J-aggregates are of significant interest for organic materials conceived by supramolecular approaches. Their discovery in the 1930s represents one of the most important milestones in dye chemistry as well as the germination of supramolecular chemistry. The intriguing optical properties of J-aggregates (in particular, very narrow red-shifted absorption bands with respect to those of the monomer and their ability to delocalize and migrate excitons) as well as their prospect for applications have motivated scientists to become involved in this field, and numerous contributions have been published. This Review provides an overview on the J-aggregates of a broad variety of dyes (including cyanines, porphyrins, phthalocyanines, and perylene bisimides) created by using supramolecular construction principles, and discusses their optical and photophysical properties as well as their potential applications. Thus, this Review is intended to be of interest to the supramolecular, photochemistry, and materials science communities.

J-aggregates: from serendipitous discovery to supramolecular engineering of functional dye materials.

Nearly one century after the birth of quantum mechanics, parity–time symmetry is revolutionizing and extending quantum theories to include a unique family of non-Hermitian Hamiltonians. While conceptually striking, experimental demonstration of parity–time symmetry remains unexplored in quantum electronic systems. The flexibility of photonics allows for creating and superposing non-Hermitian eigenstates with ease using optical gain and loss, which makes it an ideal platform to explore various non-Hermitian quantum symmetry paradigms for novel device functionalities. Such explorations that employ classical photonic platforms not only deepen our understanding of fundamental quantum physics but also facilitate technological breakthroughs for photonic applications. Research into non-Hermitian photonics therefore advances and benefits both fields simultaneously. General concepts and recent developments of parity–time symmetry in classical photonics are reviewed.

Non-Hermitian photonics based on parity-time symmetry

An arbitrary body or aggregate can be made perfectly absorbing at discrete frequencies, if a precise amount of dissipation is added under specific conditions of coherent monochromatic illumination. This effect arises from the interaction of optical absorption and wave interference, and corresponds to moving a zero of the S-matrix onto the real wavevector axis. It is thus the time-reversed process of lasing at threshold. The effect may be demonstrated in a Si slab illuminated in the 500–900nm range. Coherent perfect absorbers form a novel class of linear optical elements—absorptive interferometers—which may be useful for controlled optical energy transfer.

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Coherent perfect absorbers: Time-reversed lasers

In the time-reversed counterpart to laser emission, incident coherent optical fields are perfectly absorbed within a resonator that contains a loss medium instead of a gain medium. The incident fields and frequency must coincide with those of the corresponding laser with gain. We demonstrated this effect for two counterpropagating incident fields in a silicon cavity, showing that absorption can be enhanced by two orders of magnitude, the maximum predicted by theory for our experimental setup. In addition, we showed that absorption can be reduced substantially by varying the relative phase of the incident fields. The device, termed a “coherent perfect absorber,” functions as an absorptive interferometer, with potential practical applications in integrated optics.

https://science.sciencemag.org/content/sci/331/6019/889.full.pdf

Time-reversed lasing and interferometric control of absorption.

The optical properties of organic semiconductors are almost exclusively described using the Frenkel exciton picture1. In this description, the strong Coulombic interaction between an excited electron and the charged vacancy it leaves behind (a hole) is automatically taken into account. If, in an optical microcavity, the exciton–photon interaction is strong compared to the excitonic and photonic decay rates, a second quasiparticle, the microcavity polariton, must be introduced to properly account for this coupling2. Coherent, laser-like emission from polaritons has been predicted to occur when the ground-state occupancy of polaritons 〈ngs〉, reaches 1 (ref. 3). This process, known as polariton lasing, can occur at thresholds much lower than required for conventional lasing. Polaritons in organic semiconductors are highly stable at room temperature, but to our knowledge, there has as yet been no report of nonlinear emission from these structures. Here, we demonstrate polariton lasing at room temperature in an organic microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielectric mirrors. Polaritons in organic semiconductors are highly stable at room temperature, but so far nonlinear emission from these structures has not been demonstrated. Here, polariton lasing at room temperature in an organic microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielectric mirrors is reported.

