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Identifier

000462143

Title

Chiral metamaterials with parity-time (PT-) symmetry

Alternative Title

Χειρόμορφα μεταϋλικά με συμμετρία ισοτιμίας χρόνου

Author

Κατσαντώνης, Ιωάννης

Thesis advisor

Καφεσάκη, Μαρία

Abstract

The scope of this thesis is to provide a theoretical and computational study of photonic PT-symmetric chiral metamaterials, together with providing new and novel electromagnetic effects and possibilities which can be exploited in a wide range of applications; in particular in applications where advanced light polarization control is required, offering either novel possibilities (e.g., lasers/absorbers of circularly polarized waves) or novel optimized approaches to existing possibilities/devices (e.g., tunable polarization filters or wave-plates of highly-reduced thickness compared to the existing ones). Until now, the PT-symmetry concept has been mainly investigated in metamaterials with extreme material parameters, such as near-zero index metamaterials, or in isotropic metamaterials [1-10] without any magneto-electric coupling. Here the investigation is focused on chiral (bi-isotropic) metamaterials. In Chapter 1 we present a short introduction on metamaterials; especially chiral metamaterial and active metamaterials (metamaterials incorporating gain media). In Chapter 2 we present a detailed analysis of electromagnetic theory of chiral media. A short introduction of the PT-symmetry and the description of the most important relevant effects is shown in Chapter 3. In Chapter 4, we investigate a simple scattering model consisting of a double chiral layer with PT-symmetry. We derive the necessary conditions for a chiral system to be PT-symmetric and we calculate the scattering parameters of such a system under normal wave incidence and, we show that PT-symmetric characteristics such as exceptional points and PT-symmetry phases are independent of the chirality. Our findings are demonstrated also with realistic chiral metamaterials. In Chapter 5 we investigate the simple PT-symmetric bilayer under oblique incidence of TM or TE plane waves; by adjusting the wave impedance enabled by metamaterials we can grant access to wide control of the exceptional points and appearance of mixed phases (coexistence of PT-symmetric and PT-broken phases), not easily realizable with natural materials. In Chapter 6 we investigate in detail the influence of chirality on the PT-symmetric and PT-broken phase of chiral systems with PT-symmetry. Starting from the point that transverse magnetic (TM) and transverse electric (TE) waves experience different exceptional points, we show that with circularly polarized waves (which are linear combinations of TM and TE waves) mixed PT-symmetry phases can be realized and the extent of these phases can be highly controlled by either or both the chirality parameter and the angle of incidence. Additionally, while the transmission of both TM and TE waves in non-chiral PT-symmetric systems is the same for forward and backward propagation, we show that with chirality this symmetry can be broken. As a result, it is possible to realize asymmetric, i.e., side-dependent, optical rotation, and transmitted wave ellipticity. Our system constitutes a simple example of a chiral PT-symmetric optical system in which the various phases (full PT, mixed, broken) and the asymmetric effects can be easily tuned by adjusting the chirality parameter and/or the angle of incidence. In Chapter 7 we discuss a possible application of PT-symmetry for effective molecular chirality enhancement and detection. We present a simple three layer configuration in which a thin chiral layer is placed in between a gain and a loss medium leading to considerable chiral signal circular dichroism enhancement. We also show that circular dichroism can be controlled by tuning the gain parameter. We also find that at anisotropic transmission resonances (ATRs) and at accidental flux-conserving points (in which R+T=1) the effective chiral response is maximized. In chapter 8, we demonstrate a simple chiral metamaterial design to achieve lasing with any desired polarization state from linear to circular. The obtained laser intensity and polarization is controlled by the chiral metamaterial, which acts as a chiral optical cavity. We conclude the thesis with future directions in the field of PT-symmetric chiral metamaterials and we discuss some possible applications. We hope that the results of this thesis demonstrate the potential of PT-symmetric chiral metamaterials as a field of study for novel optical applications.

Language

English

Subject

Chirality

Exceptional points

Ιδιάζουσα σημεία

Χειρομορφία

Issue date

2024-03-22

Collection

School/Department--School of Sciences and Engineering--Department of Materials Science and Technology--Doctoral theses

Type of Work--Doctoral theses