Electrically Injected Cavity Polaritons

We have realized an electroluminescent device operating in the light-matter strong-coupling regime based on a $\mathrm{GaAs}/\mathrm{AlGaAs}$ quantum cascade structure embedded in a planar microcavity. At zero bias, reflectivity measurements show a polariton anticrossing between the intersubband transition and the cavity mode. Under electrical injection the spectral features of the emitted light change drastically, as electrons are resonantly injected in a reduced part of the polariton branches. Our experiments demonstrate that electrons can be selectively injected into polariton states up to room temperature.

Figure 1

(a) Schematic view of the sample. The arrows represent the optical path of the incident beam in the reflectivity measurements; $\theta$ is the propagation angle in the sample. The top and the bottom mirror are shaded. A 86 nm layer of $1\times {10}^{17}{\mathrm{cm}}^{-3}$ Si-doped GaAs and a 17 nm layer of $3\times {10}^{18}{\mathrm{cm}}^{-3}$ Si-doped GaAs are grown on top of the QC-LED to allow current flow towards the metallic contacts. The continuous (blue) line represents the intensity of the photon mode along the growth axis and the (blue) dotted line the zero of the mode intensity. (b) Band diagram of the QC-LED for an applied voltage of 6 V. The square moduli of the relevant wave functions are shown. The layer sequence of one period of the structure, in nm, from left to right, starting from the largest well is $6.4/\mathbf{3.6}/3.3/\mathbf{1.6}/\underline{3.2}/\underline{\mathbf{1.8}}/\underline{2.3}/\underline{\mathbf{2.0}}/\underline{1.9}/\underline{\mathbf{2.0}}/1.8/\mathbf{2.0}/2.2/\mathbf{3.9}$. ${\mathrm{Al}}_{0.45}{\mathrm{Ga}}_{0.55}\mathrm{As}$ layers are in bold, underlined layers are Si doped. This sequence has been repeated 30 times in the sample.

Figure 2

Reflectivity spectra as a function of the incident angle, at 78 K and zero voltage. The spectra result from the ratio $R$ between spectra collected with TM and TE-polarized incident light and are offset each other for clarity. A Nicolet Fourier-transform infrared spectrometer, in rapid scan mode, with a spectral resolution of $8{\mathrm{cm}}^{-1}$, has been used. A $f/10.9$ optical system has been used for collection. In the inset, the energy position of the reflectivity peaks is plotted as a function of the angle (full dots), as well as the results of transfer matrix calculations (line).

Figure 3

(a) Voltage and EL intensity as a function of the injected current density in pulsed operation (20 kHz, 500 ns) at 73.8° at 78 K (continuous line) and 300 K (dashed line). The dots indicate the working conditions for obtaining the electroluminescence spectra shown below. (b) EL spectra, offset each other for clarity, as functions of the propagating angle within the microcavity, at 78 K and 5 V (left side) and 2.8 V (right side). These voltages have been chosen as the lowest possible values to measure a spectrum at the two temperatures. In order to increase the EL signal, we worked at 50% duty cycle (100 kHz repetition rate and $5\mu \mathrm{s}$ pulse width). Spectra are collected in step scan mode, with a spectral resolution of $16{\mathrm{cm}}^{-1}$.

Figure 4

(a) Comparison between $1-R$ (i.e., absorption, see text) and EL spectra measured at 78 K, at 65° (upper panel, reflectivity 65.7°; EL 65.2°) and at 70° (lower panel: reflectivity 70.6°, EL 70.1°). (b) Comparison between the experimental (solid lines) and simulated (dashed lines) EL. The dash-dotted (blue) lines indicate the function $G(E)$.