Excitonic Structure Research Articles

New Perspectives on Photosystem II Reaction Centres

We apply the differential optical spectroscopy techniques of circular polarisation of luminescence (CPL) and magnetic CPL (MCPL) to the study of isolated reaction centres (RCs) of photosystem II (PS II). The data and subsequent analysis provide insights into aspects of the RC chromophore site energies, exciton couplings, and heterogeneities. CPL measurements are able to identify weak luminescence associated with the unbound chlorophyll-a (Chl-a) present in the sample. The overall sign and magnitude of the CPL observed relates well to the circular dichroism (CD) of the sample. Both CD and CPL are reasonably consistent with modelling of the RC exciton structure. The MCPL observed for the free Chl-a luminescence component in the RC samples is also easily understandable, but the MCPL seen near 680nm at 1.8K is anomalous, appearing to have a narrow, strongly negative component. A negative sign is inconsistent with MCPL of (exciton coupled) Qy states of either Chl-a or pheophytin-a (Pheo-a). We propose that this anomaly may arise as a result of the luminescence from a transient excited state species created following photo-induced charge separation within the RC. A comparison of CD spectra and modelling of RC preparations having a different number of pigments suggests that the non-conservative nature of the CD spectra observed is associated with the ‘special pair’ pigments PD1 and PD2.

Pump-driven normal-to-excitonic insulator transition: Josephson oscillations and signatures of BEC-BCS crossover in time-resolved ARPES

We consider a ground-state wide-gap band insulator turning into a nonequilibrium excitonic insulator (NEQ-EI) upon visiting properly selected and physically relevant highly excited states. The NEQ-EI phase, characterized by self-sustained oscillations of the complex order parameter, neatly follows from a Nonequilibrium Green's Function treatment on the Konstantinov-Perel' contour. We present the first {\em ab initio} band structure of LiF, a ground-state bulk insulator, in different NEQ-EI states and show that these states can be generated by currently available pump pulses. We highlight two general features of time-resolved ARPES spectra: (1) during the pump-driving the excitonic spectral structure undergoes a convex-to-concave shape transition and {\em concomitantly} the state of the system goes through a BEC-BCS crossover; (2) attosecond pulses shone after the pump-driving at different times $t_{\rm delay}$ generate a photocurrent which {\em oscillates} in $t_{\rm delay}$ with a pump-tunable frequency -- we show that this phenomenon is similar to the AC response of an exotic Josephson junction.

Intrinsic exciton transitions of isotopically purified 13C studied by photoluminescence and transmission spectroscopy

The intrinsic exciton transitions of isotopically purified (13C) and natural-abundance (N.A.C) diamond are studied by photoluminescence and transmission spectroscopies at liquid helium temperatures. The comparative study shows that 13C has the same fine structure splitting with N.A.C, indicating that the isotopic effect of diamond does not influence the internal excitonic structure. This approach also deduces the absolute energies of excitons and phonons in 13C and N.A.C at the Δ [k = (0.76, 0, 0) × (2π/a)] point.

The electronic and optical properties of an exciton, biexciton and charged excitons in CdSe/CdTe-based multi-shell type-II quantum dot nanocrystals

It has been recently reported that multi-shell type-II quantum dot nanocrystals (QDNCs) have higher quantum yields. Besides these higher quantum yields of multi-shell type-II QDNCs, additional second layer has been a critical influence on the formation mechanisms of the excitonic structures. Understanding of bound and unbound cases of the excitonic structures in multi-shell type-II QDNCs gives some important information for applications. In this study, we have investigated the electronic and optical properties of a single exciton (X), biexciton (XX), and positively and negatively charged excitons ( $${\mathsf {X^+}}$$ and $${\mathsf {X^-}}$$ ) in CdSe/CdTe-based multi-shell type-II QDNCs. In the study, three different structure compositions, i.e., CdSe/CdTe, CdSe/CdTe/CdS, and CdSe/CdTe/ZnTe, have been considered. We have observed that CdS and ZnTe materials have drastically changed the electronic and optical properties of the bare CdSe/CdTe type-II QDNCs.

Electronic states of (InGa)(AsSb)/GaAs/GaP quantum dots

Detailed theoretical studies of the electronic structure of (InGa)(AsSb)/GaAs/GaP quantum dots are presented. This system is unique since it exhibits concurrently direct and indirect transitions both in real and momentum space and is attractive for applications in quantum information technology, showing advantages as compared to the widely studied (In,Ga)As/GaAs dots. We proceed from the inspection of the confinement potentials for ${\bf k}\neq 0$ and ${\bf k}= 0$ conduction and ${\bf k}= 0$ valence bands, through the formulation of ${\bf k}\cdot{\bf p}$ calculations for ${\bf k}$-indirect transitions, up to the excitonic structure of $\Gamma$-transitions. Throughout this process we compare the results obtained for dots on both GaP and GaAs substrates enabling us to make a direct comparison to the (In,Ga)As/GaAs quantum dot system. We also discuss the realization of quantum gates.

