Slow Light Effect Research Articles

A triple plasmon-induced transparency terahertz sensor based on graphene metamaterials

A metamaterial-based terahertz sensor design is presented in this paper. The metamaterial unit structure comprises a graphene cross and three sets of graphene strips, which collectively achieve a triple plasmon-induced transparency (PIT) in both polarization directions. The theoretical results based on the Lorentz resonance theoretical model are in good agreement with finite difference time domain (FDTD) simulations. The sensor is polarization-independent and changes in the angle of incidence between 0° and 10° have a negligible impact on the sensing performance. Furthermore, the sensor exhibits a notable slow-light effect, with a maximum sensitivity of 1.65 THz/RIU and a FOM value of 4.74/RIU for the triple PIT, which is superior to that of comparable devices.

Tunable phonon-photon coupling induces double MMIT and enhances slow light in an atom-opto-magnomechanics

Abstract In this paper, we theoretically investigate the magnomechanically induced transparency phenomenon, Fano resonance, and the slow/fast light effect in the situation where an atomic ensemble is placed inside the hybrid cavity of an opto-magnomechanical system. The system is driven by dual optical and phononic drives. We show double magnomechanically induced transparency (MMIT) in the probe output spectrum by exploiting the phonon-photon coupling strength. Then, we study the effects of the decay rate of the cavity and the atomic ensemble on magnomechanically induced transparency. In addition, we demonstrate that effective detuning of the cavity field frequency changes the transparency window from a symmetrical to an asymmetrical profile, resembling Fano resonances. Further, the fast and slow light effects in the system are explored. Besides, we show that the slow light profile is enhanced by adjusting the phonon-photon coupling strength. This result may have potential applications in quantum information processing and communication.

Silicon photonics foundry fabricated, slow-light enhanced, low power thermal phase shifter

In this research, we developed a low-power silicon photonics foundry-fabricated slow-light thermal phase shifter (SLTPS) where the slow-light (SL) effect is achieved using an integrated Bragg grating (BG) waveguide. Heating the grating induces a red shift in the transmission spectrum, leading to an increased group index ng during operation, which facilitates a further reduction in the voltage needed for a π phase shift, i.e., Vπ. Additionally, we investigated a compact Mach–Zehnder Interferometer (MZI) that incorporates the SLTPS in both arms with a phase shifter length of 50 μm. A detailed theoretical analysis was conducted to address the non-idealities of the SL-MZI due to uneven optical power splitting and unbalanced loss in the two MZI arms. Vπ and power consumption for a π phase shift (Pπ) of the SL-MZI were quantified for operation in the slow-light regime, demonstrating a Vπ of 1.1 V and a Pπ of 3.63 mW at an operational wavelength near the photonic band edge. The figure of merit Pπ×τ is commonly used to assess the performance of thermal optical switches. The SL-MZI in this work has achieved a low Pπ×τ of 5.1 mW μs with an insertion loss of 4.4 dB, indicating a trade-off with the Vπ reduction.

Freestanding Germanium Photonic Crystal Waveguide for a Highly Sensitive and Compact Mid-Infrared On-Chip Gas Sensor.

The performance of mid-infrared (MIR) on-chip gas sensors, operating via laser absorption spectroscopy, hinges critically on light-matter interaction dynamics, significantly influenced by external confinement and the effective light path length. Conventional on-chip sensors, however, face challenges in achieving the required limit of detection for highly sensitive applications, primarily due to their intrinsically short effective light path. Furthermore, these sensors are limited in their spectral range coverage within the MIR spectrum by the constraints of standard silicon-based platforms. To overcome these limitations, our research presents a novel approach to fabricate a freestanding germanium (Ge) photonic crystal waveguide (PCW) on a germanium-on-insulator (Ge-OI) platform, utilizing yttrium oxide (Y2O3) as the buried oxide layer. This device leverages the broad transparent windows of Ge and Y2O3, broadening the spectral coverage across the MIR range. The introduction of the PCW and its slow light effect significantly elevate external confinement and light-matter interactions, enabling a notable reduction in waveguide length, which traditionally limits on-chip configurations. The freestanding structure not only expands the sensing region and enhances external confinement but also prevents the emergence of leaky modes within the PCW. As a result, our compact sensor achieves an exceptionally low LoD of 7.56 ppm for carbon dioxide (CO2) sensing at the operational wavelength of 4.23 μm, with a compact waveguide length of only 800 μm.

