Cavity Mode Energy Research Articles

Exciton-polariton dynamics of the single site-controlled quantum dot-nanocavity in the coexisting strong-weak coupling regime

Deterministic positioning single site-controlled high symmetric InGaAs quantum dots (QDs) in (111)B-oriented GaAs photonic crystal cavities with nanometer-scale accuracy provides an idea component for building integrated quantum photonic circuits. However, it has been a long-standing challenge of improving cavity Q-factors in such systems. Here, by optimizing the trade-off between the cavity loss and QD spectral quality, we demonstrate our site-controlled QD-nanocavity system operating in the intermediate coupling regime mediated by phonon scattering, with the dynamic coexistence of strong and weak coupling. The cavity-exciton detuning-dependent micro-photoluminescence spectrum reveals concurrence of a trend of exciton-polariton mode avoided crossing, as a signature of Rabi doublet of the strongly coupled system. Meanwhile, a trend of keeping constant or slight blue shift of coupled exciton–cavity mode(CM) energy across zero-detuning is ascribed to the formation of collective states mediated by phonon-assisted coupling, and their rare partial out-of-synchronization linewidth-narrowing is linked to their coexisting strong-weak coupling regime. We further reveal the pump power-dependent anti-bunching photon statistical dynamics of this coexisting strong-weak coupled system and the optical features of strongly confined exciton-polaritons, and dark-exciton-like states. These observations demonstrate the potential capabilities of site-controlled QD-cavity systems as deterministic quantum nodes for on-chip quantum information processing and provide guidelines for future device optimization for achieving the strong coupling regime.

Autotuning of Vibrational Strong Coupling for Site-Selective Reactions.

Site-selective chemistry opens new paths for the synthesis of technologically important molecules. When a reactant is placed inside a Fabry-Perot (FP) cavity, energy exchange between molecular vibrations and resonant cavity photons results in vibrational strong coupling (VSC). VSC has recently been implicated in modified chemical reactivity at specific reactive sites. However, as a reaction proceeds inside an FP cavity, the refractive index of the reaction solution changes, detuning the cavity mode away from the vibrational mode and weakening the VSC effect. Here we overcome this issue, developing actuatable FP cavities to allow automated tuning of cavity mode energy to maintain maximized VSC during a reaction. As an example, the site-selective reaction of the aldehyde over the ketone in 4-acetylbenzaldehyde is achieved by automated cavity tuning to maintain optimal VSC of the ketone carbonyl stretch during the reaction. A nearly 50 % improvement in site-selective reactivity is observed compared to an FP cavity with static mirrors, demonstrating the utility of actuatable FP cavities as microreactors for organic chemistry.

Up-conversion superfluorescence induced by abrupt truncation of coherent field and plasmonic nanocavity.

We theoretically propose a new method for generating up-converted coherent light from two-level systems (TLSs) coupled with a plasmonic nanocavity. The emission spectrum of a TLS excited by a strong laser exhibits a triplet structure called the Mollow triplet. If the lower Mollow sideband is tuned to the cavity mode energy, population inversion of a TLS occurs. When the driving laser is abruptly truncated under this condition, an up-converted photon is emitted from the TLSs. We also predict the up-converted superfluorescence from an ensemble of TLSs as a correlation effect among the excited states of the TLSs.

Interrogating the Structure of Molecular Cavity Polaritons with Resonance Raman Scattering: An Experimentally Motivated Theoretical Description

The formation of exciton polaritons by strongly coupling molecular electronic transitions to spatially confined photons may change photochemical processes. However, the fundamental physical drivers of these proposed changes remain unclear. To understand the role of changes to molecular structure caused by polariton formation in these photochemical processes we develop an experimentally motivated theoretical description of resonance Raman scattering from exciton polaritons. By modeling the structural parameters of light absorption in zinc tetraphenyl porphyrin and exciton polaritons formed from this molecule in nano-structured Fabry-Perot cavities, we simulate resonance Raman scattering excitation spectra from exciton polaritons and assess how these spectra depend on the polariton characteristics. We find the detuning of the cavity mode energy from that of the molecular transition can affect the structure of the resulting polaritonic states, as inferred from our simulated spectra. We also find previously u...

