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International audience; In pump-probe spectroscopy, two laser pulses are employed to garner dynamical information from the sample of interest. The pump initiates the optical process by exciting a portion of the sample from the electronic ground state to an accessible electronic excited state, an exciton. Thereafter, the probe interacts with the already excited sample. The change in the absorbance after pump provides information on transitions between the excited states and their dynamics. In this work we study these exciton-exciton transitions by means of an ab initio real time propagation scheme based on dynamical Berry phase formulation. The results are then analyzed taking advantage of a Fermi-golden rule approach formulated in the excitonic basis-set and in terms of the symmetries of the excitonic states. Using bulk LiF and 2D hBN as two prototype materials, we discuss the selection rules for transitions involving strongly bound Frenkel excitons, for which the hydrogen model cannot be used.

Violet phosphorus (VP), the most stable phosphorus allotrope, is a van der Waals semiconductor that can be used to construct $\textit{p}$-type nanodevices. Recently, high-quality VP crystals have been synthesized while a deep insight into their excitonic properties and bandgap tailoring approaches, which are crucial for their optoelectronic device applications, is still lacking. Here, we study the optical properties of ultrathin VP by second harmonic generation, photoluminescence, and optical absorption spectroscopy. We observed strong bound exciton emission that is 0.48 eV away from the free exciton emission, which is among the largest in 2D materials. In addition, the bandgaps of VP are highly sensitive to the number of layers and external strain, which provides convenient approaches for bandgap engineering. The strong bound exciton emission and tunable bandgaps make VP a promising material in optoelectronic devices.

Controlled insertion of electronic states within the band gap of semiconductor nanocrystals (NCs) is a powerful tool for tuning their physical properties. One compelling example is II-VI NCs incorporating heterovalent coinage metals in which hole capture produces acceptor-bound excitons. To date, the opposite donor-bound exciton scheme has not been realized because of the unavailability of suitable donor dopants. Here, we produce a model system for donor-bound excitons in CdSeS NCs engineered with sulfur vacancies (VS) that introduce a donor state below the conduction band (CB), resulting in long-lived intragap luminescence. VS-localized electrons are almost unaffected by trapping, and suppression of thermal quenching boosts the emission efficiency to 85%. Magneto-optical measurements indicate that the VS are not magnetically coupled to the NC bands and that the polarization properties are determined by the spin of the valence-band photohole, whose spin flip is massively slowed down due to suppressed exchange interaction with the donor-localized electron.

Reduced dielectric screening in two-dimensional materials enables bound excitons, which modifies their optical absorption and optoelectronic response even at room temperature. Here, we demonstrate the existence of excitons in the bandgap of the monolayer family of the newly discovered synthetic MoSi$_2$Z$_4$ (Z = N, P, and As) series of materials. All three monolayers support several bright and strongly bound excitons with binding energies varying from 1 eV to 1.35 eV for the lowest energy exciton resonances. On increasing the pump fluence, the exciton binding energies get renormalized, leading to a redshift-blueshift crossover. Our study shows that the MoSi$_2$Z$_4$ series of monolayers offer an exciting test-bed for exploring the physics of strongly bound excitons and their non-equilibrium dynamics., 10 pages, 5 figures, 2 tables. Comments and suggestions are most welcome

The in-plane anisotropy of monolayer InSe plays a critical role in the application of photodetectors. In this work, through nonequilibrium Green's function density functional theory (NEGF-DFT) and time-dependent density functional theory (TD-DFT) calculations, we investigated the anisotropic quantum transport in darkness and under linearly polarized light, and explored the role of surface-bound excitons in the anisotropic photocurrent. The anisotropic dark quantum transport is attributed to different potential barriers in the zigzag and armchair orientations (Id-zig/Id-arm = 1.2 × 102). Linearly polarized photocurrent calculations show that the extinction ratio reaches a maximum value of 105.67. Moreover, surface-bound exciton calculations via TD-DFT revealed that the strong anisotropic photocurrent derives from surface-bound excitons generated in the In 5pz, Se 4pz, and Se 4dz2 orbitals. InSe shows tremendous potential for use in field-effect transistors, flexible nano- and optoelectronics, and polarized light devices.

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 $��$s) and polarization lifetimes (> 100 ns). Resonant excitation of the free inter- and intra-valley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellites 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., to be appear in Nature Communications

Lead-free halide double perovskites have emerged as a nontoxic alternative to the heavily researched lead-based halide perovskites. However, their optical properties and the initial charge carrier relaxation processes are under debate. In this study, we apply time-resolved photoluminescence and differential transmission spectroscopy to investigate the photoexcited charge carrier dynamics within the indirect band structure of Cs2AgBiBr6 nanocrystals. Interestingly, we observe a high energetic emission stemming from the direct band gap, besides the previously reported emission from the indirect band gap transition. We attribute this emission to the radiative recombination of direct bound excitons. This emission maximum redshifts nearly 1 eV within 10 ps due to electron intervalley scattering, which leads to a transfer of direct to indirect bound excitons. We conclude that these direct bound excitons possess a giant oscillator strength causing not only a pronounced absorption peak at the optical band gap energy but also luminescence to occur at the direct band gap transition in spite of the prevailing intervalley scattering process. These results expand the understanding of the optical properties and the charge carrier relaxation in double perovskites, thus, facilitating the further development of optoelectronic devices harnessing lead-free perovskites.

