Biexciton wavefunction in bulk CuCl probed by inverse polariton series

We report the observation of the radiative decay of excitonic molecules, se lectively excited at the wavevector K-m, into polaritons associated with th e n = 1s, 2s, 3s, and 4s states of the optically allowed Z(3) exciton in bu lk CuCl. The K-m dependence of the emission series (inverse polariton serie s) is quantitatively explained by the bipolariton model, leading to unambig uous estimation of the relative contributions of the 2s, 3s, and 4s exciton states to the four-particle molecule wavefunction, both in sign and in mag nitude. The very structure of the molecule wavefunction is highly sensitive to the above relative contributions, and, in particular, we examine the Ak imoto-Hanamura and hydrogen-molecule trial wavefunctions.