The ultrafast dynamics of gain and refractive index in an electrically pumped InAs-InGaAs quantum-dot (QD) optical amplifier are measured at room temperature using differential transmission with femtosecond time resolution. Both absorption and gain regions are investigated. While the absorption bleaching recovery occurs on a picosecond time scale, the gain compression recovers with /spl sim/100-fs time constant, making devices based on such dots promising for high-speed optical communications

Bimberg, Dieter

Borri, Paola

Heinrichsdorff, F.

Hvam, Jørn Märcher

Langbein, Wolfgang

Mao, M.-H.

Langbein, Wolfgang Werner

Hvam, J. M.

Bimberg, D.

Online Research @ Cardiff

Ultrafast gain dynamics in InAs-InGaAs quantum-dot amplifiers

The ultrafast dynamics of gain and refractive index in an electrically pumped InAs-InGaAs quantum-dot (QD) optical amplifier are measured at room temperature using differential transmission with femtosecond time resolution. Both absorption and gain regions are investigated. While the absorption bleaching recovery occurs on a picosecond time scale, the gain compression recovers with ~100-fs time constant, making devices based on such dots promising for high-speed optical communications

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Ultrafast gain dynamics in InAs/InGaAs quantum dot amplifiersBorri, Paola; Langbein, Wolfgang; Hvam, Jørn Marcher; Heinrichsdorff, F.; Mao, M.-H.; Bimberg, DieterPublished in:I E E E Photonics Technology LettersLink to article, DOI:10.1109/68.849054Publication date:2000Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Borri, P., Langbein, W., Hvam, J. M., Heinrichsdorff, F., Mao, M-H., & Bimberg, D. (2000). Ultrafast gaindynamics in InAs/InGaAs quantum dot amplifiers. I E E E Photonics Technology Letters, 12(6), 594-596. DOI:10.1109/68.849054594 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 6, JUNE 2000Ultrafast Gain Dynamics in InAs–InGaAsQuantum-Dot AmplifiersP. Borri, W. Langbein, J. M. Hvam, F. Heinrichsdorff, M.-H. Mao, and D. BimbergAbstract—The ultrafast dynamics of gain and refractive indexin an electrically pumped InAs–InGaAs quantum-dot (QD) opticalamplifier are measured at room temperature using differentialtransmission with femtosecond time resolution. Both absorptionand gain regions are investigated. While the absorption bleachingrecovery occurs on a picosecond time scale, the gain compressionrecovers with 100-fs time constant, making devices based onsuch dots promising for high-speed optical communications.Index Terms—Amplifiers, modulators, quantum-dot lasers, ul-trafast optics.SEMICONDUCTOR quantum-dot (QD) lasers are at-tracting increasing interest due to their high material gain,low threshold current, and low chirp operation [1]. Recentdevelopments in the fabrication of self-organized QD lasersemitting at room temperature (RT) on the QD ground-statetransition [2] allow for first experimental tests of their perfor-mance. Some of the most important issues of QD lasers relatesto their dynamic behavior, which is conjectured to be limitedby the carrier capture into and relaxation in the QD’s [1],[3]. Previous work dedicated to characterize directly carrierdynamics (capture, escape, lifetime) in self-organized QD’s(see, for instance, [4], [5], and references therein) used mainlytime-resolved photoluminescence experiments on unprocessedsamples at low temperatures in the picosecond and nanosecondregime.In this letter, we investigate for the first time the RT timeconstants of gain and refractive index dynamics in an elec-trically pumped InAs–InGaAs QD amplifier by femtosecondtime-resolved differential transmission measurements. Bothgain and absorption regimes are considered relevant forapplications of QD’s in laser amplifiers and electro-opticalabsorbers, respectively. The investigated sample is a p-i–nstructure grown by MOCVD. Its active region consists ofthree stacked layers of binary/ternary InAs–InGaAs QD’sseparated by 21-nm-thick GaAs barriers placed in the centerof a 120 nm GaAs layer. Two Al Ga As cladding layersand a ridge structure of 8 m width and 475 m length provideoptical confinement and waveguiding [2]. The end facets weretilted, avoiding back-reflection into the waveguide mode, thusManuscript received November 29, 1999; revised March 7, 2000. This workwas supported by the Danish Technical Research Council in the framework ofSCOOP and by DFG in the framework of SFB 296.P. Borri and J. M. Hvam are with the Research Center COM, The TechnicalUniversity of Denmark, Lyngby, Denmark.W. Langbein is with the Lehrstuhl für Experimentelle Physik EIIb, UniversitätDortmund, Dortmund, Germany.F. Heinrichsdorff, M.