Slow Light Effect Research Articles

Triple plasmon-induced transparency and outstanding slow-light in quasi-continuous monolayer graphene structure

We propose a simple quasi-continuous monolayer graphene structure and achieve a dynamically tunable triple plasmon-induced transparency (PIT) effect in the proposed structure. Additional analyses indicate that the proposed structure contains a self-constructed bright-dark-dark mode system. A uniform theoretical model is introduced to investigate the spectral response characteristics and slow light-effects in the proposed system, and the theoretical and the simulated results exhibit high consistency. In addition, the influences of the Fermi level and the carrier mobility of graphene on transmission spectra are discussed. It is found that each PIT window exhibits an independent dynamical adjustability owing to the quasi-continuity of the proposed structure. Finally, the slow-light effects are investigated based on the calculation of the group refractive index and phase shift. It is found that the structure displays excellent slow-light effects near the PIT windows with high-group indices, and the maximum group index of each PIT window exceeds 1000 when the carrier mobility of graphene increases to 3.5 m2 V−1 s−1. The proposed structure has potential to be used in multichannel filters, optical switches, modulators, and slow light devices. Additionally, the established theoretical model lays a theoretical basis for research on multimode coupling effects.

Design and verification a six-band plasmonic metasurface based on plasmon-induced transparency

Abstract In this paper, a metasurface base on plasmon-induced transparency (PIT) effect is verified and measured in 8–54 THz. This metasurface achieves six transmission peaks based on PIT resonances at resonance frequencies: 21.24 THz, 27.96 THz, 32.44 THz, 37.62 THz, 46.1 THz, and 51.12 THz, respectively. Phases near zero and positive group delays of transmission peaks result in slow light effects. In the first group experiments, structural parameter W1 decrease from 18 μ m to 14 μ m , transmission peaks move to lower frequencies and amplitudes enhance. In the second group experiments, structural parameter W2 also reduce, similar resonance behaviors obtain. This proposed PIT-based metasurface demonstrates the potential for applications in sensing, optical modulation, and slow-light devices.

Control of slow-light effect in a metamaterial-loaded Si waveguide.

A metamaterial is an artificial material designed to control the electric permittivity and magnetic permeability freely beyond naturally existing values. A promising application is a slow-light device realized using a combination of optical waveguides and metamaterials. This paper proposes a method to dynamically control the slow-light effect in a metamaterial-loaded Si waveguide. In this method, the slow-light effect (i.e., group index) is controlled by changing the phase of the control light incident on the device from a direction opposite to that of the signal light. The group index of the device could be continuously controlled from 63.6 to 4.2 at a wavelength of 1.55 µm.

Z-scheme heterojunction of SnS2-decorated 3DOM-SrTiO3 for selectively photocatalytic CO2 reduction into CH4

The rapid recombination of photoinduced electron-hole pairs as well as the deficiency of high-energy carriers restricted the redox ability and products selectivity. Herein, the heterojunction of SnS2-decorated three-dimensional ordered macropores (3DOM)-SrTiO3 catalysts were in-situ constructed to provide transmit channel for high-energy electron transmission. The suitable band edges of SnS2 and SrTiO3 contribute to the Z-scheme transfer of photogenerated carrier. The 3DOM structure of SrTiO3-based catalyst possesses the slow light effect for enhancing light adsorption efficiency, and the surface alkalis strontium is benefit to the boosting adsorption for CO2. The in-situ introduced SnS2 decorated on the macroporous wall surface of 3DOM-SrTiO3 altered the primary product from CO to CH4. The Z-scheme electron transfer from SnS2 combining with the holes in SrTiO3 occurred under full spectrum photoexcitation, which improved the excitation and utilization of photogenerated electrons for CO2 multi-electrons reduction. As a result, (SnS2)3/3DOM-SrTiO3 catalyst exhibits higher activity for photocatalytic CO2 reduction to CH4 compared with single SnS2 or 3DOM-SrTiO3, i.e., its yield and selectivity of CH4 are 12.5μmolg-1 h-1 and 74.9%, respectively. The present work proposed the theoretical foundation of Z-scheme heterojunction construction for enhancing photocatalytic activity and selectivity for CO2 conversion.