The controlled insertion of electronic states within the band gap of semiconductor nanocrystals (NCs) is a powerful tool which enables to engineer their physical properties. One compelling example is represented by metal chalcogenide NCs incorporating heterovalent p-type impurities such as d10 coinage metals (Cu+, Ag+, or Au+). In this type of nanostructures the interplay between the dopant and the host semiconductor energy levels leads in the ultra-fast capture of the hole by the localized state of the dopant leading to the so-called "acceptor-bound" excitons. The control of this paradigm unlocked technologically relevant functionalities, such as Stokes-shift between the emission and the absorptionspectra, extended luminescence lifetimes, photomagnetic behaviors, and enhanced electrical transport. To date, although conceptually analogous to hole-management schemes, the opposite "donor-bound" exciton scheme using aliovalent elements adopted to n-doped NCs (e.g., Al3+ and In3+ in) has not been realized due to the natural propensity of such cations to produce shallow donor states that inject electrons directly in the CB. Here, we exploit the propensity of metal sulfides to present sulfur vacancies (VS) that introduce a localized level pinned about 1 eV below the CB to produce a model system for "donor-bound" excitons in CdSeS NCs. The investigation of the optical and magneto-optical properties of these NCs revealed that the VS state is responsible for the ultrafast capture of electron in the CB which can then either decay non-radiatively or recombine with the VB photohole leading to long-lived, Stokes-shifted emission with size-tunable energy. Moreover, VS-localized electrons are almost unaffected by trapping, and suppression of thermal quenching boosts the emission efficiency to 85%. Magneto-optical measurements indicate that the VS are not magnetically coupled to the NC bands and that the polarization properties are determined by the spin of the valence-band photo-hole, whose spin flip is massively slowed down due to suppressed exchange interaction with the donor localized electron.

Quantum light sources and spin-based qubits are essential building blocks for on-chip scalable quantum computation and information processing. To achieve scalability, information-storing qubits should exhibit long coherence times. These qubits should also be efficiently interfaced with information-carrying single photons. Semiconductors are not only able to host such qubits and single photon sources but also, they offer a platform to interface them with the help of photonic structures. Hence, optically active solid-state qubits such as quantum dots, crystal defects and color centers have been extensively studied to date in various semiconductors. However, we still lack a suitable platform to satisfy all the requirements needed to realize a scalable quantum technology. Impurities in epitaxially grown ZnSe are particularly promising single photon sources and qubit candidates due to the direct bandgap of the material and potential for isotopic purification to achieve nuclear spin-zero background. These impurities possess impurity bound electrons that can serve as spin-qubit. They also form impurity-bound excitons that can generate single photons. Various impurities have been studied in ZnSe, but only F impurities have been isolated as single emitters to date. Despite the great potential suggested by previous results, there are many impurities waiting to be explored for their quantum capabilities. In this thesis, we study isolated Cl impurities in ZnSe for their photon emission and spin properties. We utilize a ZnMgSe/ZnSe/ZnMgSe quantum well to increase the binding energies bound excitons and to better separate donor bound exciton emission from the free excitons. In the PL spectrum, we observe narrow emission lines around 440 nm, which are originated from the single bound excitons. We calculate the average binding energy as 15 meV (at least 2 times higher than bulk values) and inhomogeneous broadening as 6 meV. We confirm the single photon emission by observing clear photon antibunching in the second order autocorrelation measurements. The time-resolved photoluminescence measurements show short radiative lifetimes of 192 ps. Our results demonstrate first time isolation of donor impurities in an unstructured ZnSe and provide complete characterization of radiative properties single Cl bound excitons. The bound electron of a donor impurity atom can serve as a spin qubit. We verify that the presence of ground state electron of the Cl donor complex by observing two electron satellite emission. We also characterize the Zeeman splitting of the exciton transitions by performing polarization-resolved magnetic spectroscopy on the single emitters. We also discover the presence of single biexcitons bound to Cl impurities. We demonstrate a radiative cascade from the decay of bound biexcitons. The emission exhibits both single photon statistics and clear temporal correlations revealing the time–ordering of the cascade. Finally, we discuss the design of nanophotonic cavities in the ZnSe platform. We develop a nanofabrication recipe to create suspended photonic crystal cavities. Then, we optically characterize the fabricated cavities. The results presented in this thesis provide the first complete study of single Cl impurities in ZnSe. Based on the results discussed, single Cl impurities in ZnSe manifest themselves as promising quantum light sources and appealing solid-state qubit candidates.

