Linear Exciton Research Articles

Linear and nonlinear exciton polariton dispersion in MoS2 nanosheets and its nickel doping dependence

In this paper, we study the influence of Ni-doping on A- and B-exciton polariton dispersion in MoS2 nanosheets. In particular, we have calculated the effect of the Ni-concentration on the energy dispersion and therefore we have deduced the effective masses by adjusting the hopping parameter. The E A , E B -exciton transition energies and the hopping parameter are discerned from the PL of Ni-MoS2 nanosheets. It is shown that the Ni changes considerably the bandgap and as a result the electronic band structure, the effective masses, the optical transition, and the hopping term. We found that the increasing concentration of Ni-dopant can modulate and favors the mixture of A- and B-exciton peaks of MoS2 nanosheets. The diverging Ni-concentration of A- and B-exciton polariton dispersion are also discerned and discussed. Finaly, we can predict that these changes caused by the influence of Ni-doping can be used in most applications involving exciton polariton dispersion for photonics and optoelectronics devices.

Effect of intravalley and intervalley electron-hole exchange on the nonlinear optical response of monolayer MoSe2

The coherent third-order nonlinear response of monolayer transition-metal dichalcogenide semiconductors, such as ${\mathrm{MoSe}}_{2}$, is dominated by the nonlinear exciton response, as well as biexciton and trion resonances. The fact that these resonances may be spectrally close together makes identification of the signatures, for example in differential transmission (DT), challenging. Instead of focusing on explaining a given set of experimental data, a systematic study aimed at elucidating the roles of intravalley and intervalley long-range electron-hole $(e\ensuremath{-}\mathrm{h})$ exchange on the DT spectra is presented. Previous works have shown that the $e\ensuremath{-}\mathrm{h}$ long-range exchange introduces a linear leading-order term in the exciton dispersion. Based on a generalized Lippmann-Schwinger equation, we show that the presence of this linear dispersion term can reduce the biexciton binding energy to zero, contrary to the conventional situation of quadratic dispersion where an arbitrarily weak (well-behaved) attractive interaction always supports bound state(s). The effects of spin scattering and the spin-orbit interaction caused by $e\ensuremath{-}\mathrm{h}$ exchange are also clarified, and the DT line shape at the exciton and trion resonance is studied as a function of $e\ensuremath{-}\mathrm{h}$ exchange strength. In particular, as the exciton line shape is determined by the interplay of linear exciton susceptibility and the bound-state two-exciton resonance in the $T$ matrix, the line shape at the trion is similarly determined by the interplay of the linear trion susceptibility and the bound-state exciton-trion resonance in the $T$ matrix.

Linear and nonlinear optical excitons in primary cyclic ammonium based inorganic–organic hybrid semiconductor series

The low dimensional inorganic–organic (IO) hybrid semiconductors become a fascinating subject of interest due to their unique crystal structural packing and tunable optical exciton emission features. These naturally self-assembled 2D IO hybrid semiconductors form multiple quantum wells (MQWs) like structure due to the quantum widths of both organic moiety and inorganic network , enabling them to show stable room temperature Mott-type excitons with large exciton binding energy. A systematic co-existence of two kinds of excitons observable from one-(ħω ≥ Eg) and two-photon (2ħω ≥ Eg) excitations is studied to probe into deeper details of sensitive structural changes in the IO-hybrids thin film and single crystals.

Understanding radiative transitions and relaxation pathways in plexcitons

Summary Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of polaritonic light-matter states, called plexcitons. Such systems are tunable, scalable, easy to synthesize, and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime, promising a plethora of applications. However, the complexity of these materials prevented the understanding of their excitation and relaxation phenomena. Here, we follow the relaxation pathways in plexcitons and conclude that while the metal destroys the optical coherence, the molecular aggregate coupled to surface processes significantly contributes to the energy dissipation. We use two-dimensional electronic spectroscopy with theoretical modeling to assign the different relaxation processes to either molecules or metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be considered in the language of hot electron distributions instead of the accepted lower and upper polariton branches and establish the framework for further understanding.

Selective Photoexcitation of Finite-Momentum Excitons in Monolayer MoS2 by Twisted Light.

