https://www.ulp.ethz.ch/content/dam/ethz/special-interest/phys/quantum-electronics/ultrafast-laser-physics-dam/publications_awards/publications/2006/248__PhysRep_429__67__2006_.pdf

Passively modelocked surface-emitting semiconductor lasers

We investigate ultrashort pulse generation based on the fundamental soliton generation that is stabilized by a saturable absorber. The case of an absorber with a recovery time much longer than the pulsewidth of the generated soliton is investigated in detail. Based on soliton perturbation theory we derive equations for the soliton variables and the continuum generated in a mode-locked laser. Analytic criteria for the transition from stable to unstable soliton generation are derived. The results demonstrate the possibility of ultrashort pulse generation by a slow saturable absorber only. The theoretical results are compared with experiments. We generate pulses as short as 13 fs using only semiconductor saturable absorbers.

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Soliton mode-locking with saturable absorbers

In band-like semiconductors, charge carriers form a thermal energy distribution rapidly after optical excitation. In hybrid perovskites, the cooling of such thermal carrier distributions occurs on timescales of about 300 fs via carrier-phonon scattering. However, the initial build-up of the thermal distribution proved difficult to resolve with pump–probe techniques due to the requirement of high resolution, both in time and pump energy. Here, we use two-dimensional electronic spectroscopy with sub-10 fs resolution to directly observe the carrier interactions that lead to a thermal carrier distribution. We find that thermalization occurs dominantly via carrier-carrier scattering under the investigated fluences and report the dependence of carrier scattering rates on excess energy and carrier density. We extract characteristic carrier thermalization times from below 10 to 85 fs. These values allow for mobilities of 500 cm2 V−1 s−1 at carrier densities lower than 2 × 1019 cm−3 and limit the time for carrier extraction in hot carrier solar cells. Carrier-carrier scattering rates determine the fundamental limits of carrier transport and electronic coherence. Using two-dimensional electronic spectroscopy with sub-10 fs resolution, Richter and Branchi et al. extract carrier thermalization times of 10 to 85 fs in hybrid perovskites.

/pdf/ultrafast-carrier-thermalization-in-lead-iodide-perovskite-4t0kwgmm01.pdf

Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy

Femtosecond carrier dynamics in GaAs and AlGaAs thin films are studied by time-resolved pump-probe spectroscopy. Measurements are performed using identical pulses as well as a continuum probe. Observations of the ultrafast absorption saturation response are related to the excitation of the carriers into specific nonthermal distributions, to their rapid scattering out of these states, and to subsequent cooling of the quasi-thermal carrier distribution to the temperature of the lattice. A strong dependence of the initial scattering rates on the mole fraction of Al and the excess electron energy is observed. >

Femtosecond absorption saturation studies of hot carriers in GaAs and AlGaAs

Hot carrier dynamics critically impacts the performance of electronic, optoelectronic, photovoltaic, and plasmonic devices. Hot carriers lose energy over nanometer lengths and picosecond timescales and thus are challenging to study experimentally, whereas calculations of hot carrier dynamics are cumbersome and dominated by empirical approaches. In this work, we present ab initio calculations of hot electrons in gallium arsenide (GaAs) using density functional theory and many-body perturbation theory. Our computed electron–phonon relaxation times at the onset of the Γ, L, and X valleys are in excellent agreement with ultrafast optical experiments and show that the ultrafast (tens of femtoseconds) hot electron decay times observed experimentally arise from electron–phonon scattering. This result is an important advance to resolve a controversy on hot electron cooling in GaAs. We further find that, contrary to common notions, all optical and acoustic modes contribute substantially to electron–phonon scattering, with a dominant contribution from transverse acoustic modes. This work provides definitive microscopic insight into hot electrons in GaAs and enables accurate ab initio computation of hot carriers in advanced materials.

https://www.pnas.org/content/pnas/112/17/5291.full.pdf

Ab initio study of hot electrons in GaAs

Excited carrier dynamics in GaAs and Al0.2Ga0.8As are investigated using femtosecond pump and continuum probe techniques. We observe absorption spectral hole burning arising from excited carriers generated by transitions from the split‐off band as well as the heavy‐ and light‐hole bands. Transient absorption saturation measurements indicate that the initial nonthermal carrier distribution thermalizes on a time scale of several tens of femtoseconds.

W. Z. Lin

Papers

Femtosecond hot‐carrier energy relaxation in GaAs