Fine Structure and Spin Dynamics of Linearly Polarized Indirect Excitons in Two-Dimensional CdSe/CdTe Colloidal Heterostructures

Heterostructured two-dimensional colloidal nanoplatelets are a class of material that has attracted great interest for optoelectronic applications due to their high photoluminescence yield, atomically tunable thickness, and ultralow lasing thresholds. Of particular interest are laterally heterostructured core-crown nanoplatelets with a type-II band alignment, where the in-plane spatial separation of carriers leads to indirect (or charge transfer) excitons with long lifetimes and bright, highly Stokes shifted emission. Despite this, little is known about the nature of the lowest energy exciton states responsible for emission in these materials. Here, using polarization-controlled, steady-state, and time-resolved photoluminescence measurements, at temperatures down to 1.6 K and magnetic fields up to 30 T, we study the exciton fine structure and spin dynamics of archetypal type-II CdSe/CdTe core-crown nanoplatelets. Complemented by theoretical modeling and zero-field quantum beat measurements, we find the bright-exciton fine structure consists of two linearly polarized states with a fine structure splitting ∼50 μeV and an indirect exciton Landé g-factor of 0.7. In addition, we show the exciton spin lifetime to be in the microsecond range with an unusual B-3 magnetic field dependence. The discovery of linearly polarized exciton states and emission highlights the potential for use of such materials in display and imaging applications without polarization filters. Furthermore, the small exciton fine structure splitting and a long spin lifetime are fundamental advantages when envisaging CdSe/CdTe nanoplatelets as elementary bricks for the next generation of quantum devices, particularly given their ease of fabrication.

Ab initio simulations of x-ray emission spectroscopy with the GW+ Bethe-Salpeter equation method

In order to calculate x-ray emission spectroscopy (XES) spectra, we apply the $GW+$Bethe-Salpeter equation ($\mathrm{GW}+\mathrm{BSE}$) method on a basis of extended quasiparticle theory, which enables one to treat an arbitrary excited state as an initial state, because the initial state in the XES process is a highly excited state with a core hole. Compared to the preexisting experimental data of XES fluorescence photon energy, the calculated $\mathrm{GW}+\mathrm{BSE}$ results give values with about 1-eV accuracy, which is comparable to the previous results using the time-dependent density functional theory with the short-range corrected exchange-correlation functional, the equation of motion-coupled cluster single and double, and delta self-consistent field methods. Our $\mathrm{GW}+\mathrm{BSE}$ results reproduce corresponding experimental XES spectra without missing any peak. The method can assign the excitonic configuration of each peak in XES spectra with the quasiparticle levels. As a result, the analysis of excitonic structure for each peak gives obvious interpretation concerning the relation between excitonic states and valence states.

Exciton Spatial Coherence and Optical Gain in Colloidal Two-Dimensional Cadmium Chalcogenide Nanoplatelets.

Two-dimensional (2D) cadmium chalcogenide (CdX, X = Se, S, Te) colloidal nanoplatelets (NPLs) make up an emerging class of quantum well materials that exhibit many unique properties including uniform quantum confinement, narrow thickness distribution, large exciton binding energy, giant oscillator strength, long Auger lifetime, and high photoluminescence quantum yield. These properties have led to their great performances in optoelectrical applications such as lasing materials with a low threshold and large gain coefficient. Many of these properties are determined by the structure and dynamics of band-edge excitons in these 2D materials. Motivated by fundamental understanding of both 2D nanomaterials and their applications, the properties of 2D excitons have received intense recent interest. This Account provides an overview of three key properties of 2D excitons: how big is the 2D exciton (i.e., exciton center-of-mass coherent area); how the exciton moves in 2D NPLs (i.e., exciton in-plane transport mechanism); how multiple excitons interact with each other (i.e., biexciton Auger recombination); and their effects on the optical gain mechanism and threshold of colloidal NPLs. After a brief introduction in Section 1, the current understandings of 2D electronic structures of cadmium chalcogenide NPLs, and type-I CdSe/CdS and type-II CdSe/CdTe core/crown NPL heterostructures are summarized in Section 2. Section 3 discusses the direct measurement of exciton center-of-mass coherent area in 2D CdSe NPLs, its dependence on NPL parameters (thickness, lateral area, dielectric environment, and temperature), and the resulting giant oscillator strength transition (GOST) effect in 2D NPLs. 2D exciton diffusive in-plane transport in CdX NPLs and the comparison of exciton transport mechanisms in 2D NPLs and 1D nanorods are reviewed in Section 4. How Auger recombination lifetime depends on nanocrystal dimensions in NPLs, quantum dots, and nanorods is discussed in Section 5. The lateral area and thickness dependent Auger recombination rates of NPLs are shown to be well described by a model that accounts for the different dependence of the Auger recombination rates on the quantum confined and nonconfined dimensions. It is shown that Auger recombination rates do not follow the "universal volume scaling" law in 1D and 2D nanocrystals. Section 6 describes optical gain mechanisms in CdSe NPLs and the dependence of optical gain threshold on NPL lateral size, optical density, and temperature. The differences of optical gain properties in 0D-2D and the bulk materials are also discussed, highlighting the unique gain properties of 2D NPLs. At last, the Account ends with a summary and perspective of key remaining challenges in this field in Section 7.

