Excitonic Superradiance Research Articles

Critical spatial disorder in InGaN resonant Bragg structures

We study a disorder-induced transformation of the resonant optical reflection from a nearly periodic system of quasi-2D excitons in the InGaN quantum wells arranged as a resonant Bragg structure (RBS). We show that there is a critical deviation from the exact periodicity when the reflection by a single super-radiant exciton–polariton mode is transformed into a multimode spectrum. This critical disorder appears to be inversely proportional to the number of periods in the RBS. Using a numerical model which describes the experimentally observed optical spectra, we found the critical standard deviation from exact periodicity to be 1.76%, 1.0%, and 0.45% for InGaN-based RBS with 60, 100, and 200 periods, respectively.

Gauge fixing for strongly correlated electrons coupled to quantum light

We discuss the problem of gauge fixing for strongly correlated electrons coupled to quantum light, described by projected low-energy models such as those obtained within tight-binding methods. Drawing from recent results in the field of quantum optics, we present a general approach to write down quantum light-matter Hamiltonian in either dipole or Coulomb gauge which are explicitly connected by a unitary transformation, thus ensuring gauge equivalence even after projection. The projected dipole gauge Hamiltonian features a linear light-matter coupling and an instantaneous self-interaction for the electrons, similar to the structure in the full continuum theory. On the other hand, in the Coulomb gauge the photon field enters in a highly non-linear way, through phase factors that dress the electronic degrees of freedom. We show that our approach generalises the well-known Peierls approximation, to which it reduces when only local, on-site orbital contributions to light-matter coupling are taken into account. As an application, we study a two-orbital model of interacting electrons coupled to a uniform cavity mode, recently studied in the context of excitonic superradiance and associated no-go theorems. Using both gauges we recover the absence of superradiant phase in the ground state and show that excitations on top of it, described by polariton modes, contain instead non-trivial light-matter entanglement. Our results highlight the importance of treating the non-linear light-matter interaction of the Coulomb gauge non-perturbatively, to obtain a well-defined ultrastrong coupling limit and to not spoil gauge equivalence.

Thermodynamic Control over Molecular Aggregate Assembly Enables Tunable Excitonic Properties across the Visible and Near-Infrared.

Specific molecular arrangements within H-/J-aggregates of cyanine dyes enable extraordinary photophysical properties, including long-range exciton delocalization, extreme blue/red shifts, and excitonic superradiance. Despite extensive literature on cyanine aggregates, design principles that drive the self-assembly to a preferred H- or J-aggregated state are unknown. We tune the thermodynamics of self-assembly via independent control of the solvent/nonsolvent ratio, ionic strength, or dye concentration, obtaining a broad range of conditions that predictably stabilize the monomer (H-/J-aggregate). Diffusion-ordered spectroscopy, cryo-electron microscopy, and atomic force microscopy together reveal a dynamic equilibrium between monomers, H-aggregated dimers, and extended J-aggregated 2D monolayers. We construct a model that predicts the equilibrium composition for a range of standard Gibbs free energies, providing a vast aggregation space which we access using the aforementioned solvation factors. We demonstrate the universality of this approach among several sheet-forming cyanine dyes with tunable absorptions spanning visible, near, and shortwave infrared wavelengths.

New Nonlinear Optical Crystal of Rhodamine 590 Acid Phthalate.

The synthesis and crystal structure of rhodamine 590 acid phthalate (RhAP) have been reported. This novel solid-state rhodamine derivative not only has a longer fluorescence lifetime compared to rhodamine solid-state matrixes where emission is quenched but also possesses strong nonlinear optical characteristics. The static and dynamic first- and second-order hyperpolarizabilities were calculated using the time-dependent density functional theory at the B3LYP/6-31+G* level. The computed static values of β and γ of RhAP by the X-ray diffraction (XRD) structure were 31.9 × 10–30 and 199.0 × 10–36 esu, respectively. These values were about 62 times larger than the corresponding values in urea, an already well-known nonlinear optical material. The second-order hyperpolarizability of the compound was determined experimentally by measuring the two-photon absorption cross section using intensity-modulated light fields. The reported compound, excitable at near-infrared, exhibited frequency upconversion with the two-photon absorption coefficient enhanced by two orders of magnitude compared to that of the dye solution. Hosting the dye in the solid, at high concentrations, exploits the nonlinearity of the dye itself as well as results in significant excitonic effects including formation of broad exciton band and superradiance.

