Strong Pumping Conditions Research Articles

Theoretical study of optical pump process in solid gain medium based on four-energy-level model

A semiclassical algorithm is explored to a four-energy level model, aiming to find out the factors that affect the dynamics behavior during the pump process. The impacts of pump intensity Ωp, non-radiative transition rate γ43 and decay rate of electric dipole δ14 are discussed in detail. The calculation results show that large γ43, small δ14, and strong pumping Ωp are beneficial to the establishing of population inversion. Under strong pumping conditions, the entire pump process can be divided into four different phases, tentatively named far-from-equilibrium process, Rabi oscillation process, quasi dynamic equilibrium process and ‘equilibrium’ process. The Rabi oscillation can slow the pumping process and cause some instability. Moreover, the duration of the entire process is negatively related to Ωp and γ43 whereas positively related to δ14.

Dynamic gain induced pulse shortening in Q-switched lasers

We describe a novel mechanism of pulse shortening in a Q-switched laser induced by the gain compression effect under strong pumping conditions. The pulse shortening requires a large variation of the gain excursion during the saturation process and benefits from the large volume of the gain medium. The effect has been experimentally demonstrated using a passive Q-switched Tm/Ho-doped fiber laser that shows gain-induced pulse compression from 800 ns down to 160 ns when the pump threshold is exceeded by 15 times.

NONLINEAR FREQUENCY CONVERSION BY RAMAN COHERENCE PREPARED IN SOLID HYDROGEN FILM

We investigated the Raman coherence characteristics in the solid hydrogen film deposited on a sapphire substrate. By using Raman coherence prepared with two single-frequency pulsed lasers, we generated the multiorder coherent Raman sidebands in solid hydrogen film. High-order Raman sidebands were obtained under strong pumping conditions (≥230 MW/cm2). The generated anti-Stokes(AS)–Raman sidebands extended from ultraviolet (292 nm for AS5 band) to visible (565 nm for AS1 band) region. The multiorder Raman sideband generation is thought to be due to the parametric coupling of pump and coupling lasers. The frequency conversion efficiency shows the maximum (14%) at the pumping intensity of 360 MW/cm2. From the experiment that makes the multimode probe beam beat with the prepared Raman coherence, we found that the prepared Raman coherence replicates the probe beam to its Raman sidebands.

Observation of broadband forward hyper-Raman emission with high intensity focused laser beams

We show that the well known multiphoton destructive interference, which leads to hyper-Raman gain suppression in the weak pumping regime, no longer occurs under strong pumping conditions. In contrast to the weak pumping case, where no forward hyper-Raman can be observed and strong backward hyper-Raman occurs, forward hyper-Raman emission is found to be much stronger under strong pumping conditions. The emission spectrum acquires new characteristics due to the ac Stark effect introduced by the strong laser field. The frequency spectrum of the forward hyper-Raman is no longer determined by the laser bandwidth, instead a continuum of forward hyper-Raman emission is observed with the width being determined by the peak power density. We will present here a comparison between strong and weak pumping regimes.

Laser Emission from Natural Diamonds

ABSTRACTThe red luminescence band which is a strong broad band that occurs in natural diamonds, showing a zero phonon line at 2.145 eV and a coupling to a nearly localized phonon of 30 meV with a Huang-Rhys factor of 10, and a constant intensity up to room temperature, is shown to present tunable stimulated emission at room temperature. The stability of the band towards strong pumping conditions is discussed. Another strong luminescent band (S3) is shown to present excited state absorption.

Carrier lifetime in MBE grown Si:Sb and Si:In layers measured by the transient grating method

The carrier recombination dynamics in MBE grown silicon films, doped with Sb or In, are investigated by the transient grating method under strong pumping conditions with 35 ps laser pulses. The temporal decay of the induced gratings gives carrier lifetimes of the order 10-10-10-8 s for doping concentrations in the range 1017-1020 cm-3. A complementary theoretical evaluation of the lifetimes shows the importance of the different kinds of band-to-band Auger recombination increases with increasing doping concentration for Sb-doped Si. For In-doped (1017-1018 cm-3) Si and SRH recombination via in centres dominates. The measured lifetimes agree well with the theoretical values if it is assumed that, at the doping levels, internal stress increases the light absorption coefficient for lambda =0.53 mu m by a factor of up to 2 greater than the pure silicon value.

Picosecond carrier recombination dynamics of semiconductor-doped glasses

The carrier recombination dynamics of CdSxSe1−x-doped color glasses under strong pumping conditions were studied with 20 ps dye laser pulses and a white light continuum. The initial blue shift of the absorption spectrum was followed by a fast coherent transient and a slower decay. The results yield an electron-hole recombination time of ∼400 ps. An interesting wavelength dependence of the decay curves was also noted.

Theory of stimulated glide of dislocations in laser semiconductor crystals under strong pumping conditions

The influence of excess carriers on the overcoming of pinning centers of gliding dislocations is analyzed for the case when such carriers are captured by a dislocation near a pinning center or by the pinning center itself. Resonance defect-forming electron capture is assumed. It is shown that at subcritical densities of excess carriers a dislocation travels by successive detachment from pinning centers, but when this density (rate of pumping) reaches a critical value, the dislocation velocity increases abruptly to a value expected for obstacle-free motion.