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## Submonolayer materials

As part of the SFB787 project C8/A7, we study the properties of materials and nanoscaled quantum systems for use as lasers and/or amplifiers for fast optical interconnects. “The target of this WP is the deeper understanding of ways to control the electronic coupling between quantum structures and of fundamentals of new light-emitting materials and devices.” Superlattices out of submonolayer depositions - so-called submonolayer-stacks - on InGaAs basis provide high power conversion efficiency and low threshold current density when used as active medium in laser diodes, making them a good candidate for optical communication systems. SML-stacks offer controllable tuning parameters, e.g. the period length of the superlattice directly influences the recombination energy inside the SML-stack, as the vertical Coulomb coupling is modified. InGaAs-SML-heterostructures provide a spectral range from approximately 900 to 1050nm at room temperature. In comparison to SK-QDs the PL width of SML-stacks is narrower, enabling the creation of a narrower gain spectrum when used as active laser medium. We study the relaxation and recombination dynamics of MOCVD-grown submonolayer heterostructures with different ultrafast spectroscopy techniques. For further understanding of the dynamics we use rate equation systems to simulate the decay processes inside SML-stacks. In addition, in this WP we investigate the impact of coupling a SML-stack to a Stranski-Krastanov-grown quantum dot layer, as such a 2D-0D coupled quantum system offers parameters to control its decay dynamics.

### Gain and phase dynamics in semiconductor optical amplifiers based on submonolayers

Submonolayer quantum dots as active medium in opto-electronic devices promise to combine the high density of states of quantum wells with the fast recovery dynamics of self-assembled quantum dots. We investigate the gain and phase recovery dynamics of a semiconductor optical ampliﬁer based on InAs submonolayer quantum dots in the regime of linear operation by one- and two-color heterodyne pump-probe spectroscopy. We ﬁnd an as fast recovery dynamics as for quantum dot-in-a-well structures, reaching 2 ps at moderate injection currents. The effective quantum well embedding the submonolayer quantum dots acts as a fast and efﬁcient carrier reservoir.

### Modeling the optical response of submonolayer based optoelectronic devices

Submonolayer quantum dots combine the zero-dimensional charge-carrier conﬁnement of self-assembled quantum dots with the large density of states of a quantum well. Electroluminescence and pump-probe experiments on a submonolayer-based optical ampliﬁer show that the system exhibits a high gain of 90 cm

− 1 and an ultrafast gain recovery. We propose a rate equation system describing the microscopic carrier dynamics which quantitatively reproduces the observed behavior and provides deeper theoretical understanding of the material system. In contrast to Stranski-Krastanov quantum dots, the fast gain recovery is enhanced by a strong interdot coupling. Optically inactive submonolayer states form an efﬁcient carrier reservoir and give rise to a large nonlinear optical response.

## Publications

**2016**

Lingnau, B., Lüdge, K., Herzog, B.,Kolarczik, M., Kaptan, Y., Woggon, U. and Owschimikow, N., Physical Review B **94**, 014305 (2016). *Ultrafast gain recovery and large nonlinear optical response in submonolayer quantum dots. *DOI: 10.1103/PhysRevB.94.014305

**2015**

Herzog, B., Owschimikow, N.,Schulze, J.-H., Rosales, R., Kaptan, Y., Kolarczik, M., Switaiski, T., Strittmatter, A., Bimberg, D., Pohl, U.W. and Woggon U., Applied Physics Letters **107**, 201102 (2015). *Fast gain and phase recovery of semiconductor optical amplifiers based on submonolayer quantum dots. *DOI: 10.1063/1.4935792

Kolarczik, M., Owschimikow, N., Herzog, B., Buchholz, F., Kaptan, Y.I. and Woggon, U., Physical Review B **91**, 235310 (2015). *Exciton dynamics probe the energy structure of a quantum dot-in-a-well system: The role of Coulomb attraction and dimensionality. *DOI: 10.1103/PhysRevB.91.235310

**2014**

Grosse, N.B., Owschimikow, N., Aust, R., Lingnau, B., Koltchanov, A., Kolarczik, M., Lüdge, K. and Woggon, U., Optics Express **22**, 32520 (2014). *Pump-probe quantum state tomography in a semiconductor optical ampliﬁer. *DOI:10.1364/OE.22.032520