Polarization-insensitive terahertz spoof localized surface plasmon-induced transparency based on lattice rotational symmetry

In recent years, metasurfaces enabling a slow light effect in the terahertz band have seen considerable achievement. However, most of these advances demonstrated so far are polarization sensitive. In this work, we demonstrate polarization-insensitive terahertz slow light at spoof surface plasmon-induced transparency windows. Two types of metasurfaces based on different lattice layouts, a C2 and a C4 lattice symmetry, are compared. On the one hand, the metasurface with C2 lattice symmetry displayed a 5 ps slow light effect in a transparency window around 0.3 THz. On the other hand, the metasurface with a C4 lattice layout achieves a maximum of 28 ps slow light at 0.3 THz. The coupling coefficient and the damping ratio in the transparency window in the metasurface with C4 lattice symmetry are 5 times higher than in the metasurface with C2 lattice layout. Two eigenmode mode constructive interference introduces a positive group delay in the transparency window in the metasurface with C4 lattice symmetry, whereas the superposition of two eigenmodes in the metasurface with C2 lattice symmetry forms the transparency window without distinct coupling. Our results show that the point group symmetry or lattice structure of a metasurface has a huge impact on the group velocity of terahertz pulses and therefore introduces flexibility in the design of polarization-insensitive slow light devices for terahertz telecommunication application.

An experimental study of a nearly perfect absorber made from a natural hyperbolic material for harvesting solar energy

In this paper, the absorptance of a broadband nearly perfect absorber, consisting of a metal surface covered by a rough Bi2Te3 layer, was measured and the optical properties of such a sample are calculated by using the finite-difference time-domain (FDTD) method. The results show that the measured absorptance of a designed sample with a roughness of 794 nm is larger than 0.94 in the wavelength range of 380–1800 nm, which can also be validated and explained by the calculated results. However, the absorptance of the absorber is affected by the morphology of the Bi2Te3 layer, including the roughness and the bottom width of the Bi2Te3 nanostructures. It is shown that the calculated absorptance of the absorber increases with the increasing roughness of the Bi2Te3 layer made of sharp nanostructures (with increasing cross section in the direction of incident light) if the bottom width of the Bi2Te3 nanostructures is fixed. However, the absorptance of the absorber decreases with the increase of the bottom width of Bi2Te3 nanostructures for absorbers with the same roughness. The underlying mechanisms for perfect absorbers are owing to the slow-light effect and gradient index effect, which require tall nanostructures with suitable bottom width for the total absorption of electromagnetic (EM) waves in the solar wavelength range. Furthermore, different samples were fabricated by using an electrochemical reaction method, and the measured absorption properties of these samples are shown in good agreement with calculated results.

Dynamically tunable dual-band electromagnetically induced transparency-like in terahertz metamaterial

A dynamically tunable dual-band electromagnetically induced transparency-like (EIT-like) metamaterial in terahertz region is proposed. The EIT-like metamaterial consists of a left metal strip (LMS), a right metal strip (RMS) and an asymmetric two-gap rectangular split resonator (ARSR) based on vanadium dioxide film (VO2). Two transparency windows can be observed and the maxima of transmission amplitude are 92.5% and 90.5%, respectively. The surface current distributions can well explain the physical mechanism of the dual-band EIT-like effect. In addition, the influence of structure parameters on dual-band EIT-like effect is also be discussed. The four-level tripod (FLT) system, which is make up of a Λ-system and a control state, can validate the numerical results and explain the mechanism of the dual-band EIT-like effect better. The theoretically fitted transmission spectra are in good agreement with the simulated spectra for different conductivity of VO2. By tuning the conductivity of VO2, dual-band EIT-like effect and slow light effect can be dynamically controlled without the shift of frequency. In addition, the EIT-like metamaterial with low loss can be proved by the effective medium theory. Therefore, our dual-band EIT-like metamaterial is very promising for making multiple-band terahertz functional and slow light devices.

Optical parametric gain in CMOS-compatible sub-100 μm photonic crystal waveguides

Emerging compositionally engineered complementary metal-oxide-semiconductor (CMOS)-compatible platforms have been employed for high efficiencies in various on-chip applications, including optical parametric amplification and wavelength conversion. Combining the novel nonlinear optics platforms such as ultra-silicon-rich nitride (USRN: Si7N3) with periodic waveguide structures can lead to further enhancement of material nonlinearities via the slow light effect and enable ultra-compact devices. Four-wave mixing in a USRN-based, CMOS-compatible, photonic crystal waveguide (PhCWg) leading to on/off optical parametric signal gain reaching 3 dB, and a large instantaneous idler conversion efficiency of −1 dB is explored experimentally. Enhancement of Kerr nonlinearity in the presence of a sizable and near-constant group index allows the findings on an ultra-compact, 97 μm-long PhCWg, equivalent to a large on/off gain per unit length of 333 dB/cm.

