Large vacuum Rabi splitting in a multiple quantum well GaN-based microcavity in the strong-coupling regime

An $\mathrm{Al}\mathrm{In}\mathrm{N}∕\mathrm{Al}\mathrm{Ga}\mathrm{N}$ microcavity (MC) containing a $\mathrm{Ga}\mathrm{N}∕{\mathrm{Al}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ multiple quantum well (MQW) structure is investigated through room temperature (RT) photoluminescence and reflectivity experiments. A vacuum Rabi splitting as large as $50\mathrm{meV}$ at RT is reported for this MC structure, the highest value reported so far for a semiconductor MC containing QWs. This is shown to result from a geometry combining a state of the art nitride-based MC with a MQW system where the built-in electric field has a reduced impact on the oscillator strength of optical transitions. The contribution of Bragg modes seen in optical spectra is well accounted for by transfer matrix simulations. In addition, the sensitivity of the present system to the tuning between the various optical components of the microcavity (bottom and top Bragg reflectors and active cavity region) to maximize the strong-coupling regime is also shown through simulations. Prospects regarding the nonlinear polariton emission from such a structure indicate that this type of MCs could potentially sustain ultrafast polariton parametric amplification up to $440\mathrm{K}$, thanks to an increased exciton binding energy. More generally, it is predicted that, owing to a large exciton saturation density in excess of $1\times {10}^{12}{\mathrm{cm}}^{-2}$ per QW, such a MC structure would be suitable for the observation of nonlinear effects associated with cavity polaritons (polariton lasing and polariton Bose-Einstein condensates) at RT and above.

Figure 1

Cross-section transmission electron micrograph of the half-microcavity structure.

Figure 2

RT PL spectra of the 67 period $\mathrm{Ga}\mathrm{N}∕{\mathrm{Al}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}$ MQW structure.

Figure 3

(a) RT normal incidence $\mu \mathrm{PL}$ spectrum of the complete MQW-MC structure in a cavitylike region. (b) RT normal incidence reflectivity spectrum of the complete MQW-MC structure.

Figure 4

(a) RT angle-resolved PL spectra ranging from 0 to 50°, measured every 2° and downshifted for clarity. (b) RT angle-resolved reflectivity spectra ranging from 0 to 50°, measured every 2° (except at small angles) and downshifted for clarity. The dashed line corresponds to the uncoupled QW exciton energy. Gray lines are a guide for the eyes, showing the dispersion of polariton branches.

Figure 5

(Color online) (a) Computed angle-resolved reflectivity spectra ranging from 0 to 50°, calculated every 2° and downshifted for clarity. Empty squares correspond to the experimental RT reflectivity minima ascribed to the LPB and the UPB. (b) RT experimental dispersion curves deduced from reflectivity spectra (empty squares) and PL spectra (empty red circles) and fits of the LPB and the UPB (dashed lines). The cavity mode $\left(C\right)$ and the uncoupled exciton $\left(X\right)$ are also reported (black lines).

Figure 6

(Color online) Color map of the computed angle-resolved reflectivity spectra ranging from 0 to 50° for wavelengths ranging between 325 and $370\mathrm{nm}$. Various branches are reported, namely, the polariton branches (LPB and UPB, thick white lines), the uncoupled exciton and the cavity modes ($X$ and $C$, dashed white lines), the first Bragg mode of the dielectric DBR (BM1, dotted red line), the Bragg modes of the bottom nitride DBR (BM2-4, dotted white lines), and the coupled Bragg modes (thin white lines).

Figure 7

(Color online) (a) Computed angle-resolved reflectivity spectra ranging from 0 to 50°, calculated every 2° and downshifted for clarity, of the MQW-MC with the top dielectric DBR redshifted by 6%. The dashed line corresponds to the uncoupled QW exciton energy. Gray lines are a guide for the eyes, showing the dispersion of polariton branches. (b) Color map of the computed angle-resolved reflectivity spectra ranging from 0 to 50° for wavelengths ranging between 325 and $370\mathrm{nm}$ of the MQW-MC with the top dielectric DBR redshifted by 6%. Various branches are reported, namely, the polariton branches (LPB and UPB, thick white lines), the uncoupled exciton and the cavity modes ($X$ and $C$, dashed white lines), the first Bragg mode of the dielectric DBR (BM1, dotted red line), the Bragg modes of the bottom nitride DBR (BM2-4, dotted white lines), and the coupled Bragg modes (thin white lines).