Owschimikow, N., Kolarczik, K., Kaptan, Y.I., Grosse, N.B. and Woggon, U., APL **105**, 101108 (2014). *Crossed excitons in a semiconductor nanostructure of mixed dimensionality. *DOI: 10.1063/1.4895558

Kaptan, Y. and Röhm, A. and Herzog, B. and Lingnau, B. and Schmeckebier, H. and Arsenijevic, D. and Mikhelashvili, V. and Schöps, O. and Kolarczik, M. and Eisenstein, G. and Bimberg, D. and Woggon, U. and Owschimikow, N. and Lüdge, K., Applied Physics Letters **119**, 191105 (2014). *Stability of quantum-dot excited-state laser emission under simultaneous ground-state perturbation. *DOI: 10.1063/1.4901051

Kaptan, Y. and Schmeckebier, H. and Herzog, B. and Arsenijevic, D. and Kolarczik, M. and Mikhelashvili, V. and Owschimikow, N. and Eisenstein, G. and Bimberg, D. and Woggon, U., Applied Physics Letters **104**, 261108 (2014). *Gain dynamics of quantum dot devices for dual-state operation. *DOI: 10.1063/1.4885383

**2013**

Kolarczik, M., Owschimikow, N., Korn, J., Lingnau, B., Kaptan, Y., Bimberg, D., Schöll, E., Lüdge, K. and Woggon, U., Nature Communications **4**, 2953 (2013). *Quantum coherence induces pulse shape modiﬁcation in a semiconductor optical ampliﬁer at room temperature.* DOI: 10.1038/ncomms3953

Switaiski, T., Woggon, U., Angeles, D.E.A., Hoffmann, A., Schulze, J.-H., Germann, T.D., Strittmatter, A. and Pohl, U.W., Physical Review B **88**, 035314 (2013). *Carrier dynamics in InAs / GaAs submonolayer stacks coupled to Stranski-Krastanov quantum dots.* DOI: 10.1103/PhysRevB.88.035314

# Nano-optoelectronics

We investigate nanoscaled quantum systems for potential use as ultrafast amplifiers, fast optical switches, modulators or lasers, single photon sources or solid-state based qubits. The complex quantum structures we are particularly interested in are composite electronic quantum systems consisting of a three-dimensionally confined 0D-system and a less-confined, e.g. 2D, quantum system with a specific, controllable 0D-2D system reservoir interaction. We discuss fundamental limits for optical pulse amplification and signal processing and compare optically and electrically population-inverted quantum dot systems. The research is embedded in the collaborative research center SFB 787 at TU Berlin.

**Personal**

Bastian Herzog

Mirco Kolarczik

Sophia Helmrich

## Quantum dot dual-state devices

Ground state gain dynamics of In(Ga)As-quantum dot excited state lasers are investigated via single-color ultrafast pump-probe spectroscopy below and above lasing threshold. Two-color pump-probe experiments are used to localize lasing and non-lasing quantum dots within the inhomogeneously broadened ground state. Single-color results yield similar gain recovery rates of the ground state for lasing and non-lasing quantum dots decreasing from 6 ps to 2 ps with increasing injection current. We ﬁnd that ground state gain dynamics are inﬂuenced solely by the injection current and unaffected by laser operation of the excited state. This independence is promising for dual-state operation schemes in quantum dot based optoelectronic devices.

### Stability of quantum-dot excited-state laser emission under simultaneous ground-state perturbation

The impact of ground state ampliﬁcation on the laser emission of In(Ga)As quantum dot excited state lasers is studied in time-resolved experiments. We ﬁnd that a depopulation of the quantum dot ground state is followed by a drop in excited state lasing intensity. The magnitude of the drop is strongly dependent on the wavelength of the depletion pulse and the applied injection current. Numerical simulations based on laser rate equations reproduce the experimental results and explain the wavelength dependence by the different dynamics in lasing and non-lasing sub-ensembles within the inhomogeneously broadened quantum dots. At high injection levels, the observed response even upon perturbation of the lasing sub-ensemble is small and followed by a fast recovery, thus supporting the capacity of fast modulation in dual-state devices.