Slow Light Structures Research Articles

Futuristic elongated-hexagonal photonic crystal waveguide for slow light

The capability of photonic crystal waveguides (PCWs) to reduce the device footprint, extend the bandwidth, and control the group velocity dispersion effects, has prompted many researchers to propose new models of slow-light PCWs. An unpretentious figure of merit (FoM) to evaluate the intrinsic performances of slow-light structures, e.g., maximum buffer capacity and delay time with modest dispersion engineering, is the normalized-delay-bandwidth-product (NDBP). We proposed a unique platform from an elongated-hexagonal (e-hexagonal) lattice PCW for multi-modes and its performance has been investigated. The proposed e-hexagonal PCW has an ultra-wide bandwidth compatible with a high group index and a high FoM around 0.6456 within the flat band of slow light. We observed periodic oscillations in the field pattern of spatially confined light along the path of propagation, indicating a strong coupling among the cavities of defects. The e-hexagonal PCW is an outstanding initial point for further slow-light engineering in PCWs alongside a promising application for random-access memories and all-optical signal processors.

Tunable Optical Buffer through an Analogue to Electromagnetically Induced Transparency in Coupled Photonic Crystal Cavities

Tunable on-chip optical delay has long been a key target for the research community, as it is the enabling technology behind delay lines, signal retiming and other applications vital to optical signal processing. To date, the field has been limited by high optical losses associated with slow light or delay structures. Here, we present a novel tunable delay line, based on a coupled cavity system exhibiting an electromagnetically induced transparency-like transmission spectrum, with record low loss, around 15 dB/ns. By tuning a single cavity, the delay of the complete structure can be tuned over 120 ps, with the maximum delay approaching 300 ps.

Energy Consumption Enhancement of Reverse-Biased Silicon-Based Mach–Zehnder Modulators Using Corrugated Slow Light Waveguides

The energy efficiency of silicon modulators can be enhanced by increasing the doping level. As an alternative solution, slow light modulators based on corrugated waveguides can also be used for improving the energy efficiency. Using numerical and analytical tools, we show that for the same level of energy consumption reduction, the loss coefficient and the loss-modulation efficiency for the slow-light modulators are smaller than those for the modulators with an increased doping level. It is shown that the slow light structures with medium slow down factors (specifically slowdown factors of less than 6) can be reasonable candidates for enhancing the modulator's energy efficiency. For example, an energy consumption of less than 100 fJ/bit can be achieved at a slow-down factor of 6, using a doping level of 5 × 1017 cm-3 for acceptors and 1 × 1018 cm-3 for donors.

Theoretical investigation of intensity-dependent optical nonlinearity in graphene-aided D-microfiber

We theoretically investigate the intensity-dependent optical nonlinearity in graphene-aided D-microfiber, by tuning the chemical potential of graphene and varying radial distance and radii of the D-microfiber. Utilizing an interplay between graphene and the enhanced evanescent field of a guided mode in the waveguide of interest, the net utility of nonlinear coefficient is harnessed up to a very high value of 106 W−1m−1. Importantly, which is ∼ two orders of magnitude larger than in PMMA–graphene–PMMA waveguide. The highly dispersive nature of the waveguide, D ∼ 103 ps/nm-km, and large nonlinear figure-of-merit, FOMNL∼ 1.29, have raised the possibilities of utilizing slow light structures to operate devices at few watts power level with microscale length. These studies have opened one window towards the next-generation all fiber-optic graphene nonlinear optical devices.

Optimal interfacing with coupled-cavities slow-light waveguides: mimicking periodic structures with a compact device.

We present a design for optimal interfacing (I/O coupling) with slow-light structures consisting of coupled cavities. The I/O couplers are based on adding a small set of cavities with varying coupling coefficients at edges of the coupled cavities waveguide in order to match its impedance with that of the I/O waveguides. I/O efficiencies exceeding 99.9% are shown to be possible over a bandwidth which is larger than 50% of that of the coupled cavities waveguide. Consequently, the reflections at the edges of the slow-light structure are practically eliminated. We discuss the properties of the perfectly impedance matched slow-light structure as an effective (super-structure) cavity and study the impact of the number of cavities comprising the I/O coupler. We also consider in details the impact of errors and disorder in the I/O coupling sections.

Enhancing and inhibiting stimulated Brillouin scattering in photonic integrated circuits.

On-chip nonlinear optics is a thriving research field, which creates transformative opportunities for manipulating classical or quantum signals in small-footprint integrated devices. Since the length scales are short, nonlinear interactions need to be enhanced by exploiting materials with large nonlinearity in combination with high-Q resonators or slow-light structures. This, however, often results in simultaneous enhancement of competing nonlinear processes, which limit the efficiency and can cause signal distortion. Here, we exploit the frequency dependence of the optical density-of-states near the edge of a photonic bandgap to selectively enhance or inhibit nonlinear interactions on a chip. We demonstrate this concept for one of the strongest nonlinear effects, stimulated Brillouin scattering using a narrow-band one-dimensional photonic bandgap structure: a Bragg grating. The stimulated Brillouin scattering enhancement enables the generation of a 15-line Brillouin frequency comb. In the inhibition case, we achieve stimulated Brillouin scattering free operation at a power level twice the threshold.

