Delay-bandwidth Product Research Articles

Research on slow light transmission with wide bandwidth and large normalized delay bandwidth product

Research on slow light transmission with wide bandwidth and large normalized delay bandwidth product

Frozen mode in coupled silicon ridge waveguides for optical true time delay applications

We propose a simple photonic waveguide structure that exhibits light propagation modes with vanishing group velocity via mode degeneracy. This enables stationary inflection point dispersion leading to the frozen mode and a true time delay device suitable for ultra-wide-band beamforming for millimeter wave (mmWave) phased arrays. The structure consists of three silicon ridge waveguides in proximity with periodic gaps introduced in the outer waveguides to create a band gap. The structure is complementary metal–oxide–semiconductor compatible with a very small footprint of only about 56 µ m 2 and more resilient to fabrication uncertainties as compared to the previously studied structures. Simulation results show transmission of 70% of the incident wave for the frozen mode at the 1.55 µm (193.6 THz) wavelength through the waveguide. It also enables a delay-bandwidth product of 6.75 along with unprecedented frequency independent bandwidth of about 0.5 THz for RF-mmWave–terahertz beamforming.

Dynamically tunable slow light characteristics in graphene based terahertz metasurfaces

We propose a graphene based metasurface exhibiting dynamic slow light behavior via electromagnetically induced transparency (EIT) effect in the terahertz regime. The unit cell of the metasurface consists of bright and dark graphene rod resonators in a planar orthogonal configuration. The onset and tunability of the EIT effects are investigated through manipulating the near field electromagnetic coupling among the bright and dark resonators. An analytical method based on three level Lorentz oscillator model is employed to validate the simulated results. Further, we investigate the dynamic tunability of EIT effect by altering the Fermi energy of the graphene resonators. A blue shift of 0.59 THz in the EIT transparency peak is observed as the Fermi energy is increased from 0.3 eV to 1.0 eV. Similar dynamic tunability is investigated for the slow light characteristics (i.e., group delay, group index & group velocity) along with the delay bandwidth product. It is observed that an increase in the Fermi energy ultimately leads to an increase in group delay, group index & delay bandwidth product with their corresponding maximum values as 0.76 ps, 22.78 & 0.14 respectively for Fermi energy as 1.0 eV. For this case, the normalized group velocity reaches a minimum value of 0.04, thus, confirming the dynamic tunability of slow light behavior in the studied metasurfaces. Our findings can lead to the development of active slow light components like modulators, sensors, filters, buffers, ultrafast switches, and compact delay lines etc. in the terahertz domain.

A Full-Duplex Receiver With True-Time-Delay Cancelers Based on Switched-Capacitor-Networks Operating Beyond the Delay–Bandwidth Limit

Wideband self-interference cancellation (SIC) in full-duplex (FD) radios requires the achievement of large delays to accurately emulate the SI channel. However, compact, power-efficient, low-loss/noise/distortion nanosecond-scale delays are extremely challenging to achieve on silicon. Passive transmission lines on silicon are lossy and area-intensive and exhibit reduced bandwidths when miniaturized using inductors and capacitors, whereas active approaches are noisy and power-hungry. In this work, we present a technique that leverages switched-capacitor circuits with multiphase clocking to obtain large on-chip delays over wide bandwidths with the low area and power consumption, thus exceeding the delay-bandwidth product (DBW) limits offered by conventional linear time-invariant (LTI) circuits. This technique is demonstrated in an FD receiver with time-interleaved switched-capacitor-based delay cells in RF and BB domains. The FD receiver is implemented in a standard 65-nm CMOS process and operates from 100 MHz-1 GHz with gain tunability of 15-38 dB, a noise figure of 5.4 dB, and power consumption of 31 mW. The RF/BB canceler delay cells have real-/complex-valued weighting with delays ranging from 0.2-1.1 ns/10-75 ns while consuming 25.5 and 6.5 mW, respectively. These large tunable delays perform FIR-filtering-based cancellation, enabling 30-35-dB integrated SI cancellation over 20 MHz on top of an off-the-shelf ferrite circulator when terminated by a dipole antenna (isolation of 22 dB), and can handle TX power of up to +9 dBm. Under SIC, the RF and BB cancelers degrade the RX noise figure by 1.1 and 0.8 dB, respectively.