Exciton-polaritons in multilayer WSe2 in a planar microcavity

Due to high binding energy and oscillator strength, excitons in thin flakes of transition metal dichalcogenides constitute a perfect foundation for realizing a strongly coupled light-matter system. In this paper we investigate mono- and few-layer WSe2 flakes encapsulated in hexagonal boron nitride and incorporated into a planar dielectric cavity. We use an open cavity design which provides tunability of the cavity mode energy by as much as 150 meV. We observe a strong coupling regime between the cavity photons and the neutral excitons in direct-bandgap monolayer WSe2, as well as in few-layer WSe2 flakes exhibiting indirect bandgap. We discuss the dependence of the exciton’s oscillator strength and resonance linewidth on the number of layers and predict the exciton–photon coupling strength.

Strong exciton-photon coupling in open semiconductor microcavities

We present a method to implement 3-dimensional polariton confinement with in-situ spectral tuning of the cavity mode. Our tunable microcavity is a hybrid system consisting of a bottom semiconductor distributed Bragg reflector (DBR) with a cavity containing quantum wells (QWs) grown on top and a dielectric concave DBR separated by a micrometer sized gap. Nanopositioners allow independent positioning of the two mirrors and the cavity mode energy can be tuned by controlling the distance between them. When close to resonance, we observe a characteristic anticrossing between the cavity modes and the QW exciton demonstrating strong coupling. For the smallest radii of curvature concave mirrors of 5.6 μm and 7.5 μm, real-space polariton imaging reveals submicron polariton confinement due to the hemispherical cavity geometry.

Characterization of 2.3 μm GaInAsSb-based vertical-cavity surface-emitting laser structures using photo-modulated reflectance

We report angle dependent and temperature dependent (9 K–300 K) photo-modulated reflectance (PR) studies on vertical-cavity surface-emitting laser (VCSEL) structures, designed for 2.3 μm mid-infrared gas sensing applications. Changing the temperature allows us to tune the energies of the quantum well (QW) transitions relative to the VCSEL cavity mode (CM) energy. These studies show that this VCSEL structure has a QW-CM offset of 21 meV at room temperature. Consequently the QW ground-state transition comes into resonance with the CM at 220 ± 2 K. The results from these PR studies are closely compared with those obtained in a separate study of actual operating devices and show how the PR technique may be useful for device optimisation without the necessity of having first to process the wafers into working devices.

Monolithic microcavity with carbon nanotubes as active material

We report on the realization of monolithic optical microcavities using a single wall carbon nanotubes doped polymer as active material. Thanks to the control of the polymer thickness, a fine control of the cavity mode energy is achieved, which allows to tune it in exact resonance with a specific chiral species emission line. The quality factor of the filled cavity mode (Q = 40) allows to selectively extract the luminescence of the (7,5) chiral species. Finally, angle resolved experiments show the tunability of the emission energy within a 150 meV range.

High guided mode–cavity mode coupling for an efficient extraction of spontaneous emission of a single quantum dot embedded in a photonic crystal nanobeam cavity

We demonstrate efficient extraction of a single InAs quantum dot spontaneous emission through an air-suspended waveguide to which a photonic crystal nanobeam cavity is coupled. First, the quantum dot emission into the cavity mode is enhanced due to a high Purcell effect arising from a large quality factor and a small mode volume of the cavity mode. Second, the cavity photons can be efficiently transferred to the guided mode by optimizing the design of a structure that exhibits a high coupling rate between the two modes. This configuration has lead to the observation of a high extraction efficiency ($79%$) of the quantum dot emission through the waveguide. Moreover, as a result of the high extraction efficiency to the guided mode, the unwanted microphotoluminescence coupling to the free space modes and detected from the top surface of the sample is strongly suppressed. Indeed, we show that it exhibits a negative peak as a function of the detuning between the cavity mode energy and the quantum dot transition energy.