Twisted light carries a well-defined orbital angular momentum (OAM) of per photon. The quantum number of its OAM can be arbitrarily set, making it an excellent light source to realize high-dimensional quantum entanglement and ultrawide bandwidth optical communication structures. In spite of its interesting properties, twisted light interaction with solid state materials, particularly two-dimensional materials, is yet to be extensively studied via experiments. In this work, photoluminescence (PL) spectroscopy studies of monolayer molybdenum disulfide (MoS2), a material with ultrastrong light-matter interaction due to reduced dimensionality, are carried out under photoexcitation of twisted light. It is observed that the measured spectral peak energy increases for every increment of of the incident light. The nonlinear -dependence of the spectral blue shifts is well accounted for by the analysis and computational simulation of this work. More excitingly, the twisted light excitation revealed the unusual lightlike exciton band dispersion of valley excitons in monolayer transition metal dichalcogenides. This linear exciton band dispersion is predicted by previous theoretical studies and evidenced via this work's experimental setup.

Real-time dynamic evolution monitoring of laser-induced exciton phase flips in 2D hybrid semiconductor (C12H25NH3)2PbI4

Real-time monitoring of room-temperature exciton photoluminescence (PL) while irradiated with ultrafast laser excitations (UV and infrared) in long alkyl-chain based (C12H25NH3)2PbI4 inorganic–organic hybrid semiconductors is presented. These naturally self-assembled 2D hybrid structures show strong room-temperature Mott-type excitons confined within the lowest inorganic bandgap, which are highly sensitive to structural phase flips. Under both one-photon (E1PA ≥ Eg) and two-photon (2E2PA ≥ Eg) laser excitations, the exciton PL of unstable phase-II appears initially, and with prolonged laser exposure, the PL peak switches to a new stable blueshifted phase-I peak position. This exciton phase flip demonstrates different laser-induced structural deformations in inorganic quantum wells (PbI6 extended network) associated with orthorhombic (phase-I) and monoclinic (phase-II) unit cells. One-photon absorption induced PL shows the various time dynamics of laser exposure depending on laser characteristics (continuous wave and ultrashort pulsed lasers), mostly influenced by localized heating, ablation effects, and third-order nonlinear effects such as saturation of linear absorption and exciton–exciton annihilation. However, in two-photon absorption induced PL, the near infrared laser excitation reveals the redshifted crumpled excitons from the deeper depth of the sample, which are induced by multiphoton absorption and avalanche ionization. A series of systematic linear and nonlinear steady-state and time-resolved PL studies are presented. A simplified kinetic model further provides an understanding of the real-time evolution of laser-induced excitons and their related phase flips. These laser-induced exciton phase flips and linear and nonlinear optical probing open a new avenue for novel functional properties and nonlinear absorption–based optoelectronic devices.

Anomalous Temperature Dependence of Exciton Spectral Diffusion in Tetracene Thin Film.

In this work, an ultrafast spectral diffusion of the lowest exciton in a tetracene ultrathin film is studied by two-dimensional electronic spectroscopy. From the analysis of the nodal line slope, the frequency-fluctuation correlation function (FFCF) of the exciton band is extracted. The FFCF contains two components with decay times of 400 and 80 fs; while the former can be understood by a linear exciton-phonon coupling model, the latter shows an order of magnitude increase in its amplitude from 96 to 186 K that cannot be explained by the same model. A novel scheme of the energy-gap fluctuations is examined, in which an intramolecular high-frequency mode causes the spectral diffusion that is enhanced through an anharmonic coupling to low-frequency phonon modes. This finding provides a valuable input for future theoretical predictions on the ultrafast nonadiabatic dynamics of the molecular exciton.

Interacting Polaron-Polaritons

Two dimensional semiconductors provide an ideal platform for exploration of linear exciton and polariton physics, primarily due to large exciton binding energy and strong light-matter coupling. These features, however, generically imply reduced exciton-exciton interactions, hindering the realisation of active optical devices such as lasers or parametric oscillators. Here, we show that electrical injection of itinerant electrons into monolayer molybdenum diselenide allows us to overcome this limitation: dynamical screening of exciton-polaritons by electrons leads to the formation of new quasi-particles termed polaron-polaritons that exhibit unexpectedly strong interactions as well as optical amplification by Bose-enhanced polaron-electron scattering. To measure the nonlinear optical response, we carry out time-resolved pump-probe measurements and observe polaron-polariton interaction enhancement by a factor of 50 ($0.5 \mu$eV $\mu$m$^2$) as compared to exciton-polaritons. Concurrently, we measure a spectrally integrated transmission gain of the probe field of $\gtrsim 2$ stemming from stimulated scattering of polaron-polaritons. We show theoretically that the non-equilibrium nature of optically excited quasiparticles favours a previously unexplored interaction mechanism stemming from a phase-space filling in the screening cloud, which provides an accurate explanation of the strong repulsive interactions observed experimentally. Our findings show that itinerant electron-exciton interactions provide an invaluable tool for electronic manipulation of optical properties, demonstrate a new mechanism for dramatically enhancing polariton-polariton interactions, and pave the way for realisation of nonequilibrium polariton condensates.

