Large Group Delay Research Articles

Exceptional points in the microcavity with phonon pump enhance the transparency and slow light.

We theoretically investigated optomechanically induced transparency (OMIT) and slow light in a microcavity optomechanical system containing three nanoparticles, where the pump-probe field drives the cavity and a weak phonon pump drives the mechanical resonator. When the phonon pump frequency matches the pump-probe field frequency difference, adjusting the phonon pump's amplitude and phase can result in the transparency window exceeding unity. Tuning the relative positions of nanoparticles can periodically steer the system to exceptional points (EPs), further enhancing and modulating the transparency window. Furthermore, the phonon pump causes the phase dispersion at the transparency window to become highly steep, resulting in a large value and tunable group delay. Notably, when the system is at EPs, the slow light can be enhanced by approximately two times compared to when the system is not at EPs. Our research demonstrates a way to control optical transmission with potential applications in quantum communications and optical buffers.

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.

An epsilon‐near‐zero circuit based on half‐mode SIW with enhanced integrability for applications of narrowband large group delay

AbstractThe tunneling effect of an epsilon‐near‐zero (ENZ) is exploited to fulfill a narrowband large group delay by using a compact half‐mode substrate integrated waveguide structure built on thin substrate. The ENZ channel possesses a finite input impedance at the tunneling frequency point, and the finite input impedance can be matched to the final I/O port simply by a quarter wavelength impedance transformer. Thus, the enhanced integrability is realized in a planar form in the proposed circuit comparing to the conventional waveguide ENZ channel. Meanwhile, in the real ENZ channel, the effective impedance is observed to be close to 50 Ω, so that the ENZ can be connected with 50 Ω microstrip directly, which has been experimentally validated. The final measured net group delay is 1.36 ns at 3.08 GHz with 3.6 dB insertion loss, and the proposed method provides an alternative for the group delay engineering.

Continuously tunable silicon optical true-time delay lines with a large delay tuning range and a low delay fluctuation.

On-chip switchable optical true-time delay lines (OTTDLs) feature a large group delay tuning range but suffer from a discrete tuning step. OTTDLs with a large delay tuning range and a continuous tuning capability are highly desired. In this paper, we propose and experimentally demonstrate a silicon-based broadband continuously tunable OTTDL comprising a 7-bit delay line and a switch-based continuously tunable delay line. The group delay of the entire OTTDL can be continuously tuned from 0 to 1020.16 ps. A delay error within -1.27 ps to 1.75 ps, and a delay fluctuation of less than 2.69 ps in the frequency range of 2∼25 GHz are obtained. We analyze the causes of the delay fluctuation and its influence on beamforming. Moreover, we also propose a simplified non-invasive calibration method that can significantly reduce the complexity of the delay state calibration and can be easily extended to delay lines with more stages of optical switches. The high performance of our OTTDL chip and the calibration method drive practical applications of integrated OTTDLs.

Small attenuation negative group delay of spoof surface plasmon polaritons

Spoof surface plasmon polariton (SSPP) waveguides can be used to effectively construct on-chip microwave systems due to the single-side conductor structure. SSPP waveguides usually exhibit normal dispersion, which can introduce large group delays to transmission signal and impair waveform shapes. However, the negative group velocity (NGV) effect may occur in the split modes of coupled surface plasmon polariton waveguides. In this regard, the synthesis wave model of forward and backward transmission waves is applied, which generates a quadrupole vortex wave transmission. In this paper, the SSPP unit structure with folded multi-split rings is proposed to achieve anomalous dispersion with NGV feature in the fundamental mode. The SSPP unit can be equivalent to an epsilon-negative medium, and a maximum negative group delay (NGD) can be reached at the frequency where the NGV begins to appear. For the corresponding SSPP waveguide, a NGD band with lower attenuation can be achieved at the stopband edge. While the SSPP unit structures are loaded as dispersive materials in a microstip line, the similar NGD performance can be obtained. Clearly, the SSPP NGD with lower attenuation provides a new delay equalization method for ultra reliable and low latency communications (URLLC), and can also be used as an effective dispersion compensation means.

