Two-photon emission competing with cooperative and nonlinear processes

N. A. Enaki, Mihai Macovei

Steve Smeets, Björn Maes, Gilles Rosolen

K. J. Boller, H.-J. Lau, J. Sparbier +1 more

Two‐photon excitation (λ1,λ2) of lithium vapor generates light emission by four processes involving the 3d,3s levels: 1. Two‐photon emission, 3d−2s, 2. superfluorescence, 3d−2p, 3. parametric mixing, and 4. two−photon scattering. Upon stepwise tuning λ2, emission spectra are recorded close to 610 and 812 nm, and thus the sum frequency ω1+ω2 is scanned across the 4f resonance. Light observed at 610 nm is superfluorescence. The light at 609.2 nm varies in wavelength upon a shift of λ1, but not of λ2: It is two‐photon emission. Two more emission peaks shift in step with λ2: Parametric emission by five‐wave mixing, and hyper‐Raman‐Stokes scattering of two photons. Interference of these emissions has been observed. The pulse fluctuations of parametric emission are caused by the pump light, while quantum fluctuations dominate the other processes.

Stephen Colby Rand, S. A. Pollack

A new class of laser is reported in which inversion is due to pair-mediated upconversion, quite distinct from conventional sequential one-photon or multiphoton excitation mechanisms. Upconversion of radiation is standardly accomplished by multiphoton processes relying on linear or nonlinear susceptibilities of individual atoms. However, more efficient upconversion can occur by cooperative multiatom processes involving simultaneous transitions on two or more atoms. Cooperative pair pumping in 5% Er:LiYF4 produces laser action from upconverted energy states following pumping at 1.54 µm with a flashlamp-pumped Er:glass laser on the ground to the 4/13/2 transition. Two neighboring 4/13/2 ions decay cooperatively to yield one 4/9/2 ion (which relaxes quickly to 4/11/2), competing effectively with spontaneous emission because of strong interatom coupling and long 4/13/2 lifetime. The upconversion pair process is shown for the first time to be so effective in Er:YLF that this mechanism alone can invert the 4/11/2 state with respect to lower states, generating long delays proportional to inverse pump energy in the onset of lasing.

N. A. Enaki

The paper proposes to unify the two-photon laser and induced cooperative scattering Raman lasing in which the flying ensemble of two-level excited radiators are coupled with the bimodal cavity field in nonlinear two-photon and scattering interactions. It discusses the situation in which the induced two-quantum lasing takes place together with the cooperative scattering conversion of photons between the pump, Stokes, and anti-Stokes cavity modes. The proposed master equation describes cooperative energy exchanges between Raman and two-photon lasing processes by portions of energies multiple to the energy of the entangled photon pair. The generators of such emission and absorption processes belonging to the symmetries of Raman conversion and two-photon cooperative generation are unified. The first symmetry belongs to su(2) algebra connected to the scattering conversion in the Stokes, pump, and anti-Stokes modes of cavity (Opt. Commun.,285(5) (2012) 686–692). The second one belongs to multi-mode su(1,1) symmetry which quantifies the cooperative process belonging to two-photon emission in cavity/fiber mode components of field (see Opt. Commun. 247(4–6) (2005) 381–392). The bimodal collective photon operators are introduced describing the emission or absorption acts of the fixed portion of energy from the cavity equal to the transition energy between the states of each excited atom. A key impact of the study focuses on the statistical properties of the bimodal field and their detection possibilities are proposed for the description of the time evolution of quantum correlations between the field components of Raman conversion and two-photon emission.

Hans Kleinpoppen