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...
The carrier relaxation and escape dynamics of InAs/GaAs quantum dot waveguide absorbers is studied using heterodyne pump-probe measurements. Under reverse bias conditions, we reveal differences in intradot relaxation dynamics, related to the initial population of the dots' ground or excited states. These differences can be attributed to phonon-assisted or Auger processes being dominant for initially populated ground or excited states, respectively. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3106633͔The physics of quantum dot ͑QD͒ based optical devices has been studied intensively due to their interesting blend of atomic and solid state properties.1 Recently, attention has been focused on their absorption properties and has led to QD materials finding favor in such applications as monolithic mode-locked lasers, 2 electroabsorption modulators, 3 and saturable absorber mirrors. 4 Time-resolved pump-probe spectroscopy is a very useful technique to investigate the fundamental timescales and underlying dynamical processes occurring in such absorbers; a notable example being the demonstration of the dynamical role of different carrier types for QW devices. 5 More recently, similar techniques were applied to QD structures to explain the nature of tunneling processes at high reverse bias voltages 6 and to demonstrate the electroabsorption properties of a bilayer QD waveguide. 7In this letter, the nonlinear recovery of QD based reversed-biased waveguide absorbers is analyzed using a single color pump-probe technique. Either the dots' ground state ͑GS͒ or excited state ͑ES͒ is initially populated thus increasing the transmission, and the resulting absorption recovery dynamics is recorded. The recovery dynamics is observed to be dependent on the initial population of the dots' energy states. The difference in recovery dynamics can be understood by the ES to GS carrier relaxation process being phonon assisted when the dots are initially populated in the GS, while it is Auger related when the dots' ES is initially populated.In our experiments, we study intradot relaxation processes as a function of reverse bias using time-resolved spectroscopy. The QD waveguide absorber was 1 mm long, had 4 m width ridges together with tilted, antireflection coated facets. It was fabricated from a material that included six stacks of InAs/GaAs QDs in a dots-in-a-well structure, grown by Zia Inc. ͑see Ref. 8 for further details of the material and experimental technique͒. To summarize, the GS and ES peaks appear at 1320 and 1250 nm, respectively. The pump-probe differential transmission was measured using a heterodyne detection technique. Pulses of about 600 fs width at either 1320 or 1250 nm were obtained from a titaniumsapphire pumped, optical parametric oscillator and split into three beams: reference, pump, and probe ͑260 fJ pump pulse energy, 13 fJ probe pulse energy͒. After propagation through the waveguide absorber with suitable delays, the frequency shifted probe and reference beams were overlapped on a detector, and the amp...
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