Intradot dynamics of InAs quantum dot based electroabsorbers

Abstract

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...

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

“…The transmission is similar to the lower voltage case although as shown previously, the dynamics can now be fitted to a single exponential with $1-2 ps timescale 13 and commonly attributed to tunnelling into the wetting or barrier layers. 17 The tunnelling timescale primarily depends on the effective barrier thickness which greatly reduces at high reverse bias due to the large tilting of the band edge and so depends on the energy offset between the QD state and wetting layer.…”

“…13. It can be proved that after the fast layer, we may approximate the solutions by two exponentials, q g ðtÞ $ expðÀ2t=s esc Þ; q e ðtÞ $ expðÀt=s w Þ:…”

“…In a MMLL structure, where the lasing wavelength remain at 1320 nm as RB increases, simultaneous interaction with GS and ES absorbing transitions would become important at $8 V. It is important to note that, while QD MMLLs have demonstrated significant advantages such as reduced timing jitter and locking range, the experimentally observed locking range is usually restricted to reverse voltages less that 8 V. The performance deterioration at higher RB levels is accompanied by a strong hysteresis loop and an output power jump in the light-current characteristics, 9 increased pulsewidth, 10 increased pulse-to-pulse timing jitter, 11 decreased locking range and increased pulsewidth of the hybrid ML regime. 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.…”

“…Following Ref. 13, Auger type carrier exchange between ES and GS should dominate and so we have the following equations for the occupation probabilities of the GS (q g ) and the ES (q e ), respectively:…”

“…The dynamics may be fitted by a bi-exponential with associated fast ($1 ps) and slow ($20 ps) timescales, these low voltage timescales have been investigated previously in Ref. 13.…”