Photonic reservoir computing enabled by stimulated Brillouin scattering

Artificial Intelligence (AI) drives the creation of future technologies that disrupt the way humans live and work, creating new solutions that change the way we approach tasks and activities, but it requires a lot of data processing, large amounts of data transfer, and computing speed. It has led to a growing interest of research in developing a new type of computing platform which is inspired by the architecture of the brain specifically those that exploit the benefits offered by photonic technologies, fast, low-power, and larger bandwidth. Here, a new computing platform based on the photonic reservoir computing architecture exploiting the non-linear wave-optical dynamics of the stimulated Brillouin scattering is reported. The kernel of the new photonic reservoir computing system is constructed of an entirely passive optical system. Moreover, it is readily suited for use in conjunction with high performance optical multiplexing techniques to enable real-time artificial intelligence. Here, a methodology to optimise the operational condition of the new photonic reservoir computing is described which is found to be strongly dependent on the dynamics of the stimulated Brillouin scattering system. The new architecture described here offers a new way of realising AI-hardware which highlight the application of photonics for AI.Comment: 8 pages, 6 figures, research articl

Theory and numerical modelling of parity-time symmetric structures for photonics

This thesis presents the study of a relatively new class of photonic structures in-voking Parity-Time (PT)-symmetry. PT-symmetric structures in photonics, as a realisation of PT-symmetric Quantum Mechanics problems, are constructed by a judicious design of refractive index modulation which requires the real part of the refractive index to be an even function and the imaginary part of the refractive index to be an odd function in space. PT-symmetric structures in the form of Bragg gratings, coupled resonators and chain resonators are the main configurations studied in this thesis. These PT-symmetric structures feature a spontaneous symmetry breaking at which interesting wave behaviour such as an asymmetric response depending on the direction of the incident wave, unidirectional invisibility, simultaneous coherent-perfect absorber lasing and localised termination modes are observed; these behaviours are presented in this thesis. Theoretical and numerical studies of these PT-symmetric structures are undertaken which assume realistic material parameters,including material dispersion and material non-linearity. Moreover,in this thesis, potential applications of these PT-symmetric structures are explored. The first part of the thesis considers PT-symmetric Bragg grating structures which are formed by introducing a PT-symmetric refractive index modulation into a Bragg grating structure. If gain/loss dispersion is considered, it is shown that dispersion limits the PT-symmetric operation to just a single frequency. As such spontaneous symmetry breaking can only be achieved by varying the gain/loss parameter. Nevertheless, it is shown that by switching the gain/loss in the system, a switching operation can be achieved by using the PT-Bragg grating at a single frequency. Subsequently,anon-linear PT-Bragg grating is investigated by using a time-domain numerical method, namely the Transmission-Line modelling (TLM) method. For the present work a TLM code is developed from scratch in order to ensure full-flexibility when modelling a dispersive and non-linear material. Using the TLM solver, it is demonstrated that gain/loss saturation is an important material property which should be considered as it may impact the practical applications of a PT-symmetry-based device. In the context of a non-linear PT-Bragg grating (NPTBG), the gain/loss saturation affects the interplay between the PT-symmetric opearation and the Kerr non-linear effect. It is further shown that gain/loss saturation plays a crucial role in securing a stable operation of non-linear PT-based devices. For practical applications, it is demonstrated that a non-linear PT-symmetric Bragg grating offers an additional degree of freedom in their operation,by modulating the gain/loss and the intensity of the input signal,compared to a passive structure which can only be manipulated by the input signal intensity. Two applications based on the interplay of PT-symmetric behaviour and Kerr non-linearity are demonstrated,namely a memory device and a logic-gate device. The second part of the thesis studies PT-symmetric resonator structures as a coupled system and as a periodic chain system. For these studies, a semi-analytical method based on the Boundary Integral Equation (BIE) method is developed and used together with a two-dimensional TLM method. The impact of realistic material parameter on the spectral properties of the structure is again investigated. It is shown that the PT-symmetric behaviour can be observed at a single frequency. Moreover, it is shown that PT-symmetry-like behaviour is observed but with complex eigenfrequencies due to the radiation losses; this is a deviation of the strict definition of a PT-symmetric structure with balanced gain and loss. Lowering lasing threshold by increasing loss in the system is demonstrated; this occurs due to induced early symmetry breaking. The final part of the thesis studies the spectral properties of an infinite and finite chain of PT-symmetric resonators. It is shown that the type of modulation along the PT-chain affects the position of the breaking point of the PT-structure. For a finite PT-chain structure, and for a particular type of refractive index modulation, early PT-symmetry breaking is observed and shown to cause the presence of termination states which are localised at the edge of the finite-chain resulting in localised lasing and dissipative modes at each end of the chain