Reconfigurable electromagnetically induced transparency metamaterial simultaneously coupled with the incident electric and magnetic fields

In this paper, an electromagnetically induced transparency metamaterial simultaneously coupled with the incident electric and magnetic fields is designed and presented theoretically, whereas its reconfigurability, slow-wave effect, low-loss, and polarization insensitivity are analyzed and discussed principally. Based on the tunable solid-state plasma, there is a transmission peak with a 92.06% transmission at 0.544 THz in State 1 and a transmission peak with a 92.84% transmission at 0.7535 THz in State 2, thus achieving a frequency shift of 0.2085 THz. The maximum group delay, group index, and delay-bandwidth product in State 1 or 2, which are 723.7 ps or 494.7 ps, 1024.1 or 700.1, and 48.5 or 36.1, respectively, and the excellent slow-wave effects are discussed. In addition, the low-loss and polarization insensitivity are realized by rotating the split-ring resonators 180° and twisting the planar structure 90°. Considering the unique features of the designed metamaterial, it can be extensively applied to slow-light devices, communication, sensors, and nonlinear devices.

具有宽带宽和高归一化延迟带宽积的光子晶体波导慢光传输

具有宽带宽和高归一化延迟带宽积的光子晶体波导慢光传输

Free-space optical delay line using space-time wave packets

An optical buffer featuring a large delay-bandwidth-product—a critical component for future all-optical communications networks—remains elusive. Central to its realization is a controllable inline optical delay line, previously accomplished via engineered dispersion in optical materials or photonic structures constrained by a low delay-bandwidth product. Here we show that space-time wave packets whose group velocity is continuously tunable in free space provide a versatile platform for constructing inline optical delay lines. By spatio-temporal spectral-phase-modulation, wave packets in the same or in different spectral windows that initially overlap in space and time subsequently separate by multiple pulse widths upon free propagation by virtue of their different group velocities. Delay-bandwidth products of ~100 for pulses of width ~1 ps are observed, with no fundamental limit on the system bandwidth.

Tunable Electromagnetically Induced Transparency in Asymmetric Graphene-Based Metamaterial at Terahertz Region

We theoretically investigate the active control of electromagnetic induced transparency (EIT) in a terahertz graphene-based metamaterial, whose unit cell structure is composed two perpendicularly connected graphene cut-wires. The theoretical investigations and field distributions reveal that the EIT window arises from the destructive interference caused by asymmetric coupling between two cut-wire structures. Moreover, the strength and frequency of the EIT peak, the associated group delay and delay bandwidth product can be actively controlled. Therefore, the work presents a novel modulation strategy for controlling EIT behavior and exhibits potential applications for switching, modulation, and slow-light devices.

Enhancing optical delay using cross-Kerr nonlinearity in Rydberg atoms.

A scheme to enhance the optical delay in Rydberg atoms is proposed. In the linear case, the optical delay in a four-level system can be significantly enhanced compared to that of the three-level system. However, the width of the transparent window will decrease with an increase in the optical delay. In the nonlinear case, the nonlinear dispersion becomes steep around the transparency window. The enhanced cross-Kerr nonlinearity mainly contributes to the effective dispersion, which dramatically increases the optical delay. The simulation result shows that the optical delay of the system could be enhanced tens of times; moreover, the wide transparency window remains. So the delay-bandwidth product could be significantly improved due to nonlinear dispersion. We further examine Gaussian pulse propagation in the Rydberg atoms.

Reconfigurable slow light in phase change photonic crystal waveguide

Experimental demonstration of light propagation with ultralow group velocity, i.e., slow light, allows for revolutionary solutions for time-domain processing and buffering of optical signals. It can spatially compress optical energy, which lessens the device footprint and enhances linear and nonlinear optical effects. Photonic crystal waveguides (PCWs) are appealing for producing slow light since they can be on-chip integrated and operated under room temperature. However, most PCW slow-light devices are restricted to the narrow spectral range of material resonance, leading to a small delay-bandwidth product, which restricts the maximum data rate, operation frequency, and storage capacity. Furthermore, the lack of broadly tunable slow light hinders practical applications in tunable photonic devices. We propose a reconfigurable slow-light device using a PCW based on a prototypical chalcogenide glass, Ge2Sb2Te5 (GST225) to solve the problems. We find that the operating wavelength of the slow light within the structure can be reversibly switched between 3575 and 4905 nm by changing the structural state of GST225 between amorphous and crystalline ones. The corresponding average group indices are 40.8 and 54.4, respectively. We experimentally illustrate that the reversible phase transition of GST225 between amorphous and crystalline ones can be realized in nanoseconds. Our proof of concept may provide a platform for actively engineering slow light that might otherwise be difficult to obtain in photonic systems. We expect it to improve the device performance in the fields of nonlinearity and sensing.

