Exciton-cavity Detuning Research Articles

Condensation of 2D exciton-polaritons in an open-access microcavity

We establish a tunable open-access microcavity consisting of two planar distributed Bragg reflectors (DBRs) individually controlled by nanopositioners. By varying the cavity length, such configuration enables variation of the light–matter coupling strength by a factor of 2, while keeping in microresonators the same active region and cavity mirrors. Polariton condensation was demonstrated over a large range of Rabi splittings and the corresponding threshold diagram was derived as a function of cavity-exciton detuning, which fits well with theoretical simulations. The results show that for various light-matter coupling strengths, optimal detunings featured by the lowest condensation threshold always occur at a fixed depth of energy trap between the exciton reservoir and the polariton ground state, which enables the most efficient exciton–exciton scattering into the condensate state in the driven-dissipative polaritonic system.

Optimal condition to probe strong coupling of two-dimensional excitons and zero-dimensional cavity modes

The light-matter interaction associated with a two-dimensional (2D) excitonic transition coupled to a zero-dimensional (0D) photonic cavity is fundamentally different from coupling localized excitations in quantum dots or color centers, which have negligible spatial extent compared to the cavity-confined mode profile. By calculating the radiation-matter coupling of the exciton transition of a surface deposited 2D material and a 0D photonic crystal nanobeam mode, we found that there is an optimal spatial extent of the monolayer material that maximizes such an interaction strength due to the competition between minimizing the excitonic envelope function area and maximizing the total integrated field. This is counter to the intuition from the Dicke model, where the oscillator strength is expected to monotonically grow with the number of oscillators, which correlates to the monolayer area assuming the excitonic wavefunction is delocalized over the entire quantum well. We also found that at near zero exciton-cavity detuning, the direct transmission efficiency of a waveguide-integrated cavity can be severely suppressed, which suggests performing experiments by using a side-coupled cavity to get better performances.

Dispersive coupling between MoSe2 and an integrated zero-dimensional nanocavity

Establishing a coherent interaction between a material resonance and an optical cavity is a necessary first step to study semiconductor quantum optics. Here we report on the signature of a coherent interaction between a two-dimensional excitonic transition in monolayer MoSe2 and a zero-dimensional, ultra-low mode volume (V m ∼ 2(λ/n)3) on-chip photonic crystal nanocavity. This coherent interaction manifests as a dispersive shift of the cavity transmission spectrum, when the exciton-cavity detuning is decreased via temperature tuning. The exciton-cavity coupling is estimated to be ≈6.5 meV, with a cooperativity of ≈4.0 at 80 K, showing our material system is on the verge of strong coupling. The small mode-volume of the resonator is instrumental in reaching the strongly nonlinear regime, while on-chip cavities will help create a scalable quantum photonic platform.

Electrically tunable topological transport of moiré polaritons

Moiré interlayer exciton in transition metal dichalcogenide heterobilayer features a permanent electric dipole that enables the electrostatic control of its flow, and optical dipole that is spatially varying in the moiré landscape. We show the hybridization of moiré interlayer exciton with photons in a planar 2D cavity leads to two types of moiré polaritons that exhibit distinct forms of topological transport phenomena including the spin/valley Hall and polarization Hall effects, which feature remarkable electrical tunability through the control of exciton-cavity detuning by the interlayer bias.

Impact of temperature on exciton-cavity detuning and polariton relaxation kinetics in monolayer Tungsten Disulphide ([formula omitted]) based microcavity

Monolayer Tungsten Disulphide (WS2) with strongly confined 2D Wannier-Mott excitons exhibits a large binding energy and oscillator strength. This monolayer is currently emerging as a good candidate for studying strong light–matter coupling at high temperature. Here, we investigate the formation of exciton-polaritons in a monolayer WS2 based semiconductor microcavity under non-resonant excitation. Our results show that the cavity detuning changes from negative to positive values over a temperature range of 130–230 K, which allow the tuning of polariton dispersion. The Hopfield coefficient show that the Upper branch (UP) can be tuned from a more excitonlike (130 K), to a more photonlike (230 K) at small wave vector k. Thus, the UP states have a faster lifetime which enhances the relaxation mechanisms towards lower polariton (LP) states. A Rabi splitting of 40 meV is observed, which corresponds to energy of longitudinal optical (LO) phonon. To show how polaritons states are populated, we investigate a semiclassical model that treats the temporal dynamics of polaritons in which LO phonon-assisted polariton emission is taken into account. The results reveals that the UP occupation starts to decrease, for increasing the occupation factor in the LP branch.

