Light management in non-hermitian systems

Author: Waqas Waseem Ahmed
Title: Light management in non-hermitian systems
Director: Prof. Ramon Herrero, Prof. Muriel Botey, Prof. Diederik Wiersma
Presentation date: April 17, 2018
Link to text: https://www.tdx.cat/handle/10803/586058

Abstract: The quest for new artificial structures and materials uncovers novel light-matter interactions, and intriguing physical phenomena. Since the late 80’s purely dielectric materials have been structured, at the wavelength-scale, to develop photonic crystals and photonic crystal fibres. Recently, also the modulation of gain and losses came into play. The new platform based on complex refractive index materials, combining index and gain-loss modulations, opens the door towards new physical concepts and novel applications. One of these fascinating new insights is the realization of classical analogues of quantum systems described by non-Hermitian Hamiltonians, where the complex refractive index plays the role of a complex optical potential. Thus, by carefully combining the real and imaginary parts of the refractive index; it is possible to observe unusual features that cannot be attained in classical Hermitian systems. In particular, non-Hermitian PT-symmetric systems, invariant under the parity and time-reversal symmetries, may support unidirectional mode coupling which is at the basis of novel ideas and applications such as unidirectional invisibility, single mode microlasers or super-sensitive sensors just to name a few. Non-Hermitian photonics has overturned the conventional negative perception of losses, and offers new possibilities to utilize the gain and loss for steering optical processes. The aim of this thesis is to provide new insights into the wave dynamics in this new artificial complex media since a flexible wave control may be essential to design novel technological devices. The work has a double scope: to propose and develop new concepts in fundamental science framework; and to provide technological studies for direct applications in actual and ubiquitous devices such as broad area semiconductor lasers. In this sense, the main contribution is a novel approach to manipulate the light flow using non-Hermitian systems for light localization and enhancement, and for the control of the flow of light following arbitrary vector fields and corresponding applications to photonic devices. We apply the new fundamental concept of local PT-symmetry to one and two dimensions PT-axisymmetric configurations to obtain highly localized fields at selected position or area. Further, we extend and generalize the idea of local PT-symmetry to design systems able to control the flow of light. We develop a mathematical framework to design such 2D (or higher dimensional) complex structured material for any arbitrary vector field and desired topology, referred as a local Hilbert transform which relates the real and imaginary parts of the complex refractive index. The theory of such directionality field is tested on random, periodic, quasi-periodic and localized patterns to generate arbitrary field flows in the form of a sink, vortices or circular channel flows, both in linear and nonlinear systems. Moreover, we investigate non-Hermitian structures for technological applications. We consider 2D periodic lattices, with simultaneous transverse and longitudinal PT-symmetry to obtain self-collimated beams, useful for example in integrated optics. We also propose different schemes of index and gain/loss modulations to enhance the stability of broad area semiconductor (BAS) devices. Besides, we show how: (i) the introduction of spatiotemporal modulation on the pump of vertical-external-cavity surface-emitting lasers (VECSELs) with a simple flat mirror configuration, may stabilize its emission; (ii) in phase modulations of index and gain-loss profiles stabilizes the emission from BAS lasers and amplifiers, a proposal relying on the suppression of modulation instabilities using dispersion management; (iii) PT-axisymmetry applied either to BAS lasers or VCSELs renders these lasers into bright and narrow laser sources.