric field, that could dissociate the excitons, is cancelled out by the metal. This is exemplified for monolayer MoS$\mathrm{_{2}}$ and mono- and few-layer black phosphorus on top of a thin silver layer on a LiNbO$\mathrm{_{3}}$ piezoelectric substrate, providing Rabi splittings of 100-150 meV. Thus, we demonstrate that SAWs are powerful tools to modulate the optical properties of supported 2D semiconductors by means of the high-frequency localized deformations tailored by the acoustic transducers, that can serve as electrically switchable launchers of propagating plexcitons suitable for active high-speed nanophotonic applications.

Controlling exciton many-body states by the electric-field effect in monolayer MoS2

We report magneto-optical spectroscopy of gated monolayer MoS$_2$ in high magnetic fields up to 28T and obtain new insights on the many-body interaction of neutral and charged excitons with the resident charges of distinct spin and valley texture. For neutral excitons at low electron doping, we observe a nonlinear valley Zeeman shift due to dipolar spin-interactions that depends sensitively on the local carrier concentration. As the Fermi energy increases to dominate over the other relevant energy scales in the system, the magneto-optical response depends on the occupation of the fully spin-polarized Landau levels in both $K/K^{\prime}$ valleys. This manifests itself in a many-body state. Our experiments demonstrate that the exciton in monolayer semiconductors is only a single particle boson close to charge neutrality. We find that away from charge neutrality it smoothly transitions into polaronic states with a distinct spin-valley flavour that is defined by the Landau level quantized spin and valley texture.

Intrinsic donor-bound excitons in ultraclean monolayer semiconductors

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (>6 µs) and polarization lifetimes (>100 ns). Resonant excitation of the free inter- and intravalley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellitesʼ photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.

Collective states of excitons in semiconductors

A review of many-body effects in exciton ensembles in semiconductors is given with the emphasis on two-dimensional systems: structures with single and double quantum wells and with quantum microcavities. The Bose–Einstein condensation effect, an accumulation of a macroscopic number of excitons in the ground state of the system, is discussed. The known prohibition on condensation in low-dimensional systems can be lifted due to the disorder resulting from the chaotic potential. Manifestations of the finite exciton lifetime and, correspondingly, of the nonequilibrium of the excitonic system caused by processes of excitons entering and leaving the condensate state are analyzed. Other collective phases of excitons, namely, two-dimensional crystals of dipolar excitons and an electron–hole liquid, formed as a result of interparticle interactions, are discussed.

Nature of Optical Excitations in Porphyrin Crystals: A Joint Experimental and Theoretical Study.

The nature of optical excitations and the spatial extent of excitons in organic semiconductors, both of which determine exciton diffusion and carrier mobilities, are key factors for the proper understanding and tuning of material performances. Using a combined experimental and theoretical approach, we investigate the excitonic properties of meso-tetraphenyl porphyrin-Zn(II) crystals. We find that several bands contribute to the optical absorption spectra, beyond the four main ones considered here as the analogue to the four frontier molecular orbitals of the Gouterman model commonly adopted for the isolated molecule. By using many-body perturbation theory in the GW and Bethe–Salpeter equation approach, we interpret the experimental large optical anisotropy as being due to the interplay between long- and short-range intermolecular interactions. In addition, both localized and delocalized excitons in the π-stacking direction are demonstrated to determine the optical response, in agreement with recent experimental observations reported for organic crystals with similar molecular packing.

Monolayer Excitonic Semiconductors Integrated with Au Quasi-Periodic Nanoterrace Morphology on Fused Silica Substrates for Light-Emitting Devices

Two-dimensional transition metal dichalcogenides (TMDs) have a promising future in the nanophotonics field due to their unique optoelectronic properties such as large exciton binding energies and c...

Geometry relaxation-mediated localization and delocalization of excitons in organic semiconductors: A quantum chemical study.

Photo-induced relaxation processes leading to excimer formations or other traps are in the focus of many investigations of optoelectronic materials because they severely affect the efficiencies of corresponding devices. Such relaxation effects comprise inter-monomer distortions in which the orientations of the monomer change with respect to each other, whereas intra-monomer distortions are variations in the geometry of single monomers. Such distortions are generally neglected in quantum chemical investigations of organic dye aggregates due to the accompanied high computational costs. In the present study, we investigate their relevance using perylene-bisimide dimers and diindenoperylene tetramers as model systems. Our calculations underline the importance of intra-monomer distortions on the shape of the potential energy surfaces as a function of the coupling between the monomers. The latter is shown to depend strongly on the electronic state under consideration. In particular, it differs between the first and second excited state of the aggregate. Additionally, the magnitude of the geometrical relaxation decreases if the exciton is delocalized over an increasing number of monomers. For the interpretation of the vibronic coupling model, pseudo-Jahn-Teller or Marcus theory can be employed. In the first part of this paper, we establish the accuracy of density functional theory-based approaches for the prediction of vibrationally resolved absorption spectra of organic semiconductors. These investigations underline the accuracy of those approaches although shortcomings become obvious as well. These calculations also indicate the strength of intra-monomer relaxation effects.

