There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Orbital angular momentum (OAM), which describes the “phase twist” (helical phase pattern) of light beams, has recently gained interest due to its potential applications in many diverse areas. Particularly promising is the use of OAM for optical communications since: (i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal, (ii) inter-beam crosstalk can be minimized, and (iii) the beams can be efficiently multiplexed and demultiplexed. As a result, multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams, thereby potentially increasing the system capacity. In this paper, we review recent progress in OAM beam generation/detection, multiplexing/demultiplexing, and its potential applications in different scenarios including free-space optical communications, fiber-optic communications, and RF communications. Technical challenges and perspectives of OAM beams are also discussed.

Optical communications using orbital angular momentum beams

It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.

a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals.

Optical Waveguide Theory

Driven by the increasing demand on handing microwave signals with compact device, low power consumption, high efficiency and high reliability, it is highly desired to generate, distribute, and process microwave signals using photonic integrated circuits. Silicon photonics offers a promising platform facilitating ultracompact microwave photonic signal processing assisted by silicon nanophotonic devices. In this paper, we propose, theoretically analyze and experimentally demonstrate a simple scheme to realize ultracompact rejection ratio tunable notch microwave photonic filter (MPF) based on a silicon photonic crystal (PhC) nanocavity with fixed extinction ratio. Using a conventional modulation scheme with only a single phase modulator (PM), the rejection ratio of the presented MPF can be tuned from about 10 dB to beyond 60 dB. Moreover, the central frequency tunable operation in the high rejection ratio region is also demonstrated in the experiment.

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Photonic crystal nanocavity assisted rejection ratio tunable notch microwave photonic filter

A novel single-notch filter is experimentally demonstrated based on a compact asymmetric microfiber coupler. With an extinction ratio over 30 dB, a single narrow transmission dip in a wavelength range over 300 nm was obtained.

Single-notch filter based on a compact asymmetric microfiber coupler

We theoretically studied a two cascaded stages of microring coupled-resonator optical waveguides with corresponding
analytical model developed. Simulations show that multiple electromagnetically induced transparency-like spectra
appear in this system, which can be ascribed to the combined effects of mode splittings and instructive interferences.
Applications of such multiple electromagnetically induced transparency-like spectra are promising to be tailored for
multi-channel delaying lines.

Transparency in two cascaded stages of microring coupled-resonator optical waveguides

Encircling an exceptional point (EP) in a parity-time (PT) symmetric system has shown great potential for chiral optical devices, such as chiral mode switching for symmetric and anti-symmetric modes. However, the chiral switching for polarization states has never been reported although chiral polarization manipulation has significant applications in imaging, sensing, and communication etc. Here inspired by the anti-PT symmetry, we demonstrate an on-chip chiral polarizer by constructing polarization-coupled anti-PT symmetric system for the first time. The transmission axes of the chiral polarizer are different for forward and backward propagation. A polarization extinction ratio of over 10 dB is achieved for both propagating directions. Moreover, a telecommunication experiment is performed to demonstrate the potential applications in polarization encoding signals. It provides a novel functionality for encircling-an-EP parametric evolution and offer a new approach for on-chip chiral polarization manipulation.

Chiral polarizer based on encircling EP

A fundamental/high-order mode tunable filter is proposed and demonstrated for mode-multiplexed optical network, based on Silicon platform. This reconfigurable mode filter exhibits high extinction ratio of 18 dB and low insertion loss of 3 dB over C-band.

Integrated Tunable Mode Filter for Mode-Division Multiplexed System

We experimentally demonstrate a self-calibrating microring resonator with 7-bit high-precision configuration and resistance to temperature fluctuations by monitoring wavelengths. This work provides stable and accurate control of microring resonators for applications such as optical computing and optical information processing.

Xinliang Zhang

Papers

Single-notch filter based on a compact asymmetric microfiber coupler

Transparency in two cascaded stages of microring coupled-resonator optical waveguides

Chiral polarizer based on encircling EP

Integrated Tunable Mode Filter for Mode-Division Multiplexed System

Self-Calibrating Microring Resonator by Monitoring Wavelengths