Graphene-based active slow surface plasmon polaritons.

Finding new ways to control and slow down the group velocity of light in media remains a major challenge in the field of optics. For the design of plasmonic slow light structures, graphene represents an attractive alternative to metals due to its strong field confinement, comparably low ohmic loss and versatile tunability. Here we propose a novel nanostructure consisting of a monolayer graphene on a silicon based graded grating structure. An external gate voltage is applied to graphene and silicon, which are separated by a spacer layer of silica. Theoretical and numerical results demonstrate that the structure exhibits an ultra-high slowdown factor above 450 for the propagation of surface plasmon polaritons (SPPs) excited in graphene, which also enables the spatially resolved trapping of light. Slowdown and trapping occur in the mid-infrared wavelength region within a bandwidth of ~2.1 μm and on a length scale less than 1/6 of the operating wavelength. The slowdown factor can be precisely tuned simply by adjusting the external gate voltage, offering a dynamic pathway for the release of trapped SPPs at room temperature. The presented results will enable the development of highly tunable optoelectronic devices such as plasmonic switches and buffers.

Slow light optofluidics: a proposal.

The resonant slow light structures created along a thin-walled optical capillary by nanoscale deformation of its surface can perform comprehensive simultaneous detection and manipulation of microfluidic components. This concept is illustrated with a model of a 0.5 mm long, 5 nm high, triangular bottle resonator created at a 50 μm radius silica capillary containing floating microparticles. The developed theory shows that the microparticle positions can be determined from the bottle resonator spectrum. In addition, the microparticles can be driven and simultaneously positioned at predetermined locations by the localized electromagnetic field created by the optimized superposition of eigenstates of this resonator, thus exhibiting a multicomponent, near-field optical tweezer.

On the limitations of the first-order nonlinear Schrödinger equation in slow-light photonic crystal structures

We investigate the limitations of the first-order nonlinear Schrodinger equation for describing slow-light enhanced optical nonlinearities in photonic crystal structures. We do this by introducing the concept of generalized Bloch modes to describe light propagation in nonlinear photonic crystal structures. By considering how the nonlinear effects perturb the eigenproblem that describes such generalized Bloch modes, we derive a formalism that allows us to determine the impact of nonlinearities up to an arbitrary order. Based on our formalism, we derive expressions for the first- and second-order nonlinear terms experienced by a single generalized Bloch mode. We then theoretically demonstrate that the second-order terms can in silicon-based slow-light photonic crystal waveguides already become significant at optical powers as small as 100 mW. Additionally, we show that the significance of the second-order term is closely related with the structural group index dispersion or second-order dispersion parameter of the structures considered.

Slow light in photonic crystals waveguides

We analyzed the scattering produced by technological imperfections in a strip photonic crystal waveguide. Modeling and losses analysis of the slow light structures were carried out by plane wave expansion method using the MPB software.

Energy efficient nonlinear optics in silicon: are slow-light structures more efficient than nanowires?

We compare the energy performance of four-wave mixing in nanowires and slow-light photonic crystals and outline the regimes where each platform exhibits salient advantages and limitations, including analysis of the impact of future fabrication improvement. These results suggest a route towards energy efficient silicon integrated photonics.

Ultrahigh sensitivity of rotation sensing beyond the trade-off between sensitivity and linewidth by the storage of light in a dynamic slow-light resonator

We propose to employ the storage of light in a dynamically tuned add-drop resonator to realize an optical gyroscope of ultrahigh sensitivity and compact size. Taking the impact of the linewidth of incident light on the sensitivity into account, we investigate the effect of rotation on the propagation of a partially coherent light field in this dynamically tuned slow-light structure. It is demonstrated that the fundamental trade-off between the rotation-detection sensitivity and the linewidth will be overcome and the sensitivity-linewidth product will be enhanced by two orders of magnitude in comparison to that of the corresponding static slow-light structure. Furthermore, the optical gyroscope employing the storage of light in the dynamically tuned add-drop resonator can acquire ultrahigh sensitivity by extremely short fiber length without a high-performance laser source of narrow linewidth and a complex laser frequency stabilization system. Thus the proposal in this paper provides a promising and feasible scheme to realize highly sensitive and compact integrated optical gyroscopes by slow-light structures.

Scaling of Raman amplification in realistic slow-light photonic crystal waveguides

The prospect for low pump-power Raman amplification in silicon waveguides has recently been boosted by theoretical studies discussing the enhancement of nonlinear phenomena in slow-light structures. In principle, the slowing down of either the pump or the signal beam is equivalent in terms of Raman gain, but in the presence of losses, we show that they play different roles in determining the net signal gain. We also investigate the impact of the mode profile in realistic slow-light waveguides on the total gain, an effect that is usually neglected in the context of stimulated Raman scattering. By taking representative losses and mode shapes into account, we provide a realistic estimation of the achievable performance of slow-light photonic crystal waveguides.

