Single-color and two-color pump-probe measurements are used to analyze carrier dynamics in InAs/ GaAs quantum dot amplifiers. The study reveals that hole recovery and intradot electron relaxation occur on a picosecond time scale, while the electron capture time is on the order of 10 ps. A longer time scale of hundreds of picoseconds is associated with carrier recovery in the wetting layer, similar to that observed in quantum well semiconductor amplifiers. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2771374͔ Ultrafast spectroscopy of quantum dot ͑QD͒ semiconductor optical amplifiers ͑SOAs͒ provides valuable information about the potential of these devices for emerging applications such as multiwavelength regeneration while giving insight on their unique carrier dynamics. 1 Unlike bulk and quantum well materials, QDs have discrete energy levels; it is the carrier capture and relaxation dynamics between these levels that will constitute the intrinsic limiting device bandwidth. For this reason, initial SOA studies have concentrated on the gain and refractive index recovery of the QD ground state ͑GS͒, following a pump pulse at the same wavelength. These single-color studies have revealed the presence of several recovery time scales, related to electron capture and relaxation within a dot. 2 Similar SOA studies have been performed on the first excited state ͑ES͒ and revealed the presence of similar time scales. 3 To further investigate carrier transition times, two-color differential transmission spectroscopy has been applied to both quantum well 4 and QD ͑Ref. 5͒ structures. Time scales reported include dot capture and relaxation times of a few picoseconds and less than a picosecond respectively for InGaAs QDs, 5 dot to dot carrier scattering times of ϳ35 ps for InAs QDs, 6 and a Ͻ1 ps time scale for thermalization of holes together with a 15 ps time scale for electron GS to ES escape, also in InAs QDs. 7 Previously, we demonstrated that the carrier capture process for InAs QD structures was Auger mediated. 8 However, our analysis did not include the asymmetry that exists between QD electron and hole effective masses, which in turn leads to more closely spaced hole levels. This hole spacing is less than the thermal energy at room temperature, which in turn is less than the electron spacing, and consequently, the dots' hole states can be reduced to a single shared hole population. This has been shown to lead to GS gain compression in QD lasers 9 and subpicosecond hole capture time scales. 7,10 In this letter, we present two-color differential transmission measurements to confirm the presence of these fast hole redistribution processes and deduce the time scales of the remaining electron capture/escape processes. In addition, we develop a rate equation model of the electron and hole occupancies and demonstrate its agreement with the experimental results.Our experiment is based on the scheme presented in ͑Ref. 11͒ where pump and probe pulses of different wavelengths are filtered from a femtosecond pulse spect...

Carrier dynamics of a 1.3 m InAs/ GaAs quantum dot amplifier is studied using heterodyne pump-probe spectroscopy. Measurements of the recovery times versus injection current reveal a power law behavior predicted by a quantum dot rate equation model. These results indicate that Auger processes dominate the carrier dynamics. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2715115͔ Quantum dot ͑QD͒ photonic materials have attracted much study in recent years as they have the potential to deliver the stability and coherence of atomic sources within a compact and efficient semiconductor device. 1 Characteristics such as reduced sensitivity to optical feedback and reduced alpha parameter have made such materials attractive as laser sources. 2 Also, the suppression of pattern effects in QD semiconductor optical amplifiers ͑SOAs͒ shows promise for high speed applications. 3,4 The understanding of the high speed carrier dynamics of these materials is crucial for their optimization and exploitation. To address this issue, timeresolved spectroscopy has been used to investigate the fundamental carrier decay time scales of SOA structures and determine their suitability for high speed applications. Such pump-probe studies are usually performed using pulse widths of a few hundred femtoseconds to picoseconds in order to sufficiently resolve the relaxation dynamics of high speed devices. 5 In this letter, we apply such techniques to the study of InAs/ GaAs QD SOAs emitting near 1.3 m. Similar QD devices have been studied previously and, in general, it was shown that the carrier dynamics in QDs can be described by three characteristic time scales; an initial ultrafast component with a time scale of 100s of femtoseconds to picoseconds and commonly related to intradot scattering, 6-8 an intermediate component of up to 10s of picosecond duration, 7,8 commonly attributed to the capture of carriers into the dot and a much longer time ͑100s of picoseconds͒ related to the refilling of carriers into the wetting layer. 4,9 In addition, several authors have already pointed out the role of Auger effects in the carrier dynamics of quantum dot devices. For example, previous studies of the threshold current density of lasers as a function of temperature have shown that carrier interband relaxation times are Auger dominated. 10 Recent experimental studies of the ground and excited state recovery as a function of the injection current have also suggested that the ultrafast relaxation processes are Auger dominated. 8 Here, we present an analysis of the intermediate recovery time scale which corresponds to the dot capture time, as a function of the injection current using time-resolved spectroscopy. By focusing on the dependence of the dot capture time on bias level and performing theoretical calculations to interpret our results, we highlight the importance of Auger effects in this slower component of the gain recovery. It is worthwhile pointing out that the carrier dynamics at this time scale currently constitutes the main limitation for h...