Tunable and polarization-dependent electromagnetically induced transparency analogy in graphene-based terahertz metasurfaces

In this paper, we theoretically and numerically demonstrate a tunable and polarization-dependent electromagnetically induced transparency analogy based on graphene terahertz metasurfaces. The unit cell of the metasurface consists of three-layer graphene strips embedded in a silicon grating. The dynamic adjustment of the transparent window can be achieved by changing the coupling distance between the graphene layers and the polarization direction of the incident lights. The operation mechanism behind the phenomenon can be attributed to the near-field interaction and electromagnetic coupling of modes in graphene strips. Furthermore, the full wave electromagnetic simulations obtained by the finite-difference time-domain method agree well with the theoretical fitting results based on the three-harmonic oscillator model. In addition, by changing the Fermi levels, it can not only realize the outstanding slow-light effects with a maximum group index of 3750 but also obtain the four-frequency asynchronous optical switch function in terahertz regions. Therefore, our proposed metamaterial device may have potential applications in image switching, optical switches, slow-light device, optical communication, and optical storage.

ZIF-8-CdS photonic crystal beads for slow photon effect induced enhanced photocatalysts

The unique periodic structure of photonic crystals demonstrates an effective slow photon effect that enhances the interaction between light and matter. This study designed and prepared MOF photonic crystal beads, and significantly improved the photocatalytic efficiency by utilizing the slow light effect. Utilizing microfluidic technology, ZIF-8 was self-assembled to form photonic crystal beads (PCB-Z) to enhance adsorption properties and light absorption. The surface of PCB-Z is modified with CdS to form a heterostructure (PCB-ZC), which widens the absorption spectrum, reduces the carrier recombination rate, and increases the specific surface area, while the photocurrent intensity of PCB-ZC is about 1.8 and 29 times that of CdS and PCB-Z. Under the combined effect of photonic crystal structure, ZIF-8/CdS heterojunction and slow photon effect, the degradation rate of Rhodamine B by PCB-ZC was 15.22 times higher than the disordered ZIF-8 powder, and the photocatalytic performance was significantly improved. The dominant role of holes in Rhodamine B degradation was confirmed by free radical trapping experiments and EPR tests. This research confirms that MOF-based photonic crystals can be assembled into periodic structures to enhance adsorption properties and light absorption, which provides a new paradigm for the development of MOF-based photocatalysts.

Triple plasmon induced transparency based on multilayer graphene metamaterials

In this study, a novel multilayer terahertz metamaterial with four graphene sub-structures is designed to achieve dynamically tunable triple plasmon-induced transparency (PIT) due to interference among bright-bright modes. Coupled mode theory (CMT) and finite-difference time-domain (FDTD) simulations show a high degree of consistency in their results. Interestingly, enhanced transmission dips are observed when various graphene structures couple with the two longitudinal graphene strips at the bottom layer. Based on the dynamic modulation characteristics of graphene, the metamaterial demonstrates seven-band optical switching capabilities, achieving modulation depths (MD) of 94.6%, 88.1%, 90.8%, 90.8%, 90.4%, 86.6%, and 87.5%. Additionally, the proposed graphene metamaterial also shows excellent slow-light effects, with a group index reaching up to 1034. This proposed terahertz metamaterial holds significant theoretical implications for the development of dynamically integrated terahertz optoelectronic devices.

