Fast Light Research Articles

Fast and slow light propagations in a four-level cascade atomic medium

Under weak probe field, rotating wave and electric dipole approximations, we have derived analytic expressions for absorption and dispersion coefficients, group index and group velocity in a four-level cascade atomic medium interacting with the probe and pump laser fields. The model is applied to 85Rb atomic system to study the control of the fast and slow light propagation of the probe beam according to the intensity and frequency of the pump laser beam. With this excitation model, we find two transparent domains and thus the light group velocity is also controlled at these two frequency domains. In particular, the amplitude and the sign of the group index are easily changed by adjusting the intensity or frequency of the pump laser. That is, the probe beam propagation can be changed between fast and slow light modes by adjusting the intensity or frequency of the pump laser. Our analytical results can be useful for related experimental studies.

The fast and slow light in a hybrid spinning optomechanical system mediated by a two-level system

We present a hybrid spinning whispering-gallery-mode (WGM) optomechanical system, where the mechanical resonator is not only coupled to the WGM cavity via radiation pressure but also coupled to a two-level system (TLS) via the Jaynes–Cummings coupling. If the WGM cavity is spinning along the clockwise or counterclockwise direction, the cavity field can undergo different Sagnac effect. The transmission spectra experience the conversion from optomechanically induced transparency (OMIT) to Fano resonance by controlling the resonator-TLS coupling, the rotational direction of the WGM cavity, and the cavity-pump detuning. Furthermore, both the OMIT and Fano resonance in the transmission spectra can induce the transparency windows accompanied by the rapid normal phase dispersion, which results in the tunable delayed or accelerated probe light transmission. The numerical results indicate that the OMIT leads to slow light, while Fano resonances generate the conversion from fast to slow light (or slow to fast light), and the parameters of the resonator-TLS coupling, the Sagnac frequency shift, the pump field frequency, and the cavity-pump detuning together determine the fast and slow light effect based on the hybrid spinning optomechanical system.

Photoelectrocatalytic degradation of m-chloronitrobenzene through rGO/g-C3N4/TiO2 nanotube arrays photoelectrode under visible light: Performance, DFT calculation and mechanism

The photoelectrocatalytic (PEC) technology by combing photocatalysis and electrochemistry has attracted widespread concerns in the organic pollutant degradation due to its effective degradation performance. TiO2 nanotube arrays (TNAs) can be used not only as photocatalyst but also as electrode and is regarded as a popular photoelectrode in PEC system. However, the fast photogenerated carrier recombination and low visible light utilization of TNAs restrict practical application of PEC technology for organic pollutant degradation. In this work, the graphitic carbon nitride (g-C3N4) and reduced graphene oxide (rGO) were loaded on TNAs surface to prepare ternary photoelectrode (rGO/g-C3N4/TNAs) for PEC degradation of m-chloronitrobenzene (m-CNB) under visible light. The surface structure and composition analysis revealed that rGO and g-C3N4 were successfully deposited on TNAs surface. The good light absorption capacity of g-C3N4 and good conductivity of rGO inhibited the photogenerated carrier recombination of TNAs and g-C3N4. The rGO/g-C3N4/TNAs prepared with 60 mg/L graphene oxide (rGO/g-C3N4/TNAs-60) possessed wider visible-light absorption range, higher carrier density and photoconversion efficiency than other as-prepared photoelectrodes, which promoted PEC degradation efficiency of m-CNB. The cyclic degradation experiment indicated that rGO/g-C3N4/TNAs-60 photoelectrode had good reusability and stability. The radical scavenging experiments and electron paramagnetic resonance (EPR) spectra demonstrated that OH, O2– and h+ were major active species, and e− was auxiliary species for PEC degradation of m-CNB by rGO/g-C3N4/TNAs-60. The degradation pathways of m-CNB were proposed according to gas chromatography mass spectra and density functional theory calculation. The comprehensive toxicity of m-CNB was relieved after PEC degradation according to the toxicity prediction.

Group delay manipulation in a passive microwave-photonic fiber optic Mach-Zehnder interferometer.