A snapshot review on exciton engineering in deformed 2D materials

AbstractMost optoelectronic characteristics of two-dimensional (2D) materials are associated with excitonic effects. Excitonic effects in 2D material have been intensively investigated, and various efforts to engineer exciton behavior in 2D materials have been reported for advanced nanophotonic and optoelectronic applications. Excitons in 2D semiconductors can be controlled by external stimuli, including mechanical, electrical, thermal, and magnetic stimuli. Mechanical stimuli applied to a 2D material can generate uniform or non-uniform deformation and strain gradient in the 2D lattice, which creates a strain-induced bandgap energy gradient in the 2D material. In an inhomogeneous bandgap energy gradient generated by a non-uniform strain gradient, excitons drift across the energy gradient. Exciton engineering in deformed 2D materials aims to control exciton movement by mechanical strain. In this snapshot review, we focus on exciton engineering in a mechanically deformed 2D material and their potential towards advanced optoelectronic and photonic applications.

Room Temperature Polariton Lasing in Ladder‐Type Oligo(p‐Phenylene)s with Different π‐Conjugation Lengths

Polariton lasing is coherent emission that originates from macroscopic accumulation of polariton population in the ground state and is a promising route toward efficient coherent light sources as population inversion is not necessary. Unlike most Wannier–Mott excitons in inorganic semiconductors, Frenkel excitons created in organic semiconductors have high oscillator strength and high exciton binding energy, which sustain stable exciton–polaritons at room temperature. Herein, room temperature polariton lasing from a novel class of ladder‐type oligo(p‐phenylene)s is demonstrated. The polariton lasers exhibit a nonlinear increase of their spectrally integrated emission, a reduction in spectral linewidth, blueshift of emission peaks, and long‐range spatial coherence when the pump fluence is increased above threshold. By tuning the π‐conjugation length of the molecular structure, the polariton lasing wavelength can be changed from 430 to 457 nm. Optically pumped thresholds of 12 and 17 μJ cm−2 are observed, which are among the lowest values reported for polariton lasing in organic semiconductors.

Theory of moiré localized excitons in transition metal dichalcogenide heterobilayers

Transition-metal dichalcogenide heterostructures exhibit moir\'e patterns that spatially modulate the electronic structure across the material's plane. For certain material pairs, this modulation acts as a potential landscape with deep, trigonally symmetric wells capable of localizing interlayer excitons, forming periodic arrays of quantum emitters. Here, we study these moir\'e localized exciton states and their optical properties. By numerically solving the two-body problem for an interacting electron-hole pair confined by a trigonal potential, we compute the localized exciton spectra for different pairs of materials. We derive optical selection rules for the different families of localized states, each belonging to one of the irreducible representations of the potential's symmetry group $C_{3v}$, and numerically estimate their polarization-resolved absorption spectra. We find that the optical response of localized moir\'e interlayer excitons is dominated by states belonging to the doubly-degenerate $E$ irreducible representation. Our results provide new insights into the optical properties of artificially confined excitons in two-dimensional semiconductors.

(Invited) Manipulating Carrier Polarization in Semiconductor Nanocrystals

Degenerately-doped semiconductor nanocrystals exhibiting tunable localized surface plasmon resonance (LSPR) have attracted significant attention in recent years due to their unique optoelectronic properties. Unlike noble metal nanoparticles, colloidal plasmonic semiconductor nanocrystals have LSPR frequencies tunable in the infrared region, which makes them appealing for terahertz imaging, heat-responsive devices, and surface-enhanced infrared spectroscopic measurements. Besides expanding the LSPR frequency range, plasmonic semiconductor nanocrystals could potentially bring about numerous other opportunities related to single-phase plasmon-exciton interactions. However, non-resonant nature of the LSPR and exciton in semiconductors has been a major obstacle toward realizing such opportunities. In this talk I will discuss the results of our recent work on the structure and composition dependent plasmonic properties of colloidal semiconductor nanostructures. I will particularly focus on generating robust excitonic splitting in degenerately-doped transparent metal oxide nanocrystals, enabled by non-resonant plasmon-exciton coupling in an external magnetic field. This phenomenon allows for controlling carrier polarization in semiconductor nanocrystals using circularly polarized light. Furthermore, the ability to control carrier localization and nanocrystal morphology allows for further manipulation of the excitonic splitting pattern and quantum states in these non-magnetic semiconductor nanostructures. Possible applications of this emerging class of multifunctional materials for new electronic and quantum information technologies will also be discussed.