Optimization of nanocavity field enhancement using two-dimensional plasmonic photonic crystals

We have investigated the influence of Ag nanorod radius (r) on the resonant modes of a two-dimensional plasmonic photonic crystal (PPC) with dipole sources embedded into the central vacancy area, using finite-difference time-domain methods. Both the localized surface plasmon (LSP) mode of individual Ag nanorods and the resonant cavity mode of PPC are found to vary as a function of r. The resonant cavity mode is strongly enhanced as r is increased, while the LSP signal will eventually become no longer discernable in the Fourier spectrum of the time-evolved field. An optimized condition for the nanocavity field enhancement is found for a given PPC periodicity (e.g. d = 375 nm) with the critical nanorod radius r c = d/3. At this point the resonant cavity mode has the strongest field enhancement, best field confinement and largest Q-factor. We attribute this to competition between the blocking of cavity confined light to radiate out when the cavity resonant frequency falls inside the opened photonic stopband as r reaches r c, and the transfer of cavity mode energy to inter-particle plasmons when r is further increased.

Angular dependent luminescence behavior in one-dimensional Si-based microcavities

One-dimensional all solid-state microcavities (MCs) based on amorphous silicon-nitride films have been fabricated in plasma enhanced chemical vapor deposition system. Spectral narrowing and enhancement of room temperature photoluminescence due to the modulation effect in MCs were observed. The modulated luminescence behavior in MCs was further studied by angular dependent luminescence measurements. The distinct cavity mode energy shifting and variation of luminescence intensity with the detect angle were observed, which reflected the directionality of emission properties in the planar MCs. The deviation of the measured angular dependent energy shifting from the theory prediction was discussed as being related to the structure properties of the MCs.

Spatial structure and stability of the macroscopically occupied polariton state in the microcavity optical parametric oscillator

Transverse spatial images of the signal generated in semiconductor microcavities under conditions of resonant parametric scattering are obtained. When the system is pumped significantly above the threshold, the signal image is found to be highly structured, consisting of a set of localized bright spots. By comparison with a numerical model, it is shown that the structure arises from the blueshift generated by the nonlinear polariton-polariton interaction which drives the microcavity optical parametric oscillator, in combination with partially ordered spatial fluctuations in the cavity mode energy. An important outcome is that the form and stability of the signal is strongly influenced by the local cavity disorder.

Light mediated superconducting transistor

Polaritons composed by electron-hole pairs coupled to confined light modes in optical cavities may form a Bardeen-Cooper-Schriefer (BCS) superfluid. We propose a new transistor based on stimulated scattering of electron-hole pairs into the BCS polariton mode. A pn-junction embedded inside an optical cavity resonantly emits light into the cavity mode. If the cavity mode energy slightly exceeds the band-gap energy, it couples with electron-hole pairs with zero centre of mass wave-vector but non-zero wave-vector of relative motion. This creates a super-current in the plane of the structure. In an isotropic case, its direction is chosen by the system spontaneously. Otherwise, it is pinned to the external in-plane bias. We calculate the phase diagram for the electron-hole-polariton system. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of 1.3-μm GaInNAd vertical-cavity surface-emitting lasers (VCSELs) using temperature, high-pressure, and modeling techniques

We have investigated the temperature and pressure dependence of the threshold current (I/sub th/) of 1.3 /spl mu/m emitting GaInNAs vertical-cavity surface-emitting lasers (VCSELs) and the equivalent edge-emitting laser (EEL) devices employing the same active region. Our measurements show that the VCSEL devices have the peak of the gain spectrum on the high-energy side of the cavity mode energy and hence operate over a wide temperature range. They show particularly promising I/sub th/ temperature insensitivity in the 250-350 K range. We have then used a theoretical model based on a 10-band k.P Hamiltonian and experimentally determined recombination coefficients from EELs to calculate the pressure and temperature dependency of I/sub th/. The results show good agreement between the model and the experimental data, supporting both the validity of the model and the recombination rate parameters. We also show that for both device types, the super-exponential temperature dependency of I/sub th/ at 350 K and above is due largely to Auger recombination.

Polariton-Electron Scattering in a Semiconductor Microcavity

The effect of a classical electron gas on the cavity polariton linewidths is studied experimentally at T = 80 K in a GaAs/AlAs microcavity with an embedded quantum well. The linewidth dependence on the electron density and on the cavity mode energy is explained by a theoretical model based on the asymmetric bare exciton line shape, due to electron–exciton scattering.