The successful synthesis of Janus transition metal dichalcogenides offers new opportunities in two-dimensional materials due to its novel properties induced by structural mirror asymmetry. Herein, by using the first-principle calculations, the thermodynamical stability for monolayers MoSSe and WSSe is demonstrated by phonon dispersion with no imaginary frequencies. No longitudinal optical-transverse optical (LO-TO) splitting exists at the Γ point and phonon frequencies are red-shifted, since the 2D Coulomb screening effect is introduced to eliminate the spurious interaction between adjacent layers. An indirect-direct-indirect transition in band gaps for both MoSSe and WSSe is observed, and tunable mobilities can be realized by uniaxial strain, reaching up to 10

Defects in wide-bandgap semiconductors are promising qubit candidates for quantum communication and computation. Epitaxially grown II-VI semiconductors are particularly promising host materials due to their direct bandgap and potential for isotopic purification to a spin-zero nuclear background. Here, we show a new type of single photon emitter with potential electron spin qubit based on Cl impurities in ZnSe. We utilize a quantum well to increase the binding energies of donor emission and confirm single photon emission with short radiative lifetimes of 192 ps. Furthermore, we verify that the ground state of the Cl donor complex contains a single electron by observing two-electron satellite emission, leaving the electron in higher orbital states. We also characterize the Zeeman splitting of the exciton transition by performing polarization-resolved magnetic spectroscopy on single emitters. Our results suggest single Cl impurities are suitable as single photon source with potential photonic interface., Comment: 7 pages, 6 figures

We study photoluminescence (PL) spectra and exciton dynamics of the MoS2 monolayer (ML) grown by the chemical vapor deposition technique. In addition to the usual direct A-exciton line, we observe a low-energy line of bound excitons dominating the PL spectra at low temperatures. This line shows unusually strong redshift with an increase in the temperature and submicrosecond time dynamics suggesting indirect nature of the corresponding transition. By monitoring the temporal dynamics of exciton PL distribution in the ML plane, we observe diffusive transport of A-excitons and measure the diffusion coefficient up to 40 cm2/s at elevated excitation powers. The bound exciton spatial distribution spreads over tens of micrometers in ∼1 μs. However, this spread is subdiffusive, characterized by a significant slowing down with time. The experimental findings are interpreted as a result of the interplay between the diffusion and Auger recombination of excitons.

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with the linear-dichroic light-matter interaction and the widely tunable direct band gap in the infrared frequency range. Despite recent theoretical predictions of strongly bound excitons with unique properties, it remains experimentally challenging to probe excitonic quasiparticles due to the severe oxidation that occurs during device fabrication. In this study, we report the observation of strongly bound excitons and trions with highly anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN) in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) \ensuremath{\sim}30 meV below the optical band gap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and photonics and optoelectronics based on strongly bound excitons with twofold anisotropy.

In this work we investigated how sulphur concentration affects the electronic structure of CVD grown MoS2 samples. Understanding defect influence on optical and transport properties of MoS2 samples is of great importance since this material has promising application in optics and optoelectronics [1]. In order to have a systematic approach, we investigated samples grown under different synthesis parameters, namely, growth temperature TG, sulphur temperature TS and argon flow Φ. Using two complementary techniques, micro- photoluminescence (PL) optical mapping and AFM, at certain synthesis parameters, discrepancies between edges and central parts of MoS2 flakes were observed, as shown in Figure 1. Possible explanation for this behaviour is the increased defect concentration, primarily sulphur vacancies [2], [3], [4] at the sample edges. Line profile of the KPFM measurement implies that the edges of the MoS2 flake are negatively charged with respect to the middle part, which suggests the change in the electronic structure. We investigated sulphur vacancies and their influence on optical spectra in the low-temperature limit using PL micro- spectroscopy. Low-temperature optical spectra reveal additional feature corresponding to the bound excitons. Spectral contribution of such bound excitons in overall optical response of the crystal provides us with quantitative information on defect states, as well as their spatial distribution.

Illuminating the charge transport at photoanode-electrolyte interface is pivotal for optimizing photoelectrochemical (PEC) water oxidation performance of BiVO4. Our study demonstrates a preferentially photogenerated electrons trapping process at BiVO4 surface, followed by the formation of surface-bound excitons. A facile LSV treatment method is described, which remarkably changes the surface termination of BiVO4 photoanode. The surface electron trapping states could be passivated by LSV treatment and “free” the photogenerated holes from the surface-bound excitons. As a result, more photogenerated holes transfer to BiVO4 surface and the PEC performance is enhanced.

First-principles studies of electronic structures, excitons, and defects in Li2B12H12 reveal unique photophysical properties of Li2B12H12 and its potential as a neutron scintillator material. We identify the electronic shell structure in (B12H12)2−, which gives rise to strong chemical stability of (B12H12)2− and a large band gap of Li2B12H12. Based on hybrid functional calculations, we show that excitons in Li2B12H12 are strongly self-trapped and predict that the exciton excitation and emission are both in the UV range and are not related to the bright blue emission observed in the experiment. The experimentally observed blue emission in Li2B12H12 is likely due to the emission of excitons bound to hydrogen vacancies. The calculated strong localization and large exciton trapping energy at hydrogen vacancies indicate enhanced radiative recombination, while the calculated large Stokes shift suggests that the self-absorption and the resonance transfer of the excitation energy may be suppressed. These properties should promote efficient defect emission at room temperature. The efficient defect emission, combined with the large thermal neutron capture cross-section by Li and B and the efficient energy transfer between fast neutrons and H, suggests potential applications of Li2B12H12 as a thermal or fast neutron scintillator. Optical characterization of VH may also be used to monitor H and Li concentrations in Li2B12H12-based hydrogen storage materials and solid-state electrolytes in Li batteries.