Strong Plasmon-Exciton Coupling in Ag Nanoparticle-Conjugated Polymer Core-Shell Hybrid Nanostructures.

Strong plasmon–exciton coupling between tightly-bound excitons in organic molecular semiconductors and surface plasmons in metal nanostructures has been studied extensively for a number of technical applications, including low-threshold lasing and room-temperature Bose-Einstein condensates. Typically, excitons with narrow resonances, such as J-aggregates, are employed to achieve strong plasmon–exciton coupling. However, J-aggregates have limited applications for optoelectronic devices compared with organic conjugated polymers. Here, using numerical and analytical calculations, we demonstrate that strong plasmon–exciton coupling can be achieved for Ag-conjugated polymer core-shell nanostructures, despite the broad spectral linewidth of conjugated polymers. We show that strong plasmon–exciton coupling can be achieved through the use of thick shells, large oscillator strengths, and multiple vibronic resonances characteristic of typical conjugated polymers, and that Rabi splitting energies of over 1000 meV can be obtained using realistic material dispersive relative permittivity parameters. The results presented herein give insight into the mechanisms of plasmon–exciton coupling when broadband excitonic materials featuring strong vibrational–electronic coupling are employed and are relevant to organic optoelectronic devices and hybrid metal–organic photonic nanostructures.

Efficient UV Luminescence from Organic-Based Tamm Plasmon Structures Emitting in the Strong-Coupling Regime

Excitons in organic semiconductors possessing a large oscillator strength demonstrate strong coupling with cavity modes at room temperature. A large Stokes shift in some organic semiconductors enri...

Dynamic Exciton Funneling by Local Strain Control in a Monolayer Semiconductor.

The ability to control excitons in semiconductors underlies numerous proposed applications, from excitonic circuits to energy transport. Two dimensional (2D) semiconductors are particularly promising for room-temperature applications due to their large exciton binding energy and enormous stretchability. Although the strain-induced static exciton flux has been observed in predetermined structures, dynamic control of exciton flux represents an outstanding challenge. Here, we introduce a method to tune the bandgap of suspended 2D semiconductors by applying a local strain gradient with a nanoscale tip. This strain allows us to locally and reversibly shift the exciton energy and to steer the exciton flux over micrometer-scale distances. We anticipate that our result not only marks an important experimental tool but will also open a broad range of new applications from information processing to energy conversion.

Femtosecond exciton dynamics in WSe2 optical waveguides

Van-der Waals (vdW) atomically layered crystals can act as optical waveguides over a broad range of the electromagnetic spectrum ranging from Terahertz to visible. Unlike common Si-based waveguides, vdW semiconductors host strong excitonic resonances that may be controlled using non-thermal stimuli including electrostatic gating and photoexcitation. Here, we utilize waveguide modes to examine photo-induced changes of excitons in the prototypical vdW semiconductor, WSe2, prompted by femtosecond light pulses. Using time-resolved scanning near-field optical microscopy we visualize the electric field profiles of waveguide modes in real space and time and extract the temporal evolution of the optical constants following femtosecond photoexcitation. By monitoring the phase velocity of the waveguide modes, we detect incoherent A-exciton bleaching along with a coherent optical Stark shift in WSe2.

Optical properties of charged excitons in two-dimensional semiconductors

Strong Coulomb interaction in atomically thin transition metal dichalcogenides makes these systems particularly promising for studies of excitonic physics. Of special interest are the manifestations of the charged excitons, also known as trions, in the optical properties of two-dimensional semiconductors. In order to describe the optical response of such a system, the exciton interaction with resident electrons should be explicitly taken into account. In this paper, we demonstrate that this can be done in both the trion (essentially, few-particle) and Fermi-polaron (many-body) approaches, which produce equivalent results, provided that the electron density is sufficiently low and the trion binding energy is much smaller than the exciton one. Here, we consider the oscillator strengths of the optical transitions related to the charged excitons, fine structure of trions, and Zeeman effect, as well as photoluminescence of trions illustrating the applicability of both few-particle and many-body models.