A Compact 3–30-GHz 68.5-ps CMOS True-Time Delay for Wideband Phased Array Systems

This article presents a compact 4-bit switched-line true-time delay (TTD) circuit over a wide frequency range extending from 3 to 30 GHz using novel delay elements. The delay elements, namely, the cascading coupled all-pass network (CAPN) and noncoupled all-pass network (NCAPN), were employed in the proposed TTD circuit to improve the delay-bandwidth product (DBW) while maintaining its compact size and low delay variation (DV). For comparison, a theoretical analysis for understanding the group delay feature of the APN with various coupling coefficients is presented along with low-pass network (LPN). To verify the proposed structure, the proposed delays are applied to construct the 4-bit switched-line TTD by utilizing two single-pole double-through (SPDT) and three double-pole double-through (DPDT) switches in a 28-nm CMOS process. The circuit has a compact size of 1.7 mm ×0.2 mm, with a maximum delay of 68.5 ps and a minimum delay of 4.6 ps. The measured average insertion loss is 13.5 dB, and the in/out return loss is better than 10 dB across 3-30 GHz. The measured rms delay and gain errors are less than 2 ps and 3.2 dB, respectively, over the operating frequency range. To the best of our knowledge, the proposed TTD achieves the largest figure of merit (FoM) among the integrated TTD circuits.

Polarization-insensitive classical electromagnetically induced transparency metamaterial with large group delay by Dirac semimetal

We studied the Classical electromagnetically induced transparency (Cl-EIT) of a non-centrosymmetric metamaterial structure based on Dirac semimetal films (DSFs), which is polarization-insensitive and tunable. It is composed of a square ring and two U-shaped rings. By analyzing the distribution and intensity of the current, we find that Cl-EIT is caused by destructive interference between the resonators that produce near-field coupling. As the Fermi level of DSFs increases, the transparent window of the Cl-EIT structure blueshifts and has a good linear relationship with the Fermi level. For DSFs with different Fermi levels, we calculated their transmission phases and group delays where the maximum group delay is 31.35 ps, and the delay-bandwidth product (DBP) at this point can reach 0.658. The proposed metamaterial structure opens up a new path for various terahertz applications, such as optical storage and slow-light devices.

Broadband dispersionless topological slow light

We present a scheme to realize topological slow-light state with low group velocity and vanishing group velocity dispersion. By harnessing the strong interactions between two regular co-propagating topological photonic states in a magneto-optical photonic crystal waveguide, the energy flux transport of light exhibits a peculiar eight-shaped flowing loop within each unit cell of the waveguide. This permits the broadband pulse transporting with low group velocity (ng=13.26), broad bandwidth with a relative bandwidth of 3.08%, large normalized delay-bandwidth product (about 0.409), and vanishing group velocity dispersion. More importantly, they are robust against backscattering from obstacles. Our scheme paves the way to dig into the concept and physics of topology for solving the difficult problems of signal distortion and scattering loss in slow-light systems.

A theoretical proposal for the tunable electromagnetically induced transparency with superior properties based on the solid-state plasma

Based on the solid-state plasma (SSP), a metamaterial analog of electromagnetically induced transparency (EIT-like) with superior properties is theoretically proposed, including tunability, a large group index, and delay-bandwidth product (DBP), polarization independence, and low-loss. By exciting different specific parts of the SSP, the EIT-like peak frequency switches from 8.518 GHz with 90.92% transmissivity in state I to 11.26 GHz with 93.76% transmissivity in state II, i.e. a 2.742 GHz shift. Meanwhile, the metamaterial has 0.679 ns or 1.314 ns group delay, 453 or 876.1 group index, and 0.422 or 1 DBP in state I or III. Additionally, the transmission curves are the same under the circular polarization waves or linear polarization waves at any angle. The low-loss feature is likewise demonstrated. Therefore, the EIT-like metamaterial has superior properties for related applications, especially in slow light, sensing, switching, and communication.

A Multifrequency Electromagnetic Modulator Based on the Solid-State Plasma Metamaterial

In this article, a multifrequency electromagnetic modulator based on the solid-state plasma (SSP) metamaterial is proposed. It can achieve two modulations of electromagnetically induced transparency (EIT) and bandpass/stop filter, to realize speed and switching modulation of electromagnetic waves with specific frequencies. By exciting different SSP resonance units, the metamaterial modulator can obtain two states (states I and II), showing two different transmission spectra. When working as a tunable EIT, the state I appears as double transmission peaks with the delay-bandwidth product (DBP) reaching 0.517 and 0.421, whereas state II has a single peak with DBP up to 0.675. Since there is a one-to-one correspondence between the frequency points of transmission peaks and valleys in the two states, when state I manifests as an EIT, state II can serve as a band stop filter for the transmission peaks of state I. Conversely, when state II acts as an EIT, the state I can serve as its band stop filter to realize the switching modulation of these frequencies. Therefore, such an SSP-based metamaterial modulator can simultaneously meet the modulations of tunable EIT and corresponding bandpass/stop filter, which has great application prospects.