Generalized Kasha’s Model: T-Dependent Spectroscopy Reveals Short-Range Structures of 2D Excitonic Systems

Summary Excitonic organic materials owe their tunable optoelectronic properties to intricate microscopic configurations, challenging for conventional characterization methods to resolve. We generalize Kasha’s model by incorporating the temperature (T)-dependent absorption peak shift, in addition to the monomer-aggregate absorption peak shift that defines J- and H-aggregates, to characterize the microscopic structures. We show that the short-range interactions dominate in determining the direction of the T-dependent peak shift, which accounts for previously observed T-dependent blueshifting J-aggregates (BJ-aggregates) that were not explained and predicts the existence of redshifting H-aggregates (RH-aggregates). This defines four types of excitonic aggregates: RJ-, BH-, BJ-, and RH-aggregates, where the latter two unconventional aggregates are possible because of the two dimensionality of excitonic systems and cannot be understood with the original Kasha’s theory. Our conceptual framework is useful for elucidating structure-function relationships in molecular excitonic systems and is fully compatible with existing tools. Possible extensions to other spectroscopic observables and related systems are discussed.

Photochemical Control of Exciton Superradiance in Light-Harvesting Nanotubes.

Photosynthetic antennae and organic electronic materials use topological, structural, and molecular control of delocalized excitons to enhance and direct energy transfer. Interactions between the transition dipoles of individual chromophore units allow for coherent delocalization across multiple molecular sites. This delocalization, for specific geometries, greatly enhances the transition dipole moment of the lowest energy excitonic state relative to the chromophore and increases its radiative rate, a phenomenon known as superradiance. In this study, we show that ordered, self-assembled light-harvesting nanotubes (LHNs) display excitation-induced photobrightening and photodarkening. These changes in quantum yield arise due to changes in energetic disorder, which in turn increases/decreases excitonic superradiance. Through a combination of experiment and modeling, we show that intense illumination induces different types of chemical change in LHNs that reproducibly alter absorption and fluorescence properties, indicating control over excitonic delocalization. We also show that changes in spectral width and shift can be sensitive measures of system dimensionality, illustrating the mixed 1-2D nature of LHN excitons. Our results demonstrate a path forward for mastery of energetic disorder in an excitonic antenna, with implications for fundamental studies of coherent energy transport.

Photophysics of J-Aggregate-Mediated Energy Transfer on DNA.

Achieving nanoscale spatial and electronic control over the formation of dye aggregates is a major synthetic challenge due to their typically inhomogeneous self-assembly, which limits control over their higher-order organization. To address these challenges, synthetic DNA-templated pseudoisocyanine (PIC) J-aggregates were recently introduced. However, the dependence of the photophysics of the superradiant exciton on the underlying DNA template length and the impact of static disorder on energy transfer through these PIC J-aggregates remain unknown. We examine the delocalization length progression of superradiant PIC excitons by varying the length of poly-A DNA tracts that template PIC J-aggregates. We then investigate the energy-transfer efficiency from PIC J-aggregates with DNA duplex template length, which we found to be limited by static disorder. Utilizing the self-assembled and selective formation of superradiant excitons on DNA provides a platform to determine the function of delocalized excitons in the context of nanoscale energy transport.

Proposal of highly efficient photoemitter with strong photon-harvesting capability and exciton superradiance

We propose a system of highly efficient photoemitters comprising metal-molecule multilayered structures. In the proposed structure, the absorption in the molecular layer is greatly enhanced through quantum interference between the split modes arising from the coupling of the layered excitons and the plasmons sustained by the metal layer. Furthermore, the large interaction volume between surface plasmons and excitons causes exciton superradiance, which results in the extremely efficient photoemission. This finding indicates the possibility of designing highly efficient photoemitters based on simple layered structures.

