Sendy Phang

Ultrafast optical switching using parity-time symmetric Bragg gratings

This paper reports on time-domain modeling of an optical switch based on the parity–time (PT) symmetric Bragg grating. The switching response is triggered by suddenly switching on the gain in the Bragg grating to create a PT-symmetric Bragg grating. Transient and dynamic behaviors of the PT Bragg gratings are analyzed using the time-domain numerical transmission line modeling method including a simple gain saturation model. The on/off ratio and the switching time of the PT Bragg grating optical switch are analyzed in terms of the level of gain introduced in the system and the operating frequency. The paper also discusses the effect the gain saturation has on the operation of the PT-symmetric Bragg gratings.

Parity-time symmetric coupled microresonators with a dispersive gain/loss

The paper reports on the coupling of Parity-Time (PT)-symmetric whispering gallery resonators with realistic material and gain/loss models. Response of the PT system is analyzed for the case of low and high material and gain dispersion, and also for two practical scenarios when the pump frequency is not aligned with the resonant frequency of the desired whispering gallery mode and when there is imbalance in the gain/loss profile. The results show that the presence of dispersion and frequency misalignment causes skewness in frequency bifurcation and significant reduction of the PT breaking point, respectively. Finally, we demonstrate a lasing mode operation which occurs due to an early PT-breaking by increasing loss in a PT system with unbalanced gain and loss.

Impact of dispersive and saturable gain/loss on bistability of nonlinear parity–time Bragg gratings

We report on the impact of realistic gain and loss models on the bistable operation of nonlinear parity-time (PT) Bragg gratings. In our model we include both dispersive and saturable gain and show that levels of gain/loss saturation can have a significant impact on the bistable operation of a nonlinear PT Bragg grating based on GaAs material. The hysteresis of the nonlinear PT Bragg grating is analyzed for different levels of gain and loss and different saturation levels. We show that high saturation levels can improve the nonlinear operation by reducing the intensity at which the bistability occurs. However, when the saturation intensity is low, saturation inhibits the PT characteristics of the grating.

Localized single frequency lasing states in a finite parity-time symmetric resonator chain

In this paper a practical case of a finite periodic Parity Time chain made of resonant dielectric cylinders is considered. The paper analyzes a more general case where PT symmetry is achieved by modulating both the real and imaginary part of the material refractive index along the resonator chain. The band-structure of the finite periodic PT resonator chains is compared to infinite chains in order to understand the complex interdependence of the Bloch phase and the amount of the gain/loss in the system that causes the PT symmetry to break. The results show that the type of the modulation along the unit cell can significantly affect the position of the threshold point of the PT system. In all cases the lowest threshold is achieved near the end of the Brillouin zone. In the case of finite PT-chains, and for a particular type of modulation, early PT symmetry breaking is observed and shown to be caused by the presence of termination states localized at the edges of the finite chain resulting in localized lasing and dissipative modes at each end of the chain.

Modeling Curved Carbon Fiber Composite (CFC) Structures in the Transmission-Line Modeling (TLM) Method

A new embedded model for curved thin panels is developed in the transmission line modeling (TLM) method. In this model, curved panels are first linearized and then embedded between adjacent 2-D TLM nodes allowing for arbitrary positioning between adjacent node centers. The embedded model eliminates the necessity for fine discretization, thus reducing the run time and memory requirements for the calculation. The accuracy and convergence of the model are verified by comparing the resonant frequencies of an elliptical cylinder formed using carbon fiber composite (CFC) materials with those of the equivalent metal cylinder. Furthermore, the model is used to analyze the shielding performance of CFC airfoil NACA2415.

Theory and Numerical Modelling of Parity-Time Symmetric Structures in Photonics: Introduction and Grating Structures in One Dimension

A class of structures based on \(\mathcal {PT}\)-symmetric Bragg gratings in the presence of both gain and loss is studied. The basic concepts and properties of parity and time reversal in one-dimensional structures that possess idealised material properties are given. The impact of realistic material properties on the behaviour of these devices is then investigated. Further extension to include material non-linearity is used to study an innovative all-optical memory device.

Theory and Numerical Modelling of Parity-Time Symmetric Structures in Photonics: Boundary Integral Equation for Coupled Microresonator Structures

The spectral behaviour and the real-time operation of Parity-Time (\(\mathcal {PT}\)) symmetric coupled resonators are investigated. A Boundary Integral Equation (BIE) model is developed to study these structures in the frequency domain. The impact of realistic gain/loss material properties on the operation of the \(\mathcal {PT}\)-symmetric coupled resonators is also investigated using the time-domain Transmission-Line Modelling (TLM) method. The BIE method is also used to study the behaviour of an array of PT-microresonator photonic molecules.

Coupled Parity-Time symmetric cavities: Results from Transmission Line Modelling simulations

This paper studies the impact of a dispersive gain/loss material model on Parity-Time (PT) coupled microresonator cavity structures using the time-domain Transmission-Line Modelling method. A modal analysis is also performed to compare the modal composition in the dispersive and non-dispersive cases. Furthermore, waveguide-to-waveguide coupler based on the coupled PT-resonant microresonators is analysed to see how the resulting modal profiles are manifested in the power transmitted between input/output ports.

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.

Resolution enhancement of 2-photon microscopy using high-refractive index microspheres

Intravital microscopy using multiphoton processes is the standard tool for deep tissue imaging inside of biological specimens. Usually, near-infrared and infrared light is used to excite the sample, which enables imaging several mean free path inside a scattering tissues. Using longer wavelengths, however, increases the width of the effective multiphoton Point Spread Function (PSF). Many features inside of cells and tissues are smaller than the diffraction limit, and therefore not possible to distinguish using a large PSF. Microscopy using high refractive index microspheres has shown promise to increase the numerical aperture of an imaging system and enhance the resolution. It has been shown that microspheres can image features ~λ/7 using single photon process fluorescence. In this work, we investigate resolution enhancement for Second Harmonic Generation (SHG) and 2-photon fluorescence microscopy. We used Barium Titanate glass microspheres with diameters ∼20–30 μm and refractive index ∼1.9–2.1. We show microsphere-assisted SHG imaging in bone collagen fibers. Since bone is a very dense tissue constructed of bundles of collagen fibers, it is nontrivial to image individual fibers. We placed microspheres on a dense area of the mouse cranial bone, and achieved imaging of individual fibers. We found that microsphere assisted SHG imaging resolves features of the bone fibers that are not readily visible in conventional SHG imaging. We extended this work to 2-photon microscopy of mitochondria in mouse soleus muscle, and with the help of microsphere resolving power, we were able to trace individual mitochondrion from their ensemble.