Controllable dual resonances of Fano and EIT in a graphene-loaded all-dielectric GaAs metasurface and its sensing and slow-light applications

Abstract Active nanophotonic metasurfaces have attracted considerable attention for their promise to develop compact, tunable optical metadevices with advanced functions. In this work, we theoretically demonstrated the dynamically controllable dual resonances of Fano and electromagnetically induced transparency (EIT) using a graphene-loaded all-dielectric metasurface with U-shaped gallium arsenide (GaAs) nanobars operating in the near-infrared region. The destructive interference between a subradiant mode (i.e., a dark mode) supported by two vertical GaAs bars and two radiative modes (i.e., two bright modes) supported by a horizontal GaAs nanobar gives rise to a Fano resonance and an EIT window with high transmission and a large quality factor (Q-factor) in the transmission spectrum. Importantly, the transmission amplitudes can be flexibly modulated by adjusting the graphene Fermi levels without rebuilding the nanostructures. This modulation results from the controllable light absorption by the loaded graphene monolayer due to its interband losses in the near-infrared spectrum. Furthermore, the peak wavelengths of the Fano resonance and EIT window with high Q-factors are highly sensitive to variations in the refractive index (RI) of the surrounding medium, giving the proposed metasurface a relatively good sensitivity of ~700 nm/RIU and a high figure of merit (FOM) of 280, making it an effective RI sensor. Additionally, the metasurface features an adjustable slow light effect, indicated by the adjusted group delay time ranging from 0.12 ps to 0.38 ps. Therefore, the metasurface system proposed in this work offers a viable platform for advanced multi-band optical sensing, low-loss slow light devices, switches, and potential applications in nonlinear optical fields.

Enhancing phase modulation and group delay in reflective Huygens metasurfaces via a Fabry-Perot cavity

Huygens metasurfaces exhibit excellent optical properties such as 2π phase modulation and slow light effects. However, they face challenges including wide bandwidth and low group delay due to their high radiation losses. Here, we propose a reflective Huygens metasurface coupled with an F-P cavity. We demonstrate that F-P resonance modes can couple with magnetic-quasi-bound-state (M-QBIC) and electric-quasi-bound-state (E-QBIC) in the Huygens metasurface through constructive interference, significantly enhancing the quality factors of both QBICs. Through structural parameter optimization, our reflective Huygens metasurface achieves 4π phase modulation and a high group delay of up to 166 ps. Compared to the non-coupled Huygens metasurface with the same structural asymmetry, the group delay of the F-P coupled reflective Huygens metasurface is enhanced by up to 30 times. Our design reduces the fabrication precision requirements for Huygens metasurfaces, enabling similar group delays to be achieved in low-symmetry coupling structures as in highly symmetric non-coupling structures. Additionally, the performance of this metasurface shows robustness to changes in incident light polarization. This design highlights the potential for achieving high-quality factors, large phase modulation, and large group delay, offering new avenues for the design of highly sensitive tunable devices, efficient nonlinear optical devices, and narrowband slow light devices.

Adjustable slow light with high group index in a graphene metasurface based on plasmon-induced transparency

A relatively simple metasurface structure is proposed to achieve a dynamically tunable slow light effect. The metasurface consists of two horizontal graphene strips and two vertical continuous graphene strips. Strong interference between the bright and dark modes enables the metasurface to generate a significant triple plasmon-induced transparency (PIT). Varying the coupling distance between the horizontal strips, double-PIT and triple-PIT can be transformed into each other. During the reduction of the transparent window, electromagnetic waves undergo strong phase changes, resulting in a higher group index. After structural optimization, the maximum time delay and group index can be as high as 2.624 ps and 3934, surpassing comparable slow light devices. The proposed patterned graphene metasurface provides theoretical guidance for designing high-performance slow light devices for optical storage, nonlinear optics and quantum optics.

