Search

Disorder has long been regarded as an adversary to nano-optics and photonics designers. In the fields like metamaterials, plasmonics, and integrated photonics circuits, researchers have always striven to minimize the disorder and any deviation from the ideal design for many years. However, as more sophisticated nanostructures are designed with increasing complexities, the required fabrication precision has reached the limit of current technologies. As a result, disorder becomes a nuisance that prevents many nano-systems from reaching their full potential.Especially, hyperbolic metamaterials, also known as indefinite metamaterials, have a unique dispersion relationship that has been theoretically predicted to provide an unlimited imaging resolution. However, fabrication disorder has limited its imaging performance as it generates unavoidable random scatterings. We, on the other hand, demonstrate a random structured illumination nanoscopy that utilizes this random scattering property. Combined with the ultrathin metallic film deposition, we managed to demonstrate a record-high spatial resolution in the field of metamaterial imaging.We also proposed a new framework for metamaterial far field single-shot super-resolution imaging. The improvement in resolution, field of view, and structural complexity is sub- stantial compared with current single-shot imaging methods. The basic idea is to extract the high-resolution information of the object into the far field via the unique scattering and interfering process in metamaterials. The new scattering forward model has been proposed and an inverse algorithm inspired by the untrained Neural Network is designed to decode this seemingly complex interference pattern to reconstruct the object. This work provides a blueprint for the future of high spatiotemporal resolution imaging.Furthermore, in the field of integrated photonics, we demonstrate a strategy to improve the slow light capability by utilizing the previously unwanted structural disorder. Since slow light device typically requires a strictly periodic nanostructure, many works have shown that a slight amount of disorder introduced in the fabrication process can have a detrimental effect on the device performance, causing the backscattering effect and even Anderson localization. To solve this issue, we developed a method to help a disordered slow light system perform physical learning to reorganize a scattering landscape to have an improved slow light capability, even better than a perfectly periodic system.I believe my efforts in developing different types of metamaterial imaging modalities and random slow light structures through the combination of nanotechnology and state-of-the-art computational methods will advance future nanoscale optics and photonics.

In this paper, we realize tunable Fano and electromagnetically induced transparency (EIT)-like resonances in the outputs of a nested feedback ring resonator. Compared with the traditional add-drop ring resonator, the feedback arm introduces interference between different resonance modes. Using coupled mode theory, we reveal the output Fano resonances are the interference of discrete modes and continuum modes, whereas the EIT-like resonance only appears in the over-coupled state. We experimentally confirm the two effects using optical fiber ring resonators, finding that the Fano effect can be easily controlled with phase shifts in the feedback arm. Moreover, we achieve the double Fano resonance, which displays an X type lineshape utilizing one ring resonator, predicting it to be a good candidate in wavelength monitor. The proposed simple configuration provides a platform to study Fano and EIT-like effect, which is useful in the optical filter, fast and slow light, optical switching, and optical sensing

A light beam scanning technology for 3D sensing has been attracting much interest thanks to its signal-to-noise-ratio merit compared to conventional flash 3D sensing. A Solid-state beam scanner based on a vertical cavity surface emitting laser (VCSEL) with amplification function was invented. In this paper, though enlarging the length of scanner to 6mm, a narrow beam divergence of 4000 was obtained. Its intensity variation is < 40% and could be further improved by better DOE design and fabrication process. The DOE could be simply stacked on the scanner and thus the submodule size of the scanner could be managed within 7mm 1mm 1mm. We also discussed its integrability with a tunable seed laser and its application in 2D beam scanning for better signal to noise ratios.

We developed a thermo-optically controlled nonmechanical optical beam scanner using a Si photonic crystal slow-light waveguide with a diffraction grating to achieve on-chip light detection and ranging (LIDAR). This Letter applies pre-emphasis signals to the thermo-optic control, and the cutoff frequency increases to 500 kHz. Observing the beam scanning in the space-time domain showed that the turn-on and turn-off times of the scanner for a rectangular drive voltage were 10 µs and reduced to 2.7 µs when the pre-emphasis signals were optimized. This new, to the best of our knowledge, result enables a frame rate of 29 fps for 12,800 resolution points in LIDAR.