Lasers and light emitting diodes are found in numerous applications in our daily lives. The performance and applications of these diodes are very dependent on the choice of light emitting material. Ideally, these materials should be brightly emitting, solution-processible, low cost, and tunable. Among the vast variety of light emitting materials, three dimensional organic-inorganic halide perovskites are found to inherit these characteristics. As such, their light emitting and optical gain properties have been in the limelight over the past few years. On the contrary, not much attention was given to their low dimensional counterparts, which exhibit bright excitonic emission. In these low dimensional systems, the role of bound excitons cannot be neglected as they are an important species that influences light emitting and optical gain properties. In particular, this work explores the role of bound excitons in these perovskites for optical gain and white emission, which could be attractive for electrically pumped gated laser and white LEDs respectively. Bound excitons, in particular defect-bound excitons, were found to be one of the problems in limiting low temperature biexcitonic optical gain in (C6H5C2H4NH3)2PbI4 two dimensional perovskites. In this material, biexcitonic states were found to be populated through free-to-bound excitonic relaxation followed by bound-to-biexcitonic relaxation. Through these relaxation pathways, both free and bound excitonic recombinations can take place, which effectively compete withbiexcitonic population inversion build up. Theoretical modelling revealed a high biexcitonic optical gain threshold, which is consistent with the presence of strongly competing relaxation channels. Low optical damage threshold is another problem that is inherent in this material. This threshold, which is ∼1 order of magnitude lower than the theoretical biexcitonic optical gain threshold, could impede the path to achieving optical gain. On the other hand, bound excitons, in the form of self-trapped excitons, are responsible for white emission from low dimensional perovskites, such as (C6H5C2H4NH3)2PbCl4 and (NH3CH2C6H4CH2NH3)PbBr6. Notably, white emission is only observed in the latter thin films fabricated with excess PbBr2. Importantly, these excitons were found to undergo self-trapping at different locations in the two materials, with exciton self-trapping at the organic framework for the former and at the inorganic framework for the latter. Nonetheless, large exciton-phonon coupling strength, which is indicative of self-trapped excitons, was measured from both materials. Self-trapping phenomenon at the organic framework provide a new perspective on the origin of white emission in these systems, which deviates from conventional understanding of self-trapping in inorganic semiconductor systems. These two findings suggest that judicious selection of both organic and inorganic precursors are crucial for developing high efficiency white emitting perovskites. In retrospect, the presence of bound excitons in low dimensional perovskites serves as a double-edged sword, which either limits optical gain or gives rise to white emission. Doctor of Philosophy (IGS)

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with a linear-dichroic light-matter interaction and a widely-tunable direct-band gap in the infrared frequency range. Despite recent theoretical predictions of strongly-bound excitons with unique properties, it remains experimentally challenging to probe the excitonic quasiparticles due to the severe oxidation during device fabrication. In this study, we report observation of strongly-bound excitons and trions with highly-anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN), in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) ~ 30 meV below the optical bandgap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and novel photonics and optoelectronics based on strongly-bound excitons with two-fold anisotropy., Comment: 20 pages, 4 figures, Sangho Yoon, Taeho Kim contributed equally to this work

International audience; During the past years, one of the most iconic metal–organic frameworks (MOFs), MOF-5, has been characterized as a semiconductor by theory and experiments. Here we employ the GW many-body perturbation theory in conjunction with the Bethe–Salpeter equation to compute the electronic structure and optical properties of this MOF. The GW calculations show that MOF-5 is a wide-band-gap insulator with a fundamental gap of ∼8 eV. The strong excitonic effects, arising from highly localized states and low screening, result in an optical gap of 4.5 eV and in an optical absorption spectrum in excellent agreement with experiments. The origin of the incorrect conclusion reported by past studies and the implication of this result are also discussed.

We demonstrate that, in cavity-embedded doped quantum wells, strong light-matter interaction can create a bound excitonic state. Such a cavity-stabilised state is spectroscopically observed as a discrete resonance below the ionisation threshold.

International audience; This study aims at a quantitative understanding of the optical spectra taken from bound excitons weakly coupled to phonons in indirect semiconductors, which had been missing for decades. Insights on the properties of excitons bound at neutral acceptor impurities are obtained by analyzing the spectra reported for diamond with appropriate boron doping [Y. Kubo, Appl. Phys. Lett. 114, 132104 (2019)APPLAB0003-695110.1063/1.5089894]. We focus on the mirror symmetry holding between phonon-assisted absorption and luminescence simultaneously regarding energies, linewidths, and intensities. New analytic expressions are proposed to reproduce the spectra of phonon-assisted recombination lines. The detailed analysis reveals the contribution from excited bound-exciton states whose origin is discussed and modeled. Taking them into account improves simulation of the bound-exciton optical spectra in indirect band-gap semiconductors up to a quantitative level.