Conventional slow and fast light (SFL) systems are based on active or nonlinear media or photonic band-gap systems. Radio-frequency (RF) interferometric systems were recently explored for sustaining fast and slow light, owing to their simple structures and passive operation. This Letter introduces a simple, effective, and efficient SFL system based on a linear and passive microwave-photonic (MWP) fiber optic Mach-Zehnder interferometer (MZI). By optically tuning the light transmission ratio of the two fiber beams of the MZI (i.e., the optical carrier), we demonstrate that both delay and advancement of a RF signal (i.e., the envelope signal) can be realized. The magnitude of the signal group delay can also be adjusted using the optical tuning method. Therefore, high-degree manipulation of signal group delay can be achieved using the proposed MWP fiber optic MZI system.

Carbon nanotube resonator enhancement of Majorana fermions induced slow light in a hybrid semiconductor/superconductor device

We theoretically investigate the Rabi-like splitting in the absorption spectrum of a single electron spin mediated by Majorana fermions in a hybrid semiconducting nanowire/superconductor system. The absorption spectra can display the symmetrical splitting phenomenon, and their related optical propagation such as fast light and slow light are investigated in different parametric regimes. When we consider implanting a spin into the carbon nanotube (CNT) resonator, the Rabi-like splitting in the absorption spectrum changes into asymmetric Fano resonance, which is accompanied by the rapid steep dispersion promising the slow- or fast-light effect, and even reaching tunable fast-to-slow light propagation (or vice versa) with controlling different parameter regimes. Moreover, we also investigate the role of the CNT resonator, which behaves as a phonon cavity enhancing the fast and slow light effect.

ANALYSIS OF POLYSORBATE 80 SOLUTION STABILITY UNDER STRESS CONDITIONS TO ENSURE ITS QUALITY AS A BIOPHARMACEUTICAL EXCIPIENT

Objective: This study aimed to investigated the effect of light, temperature, pH, peroxides, trace metals, and buffer type on the chemical stability of polysorbate 80 (PS80) obtained from the three key manufactures. Methods: We used a fast liquid chromatography-evaporative light scattering detector that allowed the monitoring of PS80 decay over time. For data analysis, we investigated the change in the peak area percentage of the compound over time. Results: At pH 6.0 in histidine buffer, PS80-B was more sensitive than PS80-A and PS80-C. The PS80 from the three different sources degraded significantly with varying performance levels when exposed to light, temperature of 40 °C, peroxides, and trace metals over time. Conclusion: Our results provide an improved understanding of the stability of PS80 obtained from the three different sources under different conditions, which provides a basis for the selection of the appropriate grade of PS80 according to the specific requirements.

Comparison of fast bus and light rail transport based on SWOT analysis

The rapid expansion of urban areas requires the application of more capacious technologies of public passenger transport. New public transport technologies should provide satisfactory transport characteristics with acceptable capital investment and a short implementation period. Two basic public passenger transport technologies that can provide good transport quality with reasonable investment are Bus rapid transit (BRT) and Light rail transit (LRT). The characteristics of these public transport subsystems largely depend on the implemented technological elements. Within this paper, a comparison of BRT and LRT will be performed based on SWOT analysis, so that city administrations can make appropriate strategic decisions when choosing technology.

Tunable slow and fast light in an atom-assisted hybrid system via external mechanical driving force

Tunable slow and fast light in an atom-assisted hybrid system via external mechanical driving force

A Stray Light Detection Model for VR Head-Mounted Display Based on Visual Perception

In recent years, the general public and the technology industry have favored stereoscopic vision, immersive experience, and real-time visual information reception of virtual reality (VR) and augmented reality (AR). The device carrier, the Head-Mounted Display (HMD), is recognized as one of the next generation’s most promising computing and communication platforms. HMD is a virtual image optical display device that combines optical lens modules and binocular displays. The visual impact it brings is much more complicated than the traditional display and also influences the performance of image quality. This research investigated the visual threshold of stray light for three kinds of VR HMD devices, and proposes a qualitative model, derived from psychophysical experiments and the measurement of images on VR devices. The recorded threshold data of the psychophysical stray light perception experiment was used as the target when training. VR display image captured by a wide-angle camera was processed, through a series of image processing procedures, to extract variables in the range of interest. The machine learning algorithm established an evaluation method for human eye-perceived stray light in the study. Four supervised learning algorithms, including K-Nearest Neighbor (KNN), Logistic Regression (LR), Support Vector Machine (SVM), and Random Forest (RF), were compared. The established model’s accuracy was about 90% in all four algorithms. It also proved that different percentages of thresholds could be used to label data according to demand to predict the feasibility of various subdivision inspection specifications in the future. This research aimed to provide a fast and effective stray light qualitative evaluation method to be used as a basis for future HMD optical system design and quality control. Thus, stray light evaluation will become one of the critical indicators of image quality and will be applicable to VR or AR content design.

