Recovery time scales in a reversed-biased quantum dot absorber

Abstract

The nonlinear recovery of quantum dot based reverse-biased waveguide absorbers is investigated both experimentally and analytically. We show that the recovery dynamics consists of a fast initial layer followed by a relatively slow decay. The fast recovery stage is completely determined by the intradot properties, while the slow stage depends on the escape from the dot to the wetting layer. The physical properties of quantum dot ͑QD͒ based optical devices has been studied intensively in recent years 1 with particular attention given on their absorption properties. This has led to their use as monolithic mode-locked lasers, 2 electroabsorption modulators 3 and saturable absorber mirrors. 4 To explore the underlying carrier dynamics, timeresolved pump-probe spectroscopy has been applied to such QD structures in order to explain the nature of tunneling processes at high reverse bias voltages, 5 to demonstrate the electroabsorption properties of a bilayer QD waveguide, 6 and to illustrate the importance of Auger processes when the dots excited state ͑ES͒ is initially populated.7 In this letter, we investigate the nonlinear recovery of QD based reversebiased waveguide absorbers experimentally, analytically, and numerically. Our study highlights the role of the capture and escape processes in the formation of a fast initial layer followed by a relatively slow relaxation. We show that the fast stage is completely determined by the intradot relaxation properties while the slow recovery stage depends on the escape from the dot to the wetting layer.Our experiment uses time-resolved pump probe spectroscopy to study the nonlinear recovery of QD based reversed-biased waveguide absorbers as a function of reverse bias. The QD waveguide absorber was 1 mm long had a 4 m width ridges together with tilted antireflection coated facets. It was fabricated from 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. Ground state ͑GS͒ and ES appear at 1320 and 1250 nm, respectively. The pumpprobe differential transmission was measured at the dots' GS using a heterodyne detection technique. In reverse bias, the saturation fluence varied from ϳ0.1 J / m 2 at 0 V to 0.6 J / m 2 at 9 V. Pulses of about 600 fs width at either 1320 or 1250 nm were obtained from a titanium-sapphire pumped, optical parametric oscillator and split into three beams: reference, pump, and probe. After propagation through the waveguide absorber with suitable delays, the probe and reference beams were overlapped on a detector. The amplitude of the difference frequency was detected using a high frequency lock-in amplifier. This signal is proportional to the differential transmission ⌬T of a probe pulse at the same wavelength as the pump. The resulting data are normalized to the maximum differential transmission at zero reverse bias T 0 and therefore represented by ⌬T / T 0 .Figure 1 displays such differential transmission dynamics for reverse bias ...

Induced absorption dynamics in quantum dot based waveguide electroabsorbers

“…The values for 0 ͑V͒ and W g,e ͑V͒ are given on Table I with the other parameters remaining the same as previously used. 3,4 These values can be related to the decrease in absorption in the GS and the increase in absorption in the ES with voltage, experimentally shown on Fig. 1.…”

“…2 More recently, single-color measurements revealed fundamental differences in intradot relaxation dynamics which were attributed to phonon-assisted or Auger processes being dominant for initially populated ground states ͑GSs͒ or excited states ͑ESs͒, respectively, 3 and explained multiple recovery stages by nonlinear energy exchange between the GS and ES. 4 Two-color experiments showing pump induced absorption in the probe differential transmission near the first excited-state transition have previously been reported for InGaAs QDs at 1300 nm and revealed the role of Coulomb renormalization of the first ES in the presence of one electron-hole pair in the GS. 5 Similar experiments with InAs/ GaAs QDs in the range of 970 nm attributed induced absorption to the formation of a nondegenerate biexciton state.…”

Hybrid mode-locking in a 40 GHz monolithic quantum dot laser

“…10 The carrier exchange dynamics for the absorber section is described by the occupation probabilities gs and es of the ground ͑GS͒ and the first excited ͑ES͒ state of a dot, respectively. 11,12 The parameters q cap , q esc , and w determine the time-dependent recovery of the QD absorber. q cap =2 ps and q esc = 10 ps, w = w0 exp͉͑V͑t͒ / V 0 ͉͒, where w0 =18 ps and V 0 Ϸ −2 V. The modulation of the applied voltage results in V͑t͒ = V ͓1+a cos͑⍀t͔͒, where V is the dc component of the reverse bias applied to the absorber section.…”

Pump dependence of the dynamics of quantum dot based waveguide absorbers

“…5 The role of intradot relaxations in the formation of a two stage recovery in QD absorbers has been experimentally clarified 6,7 and described using a rate equation approach. 8 The dependence of the absorber timescales with respect to the initial QD population is an important issue for such devices. In forward bias, a dependence of these timescales on the initial population of the dots has been outlined both theoretically 9 and illustrated experimentally 10 where the gain recovery dynamics was faster after optical QD depletion by a prepump pulse.…”

“…8 where the carrier dynamics are described by occupational probabilities q g and q e of the ground state (GS) and the first excited state (ES) of a dot, respectively. The model accounts for a phonon-assisted interaction between these states, limited by Pauli blocking factors ð1 À q g;e Þ:…”