Exciton diffusion in monolayer semiconductors with suppressed disorder

Monolayer semiconductors with suppressed environmental disorder from encapsulation in high-quality insulators provide a versatile platform to study the propagation of excitonic quasiparticles at ambient conditions. Using time-resolved emission microscopy, the authors monitored linear and nonlinear exciton diffusion in single layers of tungsten disulfide, observing highly mobile excitons with nonmonotonic density dependence of the diffusion. Combining experiment and theory, the authors discuss the impact of multivalley band structure, the role of bound and free charge carriers, the interplay of nonradiative recombination and diffusion as well as the suppressed Auger recombination.

Photoluminescence saturation and exciton decay dynamics in transition metal dichalcogenide monolayers

We report a photoluminescence (PL) and transient reflection spectroscopy study of exciton dynamics in monolayer transition transition-metal dichalcogenides (TMDs). PL saturation in monolayer MoSe2 occurs an excitation intensity more than two orders of magnitude lower than in monolayer MoS2. Transient reflection shows that the nonlinear exciton-exciton annihilation is the dominant exciton decay process in monolayer MoSe2 in contrast to the previously reported linear exciton decay in monolayer MoS2. In addition, the exciton lifetime in MoSe2, > 125 ps, is more than an order of magnitude longer than the several-ps exciton lifetime in MoS2. We find that the dramatically different exciton decay mechanism and PL saturation behavior of MoSe2 and MoS2 monolayers can be explained by the difference in their exciton lifetime.

Exciton Level Structure and Dynamics in Tubular Porphyrin Aggregates

We present an account of the optical properties of the Frenkel excitons in self-assembled porphyrin tubular aggregates that represent an analog to natural photosynthetic antennae. Using a combination of ultrafast optical spectroscopy and stochastic exciton modeling, we address both linear and nonlinear exciton absorption, relaxation pathways, and the role of disorder. The static disorder-dominated absorption and fluorescence line widths show little temperature dependence for the lowest excitons (Q band), which we successfully simulate using a model of exciton scattering on acoustic phonons in the host matrix. Temperature-dependent transient absorption of and fluorescence from the excitons in the tubular aggregates are marked by nonexponential decays with time scales ranging from a few picoseconds to a few nanoseconds, reflecting complex relaxation mechanisms. Combined experimental and theoretical investigations indicate that nonradiative pathways induced by traps and defects dominate the relaxation of excitons in the tubular aggregates. We model the pump–probe spectra and ascribe the excited-state absorption to transitions from one-exciton states to a manifold of mixed one- and two-exciton states. Our results demonstrate that while the delocalized Frenkel excitons (over 208 (1036) molecules for the optically dominant excitons in the Q (B) band) resulting from strong intermolecular coupling in these aggregates could potentially facilitate efficient energy transfer, fast relaxation due to defects and disorder probably present a major limitation for exciton transport over large distances.

Two-Dimensional Infrared Spectroscopy and Electron Microscopy of Seeded and Non-Seeded Amyloid β Peptide Fibrils

Seeded and non-seeded fibrils from amyloid β 40-residue (Aβ40) peptides have been studied with two-dimensional infrared (2D-IR) spectroscopy and electron microscopy (EM). For the former, 13C=18O isotope labels were placed in various residues to probe residue-specific amide-I' vibrations of Aβ40 peptides. 2D-IR spectroscopy (2D-IR photon echo) of both kinds of fibrils shows distinct linear chain excitons of amide units due to the interamide vibrational coupling. Structural differences between the seeded and the non-seeded fibrils vary across the sequence. However, seeded fibrils tend to have a better aligned tertiary structure than non-seeded fibrils. Negatively stained EM images indicate that non-seeded fibrils had no discernible narrowing or nodes. In comparison, seeded fibrils tend to appear twisted like a ribbon, with periodic narrowing or nodes. The distances between nodes (the “internodal” distances) were relatively homogeneous distributions (∼75 nm) and the same in all isotopically labeled peptides. The apparent width of the fibrils in these images indicate that seeded fibrils are two kinds and that the width of the narrower fibrils is the same as the width of non-seeded fibrils. The mass-per-length evaluated from dark-field EM images indicates that the most prevalent number of filaments in seeded fibrils is 6, which is twice as many as the number in non-seeded fibrils (3 filaments per fibril). Overall, these results demonstrate that seeded and non-seeded fibrils have distinctly different tertiary structures.

