Strong Light-Matter Coupling in Subwavelength Metal-Dielectric Microcavities at Terahertz Frequencies

We have demonstrated that a metal-dielectric-metal microcavity combined with quantum well intersubband transitions is an ideal system for the generation of cavity polariton states in the terahertz region. The metallic cavity has highly confined radiation modes that can be tuned in resonance with the intersubband transition. In this system we were able to measure a very strong light-matter splitting (the Rabi splitting $2\hslash {\Omega }_R$), corresponding to 22% of the transition energy. We believe this result to be the first demonstration of intersubband polaritons in the terahertz frequency range.

Figure 1

The photonic structure used in our experiments. (a) Optical microscope top view of the rectangular metallic grating—with typical dimensions. (b) Scanning electron micrograph of the cleaved facet of the structure. The top and bottom metallic layers are, respectively, 0.4 and $1\mu \mathrm{m}$ thick. The bottom metallic layer is also used to bond the sample on a GaAs host wafer. (c) Schematics of the active layer, containing 15 repetitions of the $\mathrm{GaAs}/\mathrm{AlGaAs}$ quantum well. The first two energy levels of the wells are separated of 14.5 meV, corresponding to a frequency of 3.5 THz.

Figure 2

(a) Upper panel: Reflectivity spectra of the cavity at a 45° angle of incidence. Lower panel: Position of the three reflectivity minima $K=1–3$ plotted in an inverse slit $1/s$-frequency diagram. Red dotted curve: Experiment. Blue straight curve: Numerical model. (b) Spatial distribution of the electromagnetic field components $|H_y|$ and $|E_z|$ for the $K=1$ resonance. Note that the field goes to zero beyond the lines of impedance mismatch; therefore, the different double-metal regions are uncoupled. (c) Spatial distribution of $|H_y|$ for the $K=2,3$ resonances. The integer $K$ counts the number of field maxima of the standing wave pattern of $|H_y|$.

Figure 3

(a) Transmission measurements for different temperatures of the quantum well absorption region. The top surface of the sample is fully covered with gold to enhance the terahertz electric field at the metal-semiconductor interface. The terahertz radiation is injected from a 45° polished facet and reaches the quantum well region through the substrate. (b) Reflectivity measurements for different temperatures of the cavity with strip width $s=9.6\mu \mathrm{m}$, nearly resonant with the quantum well absorption (dashed line).

Figure 4

(a) Low temperature reflectivity spectra of three cavities, with modes that are, respectively, redshifted ($s=8.4\mu \mathrm{m}$), nearly resonant ($s=9.6\mu \mathrm{m}$), and blueshifted ($s=10.4\mu \mathrm{m}$) with respect to the intersubband absorption. The bare absorption peak from Fig. 3a [$-\log (\mathrm{\text{transmission}})$] is also shown and has been rescaled for clarity. (b) The dispersion diagram of the system at $T=4.5\mathrm{K}$. The $K=1$ mode is split into two polaritonic branches, with minimal energy separation $2{\Omega }_R/2\pi =0.8\mathrm{THz}$. The blue continuous lines are our simulations.