Effects of Spin-Orbit Coupling on the Electronic and Excitonic Structures of Monolayer WS<sub>2</sub>

Transition metal dichalcogenides (TMDs) display unique properties in their monolayer structures, namely a direct band-gap transition, which becomes a promising candidate for optoelectronics applications. Among them, WS2 exhibits strong spin-orbit interaction which splits the excitonic peaks as observed in the experimental data up to ~400 meV. Unlike the other TMDs, the first excitonic peak A is very sharp for WS2, while the secondary peak B is broader with smaller relative intensity. In this paper, we perform first-principles calculations on the electronic band structure and solve the Bethe-Salpeter equation for the complex dielectric function of monolayer WS2 to study the effects of spin-orbit coupling on its excitonic structures. To resolve the excitonic peaks, in particular the B peak, we implement the double-grid method. We discuss the effects of electron-hole interaction on the absorption spectrum by comparing it with that calculated at the independent-particle level.

Optical Transmission of p-Type a-Si:H Thin Film Deposited by PECVD on ITO-Coated Glass

A p-type thin film of hydrogenated amporphous silicon (a-Si:H) has successfully been fabricated by radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) technique. Substrate used in the deposition process is indium tin oxide (ITO) layer coated having size of 10 x10 cm2 and being cleaned with 97% alcohol using ultrasonic bath. According to Atomic Force Microscope (AFM) observation, the layer thickness of p-type a-Si: H film was 150 nm. The Transmission spectrum at room temperature obtained from UV-Vis measurement demonstrates a large period modulation, which is due to the interference within the film. At wavelength longer than 1000 nm (or energy <1 eV), the interference modulation in the transmission spectrum of the film is seen to broaden. It is shown in a zoomed - scale around the related band gap area that one may find an exciton structure.

Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $\Gamma$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations.

Mixed Upper Exciton State of the Special Pair in Bacterial Reaction Centers.

Excitonic interactions between two closely separated bacteriochlorophyll a molecules (BChls) in the special pair of the reaction center (RC) of purple bacteria determine the positions and relative oscillator strengths of its two excitonic components. While the absorption of the lower excitonic band is well-defined, the position and the intensity of the upper excitonic band ( PY+) are still under debate. Recent 77 K two-dimensional electronic spectroscopy data on Rba. capsulatus suggested that the PY+ component absorbs at ∼840 nm, i.e., at a significantly lower energy than previously suggested. In the present work, we argue that the PY+ state is mixed with the excited states of the accessory BChls ( B*/ P Y+) leading to excitons contributing to the 785-825 nm spectral region which is consistent with previously published data. This conclusion is based on hole-burning/linear dichroism data and modeling studies of the excitonic structure of the RC using a non-Markovian reduced density matrix approach.

Dynamics of A-exciton and spin relaxation in WS<sub>2</sub> and WSe<sub>2</sub> monolayer