Tunable fast to slow light and second-order sideband generation in an optomechanical system with phonon pump

We theoretically demonstrate optomechanically induced transparency (OMIT) and its related coherent optical propagation properties such as fast and slow light effects in a cavity optomechanical system driven by a weak coherent phonon pump for suitable parametric and detuning regimes. The probe transmission spectrum experiences different processes under different detuning regimes, and the numerical simulations indicating a tunable and controllable fast-to-slow light propagation can be achieved by controlling the driving amplitudes and phase of the phonon pump. Further, the second-order sideband (SOS) generation is also demonstrated in the system, and it is found that the efficiency of the SOS generation can be significantly enhanced with manipulating the phonon pump in the resonance regime with numerical simulations. More interestingly, the SOS generation is also sensitive to the detuning (the off-resonance regime), and a robust SOS can be induced and the efficiency of the SOS generation even presents mode splitting which is similar to the linear OMIT. These results lead to good tunability of SOS generation and may find applications in manipulation of light propagation.

Design of a ZnO/Poly(vinylidene fluoride) inverse opal film for photon localization-assisted full solar spectrum photocatalysis

Owing to its photonic band gap (PBG) and slow light effects, aniline black (AB)-poly(vinylidene fluoride) (PVDF) inverse opal (IO) photonic crystal (PC) was constructed to promote the utility of light and realize photothermal synergetic catalysis. As a highly efficient reaction platform with the capability of restricting heat, a microreactor was introduced to further amplify the photothermal effects of near infrared (NIR) radiation. The photocatalytic efficiency of ZnO/0.5AB-PVDF IO (Z0.5A) increases 1.63-fold compared to that of pure ZnO film under a full solar spectrum, indicating the effectiveness of synergetic promotion by slow light and photothermal effects. Moreover, a 5.85-fold increase is achieved by combining Z0.5A with a microreactor compared to the film in a beaker. The photon localization effect of PVDF IO was further exemplified by finite-difference time-domain (FDTD) calculations. In conclusion, photonic crystal-microreactor enhanced photothermal catalysis has immense potential for alleviating the deteriorating water environment.

Tunable plasmon-induced transparency and slow light in terahertz chipscale semiconductor plasmonic waveguides

We numerically propose plasmon-induced transparency (PIT) and the slow light effect based on a terahertz (THz) chipscale plasmonic semiconductor-insulator-semiconductor waveguide system. Via the coupling between two stub resonators, PIT is introduced due to the destructive interference, which can be theoretically described by the transmission-line method. Meanwhile, the strong dispersion within the transparent window will lead to the slow light effect. Via tuning the geometric parameters, the profile of PIT as well as group delay line can be arbitrarily tailored. Moreover, active control of both PIT and slow light can be realized by changing the ambient temperature. Especially, by integrating monolayer graphene into the structure, PIT and slow light can also be electrically modulated via applying voltage. This work provides geometrical, thermal and electrical approaches to manipulate THz transmission and group delay in the subwavelength scale, and can find potential applications as filters, sensors, modulators and active optical delay lines in THz ultracompact circuits.

BER evaluation in a multi-channel graphene-silicon photonic crystal hybrid interconnect: a study of fast- and slow-light effects.

A comprehensive theoretical investigation on the bit-error ratio (BER) performance of multi-channel photonic interconnects operating in pulsed regimes is presented. Specifically, the optical link contains either a silicon photonic crystal (SiPhC) or a SiPhC-graphene (SiPhC-GRA) waveguide, possessing slow-light (SL) and fast-light (FL) regimes. A series of Gaussian pulses plus complex white noise are placed at input of each channel, with output signals demultiplexed and analyzed by a direct-detection receiver. Moreover, a rigorous theoretical model is proposed to measure signal propagation in SiPhC and SiPhC-GRA, which incorporates all crucial linear and nonlinear optical effects, as well as influences of free-carriers and SL effects. BER results of multi-channel systems are evaluated by utilizing the Fourier series Karhunen-Loeve expansion method. Our findings reveal that good BER performance is acquired at SiPhCs and SiPhC-GRAs in SL regimes but with their footprint about 2.5-fold smaller than FL waveguides. Moreover, the enhanced nonlinearity in SiPhC-GRAs induced by strong graphene-SiPhC coupling causes extra signal degradation than SiPhCs at the same length. This work provides additional insights into the coupling effect between SiPhCs operating in SL regimes and graphene, and their influence on WDM signal transmission, highlighting the potential applications of SiPhC-GRA interconnects in next-generation super-computing systems.

