Rabi oscillations and self-induced transparency in InAs/InP quantum dot semiconductor optical amplifier operating at room temperature Ouri Karni,1,* Amir Capua,1 Gadi Eisenstein,1 Vitalii Sichkovskyi2, Vitalii Ivanov2 and Johann Peter Reithmaier2 2

1 Department of Electrical Engineering, Technion- Israeli Institute of Technology, Haifa, 32000 Israel Technische Physik, Institute of Nanostructure Technologies and Analytics, CINSaT, University of Kassel, Kassel, Germany * [email protected]

Abstract: We report direct observations of Rabi oscillations and selfinduced transparency in a quantum dot optical amplifier operating at room temperature. The experiments make use of pulses whose durations are shorter than the coherence time which are characterized using CrossFrequency-Resolved Optical Gating. A numerical model which solves the Maxwell and Schrödinger equations and accounts for the inhomogeneously broadened nature of the quantum dot gain medium confirms the experimental results. The model is also used to explain the relationship between the observability of Rabi oscillations, the pulse duration and the homogeneous and inhomogeneous spectral widths of the semiconductor. ©2013 Optical Society of America OCIS codes: (230.5590) Quantum-well, -wire and –dot devices; (250.5960) Semiconductor optical amplifiers; (270.0270) Quantum optics; (270.5580) Quantum electrodynamics; (320.2250) femtosecond phenomena; (320.7100) Ultrafast measurements.

References and links 1.

Capua, O. Karni, G. Eisenstein, and J. P. Reithmaier, “Electron wavefunction probing in room-temperature semiconductors: direct observation of Rabi oscillations and self-induced transparency,” arXiv:1210.6803, October 2012. 2. G. Eisenstein, A. Capua, O. Karni, and J. P. Reithmaier, “Highly Nonlinear phenomena and coherent effects in 1550 nm QD lasers and amplifiers,” The 25th International Conference on Indium Phosphide and Related Materials (IPRM) May 2013. 3. A. Capua, O. Karni, and G. Eisenstein, “A Finite-difference time-domain model for quantum-dot lasers and amplifiers in the Maxwell–Schrödinger framework,” IEEE J. Sel. Top. Quantum Electron. 19(5), 1900410 (2013). 4. M. Bayer and A. Forchel, “Temperature dependence of the exciton homogeneous linewidth in In0.60Ga0.40As/GaAs self-assembled quantum dots,” Phys. Rev. B 65(4), 041308 (2002). 5. R. Trebino, Frequency-Resolved Optical Gating: The Measurement Of Ultrashort Laser Pulses (Kluwer Academic Publishers, 2002). 6. H. Choi, V. M. Gkortsas, L. Dieh, D. Bour, S. Corzine, J. Zhu, G. Hoefler, F. Capasso, F. X. Kartner, and T. B. Norris, “Ultrafast Rabi floopping and coherent pulse propagation in a quantum cascade laser,” Nat. Photonics 4(10), 706–710 (2010). 7. C. Fürst, A. Leitenstorfer, A. Nutsch, G. Tränkle, and A. Zrenner, “Ultrafast Rabi oscillations of free-carrier transitions in InP,” Phys. Status Solidi, B Basic Res. 204(1), 20–22 (1997). 8. S. T. Cundiff, A. Knorr, J. Feldmann, S. W. Koch, E. O. Göbel, and H. Nickel, “Rabi flopping in semiconductors,” Phys. Rev. Lett. 73(8), 1178–1181 (1994). 9. A. Scholzgen, R. Binder, M. E. Donovan, M. Lindberg, K. Wundke, H. M. Gibbs, G. Khitrova, and N. Peyghambarian, “Direct observation of excitonic Rabi oscillations in semiconductors,” Phys. Rev. Lett. 82(11), 2346–2349 (1999). 10. T. H. Stievater, X. Li, D. G. Steel, D. Gammon, D. S. Katzer, D. Park, C. Piermarocchi, and L. J. Sham, “Rabi oscillations of excitons in single quantum dots,” Phys. Rev. Lett. 87(13), 133603 (2001). 11. H. Htoon, T. Takagahara, D. Kulik, O. Baklenov, A. L. Holmes, Jr., and C. K. Shih, “Interplay of Rabi oscillations and quantum interference in semiconductor quantum dots,” Phys. Rev. Lett. 88(8), 087401 (2002). 12. P. Borri, W. Langbein, S. Schneider, U. Woggon, R. L. Sellin, D. Ouyang, and D. Bimberg, “Rabi oscillations in the excitonic ground-state transition of InGaAs quantum dots,” Phys. Rev. B 66(8), 081306 (2002).