Concentration and excitation effects on the exciton dynamics of poly(3-hexylthiophene)/PbS quantum dot blend films

The dynamics of photoexcitations in hybrid blends of poly(3-hexylthiophene) (P3HT) conjugated polymer donor and oleic-acid capped lead sulfide (PbS) quantum dot (QD) acceptors of different concentrations—for light harvesting applications—were investigated using time-resolved transmission and photoluminescence spectroscopies. Following excitation at 400 nm and probing in the 500–1000 nm region, we find that geminate excitation recombination in the blend of P3HT/PbS QDs dominates the transient decays at sub-ns times while intermaterial interactions such as charge transfer processes appear at longer times in the 1–50 ns regime. For the hybrid blend films with lower QD concentrations (<67% wt), polymer exciton recombination dominates the overall transient absorption signal. For higher QD contents, QD state relaxation effects become visible. Excitation density studies reveal the presence of linear exciton relaxation effects in the P3HT region while carrier decay for films with high PbS QD concentration is influenced by QD Auger recombination. Time-resolved luminescence shows that electron transfer from the P3HT/PbS QDs appears relatively inefficient in comparison to the geminate recombination, while hole transfer competes favorably to intrinsic QD recombination.

Temperature-insensitive optical alignment of the exciton in nanowire-embedded GaN quantum dots

We report on the exciton spin dynamics of nanowire-embedded GaN/AlN quantum dots (QDs) investigated by time-resolved photoluminescence spectroscopy. Under a linearly polarized quasiresonant excitation, we evidence the quenching of the exciton spin relaxation and a temperature-insensitive degree of the exciton linear polarization, demonstrating the robustness of the optical alignment of the exciton spin in these nanowire-embedded QDs. A detailed examination of the luminescence polarization angular dependence shows orthogonal linear exciton eigenstates.

Linear and Two-Dimensional Infrared Spectroscopic Study of the Amide I and II Modes in Fully Extended Peptide Chains

We have carried out structural determination of capped C(α,α)-diethylglycine (Deg) homopeptides with different chain lengths, Ac-(Deg)(n)-OtBu (n = 2-5), solvated in CDCl(3), and investigated vibrational properties of the amide I and II modes by linear and 2D IR spectroscopy, ONIOM calculations, and molecular dynamics simulations. 2D IR experiments were performed in the amide I region using the rephasing pulse sequence under the double-crossed polarization and the nonrephasing sequence under a new polarization configuration to measure cross-peak patterns in the off-diagonal regions. The 2D IR spectra measured in the amide I and II regions reveal complex couplings between these modes. Model spectral calculations finely reproduced the measured spectral profiles by using vibrational parameters that were very close to the values predicted by the ONIOM method. The agreement led to a conclusion that peptide backbones are fully extended with the dihedral angles (ϕ,ψ) ≈ (±180°,±180°) and that a sequence of intramolecular C(5) hydrogen bonds forms along the entire chain regardless of the chain length. This conclusion was endorsed by analysis of the molecular dynamics trajectories for n = 3 and 5 that showed an exclusive population of the C(5) conformation. The conformationally well-restrained Deg homopeptides serve as an ideal linear exciton chain, which is scarcely obtainable by protein amino acids. We investigated excitonic properties of the linear chain through analytic modeling and compared the measurement and calculation results of the amide I and II modes. The integrated intensity of the amide II band is larger than that of the amide I for the C(5) structure, untypical behavior in contrast with other secondary structures. This comprehensive study characterized the amide I and II spectral signatures of the fully extended conformation, which will facilitate the conformational analysis of artificial oligopeptides that contain such structural motifs.

Photogenerated Free Carrier Dynamics in Metal and Semiconductor Single-Walled Carbon Nanotube Films

Time-resolved THz spectroscopy (TRTS) is employed to study the photogenerated charge-carrier dynamics in transparent films of single-walled carbon nanotubes (SWNTs). Two films were investigated: a film with 94% semiconducting-type tubes (s-SWNTs) and a film with only 7% s-SWNT and 93% metal-type tubes (m-SWNTs). We conclude that charge-carriers are generated with >60% yields at low light intensities in both films. Free-carriers are generated by a linear exciton dissociation process that occurs within -1 ps and is independent of excitation wavelength or tube type.