We study three-body Schrödinger operators in one and two dimensions modelling an exciton interacting with a charged impurity. We consider certain classes of multiplicative interaction potentials proposed in the physics literature. We show that if the impurity charge is larger than some critical value, then three-body bound states cannot exist. Our spectral results are confirmed by variational numerical computations based on projecting on a finite dimensional subspace generated by a Gaussian basis.

We study the quantum beats in the polarization of the photon echo from donor-bound exciton ensembles in semiconductor quantum wells. To induce these quantum beats, a sequence composed of a circularly polarized and a linearly polarized picosecond laser pulse in combination with an external transverse magnetic field is used. This results in an oscillatory behavior of the photon echo amplitude, detected in the $\sigma^+$ and $\sigma^-$ circular polarizations, occurring with opposite phases relative to each other. The beating frequency is the sum of the Larmor frequencies of the resident electron and the heavy hole when the second pulse is polarized along the magnetic field. The beating frequency is, on the other hand, the difference of these Larmor frequencies when the second pulse is polarized orthogonal to the magnetic field. The measurement of both beating frequencies serves as a method to determine precisely the in-plane hole $g$ factor, including its sign. We apply this technique to observe the quantum beats in the polarization of the photon echo from the donor-bound excitons in a 20-nm-thick CdTe/Cd$_{0.76}$Mg$_{0.24}$Te quantum well. From these quantum beats we obtain the in-plane heavy hole $g$ factor $g_h=-0.143\pm0.005$., Comment: 4 pages, 4 figures

Lead-free halide double perovskites have emerged as a nontoxic alternative to the heavily researched lead-based halide perovskites. However, their optical properties and the initial charge carrier relaxation processes are under debate. In this study, we apply time-resolved photoluminescence and differential transmission spectroscopy to investigate the photoexcited charge carrier dynamics within the indirect band structure of Cs

The present work reports on the tunable optical emission from oxygen vacancies in ZnO nanorods. The common green luminescence at room temperature originated from oxygen vacancy defects can be mediated by simply controlling pH values of alkaline solutions without the introduction of any intentional dopants. The peak energies of the green band are found to be monotonously tuned in the range from 2.17 to 2.47 eV, which could promote the applications of tunable wavelength ZnO optoelectronic devices. The temperature dependent behavior of exciton transitions and oxygen vacancy recombinations in ZnO nanorods is investigated in detail. It is suggested that the localized bound exciton (LBX) attached near the lateral facets of ZnO nanorods could survive up to nearly room temperature due to the large binding energy.

Anatase TiO2 is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.Here the authors combine steady-state angle-resolved photoemission spectroscopy, ellipsometry and ultrafast two-dimensional ultraviolet spectroscopy to examine the role of many-body correlations in anatase TiO2, revealing the existence of strongly bound excitons in single crystals and nanoparticles.

We report the ultraviolet absorption of boron-bound excitons at low temperature in a single crystal of diamond grown by chemical vapor deposition. The no-phonon (NP) and phonon-assisted lines are identified by comparison with cathodoluminescence. The oscillator strength of the NP lines was found to be 3.0 × 10−5 based on the measured absorption cross-section. This value is discussed in terms of the scaling law known for doped silicon, where the oscillator strength varies proportionally to E loc 2.5, with Eloc being the localization energy of excitons on acceptors. More importantly, we also could assess the oscillator strength of the dominant transverse optical phonon-assisted transition, which is found to be equal to 1.2 × 10 − 3. The associated radiative lifetime for the boron-bound exciton is 1.8 μs, which is much longer than the non-radiative Auger lifetime that governs its decay.We report the ultraviolet absorption of boron-bound excitons at low temperature in a single crystal of diamond grown by chemical vapor deposition. The no-phonon (NP) and phonon-assisted lines are identified by comparison with cathodoluminescence. The oscillator strength of the NP lines was found to be 3.0 × 10−5 based on the measured absorption cross-section. This value is discussed in terms of the scaling law known for doped silicon, where the oscillator strength varies proportionally to E loc 2.5, with Eloc being the localization energy of excitons on acceptors. More importantly, we also could assess the oscillator strength of the dominant transverse optical phonon-assisted transition, which is found to be equal to 1.2 × 10 − 3. The associated radiative lifetime for the boron-bound exciton is 1.8 μs, which is much longer than the non-radiative Auger lifetime that governs its decay.

Excitation of perovskite CH3NH3PbBr3 crystal generates long-lived carriers, which radiatively recombine via free electrons and holes or localized excitons. Here, we investigate the competitive recombination of free carriers and bound excitons by tuning two-photon excitation wavelength. At excitation far from resonance, the CH3NH3PbBr3 perovskite single crystal shows the free carrier up-conversion emission because the exciton binding energy is comparable with the thermal energy at room temperature. At excitation near resonance (1.97 eV < hνexc < 2.16 eV), the anti-Stokes process with a consistent phonon energy of ∼186 meV is reported for the first time. Furthermore, when the anti-Stokes transition is resonant with localized exciton level below the band, highly efficient up-converted luminescence from the bound exciton recombination is observed. The finding that the excited state recombination kinetics vary versus excitation wavelength, in the present work, is helpful for developing the high-performance opt...

