Observation of a dynamical mixing process of exciton-polaritons in a ZnSe epitaxial layer using four-wave mixing spectroscopy

We have observed a coherent spectral change of exciton-polaritons in a ZnSe epitaxial layer through spectrally resolved four-wave mixing spectroscopy. The spectra exhibit an exchange of the dominant peak position between the different polariton branches depending on the delay time of the second pulse. This result reflects the initial creation process of polaritons with many-body interactions. The calculation based on the exciton-photon microscopic model reveals that the spectral change occurs due to the four-particle correlations between heavy-hole and light-hole excitons; it clearly shows the dynamical mixing process of exciton-polaritons in the initial creation.

Figure 1

(a) Calculated dispersions of a three-branch polariton (black solid lines), upper bright exciton-polariton (UBXP) (green solid line), lower bright exciton-polariton (LBXP) (red solid line), and dark exciton (DX) (blue solid line) in the ZnSe epitaxial layer. The FWM signal (filled spectrum) was experimentally measured at the delay time of 4.4 ps. Dashed lines indicate the peaks of the spectrum. Squared coefficients are calculated for UBXP, LBXP, and DX in (b) upper, (c) middle, and (d) lower polaritons (UP, MP, and LP).

Figure 2

(a) Experimental and (b) theoretical spectrally resolved FWM signal for delay times from 0.0 to 4.8 ps in 0.4-ps steps as indicated. The energy of 2802.2 meV is set to the zero of the energy axis as the energy reference. The calculation is performed with $U_1:U_2=1:-0.2$. The signal indicated by the dotted line is calculated without the fast-decay component. The spectra are normalized for each delay time.

Figure 3

(a) Experimental and (b) theoretical time-integrated FWM signal as a function of delay time. The calculation includes the fast-decay component (dotted line), LP and MP (dashed line), and total signal (solid line) conditions.

Figure 4

Calculation of spectrally resolved FWM signal for delay times from 0.0 to 4.8 ps in 0.4-ps steps as indicated. (a) $U_1:U_2=1:0$ and (b) $U_1:U_2=0:1$. The signal indicated by the dotted line is calculated without the fast-decay component. The spectra are normalized for each delay time.