基于耦合谐振系统的慢光结构及其应用

基于耦合谐振系统的慢光结构及其应用

Silicon coupled-ring resonator structures for slow light applications: potential, impairmentsand ultimate limits

Coupled-ring resonator-based slow light structures are reported and discussed. Bycombining the advantages of high index contrast silicon-on-insulator technology withan efficient thermo-optical activation, they provide an on-chip solution with abandwidth of up to 100 GHz and a slowdown factor of up to 16, as well as a continuousreconfiguration scheme and a fine tunability. The performance of these devices isinvestigated in detail for both static and dynamic operation, in order to evaluate theirpotential in optical signal processing applications at high bit rate. The mainimpairments imposed by fabrication imperfections are also discussed in relation to theslowdown factor. In particular, the analysis of the impact of backscatter, disorderand two-photon absorption on the device transfer function reveals the ultimatelimits of these structures and provides valuable design rules for their optimization.

Nonlinear control of tunneling through an epsilon-near-zero channel

The epsilon-near-zero (ENZ) tunneling phenomenon allows full transmission of waves through a narrow channel even in the presence of a strong geometric mismatch. Here we experimentally demonstrate nonlinear control of the ENZ tunneling by an external field, as well as self-modulation of the transmission resonance due to the incident wave. Using a waveguide section near cut-off frequency as the ENZ system, we introduce a diode with tunable and nonlinear capacitance to demonstrate both these effects. Our results confirm earlier theoretical ideas on using an ENZ channel for dielectric sensing and their potential applications for tunable slow-light structures.

System Performances of On-Chip Silicon Microring Delay Line for RZ, CSRZ, RZ-DB and RZ-AMI Signals

We theoretically study the group-delay characteristics of a silicon microring resonator based on the coupled mode theory, and experimentally demonstrate error-free operations of an on-chip delay line using a silicon-on-insulator (SOI) microring resonator with a 20-mum radius. Four signals of different modulation formats are examined at 5 Gb/s, including return-to-zero (RZ), carrier-suppressed return-to-zero (CSRZ), return-to-zero duobinary (RZ-DB), and return-to-zero alternate-mark-inversion (RZ-AMI). Bit error rate (BER) measurements show that the maximal delay times with error-free operations are 80, 95, 110, and 65 ps, respectively, corresponding to a fractional group delay of ~0.4, ~0.5, ~0.55, and ~0.35. The differences in delay and signal degradations have been investigated based on the signal spectra and pattern dependences. Although the delays are demonstrated in a single ring resonator, the analysis is applicable in slow-light resonance structures such as all-pass filters (APF) and coupled resonator optical waveguides (CROW).

Coded output photonic A/D converter based on photonic crystal slow-light structures

A photonic analog-to-digital converter (PADC) utilizing a slow-light photonic crystal Mach-Zehnder interferometer (MZI) is proposed, to enable the optically coded output of a PADC with reduced device size and power consumption. Assuming an index modulation for the MZI on the Taylor's PADC structure, limiting factors in device size, speed, and effective number of bits are derived considering the signal transition time of the light and the slow light dispersion effects. Details of the device design and results of a time domain assessment of the device performance is described with discussions on the feasibility of sub-mm size, 20GS/s operation of the device having the ENOB (effective number of bits) > 5.

Slow light in photonic crystals

Slow light with a remarkably low group velocity is a promising solution for buffering and time-domain processing of optical signals. It also offers the possibility for spatial compression of optical energy and the enhancement of linear and nonlinear optical effects. Photonic-crystal devices are especially attractive for generating slow light, as they are compatible with on-chip integration and room-temperature operation, and can offer wide-bandwidth and dispersion-free propagation. Here the background theory, recent experimental demonstrations and progress towards tunable slow-light structures based on photonic-band engineering are reviewed. Practical issues related to real devices and their applications are also discussed. The unique properties of wide-bandwidth and dispersion-free propagation in photonic-crystal devices have made them a good candidate for slow-light generation. This article gives the background theory of slow light, as well as an overview of recent experimental demonstrations based on photonic-band engineering.

Fundamental Limit to Linear One-Dimensional Slow Light Structures

Using a new general approach to limits in optical structures that counts orthogonal waves generated by scattering, we derive an upper limit to the number of bits of delay possible in one-dimensional slow light structures that are based on linear optical response to the signal field. The limit is essentially the product of the length of the structure in wavelengths and the largest relative change in dielectric constant anywhere in the structure at any frequency of interest. It holds for refractive index, absorption, or gain variations with arbitrary spectral or spatial form. It is otherwise completely independent of the details of the structure's design, and does not rely on concepts of group velocity or group delay.