GaN-Nanodrähte können mit einer hohen strukturellen Perfektion auf verschiedenen kristallinen und amorphen Substraten gewachsen werden. Sie bieten somit faszinierende Möglichkeiten, sowohl zur Untersuchung von fundamentalen Eigenschaften des Materialsystems, als auch in der Anwendung in optoelektronischen Bauteilen. Obwohl bereits verschiedene Prototypen solcher Bauteile vorgestellt wurden, sind viele grundlegende Eigenschaften von GaN-Nanodrähten noch ungeklärt, darunter die interne Quanteneffizienz (IQE), welche ein wichtiges Merkmal für optoelektronische Anwendungen darstellt. Die vorliegende Arbeit präsentiert eine detaillierte Untersuchung der Rekombinationsdynamik von Exzitonen, in selbst-induzierten und selektiv gewachsenen GaN Nanodraht-Proben, welche mit Molekularstrahlepitaxie hergestellt wurden. Die zeitaufgelösten Photolumineszenz (PL)-Experimente werden durch Simulationen ergänzt, welche auf Ratengleichungs-Modellen basieren. Es stellt sich heraus, dass die Populationen von freien und gebundenen Exzitonen gekoppelt sind und zwischen 10 und 300 K von einem nichtstrahlenden Kanal beeinflusst werden. Die Untersuchung von Proben mit unterschiedlichem Nanodraht-Durchmesser und Koaleszenzgrad zeigt, dass weder die Nanodraht-Oberfläche, noch Defekte als Folge von Koaleszenz diesen nichtstrahlenden Kanal induzieren. Daraus lässt sich folgern, dass die kurze Zerfallszeit von Exzitonen in GaN-Nanodrähten durch Punktdefekte verursacht wird, welche die IQE bei 10 K auf 20% limitieren. Der häufig beobachtete biexponentiellen PL-Zerfall des Donator-gebundenen Exzitons wird analysiert und es zeigt sich, dass die langsame Komponente durch eine Kopplung mit Akzeptoren verursacht wird. Motiviert durch Experimente, welche eine starke Abhängigkeit der PL-Intensität vom Nanodraht-Durchmesser zeigen, wird die externen Quanteneffizienz von geordneten Nanodraht-Feldern mit Hilfe numerischer Simulationen der Absorption und Extraktion von Licht in diesen Strukturen untersucht. GaN nanowires (NWs) can be fabricated with a high structural perfection on various crystalline and amorphous substrates. They offer intriguing possibilities for both fundamental investigations of the GaN material system as well as applications in optoelectronic devices. Although prototype devices based on GaN NWs have been presented already, several fundamental questions remain unresolved to date. In particular, the internal quantum efficiency (IQE), an important basic figure of merit for optoelectronic applications, is essentially unknown for GaN NWs. This thesis presents a detailed investigation of the exciton dynamics in GaN NWs using continuous-wave and time-resolved photoluminescence (PL) spectroscopy. Spontaneously formed ensembles and ordered arrays of GaN NWs grown by molecular-beam epitaxy are examined. The experiments are combined with simulations based on the solution of rate equation systems to obtain new insights into the recombination dynamics in GaN NWs at low temperatures. In particular, the free and bound exciton states in GaN NWs are found to be coupled and affected by a nonradiative channel between 10 and 300 K. The investigation of samples with different NW diameters and coalescence degrees conclusively shows that the dominating nonradiative channel is neither related to the NW surface nor to coalescence-induced defects. Hence, we conclude that nonradiative point defects are the origin of the fast recombination dynamics in GaN NWs, and limit the IQE of the investigated samples to about 20% at cryogenic temperatures. We also demonstrate that the frequently observed biexponential decay for the donor-bound exciton originates from a coupling with the acceptor-bound exciton state in the GaN NWs. Motivated by an experimentally observed, strong dependence of the PL intensity of ordered GaN NW arrays on the NW diameter, we perform numerical simulations of the light absorption and extraction to explore the external quantum efficiency of these samples.

The steady-state and time-resolved photoluminescence (PL) of excitons bound on intercalated iodine molecules was studied for the first time in MoSe2:I2 layered single crystals. Along with narrow exciton spectral lines located near the energy of the MoSe2 indirect bandgap in the region of 0.98–1.06 eV, an IR broadband radiation centered at 0.78 eV and caused by the recombination of photoexcited carriers through the intrinsic lattice defects were found. To describe the temperature dependences of the intensity of steady-state PL, as well as its temporal characteristics, a kinetic model was proposed that takes into account the radiative and non-radiative recombination channels present in this quasi-two-dimensional semiconductor.

