Type-II heterostructures as active layers for semiconductor laser devices combine the advantages of a spectrally broad, temperature stable, and efficient gain with the potential for electrical injection pumping. Their intrinsic charge carrier relaxation dynamics limit the maximum achievable repetition rates beyond any constraints of cavity design or heat dissipation. Of particular interest are the initial build up of gain after high-energy injection and the gain recovery dynamics following depletion through a stimulated emission process. The latter simulates the operation condition of a pulsed laser or semiconductor optical amplifier. An optical pump pulse injects hot charge carriers that eventually build up broad spectral gain in a model (Ga,In)As/GaAs/Ga(As,Sb) heterostructure. The surplus energies of the optical pump mimic the electron energies typical for electrical injection. Subsequently, a second laser pulse tuned to the broad spectral gain region depletes the population inversion through stimulated emission. The spectrally resolved nonlinear transmission dynamics reveal gain recovery times as fast as 5 ps. These data define the intrinsic limit for the highest laser repetition rate possible with this material system in the range of 100 GHz. The experimental results are analyzed using a microscopic many-body theory identifying the origins of the broad gain spectrum.

Semiconductors are amongst the most efficient active laser media as they yield extreme wall-plug efficiencies. Their broad gain bandwidth also promise short-pulse operation. Yet, intrinsic charge-carrier relaxation dynamics limit the feasible repetition rates beyond constraints of cavity design and heat removal. In lieu of studying an operation device we monitor the population dynamics, i.e., the initial buildup of gain after optical excitation as well as its recovery after a stimulated emission process using multiple pump-probe spectroscopy. The first optical pulse injects hot charge carriers that eventually build up spectral gain in the sample. The energies are chosen such to mimic typical electrical injection surplus energies. Subsequently, a second laser pulse tuned to the broad spectral region in which gain is observed is used to stimulate emission and thus eliminate the gain. Analysis of the absorption spectra after stimulated emission reveals gain recovery times in the picosecond regime.

Li vacancy diffusion steps in Li4Ti5O12 mainly occur as 8ainit ↔ 16c ↔ 8afinal or the corresponding back diffusion step 8ainit ↔ 16c ↔ 8ainit depending on the 16c lifetime.