Whispering-Gallery Quantum-Well Exciton Polaritons in an Indium Gallium Arsenide Microdisk Cavity

Despite appealing high-symmetry properties that enable strong spatial confinement and ultrahigh-$Q$, optical whispering-gallery modes of spherical and circular resonators have been absent from the field of quantum-well exciton polaritons. Here we observe whispering-gallery exciton polaritons in a gallium arsenide microdisk cavity filled with indium gallium arsenide quantum wells, the test bed materials of polaritonics. Strong coupling is evidenced in photoluminescence and resonant spectroscopy accessed through concomitant confocal microscopy and near-field optical techniques. Excitonic and optical resonances are tuned by varying temperature and disk radius, revealing Rabi splittings between 5 and 10 meV. A dedicated analytical quantum model for such circular whispering-gallery polaritons is developed, which reproduces the measured values. At high power, lasing is observed and accompanied by a blueshift of the emission consistent with the regime of polariton lasing. With experimental methods and theory now established, whispering-gallery-mode polaritons in round dielectric resonators appear as a new viable platform toward low loss polaritonics.

Figure 1

(a) Coupling between an optical cavity mode (red) and the excitons of a quantum well (green). (b) Disk resonator supporting a WGM, and MQW structure within the disk. (c) SEM micrograph of the sample with eight disks and coupling waveguides. Each waveguide suspended by its square anchoring pads enables evanescent coupling to an individual disk and is terminated by inverted tapers. The latter are protruding over the mesa edges (extreme left and right of the image). (d) 3D rendering illustrating the three experimental configurations of this work: 1, confocal microphotoluminescence ($\mu \mathrm{PL}$) under nonresonant excitation; 2, nonresonant excitation through confocal objective and emission collection through waveguide; 3, in-plane resonant laser spectroscopy through waveguide.

Figure 2

Energy shift of the exciton ($E_x$, green) and WGMs ($E_c$, red) as a function of the temperature. Dots, experiments; green solid line, Viña’s model [29] ($r^2\simeq 0.999$); red solid line, custom minimal model; see text ($r^2\simeq 0.992$). Inset: confocal $\mu \mathrm{PL}$ spectra taken at different temperatures from 4 to 60 K and shifted for clarity.

Figure 3

PL spectra as a function of the temperature for three different disks of decreasing radius: (a) $R=1.992\mathrm{\mu }\mathrm{m}$, (b) $R=1.989\mathrm{\mu }\mathrm{m}$, (c) $R=1.984\mathrm{\mu }\mathrm{m}$. Each spectrum displays the exciton signal (labeled as $X$) and several lower-polariton branches labeled with numbers. The temperature starts at 4 K and is incremented in steps of 2 K.

Figure 4

(a) Energy dispersion as a function of the temperature for LP3 of a given disk ($R=1.992\mathrm{\mu }\mathrm{m}$). Maroon circles, measured LP3 emission; open green circles, measured exciton emission; solid maroon lines, polariton dispersion derived from the Hopfield model; dashed red line, WGM dispersion obtained from ${\delta }_0$ output by the fitting procedure. (b) LP3 PL as function of the temperature for three disks of decreasing radius (maroon, $R=1.992\mathrm{\mu }\mathrm{m}$; orange, $R=1.989\mathrm{\mu }\mathrm{m}$; yellow, $R=1.984\mathrm{\mu }\mathrm{m}$). Circles, experiments; solid lines, LP branches predicted by the model. Green dashed lines are bare exciton dispersion curves shown with an offset on each LP branch.

Figure 5

PL intensity as a function of the excitation power. Emitted power measured by the optical spectrum analyser (red circles, main plot). Emission blueshift (black circles, inset). Emission linewidth (blue circles, inset). The black dotted lines in the inset represent the spectral resolution of the optical spectrum analyser. The experiments are run under cw optical pumping in configuration 2 (pump through the confocal path, and emission collection through the waveguide). The pump optical spot has a radius of $2\mathrm{\mu }\mathrm{m}$; hence, the pump power density at threshold is $3.4\mathrm{kW}/{\mathrm{cm}}^2$.