Two-dimensional infrared spectra of isotopically diluted amyloid fibrils from Aβ40

The 2D IR spectra of the amide-I vibrations of amyloid fibrils from Abeta40 were obtained. The matured fibrils formed from strands having isotopic substitution by (13)C (18)O at Gly-38, Gly-33, Gly-29, or Ala-21 show vibrational exciton spectra having reduced dimensionality. Indeed, linear chain excitons of amide units are seen, for which the interamide vibrational coupling is measured in fibrils grown from 50% and 5% mixtures of labeled and unlabeled strands. The data prove that the 1D excitons are formed from parallel in-register sheets. The coupling constants show that for each of the indicated residues the amide carbonyls in the chains are separated by 0.5 +/- 0.05 nm. The isotope replacement of Gly-25 does not reveal linear excitons, consistent with the region of the strand having a different structure distribution. The vibrational frequencies of the amide-I modes, freed from effects of amide vibrational excitation exchange by 5% dilution experiments, point to there being a component of an electric field along the fibril axis that increases through the sequence Gly-38, Gly-33, Gly-29. The field is dominated by side chains of neighboring residues.

Nonmonotonic energy harvesting efficiency in biased exciton chains

We theoretically study the efficiency of energy harvesting in linear exciton chains with an energy bias, where the initial excitation is taking place at the high-energy end of the chain and the energy is harvested (trapped) at the other end. The efficiency is characterized by means of the average time for the exciton to be trapped after the initial excitation. The exciton transport is treated as the intraband energy relaxation over the states obtained by numerically diagonalizing the Frenkel Hamiltonian that corresponds to the biased chain. The relevant intraband scattering rates are obtained from a linear exciton-phonon interaction. Numerical solution of the Pauli master equation that describes the relaxation and trapping processes reveals a complicated interplay of factors that determine the overall harvesting efficiency. Specifically, if the trapping step is slower than or comparable to the intraband relaxation, this efficiency shows a nonmonotonic dependence on the bias: it first increases when introducing a bias, reaches a maximum at an optimal bias value, and then decreases again because of dynamic (Bloch) localization of the exciton states. Effects of on-site (diagonal) disorder, leading to Anderson localization, are addressed as well.

Probing Quantum-Mechanical Level Repulsion in Disordered Systems by Means of Time-Resolved Selectively Excited Resonance Fluorescence

We argue that the time-resolved spectrum of selectively-excited resonance fluorescence at low temperature provides a tool for probing the quantum-mechanical level repulsion in the Lifshits tail of the electronic density of states in a wide variety of disordered materials. The technique, based on detecting the fast growth of a fluorescence peak that is redshifted relative to the excitation frequency, is demonstrated explicitly by simulations on linear Frenkel exciton chains.

Vibronic spectra of mixed Frenkel and charge transfer excitons

The excitonic and vibronic spectra of a molecular chain and of a crystal of $N{N}^{\ensuremath{'}}$-dimetyl 3,4,9,10-perylene tetracarboxylic diimide (MePTCDI) are studied in the case when: (i) a Frenkel exciton (FE) and charge transfer excitons (CTEs) mix strongly; and (ii) two mechanisms of coupling between these mixed excitons and intramolecular vibrations, notably linear and quadratic coupling, are acting. Using a convenient canonical transformation and the Green function method, we calculate the linear optical susceptibility: (a) in the exciton region taking into account the contribution of the transition dipoles of FE; and (b) in one-phonon vibronic regime. The spectra of linear absorption in the excitonic and vibronic regions have been calculated introducing the exciton parameters of MePTCDI. These spectra exhibit: (i) a relative separation of vibronics of FE and CTEs; (ii) a stronger impact of the linear coupling on the intensity of the excitonic and vibronic spectra; and (iii) the appearance of a spectral doublet of vibronics of the CTEs---its splitting depends on the parameters of linear and quadratic exciton--phonon coupling in the neutral excited molecule and ions. Moreover, in the case of weak linear exciton--phonon coupling the vibronic line of FE, being wide and flat, lies in many-particle continuum while in the case of intermediate and strong linear coupling the linear absorption is dominated by the bound exciton--phonon states and their narrow Lorentzian maxima depend strongly on the quadratic coupling.