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Collective Effects in Linear Spectroscopy of Molecular Aggregates

Aleksey A. Kocherzhenko, Jahan M. Dawlaty, B. P. Abolins, Felipe Herrera, Douglas B. Abraham, K. Birgitta Whaley

Published Jul 10, 2013• 0 citations
OpenAlex

Abstract

A consistent analysis of linear spectroscopy for arrays of dipole-coupled two-level molecules reveals distinct signatures of weak and strong coupling regimes separated by a critical point. Multiple molecular excitations (odd(even) in weak(strong) coupling) are accessed from the ground state. As the coupling increases, the single excitation oscillator strength rapidly exceeds the Heitler-London value and diverges at the critical point, returning to a quadratic size scaling in strong coupling, where also the photon frequency decreases with size. The lowest accessible excitation is found to show a one-photon hyperradiance.

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A consistent analysis of linear spectroscopy for arrays of dipole-coupled two-level molecules reveals distinct signatures of weak and strong coupling regimes separated by a critical point. Multiple molecular excitations (odd(even) in weak(strong) coupling) are accessed from the ground state. As the coupling increases, the single excitation oscillator strength rapidly exceeds the Heitler-London value and diverges at the critical point, returning to a quadratic size scaling in strong coupling, where also the photon frequency decreases with size. The lowest accessible excitation is found to show a one-photon hyperradiance.
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.
Interaction of the vacuum field of an optical mode with an electronic two-level system can drive non-perturbative light-matter dynamics, resulting in an exotic quantum ground state containing a sizeable population of virtual excitations. In the deep-strong coupling (DSC) regime, where the Rabi frequency,ΩR\Omega_{\mathrm{R}}ΩR​, exceeds the eigen-frequencies of the coupled systems, this extreme ground state can result in spectacular effects of cavity-QED, such as the expected release of the virtual photon ground state population as vacuum radiation by non-adiabatic modulation of the coupling mechanism [1], [2]. However, for resonant coupling of two single modes [3], the finite oscillator strength of resonant electronic excitation sets a natural limit to the achievable coupling strength, hindering further exploration of this exciting field.
Ordered atomic arrays feature an enhanced collective optical response compared to random atomic ensembles due to constructive interference in resonant dipole-dipole interactions. One consequence of this is the existence of a large shift of the transition with respect to the bare atomic frequency. In the linear optics regime (low light intensity), one observes a spectroscopic shift of the Lorentzian atomic line often called the collective Lamb shift. For stronger driving, many excitations are present in the system rendering the calculation of this shift theoretically challenging, but its understanding is important for instance when performing Ramsey spectroscopy in optical clocks. Here we report on the study of the collective optical response of a one-dimensional array of 30 dysprosium atoms. We drive the atoms on the narrow intercombination transition isolating a two-level system, and measure the atomic state with single-shot state readout using a broad transition. In the linear optics regime, we measure the shift of the resonance in steady state due to dipole interactions, and measure how this shift depends on the interatomic distance. We further resolve at the single atom level how the excitation is distributed over the array. Then, on the same transition we perform Ramsey spectroscopy i.e., away from the linear regime. We observe a time-dependent shift, that allows us to draw the connection between the collective Lamb shift observed in the linear optics regime and in the large-excitation case.
The collective behavior of ensembles of atoms has been studied in-depth since the seminal paper of Dicke [R. H. Dicke, Phys. Rev. 93, 99 (1954)], where he demonstrated that a group of emitters in collective states is able to radiate with increased intensity and modified decay rates in particular directions, a phenomenon which he called superradiance. Here, we show that the fundamental setup of two atoms coupled to a single-mode cavity can be distinctly exceeding the free-space superradiant behavior, a phenomenon which we call hyperradiance. The effect is accompanied by strong quantum fluctuations and surprisingly arises for atoms radiating out-of-phase, an alleged non-ideal condition, where one expects subradiance. We are able to explain the onset of hyperradiance in a transparent way by a photon cascade taking place among manifolds of Dicke states with different photon numbers under particular out-of-phase coupling conditions. The theoretical results can be realized with current technology and thus should stimulate future experiments.
Abstract. We apply our recently developed theory of frequency-filtered and time-resolved N-photon correlations [1] to study the two-photon spectra of a variety of systems of increasing complexity: single mode emitters with two limiting statistics (one harmonic oscillator or a two-level system) and the various combinations that arise from their coupling. We consider both the linear and nonlinear regimes under incoherent excitation. We find that even the simplest systems display a rich dynamics of emission, not accessible by simple single photon spectroscopy. In the strong coupling regime, novel two-photon emission processes involving virtual states are revealed. Furthermore, two general results are unraveled by two-photon correlations with narrow linewidth detectors: i) filtering induced bunching and ii) breakdown of the semi-classical theory. We show how to overcome this shortcoming in a fully-quantized picture. PACS numbers: 42.50.Ar,42.50.Lc,05.10.Gg,42.25.Kb,42.79.Pw ar X iv
We explore how collective states of light and matter differ when the multi-level nature of the quantum emitters is fully taken into account. For closely spaced emitters, interferences between near-resonant transitions completely change the character of the collective states compared to two-level approximations. In particular, we find a lower bound on the emitter separation for superradiance to occur which does not exist for two-level emitters. By contrast, for larger separations between the emitters, the collective states resemble those obtained within the widely used two-level approximation of the emitters. Both regimes may be realized by molecules trapped in optical lattices. We therefore propose molecular candidates and describe experimental signatures of emerging multi-level interference.