Inducing and Probing Localized Excitons in Atomically Thin Semiconductors via Tip‐Enhanced Cavity‐Spectroscopy (Adv. Funct. Mater. 33/2021)
Hyeongwoo LeeInki KimChulho ParkMingu KangJinseong ChoiKwang‐Yong JeongJungho MunYeseul KimJeong‐Hoon ParkMarkus B. RaschkeHong‐Gyu ParkMun Seok JeongJunsuk RhoKyoung‐Duck Park
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Atomically Thin Semiconductors In article number 2102893, Junsuk Rho, Kyoung-Duck Park, and co-workers develop a triple-sharp-tips nano-antenna to facilitate radiative emissions of the localized states in 2D semiconductors. The localized exciton is deterministically induced and probed at room temperature through this novel concept of tip-enhanced cavity-spectroscopy. This new approach provides a practical way to control single-photon generation at room temperature, which is the centerpiece of quantum optical communications.Biexciton
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Abstract The binding of two free excitons into a bi‐exciton due to direct exciton‐exciton interaction is studied. The process of bi‐exciton formation via exciton–phonon interaction is not taken into account. By group theory methods the wave functions of two interacting excitons are constructed for a model applicable to CuCl and Cu 2 O crystals. Making use of these functions, the Fourier transform of exciton‐exciton interaction energy is calculated. By the Green's function method the dissociation energy of the bi‐exciton is obtained in the adiabatic approximation (the exciton‐exciton interaction energy is much smaller than the ionization energy of the exciton). A numerical estimate and comparison with experimental values is made for CuCl crystals.
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One- to two-exciton transitions have been examined in molecular aggregates with linear and circular geometries at various strengths of the exciton–exciton interaction. For the interaction parameter a sufficiently different from its critical value acrit=1, the exciton–exciton interaction has been shown to have little influence on the transition dipole moments, as well as on the corresponding transition energies between the one-exciton states and the dissociated two-exciton states. The interaction between the excitons then may be represented in an effective manner by the replacement of the actual number N of molecules per aggregate by a nearby effective number Neff, the latter being a-dependent. Hence, inclusion of the exciton–exciton coupling does not affect substantially the previous analysis of one- to two-exciton transitions based on the model of noninteracting one-dimensional excitons. That is, effects such as the blue shift of the excited-state absorption and the enhancement of nonlinear susceptibilities are not sensitive to the exciton–exciton interaction. These findings are relevant, inter alia, to J-aggregates in which there is no evidence for the coupling parameter a to be in the critical region or beyond. On the other hand, for the critical value of the exciton–exciton interaction (a=acrit), the blue shift is either totally absent in the excited-state absorption, or extremely small compared with the ordinary case. The above is in full agreement with earlier calculation of the pump–probe spectrum showing a weak dependence on the exciton–exciton interaction for a<1, as well as a strong bleaching of the exciton band in the critical region.
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Summary form only given. Spectroscopy at single, localized excitons in semiconductor quantum structures is presently a very interesting field of research. Much attention is paid to single-dot spectroscopy on quasi-0D excitons, which are localized e.g., at well width fluctuations in ultrathin quantum wells or within self-assembled islands. The microscopic origin of three-dimensional quantum confinement is a strong local potential fluctuation given by growth conditions. In a similar way, impurities ties and extended defects can provide such local potentials. In bulk semiconductors, excitons or electrons bound at some defects often represent well-defined, discrete two-level systems. Optical spectroscopy at single impurity states has to solve the problem of how to optically address an exciton localized at any defect. At very low impurity concentrations, imaging spectroscopy allows spatially and spectrally to separate single emission centers at micrometer length scales. In the contribution we study impurity-related single exciton emission in ZnSe layers homoepitaxially grown on ZnSe substrates. Well-known defects in ZnSe are donor-bound excitons, donor-acceptor pair transitions and the Y-defect, a structural defect connected with dislocations. By imaging spectroscopy we analysed the spatially resolved emission. The emission at the free-exciton energy (FE) is characterized by a homogeneously distributed signal intensity extended over the whole detection area of the CCD. No effects of localization are seen as expected for a free exciton emission.
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Abstract In first order of perturbation theory the probability of binding of two excitons into a bi‐exciton accompanied by light emission and caused by exciton–exciton collisions is calculated. Firstly the case is considered where two excitons of 1s‐type and n ‐p‐type form a bi‐exciton in the ground state consisting of two excitons of 1s‐type. The binding is accompanied by light emission. The luminescence begins with a threshold and the luminescence band has the form of a sharp asymmetrical peak. The exciton–exciton collisions lead to the binding of two excitons of 1s‐type with no internal excitation. Numerical estimations of the processes under consideration are made for a set of crystals.
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This paper is a review of already published work. Exciton-relaxation processes in organic solids are discussed. Pyrene and poly(phenylenevinylene) (PPV) crystals are investigated as two examples of strong exciton–phonon coupled systems. Free-exciton luminescence is observed simultaneously with self-trapped exciton luminescence in these crystals. Since the presence of a self-trapping (a barrier that separates the free-exciton state from the self-trapped exciton state) directly affects the exciton-relaxation process and excimer (self-trapped exciton) -formation time, attention is focused to reveal the presence of the barrier. The intensity and the decay time of transient free-exciton luminescence are discussed, as is the rise of self-trapped exciton luminescence. The exciton-relaxation path within the free-exciton band is also discussed. In pyrene above 120 K, the structural phase-transition point, most of the photoproduced excitons relax directly toward the self-trapped state, and only a small fraction of them relax to the bottom of the free-exciton band. At temperatures below 60 K no exciton relaxes to the bottom of the free-exciton band, except in the case when excitons are produced close to the bottom of the free-exciton band. In PPV all the photoproduced excitons relax to the bottom of the free-exciton band.
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This chapter contains sections titled: Excitons in bulk Excitons in heterostructures Exciton binding energies 1s exciton The two-dimensional and three-dimensional limits Excitons in single quantum wells Excitons in multiple quantum wells Stark Ladders Self-consistent effects Spontaneous symmetry breaking 2s exciton
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Based on the framework of the tight binding approach,the formation of an exciton in inter-coupled polymer chains is studied.Both a localized exciton in one single chain and a spread exciton between chains are obtained.It is found that an excited electron-hole pair is more inclined to evolve into a localized exciton.By analyzing the effect of the interchain couplings on the exciton binding energy,it is shown that the photoluminescence efficiency of the polymer in a dilute solution state is higher than that in a solid state.
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