Reconfigurable multi-band electromagnetically induced transparency metamaterial based on graphene

By utilizing the monolayer graphene, we propose a reconfigurable multiband EIT 3D structure in the THz region, which exhibits eight consecutive transparency windows spanning from 1.16 to 2.80 THz, and the according transmission intensities were in the range of 0.75–1.0. Furthermore, with the help of graphene and VO2, the transmission curves can be modulated efficiently. The research results demonstrate that the structure proposed can generate high-intensity slow light effects at multiple frequency points within the terahertz range. Compared to existing research on EIT metamaterials, this structure offers advantages such as operation in multiple frequency bands, high transmission coefficients, and flexible modulation capabilities. Therefore, this study helps design novel tunable THz devices.

Photoreduction of carbon dioxide into methane and ethylene using TiO2 inverse opal-derived PCN-222(M)

Solar-driven photocatalytic CO2 reduction has significant potential to address the energy crisis and mitigate greenhouse gas emissions by producing valuable organic compounds. Narrow bandgap materials enhance photon absorption within the visible light spectrum, thereby improving exciton generation. Both the inverse opal (IOT) and the porphyrin-based MOF (PCN-222) exhibit biomimetic design concepts. The IOT has a highly ordered porous structure that increases the light-absorbing surface area, facilitating efficient light harvesting. Conversely, the PCN-222 structure, which is modelled on biological porphyrin molecules, has a highly ordered pore structure and abundant active centers. Under hydrothermal conditions, PCN-222 self-assembles on the IOT surface to form a P(Cu, Fe, Zn)/IOT heterostructure. The tight interface of this heterostructure promotes rapid electron-hole separation and transport. In addition, the slow-light effect is exploited to increase the light absorption efficiency, thus improving the overall photocatalytic reaction efficiency and stability. In this study, the morphology, structure and photoelectric properties of P(Cu, Fe, Zn)/IOT heterostructures are investigated, revealing enhanced photocatalytic efficiency. The P(Fe)/IOT composite demonstrated outstanding CO2 conversion rates under simulated sunlight, yielding carbon monoxide (62 %), methane (0.11 %), and ethylene (0.09 %). The Z-scheme charge transfer mechanism significantly enhances photocatalytic performance by improving the efficiency of carrier separation and transfer, thereby increasing the overall photocatalytic reaction efficiency. This advance offers significant benefits for environmental and renewable energy remediation. The study provides key insights and recommendations for enhancing the efficiency of Z-scheme photocatalytic systems in future research and development.

Enhanced Fluorescence Based on Slow Light Effect of ZIF-8 Photonic Crystals for Trace 2,4,6-Trinitrophenol Detection.

In response to growing concerns about public safety and environmental conservation, it is essential to develop a precise identification method for trace explosives. To improve the stability and detection sensitivity of perovskite quantum dots (PQDs) and address the issue of low porosity in traditional polymer-based photonic crystals (PhCs), this study proposed a PQD photoluminescence (PL) enhancement strategy based on the slow light effect of ZIF-8 PhCs for highly sensitive, selective, and convenient detection of 2,4,6-trinitrophenol (TNP). The slow light effect at the photonic band gap edge is the basis of amplifying the PL signal. PhCs were fabricated by the evaporation-induced self-assembly method. The diffraction wavelength overlapping the whole visible region was designed to match the emission wavelength of PQDs. Results showed that PhCs matching the PBG edge with PQDs' emission peak amplified the PL signal 11.3 times, significantly improving sensitivity for trace TNP detection with a limit as low as 2.52 nM. Moreover, there was a 13.3-fold enhancement of PQDs' fluorescence lifetime when the emission wavelength fell in the PBG range. The hydrophobic surface of ZIF-8 PhCs enhanced the PQDs' stability and moisture resistance. Furthermore, the selective quenching mechanism of TNP by the sensor was photoinduced electron transfer (PET) verified by DFT calculations and time-resolved PL decay dynamics measurements. This study demonstrated great potential for manipulating light emission enhancement by PhCs in developing efficient fluorescent sensors for trace environmental pollutant detection.

