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Photocreation of excitons

Monique Combescot, Shiue-Yuan Shiau

Published Aug 5, 2026• 0 citations
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Abstract

Abstract The chapter « Photocreation of excitons » addresses the creation of excitons by photon absorption. It explains why, for photons to be absorbed in a semiconductor, it is necessary to prevent the formation of long-lived exciton-polaritons by changing the exciton center-of-mass wave vector through collisions with crystal defects or lattice thermal vibrations. The exciton lifetime associated with this broadening dresses the coupled photon with a finite lifetime that ends by producing a photon number decrease, which fundamentally corresponds to photon absorption. The exciton broadening is handled via an energy imaginary part, that is, a nonhermitian photon-exciton Hamiltonian with ket and bra eigenstates. The time evolution of the photon number leads to an optimum absorption that results from the competition between the photon coupling to exciton and the decrease of the coupled exciton number due to wave vector relaxation.

DOI
10.1093/oso/9780198926641.003.0014

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•Aug 5, 2026•0 citations
Abstract The chapter « Photocreation of excitons » addresses the creation of excitons by photon absorption. It explains why, for photons to be absorbed in a semiconductor, it is necessary to prevent the formation of long-lived exciton-polaritons by changing the exciton center-of-mass wave vector through collisions with crystal defects or lattice thermal vibrations. The exciton lifetime associated with this broadening dresses the coupled photon with a finite lifetime that ends by producing a photon number decrease, which fundamentally corresponds to photon absorption. The exciton broadening is handled via an energy imaginary part, that is, a nonhermitian photon-exciton Hamiltonian with ket and bra eigenstates. The time evolution of the photon number leads to an optimum absorption that results from the competition between the photon coupling to exciton and the decrease of the coupled exciton number due to wave vector relaxation.
An exciton-phonon mechanism for producing cavityless optical bistability is suggested. We consider the influence of the oscillating subsystem of a crystal on the absorption process of a laser wave in the region of exciton frequencies. It is shown that exciton-phonon interaction leads to nonlinear dependence of the absorption coefficient on the density of the exciton gas. The increase of absorption with increasing exciton density, however, is possible only in the frequency region on the long-wavelength tail of the exiton band. As a result, the exciton density of the crystal becomes a hysteresis function of external parameters (intensity of the incident wave, temperature, etc.). We calculate the absorption coefficient of excitons and define the frequency and temperature regions where optical bistability may exist. We analyze different relaxation mechanisms of excitons and explain the experimental results of observing optical bistability in different semiconductor crystals.
The strong coupling of photons and quantum well excitons in semiconductor microcavities leads to the formation of new eigenstates called polaritons. Due to their peculiar dispersion curve and the convenient experimental accessibility, microcavity polaritons have gained considerable interest in the recent years. Polariton states can be selectively populated by choosing the excitation angle and the wavelength of the excitation laser. After a lifetime of a few picoseconds the polariton decays and a photon is emitted from the cavity. This photon carries the whole information on the underlying polariton state.
We review the nonlinear-optical effects observed at room temperature in semiconductor quantum-well structures photoexcited near the band gap. A comprehensive discussion of optical transitions in these microstructures is given, including excitonic effects and the specific features of room-temperature exciton resonances. Experimental investigations using continuous-wave, picosecond-, and femtosecond-laser sources are presented. They show extremely efficient nonlinear processes. In the case of excitations that are long compared with the exciton-ionization time, the induced changes in absorption and refraction do not depend on the wavelength or on the duration of excitation. These changes depend only on the density of absorbed photons and are interpreted in terms of electron–hole plasma screening and band filling. In contrast, for ultrashort excitation, nonlinear processes depend critically on the excitation wavelength. The selective generation of excitons is found to produce effects larger than a plasma of the same density. This unexpected result is shown to arise from the low temperature of the exciton gas before it interacts with the lattice and from the decrease of screening that is the reduced dimensionality of quantum-well structures.
We rederive the dressed atom and the exciton polariton within the same framework to make clear that their major difference only comes from the number of electrons available for photoexcitations. This allows to analytically show how the decrease of the excited-state lifetime transforms the photon number time evolution from oscillations—at the stimulated Rabi frequency for an atom and the vacuum one for semiconductors—to an exponential decay identical for both. Although the matter ground state is in both cases coupled not to a continuum but to a discrete state , broadened by lifetime, this decay yet follows a Fermi golden rule, however having a different scale for the energy conservation. Our work actually provides a clean justification for using the golden rule for exciton absorption.
BEC of exciton-polaritons and related effects such as superfluidity1,2, spontaneous symmetry breaking3,4 and quantised vortices5,6 open way to creation of novel light sources7 and optical logic elements8. Remarkable observations of exciton-polariton BEC in microcavities9-12 have been reported in the recent ten years. Very recently, thermalisation and subsequent condensation of cavity photons in a dye-filled microcavity have been observed13. Here we show that BEC of both exciton-polaritons and photons can be created in the same system under different optical excitation conditions. A dynamic phase transition between a photon and a polariton BEC takes place after a single high-power excitation pulse and we find both condensed states in thermal equilibrium with the excited states. At the crossover, photons and polaritons coexist, which results in a decrease in the long-range spatial coherence. Build-up and successive depinning of polarisation is observed at the threshold of both polariton and photon condensation.
Abstract The chapter « Photon absorption » tackles a fundamental question: as excitons with a given center-of-mass wave vector are well separated in energy at the exciton Rydberg scale, while the Fermi golden rule requires a final-state continuum to be valid, photons cannot be absorbed in a semiconductor along this rule. An extrinsic exciton broadening must exist for photons to be absorbed. This chapter also discusses the possibility to go beyond the Fermi golden rule, as this rule fundamentally is second order in coupling.