Nonlinear physics of solitary and coupled microcavities.

Project Details


INTRODUCTION The physics of microcavities is a flourishing research topic with moreover a huge potential for applications [1-2]. Thanks to, on the one hand, revolutions in the epitaxial techniques allowing for the introduction of micro- and nanostructures in photonic semiconductors devices, and on the other hand, to the development of new artificially structured composite materials (the so-called metamaterials), one is now able to tailor the properties of microcavities and to harness new photonic functionalities. For example, optical semiconductor microcavities allow for tailoring the laser emission process. Both active and passive microcavities are currently being developed to realize low-power, optical nonlinear, integrated components (e.g. all-optical memories). In the case of passive devices, microcavities can store enormous optical intensities in very small volumes making it easier and more realistic to bring nonlinear effects into practice at lower powers. In the case of active cavities, such as lasers, light can be confined by micro- or even nanostructures allowing for the fabrication of extremely small lasers. Not only have these micro- or nanolasers an ultralow threshold current [3], but also very specific spectral, spatial, dynamical and coherence properties can be designed. The breakthroughs in this field are not only due to the development of novel structures such as microcavities, but also due to novel so-called metamaterials such as quantum dot materials, optically left-handed materials and photonic crystal structures [4]. However, not only individual cavities offer interesting perspectives: recently it was shown that networks of mutually optically coupled nonlinear microcavities can exhibit a fascinating collective dynamical behavior with perspectives for applications in encrypted data communication [5], key exchange and nonlinear photonic information processing [6]. And this is only the start: further in-depth research must lead to unexpected new insights and new applications. To realize these challenges, we propose here to undertake a deep study of both the physics of these novel photonic metamaterials, of the characteristics of microcavities and of the collective dynamical behavior of coupled microcavities. We dare to state that the expertise, obtained during the last years in modeling of material properties (an analytical study of the quantum well susceptibility [7] and of photonic crystals [8]), laser cavities (VCSELs [9]) and in laser dynamics [10], forms a unique and solid base for this research. Because breakthroughs in one of these topics will have an impact on the others, we wish to undertake in this project an integrated theoretical study -closely linked to experimental progress- of both photonic metamaterials, microcavities and integrated microcavity networks. 2. NOVEL PHOTONIC METAMATERIALS Semiconductor lasers, which formed the object of research of our doctorate work, contain in their active layer an additional layered structure in the form of quantum wells, which ensure a better confinement of the charge carriers. A natural improvement of this confinement can be to replace these two-dimensional structures by quasi-zero-dimensional structures: quantum dots [11]. Quantum dots are quasi-atomic systems with specific spectral properties. A large quantity of these artificial atoms can be arranged to form a sophisticated active metamaterial. Quantum dots have a density of states far sharper than quantum wells. Because of this, they have superior optical properties and their nonlinearities differ strongly from traditional quantum well systems. By the phenomenon of Pauli blocking, this material has also been also designated for single photon processing. These innovations demand for new theoretical models. For this reason, within the framework of this project, we will devise a realistic optical susceptibility for the spectra of the optical gain and the refractive index of these metamaterials. This, with a suitable modeling of the intraband relaxation effects and the unique dynamics in the wetting layer, must lead to original laser rate equations for a quantum dot laser containing all the essential ingredients, but which are still simple enough to be solved analytically. Then, we will be able to apply the tools of the world of nonlinear dynamics -which we have intensively used during our doctorate- on these new semiconductor lasers. One aim is to examine the characteristics of co-lasing of several energy levels [12]. Beside the new lines of research concerning active (laser) materials, novel passive media, such as the so-called left-handed materials, also show very promising perspectives. These are metamaterials with both negative permittivity and permeability. These metamaterials show some remarkable properties, such as negative refraction (the light refracts on the same side of the normal on the incident surface). Initially, these materials were developed in the microwave range [13], but their working wavelength shifts more and more to the interesting telecom wavelengths [14]. In previous work, we have shown that if a layer of left-handed material is placed strategically in a microcavity (which is filled for the remainder with active or passive right-handed material), the effects of diffraction can be reduced, compensated and even be made negative [15]. By such diffraction compensation, one is able to