Collective effects in linear spectroscopy of dipole-coupled molecular arrays

Abstract

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.

Quantum phases of dipolar rotors on two-dimensional lattices

“…This should allow these self-assembled crystals to be probed without fear of entering a phase with in-plane ordering which in the absence of an external lattice potential, would lead to a collapse of the observed crystal due to attractive interactions. This effect may also be important in systems of dipolar coupled pseudo-spins or excitons within a strongly interacting regime [78]. Such a strongly interacting regime of dipolar coupled pseudospins [78] is precisely the same as the high density regime where the dipoles are likely to interact strongly with one another.…”

“…This effect may also be important in systems of dipolar coupled pseudo-spins or excitons within a strongly interacting regime [78]. Such a strongly interacting regime of dipolar coupled pseudospins [78] is precisely the same as the high density regime where the dipoles are likely to interact strongly with one another. More directly relevant to dipolar molecules, as was pointed out in Section 1, new trapping schemes [47,48,49,50,51] may lead to lattices with much smaller lattice constants than are currently implemented by optical lattices.…”

Absorption Spectra for Disordered Aggregates of Chromophores Using the Exciton Model

“…, which is typically the case in organic materials. 48,49 Second, because under typical experimental conditions the spatial density of excitations is relatively low, we assumed that interaction between excitons is negligible. Third, we assumed that the lowest excited bright state for each YLD124 chromophore is well-separated in energy from its higher excited states.…”

“…To enable calculation of the linear absorption spectrum, we simplify this Hamiltonian by making a number of approximations. First, we assumed that the excitation energies are considerably greater than absolute values of the interchromophore excitonic couplings (ε i v i , ε j v j ≫ b ij v i v j ∀ i , j , v i , v j ), which is typically the case in organic materials. , Second, because under typical experimental conditions the spatial density of excitations is relatively low, we assumed that interaction between excitons is negligible. Third, we assumed that the lowest excited bright state for each YLD124 chromophore is well-separated in energy from its higher excited states.…”