Exciton Photon Research Articles

Guided Bloch surface wave polaritons

The authors report on a theoretical investigation of guided polariton states arising from the strong coupling between quantum-well excitons and a Bloch surface wave confined at the interface between a uniform dielectric medium and a Bragg mirror. It is shown that the exciton–photon coupling is almost doubled as compared to a similar structure made in a conventional planar microcavity. It is also shown that, by simple engineering of the sample surface with silicon oxide deposition, one can efficiently produce one-dimensional polaritons propagating within the structure with extremely low losses. The latter result evidences the usefulness of Bloch surface waves as a key component for the realization of “polaritonic integrated circuits.”

Anomalous reflection properties in high density limit fibril shaped PIC‐J aggregates in microcavity structure

AbstractWe report our recent observations on anomalous reflection properties of organic microcavity containing PIC J‐aggregates in fibril‐shaped structures as one of their highest‐density limiting case. Even in ordinary low density PIC‐Js dispersed in polymer matrices, they easily go into the exciton‐photon strong coupling regime and new quantum states, i.e. exciton‐polaritons, are formed. Vacuum Rabi‐splitting, an indicator of the coupling strength, can be controlled by the density of PIC‐Js in the microcavities in low density regime. Fibril‐shaped PIC‐J aggregates are bundle‐like, self‐assembled higher‐order structure of Js and are considered as a high‐density limiting case. Angular dependence of the local reflection spectra are observed by our improved microscopy. While in the low density cases, clear anti‐crossing behaviours of the two branches appear, which is the evidence of the polariton formations, the highest‐density limiting cases show one or two anomalous reflection dip structures with almost no angular dependence. The result is inconsistent with neither ordinary polariton reflection property nor bare exciton nor cavity photon properties. We tentatively consider that the anomaly is inherent to the fibril formation and is caused by possible breakdown of polariton coherence due to the inter‐fiber energy transfer. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Density operator of a system pumped with polaritons: a Jaynes–Cummings-likeapproach

We investigate the effects of considering two different incoherent excitation mechanisms onmicrocavity quantum dot systems modeled using the Jaynes–Cummings Hamiltonian.When the system is incoherently pumped with polaritons it is able to sustain a largenumber of photons inside the cavity with Poisson-like statistics in the stationary limit, andit also leads to a separable exciton–photon state. We also investigate the effects ofboth types of pumpings (excitonic and polaritonic) in the emission spectrumof the cavity. We show that the polaritonic pumping considered here is unableto modify the dynamical regimes of the system at variance with the excitonicpumping. Finally, we obtain a closed form expression for the negativity of thedensity matrices that the quantum master equation considered here generates.

Scalable quantum computing with atomic ensembles

Atomic ensembles, comprising clouds of atoms addressed by laser fields, provide an attractive system for both the storage of quantum information and the coherent conversion of quantum information between atomic and optical degrees of freedom. We describe a scheme for full-scale quantum computing with atomic ensembles, in which qubits are encoded in symmetric collective excitations of many atoms. We consider the most important sources of error—imperfect exciton–photon coupling and photon losses—and demonstrate that the scheme is extremely robust against these processes: the required photon emission and collection efficiency threshold is ≳86%. Our scheme uses similar methods to those already demonstrated experimentally in the context of quantum repeater schemes and yet has information processing capabilities far beyond those proposals.

Excitonic effects on radial breathing mode intensity of single wall carbon nanotubes

We develop exciton–photon and exciton–phonon interaction matrix elements for single wall carbon nanotubes within extended tight binding method. The exciton–photon matrix elements using the extended tight binding wave function have a large chirality dependence compared with the previous calculation by simple tight binding method. Using the exciton–photon and exciton–phonon matrix elements presented here, we calculate resonance Raman intensity for radial breathing mode as a function of diameter and chiral angle. Excitonic effect of resonance Raman intensities for the radial breathing mode is enhanced by the curvature and trigonal warping effects.

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

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.

Vertical electric field tuning of the exciton fine structure splitting and photon correlation measurements of GaAs quantum dot

The effects of a vertical electric field on the fine structure splitting of neutral exciton of monolayer fluctuation GaAs quantum dots were investigated. Using the gate voltage between the top gate electrode and the bottom n-GaAs substrate, the fine structure splitting of the neutral exciton was tuned to 15–30 μeV. Photon correlation measurements demonstrate neutral exciton single photon emission and neutral exciton—biexciton cascaded emission.