Excitonic stimulated emission provides a promising mechanism and route to achieve low-threshold semiconductor lasers for micro-nano optoelectronic integrations. However, excitonic stimulated emission from quantum structure-free semiconductors has rarely been realised at room temperature due to the phase transition between excitonic and electron-hole plasma states. Herein, we show that through trap-state and band-edge engineering, bound exciton states can be stabilised within the hybrid lead bromide perovskite. Under modest pumping conditions, these states enable stimulated emission behaviour that exhibits a low threshold carrier density of only 1.6*1017 cm-3, as well as a high peak gain coefficient of ~1300 cm-1. This is the first time that bound exciton stimulated emission has been realised at room temperature from a quantum structure-free semiconductor. Not only does this open up new research horizons for perovskite materials, but also it has important implications for semiconductor excitonic physics and the development of next-generation optoelectronic applications., Comment: 5 figures

The electronic structure and optical spectra of anatase titanium dioxide is computed by combining state-of-the-art density functional theory (DFT) and many-body perturbation theory (MBPT). Excitonic optical spectra is computed by solving Bethe-SalpeterEquation (BSE). From the solution of BSE and also from the analysis of charge distribution the photo-generated excitons in anatase is shown to be strongly bound and localized. Such typical behavior of intrinsic excitons in bulk anatase makes it a material which shows superior performance in photo catalysis, photovoltaics, optoelectronics and in nonlinear optical regime.

The spin of electrons bound to neutral phosphorus donors in isotopically enriched silicon is a promising candidate for future quantum information processing. In this thesis, the intriguing properties of the associated optical transition, i.e., the donor bound exciton (D0X) transition are investigated by means of high precision laser absorption spectroscopy. The ultra-narrow spectral linewidth of the D0X transition allows for individual optical addressability of the electron spin and the phosphorus nuclear spin which is used to unambiguously quantify the microscopic origin of the enhanced donor electron spin lattice relaxation rate caused by optical excitation. For this purpose, the transient decay of the donor electron polarization is studied via a time-resolved pump-probe absorption spectroscopy technique where a significant shortening of the polarization decay with increasing laser excitation is observed. The theoretical analysis of the complete optically driven donor system shows that this shortening is caused by the creation of free electrons via the ubiquitous D0X Auger recombination. It is shown that, in addition to electron-phonon interaction, the hot Auger electrons relax their excess energy via inelastic collisions with donors and promote the donor electron from the ground state to a spin-mixed excited state giving rise to an Orbach-type spin relaxation mechanism which sets a fundamental limit to the spin relaxation and spin coherence time of optically driven donor systems. Furthermore, the ultra-narrow linewidth of the D0X transition enables the test of fundamental semiconductor physics such as the low temperature behavior of the silicon bandgap and the extraction of material parameters like the Landé g-factors.

We experimentally demonstrate the existence of bound excitonic resonances in doped quantum wells whose electron and hole are bind by the exchange of virtual cavity photons.

The excitonic effects in two-dimensional transition metal dichalcogenides and their heterostructures have been extensively investigated. Significantly, the moire excitons, induced by a moire superlattice in a twisted heterostructure, have triggered tremendous attention, demonstrating the practicability of artificial excitonic crystals. Besides, recent works have shown that the excitonic states in homostructures also exhibit novel properties worthy of further development. Here, we present a tear-and-stack technique for fabricating a regular or a twisted homostructure. Such a strategy was utilized to reassemble a monolayer WSe2 into a twisted homostructure, and various excitons from this structure have been identified in the photoluminescence spectra. Interestingly, the unusual equidistant splitting was first observed in the defect-bound excitons. It is believed that this phenomenon is attributed to the various defect states and the local stacking patterns. This interpretation is supported by theoretical calculations, which show the dependence of the energy band structure on different defect states and local stacking patterns. These unconventional defect-bound excitons are key building blocks in the research of homostructures that highlight the feasibility of artificially manipulating local excitons for practical applications.

CH3NH3PbCl3 (MAPbCl3) perovskite single crystal is attractive for the ultra-violet detector due to its wide bandgap and comparative stability over iodine or bromine systems. Single crystals of MAPbCl3 perovskite are also intriguing owing to its generic type of materials for fundamental photophysical properties and excitonic behaviors for its use in devices. Furthermore, recent progress using crystal-based device fabrication will shed light on semiconducting devices like III–V compounds. In this study, a structurally well-defined crystal is grown and examined to reveal free and bound excitonic behaviors depending on the structural phase transition. We classified the free and bound excitonic behaviors by temperature- and power density-dependent photoluminescence and optical transmission spectra. The single emission peak located at 3.1 eV and blueshift depending on decreasing the temperature is attributed to the radiative recombination of the free exciton at the cubic and the tetragonal phases, whereas the several peaks from the bound excitonic transition are just revealed under 120 K of the orthorhombic phase. We also determined the work function distribution and band structures with excitonic bound states via Kelvin probe force microscopy. The optoelectronic properties resulted in the excitonic behaviors can be a fundamental approach for the construction of perovskite-based optoelectronic and photonic applications.

