Non-thermal separation of electronic and structural orders in a persisting charge density wave.

The simultaneous ordering of different degrees of freedom in complex materials undergoing spontaneous symmetry-breaking transitions often involves intricate couplings that have remained elusive in phenomena as wide ranging as stripe formation, unconventional superconductivity or colossal magnetoresistance. Ultrafast optical, X-ray and electron pulses can elucidate the microscopic interplay between these orders by probing the electronic and lattice dynamics separately, but a simultaneous direct observation of multiple orders on the femtosecond scale has been challenging. Here we show that ultrabroadband terahertz pulses can simultaneously trace the ultrafast evolution of coexisting lattice and electronic orders. For the example of a charge density wave (CDW) in 1T-TiSe2, we demonstrate that two components of the CDW order parameter--excitonic correlations and a periodic lattice distortion (PLD)--respond very differently to 12-fs optical excitation. Even when the excitonic order of the CDW is quenched, the PLD can persist in a coherently excited state. This observation proves that excitonic correlations are not the sole driving force of the CDW transition in 1T-TiSe2, and exemplifies the sort of profound insight that disentangling strongly coupled components of order parameters in the time domain may provide for the understanding of a broad class of phase transitions.

Bragg polaritons: strong coupling and amplification in an unfolded microcavity.

Periodic incorporation of quantum wells inside a one-dimensional Bragg structure is shown to enhance coherent coupling of excitons to the electromagnetic Bloch waves. We demonstrate strong coupling of quantum well excitons to photonic crystal Bragg modes at the edge of the photonic band gap, which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg polariton branches are in good agreement with the theory and allow demonstration of Bragg polariton parametric amplification.

Indirect excitons in elevated traps.

We report on the study of indirect excitons in elevated traps. The transition from a normal to elevated trap results in the appearance of narrow lines in the emission spectrum. The density, temperature, and voltage dependences indicate that these lines correspond to the emission of individual states of indirect excitons in a disorder potential in the elevated trap.

Anti-trapping of indirect excitons by a current filament.

In order to explain the photoluminescence (PL) of indirect excitons collected from localized spots in experiments with a defocused laser excitation of coupled quantum wells (QWs), we model the in-plane carrier transport and charge distribution with a set of drift-diffusion, Poisson and thermalization equations. The quantum statistical corrections are included in our description via a generalized Einstein relationship and quantum mass action law. The PL spots are attributed to transverse current filaments crossing the structure and injecting electrons in the coupled QWs. The accumulated electron charge forms an anti-trap for indirect excitons. Our model quantitatively reproduces the whole set of the relevant experimental data.