Room-temperature polariton lasing in an organic single-crystal microcavity

The strong coupling limit of cavity QED is reached when matter inserted inside a microcavity exchanges energy with the resonant mode of the cavity more rapidly than the combined rate at which light leaves the cavity and the matter wave function loses its phase information [1,2]. In this limit, the microcavity and matter form a composite quantum system with two new eigenstates that are superpositions of the initial uncoupled states, with new eigenenergies separated in energy by the Rabi splitting. The matter component of the coupled system can be a gas of atoms trapped inside the cavity [3], a superconducting qubit [4], or a solid state thin film containing excitons, in the form of an inorganic quantum well [5], quantum dot [6,7], or organic material [8], in which case the superposition states are referred to as exciton polaritons. Applications of strong coupling in atomic and semiconductor systems have led to one-atom zero threshold lasers [9], high gain polariton parametric amplifiers [10], and predictions that strong coupling may play a key role in future quantum information processors [11]. These experiments have all relied on optical pumping. Here we demonstrate electrically pumped exciton-polariton emission, the first device in which strongly coupled states of light and matter are electrically excited. The matter component of our device is a 6 � 1n mthick film of J aggregated dye. The film consists of 4 bilayers [12] of the cationic polyelectrolyte PDAC (poly diallyldimethylammonium chloride) and J aggregates of the anionic cyanine dye TDBC (5,6-dichloro-2-[3-[5,6-dichloro-1-ethyl-3-(3-sulfopropyl)-2(3H)-benzimidazolidene]-1-propenyl]-1-ethyl-3-(3-sulfopropyl) benzimidazolium hydroxide, inner salt, sodium salt), molecular structures shown in Fig. 1(a). The J aggregates are crystallites of dye in which the transition dipoles of the constituent molecules strongly couple to form a collective narrow linewidth optical transition possessing oscillator strength derived from all the aggregated monomers [13]. The bilayer films contain a high density of J aggregated TDBC and therefore have a very large peak absorption

Strong coupling in a microcavity LED.

Highly efficient resonant coupling of optical excitations in hybrid organic/inorganic semiconductor nanostructures

A (5.1±0.5) nm thick film of high oscillator strength J-aggregated dye critically couples to a single dielectric mirror, absorbing more than 97% of incident λ=591 nm wavelength light, corresponding to an effective absorption coefficient of (6.9±0.7)×106 cm−1 for (film thickness)/λ<1%.

Critically coupled resonators in vertical geometry using a planar mirror and a 5 nm thick absorbing film

Solid state cavity QED: Strong coupling in organic thin films

We report a comprehensive study of photoluminescence (PL) quenching of tris-(8-hydroxyquinoline) aluminum $({\text{Alq}}_{3})$ at interfaces with thin films of tin oxide $({\text{SnO}}_{2})$ using both steady-state and time-resolved measurements. Quenching of excitons generated in the ${\text{Alq}}_{3}$ layer increased with increased conductivity of the ${\text{SnO}}_{2}$ films, which we relate with the presence of nonradiative energy transfer from excitons in ${\text{Alq}}_{3}$ to transitions in ${\text{SnO}}_{2}$. In addition, due to the semitransparency of ${\text{SnO}}_{2}$, the effects of optical interference on the steady-state PL quenching of ${\text{Alq}}_{3}$ are determined. We demonstrate that without accounting for the interference effects in the excitation, the extracted exciton diffusion length $({L}_{d})$ in ${\text{Alq}}_{3}$ is in the range of 10--20 nm. However, when using a numerical model to account for the optical interference effects, we find that ${L}_{d}$ is in the range of 5--9 nm, which agrees with ${L}_{d}$ extracted from time-resolved measurements (4--6 nm). We conclude that time-resolved measurements are least affected by optical interference, yielding the most accurate measurement of ${L}_{d}$.

M. Scott Bradley

Papers

Strong coupling in a microcavity LED.

Highly efficient resonant coupling of optical excitations in hybrid organic/inorganic semiconductor nanostructures

Critically coupled resonators in vertical geometry using a planar mirror and a 5 nm thick absorbing film

Solid state cavity QED: Strong coupling in organic thin films

Photoluminescence quenching of tris-(8-hydroxyquinoline) aluminum thin films at interfaces with metal oxide films of different conductivities