Transparency and Enhancement in Fast and Slow Light in q‐Deformed Optomechanical System

AbstractNonclassical phenomena can be enhanced by introducing q‐deformation in optomechanical systems. This motivates investigation of the optical response in a q‐deformed linearly coupled optomechanical system. The system consists of two deformed cavities that are linearly coupled to the motion of mechanical mirrors, and the cavities are coupled to each other by transmission strength parameter. This study shows that compared to non‐deformed cases, the deformed system exhibits more rapid phase transition at the positions of transparency windows, causing stronger and enhanced fast and slow light phenomena. Moreover, in the region , the optomechanical system results in gain. Additionally, for a fixed deformation of cavities, by tuning the tunneling strength and optomechanical coupling, one can observe a delay and advancement in probe field in the order of milliseconds and even above milliseconds for fine‐tuning of the coupling parameters. Finally, the bridge between mathematical and physical models is built by assuming the deformation to be a primitive root of unity, which indicates a class of anyon models. These results demonstrate that q‐deformation provides a novel method for manipulating and significantly enhancing optical phenomena not only in arbitrarily deformed optomechanical systems but also in anyon models.

Controllable fast light in quantum dot molecules assisted hybrid optomechanical system

AbstractWe investigate the superluminal effect of transmitted probe field in three‐level quantum dot molecules (QDMs) assisted optomechanical system which consist of mechanical resonator. We show that the superluminal behavior of transmitted probe field can be controlled by changing the tunneling strength and number of QDMs inside the cavity. Furthermore, it is shown that in the absence of tunneling strength, the transmitted probe field shows the fast light effect and by increasing the number of QDMs, the enhancement in superluminal behavior is decreased and converts into slow light. While, in the presence of the tunneling strength, with the increase of the number of QDMs the superluminal behavior of transmitted field is obtained at smaller detuning frequency. The influence of tunneling strength and number of QDMs on superluminal part of the transmitted probe field is quite useful in optical memory, optical buffers, and quantum information processing.

Sub-megahertz spectral dip in a resonator-free twisted gain medium

Ultra-narrow optical spectral features resulting from highly dispersive light–matter interactions are essential for a broad range of applications such as spectroscopy, slow-light and high-precision sensing. Features approaching sub-megahertz or, equivalently, Q-factors up to one billion and beyond, are challenging to obtain in solid-state systems, ultimately limited by loss. We present a novel approach to achieve tunable sub-megahertz spectral features at room temperature without resonators. We exploit gain-enhanced polarization pulling in a twisted birefringent medium where polarization eigenmodes are frequency-dependent. Using Brillouin gain in a commercial spun fibre, we experimentally achieve a 0.72 MHz spectral dip, the narrowest backward Brillouin scattering feature ever reported. Further optimization can potentially reduce the linewidth to <0.1 MHz. Our approach is simple and broadly applicable, offering on-demand tunability and high sensitivity, with a wide range of applications such as microwave photonic filters, slow and fast light, and optical sensing. Tunable sub-megahertz spectral features are demonstrated without resonators. The approach, which exploits gain-enhanced polarization pulling in a twisted birefringent medium, may be useful in applications such as microwave photonic filters, slow and fast light, and optical sensing.

Unbiased Plasmonic-Assisted Integrated Graphene Photodetectors.