-H. Mao, and D. Bimberg are with the Institut für Fes-tkörperphysik, Technische Universität Berlin, Berlin, Germany.Publisher Item Identifier S 1041-1135(00)04605-X.Fig. 1. Device gain measured from the transmission of a 150-fs optical pulsethrough the QD amplifier versus the input pulse energy for different biascurrents, as indicated. The maximum gain and absorption values are indicated.In the inset, the corresponding modal gain and a linear fit to the modal gainversus current before saturation are shown.inhibiting lasing. Photoluminescence measurements at RTshow a ground-state dot transition at 1.148 eV (1.08 m) with abroadening of about 60 meV and a wetting layer transition sep-arated by 110 meV. Laser action at RT on the ground-state dottransition of this type of structure has been demonstrated [2].The investigated structure shows RT lasing for a 1.5-mm cavitylength at 1.057- m wavelength, with a threshold current density[6] of 417 A/cm . Note that the density of states deduced fromthe photoluminescence spectrum corrected by a Boltzmannpopulation factor at RT results in a dot ground-state maximumat 1.06 m. We performed both single-pulse transmission andpump-probe experiments, using optical pulses from the idlerof an optical parametric amplifier, providing Fourier-limited150-fs pulses centered at 1.08- m wavelength with 13 nm(14 meV) spectral width at a 300-kHz repetition rate. The lightwas coupled into and out of the device by aspheric lenses withhigh numerical aperture. The transmission of single pulsesthrough the device was measured by a cooled Ge-detector andlock-in technique. The pump-probe experiment was performedusing a recently proposed novel version of a heterodynedetection scheme that allowed us to use copolarized (in the TEmode) and co-propagating pump and probe beams with a lowrepetition rate laser system [7].The transmission (ratio between the pulse energies at theoutput and at the input of the device) versus the input pulse1041–1135/00$10.00 © 2000 IEEEAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 30, 2009 at 10:01 from IEEE Xplore.  Restrictions apply. BORRI et al.: ULTRAFAST GAIN DYNAMICS IN InAs–InGaAs QD AMPLIFIERS 595Fig. 2. Pump-induced gain change measured in a heterodyne pump-probeexperiment at maximum gain (20 mA) and absorption (0 mA) in the device. Inthe inset, a zoom of the dynamics up to 1.5 ps.energy is shown in Fig. 1 for different bias currents. Thetransmission is corrected from coupling, waveguide, andreflection losses. For small input energies, an absorption of3.35 dB is found at zero bias, transparency occurs at 4 mA,and a maximum gain of 1.85 dB is reached at 10 mA. From thecorresponding modal gain (see inset of Fig. 1), a differentialgain of 4.2 cm mA before saturation is deduced. Withincreasing input energy, a bleaching of the absorption and a de-pletion of the gain occurs due to stimulated transitions inducedby the pulse. For high input energies ( pJ), an additionalreduction of the transmission due to two-photon absorption(TPA) is present at all bias currents. Amplified spontaneousemission spectra indicate a strong filling of the ground-state dottransition above 10 mA and the occupation of higher excitedstates, which is in agreement with the saturation of the gain.For a pump input energy of 0.27 pJ, the induced change in theprobe transmission, resonant to the dot ground-state, has beeninvestigated for different bias currents. Note that this pump en-ergy is in the small signal regime, with minor contribution ofTPA (see dotted line in Fig. 1 and the indicated values). In Fig. 2,the gain change in decibels deduced from the probe transmissionchange is shown at 0 and 20 mA as a function of the pump-probedelay. A striking difference between the absorption and the gainrecovery dynamic appears. At 0 mA, pump-induced absorptionbleaching occurs, which reaches a maximum value of 2.8 dBand then recovers over several picoseconds. At 20 mA bias, again compression up to 1.3 dB that recovers in less than 0.3 psis observed. The data show that the pump-induced spectral-holeburning (SHB) in the absorption case builds up with the timeintegral of the pulse, according to a recovery time much longerthat the pulse duration, whereas in the gain case, it recovers ona time that is either comparable to or shorter than pulse dura-tion. In the inset, a zoom of the dynamics over the first 1.5 psis shown. It can be clearly seen that the maximum absorptionbleaching occurs after the pump pulse, whereas the gain com-pression nearly follows the pulse intensity autocorrelation. ThisFig. 