We study the feedback-induced instabilities in a quantum dot semiconductor laser emitting in both ground and excited states. Without optical feedback the device exhibits dynamics corresponding to antiphase fluctuations between ground and excited states, while the total output power remains constant. The introduction of feedback leads to power dropouts in the ground state and intensity bursts in the excited state, resulting in a practically constant total output power. Self-assembled quantum dot (QD) lasers and amplifiers have attracted much interest in recent years. 1 For example, their low sensitivity to optical feedback, resulting from a relatively low linewidth enhancement factor 2-4 and a strong relaxation oscillation damping rate, 5 makes them desirable as directly modulated, 6 isolator-free semiconductor lasers. 7 The aim of this Letter is to study both the free-running and the feedback-induced behavior of a QD laser that simultaneously emits in both the ground state (GS) and the excited state (ES). The three-dimensional quantum confinement of a quantum dot (QD) gives rise to discrete energy levels for both electrons and holes. GS emission, resulting from the recombination of a GS electron-hole pair, generally occurs at low injection currents. As the injection current is increased, the populations of the ESs increase and a secondary threshold is observed. 8 The appearance of this second threshold leading to dual lasing has been the subject of much experimental and theoretical analysis and could be used for possible novel applications such as frequency modulation. 9 The devices investigated were as-cleaved Fabry-Perot ridge waveguide lasers, with a ridge width of 3-5 m. The active region consisted of a sixfold stack of InAs QDs embedded in a quantum-well by use of dots-in-well technology. 3,10 Typical devices of 1 mm length had threshold currents around 30 mA at room temperature for GS emission at 1310 nm. At a current of around 85 mA, a secondary threshold for the ES occurred, and both the GS and the ES exhibited the same power at around 110 mA 4I th , as reported in Ref. 11. The operating point for equal GS and ES emission could be tuned by changing the heatsink temperature to reduce the pump current required for simultaneous GS and ES lasing. In any of these situations, both the GS and ES optical spectra exhibited multiple longitudinal Fabry-Perot modes ranging from a few modes close to threshold to up to 30 modes at high injection currents. To analyze the laser dynamics, a grating was used to separate the GS and ES emission. The fluctuations of the output power of each state were measured experimentally by focusing the GS and ES output intensities onto two 50 MHz bandwidth detectors (Thorlabs PDA255) and recording the corresponding dynamics on a digital oscilloscope, while the total output power was monitored by a 1.5 GHz bandwidth detector (Newport AD-300). The free-running laser was biased at room temperature , at the level where equal GS and ES emissions occurred. Antiphase fluctuations between GS an...

No abstract

Abstract:We analyse the properties of GaAs based quantum dot semiconductor lasers emitting near 1310 nm. The line-width enhancement factor is shown to depend strongly on device temperature, ranging from 1.5 at 20 o C to 5 at 50 o C. With optical feedback from a distant reflector, devices remained stable at 20 o C but displayed a range of instabilities at 50 o C, including irregular power drop-outs and periodic pulsations, before entering a chaotic regime. Such dynamical features are unique to quantum dot lasers -quantum well lasers are significantly more unstable under optical feedback making such a clear route to chaos difficult to observe.

The gain and phase dynamics of InAs/ GaAs quantum dot amplifiers are studied using single and two-color heterodyne pump probe spectroscopy. The relaxation of the wetting layer carrier density is shown to have a strong effect on the phase dynamics of both ground and excited state transients, while having a much weaker effect on the gain dynamics. In addition, the dynamical alpha factor may also display a constant value after an initial transient. Such behavior is strongly encouraging for reduced pattern effect operation in high speed optical networks. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2823589͔The physics of quantum dot ͑QD͒ photonic devices has been a subject of great interest since the development of self-assembly growth techniques. One of the principle motivating factors for the study of QD lasers was the possibility of substantially reducing the phase-amplitude coupling ͑␣ factor͒ of semiconductor lasers from values in the region of 3-5 for quantum well devices. 1 Among the expected benefits of such a reduction were low modulation chirp, reduced filamentation in high power devices, and reduced sensitivity to optical feedback. 1 To date, QD lasers have displayed a wide range of ␣ factors, the value displaying a strong dependence on the measurement conditions. 2,3 Recent analysis has shown that the low ground state ͑GS͒ gain saturation of QD devices coupled with a large amount of nonresonant carriers in the QD's excited states ͑ES͒ and barrier states can result in large values above threshold 4 and as a consequence, directly phase modulated devices have been demonstrated. 5 In addition, it has been proposed that the unique carrier dynamics of QD semiconductor optical amplifiers ͑SOAs͒ would lead to a reduction of patterning effects compared to conventional devices, for both linear and nonlinear applications. 6,7 As the understanding of relevant carrier relaxation processes in QD structures improves, additional device functionalities will be realized.Heterodyne pump probe spectroscopy has been established as an ideal tool to directly record the ultrafast gain and refractive index dynamics of SOA devices. Previous studies of the gain and phase dynamics of QD SOAs have been performed by Borri et al., who investigated the effects of carrier heating and spectral hole burning in InAs/ InGaAs QDs, 8 while related techniques have been used to measure the ␣ factor in various conditions. 9 Previously on InAs/ GaAs QDs, we demonstrated the importance of Augermediated carrier capture using a single color pump probe technique 10 and illustrated the effects of ultrafast hole relaxation on the gain recovery 11 using a two-color technique. In this letter, we report on two-color phase measurements of the same structure, and note the presence of a strong phase recovery component at the same time scale as the wetting layer carrier recovery. In addition, we combine these phase measurements with two-color gain measurements to calculate an ␣ factor for the device as a function of pump probe delay. The experimental...

No abstract

O'Driscoll, Ian; Smowton, P. M.; Blood, P. Publication date 2014Original citation Hutchings, M., O'Driscoll, I., Smowton, P. M. and Blood, P. (2014) 'Fermi-dirac and random carrier distributions in quantum dot lasers',