A Reconfigurable Three-Dimensional Electromagnetically Induced Transparency Metamaterial with Low Loss and Large Group Delay

In this paper, a solid-state plasma (SSP) metamaterial for an analog of the electromagnetically induced transparency phenomenon is designed and investigated. This electromagnetically induced transparency metamaterial has the ability to interact with both incident electric and magnetic fields, and its low-loss characteristics, slow-wave effect, band reconfigurability, and polarization-insensitive characteristics are researched and explored. According to the tunable SSP, we have successfully implemented two modes of operation (mode 1 and mode 2) by whether the SSP resonance unit is excited or not. Low-loss characteristics and polarization-insensitive properties are achieved by rotating the split-ring resonator (SRR) by 180° in the plane and rotating the overall plane framework 90° to form a three-dimensional structure. After that, the maximum group delay of 261.51 ps and 785.09 ps as well as the delay bandwidth product of 17.51 and 62.96 at mode 1 and mode 2, respectively, are discussed respectively. This indicates a good slow-wave effect as well as a high efficiency of communication devices. After all, in mode 1, a transmission peak at 0.541 THz is observed for a transmission ratio of 92.05%; and in mode 2, a transmission peak at 0.741 THz is observed for a transmission ratio of 93.01%, resulting in a bandwidth shift of 0.2 THz. Due to the uniqueness of the developed metamaterial, it holds potential for a wide range of applications in slow-wave devices, modulators, sensors, and communications equipment.

Tunable slow light device based on a graphene metasurface.

Slow light devices have significant applications in memory, switching, and quantum optics. However, the design and fabrication of slow light devices with large tunable group delay are still challenging. Here, a graphene-based slow light device that can electrically modulate the group delay of terahertz (THz) waves is proposed and experimentally demonstrated. The unit cell of the device consists of a U-shaped metal resonator and an Ω-shaped metal resonator, with three graphene ribbons embedded between the two resonators. Under electrical stimuli, a relatively high amplitude modulation depth of 74% is achieved and the maximum transmission amplitude is as high as 0.7 at the transmission peak of 0.6 THz. Most importantly, the maximum group delay variation reaches 5 ps at 0.76 THz and the maximum group delay amplitude is as high as 8.8 ps. The experiment shows good agreement with simulation. This study paves a new way for developing novel switchable nanophotonic devices and slow light devices.

Lasing at a stationary inflection point

The concept of lasers based on the frozen mode regime in active periodic optical waveguides with a 3rd-order exceptional point of degeneracy (EPD) is advanced. The frozen mode regime in a lossless and gainless waveguide is associated with a stationary inflection point (SIP) in the Bloch dispersion relation, where three Bloch eigenmodes coalesce forming the frozen mode. As a practical example, we consider an asymmetric serpentine optical waveguide (ASOW). An ASOW operating near the SIP frequency displays a large group delay of a non-resonant nature that scales as the cube of the waveguide length, leading to a strong gain enhancement when active material is included. Therefore, a laser operating in the close vicinity of an SIP has a gain threshold that scales as a negative cube of the waveguide length. We determine that this scaling law is maintained in the presence of small distributed losses, such as radiation associated with waveguide bends and roughness. In addition, we show that although gain causes a distortion in the modes coalescing at the SIP, the properties of the frozen mode are relatively resistant to such small perturbations and we still observe a large degree of exceptional degeneracy for gain values that bring the system above threshold. Finally, our study also reveals that lasing near an SIP is favored over lasing near a photonic band edge located in close proximity to the SIP. In particular, we observe that an SIP-induced lasing in an ASOW displays lower gain threshold compared to lasing near the photonic regular band edge (RBE), even though the SIP resonance has a lower quality factor than the RBE resonance.