Design and modulation of the plasmon-induced transparency based on terahertz metamaterials

In this paper, an electromagnetic metamaterial with single metal layer is proposed and measured based on plasmon-induced transparency effect (PIT). A transmission window with a full width at half maximum (FWHM) 0.17 THz is achieved in 5–8 THz due to the hybridization between bright-bright modes. In this transmission window, the slow-light effect can be found at resonance frequency 6.36THz. Positive group delay and phase delay are obtained by the metamaterial structure. In experiments, this transmission peak is shifted to higher frequency with the radius R increasing, and moved to lower frequency with the lattice period P increasing. Moreover, the transmission peak can be controlled by changing temperature in experiments. This proposed metamaterial shows the the potential for temperature sensing.

High‐Responsivity Mid‐Infrared Black Phosphorus Slow Light Waveguide Photodetector

AbstractBlack phosphorus (BP) offers unique opportunities for mid‐infrared (MIR) waveguide photodetectors due to its narrow direct bandgap and layered lattice structure. Further miniaturization of the photodetector will improve operation speed, signal‐to‐noise ratio, and internal quantum efficiency. However, it is challenging to maintain high responsivities in miniaturized BP waveguide photodetectors because of reduced light–matter interaction lengths. To address this issue, a method utilizing the slow light effect in photonic crystal waveguides (PhCWGs) is proposed and experimentally demonstrated. A shared‐BP photonic system is proposed and utilized to fairly and precisely characterize the slow light enhancement. Close to the band edge around 3.8 µm, the responsivity is enhanced by more than tenfold in the BP photodetector on a 10 µm long PhCWG as compared with the counterpart on a subwavelength grating waveguide. At a 0.5 V bias, the BP PhCWG photodetector achieves a 11.31 A W−1 responsivity and a 0.012 nW Hz−1/2 noise equivalent power. The trap‐induced photoconductive gain is validated as both the dominant photoresponse mechanism and the major limiting factor of the response speed. The BP slow light waveguide photodetector is envisioned to realize miniaturized high‐performance on‐chip MIR systems for widespread applications including environmental monitoring, industrial process control, and medical diagnostics.

Coherently controllable terahertz plasmon-induced transparency using a coupled Fano-Lorentzian metasurface.

Metamaterials have been engineered to achieve electromagnetically induced transparency (EIT)-like behavior, analogous to those in quantum optical systems. These meta-devices are opening new paradigms in terahertz communication, ultra-sensitive sensing and EIT-like anti-reflection. The controlled coupling between a sub-radiant and a super-radiant particle in the unit cells of these metamaterial can enable multiple narrow plasmon induced transparency (PIT) windows over a broad band, with considerable group delay of electromagnetic field (slow light effect). Phase coherence between these PIT windows is highly desired for next-generation multichannel communication network. Herein, we numerically and experimentally validate a controllable frequency hopping mechanism between "slow light" windows in the terahertz (THz) regime. The effective media are composed of plasmonic "molecules" in which an asymmetric split-ring resonator (ASRR) or Fano resonator is displaced on the side of a cut-wire (Lorentz oscillator). Two metasurfaces where ASRR is on opposite side of the cut-wire are investigated. In these two cases, the proximity of the cut-wire to the gap on the ASRR having asymmetry is different. On one side, when the gap is nearer to the cut wire, displacing the ASRR along the cut-wire, produces only one narrow transparency window at 0.8 THz, corresponding to 20 ps group delay. When the ASRR is positioned on the opposite side, such that the gap is further, two transparency windows are observed when the ASRR is displaced along the cut-wire. That is, the transparency window hops from 0.8 THz to 1.2 THz. This corresponds to an increase from 20 to 30 ps in slow light effect. Numerical simulations suggest these single or multiple PIT windows occur if the couplings between the plasmonic modes in the different arrangements are either in-phase or out-of-phase, respectively.