Tunable triple plasmon-induced transparency in E-type graphene metamaterials.

Enhancing light-matter interaction is crucial for boosting the performance of nanophotonic devices, which can be achieved via plasmon-induced transparency (PIT). This study introduces what we believe to be a novel E-type metamaterial structure crafted from a single graphene layer. The structure, comprising a longitudinal graphene ribbon and three horizontal graphene strips, leverages destructive interference at terahertz frequencies to manifest triple plasmon-induced transparency (triple-PIT). Through a comparison of simulations using the finite difference time domain (FDTD) method and theoretical coupled-mode calculations, we elucidate the physical mechanism behind triple-PIT. Our analysis shows that the PIT effect arises from the interplay between two single-PITs phenomena, further explored through field distribution studies. Additionally, we investigate the impact of varying Fermi levels and carrier mobility on the transmission spectrum, achieving amplitude modulation in photoelectric switches of 85.5%, 99.2%, and 93.8% at a carrier mobility of 2 m2/(V·s). Moreover, we explore the relationship between Fermi levels and carrier mobility concerning the slow light effect, discovering a potential group index of up to 1021 for the structure. These insights underscore the significant potential of this graphene-based metamaterial structure in enhancing optical switches, modulators, and slow light devices.

Electromagnetically induced transparent bimodal to unimodal conversion realized by the nematic liquid crystal

In this paper, under the action of a gigahertz (GHz) band circularly polarized (CP) wave, a kind of CP metastructure (MS) is proposed. Recently, the nematic liquid crystal (NLC) has received great attention, but it is still difficult to realize the electromagnetically induced transparency (EIT) bimodal to unimodal under the control of NLC. Through the direct mutual coupling of the metal cross resonator, metal ring resonator, and dielectric cross resonator, modes coupling can be formed and generate transparent windows. Two different EIT states (unimodal and bimodal EIT) accompanied by slow light effects can be observed by regulating the NLC with the applied bias voltage (V bias). When the V bias is 0 V, two transparent windows can be achieved in the 6.595–8.443 GHz and 8.443–10.161 GHz bands. When V bias is 20 V, only one transparent window can be achieved in the 6.891–8.682 GHz band. Due to the high symmetry, the MS has the polarization insensitivity to CP waves. These extremely excellent properties make this MS have a wide application prospect. Noteworthy, the theoretical calculation result of EIT is verified by the two-oscillator theory model and circuit model, the results obtained are basically consistent with the simulation results.

Multifunctional terahertz device based on plasmon-induced transparency

Enhancing light-matter interaction is crucial in optics for boosting nanophotonic device performance, which can be achieved via plasmon-induced transparency (PIT). In this study, a polarization-insensitive PIT effect at terahertz frequencies is achieved using a novel metasurface composed of a cross-shaped graphene structure surrounded by four graphene strips. The high symmetry of this metasurface ensures its insensitivity to changes in the polarization angle of incident light. The PIT effect, stemming from the coupling of graphene bright modes, was explored through finite difference time domain (FDTD) simulations and coupled mode theory (CMT) analysis. By tuning the Fermi level in graphene, we effectively modulated the PIT transparent window, achieving high-performance optical switching with a modulation depth (88.9% < MD < 98.0%) and insertion losses (0.17 dB < IL < 0.51 dB) at a carrier mobility of 2 m2/(V·s). Furthermore, the impact of graphene carrier mobility on the slow-light effect was examined, revealing that increasing the carrier mobility from 0.5 m2/(V·s) to 3 m2/(V·s) boosts the group index from 126 to 781. These findings highlight the potential for developing versatile terahertz devices, such as optical switches and slow-light apparatus.