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26786

13. M. Jutte and W. von der Osten, “Self-induced transparency at bound excitons in CdS,” J. Lumin. 83–84, 77–82 (1999). 14. S. Schneider, P. Borri, W. Langbein, U. Woggon, J. Förstner, A. Knorr, R. L. Sellin, D. Ouyang, and D. Bimberg, “Self-induced transparency in InGaAs quantum-dot waveguides,” Appl. Phys. Lett. 83(18), 3668–3670 (2003). 15. J. A. Davis, L. Van Dao, X. Wen, P. Hannaford, V. A. Coleman, H. H. Tan, C. Jagadish, K. Koike, S. Sasa, M. Inoue, and M. Yano, “Observation of coherent biexcitons in ZnO/ZnMgO multiple quantum wells at room temperature,” Appl. Phys. Lett. 89(18), 182109 (2006). 16. S. Hughes, P. Borri, A. Knorr, F. Romstad, and J. M. Hvam, “Ultrashort pulse-propagation effects in a semiconductor optical amplifier: microscopic theory and experiment,” IEEE J. Sel. Top. Quantum Electron. 7(4), 694–702 (2001). 17. C. Gilfert, E. Pavelescu, and J. P. Reithmaier, “Influence of the As2/As4 growth modes on the formation of quantum dot-like InAs islands grown on InGaAs/InP (100),” Appl. Phys. Lett. 96(19), 191903 (2010). 18. C. Gilfert, V. Ivanov, N. Oehl, M. Yacob, and J. P. Reithmaier, “High gain 1.55 μm diode lasers based on InAs quantum dot like active regions,” Appl. Phys. Lett. 98(20), 201102 (2011). 19. A. Marynski, G. Sek, A. Musial, J. Andrzejewski, J. Misiewicz, C. Gilfert, J. P. Reithmaier, A. Capua, O. Karni, D. Gready, G. Eisenstein, G. Atiya, W. D. Kaplan, and L. Wilde, “Electronic Structure, Morphology and Emission Polarization of Enhanced Symmetry InAs Quantum-Dot-Like Structures Grown on InP Substrates by Molecular Beam Epitaxy,” J. Appl. Phys. 114(9), 094306 (2013). 20. F. Romstad, P. Borri, W. Langbein, J. Mørk, and J. M. Hvam, “Measurement of pulse amplitude and phase distortion in a semiconductor optical amplifier: from pulse compression to breakup,” IEEE Photon. Technol. Lett. 12(12), 1674–1676 (2000). 21. S. L. McCall and E. L. Hahn, “Self-induced transparency,” Phys. Rev. 183(2), 457–485 (1969). 22. A. J. Zilkie, J. Meier, M. Mojahedi, A. S. Helmy, P. J. Poole, P. Barrios, D. Poitras, T. J. Rotter, Y. Chi, A. Stintz, K. J. Malloy, P. W. E. Smith, and J. S. Aitchison, “Time-resolved linewidth enhancement factors in quantum dot and higher-dimensional semiconductor amplifiers operating at 1.55 μm,” J. Lightwave Technol. 26(11), 1498–1509 (2008). 23. O. Karni, A. Capua, G. Eisenstein, D. Franke, J. Kreissl, H. Kuenzel, D. Arsenijević, H. Schmeckebier, M. Stubenrauch, M. Kleinert, D. Bimberg, C. Gilfert, and J. P. Reithmaier, “Nonlinear pulse propagation in a quantum dot laser,” Opt. Express 21(5), 5715–5736 (2013). 24. D. Hadass, A. Bilenca, R. Alizon, H. Dery, V. Mikhelashvili, G. Eisenstein, R. Schwertberger, A. Somers, J. P. Reithmaier, A. Forchel, M. Calligaro, S. Bansropun, and M. Krakowski, “Gain and Noise Sarturation of WideBand InAs-InP Quantum Dash Optical Amplifiers: Model and Experiment,” IEEE J. Sel. Top. Quantum Electron. 11(5), 1015–1026 (2005). 25. L. Allen and J. H. Eberly, “Optical Resonance and Two-Level Atoms” (Dover books, 1987), Chap. 3.