Integrated contra-directionally coupled chirped Bragg grating waveguide with a linear group delay spectrum

Due to the advantages of low propagation loss, wide operation bandwidth, continuous delay tuning, fast tuning speed, and compact footprints, chirped Bragg grating waveguide has great application potential in wideband phased array beamforming systems. However, the disadvantage of large group delay error hinders their practical applications. The nonlinear group delay spectrum is one of the main factors causing large group delay errors. To solve this problem, waveguides with nonlinear gradient widths are adopted in this study to compensate for the nonlinear effect of the grating apodization on the mode effective index. As a result, a linear group delay spectrum is obtained in the experiment, and the group delay error is halved.Graphical

A Hybrid Cladding Ring-Core Photonic Crystal Fibers for OAM Transmission with Weak Spin–Orbit Coupling and Strong Bending Resistance

A hybrid cladding ring-core photonic crystal fiber (PCF) for transmitting orbital angular momentum (OAM) modes is proposed, which breaks the circular symmetry of the fiber structure to suppress the spin–orbit coupling and promotes bending resistance. Through the optimization of fiber structure parameters, the designed fiber can support 22 OAM modes (6 OAM mode groups) over a 200-nm wide bandwidth (covering the whole C + L band) with large effective refractive index separation between adjacent modes (>10−4) and mode groups (>3.6 × 10−3), low confinement losses (<3.5 × 10−9 dB/m), and high mode purity (>98.3%). Meanwhile, the phase of the OAM modes varies periodically and uniformly with an increase in the azimuth angle, and the polarization of OAM modes maintain nearly circular polarization in the designed fiber, which also demonstrates that the fiber has weak spin–orbit coupling. Moreover, the confinement losses of all vector modes are less than 10−7 dB/m when the bending radius is larger than 0.8 mm, indicating strong bending resistance. Furthermore, the fiber also exhibits large differential group delay, relatively low and flat dispersion, and low nonlinear coefficients (<2.0 W−1/km). Therefore, the novel fiber structure has great potential in the application of mode division multiplexing (MDM) based on OAM modes.

Tunable terahertz slow light with hybrid coupling of a magnetic toroidal and electric dipole metasurface

We present a hybrid coupling scheme of a magnetic toroidal and electric dipole metasurface with suppressed radiation loss, which can produce the tunable plasmon-induced transparency (PIT) with an enhanced slow-light effect in the terahertz regime. The terahertz metasurface is constructed by nesting a dual-split ring resonator (DSRR) inside a ring resonator (RR) to exploit the destructive coherence of hybrid electromagnetic mode coupling at the PIT resonance. The polarization-dependence excitation performs the active tunability of a PIT-induced group slowing down by rotating the polarization angle, experimentally achieving a maximum group delay of 3.5 ps. Furthermore, the modified terahertz metasurface with a four-split ring resonator (FSRR) nested in an RR is prepared on photoconductive silicon, demonstrating the pump-controllable group delay effect at the PIT resonance. The large group delay from 2.2 to 0.9 ps is dynamically tunable by adjusting the pump power. The experimental results are in good accord with the theoretical simulations.

Large group delay and low loss optical delay line based on chirped waveguide Bragg gratings.

On-chip optical delay lines (ODLs) based on chirped waveguide Bragg gratings (CWBG) have attracted much attention in recent years. Although CWBGs are well developed, the CWBG which have large group delay (GD), large delay-bandwidth product and low loss while is circulator-free have little been investigated so far. In this work, we propose and experimentally demonstrate such a CWBG-based ODL. This device is fabricated on a low-loss 800-nm-height silicon nitride platform, combining 20.11-cm long index-chirped multi-mode spiral waveguide antisymmetric Bragg gratings with a directional coupler. The bandwidth of this circulator-free ODL is 23 nm. The total GD is 2864 ps and the delay-bandwidth product is 65.87 ns·nm, which both are the largest values achieved by on-chip CWBG reported to our knowledge. Its loss is 1.57 dB/ns and the total insertion loss of the device is 6 dB at the central wavelength near 1550 nm. This integrated CWBG can be explored in practical applications including microwave photonics, temporal optics, and optical communication.

