Collective effects in linear spectroscopy of dipole-coupled molecular arrays

We present a consistent analysis of linear spectroscopy for arrays of nearest-neighbor dipole-coupled two-level molecules that reveals distinct signatures of weak- and strong-coupling regimes separated for infinite-size arrays by a quantum critical point. In the weak-coupling regime, the ground state of the molecular array is disordered, but in the strong-coupling regime, it has (anti) ferroelectric ordering. We show that multiple molecular excitations [odd (even) in the weak- (strong-) coupling regime] can be accessed directly from the ground state. We analyze the scaling of absorption and emission with system size and find that the oscillator strengths show enhanced superradiant behavior in both ordered and disordered phases. As the coupling increases, the single-excitation oscillator strength rapidly exceeds the well-known Heitler-London value. In the strong- coupling regime we show the existence of a unique spectral transition with excitation energy that can be tuned by varying the system size and that asymptotically approaches zero for large systems. The oscillator strength for this transition scales quadratically with system size, showing an anomalous one-photon superradiance. For systems of infinite size, we find a novel singular spectroscopic signature of the quantum phase transition between disordered and ordered ground states. We outline how arrays of ultracold dipolar molecules trapped in an optical lattice can be used to access the strong- coupling regime and observe the anomalous superradiant effects associated with this regime.