1. Introduction Direct observations of Rabi oscillations and self-induced transparency in a room temperature electrically driven Quantum Dash (QDash) laser-amplifier were recently demonstrated in a series of experiments [1,2], which were confirmed by a detailed Maxwell-Schrödinger model [1,3]. Two key elements enable those observations. The first is operation with pulses whose duration (~150 fs) is shorter than the room temperature coherence time [4]. The second is the ability to fully characterize the pulse complex electric field, following propagation along the amplifier waveguide, using a high resolution Cross-Frequency-Resolved Optical Gating (XFROG) system [5]. The measured X-FROG traces exhibit clear oscillations in amplitude and phase when the laser-amplifier is biased in the gain regime (where the oscillations represent Rabi oscillations), while in absorption, the combination of pulse narrowing and the phase profile signify self-induced transparency. In [6], Rabi oscillations were demonstrated in a quantum cascade laser operating at 30K. Many other experiments identified coherent effects such as Rabi oscillations [7–12] and selfinduced transparency [13,14] in low temperature semiconductors. At room temperature, clear signatures of coherent biexcitons were observed in in ZnO/ZnMgO quantum wells and reported in [15] while off-resonance interactions in a quantum well optical amplifier, which were explained by the process of adiabatic following are described in [16]. This paper reports on direct observation of Rabi oscillations and self-induced transparency in a Quantum Dot (QD) optical amplifier. The device is based on a newly developed high gain InAs/InP QD medium [17–19]. We demonstrate a systematic evolution of Rabi oscillations with increasing input pulse energy and bias. The results resemble those for the QDash amplifier [1] proving that in both types of gain material, the device behaves as an

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26787

effective two-level system, an observation made also in [6] for the quantum cascade laser. The ability to induce and experience quantum-coherent effects, in a room temperature QD optical amplifier, designed for conventional telecomm systems, opens the way to exploit such practical devices in a variety of applications such as quantum communication, data processing, storage and sensing. This paper also describes an extension of the model presented in [1,3] to include the inhomogeneously broadened nature of the self-assembled QDs. The new model confirms the experimental results. In addition, it enables to resolve a peculiarity stemming from the fact that the spectral width of the 150 fs pulse (about 25 nm) is much wider than the room temperature homogeneous linewidth of the semiconductor (7-8 nm). The pulse therefore excites resonantly several transitions, from different QD ensembles within the inhomogeneous linewidth, while off-resonantly exciting several others, which are expected to exhibit somewhat different Rabi frequencies. This should in principle distort the overall response to a degree where no oscillations are observable. Clear oscillations are experimentally observed however and the new inhomogeneous model resolves this ambiguity. 2. Experimental observations Important developments in MBE growth of InP QDs [17] emitting in the telecom wavelength range (1550 nm) enable control over the dots morphology [19] yielding symmetric QDs with high density (approximately 5 ⋅ 1010 cm −2 , obtained from atomic force analysis and atom probe tomography [19]) which offer high optical gain [18]. The experiments we report here used a 1.5 mm-long optical amplifier based on such QDs. The ridge width was 2.5 μm and the facet reflectivity was reduced to 0.01% using a multi-layer AR coating. The active region comprised four InAs dot layers formed by a nominal deposition of five monolayers of InAs separated by 20 nm In0.528Al0.238Ga0.234As barrier layers. The high dot density plays a crucial role in the ability to observe Rabi oscillations which are imprinted on an intense pulse since the criteria for their observability has been shown [3] to be strongly related to number of emitters in the material. The propagation of 150 fs wide pulses was analyzed by characterizing the complex envelope of the electric field (amplitude and phase) at the amplifier output. High-resolution X-FROG measurements [5,20] were used. A schematic description of the X-FROG set-up is shown in Fig. 1. A 150 fs long pulse from an optical parametric oscillator is injected to the waveguide of the QD amplifier. A replica of the pulse (separately characterized by standard FROG) is used to gate the pulse at the amplifier output using sum-frequency generation in a non-linear crystal. The product is measured by a spectrometer. Controlling the delay between the gating pulse and the output pulse allows generating a map of spectra versus delay, from which the complex envelope of the output pulse is retrieved by a computerized algorithm.

