It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.

a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals.

Multicolor bandgap fluorescent carbon quantum dots (MCBF-CQDs) from blue to red with quantum yield up to 75% are synthesized using a solvothermal method. For the first time, monochrome electroluminescent light-emitting diodes (LEDs) with MCBF-CQDs directly as an active emission layer are fabricated. The maximum luminance of blue LEDs reaches 136 cd m-2 , which is the best performance for CQD-based monochrome electroluminescent LEDs.

Bright Multicolor Bandgap Fluorescent Carbon Quantum Dots for Electroluminescent Light-Emitting Diodes

We synthesize the colloidal carbon/graphene quantum dots 1–9 nm in diameter and study their photoluminescence properties. Surprisingly, the luminescence properties of a fixed collection of colloidal carbon quantum dots can be systematically changed as the concentration varies. A model based on photon reabsorption is proposed which explains well the experiment. Infrared spectral study indicates that the surfaces of the carbon quantum dots are substantially terminated by oxygen atoms, which causes their ultra-high hydrophilicity. Our result clarifies the mystery of distinct emission colors in carbon quantum dots and indicates that photon reabsorption can strongly affect the luminescence properties of colloidal nanocrystals.

http://absimage.aps.org/image/MAR16/MWS_MAR16-2015-000147.pdf

Red shift in the photoluminescence of colloidal carbon quantum dots induced by photon reabsorption

Carbon quantum dots-based semiconductor preparation methods, applications and mechanisms in environmental contamination

Fabrication of S-scheme TiO2/g-C3N4 nanocomposites for generation of hydrogen gas and removal of fluorescein dye

Charge transfer mediated photoluminescence enhancement in carbon dots embedded in TiO2 nanotube matrix

Resonant light–matter interaction between a molecular transition and a confined electromagnetic field can result in strong coupling where a coherent exchange of energy between light and matter occurs to form a new set of particles called polaritons. Being hybrid particles, polaritons exhibit a wide variety of quantum phenomena such as Bose–Einstein condensation, superfluidity, quantum phase transitions and many others. Recent progress in fundamental understanding and technological advancement in the field of polariton physics has allowed scientists to design and develop many impressive experimental configurations to obtain new insights into the strong interaction of light and matter in different material systems. Among all polariton configurations, microcavity polaritons based on organic materials have been emerging as a promising platform to easily achieve strong light–matter coupling at ambient conditions due to their unique properties compared to the conventional inorganic polariton systems. This mini review covers design principles and typical configurations of organic microcavity polaritons with a short tutorial on essential conditions to be satisfied for the strong coupling regime.

Strong light–matter interaction in organic microcavity polaritons: essential criteria, design principles and typical configurations

Organic field effect transistors (OFETs) of poly(p-phenylenevinylene) fabricated by chemical vapor deposition (CVD) with improved hole mobility

Investigation of non-linear dependence of exciton recombination efficiency on PCBM concentration in P3HT:PCBM blends

Room‐temperature interaction between light–matter hybrid particles such as exciton–polaritons under extremely low‐pump plays a crucial role in future coherent quantum light sources. However, the practical and scalable realization of coherent quantum light sources operating under low‐pump remains a challenge because of the insufficient polariton interaction strength. Here, at room temperature, a very large polariton interaction strength is demonstrated, g ≈ 128 ± 21 µeV µm2 realized in a 2D nanolayered metal–organic framework (MOF). As a result, a polariton lasing at an extremely low pump fluence of P1 ≈ 0.01 ± 0.0015 µJ cm−2 (first threshold) is observed. Interestingly, as pump fluence increases to P2 ≈ 0.031 ± 0.003 µJ cm−2 (second threshold), a spontaneous transition to a polariton breakdown region occurs, which has not been reported before. Finally, an ordinary photon lasing occurs at P3 ≈ 0.11 ± 0.077 µJ cm−2 (third threshold), or above. These experiments and the theoretical model reveal new insights into the transition mechanisms characterized by three distinct optical regions. This work introduces MOF as a new type of quantum material, with naturally formed polariton cavities, that is a cost‐effective and scalable solution to build microscale coherent quantum light sources and polaritonic devices.

Anu Babusenan

Papers

Charge transfer mediated photoluminescence enhancement in carbon dots embedded in TiO2 nanotube matrix

Strong light–matter interaction in organic microcavity polaritons: essential criteria, design principles and typical configurations

Organic field effect transistors (OFETs) of poly(p-phenylenevinylene) fabricated by chemical vapor deposition (CVD) with improved hole mobility

Investigation of non-linear dependence of exciton recombination efficiency on PCBM concentration in P3HT:PCBM blends

Triple Threshold Transitions and Strong Polariton Interaction in 2D Layered Metal–Organic Framework Microplates