Doubly Dressed Bosons: Exciton Polaritons in a Strong Terahertz Field

We demonstrate the existence of a novel quasiparticle, an exciton in a semiconductor doubly dressed with two photons of different wavelengths: a near infrared cavity photon and terahertz (THz) photon, with the THz coupling strength approaching the ultrastrong coupling regime. This quasiparticle is composed of three different bosons, being a mixture of a matter-light quasiparticle. Our observations are confirmed by a detailed theoretical analysis, treating quantum mechanically all three bosonic fields. The doubly dressed quasiparticles retain the bosonic nature of their constituents, but their internal quantum structure strongly depends on the intensity of the applied terahertz field.

Figure 1

(a) Scheme of the internal structure of an atomlike exciton. The vacuum state ($|0⟩$) is coupled to a ground $1s$ heavy hole (hh) exciton via the NIR photon. The $1s$ is coupled to the excited $2p$ exciton via THz photon. In our theoretical model we neglect other excited electronic states, but we include the exciton ionization and blueshift from the dynamical Franz-Keldysh effect. (b) The bare exciton-polariton energies (black solid lines) and the double dressed quasiparticle energy diagram (yellow-orange lines). The THz energy resonant to the $1s\text{−}2p$ transition is marked with a blue arrow. (c) Scheme of the creation and annihilation of the $2p$ exciton state. This process involves two photons, one from NIR and one from THz spectral ranges.

Figure 2

Transmission spectra of LPs and UPs (at normal incidence) as a function of the delay between the NIR and THz pulses (in equidistant time intervals). The color scale indicates the intensity in arbitrary units. The THz photon energy is 6.8 meV, slightly detuned from the $1s\text{−}2p$ excitonic transition ($6.5\pm 0.5\mathrm{meV}$). The appearance of the THz-induced MP branch at the maximum of THz pulse (zero delay) is visible. This is accompanied by an energy shift of LP and UP branches. (a)–(d) illustrates the behavior with increasing average THz power as marked directly in the image. Positive delays correspond to the NIR pulse preceding the THz pulse, negative delays to the reversed situation. [(e)–(h)] Corresponding theoretical model. The color scale is nonlinear to enhance the low-intensity signal at zero delay time and the background signal was removed from the figures.

Figure 3

Transmission spectra (at normal incidence) of semiconductor microcavity revealing dressed polaritons and approaching the ultrastrong coupling regime. (a) Spectra illustrate the cross section of the results shown in Fig. 2 for the time delays from 15 to 0 ps. At weak perturbation (at 15 ps time delay, which corresponds to the onset of THz pulse) the transmission spectra reveal unperturbed LP and UP states. The time delay was converted to instantaneous THz power, as indicated directly in the image, based on Fig. 3 from SI [17]. As THz power increases, a redshift of the LP and a blueshift of the UP are observed and the additional MP line is visible (the dotted lines are guides to the eye to compare the energy shifts, for vertical lines the energy is constant, and the middle dotted line follows the MP energy shift). The THz-induced opacity of the sample is revealed by the decrease of the overall intensity of the transmitted signal. (b) Comparison of the experimental data from Fig. 2 (colored map) with the theoretical model from Fig. 2 (dots). The interaction energy with the THz field is illustrated in the top scale.