Decay and coherence of two-photon excited yellow orthoexcitons inCu2O

Abstract

Photoluminescence excitation spectroscopy has revealed a novel, highly efficient two-photon excitation method to produce a cold, uniformly distributed high density excitonic gas in bulk cuprous oxide. A study of the time evolution of the density, temperature and chemical potential of the exciton gas shows that the so called quantum saturation effect that prevents Bose-Einstein condensation of the ortho-exciton gas originates from an unfavorable ratio between the cooling and recombination rates. Oscillations observed in the temporal decay of the ortho-excitonic luminescence intensity are discussed in terms of polaritonic beating. We present the semiclassical description of polaritonic oscillations in linear and non-linear optical processes.

Magneto-optical readout of dark exciton distribution in cuprous oxide

“…Figure 4 ͑symbols͒ shows the experimental ⌫ 3 − and ⌫ 5 − phononassisted lines at T = 2 K for B = 32 T. Assuming energyindependent exciton-phonon coupling matrix elements and dispersionless phonons with energy ប⍀, the emission at a certain energy is given by 3,9 I͑E͒…”

“…It appears, however, that the lifetime of the orthoexciton, which is intrinsically limited by down conversion to the lower lying paraexciton state, 7 is too short to reach the condensed state. [8][9][10][11] In addition it has been proposed that the so-called "quantum saturation effect" prevents Bose-Einstein condensation of the orthoexciton gas in cuprous oxide. 8,9,12,13 In contrast, the lifetime for the paraexciton state, which is not optically active, is expected to be long.…”

“…[8][9][10][11] In addition it has been proposed that the so-called "quantum saturation effect" prevents Bose-Einstein condensation of the orthoexciton gas in cuprous oxide. 8,9,12,13 In contrast, the lifetime for the paraexciton state, which is not optically active, is expected to be long. The exact value for the paraexciton lifetime varies widely from nanoseconds to milliseconds, 11,[14][15][16][17][18] depending strongly on the quality of the sample, i.e., on the efficiency of nonradiative processes determined by the impurity and defect concentration.…”

Exciton dynamics in cuprous oxide

“…The obvious advantage of the two-photon process is that the exciton density will be rather uniform throughout the bulk, whereas the one-photon excitation occurs near the surface due to the strong absorption near the band gap. For higher excitation densities, one expects that eventually the two photon process becomes more efficient [6]. Another difference between the two excitation methods is that for direct excitation one expect to initially (that is after electron-hole relaxation) excite ortho-excitons only.…”

“…The excitation pulse was slightly focused on the sample to a spot size of about 1 mm (~700 µJ/cm 2 ). As a result of the applied excitation method we achieve a high-density gas of yellow excitons uniformly distributed over the whole volume of the crystal [6]. The induced absorption spectrum in the region of the 1s-np intra-exciton transitions was probed by a mid-infrared probe pulse (110-130 meV) produced using difference frequency generation from the idler and signal pulses of a second parametric amplifier.…”