Induced absorption dynamics in quantum dot based waveguide electroabsorbers

Abstract

Two-color pump-probe measurements are used to study the carrier dynamics of InAs/GaAs quantum dots in a waveguide structure under reverse bias conditions. For the case of initially populating the ground state ͑GS͒, we find relaxation dynamics that include both absorptive and bleaching components in the excited state ͑ES͒ wavelength range. We reproduce the main features of this induced absorption dynamics using a simple model with an additional term for induced absorption at the ES due to carriers injected at the GS. The induced absorption dynamics includes multiple recovery timescales which can be attributed to phonon-assisted processes of GS/ES interaction. © 2010 American Institute of Physics. ͓doi:10.1063/1.3491805͔The carrier dynamics of semiconductor quantum dot ͑QD͒ absorbers have recently been investigated using timeresolved pump-probe spectroscopy. In particular, these techniques were applied to QD structures to explain the nature of tunneling processes at high reverse bias voltages 1 and to demonstrate the electroabsorption properties of a bilayer QD waveguide. 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. 6 More recently, experiments at 1100 nm reported on anomalous induced absorption at the InAs QD ES at high current densities and time-dependent energy level shifts due to Coulomb carrier-carrier interactions. 7 In this letter, the nonlinear recovery of QD based reversed-biased waveguide absorbers is analyzed using a two-color heterodyne pump-probe technique. Either the dots' GS or ES is initially populated using a strong pump pulse. The probe pulse, usually at a different wavelength, samples the recovery dynamics of initially empty states. In the ES pump GS probe case, the dynamics consist of an initial absorption bleaching followed by absorption recovery dynamics similar to the single color case and related to the electron dynamics. The recovery dynamics in GS pump ES probe experiments is more complicated and includes both absorptive and bleaching features.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 material that included six stacks ...

Mixed state effects in waveguide electro-absorbers based on quantum dots

“…In forward bias, inhomogeneously broadened transitions (due to QD size dispersion) are apparent at 1320 nm (GS) and 1250 nm (ES). The heterodyne pump probe technique 14 for tracing the transmission and phase dynamics was extended to a double pump pulse configuration to more closely resemble the operating conditions in an actual 125 GHz MMLL, where both pump pulses have the same energy (see Ref. 7 for a very similar approach).…”

“…12 Previous studies on the dynamics of QD absorber transitions have focused attention on studying the GS and ES dynamics separately 13 or using two color techniques to examine their interplay. 14 The QD waveguide absorber device studied was 1 mm long, had 4 lm width ridges together with tilted, anti-reflection coated facets. Its active region included six stacks of InAs/ GaAs QDs in a dots-in-a-well structure, grown by Zia, Inc. (see Ref.…”