Fig. 1. Schematic description of the X-FROG set-up characterizing the complex envelope of the electric field of the output pulses from the QD amplifier.

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26788

Fig. 2. Normalized amplitude (upper plot) and instantaneous frequency (lower plot) profiles of the pulses at the output of the SOA biased in the gain regime at 250 mA, for various input energies. Pulses are centered at 1530 nm. The different traces are separated for clarity.

Measurements of input energy dependent amplitude and instantaneous frequency (chirp) profiles for a bias level of 250 mA are presented in Fig. 2. The profiles evolve from a single peaked pulse whose instantaneous frequency exhibits one valley to a double peaked pulse with an oscillatory chirp. Such profiles have been shown [1] to unequivocally represent the evolution from classical saturation (under moderate input pulse energy) to Rabi oscillations when the pulses are sufficiently energetic. The original observations [1,2] were for a QDash gain medium and the present results show that the same physical processes occur in a QD optical amplifier. The evolution of the instantaneous frequency profiles originates from the fact that the electronic state of a semiconductor is imprinted on the instantaneous frequency profile by the plasma effect as detailed in [1]. The refractive index of the semiconductor is inversely proportional to the excited carrier population. When carriers are depleted, due to stimulated emission induced by an amplified pulse, the index of refraction increases, thereby decreasing the instantaneous frequency. The pulse leading edge is therefore red shifted and conversely the trailing edge experiences a blue shift. The overall effect appears as a valley in the instantaneous frequency profile. The lowest energy pulse in Fig. 2 represents a simple single gain event; it shows no amplitude distortion and a corresponding single instantaneous frequency valley. Such a pulse is said to have an area [21] smaller than π so it does not cause a complete Rabi oscillation. As the input pulse energy (area) increases, the amplitude deforms gradually. A pulse with an area of approximately 4π yields two complete Rabi flops causing the pulse to break up and obtain

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26789

an amplitude profile with two peaks. Consistent with the results of [1], the instantaneous frequency also evolves into two distinct valleys signifying the occurrence of two distinct gain events. Furthermore, the first gain event (valley) occurs earlier for larger pulse energies. This is due to the higher Rabi frequency, Ω = μ E /  , of the more intense pulses.

Fig. 3. Amplitude (upper plot) and instantaneous frequency (lower plot) profiles of the pulses at the output of the SOA biased in the gain regime at 150 mA, for various input energies. Pulses are centered at 1530 nm. The different traces are separated for clarity.

Figure 3 shows similar results but for a lower bias – 150 mA. Once more, the systematic input pulse energy dependent measurements show clearly that the system evolves from simple saturation to one that is governed by a complete flopping of the populations. Comparing the intensity profiles of Fig. 3 to those of Fig. 2 reveals that the second peak exhibits a lower intensity than the leading peak. Recalling that the pulse breakup is caused by the alternating sign of the gain due to the population flopping, the appearance of the second peak at lower relative intensity is due to the fact that the pulse develops a smaller area upon propagation and hence the second gain event occurs at a later time, and the Rabi cycle is incomplete. The instantaneous frequency traces of Fig. 3 exhibit once more the expected profiles with clear signatures of almost two complete Rabi oscillations.

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26790

Fig. 4. Normalized amplitude (upper plot) and instantaneous frequency (lower plot) profiles of the pulses at the output of the SOA biased in the absorption regime at 60 mA, for various input energies. Pulses are centered at 1530 nm. The different traces are separated for clarity.