Photonic integrated circuits (PICs) for next-generation optical communication interconnects and all-optical signal processing require efficient (∼A/W) and fast (≥25 Gbs–1) light detection at low (<pJbit–1) power consumption, in devices compatible with Si processing, so that the monolithic integration of electro-optical materials and electronics can be achieved consistently at the wafer scale. Graphene-based photodetectors can meet these criteria, thanks to their broadband absorption, ultra-high mobility, ultra-fast electron interactions, and strong photothermoelectric effect. High responsivities (∼ 1 A/W), however, have only been demonstrated in biased configurations, which introduce dark current, noise, and power consumption, while unbiased schemes, with low noise and zero consumption, have remained in the ∼ 0.1 A/W regime. Here, we consider the unbiased asymmetric configuration and show that optimized plasmonic enhanced devices can reach for both transverse-electric and transverse-magnetic modes (at λ = 1550 nm), ∼A/W responsivity, and ∼ 100 GHz operation speed at zero power consumption. We validate the model and material parameters by simulating experimental devices and derive analytical expressions for the responsivity. Our comprehensive modeling paves the way for efficient, fast, and versatile optical detection in PICs with zero power consumption.

A 25 MHz Fast Transient Adaptive-On/Off-Time Controlled Three-Level Buck Converter

This work presents a 25 MHz adaptive on/off-time-controlled three-level buck converter with a fast reference tracking speed and high light load efficiency. The self-balancing scheme of the flying capacitor voltage is revised and extended for a broad conversion ratio range from 0.17 to 0.833. To reduce the error of the self-balancing scheme when applied to a high frequency three-level buck converter with an on-chip flying capacitor, a real-time calibration loop on the flying capacitor voltage is introduced, effectively reducing the balancing error from 340 mV to 20 mV. A turning-point output voltage prediction for the three-level buck converter is presented to achieve near-optimal reference tracking with a fast tracking speed and negligible output voltage undershoot or overshoot. The up- and down-tracking takes 81 ns and 72 ns, respectively, for a 1-V step with an inductor of 47 nH and an output capacitor of 47 nF. The three-level converter achieves a precise discontinuous conduction mode (DCM) operation with an adaptive switching frequency at light load by means of a feedback loop-based method. An approximately 28.2% increment in efficiency is achieved with a load of 10 mA. The three-level buck converter has a peak efficiency of 92.8% and an over 85% efficiency in most of the operation regions.

Luminous Millimeter, Radio, and X-Ray Emission from ZTF 20acigmel (AT 2020xnd)

Observations of the extragalactic (z = 0.0141) transient AT 2018cow established a new class of energetic explosions shocking a dense medium, producing luminous emission at millimeter and submillimeter wavelengths. Here we present detailed millimeter- through centimeter-wave observations of a similar transient, ZTF 20acigmel (AT 2020xnd), at z = 0.2433. Using observations from the NOrthern Extended Millimeter Array and the Very Large Array, we model the unusual millimeter and radio emission from AT 2020xnd under several different assumptions and ultimately favor synchrotron radiation from a thermal electron population (relativistic Maxwellian). The thermal electron model implies a fast but subrelativistic (v ≈ 0.3c) shock and a high ambient density (n e ≈ 4 × 103 cm−3) at Δt ≈ 40 days. The X-ray luminosity of L X ≈ 1043 erg s−1 exceeds simple predictions from the radio and UVOIR luminosity and likely has a separate physical origin, such as a central engine. Using the fact that month-long luminous (L ν ≈ 2 × 1030 erg s−1 Hz−1 at 100 GHz) millimeter emission appears to be a generic feature of transients with fast (t 1/2 ≈ 3 days) and luminous (M peak ≈ −21 mag) optical light curves, we estimate the rate at which transients like AT 2018cow and AT 2020xnd will be detected by future wide-field millimeter transient surveys such as CMB-S4 and conclude that energetic explosions in dense environments may represent a significant population of extragalactic transients in the 100 GHz sky.

Time advancement and distortionless of optical pulses via stimulated Brillouin scattering by broadband pump in optical fibers

The broadening Brillouin absorption spectrum of stimulated Brillouin scattering (SBS) in optical fibers by multiple monochromatic pumps is investigated. The results show that the bottom of the effective absorption spectrum becomes flatter with increasing the number of spectrum lines, and the spectral bandwidth is effectively expanded by multiple monochromatic pumps. but the spectral spacing should be tailored reasonably to ensure a broad linear range of the refractive index to reduce the pulse distortion. The time advancement and the pulse distortion induced by SBS pumped by broadband Gaussian and rectangular spectra are demonstrated respectively. The simulation results indicate that the pulse distortion induced by broadband rectangular spectral pump is smaller than that of broadband Gaussian spectral pump, and the time advancement can be improved effectively by increasing signal light power. We believe that these results are very promising for designing optical buffering or sensing components based on SBS fast light.

