Femtosecond carrier dynamics and modelocking in monolithic CPM lasers

Effects of carrier cooling and carrier heating in saturation dynamics and pulse propagation through bulk semiconductor absorbers

“…Conversely, carrier cooling leads to an increase in absorption saturation. This simple picture is the basis of several earlier studies [4]- [7], [9]- [11], [13]- [15], [22]- [27]. In contrast to amplifiers and QW absorbers, in reverse-biased bulk absorbers the electric field in the absorbing region provides an additional source of carrier heating and can significantly affect the absorption dynamics.…”

“…It explains why in the last few years the dynamics of saturation in semiconductor absorbers, particularly the effects of intraband carrier kinetics on absorption, have been studied [21], [24]- [29] in the same detail as gain dynamics in amplifiers. Complicated saturation dynamics based on intraband carrier kinetics have been demonstrated for both bulk [24]- [26] and quantumwell (QW) [21], [24], [27], [28] saturable absorbers. Those studies demonstrated that various mechanisms influence absorption dynamics in semiconductors, including nonthermal energy distribution of photogenerated carriers (like spectral hole burning in amplifiers) [21], [24]- [27], carrier cooling and heating [24]- [27], quantum-confined Stark effect [28], and the dynamics of excitonic absorption [29].…”

“…Complicated saturation dynamics based on intraband carrier kinetics have been demonstrated for both bulk [24]- [26] and quantumwell (QW) [21], [24], [27], [28] saturable absorbers. Those studies demonstrated that various mechanisms influence absorption dynamics in semiconductors, including nonthermal energy distribution of photogenerated carriers (like spectral hole burning in amplifiers) [21], [24]- [27], carrier cooling and heating [24]- [27], quantum-confined Stark effect [28], and the dynamics of excitonic absorption [29]. All these mechanisms are related to intraband carrier kinetics-transport of carriers in both energy and real spaces within the same energy band-and their characteristic times are of picosecond and femtosecond time scales, much shorter than interband carrier relaxation times.…”

“…Thus, electric field carrier heating principally distinguishes the saturation dynamics in bulk and QW absorbers. The temperature of the carriers in the QW, generated during absorption of light, is lower than lattice temperature; these "cold" carriers lead to "fast" saturation of absorption in addition to "slow" saturation [21]- [23], [27]. The fast saturation disappears on a picosecond time scale due to heating of the cold carriers to the lattice temperature.…”

“…Note that at (6)-(9) are reduced to the usual models [4]- [7], [10], [13], [15] used for bulk semiconductor optical amplifiers. In [22], [23], and [27], the model of (6)-(7) with corresponding changes for the two-dimensional (2-D) case in (8) and 9also has been used to describe the absorption dynamics in QW absorbers in modeling mode-locked lasers. In this case, photogenerated carriers are confined in the QW in the direction of the external field , so the field is not able to heat the carriers in the QW.…”