Theory and numerical modelling of parity-time symmetric structures for photonics

This thesis presents the study of a relatively new class of photonic structures in-voking Parity-Time (PT)-symmetry. PT-symmetric structures in photonics, as a realisation of PT-symmetric Quantum Mechanics problems, are constructed by a judicious design of refractive index modulation which requires the real part of the refractive index to be an even function and the imaginary part of the refractive index to be an odd function in space. PT-symmetric structures in the form of Bragg gratings, coupled resonators and chain resonators are the main configurations studied in this thesis. These PT-symmetric structures feature a spontaneous symmetry breaking at which interesting wave behaviour such as an asymmetric response depending on the direction of the incident wave, unidirectional invisibility, simultaneous coherent-perfect absorber lasing and localised termination modes are observed; these behaviours are presented in this thesis. Theoretical and numerical studies of these PT-symmetric structures are undertaken which assume realistic material parameters,including material dispersion and material non-linearity. Moreover,in this thesis, potential applications of these PT-symmetric structures are explored. The first part of the thesis considers PT-symmetric Bragg grating structures which are formed by introducing a PT-symmetric refractive index modulation into a Bragg grating structure. If gain/loss dispersion is considered, it is shown that dispersion limits the PT-symmetric operation to just a single frequency. As such spontaneous symmetry breaking can only be achieved by varying the gain/loss parameter. Nevertheless, it is shown that by switching the gain/loss in the system, a switching operation can be achieved by using the PT-Bragg grating at a single frequency. Subsequently,anon-linear PT-Bragg grating is investigated by using a time-domain numerical method, namely the Transmission-Line modelling (TLM) method. For the present work a TLM code is developed from scratch in order to ensure full-flexibility when modelling a dispersive and non-linear material. Using the TLM solver, it is demonstrated that gain/loss saturation is an important material property which should be considered as it may impact the practical applications of a PT-symmetry-based device. In the context of a non-linear PT-Bragg grating (NPTBG), the gain/loss saturation affects the interplay between the PT-symmetric opearation and the Kerr non-linear effect. It is further shown that gain/loss saturation plays a crucial role in securing a stable operation of non-linear PT-based devices. For practical applications, it is demonstrated that a non-linear PT-symmetric Bragg grating offers an additional degree of freedom in their operation,by modulating the gain/loss and the intensity of the input signal,compared to a passive structure which can only be manipulated by the input signal intensity. Two applications based on the interplay of PT-symmetric behaviour and Kerr non-linearity are demonstrated,namely a memory device and a logic-gate device. The second part of the thesis studies PT-symmetric resonator structures as a coupled system and as a periodic chain system. For these studies, a semi-analytical method based on the Boundary Integral Equation (BIE) method is developed and used together with a two-dimensional TLM method. The impact of realistic material parameter on the spectral properties of the structure is again investigated. It is shown that the PT-symmetric behaviour can be observed at a single frequency. Moreover, it is shown that PT-symmetry-like behaviour is observed but with complex eigenfrequencies due to the radiation losses; this is a deviation of the strict definition of a PT-symmetric structure with balanced gain and loss. Lowering lasing threshold by increasing loss in the system is demonstrated; this occurs due to induced early symmetry breaking. The final part of the thesis studies the spectral properties of an infinite and finite chain of PT-symmetric resonators. It is shown that the type of modulation along the PT-chain affects the position of the breaking point of the PT-structure. For a finite PT-chain structure, and for a particular type of refractive index modulation, early PT-symmetry breaking is observed and shown to cause the presence of termination states which are localised at the edge of the finite-chain resulting in localised lasing and dissipative modes at each end of the chain

Pseudo-Waveform-Selective Metasurfaces and their Limited Performance

Abstract In recent years, metasurfaces composed of lumped circuit components, including nonlinear Schottky diodes, have been reported to be capable of sensing particular electromagnetic waves even at the same frequency depending on their waveforms, or more specifically, their pulse widths. In this study, analogous waveform-selective phenomena using only linear circuits and linear media are reported. Although such linear metasurfaces are analytically and numerically demonstrated to exhibit variable absorption performance, it cannot strictly be categorized as waveform-selective absorption. It is due to the fact that the waveform-selective responses in the linear metasurfaces originated from the dispersion behaviors of the structures rather than the frequency conversion seen in nonlinear waveform-selective metasurfaces. These linear structures are thus referred to as pseudo-waveform-selective metasurfaces. Additionally, it is shown that the pseudo-waveform-selective metasurfaces have limited performance unless nonlinearity is introduced. These results and findings confirm the advantages of nonlinear waveform-selective metasurfaces, which can be exploited to provide an additional degree of freedom to address existing electromagnetic problems/challenges involving even waves at the same frequency

Modelling of dispersive PT-symmetric Bragg grating

This paper reports on the time-domain numerical model of a parity-time Bragg grating with saturated and dispersive gain. The model is compared against the ideal PT scenario where the gain is constant and unsaturated for all frequencies

Modelling of dispersive PT-symmetric Bragg grating

This paper reports on the time-domain numerical model of a parity-time Bragg grating with saturated and dispersive gain. The model is compared against the ideal PT scenario where the gain is constant and unsaturated for all frequencies

Practical limitation on operation of nonlinear parity-time Bragg gratings

The paper analyses the operation of PT Bragg gratings when the dielectric material is considered to be both dispersive and nonlinear and gain and loss are saturable. The paper demonstrates the application of the nonlinear PT Bragg Grating as an optical logic gate and an optical switch

Practical limitation on operation of nonlinear parity-time Bragg gratings

The paper analyses the operation of PT Bragg gratings when the dielectric material is considered to be both dispersive and nonlinear and gain and loss are saturable. The paper demonstrates the application of the nonlinear PT Bragg Grating as an optical logic gate and an optical switch

Saturable and dispersive parity-time symmetric directional coupler: a transmission-line modelling study

In this paper, numerical modelling of Parity-Time (PT) coupled waveguides is reported. The PT coupled waveguide structure consists of two coupled slab waveguides based on GaAs material with gain/loss material parameter models, including both dispersion and saturation. The numerical model used analyses the impact of dispersion and saturation on the eigenmode extracted by a curve fitting approach. The results show that the presence of saturation may prohibit the appearance of the threshold point above which the PT system becomes unstable

Parity-time symmetric chain resonators

A simple tight-binding model to study the band-structure of an infinite length Parity-time (PT) symmetric chain of resonators is presented in this paper. In the talk, we will investigate the impact of having a structure of finite length and consider the effect of a modulation of the real part of the refractive index on the complete band-structure of the PT-chain system. For a finite PT-chain structure under certain modulation, we observe the existence of a localised mode which is either lasing or dissipatin