Structural Slow Light Research Articles

Slow Light in Subwavelength Grating Waveguides

Structural slow light is the dispersion engineering process by which the group velocity of light can be drastically reduced in a periodic waveguide structure. Enabling large group delay and enhancing the light-matter interaction on a subwavelength scale, on-chip slow light is of great interest in a vast array of fields such as non-linear optic, sensing, laser physics, telecommunication and computing. In this work, we experimentally demonstrate, for the first time, slow light in subwavelength grating waveguides on the silicon-on-insulator platform. We present a comprehensive numerical study in analytical modelling and 3-D FDTD. Multiple waveguides variations were fabricated using an electron-beam lithography process. Figures of merit such as group index, bandwidth and loss-per-delay are examined in both theory and experiment. A maximum measured group index of 47.74 with a loss-per-delay of 103.37 dB/ns has been achieved near the wavelength of 1550 nm. A broad bandwidth of 8.82 nm was measured, in which the group index remains larger than 10. We also show that the region of slow light operation can be shifted over a large wavelength span by controlling a single design parameter.

Perspective: Photonic flatbands

Flatbands are receiving increasing theoretical and experimental attention in the field of photonics, in particular in the field of photonic lattices. Flatband photonic lattices consist of arrays of coupled waveguides or resonators where the peculiar lattice geometry results in at least one completely flat or dispersionless band in its photonic band structure. Although bearing a strong resemblance to structural slow light, this independent research direction is instead inspired by analogies with “frustrated” condensed matter systems. In this Perspective, we critically analyze the research carried out to date, discuss how this exotic physics may lead to novel photonic device applications, and chart promising future directions in theory and experiment.

On-chip optical true time delay lines featuring one-dimensional fishbone photonic crystal waveguide

In this paper, we present on-chip optical true time delay lines based on slow light one-dimensional (1D) fishbone photonic crystal waveguides (FPCWs). The structural slow light is generated by modulating the index guided optical mode with periodically arranged sidewalls along the propagation direction. Due to the reduced mode overlap with the rough etched surface, the propagation loss of the 1D FPCW is significantly reduced compared to the two-dimensional photonic crystal waveguide. A delay time of 65 ps/mm is observed experimentally.

Implementing structural slow light on short length scales: the photonic speed bump

One-dimensional (1D) infinite periodic systems exhibit vanishing group velocity and diverging density of states (DOS) near band edges. However, in practice, systems have finite sizes and inevitably this prompts the question of whether helpful physical quantities related to infinite systems, such as the group velocity that is deduced from the band structure, remain relevant in finite systems. For instance, one may wonder how the DOS divergence can be approached with finite systems. Intuitively, one may expect that the implementation of larger and larger DOS, or equivalently smaller and smaller group velocities, would critically increase the system length. Based on general 1D-wave-physics arguments, we demonstrate that the large slow-light DOS enhancement of periodic systems can be observed with very short systems, whose lengths scale with the logarithm of the inverse of the group velocities. The understanding obtained for 1D systems leads us to propose a novel sort of microstructure to enhance light-matter interaction, a sort of photonic speed bump that abruptly changes the speed of light by a few orders of magnitude without any reflection. We show that the DOS enhancements of speed bumps result from a classical electromagnetic resonance characterized by a single resonance mode and also that the nature and the properties of the resonance are markedly different from those of classical defect-mode photonic-crystal cavities.

Enhanced spectral sensitivity of a chip-scale photonic-crystal slow-light interferometer.

We experimentally demonstrate that the spectral sensitivity of a Mach-Zehnder (MZ) interferometer can be enhanced through structural slow light. We observe a 20-fold resolution enhancement by placing a dispersion-engineered, slow-light, photonic-crystal waveguide in one arm of a fiber-based MZ interferometer. The spectral sensitivity of the interferometer increases roughly linearly with the group index, and we have quantified the resolution in terms of the spectral density of interference fringes. These results show promise for the use of slow-light methods for developing novel tools for optical metrology and, specifically, for compact high-resolution spectrometers.

Theoretical investigation and optimization of fiber grating based slow light

On the edge of bandgap in a fiber grating, narrow peaks of high transimittivity exist at frequencies where light interferes constructively in the forward direction. In the vicinity of these transmittivity peaks, light reflects back and forth numerous times across the periodic structure and experiences a large group delay. In order to generate the extremely slow light in fiber grating for applications, in this research, the common sense of formation mechanism of slow light in fiber grating was introduced. The means of producing and operating fiber grating was studied to support structural slow light with a group index that can be in principle as high as several thousand. The simulations proceeded by transfer matrix method in the paper were presented to elucidate how the fiber grating parameters effect group refractive index. The main parameters that need to be optimized include grating length, refractive index contrast, grating period, loss coefficient, chirp and apodization functions, those can influence fiber grating characteristics.

Theoretical and experimental study of structural slow light in a microfiber coil resonator

In this paper, a compact slow-light microfiber coil resonator (MCR) is fabricated and the slow-light properties of it are analyzed and tested. Based on coupled-wave theory, a theoretical model for describing the slow-light propagation in the MCR is established. Experimentally, the MCR slow-light element is fabricated and its relative slow-light time delay is measured. The group velocity of the light pulse in the MCR slow-light element can be reduced to about 0.47c (c is the speed of light in vacuum) and the shape of the light pulse passing through the MCR is well preserved.

Material slow light and structural slow light: similarities and differences for nonlinear optics [Invited]: comment

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Euvgenii Aleksandrov and Valerii Zapasskii, "Material slow light and structural slow light: similarities and differences for nonlinear optics [Invited]: comment," J. Opt. Soc. Am. B 29, 2643-2643 (2012) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Material slow light and structural slow light: similarities and differences for nonlinear optics [Invited]: reply

Material slow light and structural slow light: similarities and differences for nonlinear optics [Invited]: reply

Sensing With Slow Light in Fiber Bragg Gratings

On the edge of the bandgap in a fiber Bragg grating (FBG) narrow peaks of high transmission exist at frequencies where light interferes constructively in the forward direction. In the vicinity of these transmission peaks, light reflects back and forth numerous times across the periodic structure and experiences a large group delay. Since the sensitivity of a phase sensor to most external perturbations is proportional to the reciprocal of group velocity, in these slow-light regions the sensitivity of an FBG is expected to be significantly enhanced over traditional FBG sensors operated around the Bragg wavelength. In this paper, we describe means of producing and operating FBGs that support structural slow light with a group index that can be in principle as high as several thousand. We present simulations elucidating how to select the FBG parameters, in particular index modulation, length, and apodization, to generate such low group velocities, and quantify the very large improvement in strain and temperature sensitivities resulting from these new slow-light configurations. As a proof of concept, we report an FBG with a group index of 127, or a group velocity of ~2,360 km/s. This is by far the lowest group velocity reported to date in an FBG. Used as a strain sensor, this slow-light FBG is shown to be able to detect a strain as small as 880 fe/ √Hz , the lowest value reported for a passive FBG sensor.

Material slow light and structural slow light: similarities and differences for nonlinear optics [Invited

There are two standard methods for controlling the group velocity of light. One makes use of the dispersive properties associated with the resonance structure of a material medium. The other makes use of structural resonances, such as those that occur in photonic crystals. Both procedures have proved useful in a variety of situations. In this work we contrast these two approaches, especially in terms of issues such as the kinematics of energy flow though the system and the resulting implications for the behavior of nonlinear optical processes in these situations. Stated differently, this paper addresses the question of when nonlinear optical processes are enhanced through use of slow-light interactions and when they are not.