Large angle reciprocal electromagnetically induced transparency on fano resonance in metamaterials

The electromagnetically induced transparency (EIT) effect has been widely studied in metamaterials, which has potential applications in optical storage and sensors due to its less stringent implementation conditions. Especially, EIT with reciprocity is flexibility which is not limited by the incident direction of electromagnetic field. In this paper, a reciprocal and large angle electromagnetically induced transparency (EIT) effect in the terahertz range is proposed. The tri-layer metamaterial structure is consisted of one opening ring and four sectors which represented the bright and dark modes, respectively. The physical mechanism of the EIT effect can be attributed to Fano resonance coupling with the bright and dark modes. Influence of structure parameters on the performance of the structure is discussed. The EIT effect is affected by the length of the gaps and environmental refractive index. In addition, changing the incident angle, the EIT effect always exists until the angle of the incidental electromagnetic wave is 60°. The group delay is calculated and shown potential application in slow light effect. These results have many potential applications for slow light devices, terahertz filtering, large-angle switching and sensors.

Trapping and releasing bidirectional rainbow at terahertz frequencies

Rainbow trapping arisen from slow light effect has been attracted lots of attention for the past two decades. However, until now only unidirectional rainbow trapping has been proposed, which relies on a complicated configuration or technique. Here we propose and investigate a metal–dielectric-semiconductor-dielectric-metal structure, where the semiconductor layer is applied with an external DC magnetic field. We show that in such a waveguide configuration there exist branches of dispersion curves showing slow-light bands at finite propagation constants. The positions of the slow-light bands can be tuned by varying the thickness of the semiconductor or the dielectric layers in the waveguide. By tapering the waveguide, we first demonstrate bidirectional rainbow trapping in such a simple waveguide configuration, and more importantly the trapped rainbow can be easily released by changing the external magnetic field.

Majorana fermions induced Fano resonance and fast-to-slow light in a hybrid semiconductor/superconductor ring device

We demonstrate theoretically the Fano resonance and the conversion from fast to slow light in a hybrid quantum dot (QD)–semiconductor/superconductor (S/S) ring device, where the QD is coupled to a pair of Majorana fermions (MFs) appearing in the hybrid QD–S/S ring device. The absorption spectra of the weak probe field can exhibit a series of asymmetric Fano line shapes, and their related optical propagation properties such as fast- and slow-light effects are investigated based on the hybrid system for suitable parametric regimes. The positions of the Fano resonances can be determined by the parameters, such as different detuning regimes and QD–MFs coupling strengths. Further, the transparency windows (i.e., the absorption dip approaches zero) in the probe absorption spectra are accompanied by the rapid steep dispersion of the Fano resonance profile, which promises the slow- or fast-light effect, and even tunable fast-to-slow light propagation (or vice versa) can be achieved by controlling different parameter regimes. Therefore, Fano resonance may indicate another means to investigate MFs and the investigation may open up promising applications in quantum information processing based on MFs in solid-state devices.

Auxiliary cavity enhanced dipole induced transparency and fast to slow light using cavity quantum electrodynamics in a photonic crystal nanocavity

We investigate the probe-field transmission in a hybrid cavity quantum electrodynamic (CQED) system, where one optical cavity containing a quantum dot (QD) with high cavity dissipation is coupled to another auxiliary cavity with a high quality factor. We also investigate the hybrid system operating in the weak coupling regime of the light-matter interaction via comparing the QD photon interaction with the dipole decay rate and the cavity field decay rate. It is shown that the dipole induced transparency (DIT) regime similar to electromagnetically induced transparency (EIT) can be achieved due to the destructive interference of the cavity field in the weak coupling regime, which is extremely significant for the field of semiconductor CQED. The auxiliary cavity plays a key role in the hybrid system, which affords a quantum channel to affect the probe transmission leading to enhanced DIT. Further, DIT induced coherent optical propagation properties such as fast and slow light effects are also investigated based on the hybrid system for suitable parametric regimes. By controlling the coupling strength J and the decay rate ratio δ of the two cavities, tunable and controllable fast-to-slow light propagation can be achieved. This study provides a promising platform for understanding the dynamics of QD-CQED systems and may open up promising on chip applications in quantum information processing.

Numerical study on slow-light effects in an ultra-compact grating-based Fabry–Perot cavity

In this paper, we propose a slow-light element based on a grating-based Fabry–Perot cavity (GBFPC). Using three-dimensional finite-difference time-domain simulation, the slow-light properties that depend on its structural parameters are systematically investigated. A slow-light effect with the maximum group index about 14.6 and the slow-light bandwidth about 874 GHz at near 1550 nm wavelength is achieved. Moreover, we find that changing of the GBFPC’s structural parameters, such as bus waveguide width W, grating period p, and period number N, has greater impact on its slow-light properties than its other parameters. This study helps lay a solid foundation for designing excellent slow-light elements based on a Fabry–Perot cavity.