Coherent control of the dynamics of a single quantum-dot exciton qubit in a cavity

In this work we demonstrate theoretically how to use external laser field to control the population inversion of a single quantum dot exciton qubit in a nanocavity. We consider the Jaynes-Cummings model to describe the system, and the incoherent losses were take into account by using Lindblad operators. We have demonstrated how to prepare the initial state in a superposition of the exciton in the ground state and the cavity in a coherent state. The effects of exciton-cavity detuning, the laser-cavity detunings, the pulse area and losses over the qubit dynamics are analyzed. We also show how to use a continuous laser pumping in resonance with the cavity mode to sustain a coherent state inside the cavity, providing some protection to the qubit against cavity loss.

Quantum theory of the emission spectrum from quantum dots coupled to structured photonic reservoirs and acoustic phonons

Electron-phonon coupling in semiconductor quantum dots plays a significant role in determining the optical properties of excited excitons, especially the spectral nature of emitted photons. This paper presents a comprehensive theory and analysis of emission spectra from artificial atoms or quantum dots coupled to structured photon reservoirs and acoustic phonons, when excited with incoherent pump fields. As specific examples of structured reservoirs, we chose a Lorentzian cavity and a coupled cavity waveguide, which are of current experimental interest. For the case of optical cavities, we directly compare and contrast the spectra from three distinct theoretical approaches to treat electron-phonon coupling, including a Markovian polaron master equation, a non-Markovian phonon correlation expansion technique and a semiclassical linear susceptibility approach, and we point out the limitations of these models. For the cavity-QED polaron master equation, we give closed form analytical solutions to the phonon-assisted scattering rates in the weak excitation approximation, fully accounting for temperature, cavity-exciton detuning and cavity dot coupling. We show explicitly why the semiclassical linear susceptibility approach fails to correctly account for phonon-mediated cavity feeding. For weakly coupled cavities, we calculate the optical spectra using a more general reservoir approach and explain its differences from the above approaches in the low Q limit of a Lorentzian cavity. We subsequently use this general reservoir to calculate the emission spectra from quantum dots coupled to slow-light photonic crystal waveguides, which demonstrate a number of striking photon-phonon coupling effects. Our quantum theory can be applied to a wide range of photonic structures including photonic molecules and coupled-cavity waveguide systems.

Determination of operating parameters for a GaAs-based polariton laser

We report on a systematic study of the phase transitions to polariton condensation (PC) and further to cavity lasing in a GaAs-based microcavity with respect to exciton-cavity detuning and lattice temperature. Using far field and time-resolved spectroscopy, we determined the parameter space in which PC can be achieved and the corresponding variation of PC threshold power. We found a lower bound of −12 meV for the exciton-cavity detuning and an upper bound of 90 K for the lattice temperature.

Polaron master equation theory of the quantum-dot Mollow triplet in a semiconductor cavity-QED system

