Waqas Waseem Ahmed
Universitat Politècnica de Catalunya
He is a joint PhD student at the Universitat Politècnica de Catalunya (UPC), Barcelona, Spain and European Laboratory for Non-Linear Spectroscopy (LENS), University of Florence, Italy. Since 2014, he has started his PhD in Europe, sponsored by European Commission under Erasmus Mundus Joint Doctorate (EMJD) Europhotonics program. His PhD study is mainly on dynamics of non-Hermitian and complex modulated optical systems.
Optical Parity-Time (PT-) symmetric systems support unusual properties when the symmetric coupling between internal modes is broken [1,2]. The simplest PT-symmetric 1D optical potential may be taken as: n(x) = n0 exp(iqx),... [ view full abstract ]
Optical Parity-Time (PT-) symmetric systems support unusual properties when the symmetric coupling between internal modes is broken [1,2]. The simplest PT-symmetric 1D optical potential may be taken as: n(x) = n0 exp(iqx), which at resonance, asymmetrically couples the left-propagating mode, exp(-iq/2x), to the right-propagating mode exp(iq/2x), but not vice versa. The question that arises is what happens if such PT-symmetry condition is not met globally. A complex optical potential in the form: n(x) = n0 [cos(iq│x│)-i sin(iq│x│)] imposes a unidirectional coupling towards a selected position, x = 0. Here, we propose a new class of PT-symmetric potentials with axial symmetry by radial dephased modulations of index and gain/loss: n(r)=n0[cos(iqr)-i sin(iqr)]. This axial potential intrinsically generates an exceptional central point leading to an extraordinary field enhancement and simultaneous confinement [3].
Broad aperture lasers, and VCSELs among them, are relevant laser sources however suffering from a major drawback: a poor beam quality due to the lack of an intrinsic transverse mode selection mechanism. Therefore, as a direct application of the proposed optical potential, we explore the spatio-temporal field dynamics of a 2D complex PT-axysimmetric VCSELs which could have physical realizations present nanophotonic techniques. Indeed, we show that field localization and enhancement is possible at r = 0, revealing that this 2D axisymmetric system efficiently turns such microlasers into bright and narrow sources.
We expect the proposed PT-axisymmetric configuration to find remarkable applications for the beam quality emission improvement from VCSEL. Besides, the reported effect is universal, and could be extended to different linear or nonlinear physical systems, from optics to Bose condensates, or acoustics among others.
[1] Phys. Rev. Lett. 80, 5243–5246 (1998).
[2] Nat. Mater. 12, 108–113 (2012).
[3] Phys. Rev. A 94, 053819 (2016).
Photonic & plasmonic nanomaterials , Optical properties of nanostructures , Nonlinear nano-optics