Asymmetric frequency multiplexing topological devices based on a floating edge band

Topological photonics provides a platform for robust energy transport regardless of sharp corners and defects. Recently, the frequency multiplexing topological devices have attracted much attention due to the ability to separate optical signals by wavelength and hence the potential application in optical communication systems. Existing frequency multiplexing topological devices are generally based on the slow light effect. However, the resulting static local spatial mode or finely tuned flat band has zero-group velocity, making it difficult for both experimental excitation and channel out-coupling. Here, we propose and experimentally demonstrate an alternative prototype of asymmetric frequency multiplexing devices including a topological rainbow and frequency router based on floating topological edge mode (instead of localized ones); hence the multiple wavelength channels can be collectively excited with a point source and efficiently routed to separate output ports. The channel separation in our design is achieved by gradually tuning the band gap truncation on a topological edge band over a wide range of frequencies. A crucial feature lies in that the topological edge band is detached from bulk states and floating within the upper and lower photonic band gaps. More interestingly, due to the sandwiched morphology of the edge band, the top and bottom band gaps will each truncate into transport channels that support topological propagation towards opposite directions, and the asymmetrical transportation is realized for the frequency multiplexing topological devices.

Multiple optomechanically induced transparency generated slow light in an array of optomechanical system

We present an array of optomechanical systems which consist of an optical cavity and three nanomechanical resonators. Introducing Jaynes-Cumming (J-C) coupling in the nearest neighbor resonator allows for the formation of optomechanical systems with arrays of N nanomechanical resonators. Using a strong pump field and a weak probe field to drive the optical cavity simultaneously, we investigated the coherent optical response and the probe transmission spectrum manifested as multiple optomechanically induced transparency. We illustrate the impact of probe-cavity detuning, J-C coupling strength, and the number of nanomechanical resonators on the transparent window. In particular, we analyze nanomechanical resonators and find that increasing their number results in multiple transparent windows and slow light at different frequencies. We also find that the slow light effect can be enhanced by increasing the number of nanomechanical resonators with odd numbers at resonance. These findings imply that the arrays of optomechanical system can be used for multichannel optical communications and multichannel quantum information processing.

Double ultraviolet to visible high-Q magnetic plasmon induced reflection with ultralarge Rabi splitting for optical detecting

We theoretically explore double ultraviolet (UV) to visible high-Q magnetic plasmon induced reflections (PIRs) through the coupling between the fundamental magnetic plasmon (MP) mode in single aluminium (Al) nanogroove and the first (second) order surface plasmon polaritons (SPPs) launched by periodic nanogroove arrays, which is predicted analytically by a Fano interference model. The ultrastrong coupling effects between MP and SPP with ultralarge Rabi splitting energy up to 868 meV and 842 meV are obtained in the UV and visible ranges. Due to the high Q-factors of the double PIRs, a significant slow light effect is achieved with the group index up to 65 and 77 in the UV and visible ranges, respectively. The double PIRs show the high sensitivity of 510 nm/RIU and the figure of merit (FoM) of 42, and could be used to monitor analyte layer with a few nanometers, suggesting great potential for label-free biosensing.

Active dual-control electromagnetically induced transparency analog in metal- vanadium dioxide-photosensitive silicon hybrid metamaterial

We investigate an active dual-control metamaterial leveraging electromagnetically induced transparency (EIT), exploiting near-field interactions between electric and magnetic dipole resonances. Our hybrid strip element, combining metal and vanadium dioxide, generates electric dipole resonance, while split-ring resonators integrating metal and photosensitive silicon induce magnetic dipole resonance. Simulations confirm coupling validity and demonstrate dynamic adjustability of EIT via temperature and light intensity changes. EIT modulation transitions between transparent and non-resonant states due to temperature fluctuations, or resonant states with varying light intensity. Temperature adjustments dominate when both factors are altered. Analysis via a coupled oscillator model reveals modulation of damping rates as the origin of disappearance curve variations. This innovative design enhances tunable EIT metamaterial versatility, with implications for high transmission ratios and adaptable slow-light effects in terahertz applications.