Upconverted photoluminescence induced by radiative coupling between excitons

We propose an unconventional scheme of photoluminescence in a semiconductor thin film, where the nonlocal correlation between an excitonic wave and a light wave prominently enhances the exciton-light coupling beyond the long-wavelength approximation (the so-called excitonic superradiance regime). On the basis of the developed method extending input-output theory, we elucidate atypical photoluminescence effects due to the strong wave-wave correlation. In particular, the up-converted photoluminescence based on the coherent superposition of excitons is found to be highly efficient, i.e., it can be realized by weak pumping without auxiliary systems such as cavities or photonic antennas.

Cooperative robustness to dephasing: Single-exciton superradiance in a nanoscale ring to model natural light-harvesting systems

We analyze a 1-d ring structure composed of many two-levels systems, in the limit where only one excitation is present. The two-levels systems are coupled to a common environment, where the excitation can be lost, which induces super and subradiant behavior. Moreover, each two-levels system is coupled to another independent environment, modeled by a classical white noise, simulating a dephasing bath and described by the Haken-Strobl master equation. Single exciton Superradiance, an example of cooperative quantum coherent effect, is destroyed at a critical dephasing strength proportional to the system size, showing robustness of cooperativity to the action of the dephasing environment. We also show that the coupling to a common decay channel contrasts the action of dephasing, driving the entanglement decay to slow down on increasing the system size. Moreover, after a projective measurement which finds the excitation in the system, the entanglement reaches a stationary value, independent of the initial conditions.

Enhanced phosphorescence from nanocrystalline/microcrystalline materials based on (CH3NH3)(1-naphthylmethyl ammonium)2Pb2Cl7 and similar compounds

A broad phosphorescence band, with three main maxima at ca 500, 536 and 577nm, was observed at room temperature, either from a mixture of (1-naphthylmethyl ammonium)2PbCl4 and CH3NH3PbCl3, or from (CH3NH3)(1-naphthylmethyl ammonium)2Pb2Cl7, or some Cl–Br analogues, in the form of suspensions in toluene or in the form of grinding deposits on quartz plates. It was attributed to supermigration of energy from the Wannier excitons of PbCl3 component to the triplet state of organic component (i.e., 1-naphthylmethyl group) of particulate material. This new effect seems to be a consequence of excitonic superradiance of particulate PbCl3.

Indirect exciton superradiant recombination in diamond: Theory and the perspectives of observation

A method of generation of powerful pulses of ultraviolet and soft X-ray coherent radiation is considered. It consists in employing superradiant indirect exciton recombination in large enough high quality diamond samples. The theory of this transient process is developed and its dynamics is simulated. In result it is shown that this method may probably provide the generation of picosecond pulses of coherent electromagnetic field with peak powers larger than 30W at wavelength 235nm.

Disorder-induced exciton localization and violation of optical selection rules in supramolecular nanotubes

Using numerical simulations, we study the effect of disorder on the optical properties of cylindrical aggregates of molecules with strong excitation transfer interactions. The exciton states and the energy transport properties of such molecular nanotubes attract considerable interest for application in artificial light-harvesting systems and energy transport wires. In the absence of disorder, such nanotubes exhibit two optical absorption peaks, resulting from three super-radiant exciton states, one polarized along the axis of the cylinder, the other two (degenerate) polarized perpendicular to this axis. These selection rules, imposed by the cylindrical symmetry, break down in the presence of disorder in the molecular transition energies, due to the fact that the exciton states localize and no longer wrap completely around the tube. We show that the important parameter is the ratio of the exciton localization length and the tube's circumference. When this ratio decreases, the distribution of polarization angles of the exciton states changes from a two-peak structure (at zero and ninety degrees) to a single peak determined by the orientation of individual molecules within the tube. This is also reflected in a qualitative change of the absorption spectrum. The latter agrees with recent experimental findings.

Entangled-Photon Generation in Nano-to-Bulk Crossover Regime

We have theoretically investigated the generation of entangled photons from biexcitons in a semiconductor film with a thickness in the nano-to-bulk crossover regime. In contrast with the cases of quantum dots and bulk materials, we can highly control the generated state of entangled photons through the design of a peculiar energy structure of exciton-photon coupled modes in the thickness range between nanometers and micrometers. Owing to the enhancement of the radiative decay rate of excitons (exciton superradiance), the statistical accuracy of generated photons can be increased beyond the trade-off problem with signal intensity. By implementing an optical cavity structure in the strong-coupling regime, the generation efficiency can be enhanced while maintaining the high statistical accuracy.