Theoretical design of quantum memory unit for under water quantum communication using electromagnetically induced transparency protocol in ultracold 87Rb atoms

In this paper, we present a theoretical proposal to realize Quantum Memory (QM) for storage of blue light pulses (420 nm) using Electromagnetically Induced Transparency (EIT). Three-level lambda-type EIT configuration system is solved in a fully quantum mechanical approach. Storing blue light has the potential application in the field of underwater quantum communication as it experiences less attenuation inside the sea water. Our model works by exciting the relevant transitions of [Formula: see text]Rb atoms using a three-level lambda-type configuration in a Two-Dimensional Magneto-Optical Trap (2D MOT) with an optical cavity inside it. We have estimated Optical Depth inside the cavity (ODc) of [Formula: see text], group velocity ([Formula: see text]) [Formula: see text][Formula: see text]ms[Formula: see text], Delay Bandwidth Product(DBP) of 23 and maximum storage efficiency as [Formula: see text] in our system.

Dynamic trapping and releasing photonics beyond delay-bandwidth limit in cascaded photonic crystal nanocavities

Controlling the flow of light on-chip is of great importance for quantum computing and optical signal processing. In this paper, we present a theoretical study to reveal the underlying physics of how to effectively trap, store and release a signal pulse, and eventually break the delay-bandwidth limit, based on controllable EIT-like effect in dynamically tuned standing-wave cascaded nanocavities. Using this mechanism, we design a compact silicon photonic crystal system with long storing time and a delay-bandwidth product over 460, which is about two orders of magnitude greater than the reported results obtained by other methods based on static resonator system, and the trapped signal pulse can be released on demand.

Dispersionless one-way slow wave with large delay bandwidth product at the edge of gyromagnetic photonic crystal

We theoretically, demonstrate a high delay bandwidth product (DBP) and zero group velocity dispersion (GVD) in a two-dimensional one-way slow light waveguide. The waveguide consists of gyromagnetic photonic crystal (GMPC) and a cladding formed by silicon photonic crystal. At the edge of the band, weak interactions (“semi-anticrossing”) between the chiral edge state (CES) mode and the mode localized at the surface of cladding are observed. The group velocity of CES wave can be tuned by adjusting the modal field distribution. As a result, an extraordinarily large value of normalized DBP of 0.63 with a group index of 10.32 and a bandwidth ranging from [Formula: see text] to [Formula: see text] is obtained. This result may contribute to one-way slow light applications in information communication systems.

Photon-photon interactions in dynamically coupled cavities

We study theoretically the interaction between two photons in a nonlinear cavity. The photons are loaded into the cavity via a method we propose here, in which the input/output coupling of the cavity is effectively controlled via a tunable coupling to a second cavity mode that is itself strongly output-coupled. Incoming photon wave packets can be loaded into the cavity with high fidelity when the timescale of the control is smaller than the duration of the wave packets. Dynamically coupled cavities can be used to avoid limitations in the photon-photon interaction time set by the delay-bandwidth product of passive cavities. Additionally, they enable the elimination of wave packet distortions caused by dispersive cavity transmission and reflection. We consider three kinds of nonlinearities, those arising from $\chi^{\scriptscriptstyle(2)}$ and $\chi^{\scriptscriptstyle(3)}$ materials and that due to an interaction with a two-level emitter. To analyze the input and output of few-photon wave packets we use a Schr\"odinger-picture formalism in which travelling-wave fields are discretized into infinitesimal time-bins. We suggest that dynamically coupled cavities provide a very useful tool for improving the performance of quantum devices relying on cavity-enhanced light-matter interactions such as single-photon sources and atom-like quantum memories with photon interfaces. As an example, we present simulation results showing that high fidelity two-qubit entangling gates may be constructed using any of the considered nonlinear interactions.

High Precision CMOS Integrated Delay Chain for X-Ku Band Applications

A high-precision delay chain circuit integrated in a 0.18- $\mu \text{m}$ CMOS technology working in the frequency bandwidth of 8–18 GHz has been designed and tested. The designed delay control integrated circuit with 5-bit delay control provides a maximum delay of 125 ps and has a delay resolution of 3.9 ps. Measured delay error of the fabricated chip is less than 9.3%, making it a considerably accurate delay control circuit. Low delay-error performance has resulted from incorporating a novel delay cell in this delay chain circuit. This newly proposed delay cell is a lumped-element coupled transmission line loaded with a second-order all-pass network (APN). The APN-loaded coupled line delay cell has larger delay-bandwidth product and is less prone to parasitic capacitances of inductor elements compared with conventional passive second-order APN delay cells. The fabricated chip utilizing the novel delay cell occupies an area of $0.9\times3.6$ mm2 and consumes 115.5- and 13.5-mW dc power from a 3.3-V voltage supply in its high- and low-power modes, respectively.