The coherent light-matter interactions were also examined in the absorption regime with the results shown in Fig. 4. Here the initial occupation probability of the electrons is higher in the valence band state than in the conduction band state. This means that a complete flopping of the populations transforms the medium into gain conditions. An interaction of this kind is called self-induced transparency (SIT) and was observed for the first time in a room temperature semiconductor optical amplifier in [1]. Propagation in the SIT regime causes pulse compression since its central portion experiences gain while the leading and trailing edges are absorbed. The amplitude traces of Fig. 4 reveal that the pulses are somewhat wider than the input pulse. This is due to a competing effect: two-photon absorption (TPA). Since TPA is a highly nonlinear process, it affects more the peak of the pulse and less its wings, causing some pulse broadening which opposes the compression due to SIT. Nevertheless, comparing the pulse widths of the three traces in Fig. 4 we note that the pulses do compress as the input power increases. Moreover, they are clearly narrower than the ones under gain conditions (Figs. 2 and 3) proving that significant pulse shortening is caused by the state of the material. Extracting information from the instantaneous frequency under the absorption conditions is more difficult since the lack of excited carriers makes their effect on the refractive index less apparent. Additional processes such as TPA play a significant role in determining the index. This was studied thoroughly by Zilkie [22] where it was shown that under absorbing conditions, changes in the refractive index due to changes in carrier density (known as the alpha parameter) have the opposite sign compared to the gain regime. The measurements of

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26791

the instantaneous frequency profiles in Fig. 4 show a valley, similar to the gain regime, are consistent with the principles laid out in [22]. 3. Numerical investigation

The observed signatures of coherent light matter interactions were confirmed by a numerical model, which solves simultaneously Maxwell and Schrödinger equations. The experiments in [1] were confirmed using a homogeneous model [1,3], which assumed the laser-amplifier to comprise a series of two-level systems having a single effective transition energy. The twolevel systems were fed through incoherent relaxations from a high-energy carrier reservoir and the assumed coherence time was uniform. While that model [1,3] contained several simplifying assumptions, it successfully reconstructed the measured complex electric field proving the existence of Rabi oscillations and self-induced transparency [1]. The model also converged to the classical limit, describing, for example, the build-up [3] of Beer's law for a step-function excitation of an optical amplifier, or reconstructing four-wave mixing in a QD laser [23]. The homogeneous model [1,3] was extended now to account for the inhomogeneous nature of the gain broadening that stems from size distribution of the self-assembled QDs. This extension offers a more realistic model of the medium and leads also to better insights into the interaction between the broadband pulse and many, relatively narrow-banded, components across the gain spectrum of the device. The inhomogeneous gain spectrum comprises a distribution of resonant transitions having the same homogeneous linewidths as depicted schematically in Fig. 5(a). These two-level systems do not interact directly, but only via capture and escape processes (whose rates are dictated by the principle of detailed balance [24]) to and from a common carrier reservoir that feeds them, as shown schematically in Fig. 5(b). The valance band of the QDs comprises many closely separated energy levels which are strongly coupled to each other and are therefore considered as a single state.

Fig. 5. (a) A schematic description of the distribution of homogeneous resonant transitions, which construct a Gaussian shaped inhomogeneously broadened gain spectrum. The spectrum of a short electromagnetic pulse is also illustrated. (b) An ensemble of two-level systems having different transition energies placed within a heterostructure through which the carriers are injected. Gray arrows represent resonant transitions while black arrows describe some of the incoherent relaxations.

The density of dots with a particular transition frequency f i is distributed according to N d ( f i ) ≡ N di = Ae



(

4ln 2 f i − f peak Δfin2 hom

)

2

(1)

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26792

Where f peak is the frequency of the gain peak, Δf in hom is the inhomogeneously broadened gain spectral width (FWHM) and A is a normalization factor that fixes the total density of QDs to a value N d _ total . The shape of this distribution can be estimated by amplified spontaneous emission (ASE) measurements. The excitation level under the electrical drive used in all the experiments we report was such that no emission was detected from any level other than the ground state. This is consistent with recent photoluminescence measurements [19] which show excited state emission in similar QDs only under very high excitation levels. Therefore, the present model assumes that the QDs comprise only one bound state. The incoherent relaxations in the model are introduced by a set of rate equations for the electron and hole populations of the reservoir ( N res and Pres ) and for the occupation i probabilities in the conduction and valence bands of each QD type, noted by ρ11i or ρ 22 , respectively, as in the density-matrix formalism:

∂N res ηi J N res N res = − − qd τ res N d _ total ⋅τ cap ∂t

∂ρ i11 = ∂t = −γ c ρ 11 + i

N res

2 N d (ωi )τ cap

∂ρ i 22 = ∂t = −γ v ρ i 22 −

M

i VD  N res  M i ρ11 (2) 1 −   2Nd i Dres  i =1 τ esc res 

 N (1 − ρ ) + V i =1

i d

i 11

  (3) Vres ρ i11  N res  μ⋅E i 1 − ρ 11 ) − i  1 − ( ρ12 − ρ 21 ) ( − j τ esc  Dres  VD 

(1 − ρ i 22 ) 1 − Pres  + j μ ⋅ E ( ρ − ρ ) Pres Vres ρ i 22 +   12 21  τ h cap VD 2 N d _ total τ h esc  Dres 

∂Pres ηi J N res VD M = − −  2 N di (γ c ρ i11 + γ v ρ i 22 ) ∂t qd τ res Vres i =1 M P P  1 1 V  − hres N di ρ i 22 + D  1 − res  h  τ cap N d _ total i =1 Vres  Dres  τ esc

M

 2 N di (1 − ρ i 22 )

(4)

(5)

i =1

Where J is the applied current density (with an injection efficiency ηi ), q is the electron charge and d is the dot layer thickness. The time constants: τ res , γ c−1 and γ v−1 account for the i non-radiative recombination rates. τ cap and τ esc represent the capture and escape processes h which couple the i-th QD type and the reservoir electron populations, whereas τ cap and h τ esc are the corresponding time constants for the holes (which are short and assumed here to

be energy independent). Dres is the density of states in the reservoir (considering spin degeneracy).   μ⋅E i The stimulated emission term, j ( ρ12 − ρ 21i ) originates form the Schrödinger  equation of the two-level systems under the dipole approximation for the interaction with the   electromagnetic field. E is the electric field, μ the dipole moment of the QDs and  Planck's factor. ρ12i and ρ12i are the coherence terms (off-diagonal) in the density-matrix formalism. The part of the Schrödinger equation describing the evolution of the coherence terms is: #198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26793

  ∂ρ12i μ⋅E i i = − ( j 2π f i + γ h ) ρ12 − j ( ρ11 − ρ 22i ) ∂t 

(6)

i with ρ12i = ( ρ 21 ) . In these equations, γ h is the coherence decay rate, which sets the *

homogeneous linewidth and determines the approximate period during which coherent phenomena may be observed. The coherence terms are responsible for the induced polarization of the QD: i  i (7) PQD = 2 μ ( ρ12i + ρ 21 ) This induced polarization is superimposed, together with the contribution of the other QD  types, on the background material polarization ( Pbgnd ). The polarization is coupled to the electromagnetic fields, as expressed by Maxwell's equations:   ∂H (8) ∇ × E = − μ0 ∂t  i    ∂PQD ∂E ∂Pbgnd (9) ∇ × H = σ E + ε0 + + ∂t ∂t ∂t i  Here H is the magnetic field vector, μ0 , ε 0 are the vacuum permeability and permittivity, and σ represents ohmic losses if necessary. With the inhomogeneously broadened model at hand, it is possible to explain why the homogeneous model [1,3] (with single resonant frequency and Rabi oscillation period) can reconstruct the interaction of a short pulse (having a broad spectrum, represented schematically in Fig. 5(a)) with an inhomogeneously broadened medium. We recall that a 150 fs input pulse has a spectral width of about 25 nm (13 meV) and hence it interacts resonantly with several separate resonances whose room temperature homogeneous linewidth is about 8 nm (4 meV) [4]. The inhomogeneous model calculates the occupation probability of each level in each sub-system. Figure 6(a) describes the corresponding population inversion during the interaction with the pulse and across the gain spectrum. The interaction acts as if in resonance with that part of the gain, which overlaps the pulse spectrum. Only a few Rabi flops take place, and their periods are rather uniform across the interacting spectrum of sub-levels. The resultant overall calculated signature (on the pulse amplitude and instantaneous frequency profiles), depicted by the blue curves in Fig. 7, exhibits a double peaked amplitude profile and correspondingly two minima in the chirp profile. These resemble well the signatures obtained with the homogeneous model [1,3] (under the condition that its homogeneous linewidth is taken to be roughly equal the spectral width of the pulse). This plot also reproduces qualitatively the measurements described in Figs. 2 and 3.