Photoluminescence and X-ray diffraction are used to investigate 2H and 3R polytypes of MoS2 layered crystals intercalated with Cl2 molecules. The spectral structure of the observed bound excitons emission is individual for each polytype, which can be used for unambiguous identification of the 2H and 3R polytypes. The DFT band structure calculations have been done for three molecular layers of 2H-MoS2 and 3R-MoS2 polytypes with and without intercalation of Cl2 molecules. The spectroscopic experimental results confirmed theoretical calculations performed for both cases and it has been demonstrated that halogen intercalation completely changes the local band structure of both 2H- and 3R-MoS2 polytypes.

In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-principles calculations and time-resolved spectroscopy measurements of monolayer ${\mathrm{WSe}}_{2}$ reveal that these defect-bound excitons exhibit exceptional optical properties including a recombination lifetime approaching 200 ns and a valley lifetime longer than $1\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$. The ability to engineer the crystal lattice through electron irradiation provides a new approach for tailoring the optical response of TMDs for photonics, quantum optics, and valleytronics applications.

International audience; Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where A, A’ are cations, M is a metal, X is a halide, and their physical properties can be tuned by varying the perovskite layer thickness (n value). They have recently emerged as efficient semiconductors for optoelectronics [1-3]. However, fundamental questions concerning the nature of optical resonances, their scaling with quantum well thickness, and the physics behind the exciton properties, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modelling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with binding energies varying from 470 meV to 125 meV with increasing thickness from n=1 to 5 [4]. Comprehensive modelling of exciton states enable the understanding of dielectric confinement effects which prevail over quantum confinement in 2D perovskites. From these results we produce a general scaling behaviour for the binding energy of exciton states in Ruddlesden-Popper perovskites.[1] Tsai et al., Nature (2016), 536, 312-316.[2] M. Yuan et al., Nat. Nanotechnol. (2016), 11, 872-877.[3] Blancon et al., Science (2017), 355, 1288-1292.[4] Blancon et al., arXiv:1710.07653.

When two sheets of graphene stack in a twisted bilayer graphene (tBLG) configuration, the resulting constrained overlap between interplanar 2p orbitals produce angle-tunable electronic absorption resonances. By applying a novel combination of multiphoton transient absorption (TA) microscopy and TEM, we resolve the electronic structure and ensuing relaxation by probing resonant excitations of single tBLG domains. Strikingly, we find that the transient electronic population in resonantly excited tBLG domains is enhanced many fold, forming a major electronic relaxation bottleneck. Two-photon TA microscopy shows this bottleneck effect originates from a strongly bound, dark exciton state lying ∼0.37 eV below the 1-photon absorption resonance. This stable coexistence of strongly bound excitons alongside free-electron continuum states has not been previously observed in a metallic, 2D material.

Bound exciton is a signature of sulfur vacancies, and thus, it can be used to investigate defects in atomically thin materials., Defects play a significant role in tailoring the optical properties of two-dimensional materials. Optical signatures of defect-bound excitons are important tools to probe defective regions and thus interrogate the optical quality of as-grown semiconducting monolayer materials. We have performed a systematic study of defect-bound excitons using photoluminescence (PL) spectroscopy combined with atomically resolved scanning electron microscopy and first-principles calculations. Spatially resolved PL spectroscopy at low temperatures revealed bound excitons that were present only on the edges of monolayer tungsten disulfide and not in the interior. Optical pumping of the bound excitons was sublinear, confirming their bound nature. Atomic-resolution images reveal that the areal density of monosulfur vacancies is much larger near the edges (0.92 ± 0.45 nm−2) than in the interior (0.33 ± 0.11 nm−2). Temperature-dependent PL measurements found a thermal activation energy of ~36 meV; surprisingly, this is much smaller than the bound-exciton binding energy of ~300 meV. We show that this apparent inconsistency is related to a thermal dissociation of the bound exciton that liberates the neutral excitons from negatively charged point defects. First-principles calculations confirm that sulfur monovacancies introduce midgap states that host optical transitions with finite matrix elements, with emission energies ranging from 200 to 400 meV below the neutral-exciton emission line. These results demonstrate that bound-exciton emission induced by monosulfur vacancies is concentrated near the edges of as-grown monolayer tungsten disulfide.

The coherent optical response from 140 nm and 65 nm thick ZnO epitaxial layers is studied using four-wave-mixing spectroscopy with picosecond temporal resolution. Resonant excitation of neutral donor-bound excitons results in two-pulse and three-pulse photon echoes. For the donor-bound A exciton (${\mathrm{D}}^{0}{\mathrm{X}}_{\text{A}}$) at temperature of 1.8 K we evaluate optical coherence times ${T}_{2}=33--50$ ps corresponding to homogeneous line widths of $13--19\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}\mathrm{eV}$, about two orders of magnitude smaller as compared with the inhomogeneous broadening of the optical transitions. The coherent dynamics is determined mainly by the population decay with time ${T}_{1}=30--40$ ps, while pure dephasing is negligible. Temperature increase leads to a significant shortening of ${T}_{2}$ due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (${\mathrm{D}}^{0}{\mathrm{X}}_{\text{B}}$) is significantly faster (${T}_{2}=3.6\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$) and governed by pure dephasing processes.