considerably reduce diffraction losses in the microcavity or even to remove them completely. In the case of negative diffraction, we have found that the switching behavior in nonlinear cavities are considerably accelerated and are paired with specific regions of pattern formation. When the diffraction is reduced, it is possible to tailor the typical dimensions of dissipative structures to be smaller than even the wavelength of the emitted light. In that way, we are able to circumvent the diffraction limit. We will establish new tools to study the still uncultivated characteristics of left-handed materials and the restrictions of these new photonic functionalities. Based on the expertise acquired during our PhD research, we will examine further the characteristics of the formations of patterns and other dissipative spatial structures such as solitons in these microcavities. In this work, we will be assisted by PhD student P. Tassin, who started his PhD research in October 2005 (supervisors: I. Veretennicoff and J. Danckaert). 3. NOVEL MICROCAVITY STRUCTURES Not only the material in the cavity, but also the microcavity configuration is important. There are different cavity configurations possible, but some can be fabricated only just now. One example is the whispering-gallery cavity, such as the silica microspheres and microtoroids. In our project, we will concentrate on semiconductor lasers with a ring configuration, semiconductor ring lasers (SRL). They consist of a waveguide structure, which can have several different shapes and which can be etched in GaAs or InP. They are characterized by a bistable behavior between the two counter-propagating modes in the ring. A switch from one mode to the other can be induced optically. Therefore, these SRLs are serious candidates for very fast, low-power digital optical memories. The development from SRLs to useful all-optical switching devices or memories requires a deep understanding of all nonlinear processes in the SRL which have an important influence on the competition between these two modes (such as anisotropies, backscattering ...). Our aim is to develop a complete and deep description of SRLs based on complete spatio-temporal Maxwell-Bloch equations of the active material similar to the ones studied in our doctorate work. These complex rate equations will then be analyzed with stability techniques developed during our PhD. This modeling of SRLs and the study of their modal stability will allow us to identify the speed restrictions on the switching behavior and to propose possible modifications to the ring structure to improve the performance even more. This work will be done in collaboration with Lendert Gelens, who is currently applying for a FWO PhD fellowship (supervisor: J. Danckaert). The research on these semiconductor ring lasers will be performed in close conjunction with the IST-STREP project entitled IOLOS. 4. MICROCAVITY NETWORKS Beside the advantageous properties of individual isolated microcavities, the coupling of two or more of such nonlinear components can give rise to captivating nonlinear collective dynamical behavior. This collective behavior was first considered as a drawback. However, new insights have recently conducted the notion that this behavior can strongly improve the photonic functionalities and that new concepts can become possible due to this. Two coupled SRLs were already presented in the literature as an ultrafast switching memory element (a switching time of 20 ps) with a minimum switching energy (5.5 fJ) [6]. A second example can be found in the field of the chaos encryption where one also couples two active microcavities. Using the synchronization properties of these two semiconductor lasers, one can insure a high degree of data protection in data transmission on a commercial optical network [5]. The coupling of two nonlinear microcavities creates also very interesting new filter phenomena. This recent development will continue with the coupling of more than two microcavities in incorporated networks. Such coupled systems of microlasers show a (yet to be studied) rich dynamical behavior. A key phenomenon in these complex systems is the synchronization which provides the collective behavior of separate nonlinear systems. In this project, we will characterize those dynamic properties of these microcavity networks. We will classify different types of synchronization and we will analyze their stability and this theoretical study will allow for an accurate estimation of their potential. We wish to carry out this study during our one year mobility to the IMEDEA (UIB, Palma de Mallorca, Spain), a world institute in this field. The impact of this work can reach far broader than just photonics, since these systems can serve as a metaphor for synchronization properties occurring in physics and in nature. 5. CONCLUSION In this project, we aim to contribute to the search for advanced microcavities with an increasing number of specific, sophisticated characteristics. By placing new photonic metamaterials in the microcavity and by the development of novel (better) cavity configurations, it must become possible to tailor their intensity, spectral en spatial energy distribution at will. Moreover, in this way, we hope to open new avenues to ultrafast, low-power and secure photonic information processing and communication.
Effective start/end date1/01/0831/12/08


  • physics

Flemish discipline codes

  • Other engineering and technology