Exciton

About: Exciton is a research topic. Over the lifetime, 31603 publications have been published within this topic receiving 810642 citations.

Theory of Surface-Enhanced Raman Scattering in Semiconductors

Abstract: We develop an analytical expression for the lowest order nonzero contribution to the surface-enhanced Raman spectrum from a system composed of a molecule adsorbed on a semiconductor nanoparticle. We consider a combined molecule-semiconductor system and include Herzberg–Teller vibronic coupling of the zero-order Born–Oppenheimer states. This follows a previous derivation for metallic SERS, but instead of a Fermi level, the semiconductor system involves a band gap and we find that the SERS enhancement is maximized at either the conduction or valence band edge. The resulting expression may be regarded as an extension of the Albrecht A-, B-, and C-terms and show that the SERS enhancement is caused by several resonances in the combined system, namely, surface plasmon, exciton, charge-transfer, and molecular resonances. These resonances are coupled by terms in the numerator, which provide strict selection rules that enable us to test the theory and predict the relative intensities of the Raman lines. Furthermor...

Exciton–plasmon interactions in molecular spring assemblies of nanowires and wavelength-based protein detection

Abstract: Electronic interactions at the nanoscale represent one of the fundamental problems of nanotechnology. Excitons and plasmons are the two most typical excited states of nanostructures, which have been shown to produce coupled electronic systems. Here, we explore these interactions for the case of nanowires with mobile excitons and nanoparticles with localized plasmons and describe the theoretical formalism, its experimental validation and the potential practical applications of such nanoscale systems. Theory predicts that emission of coupled excitations in nanowires with variable electronic confinement is stronger, shorter and blue-shifted. These predictions were confirmed with a high degree of accuracy in molecular spring assemblies of CdTe nanowires and Au nanoparticles, where we can reversibly change the distance between the exciton and the plasmon. The prepared systems were made protein-sensitive by incorporating antibodies in the molecular springs. Modulation of exciton-plasmon interactions can serve as a wavelength-based biodetection tool, which can resolve difficulties in the quantification of luminescence intensity for complex media and optical pathways.

Optical Properties of Molecular Aggregates. I. Classical Model of Electronic Absorption and Refraction

Abstract: A theoretical classical model is developed to predict the absorption and refraction of an aggregate of monomer units (a molecular aggregate, molecular crystal, or polymer) at any frequency. The monomers are treated as having complex electronic polarizabilities whose frequency dependence is determined by the absorption bands of the isolated monomers.Polarizations in the aggregate induced by incident light are modified by Coulombic interactions between the monomers. No first‐order approximation is involved as in exciton theory. The molar extinction coefficient and molar refraction are obtained from normal modepolarizabilities found by solving an eigenvalue problem. The predicted absorption spectra agree (to first order in interaction energy) with exciton theory in the limit of weak coupling, with the hypochromism theory of Tinoco and Rhodes, and (for a classical oscillator model) with exciton theory for strong coupling. The oscillator strength sum rule is obeyed. The predicted spectrum of a pair of dyelike monomers is illustrated for the cases of weak, intermediate, and strong coupling.

Probing dark excitons in atomically thin semiconductors via near-field coupling to surface plasmon polaritons

Abstract: Near-field coupling to surface plasmon polaritons enables the observation of spin-forbidden dark excitonic states in monolayer WSe2. Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin–orbit coupling and spin–valley degrees of freedom1,2,3,4. Depending on the spin configuration of the electron–hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark5,6,7,8. Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment9, making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe2 monolayer is placed on top of a single-crystal silver film10, its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication11.

Ultrafast coherent control and destruction of excitons in quantum wells.

Abstract: We demonstrate the coherent destruction of photogenerated excitons in semiconductor quantum wells within a few hundred femtoseconds of their excitation. Coherent control of carrier dynamics is achieved with phase-locked pairs of 100 fs infrared pulses. The technique induces an optical response which is faster than the inverse of the exciton linewidth superseding Fourier limits for a single pulse. Energy selectivity enables the coherent transfer of angular momentum between hole states. Such phase-tailored pulse trains can be utilized to investigate the generation process and intermediate virtual states in quantum structures.