Two-dimensional transitional metal dichalcogenide (2D TMD) emerges as a good candidate material in optoelectronics and valleytronics due to its particular exciton effect and strong spin-valley locking. Owing to the enhancement of quantum confinement effect and the decline of dielectric shielding effect, the optical excitation of electron-hole pair is enhanced substantially, which makes large TMD exciton binding energy and makes excitons observed easily at room temperature or even higher temperature. Optical response of 2D TMD is dominated by excitons at room temperature, which provides an ideal medium for studying the generation, relaxation and interaction of excitons or trions. By employing ultrafast time resolved spectroscopy, we investigate experimentally the dynamic behaviors of A-exciton and spin relaxations for two types of TMDs, i.e. WS<sub>2</sub> and WSe<sub>2</sub> monolayers, respectively. By tuning the excitation wavelength of the degenerate pump and probe laser beam, the WS<sub>2</sub> monolayer and WSe<sub>2</sub> monolayer are excited at their A-exciton resonance transition position or near their A-exciton resonance transition position in order to compare the dynamical evolutions of band structure and exciton polarization of the two similar WS<sub>2</sub> and WSe<sub>2</sub> monolayer structures. Our experimental results reveal that the relaxation of A exciton in WS<sub>2</sub> shows biexponential decay, while that of WSe<sub>2</sub> shows triexponential decay, and the A-exciton life time in WSe<sub>2</sub> is much longer than that of WS<sub>2</sub> counterpart. The spin relaxation of A exciton in WS<sub>2</sub> shows a monoexponential feature with a lifetime of 0.35 ps, which is dominated by the electron-hole exchange interaction. For the case of WSe<sub>2</sub>, the spin relaxation can be well fitted with biexponential function, the fast component has a lifetime of 0.5 ps and the slow one has a lifetime of 28 ps. The fast relaxation is dominated by the electron-hole exchange interaction, and the slow one comes from the formation of dark exciton via spin-lattice coupling. By tuning the excitation wavelength around A-exciton transition, the formation of dark exciton in WSe<sub>2</sub> is demonstrated to be much more effective than that in WS<sub>2</sub> monolayer. Our experimental results provide qualitative physical images for an in-depth understanding of the relationship between exciton and TMD structure, and also provide reference for further designing and regulating the TMDs based optoelectronic devices.

Excitons and trions in two-dimensional semiconductors based on transition metal dichalcogenides

Theoretical and experimental results on excitonic effects in monomolecular layers of transition metal dichalcogenides are reviewed. These two-dimensional semiconductors exhibit a direct bandgap of about 2 eV at the Brillouin zone edges, and the binding energies of their neutral and charged excitons are in the range of hundreds and tens of millielectron-volts, respectively. This implies that electron–hole complexes determine the optical properties of transition metal dichalcogenide monolayers. Topics discussed in this review include the band structure details needed to understand the excitonic effects in these materials, the structure and fine structure of exciton and trion energy levels, the features of the spin and valley dynamics of Coulomb complexes, and how neutral and charged excitons manifest themselves in linear and nonlinear optical effects.

Triplet exciton fine structure in Pt-rich polymers studied by circularly polarized emission under high magnetic field

Incorporating heavy atoms into polymer chains represents an effective way to generate emissive triplets. Here we used magneto-optical emission spectroscopy up to 17.5 Tesla for studying the fine structure of the triplet exciton in a series of Pt-rich \ensuremath{\pi}-conjugated polymers with various intrachain Pt concentrations. We found that their phosphorescence emission band shows substantial field-induced circular polarization (FICPO) up to 50% with an unusual, nonmonotonic field dependence at cryogenic temperature. From the field-induced energy splitting between left and right circularly polarized phosphorescence we obtained the effective $g$ factor of triplet exciton varying in the range of $\ensuremath{-}0.13\text{\ensuremath{-}}0.85$, which depends on the Pt concentration in the polymer chains. The FICPO of triplet emission originates from the population difference in spin sublevels, which is determined by thermal equilibrium subjected to spin-orbit coupling (SOC), exchange, and Zeeman interactions. Surprisingly we also observed FICPO in the fluorescence emission that results from the singlet-triplet interaction caused by the strong SOC. From these results we extracted the various interaction parameters that describe the exciton fine structure in these Pt-contained compounds.

Spatially-resolved fluorescence-detected two-dimensional electronic spectroscopy probes varying excitonic structure in photosynthetic bacteria

Conventional implementations of two-dimensional electronic spectroscopy typically spatially average over ~1010 chromophores spread over ~104 micron square area, limiting their ability to characterize spatially heterogeneous samples. Here we present a variation of two-dimensional electronic spectroscopy that is capable of mapping spatially varying differences in excitonic structure, with sensitivity orders of magnitude better than conventional spatially-averaged electronic spectroscopies. The approach performs fluorescence-detection-based fully collinear two-dimensional electronic spectroscopy in a microscope, combining femtosecond time-resolution, sub-micron spatial resolution, and the sensitivity of fluorescence detection. We demonstrate the approach on a mixture of photosynthetic bacteria that are known to exhibit variations in electronic structure with growth conditions. Spatial variations in the constitution of mixed bacterial colonies manifests as spatially varying peak intensities in the measured two-dimensional contour maps, which exhibit distinct diagonal and cross-peaks that reflect differences in the excitonic structure of the bacterial proteins.