We present a comprehensive theoretical study of the resonance fluorescence spectra of an coherently-driven quantum dot (QD) placed inside a high-$Q$ semiconductor cavity and interacting with an acoustic-phonon bath. We derive a quantum master equation (ME) in the polaron frame, which includes exciton-phonon and exciton-cavity coupling to all orders. This work details and extends the theory used in a recent paper [C. Roy and S. Hughes, Phys. Rev. Lett. 106, 247403 (2011)] to describe the QD Mollow triplet in the regime of semiconductor cavity QED. Here, we introduce two ME forms, Nakajima-Zwanzig and time convolutionless (TC), both to second order in the system--phonon-reservoir perturbation. In the polaron frame, these two ME forms are shown to yield equivalent population dynamics and fluorescence spectra for a continuous wave (cw) driving field. We also demonstrate that a Markov approximation is valid for computing the incoherent scattering processes, and we subsequently exploit the TC ME to explore the resonance fluorescence spectra of an exciton-driven QD. Both cavity-emitted and exciton-emitted spectra are studied, and these are found to have qualitatively different spectral features. Using a coherent driving field, the well-known characteristics of the atomic Mollow triplet are shown to be considerably modified with electron--acoustic-phonon scattering, and we highlight the key effects arising from both cavity coupling and electron-phonon coupling. Regimes of pronounced cavity feeding and anharmonic cavity QED are exemplified, and we find that the cavity coupling depends sensitively on the exciton-cavity detuning and the temperature of the phonon bath. We show how the full width at half maximum (linewidth) of the Mollow-triplet sidebands varies as a function of the square of the Rabi frequency of the cw pump. Phonon-mediated cavity coupling also contributes to the spectral broadening of the Mollow triplet, depending upon the exciton-cavity detuning and the strength of the exciton-cavity coupling rate. Finally, we calculate the fluorescence spectra for off-resonance cw driving and investigate the resulting Mollow-triplet linewidths.

Two-mode squeezing in polariton four-wave mixing

We present an analytical model describing polariton four-wave mixing oscillation in semiconductor microcavities. The noise spectra of the intensity difference of the light emitted by the oscillating polariton modes are calculated. It is shown that the two-mode squeezing depends essentially on the ratio of the photonic to the excitonic fraction of the polariton modes. This ratio can be adjusted freely by changing the cavity-exciton detuning. A comparison with available experimental results indicates that a significant intensity difference squeezing can be expected for realistic experimental situations.

Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature

Wide-band gap ZnO semiconductors are attractive materials for the investigation of microcavity exciton polaritons due to the large exciton binding energy and oscillator strength. We report the growth and characterization of bulk ZnO-based hybrid microcavity. The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based hybrid microcavity structure, which consists of 30 pair epitaxially grown AlN/AlGaN distributed Bragg reflector (DBR) on the bottom side of the 3/2λ thick ZnO cavity and 9 pair SiO2/HfO2 DBR as the top mirror. The cavity quality factor is about 221. The experimental results show good agreement with theoretically calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be about 58 meV in the ZnO-based hybrid microcavity.

Twin polaritons in semiconductor microcavities

The quantum correlations between the beams generated by polariton pair scattering in a semiconductor microcavity above the parametric oscillation threshold are computed analytically. The influence of various parameters like the cavity-exciton detuning, the intensity mismatch between the signal and idler beams and the amount of spurious noise is analyzed. We show that very strong quantum correlations between the signal and idler polaritons can be achieved. The quantum effects on the outgoing light fields are strongly reduced due to the large mismatch in the coupling of the signal and idler polaritons to the external photons.

Dispersion mapping of spin-dependent polariton dynamics in CdTe microcavities

We have studied the polarization of the light emitted by a semiconductor microcavity as a function of the detuning between the cavity mode and the exciton. Under high excitation conditions, when the cavity is in a non-linear regime, the emission originates from the cavity-like branch of the polaritons, i.e. the lower polariton branch for negative detuning and the upper polariton branch for positive detuning. The time dependence of the polarization, which represents the spin dynamics of the polaritons, shows a very rich and novel behaviour in this non-linear regime. The polarization initially is not at its maximum, like in the linear regime, but reaches it after a finite time and, furthermore, its sign is strongly dependent on the cavity–exciton detuning (δ = EC − EX): it is positive for δ > 0 and negative for δ < 0. The negative polarization is directly related with an energy splitting between the σ+- and σ−-polarized components of the emission, which appears when the excitation density drives the cavity into the non-linear regime.

