Exciton polaritons are the quasiparticles resulted from the coupling between photons confined in a microcavity and excitons. A polariton laser is not based on the stimulated emission of photons but on the coherence condensation process of exciton polaritons in the lowest energy state.^[1,2] Therefore, the lasing threshold is expected to be very low. In the past ten years, the properties of polariton have been very extensively studied, and a set of striking phenomena have also been demonstrated. Such as parametric optical scattering,^[3] spontaneous Bose–Einstein condensation (BEC) at high temperature,^[4,5] superfluidity,^[6] and even quantized vortex.^[7] In principle, these unique properties serve as a basis for development of series novelty optoelectronic devices: polariton lasers,^[8,9] polariton switches.^[9] A key principle advantage of exciton polaritons is the achievement of the strong coupling regime (SCR) at room temperature (RT) and realization of the electrical injected polariton devices. Although the BEC lasing has been observed in organic materials,^[10,11] the electrical injection is still its bottleneck in the operation. Therefore special attention and huge technological efforts have turned to wide-band gap semiconductors, such as GaN and ZnO. The advantage of using such semiconductors with a large exciton binding energy and oscillator strength is that a polariton laser based on such a material would be able to operate at RT, which is an absolute requirement for a commercial device.

For ZnO, both exciton oscillator strength and binding energies (60 meV) are much larger than those for nitrides.^[12–15] As a result, the energy splitting between the polariton eigenmodes can reach 100–200 meV, a record for inorganic structures. Up to date, some self-constructed cavities based on ZnO nanosturcture, such as microwires, have demonstrated the SCR at RT.^[16–18] It has been anticipated that the two-dimensional (2D) ZnO quantum structure will be an optimum choice for polariton devices in the blue-UV range.^[19–21] However, the vertical cavity structure with bottom dielectric distributed Bragg reflector (DBR) deteriorates the crystalline quality dramatically and results to huge spatial inhomogeneous broadening. In order to realize the ZnO polariton in SCR at RT, the broadening effect resulted from interface disorder scattering in 2D QW must be suppressed.

In this work, we investigate microcavity polariton that exhibits the excellent exciton properties of ZnO/MgZnO QWs grown on sapphire substrates enabled by single top dielectric DBRs. The complex buffer layer for QW deposition is introduced, thereby the crystalline quality of active layer is enhanced. It is demonstrated that the excellent crystalline quality of the QWs in this structure can be used to explore the dispersion properties of polariton at RT. The tapered cavities also allow the observation of polariton dispersion behavior at different detunings.

The microcavity samples used in our experiment, consisting four-period ZnO/Mg_0.15Zn_0.75O QWs, were grown by MBE on sapphire substrates. Here the bottom MgZnO spacer layer was tapered along one direction of the sample so that the resonant photon frequency of the cavity varied with sample position. Then 20 pairs Zr₂O₃/SiO₂ dielectric top DBR (reflectivity of 99.5%) were deposited on the sample, hence the thickness of cavity was gradient whose value depended on the exact location of the sample as shown by the schematic diagram of the device (Fig. 1(a)). Considering the total reflection mirror of top DBR, the output of polariton emission signal was collected from the bottom double-polish Al₂O₃ substrate. Then, the light was analyzed by a 4f angle-resolved PL system. Such a thickness gradient cavity allows for a tuning of the energies of the cavity modes and the resulting dispersion of polariton branch.

In contrast to the bare ZnO film, the PL spectrum of QW sample at RT displays a dominant peak from the well region without the defect related emitting band (Fig. 1(b)). Meanwhile, the emission shoulder at the higher energy that resulted from the barrier layer can also be seen obviously. Furthermore, low-temperature (77 K) PL spectrum of QW is present (Fig. 1(c)), which shows the precise free-exciton (FX) peak and three phonon replica from the QW. The energy of FX peak increases because of spatial confinement for exciton state in the 2D well structure. The appearance of three-order LO-phonon replicas confirms the excellent optical properties of QWs. Benefited from high exciton binding energy (E_b), the excitons will be interacted strongly with confined photons in cavity. On the other hand, the larger oscillator strength can override the leakage loss of photon, it is anticipated that the strong coupling with larger Rabi splitting will be realized in the plate microcavity.

The typical angle-resolved polariton dispersion characteristics of our sample under nonresonance excitation were investigated (Fig. 2). Four emission bands can be observed, generally the obtained emission of polariton is from the lower polariton (LP) branch and the upper polariton (UP) is absent (left panel). One reasonable explanation of the absence of the UP branch is that the interaction between polariton and phonon via the exciton component is very strong at RT. Therefore, the polaritons in the UP branch are quickly decayed to the LP, hence the emission from the UP branch is strongly suppressed. It is remarkable that the dispersion of polariton based on ZnO QWs can be clearly resolved even upon RT. The exciton-like dispersion is spectrally narrow, being very weakly affected by the photonic inhomogeneous broadening induced by the thickness gradient. On the other hand, the more photonic-like dispersion exhibits a large inhomogeneous broadening, with several neighboring modes visible.

