Electrically Tunable Single Polaritonic Quantum Dot at Room Temperature

Exciton-polaritons confined in plasmonic cavities are hybridized light-matter quasiparticles, with distinct optical characteristics compared to plasmons and excitons alone. Here, we demonstrate the electric tunability of a single polaritonic quantum dot operating at room temperature in electric-field tip-enhanced strong coupling spectroscopy. For a single quantum dot in the nanoplasmonic tip cavity with variable dc local electric field, we dynamically control the Rabi frequency with the corresponding polariton emission, crossing weak to strong coupling. We model the observed behaviors based on the quantum confined Stark effect in the strong coupling regime.

Figure 1

$e$-TESC spectroscopy and electrical control of polaritonic states. (a) Illustration of $e$-TESC spectroscopy. Under optical excitation ${\overrightarrow{E}}_{\mathrm{inc}}$, single isolated QD confined to nanoplasmonic tip-cavity can reach strong coupling regime and generates polariton emission even at room temperature. Inducing tip-sample bias results in localized external electric field ${\overrightarrow{E}}_{\mathrm{ext}}$ to strongly coupled QD in tip cavity, modifying polaritonic states. Scale bar is 50 nm. (b) PL spectra of single QD uncoupled (black, without tip), weakly coupled (orange, tip-QD distance $\sim 10\mathrm{nm}$), and strongly coupled (red, tip-QD distance $<3\mathrm{nm}$) to tip-cavity induced plasmon (green). The PL spectrum of tip-cavity induced plasmon is independently measured without the QD. (c) Topography image of single isolated QD and illustration of three-dimensional plasmonic tip control. Scale bar is 50 nm. (d) Illustration of single isolated QD with upward built-in electric field ${\overrightarrow{E}}_{\mathrm{q}}^{+}$. (e) TEPL spectra with (red) and without (black) downward external electric field ${\overrightarrow{E}}_{\mathrm{ext}}^{-}$. In this case, the external electric field is applied in the opposite direction. Dynamic electrical switching of polariton emission (f) and corresponding coupling strength (g). (h) Illustration of a single isolated QD with downward built-in electric field ${\overrightarrow{E}}_{\mathrm{q}}^{-}$. (i) TEPL spectra with (red) and without (black) downward external electric field ${\overrightarrow{E}}_{\mathrm{ext}}^{-}$. Dynamic electrical switching of polariton emission (j) and corresponding coupling strength (k).

Figure 2

Polariton emission of single QDs with different built-in electric fields. Illustration of coupling strength $g$ as a function of external electric bias for ${\overrightarrow{E}}_{\mathrm{q}}\approx 0$ (a), upward ${\overrightarrow{E}}_{\mathrm{q}}^{+}$ (c), and downward ${\overrightarrow{E}}_{\mathrm{q}}^{-}$ (e) built-in electric fields. Experimental TEPL spectra as continuously increasing external electric bias from $-1.5\mathrm{V}$ to 1.5 V for ${\overrightarrow{E}}_{\mathrm{q}}\approx 0$ (b), upward ${\overrightarrow{E}}_{\mathrm{q}}^{+}$ (d), and downward ${\overrightarrow{E}}_{\mathrm{q}}^{-}$ (f) built-in electric fields. Strongly coupled TEPL spectra are fitted to coupled harmonic oscillator model (solid line) to clearly present tendency as a function of external electric field.

Figure 3

Quantitative analysis of electrically tunable polaritonic states. (a) Band diagram of QD and corresponding wave function overlap with existence of built-in electric field ${\overrightarrow{E}}_{\mathrm{q}}$ in QD. Depending on the direction of ${\overrightarrow{E}}_{\mathrm{ext}}$ with respect to ${\overrightarrow{E}}_{\mathrm{q}}$, spatial distributions of electron and hole wave function can be bidirectionally modulated. (b) Rabi splitting and corresponding coupling strength with changing wave function overlap. Polaritonic parameters (QD energy and $g$) as function of external electric field for zero (c),(d), upward (e),(f), and downward (g),(h) built-in electric fields, derived from TEPL spectra in Fig. 2. Stark shifts (c),(e),(g) and coupling strengths (d),(f),(h) are quantitatively compared with theoretical model (black line).