Cooperative exciton–photon dynamics of a linear chain of quantum dots in a perfect microcavity

Cooperative exciton–photon dynamics of a linear chain consisting of closely spaced quantum dots has been studied numerically in the simplified Jaynes–Cummings model with dipole–dipole interaction and only nearest neighbor interactions. In the aggregate, quantum dots are assumed to be coupled with a single mode field of a perfect microcavity. In the chain, the transport of the excitation energy via resonance dipole–dipole interaction generates propagating exciton-waves which interfere with themselves in the mediation of photon field. We show that the absorption and emission of the field is inhibited or enhanced depending on system parameters and the excitation of specified quantum dots is also suppressed or enforced depending on dipole–dipole coupling constant.

Observation of strong exciton–photon coupling at temperatures up to 410 K

We report on the observation of strong exciton–photon coupling in a ZnO-based microresonator consisting of a half medium wavelength ZnO cavity embedded between two dielectric Bragg reflectors made of 10.5 layer pairs of yttria stabilized zirconia and Al2O3. The microresonator was investigated by photoluminescence and reflectivity measurements in a wide temperature range between 10 and 550 K. With both techniques a lower polariton branch (LPB) was observable. As expected no signal from an upper polariton branch could be detected caused by the strong absorption of ZnO in this spectral range. The dispersion behaviour of the LPB (in both energy and broadening) is well described by a model that takes into account the coupling between one exciton mode and one cavity-photon mode. From this analysis we can conclude that the microresonator is in the strong coupling regime up to 410 K. Maximum values of the coupling strength at 10 K of 51 meV, respectively 55 meV, could be derived from the photoluminescence and from the reflectivity. These results demonstrate the high potential of ZnO microresonators for the realization of a Bose–Einstein condensation at room temperature and above.

Coupling of polaritons to vibrational modes of ultracold atoms in an optical lattice

The coupling of internal electronic excitations to vibrational modes of the external motion of ultracold atoms in an optical lattice is studied here in using a perturbation expansion in small atomic displacements. In the Mott insulator case with one atom per site, the resonance dipole–dipole coupling between neighboring sites can induce emission and absorption of vibrational quanta. Within a cavity in the strong exciton–photon coupling regime such coupling results in polariton–vibration interactions, which create a significant thermalization mechanism for polaritons toward their minimum energy, and leading to motional heating of the lattice atoms.

Strong exciton-photon coupling in ZnO based resonators

The authors report on the fabrication of high quality all-oxide Bragg reflectors (BRs) and ZnO based resonators. The resonator consists of a bulk half-wavelength ZnO microcavity embedded between two BRs, each made of 10.5 layer pairs of yttria stabilized zirconia and Al2O3. Scanning transmission electron microscopy and atomic force microscopy, yield smooth interfaces and low surface roughness for the BR as well as the resonator. For the BR with 10.5 layer pairs the authors obtain reflectivities up to 99.2% within the Bragg stop band. The exciton-polariton dispersion was determined by both, polarization- and angle-resolved photoluminescence (PL) and reflectivity (R) measurements. The detuning between the uncoupled exciton mode and photon mode was changed by shifting the exciton mode energy in the temperature range of 10–290 K. Thereby we observed that a strong exciton-photon coupling regime up to room temperature is present in our resonators with maximum values of the Rabi splitting of about 68 meV (PL, T=10 K) and 76 meV (R,T=10 K).

Breakdown of Fermi's Golden Rule in Exciton–Photon Interaction

According to Fermi's golden rule, the radiative decay rate of excitons in a thin film becomes higher with increasing film thickness because of the increment of the interaction volume between excitons and photons. However, for a thick film, the decay rate is inversely proportional to the film thickness, because the excitons behave as polaritons and the decay time is proportional to the time of flight. This contradiction reflects a breakdown of Fermi's golden rule in the exciton–photon interaction. We have revealed the breakdown condition by a rigorous calculation method that connects the two radiative decay schemes.

Exciton–photon interaction in a quantum dot embedded in a photonic microcavity

We present a detailed analysis of exciton–photon interaction in a microcavity made out of a photonic crystal slab. Here we have analysed a disc-like quantum dot where an exciton is formed. Excitonic eigen functions in addition to their eigen energies are found through direct matrix diagonalization, while wavefunctions corresponding to unbound electron and hole are chosen as the basis set for this procedure. In order to evaluate these wavefunctions precisely, we have used the 6 × 6 Luttinger Hamiltonian in the case of hole while ignoring bands adjacent to the conduction band for electron states. After analysing excitonic states, a photonic crystal-based microcavity with a relatively high quality factor mode has been proposed and its lattice constant has been adjusted to obtain the prescribed resonant frequency. We use a finite-difference time-domain method in order to simulate our cavity with sufficient precision. Finally, we formulate the coupling constants for the exciton–photon interaction both where intra-band and inter-band transitions occur. By evaluating a sample coupling constant, it has been shown that the system can be in a strong-coupling regime and Rabi oscillations can occur.