By taking the cavity resonance condition and the polariton dispersion into account, one can get an exact expression for the polariton dispersion in the planar cavity as^[22]

$$ \begin{eqnarray}\begin{array}{lll}{E}_{{\rm{LP}},{\rm{UP}}}({k}_{\parallel }) & = & \frac{1}{2}[{E}_{{\rm{ph}}}({k}_{\parallel })+{E}_{{\rm{ex}}}({k}_{\parallel })]\\ & & \pm \frac{1}{2}\sqrt{{[{E}_{{\rm{ph}}}({k}_{\parallel })-{E}_{{\rm{ex}}}({k}_{\parallel })]}^{2}+4{g}_{0}^{2}},\end{array}\end{eqnarray}$$ (1)

It is noted that the dominant emission spot located at high k_∥ is bright than that at low energy state (k_∥ = 0), here the intensities of spectra have been normalized. This phenomenon can be attributed to phonon bottleneck effect in the low-dimensional quantum structure. Although the relaxation of LP is hindered due to the pronounced photon-like character of LP, this bottleneck can be overcome directly by increasing the temperature to favor polariton-phonon interactions or polariton-polariton scattering in the medium.^[23]

To illustrate the exciton-polariton comprehensively, the detailed numerical calculation results about dispersion with damping term are given in Fig. 3. In the simulation, we consider the decay of the exciton and photon in the cavity with the Lorentz model. The measurement data (left panel) is surprisingly in agreement with the results of rigorous coupled-wave analysis (right panel). Color maps of the measured PL spectra at RT are normalized for the purpose of highlighting the variation of polariton dispersion curves. It is given that three polariton modes are accompanied with very different excitonic contents. The distribution of polariton particles in LP branches will reach the bottom of LP through thermodynamically process.

Furthermore, for a given exciton-photon detuning, the polariton dispersion pattern versus externally pumping power is investigated (Fig. 4). When the pump intensity is lower, the distributions of polaritons in k-space in LP and UP branches are equal (Fig. 4(a)). Meanwhile, the color maps of dispersions indicate that the exciton fraction is about 11% with the detuning of δ = −110 meV. With increasing excitation power, the LP exhibits an intense intensity than UP accounting for the relaxation between two branches (Figs. 4(b) and 4(c)). It is noted that the relaxation process of polariton in LP branch appears well and is wide in k-space due to its interaction with the excitonic reservoir. With further increase of the pump intensity, the enhanced exciton-exciton scattering processes will stimulate the faster relaxation of polariton to reach the bottom of LP branch (Fig. 4(d)).

By scanning the excitation spot along the tapered cavity, the dispersions of exciton-polariton can be manipulated (Fig. 5). Experimentally, the PL signal was collected from normal direction of the sample at different positions. The height change of expitaxy layer results in the variation of cavity thickness, which corresponds to the different detunings between exciton and photon. A simulated features based on the numerical solution give further insight into the behavior of polariton distribution in these cavities. The detuning ranges from −22 meV to 4.5 meV, meanwhile the Rabi splitting ranges from 50 meV to 80 meV, extracted from the theoretical fitting of the dispersions. The k-space distribution of polariton in the LP for δ = −22 meV shows a wide homogeneous due to the interaction between polaritons (Fig. 5(a)). With the detuning changing, a remarkable variation can be achieved in polariton distribution, where bottleneck in LP branch will be eliminated efficiently (Fig. 5(b)).

At intermediate detuning (δ = −8 meV), the distribution merges to form a single LP branch (Fig. 5(c)). Subject to the trade-off between the thermodynamic (negative detuning) and kinetic (positive detuning) requirements, polaritons will select the optimal branch to condense.^[23] Remarkable, just a single UP branch can be seen clearly as the detuning (δ = 4.5 meV) going to less positive (Fig. 5(d)), which is continuously observed from the same mode. As the hybrid state becomes too much photonic-like with higher excitation power, the system is closed to conventional stimulated emission. In contrast with the result in GaN based cavities^[25] under the same temperature condition, the optimal detuning of our QW structure is positive, which is consistent with the result theoretically expected.^[26] This behavior may be due to the large Rabi splitting value and a very deep energy minimum at the bottom of the LP branch. The intrinsic scheme about our achievement will require further investigation on the polariton–polariton interaction in ZnO-based QW microcavities.

In summary, we have demonstrated the 2D ZnO-based QWs combining high exciton binding energy coupling with photons in a dielectric plate microcavity, which allows us to explore polariton dispersion at RT and to tune the detuning between photons and excitons. Beyond the technological interest, these new hybrid dielectric DBR cavity structures open a very interesting perspective to perform lasing experiments and to study the temperature and exciton fraction dependences of the BEC behavior in polariton.

The ZnO/MgZnO QW samples under investigation were grown on (001) sapphire substrates by a plasma-assisted molecular beam epitaxy system. Elemental Mg (6N) and Zn (6N) were supplied by solid source effusions cells. Oxygen was cracked by a radio-frequency (RF) plasma generator (Oxford Applied Research HD 25) for O source. Before growth, the substrate was annealed at 750 °C under vacuum ∼10⁻⁹ Torr for 15 min, and subsequently exposed in oxygen plasma to make the surface of (0001) Al₂O₃ O-terminated. A designed four-step buffer layers^[27] were deposited in sequence for reducing the mismatch between the epitaxial films and the substrate. For the QW, four periodic intrinsic ZnO/Mg_0.15Zn_0.75O (6 nm/12 nm) layers were deposited at 600 °C after the buffer layers growth, then 60-nm-thick MgZnO was used to form a cap layer.

The whole growth process was monitored by an in situ reflection high energy electron diffraction device in MBE. In the experiment, the PL spectra of the QW sample were measured with a He-Cd laser (325 nm) through a μ-PL system. Low temperature (77 K) PL was carried out using an He-Cd laser with the samples in an open-cycle liquid N₂ cryostat. The dispersion features from the device were excited by nonresonant continuous waves and collected by an angle-resolved μ-PL system, then was analyzed by a spectrometer equipped with a silicon charge-coupled device.

FDTD simulations were carried out to provide numerical calculation results. The excited source in QWs was selected as dipole, and the dispersion characteristics of polaritons were calculated using the coupling wave equation.