Polarized superradiance from delocalized exciton transitions in tetracene single crystals

Polarized superradiant emission and exciton delocalization in tetracene single crystals are reported. Polarization-, time-, and temperature-resolved spectroscopy evidence the complete polarization of the zero-phonon line of the intrinsic tetracene emission from both the lower (F state) and the upper (thermally activated) Davydov excitons. The superradiance of the F emission is substantiated by a nearly linear decrease of the radiative lifetime with temperature, being fifteen times shorter at 30 K compared to the isolated molecule, with an exciton delocalization of about 40 molecules.

Coherent homogenization of periodic structures: Transformation from structurally periodic (photonic band gap) structures to optically uniform materials

A relevant feature of periodic structures that serves as the cornerstone of many optical devices and photonic systems is alternative variations of refractive index and/or gain (loss). In this paper we will show how in Bragg multiquantum well active photonic gap structures, wherein refractive index periodicity comes with super-radiant excitons, one can use an infrared laser to design optical materials with fundamentally unique features. This includes coherent formation of a state of optical materials wherein within a frequency range around the Bragg wavelength the periodic structure becomes optically homogeneous, i.e., its material and structural periodicity are concealed from a probe field. For active photonic band structures containing structural defects, we show that this process can lead to full removal of the optical mode associated with the defect, making a uniform defectless optical material, or to generation of another defect mode at a different frequency. Applications of the results for ultrafast switching and time delay control are discussed.

Exciton-phonon scattering and photoexcitation dynamics inJ-aggregate microcavities

We have developed a model accounting for the photoexcitation dynamics and the photoluminescence of strongly coupled $J$-aggregate microcavities. Our model is based on a description of the $J$-aggregate film as a disordered Frenkel exciton system, in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. In a strongly coupled microcavity exciton polaritons are formed, mixing super-radiant excitons and cavity photons. The calculation of the microcavity steady-state photoluminescence, following a cw nonresonant pumping, is carried out. The experimental photoluminescence intensity ratio between upper and lower polariton branches is accurately reproduced. In particular, both thermal activation of the photoluminescence intensity ratio and its Rabi splitting dependence are a consequence of the bottleneck in the relaxation, occurring at the bottom of the excitonic reservoir. The effects due to radiative channels of decay of excitons and to the presence of a particular set of discrete optical molecular vibrations active in relaxation processes are investigated.

Radiative decay theory: What suppresses exciton superradiance?

AbstractWe have theoretically studied the exciton superradiance which occurs under a phase‐match between the exciton center‐of‐mass motion and the radiation field in a film with thickness over a light wavelength. With continuously increasing the film thickness, the radiative decay rates of excitons are calculated from extreme values of exciton correlation functions renormalizing the excitonphoton interaction in the film with finite thickness.Due to a consideration of intra‐ and inter‐state retarded interactions, in the simplest system, we have derived the correct suppression thickness of the exciton superradiance. The correctness is verified from the general suppression condition derived from the polariton dispersion relation consisting of complex frequency and wavenumber for finite thickness. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Super-radiance of excitons in a single ZnO nanostructure

Super-radiance of excitons in a single ZnO tetrapod nanostructure has been observed. The emitted pulses exhibited very short duration, and the emission peak intensity exhibited exponential increase with the increased pump density. Multiple narrow emission peaks with short peak spacing have been observed in the emission spectra of a single ZnO tetrapod nanostructure.

Photophysics of J‐aggregate microcavities

AbstractWe have developed a model in order to account for the photo‐excitation dynamics of J‐aggregate films and strongly coupled J‐aggregate microcavities. The J‐aggregates are described as a disordered Frenkel exciton system in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. The link between the J‐aggregate film and the microcavities is obtained introducing a model polariton wavefunction mixing cavity photon modes and J‐aggregate superradiant excitons. With the same approach we have calculated both absorption and luminescence spectra for the J‐aggregate film and the photoluminescence of strongly coupled organic microcavities. The model is able to explain the organic microcavities fast relaxation dynamics and polariton luminescence following non resonant pumping starting from the excitonic properties of the optically active layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)