Comb-Like Hybrid Plasmonic Ring Resonator for Large and Voltage Tunable Group Delay

A nanophotonic ring resonator based on hybrid plasmonic waveguide is numerically proposed. Introduction of a comb-like structure in the resonator results in large group delay which can be tuned electrically. The proposed silicon-based nanophotonic structure incorporates comb-like design with copper-air subwavelength grating on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> which supports resonance enhanced propagation of hybrid plasmonic mode confined in 50 nm thick SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The device with its engineered design shows an ultra-large group delay of 208 ps for 1 mm length. The proposed device also exhibits a voltage tunable group delay with a tuning of 48 ps on varying the voltage from −5 V to 5 V. The proposed device with its CMOS compatible and scalable design is useful for integrated optoelectronic circuits. The reported large delay and wide tuning make it a perfect candidate for applications in optical delay lines, signal processing, beam steering in phase array antennas and in microwave photonics.

High-transmission and large group delay terahertz triple-band electromagnetically induced transparency in a metal-perovskite hybrid metasurface.

A high-transmission and large group delay terahertz triple-band electromagnetically induced transparency (EIT) effect is obtained in a metal-perovskite hybrid metasurface, which consists of a cross metal (CM), a pair of square metal frames (SMFs), and a pair of square split rings (SSRs). The results reveal that the transmission amplitudes of three transparent windows are 0.83, 0.9, and 0.89. The maximum values of group delays at three transparent windows are 7.64 ps, 4.07 ps, and 4.27 ps. The multipole scattering theory shows that the first and third transparent windows are created by the coupling between the electric dipole and toroidal dipole, and the second transparent window is created by the electric dipoles. The triple-band EIT effect can be dynamically controlled by adjusting the conductivity of perovskite while the modulation depths are 49.4%, 41%, and 31.5%. Moreover, the slow light effect can also be tunable by tuning the conductivity of perovskite while the modulation depths are 87.8%, 65.6%, and 68.4%. Our study puts forward a novel design concept for multi-band EIT effect and shows great prospects in the application of multi-band devices.

Design of Microstrip Transversal Signal-Interference Circuit With Large Group Delay

This letter presents the group-delay (GD) analysis and design of a microwave stub-loaded structure. This circuit can be designed using transversal signal-interference (TSI) principles, which were previously applied to realize sharp-rejection microwave filters with transmission zeros (TZs) through constructive/destructive signal-interaction effects. The engineered circuit topology consists of a bi-path in-parallel transmission-line (TL) filtering stage loaded with an open-ended TL segment or stub at one of its electrical paths. This type of circuit exploits the spectral phase-slope variation of its transfer function, which is caused by modifying the frequency spacing between the two created notched bands or TZs below/above its main passband to generate a large GD within it. A microstrip prototype of the devised TSI-based GD circuit is built and tested for experimental-validation purposes. It exhibits among the highest GD of 10.3 ns at its operational frequency of 1 GHz compared to prior-art large-GD structures reviewed in the open literature.

Slow light generation by using one-dimensional photonic crystals for quantum memory applications

In this study, an efficient and compact optical device for slowing light in corrugated photonic crystals with different corrugation patterns are discussed. The proposed structure shows relatively large group delay with wide bandwidth and approximately zero group velocity dispersion in near-infrared region. Also, due to the zero group velocity dispersion applied on transmitted pulse, high quality pulse can be obtained by using this approach. For comparison, three different photonic crystal structures containing triangular, sinusoidal and graded sinusoidal corrugation patterns were investigated. The group index as much as 5 with the bandwidth about 50 nm is achieved in the sinusoidal corrugated photonic crystal with 8 μm length. This slow-light structure is very promising for application in quantum memories.

Compact optical grating compressor.

