Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - International Research Training Group 1740

Project 02

Lasing Networks in Semiconductors

Research Team:

  • F. Henneberger and H.-J. Wünsche (HU Berlin), Germany
  • E. Macau (INPE) and T. Pereira (UFABC), Brazil
  • M. Höfner (HU Berlin, PhD-student)

About the project:

Coupled semiconductor lasers exhibit novel nonlinear dynamics. Due to their fast internal gain-photon dynamics, the propagation times between the lasers become essential, even over short distances and govern the overall behavior. Experimental studies so far covered only a few active lasers (mostly two) with comparatively weak and passive connections so that the network aspect is only sparsely developed. The investigation of larger laser arrays is faced by immense experimental difficulties as complex and costly setups are required. In the frame of the IRTG, we intend to focus on systems where the laser action itself is generated by the cooperative coupling in an optical network.

Random lasers (RLs) [1] constitute such a network. The feedback is here not provided by an external resonator, but by scatterers which are randomly distributed in an active medium or which by themselves act as optical amplifiers. The theory of RL is still developing [2]. We intend to contribute a new perspective by considering the RL as a complex random network. The scatters constitute the nodes and connections are formed through mutually impinging scattered light fields.

In the German group, planar ZnO-based semiconductor quantum structures are studied experimentally, where random lasing is of direct practical relevance [3]. Defects in the semiconductor act as scattering centers. Their size and distribution can be manipulated by the growth process or by external nanostructuring, even producing regular arrays. Using optical pumping, gain of adjustable magnitude is generated in a certain region of the semiconductor defined by the excitation geometry. In this way, a configurable network is created where number, separation, and arrangement of the nodes as well as their connection strength can be controlled. The RL structures fabricated in the group’s own growth facilities will be analyzed with the appropriate temporal, spatial, and spectral resolution under pulsed optical pumping.

Preliminary work has uncovered multimode spectra which strikingly changed from shot to shot. The main goal is to unravel the statistical properties of the RL emission, in both time and spectral domain. Questions to be addressed concern (i) the response during a single shot, (ii) the existence of repetitions under excitation with pulse trains, and (iii) how the laser emission depends on number, arrangement, and connection strength of the scatterers. We will elucidate if these features can be understood in terms of a plastic, self-organizing, and perhaps multistable network. The interaction between two or more adjacent pump spots is a further possible direction of research. The experimental work is supplemented by the development and validation of simple but realistic models in cooperation with the Brazilian partner group.

Schema of a lasing network created by a finite pump spot and photograph of a lasing sample with hundreds of (invisible) scatterers.

[1] D.S. Wiersma, The physics and applications of random lasers, Nature Physics 4, 359 (2008).
[2] O. Zaitsev and L. Deych, Recent developments in the theory of multimode random lasers, J. Opt. 12, 024001 (2010).
[3] S. Kalusniak, H.-J. W ̈nsche, and F. Henneberger, Random Semiconductor Lasers: Scattered versus Fabry-Perot Feedback, Phys. Rev. Lett. 106, 013901 (2011)