Scattering of polaritons by a two-dimensional electron gas in a semiconductor microcavity

The effect of a classical electron gas on the cavity polariton linewidths is studied experimentally in a GaAs/AlAs microcavity with an embedded quantum well that contains photogenerated electrons (at $T=80 \mathrm{K}).$ The linewidth dependence on the electron density and on the cavity mode energy is explained by a theoretical model based on calculating the asymmetric bare exciton line shape, due to electron-exciton scattering. For a given electron density, the exciton line shape is inserted into the linear dispersion theory of the coupled excitons and cavity mode, yielding the reflection spectra that are formed of the three polariton branches. This model reproduces the reduced lower-polariton linewidth and the complex dependence of the middle- and upper-polariton linewidths on the cavity mode energy.

Instantaneous optical modulation in bulk GaAs semiconductor microcavities

Picosecond pump–probe experiments at room temperature on a bulk GaAs microcavity are presented. The microcavity device is designed to adjust the cavity mode energy 15 meV below the band gap energy of the intracavity bulk GaAs material. For low pump-energy densities (∼μJ/cm2), ultrafast modulation of the reflectivity is demonstrated due to the purely coherent refractive index change. A 5:1 contrast ratio is achieved and shows the potential of the semiconductor microcavities for implementation in ultrafast all optical switching.

Cavity mode polarisation splitting in organic semiconductor microcavities

Angular-dependent reflectivity techniques are employed to probe the optical structure of microcavities containing a passive spacer layer, and also a fluorescent polymer. For the fluorescent polymer, we used the polymer poly(9,9-dioctylfluorene) (PFO), which was deposited between a dielectric and an aluminium mirror. The reflectivity spectra exhibit a strong polarisation dependence with the cavity mode energy and photon linewidths of the cavity mode different for the TE (transverse electric) and TM (transverse magnetic) polarisations. We find that as the cavity mode is moved away from the centre of the dielectric mirror stop-band, the difference between the energy of the TE and TM modes grows. This results in a significant splitting of up to 110 meV between the TE and TM polarisations detected at large angle. We investigate this effect by varying the energy of the cavity mode with respect to the centre of the mirror stop-band. Results are analysed using a transfer-matrix model. Photoluminescence emission from the PFO containing microcavity was also measured. We find a splitting between TE and TM modes, which is also manifested in the photoluminescence (PL) emission. This allows control of polarisation from an isotropic film, presenting a new method to control exciton emission.

Resonances between the cavity mode and five excitonic transitions in an InxGa1−xAs/GaAs/AlAs/AlGaAs vertical-cavity surface-emitting laser structure using photomodulated reflectance

An InxGa1−xAs/GaAs/AlAs/AlGaAs vertical-cavity surface-emitting laser structure has been studied by conventional reflectance and photomodulated reflectance (PR) spectroscopies. Slight fluctuations in molecular beam epitaxy growth conditions led to thickness variations of <∼12% along the wafer radius. While this did not appreciably affect the energy of the quantum well (QW) transitions, it did give rise to a significant but smooth variation in the cavity mode energy. PR spectroscopy was used to study the interaction between the cavity mode and QW excitons, as the overlap between them was varied by probing different wafer regions. The PR signal was strongly enhanced when the cavity mode and a QW transition were in good alignment. We were able to investigate five distinct such resonances between the cavity mode and the ground-state and four other, higher-order, QW transitions. A theory has already been developed for the PR modulation of the coupled cavity and exciton modes, based on energy-dependent Seraphin coefficients. A similar but simplified model was used to fit all the PR spectra, and the resulting QW transition energies then compared with those predicted by a theoretical model which includes excitonic binding energy effects.

Mechanism of Polariton-Stimulation in a CdTe-Based Microcavity

At low temperature and for excitation densities well below the exciton saturation density in CdTe microcavities, the photoluminescence (PL) intensity of the lower polariton mode strongly increases with a marked threshold behaviour. We present results of pump–probe experiments which show that the stimulation of the lower polariton PL actually occurs in the strong exciton–photon coupling regime. We present also a detailed study of this stimulation effect as a function of the number of quantum wells in the microcavity and as a function of the detuning δ = EC — EX between exciton energy EX and the cavity mode energy EC. We show that: (i) our results cannot be interpreted in terms of the localised exciton model; (ii) phonon relaxation and exciton–exciton collisions or polariton–exciton collisions are involved in the stimulation.