3. (a) Differential probe transmission change in absorption versus delaytime. The solid line is a bi-exponential fit to the data proportional to the indicatedformula. The inset shows the corresponding phase change. (b) Pump-inducedgain change close to transparency (4 mA) and at maximum gain (20 mA). In theinset the phase dynamic at 20 mA. Solid lines are fit to the data.is also reflected in the higher maximum absorption bleachingcompared with the gain compression.We interpret these dynamics in the following physicalpicture. In the absorption case, the dots are initially empty.When the pump arrives, bound electron-hole pairs (excitons)are created in the dot ground state, leading to a bleaching ofthe absorption. This recovers by spontaneous carrier recombi-nation or by carrier escape from the dot ground state. Typicalspontaneous lifetimes in QD’s are of several hundreds ofpicoseconds, as measured by time-resolved photoluminescence[5] (much longer than the measured absorption bleachingrecovery), which should be then attributed to carrier escape byphonon absorption. In the top part of Fig. 3, a fit to the datais shown by convoluting the pulse intensity autocorrelationwith a response function [8]. We obtain a good fit using abi-exponential response function with two equally weightedtime constants of 1.25 and 5.9 ps. They can be attributed tohole and electron escape times since the absorption bleachingis proportional to the sum of electron and hole occupationnumbers [9]. The escape, which represents a heating of theoptically excited excitons, also appears in the phase dynamicsAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 30, 2009 at 10:01 from IEEE Xplore.  Restrictions apply. 596 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 6, JUNE 2000(see inset of Fig. 3) as a heating process [8]. The lack of along-lived population in the differential transmission, whichwe would expect once the thermal equilibrium is reached, isdue to the diode structure of the sample. Once the carriers haveescaped from the ground state, the built-in electric field ofthe diode at zero bias removes them out of the active region.When a flat band configuration is realized by a small appliedbias current, long-lived population effects are observed. In thegain case, the dots are initially occupied, and, by stimulatedemission, the pump leads to a reduction of the exciton popula-tion (gain compression) that recovers by relaxation of carriersinto the dot ground state. The measured recovery time is,therefore, a direct measure of the carrier relaxation time fromexcited states into the dot-ground state in a QD amplifier underworking conditions. Note that the used bias current of 20 mAcorresponds to a maximum modal gain that is just enough forground state lasing of the investigated structure, and it is not anunrealistic working point for QD lasers. In the bottom of Fig. 3,a fit to the gain data is shown. We should comment that eventhough the pump input energy was far from a strong TPA, asmall contribution in the measured gain compression is still dueto TPA of the probe and pump pulses, resulting in a reducedprobe transmission that follows the intensity autocorrelation[8]. Since the TPA is bias independent (i.e., mainly not in theactive region), according to Fig. 1, we can estimate this contri-bution by comparison with the differential transmission at thetransparency current, where the net stimulated transition rateis zero, and only the TPA effect should appear. In addition, inthe bottom of Fig. 3, the data at 4 mA are shown together withtheir fit, from which we deduce a TPA at zero delay of 0.5 dBwhen taking into account the small absorption bleaching stillpresent. This estimate is consistent with measurements at highinput power where the TPA dominates the response. The fitat 20 mA is then performed by fixing the amplitude of theTPA contribution and by including the so-called coherentartefact as half of the signal at zero pump-probe delay [8].The amplitude of the SHB effect is given by comparison withthe maximum absorption bleaching that give the strength ofthe stimulated transitions induced by the pump. The only freeparameter is the SHB recovery time constant, and we obtainfrom the best fit fs, which is slightly shorter than thepulse duration. Note that the fit procedure convolutes the pulseintensity autocorrelation with the response function that canhave a time constant shorter than the pulse duration, resultingin a small shift and a slight asymmetry