Exciton–Phonon Interactions Govern Charge-Transfer-State Dynamics in CdSe/CdTe Two-Dimensional Colloidal Heterostructures

CdSe/CdTe core-crown type-II nanoplatelet heterostructures are two-dimensional semiconductors that have attracted interest for use in light-emitting technologies due to their ease of fabrication, outstanding emission yields, and tunable properties. Despite this, the exciton dynamics of these complex materials, and in particular how they are influenced by phonons, is not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and temperature-dependent structural measurements to investigate CdSe/CdTe nanoplatelets with a thickness of four monolayers. We show that charge-transfer (CT) excitons across the CdSe/CdTe interface are formed on two distinct time scales: initially from an ultrafast (∼70 fs) electron transfer and then on longer time scales (∼5 ps) from the diffusion of domain excitons to the interface. We find that the CT excitons are influenced by an interfacial phonon mode at ∼120 cm-1, which localizes them to the interface. Using low-temperature PL spectroscopy we reveal that this same phonon mode is the dominant mechanism in broadening the CT PL. On cooling to 4 K, the total PL quantum yield reaches close to unity, with an ∼85% contribution from CT emission and the remainder from an emissive sub-band-gap state. At room temperature, incomplete diffusion of domain excitons to the interface and scattering between CT excitons and phonons limit the PL quantum yield to ∼50%. Our results provide a detailed picture of the nature of exciton-phonon interactions at the interfaces of 2D heterostructures and explain both the broad shape of the CT PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that to maximize the PL quantum yield both improved engineering of the interfacial crystal structure and diffusion of domain excitons to the interface, e.g., by altering the relative core/crown size, are required.

Energy reversal of light- and heavy-hole excitons bound to isoelectronic centers

Isoelectronic centers in semiconductor hosts offer convenient optical access to both heavy- and light-hole states, thus providing powerful resources for the ultrafast control of electron or hole spins. From a detailed analysis of the fine structure of excitons bound to isoelectronic defects of ${C}_{s}$ symmetry formed by two nitrogen impurities in GaAs, we demonstrate that the lowest energy states, normally exhibiting a strongly dominant heavy-hole character, can be reversed to a dominant light-hole character by the strain found in the vicinity of the emitter. This reversal suggests that strain could be used to engineer the orbital composition and ordering of excitonic states, allowing enhanced optical control over spin states.

Investigating exciton structure and dynamics in colloidal CdSe quantum dots with two-dimensional electronic spectroscopy.

Two-Dimensional Electronic Spectroscopy (2DES) is performed on CdSe colloidal quantum dots. These experiments reveal new observations on exciton structure and dynamics in quantum dots, expanding upon prior transient absorption measurements of excitonics in these systems. The 2DES method enables the separation of line broadening mechanisms, thereby better revealing the excitonic lineshapes and biexcitonic interactions. 2DES enables more information rich spectral probing of coherent phonons and their coupling to excitons. The data show spectral modulations and drifts, with differences based upon whether one monitors the excitation energy (E1) or emission energy (E3). These measurements reveal both homogeneous and inhomogeneous broadenings, as well as static and dynamic line broadening. The longitudinal optical phonon modulates the dynamic absorption spectrum both in energy and linewidth. These experiments enable measurement of hot exciton cooling with improved resolution in energy and time. These 2DES results are consistent with prior excitonic state-resolved transient absorption measurements, albeit with the addition of contributions due to coherent phonons. Finally these 2DES experiments enable disentangling of coupling versus relaxation contributions to the signals, further offering a test of electronic structure theory.

Fine Structure of Rydberg Excitons in Cuprous Oxide

In 1952, E.F. Gross and N.A. Karryev discovered excitons of big radius also called the Wannier–Mott excitons. Their energy spectrum, response to external electric and magnetic fields, and also elastic deformations of a crystal were extensively studied in the 1960s–1970s. The second wave of interest to excitons in Cu2O crystals appeared comparatively recent, in 2014, after the “giant” highly excited exciton states had been observed in this material. A theoretical description of highly excited exciton states needs, as a rule, new approaches, because, for such states, a deviation from the exactly solved hydrogen-like model becomes substantial and a numerical solution of the Schrodinger equation with allowance made for the features of the crystal energy band structure becomes extremely resource consuming. This report is a brief review of recent theoretical and experimental studies of the fine structure of the exciton energy spectrum in copper protoxide.