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26794

Fig. 6. (a) Calculated spectral profile of population inversion for the entire spectrum of accounted two-level systems, along the interaction with a 150 fs pulse. (b) The same, interacting with a 500 fs pulse. Initial conditions are of population inversion (red shades). Blue shades represent negative population inversion.

In contrast, when simulating a longer, 500 fs input pulse (whose spectral width is only 8 nm, 4 meV) having the same peak power, such that the Rabi period at resonance is the same, the occupation probabilities undergo more oscillation cycles during the pulse and a change in oscillation periods is clearly observed for off-resonant sub-levels, as depicted in Fig. 6(b). The latter observation is consistent with the expectations of the fundamental theory [25]. The overall response is generated in this case by a superposition of various contributions whose frequencies are sufficiently different so that their interference during the long, 500 fs, pulse duration smears the oscillation patterns imprinted on both the amplitude and instantaneous frequency profiles as seen in the green traces of Fig. 7.

Fig. 7. Calculated amplitude envelopes (upper plot) and instantaneous frequency profiles (lower plot) of the output pulses. The short pulse (blue trace) shows clear Rabi oscillations. The longer pulse (green trace) shows the decay of the oscillations along the pulse.

Stating this differently, pulses whose durations approach the inverse of the inhomogeneously broadened spectral width (usually termed T2*), overlap with most of the gain spectrum, thus interacting resonantly with an effectively two-level-like system. For long pulses however, the Rabi oscillations decay after T2* due to the destructive interference of the different polarizations which originate from different transitions, and the overall oscillatory nature of the response is destroyed. This de-phasing is another manifestation of convergence from the semi-classical description (where coherent effects dominate) to the classical saturation regime. 4. Conclusion

This work presents a direct observation of room temperature coherent light matter interactions in an InAs/InP QD amplifier operating near 1550 nm. This may have a major impact on the practical implementation of many proposed quantum effects in communication, processing, storage and sensing. The interactions are expressed as Rabi oscillations and self-induced

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26795

transparency imprinted on a 150 fs long pulse after propagating along the amplifier. The observations resemble earlier results in QDash amplifiers. We also present a numerical Maxwell-Schrödinger model, which extends the one in [3], to account for the inhomogeneously broadened nature of the resonant medium. This new model is used to confirm the experimental signature of Rabi oscillations and also to clarify the relationship between the observability of coherent interactions in an inhomogeneously broadened gain spectrum and the pulse width. It asserts that for a pulse whose spectral width is close to the inhomogeneous linewidth, the signature of Rabi oscillations may be regarded as that obtained from an effectively homogeneously broadened gain medium. In contrast, for long pulses, the overall response comprises different contributions from various spectral regions, each having a somewhat different Rabi frequency. These last for a sufficiently long time so that the overall response is smeared. Its exact nature can only be determined by considering the inhomogeneity of the gain broadening. Acknowledgment

This work was partially supported by the Israel Science Foundation. Ouri Karni acknowledges financial support of the Gutwirth Foundation. Amir Capua thanks the Wolf and Clore foundations for their financial support. The material growth and device fabrication was partially supported by the EU project “DeLight” and the Marie-Curie Project “Mitepho”. The technical assistance by Anna Rippien and Florian Schnabel is acknowledged.

#198043 - $15.00 USD Received 24 Sep 2013; revised 21 Oct 2013; accepted 21 Oct 2013; published 29 Oct 2013 (C) 2013 OSA 4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026786 | OPTICS EXPRESS 26796

InP quantum dot semiconductor optical amplifier operating at room temperature.

We report direct observations of Rabi oscillations and self-induced transparency in a quantum dot optical amplifier operating at room temperature. The...
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