Fast light propagating waveguide composed of heterogeneous metamaterials

A symmetrical three-layer fast light waveguide based on photonic crystal heterostructure is proposed in this article. Two types of photonic crystal where their effective optical parameters such as permittivity and permeability are variable from negative to positive at vicinity of the “Dirac-like” point frequency, are designed. Then the symmetrical three-layer light waveguide composed of these two different photonic crystals (heterogeneous structure at the junction between the cladding and the core) are constructed. By analyzing the anomalous dispersion mode of this waveguide, numerically calculated the group velocity to be negative. The light propagation effect of the waveguide is simulated, and it shows that the waveguide can confine light to the core layer well. In addition, observing the backward moving behavior of the wave packet by incident modulated gaussian pulses envelope, further verifying the generation of fast light. These numerical simulation results agree well the analytical analysis. The proposed waveguide structures in this article may improve the sensitivity of optical instruments and offer potential applications in the fields of integrated photonic circuits, on-chip optical interconnect, and fast optical communication and transmission.

Optimizing Interfacial Energetics for Conjugated Polyelectrolyte Electron Injection Layers in High Efficiency and Fast Responding Polymer Light Emitting Diodes.

Modification of the π-conjugated backbone structure of conjugated polyelectrolytes (CPEs) for use as electron injection layers (EILs) in polymer light emitting diodes (PLEDs) has previously brought conflicted results in the literature in terms of device efficiency and turn-on response time. Herein, we determine the energetics at the CPE and the light emitting polymer (LEP) interface as a key factor for PLED device performance. By varying the conjugated backbone structure of both the LEP and CPE, we control the nature of the CPE/LEP interface in terms of optical energy gap offset, interfacial energy level offset, and location of the electron–hole recombination zone. We use a wide gap CPE with a shallow LUMO (F8im-Br) and one with a smaller gap and deeper LUMO (F8imBT-Br), in combination with three different LEPs. We find that the formation of a type II heterojunction at the CPE/LEP interfaces causes interfacial luminance quenching, which is responsible for poor efficiency in PLED devices. The effect is exacerbated with increased energy level offset from ionic rearrangement and hole accumulation occurring near the CPE/LEP interface. However, a deep CPE LUMO is found to be beneficial for fast current and luminance turn-on times of devices. This work provides important CPE molecular design rules for EIL use, offering progress toward a universal PLED-compatible CPE that can simultaneously deliver high efficiency and fast response times. In particular, engineering the LUMO position to be deep enough for fast device turn-on while avoiding the creation of a large energy level offset at the CPE/LEP interface is shown to be highly desirable.

Tunable optical response and fast (slow) light in optomechanical system with phonon pump

We theoretically investigate the tunable optical response of a weak probe field in a single phonon mechanical driven cavity optomechanical system where an oscillating membrane acts as a mechanical mode. This membrane is also coupled to the optical cavity mode through both the linear optomechanical coupling (LOC) and the quadratic optomechanical coupling (QOC) simultaneously. An external single phonon mechanical driving field present in our scheme gives an additional coherence effect as well as its relative phase and intensity can control significantly the absorption (amplification) profile of the output probe field. We have also shown that the phase dispersion and group delay of the probe field can be controlled flexibly through both types of optomechanical coupling strengths as well as the mean number of thermal phonons. Our results provide a flexible and generalised route to control the light propagation in such kinds of complex cavity optomechanical systems with coherent mechanical excitation.

Visible light responsive spiropyran derivatives based on dynamic coordination bonds

Spiropyrans (SPs) are a well-known class of photochromic compounds and have found widespread application due to their unique properties. However, for many conventional SPs, high energy ultraviolet (UV) light is commonly essential to drive photoisomerization, leading to poor fatigue resistance. Moreover, the practical application of spiropyrans is hindered by their fast fading speed due to the instability of closed forms (SP) or open forms (MC). Herein, we disclose a novel strategy to address these challenges through introducing both electron-donating substituents to stabilize the SP and dynamic coordination bonds to stabilize the MC. The resulting new spiropyrans complexes exhibit negative photochromic properties, with fast visible light response, good stability of both SP and MC, and significantly improved fatigue resistance.