Suppression of the bottleneck in semiconductor microcavities

The relaxation kinetics of cavity polaritons by scattering with thermal acoustic phonons is studied within the rate equation approximation. Numerical results show that a suppression of the bottleneck of lower polariton states occurs at high polariton densities. We have found that the long decay time of the photon-like polaritons, the thin width of the embedded quantum wells and the small value of exciton-cavity detuning are favorable for the suppression of the bottleneck.

Polarization control of the nonlinear emission of semiconductor microcavities.

The degree of circular polarization ( Weierstrass p ) of the nonlinear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards K approximately 0 cavitylike states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the +/-1 spin levels at K approximately 0. At short times after a pulsed excitation Weierstrass p reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy (+1/-1 spin for positive/negative detuning).

Polariton Spin Dynamics in II-VI Microcavities

The spin dynamics of cavity polaritons, in the non-linear regime, present a novel and striking behaviour of the degree of polarization of photoluminescence. It reaches its maximum value at a finite time after excitation with a light pulse, and is strongly influenced by the exciton-cavity detuning. In the case of negative detuning, the relaxation towards K ∼ 0 states is governed by the polariton final state stimulated scattering, the emission is counter-polarized with the excitation and the polarization reaches very large negative values. In the case of positive detuning, the emission arises from the bare cavity mode, indicating a transition to the weak coupling regime; nevertheless, a very large positive polarization is observed. In addition to the large degrees of polarization of the emission, an energy splitting is observed between the two circularly polarized components of the photoluminescence, which is directly related to the anomalous behaviour of the polarization.

Polaritonic coupling and spin dynamics in GaAs microcavities

We have used time-resolved photoluminescence spectroscopy to study the light emission dynamics in a semiconductor microcavity as a function of excitation density and exciton-cavity detuning. We paid special attention to polariton spin relaxation by using circularly polarized excitation. We have found a striking behavior of the photoluminescence degree of polarization, which reaches its maximum value at a finite time. As the excitation density is increased and the system enters the stimulated emission regime, this maximum is followed by a negative dip, whose depth strongly depends on exciton-cavity detuning.

Resonant Raman Scattering in Semiconductor Microcavities

We present first-order resonant Raman scattering results on III–V and II–VI semiconductor QW embedded planar microcavities, as a function of both laser incidence angle and cavity–exciton detuning. We show that the results can be well described by a simple expression for the scattering efficiency, σpol ∝ (SpiSxi) (SpsSxs), where Sp and Sx are the photonic and excitonic strength, respectively, of the incoming and scattered cavity polaritons. We discuss the assumptions leading to this expression in terms of existing theories of polariton mediated scattering in bulk, adapted to optically confined structures.

Exciton-exciton scattering dynamics in a semiconductor microcavity and stimulated scattering into polaritons

We study the polariton dynamics in a semiconductor microcavity for small temperatures and zero exciton-cavity detuning, including the exciton-exciton and exciton-phonon scatterings. A bottleneck in the relaxation of excitons into lower polariton is found, which persists up to high densities. We then consider injection of large populations of lower polaritons with an external pump. All scatterings featuring the lower polariton as the final state become stimulated. In particular, scattering of two excitons into both lower and upper polaritons shows direct evidence of stimulation. Within the rate equation approach, we also predict sizable saturation effects due to the stimulated emission.

Polariton effects on first-order Raman scattering in II-VI microcavities

The polariton modes of a II-VI semiconductor quantum-well-embedded planar microcavity are studied by resonant Raman scattering as a function of cavity-exciton detuning. A maximum in the Raman efficiency is observed at the mode anticrossing, with minima in the pure photon and exciton limits. The results are compared with an optically nonconfined structure and discussed within the framework of existing theories of polariton-mediated Raman scattering, adapted to optically confined structures. It is shown that polariton effects are apparent in planar microcavities due to ${k}_{z}$ quantization and a peculiar polariton density-of-states cancellation in the scattering efficiency. The detuning dependence of the Raman efficiency reflects the varying photon and exciton strengths of the coupled-mode wave function, unambiguously revealing the involvement of cavity polaritons as intermediary states in the scattering process.