Excitonic Polaritons in Semiconductor Micropillars

We describe the physics of cavity polaritons in semiconductor micropillars. Cavity polaritons are exciton–photon entangled states arising from the strong coupling between excitons and the optical modes of a cavity. In micropillars, the photon three-dimensional confinement results in a discrete spectrum of 0D polariton states. Characterization of the linear properties of these micropillars will be presented. Then we will show how this system can be used to generate parametric photons and to obtain polariton lasing.

Strong exciton–photon coupling at room temperature in microcavities containing two-dimensional layered perovskite compounds

We have realized Perot–Fabry microcavities containing a two-dimensional layered perovskite-type semiconductor: (C6H5C2H4–NH3)2PbI4 between a dielectric mirror and a metallic mirror. A strong coupling regime between the perovskite exciton and the confined photon mode has been evidenced at room temperature from angular-resolved reflectivity experiments, anticrossings as large as 190 meV are observed between the excitonic and cavity modes. We have shown that the design of the microcavity can be varied at will, so that the detuning or the Rabi splitting can be precisely chosen. The emission of the polaritonic low energy branch has been observed.

Exciton–polariton scattering as signature of defects in cold atom optical lattices

We study the effect of defects in the Mott insulator phase of ultracold atoms in an optical lattice on the dynamics of resonant electronic excitations. These excitations can be described analogous to Frenkel excitons in solids and form polaritons, when coupled to an optical resonator. Defects, which are empty sites in a singly occupied Mott insulator state or singly occupied sites for a filling factor two, change the exciton dynamics. We show that vacancies behave like hard sphere scatterers, while singly occupied sites in a doubly filled region generate either attractive or repulsive interaction potentials. We suggest cavity polaritons as tools which detect such defects and show how the scattering can be controlled by changing the exciton–photon detuning. In the case of aspherical individual lattice sites, we calculate the effective scattering potential as a function of the cavity photon polarization direction which exhibits a crossover from a repulsive into an attractive potential and should give a clearly observable signal.

Parametric generation of twin photons in vertical triple microcavities

We report the realization of a monolithic vertical-cavity, surface emitting micro-optical parametric conversion nanostructure, triply resonant with the parametric frequencies, allowing parametric oscillation with ultra-low pump power threshold. The photonic phase-space naturally provides triple resonance for the parametric frequencies, together with built-in cavity phase-matching for the pump wave at normal incidence. Parametric oscillation is observed in both the strong and weak exciton–photon coupling regime, allowing a high operating temperature. Signal and idler beams can be collected at 0° or at finite angles. The OPO threshold is low enough to envisage the realization of an all-semiconductor electrically-pumped micro-parametric oscillator. To cite this article: C. Diederichs et al., C. R. Physique 8 (2007).

First and second optical transitions in single‐walled carbon nanotubes: a resonant Raman study

AbstractResonant Raman spectroscopy was performed to study electron–phonon coupling in single‐walled carbon nanotubes separated in solution. By varying the excitation energy from 1.26 eV to 1.93 eV we obtained radial breathing mode resonance profiles of the first and second optical transitions E11 and E22 of the (9,1) and (8,3) tubes. We observe up to 16 times stronger Raman intensities for the E11 transitions which can mostly be attributed to a two times broader linewidth of the E22 transition. Comparison of the matrix element ratio ℳ︁11/ℳ︁22 to theoretical predictions on the electron–phonon coupling show a deviation of a factor 1.7 which might be associated with the change of the exciton–photon matrix element. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence asymmetry with quantum state preparation

We show that the number of photons in a strongly coupled exciton–photon system is asymmetric with the detuning of the modes when, in the spontaneous emission regime, the two modes are entangled. As changing the detuning is easy in semiconductor microcavities–where on the other hand the nature of the strong-coupling in terms of single-particle effects is not yet resolved–we propose this effect as a test of the quantum character of microcavity polaritons.

Exciton–photon coupling in a ZnSe-based microcavity fabricated using epitaxial liftoff

We report the observation of strong exciton–photon coupling in a ZnSe-based microcavity fabricated using epitaxial liftoff. Molecular beam epitaxial grown ZnSe/Zn0.9Cd0.1Se quantum wells with a one wavelength optical length at the exciton emission were transferred to a SiO2/Ta2O5 mirror with a reflectance of 96% to form finesse matched microcavities. Analysis of our angle-resolved transmission spectra reveals key features of the strong coupling regime: anticrossing with a normal mode splitting of 23.6 meV at 20 K, composite evolution of the lower and upper polaritons and narrowing of the lower polariton linewidth near resonance. The heavy-hole exciton oscillator strength per quantum well is also deduced to be 1.78 × 1013 cm−2.