The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

An overview is presented of the current state-of-the-art in silicon nanophotonic ring resonators. Basic theory of ring resonators is discussed, and applied to the peculiarities of submicron silicon photonic wire waveguides: the small dimensions and tight bend radii, sensitivity to perturbations and the boundary conditions of the fabrication processes. Theory is compared to quantitative measurements. Finally, several of the more promising applications of silicon ring resonators are discussed: filters and optical delay lines, label-free biosensors, and active rings for efficient modulators and even light sources.

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Silicon microring resonators

Non-Hermitian degeneracies, also known as exceptional points, have recently emerged as a new way to engineer the response of open physical systems, that is, those that interact with the environment. They correspond to points in parameter space at which the eigenvalues of the underlying system and the corresponding eigenvectors simultaneously coalesce. In optics, the abrupt nature of the phase transitions that are encountered around exceptional points has been shown to lead to many intriguing phenomena, such as loss-induced transparency, unidirectional invisibility, band merging, topological chirality and laser mode selectivity. Recently, it has been shown that the bifurcation properties of second-order non-Hermitian degeneracies can provide a means of enhancing the sensitivity (frequency shifts) of resonant optical structures to external perturbations. Of particular interest is the use of even higher-order exceptional points (greater than second order), which in principle could further amplify the effect of perturbations, leading to even greater sensitivity. Although a growing number of theoretical studies have been devoted to such higher-order degeneracies, their experimental demonstration in the optical domain has so far remained elusive. Here we report the observation of higher-order exceptional points in a coupled cavity arrangement-specifically, a ternary, parity-time-symmetric photonic laser molecule-with a carefully tailored gain-loss distribution. We study the system in the spectral domain and find that the frequency response associated with this system follows a cube-root dependence on induced perturbations in the refractive index. Our work paves the way for utilizing non-Hermitian degeneracies in fields including photonics, optomechanics, microwaves and atomic physics.

Enhanced sensitivity at higher-order exceptional points

Trapped ions are among the most promising systems for practical quantum computing (QC). The basic requirements for universal QC have all been demonstrated with ions, and quantum algorithms using few-ion-qubit systems have been implemented. We review the state of the field, covering the basics of how trapped ions are used for QC and their strengths and limitations as qubits. In addition, we discuss what is being done, and what may be required, to increase the scale of trapped ion quantum computers while mitigating decoherence and control errors. Finally, we explore the outlook for trapped-ion QC. In particular, we discuss near-term applications, considerations impacting the design of future systems of trapped ions, and experiments and demonstrations that may further inform these considerations.

Trapped-ion quantum computing: Progress and challenges

Stability and function of eukaryotic genomes are closely linked to chromatin structure and organization. During cell division the entire genome must be accurately replicated and the chromatin landscape reproduced on new DNA. Chromatin and nuclear structure influence where and when DNA replication initiates, whereas the replication process itself disrupts chromatin and challenges established patterns of genome regulation. Specialized replication-coupled mechanisms assemble new DNA into chromatin, but epigenome maintenance is a continuous process taking place throughout the cell cycle. If DNA synthesis is perturbed, cells can suffer loss of both genome and epigenome integrity with severe consequences for the organism.

Chromatin Replication and Epigenome Maintenance

We report the fabrication and experimental characterization of an ultra-high Q microdisk resonator in a silicon-on-insulator (SOI) platform. We examine the role of the substrate in the performance of such microdisk resonators. While substrate leakage loss has warranted the necessity of substrate undercut structures in the past, we show here that the substrate has a very useful role to play for both passive chip-scale device integration as well as active electronic device integration. Two device architectures for the disk-on-substrate are studied in order to assess the possibility of such an integration of high Q resonators and active components. Using an optimized process for fabrication of such a resonator device, we experimentally demonstrate a Q~3×106, corresponding to a propagation loss ~0.16 dB/cm. This, to our knowledge, is the maximum Q observed for silicon microdisk cavities of this size for disk-on-substrate structures. Critical coupling for a resonance mode with an unloaded Q~0.7×106 is observed. We also report a detailed comparison of the obtained experimental resonance spectrum with the theoretical and simulation analysis. The issue of waveguide-cavity coupling is investigated in detail and the conditions necessary for the existence or lack of critical coupling is elaborated.

Ultra-high Q planar silicon microdisk resonators for chip-scale silicon photonics.

Here, we demonstrate a compact photonic crystal wavelength demultiplexing device based on a diffraction compensation scheme with two orders of magnitude performance improvement over the conventional superprism structures reported to date. We show that the main problems of the conventional superprism-based wavelength demultiplexing devices can be overcome by combining the superprism effect with two other main properties of photonic crystals, i.e., negative diffraction and negative refraction. Here, a 4-channel optical demultiplexer with a channel spacing of 8 nm and cross-talk level of better than -6.5 dB is experimentally demonstrated using a 4500 microm(2) photonic crystal region.

Compact wavelength demultiplexing using focusing negative index photonic crystal superprisms.

We experimentally demonstrate a high resolution integrated spectrometer on silicon on insulator (SOI) substrate using a large-scale array of microdonut resonators. Through top-view imaging and processing, the measured spectral response of the spectrometer shows a linewidth of ~0.6 nm with an operating bandwidth of ~50 nm. This high resolution and bandwidth is achieved in a compact size using miniaturized microdonut resonators (radius ~2 μm) with a high quality factor, single-mode operation, and a large free spectral range. The microspectrometer is realized using silicon process compatible fabrication and has a great potential as a high-resolution, large dynamic range, light-weight, compact, high-speed, and versatile microspectrometer.

High resolution on-chip spectroscopy based on miniaturized microdonut resonators

High quality factor (Q approximately 3.4 x 10(6)) microdisk resonators are demonstrated in a Si(3)N(4) on SiO(2) platform at 652-660 nm with integrated in-plane coupling waveguides. Critical coupling to several radial modes is demonstrated using a rib-like structure with a thin Si(3)N(4) layer at the air-substrate interface to improve the coupling.

High quality planar silicon nitride microdisk resonators for integrated photonics in the visible wavelength range.

High quality (Q ≈ 6 × 105) microdisk resonators are demonstrated in a Si3N4 on SiO2 platform at 652–660 nm with integrated in-plane wrap-around coupling waveguides to enable critical coupling to specific microdisk radial modes. Selective coupling to the first three radial modes with >20dB suppression of the other radial modes is achieved by controlling the wrap-around waveguide width. Advantages of such pulley-coupled microdisk resonators include single mode operation, ease of fabrication due to larger waveguide-resonator gaps, the possibility of resist reflow during the lithography phase to improve microdisk etched surface quality, and the ability to realize highly over-coupled microdisks suitable for low-loss delay lines and add-drop filters.

Mohammad Soltani

Papers

Ultra-high Q planar silicon microdisk resonators for chip-scale silicon photonics.

Compact wavelength demultiplexing using focusing negative index photonic crystal superprisms.

High resolution on-chip spectroscopy based on miniaturized microdonut resonators

High quality planar silicon nitride microdisk resonators for integrated photonics in the visible wavelength range.

Systematic design and fabrication of high-Q single-mode pulley-coupled planar silicon nitride microdisk resonators at visible wavelengths.