toward positive delay,as observed. The measured ultrafast SHB recovery in the gaincase thus indicates ultrafast carrier relaxation in electricallypumped QD amplifiers, which is very promising for high-speedapplications. The carrier relaxation occurs probably via Augerscattering from the excited states, partly occupied at 20 mA,into the dot ground state, as recently theoretically proposed[10] and experimentally shown in [9] and [11]. Note thatthe occurrence of such ultrafast scattering processes implyfast dephasing times, i.e., large homogeneous broadening(10–20 meV), which we have confirmed by four-wave mixingexperiments [12], and which influence the laser performanceat RT [13].We should comment that the fit of the gain data at large delay(up to 6 ps) also shows a minor contribution with slower expo-nential decay of ps time constant, which is possibly dueto carrier heating [8] (CH). Such CH effect also appears in thephase dynamic (see inset) but with a recovery time of ps.The interpretation of this behavior is beyond the scope of thisletter and up to future investigations.In conclusion, we have measured the recovery time constantsof gain dynamics in an electrically pumped QD amplifier.While the absorption bleaching recovers bi-exponentiallyover few picoseconds due to carrier escape from the dotground-state, the gain compression recovers in only 100 fs,due to carrier relaxation into the dot ground state, makingdevices based on such QD structures very promising forhigh-speed optical communications.ACKNOWLEDGMENTThe authors thank J. Mørk for discussions.REFERENCES[1] D. Bimberg, M. Grundmann, and N. N. Ledentsov, Quantum Dot Het-erostructures. New York: Wiley, 1998.[2] F. Heinrichsdorff, M.-H. Mao, N. Kirstaedter, A. Krost, D. Bimberg, A.O. Kosogov, and P. Werner, “Room-temperature continuous-wave lasingfrom stacked InAs/GaAs quantum dots grown by metalorganic chemicalvapor deposuition,” Appl. Phys. Lett., vol. 71, pp. 22–24, 1997.[3] D. Klotzkin, K. Kamath, K. Vineberg, P. Bhattacharya, R. Murty,and J. Laskar, “Enhanced modulation bandwidth (20 GHz) ofIn Ga As/GaAs self-organized quantum dot lasers at cryogenictemperature: Role of carrier relaxation and differential gain,” IEEEPhoton. Technol. Lett., vol. 10, pp. 932–934, 1998.[4] S. Raymond, X. Guo, J. L. Merz, and S. Fafard, “Excited-state radia-tive lifetime in self-assembled quantum dots obtained from state-fillingspectroscopy,” Phys. Rev. B, vol. 59, pp. 7624–7631, 1999.[5] W. Yang, R. R. Lowe-Webb, H. Lee, and P. C. Sercel, “Effect of car-rier emission and retrapping on luminescence time decays in InAs/GaAsquantum dots,” Phys. Rev. B, vol. 56, pp. 13 314–13 320, 1997.[6] F. Heinrichsdorff, “MOCVD growth and laser applications ofIn(Ga)As/GaAs qunatum dots,” Ph.D. dissertation, Insitut für Festkör-perphysik, Techn. Univ. Berlin, Berlin, Germany, 1997.[7] P. Borri, W. Langbein, J. Mørk, and J. M. Hvam, “Heterodynepump-probe and four-wave mixing in semiconductor optical amplifiersusing balanced lock-in detection,” Opt. Commun., vol. 169, pp.317–324, 1999.[8] K. L. Hall, G. Lenz, A. M. Darwish, and E. P. Ippen, “Subpicosecondgain and index nonlinearities in InGaAsP diode lasers,” Opt. Commun.,vol. 111, pp. 589–612, 1994.[9] T. S. Sosnowski, T. B. Norris, H. Jiang, J. Singh, K. Kamath, and P.Bhattacharya, “Rapid carrier relaxation in In Ga As/GaAs quantumdots characterized by differential transmission spectroscopy,” Phys. Rev.B, vol. 57, pp. R9423–R9426, 1998.[10] R. Ferreira and G. Bastard, “Phonon-assisted capture and intradot Augerrelaxation in quantum dots,” Appl. Phys. Lett., vol. 74, pp. 2818–2820,1999.[11] D. Morris, N. Perret, and S. Fafard, “Carrier energy relaxation by meansof Auger processes in InAs/GaAs self-assembled quantum dots,” Appl.Phys. Lett., vol. 75, pp. 3593–3595, 1999.[12] P. Borri, W. Langbein, J. Mørk, J. M. Hvam, F. Heinrichsdorff, M.-H.Mao, and D. Bimberg, “Dephasing in InAs/GaAs quantum dots,” Phys.Rev. B, vol. 60, pp. 7784–7787, 1999.[13] M. Sugawara, K. Mukai, and Y. Nakata, “Light emission spectra ofcolumnar-shaped self assembled InGaAs/GaAs quantum dot lasers: Ef-fect of homogeneous broadening of the optical gain on lasing character-istics,” Appl. Phys. Lett., vol. 74, pp. 1561–1563, 1999.Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 30, 2009 at 10:01 from IEEE Xplore.  Restrictions apply. 

Ultrafast gain dynamics in InAs/InGaAs quantum dot amplifiers

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Ultrafast gain dynamics in InAs/InGaAs quantum dot amplifiers

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