A novel design of a grating-based optical pulse compressor is proposed. The proposed compressor provides a large group delay dispersion while keeping the compressor linear size small. The design of the proposed compressor is based on a traditional Treacy compressor with a straightforward modification of inserting two lenses between the compressor's gratings. This simple alternation aims to substantially increase group delay dispersion of the compressor or alternatively to decrease the compressor size while maintaining its group delay dispersion. A theoretical description of the enhanced compressor has been developed in the paraxial approximation. A detailed numerical model has been built to calculate the compressor parameters more accurately. These theoretical studies have revealed that the enhanced optical compressor provides a significant increase in the group delay dispersion compared to a standard Treacy compressor.

Investigation of the characteristics of the electromagnetic induction transparent-like spectrum with counter-propagating waves coupling mechanism for waveguide and micro-ring coupled system

In this paper, a new counter-propagation waves coupling mechanism is proposed which is expected to realize an electromagnetically induced transparency (EIT)-like effect. Comparing the travelling waves coupling mechanism (see J. Mod. Opt. 2015,62:313-320 [9]) with the counter-propagating waves coupling mechanism, we find out that the transparency window breadths of transmission spectra are greatly enhanced and the corresponding phase shift spectra possess a flat profile or a square profile. Our numerical simulated results are in good agreement with the theoretical analysis. The EIT-like effect can significantly reduce the group velocity near the edge of the square profile transparent window. We believe that the counter-propagating waves coupling mechanism is particularly beneficial for the realization of active manipulation of slow light devices (such as delay lines) required in the conventional EIT scheme. In the vicinity of the transparency peak, we can obtain a large group delay, may gain more significant potential applications in slow-light transmission and optical storage.

Low bending loss few-mode hollow-core anti-resonant fiber with glass-sheet conjoined nested tubes.

A novel hollow-core anti-resonant fiber (HC-ARF) with glass-sheet conjoined nested tubes that supports five core modes of LP01-LP31 with low mode couplings, large differential group delays (DGDs), and low bending losses (BLs) is proposed. A novel cladding structure with glass-sheet conjoined nested tubes (CNT) is induced for the proposed HC-ARF which can suppress mode couplings between the LP01-LP31 modes and the cladding modes. The higher-order modes (HOMs) which are LP11-LP31 modes also have very low loss by optimizing the radius of the nested tube and the core radius. Moreover, the large effective refractive index differences Δneff between HOMs are all larger than 1 × 10-4 which contributes to a large DGD in the wavelength range from 1.3 to 1.7 µm. The bending loss of the HC-ARF is analyzed and optimized emphatically. Our calculation results show that bending losses of LP01-LP31 modes are all lower than 3.0 × 10-4 dB/m in the wavelength range from 1.4 to 1.61 µm even when the fiber bending radius of the HC-ARF is 6 cm.

Effects of quantum interference in multiwaveguide systems bridged by Jaynes-Cummings emitters

Quantum interference, responsible for a number of resonant optical phenomena, always intrigues researchers because of its application in optical devices. This work studies it in multiwaveguide systems bridged by Jaynes-Cummings emitters (JCEs) based on the scattering matrix theory. Two types of quantum interference are distinguished here. The first is between the incident wave and those scattered from the JCEs, while the second is only among those scattered waves. The first type leads to the two transmission valleys at the two eigenfrequencies of the JCEs in single-waveguide--single-JCE coupled systems. However, the second type is responsible for the narrow transmission peaks in single-waveguide--multi-JCE coupled systems, locating in the above transmission valleys. This work first shows the properties of the second type of quantum interference in detail and then discusses its two applications. On the one hand, the second type of quantum interference can be used to tailor the transmission spectra to achieve the electromagnetically-induced-transparency-like line shapes with a large group delay. On the other hand, it can also lead to the single-photon jumping between two certain waveguides with a nearly $100%$ chance in multiwaveguide systems by switching on or off some of the couplings of the JCEs to the waveguides. These applications with small peak widths commonly require a small loss and might have potential in quantum informatics.