In order to obtain excellent slow-light performance, we propose a photonic crystal waveguide (PCW) that introduces extrinsic defect rods in the center row of a complete square lattice rotated 45° counterclockwise and the second row adjacent to it. The continuous cavities are used as a storage of electromagnetic energy and a speed reducer of light speed, used for slow optical transmission in PCWs. Then, the plane wave expansion method (PWE) is used to study the slow light transmission characteristics of the proposed structure, and the influence of the structure parameters on the slow light performance is analyzed. Finally, the bandwidth is obtained at 23.37 nm when the normalized delay bandwidth product (NDBP) reaches 0.40. In addition, considering the effect of material properties on slow light performance, NDBP is further optimized to 0.44, and the bandwidth reaches 27.63 nm. A simple but universal structure is designed to provide an important theoretical basis for further improving the storage capacity with high bandwidth and high NDBP slow light.

Photon echo is a fundamental tool for the manipulation of electromagnetic fields. Unavoidable spontaneous emission noise is generated in this process due to the strong rephasing pulse, which limits the achievable signal-to-noise ratio and represents a fundamental obstacle towards their applications in the quantum regime. Here we propose a noiseless photon-echo protocol based on a four-level atomic system. We implement this protocol in a Eu3+:Y2SiO5 crystal to serve as an optical quantum memory. A storage fidelity of 0.952 ± 0.018 is obtained for time-bin qubits encoded with single-photon-level coherent pulses, which is far beyond the maximal fidelity achievable using the classical measure-and-prepare strategy. In this work, the demonstrated noiseless photon-echo quantum memory features spin-wave storage, easy operation and high storage fidelity, which should be easily extended to other physical systems., Photon echo techniques are difficult to implement in the quantum regime due to coherent and spontaneous emission noise. Here, the authors propose a low-noise photon-echo quantum memory approach based on all-optical control in a four-level system, and demonstrate it using a Eu3+:Y2SiO5 crystal.

In this paper, a terahertz metasurface supercell consists of two ring resonators with slightly different radii is proposed. Electromagnetic-induced transparency (EIT)–like resonance is excited with an ultrahigh figure of merit. Furthermore, the impact of the coupling between the two rings is investigated on the transmission amplitude response, the quality factor, and the EIT peak amplitude by varying the radius of the top resonator. The achieved figure of merit of the EIT peak reaches 88.65 and almost 20,000 when gold and a perfect electric conductor are used for the metallic layer, respectively. The simplicity and unique properties of the proposed design could render it to be a desirable candidate for filtering, slow light applications, and sensing.

Optical beam scanning is a widely utilized function in optical systems and a compact nonmechanical solid-state device has long been anticipated. Here, we have studied such a device consisting of photonic crystal slow-light waveguides and switch trees, fabricated by a Si photonics process, employing a bespoke prism lens for beam collimation. Further in this study, we particularly demonstrated the operation of the device only by the thermo-optic (TO) tuning of its components, at a fixed wavelength of light. A spot beam of ∼0.1° divergence, was scanned in two dimensions, in the angular range of 40° × 8.8° and average power consumption of < 0.7 W. Neglecting some disordered beams caused by the non-uniformity of the fabricated device, the estimated number of resolution points was 400 × 32 = 12,800, which required a significant effort in the device fabrication and calibration, utilizing optical-phased arrays, if the same performance was targeted.

Plasmon-induced transparency (PIT) is a spotlight technique for environmental monitoring. In this regard, a highly sensitive and tunable multilayer sensor including Ag–SiO2–Ag is presented in near-infrared range for both transverse mode (TE) and transverse mode (TM) modes at different incident waves. Actually, the proposed multilayer plasmonic sensor is presented to study the optical and sensing properties at near-infrared frequencies based on 3D finite-difference time-domain (FDTD). Sensitivity and tunability of all optical sensors are important parameters in their design, which are calculated based on propagation properties including absorption and reflection spectra, numerically and analytically. Results of absorption and reflection show that the proposed sensor has max sensitivity of 693.8 nm/RIU by Δn = 0.05 changing of middle refractive index and figure of merit (FoM) equal to 9.8. Also, the proposed nano-scale sensor can operate as a light propagation controlling with slow and fast light and multispectral sensor. By using silver metal in the sensor structure, the fast and slow light coefficient is obtained at 220 and 70, respectively. In addition, by increasing the incident angle of light, multiabsorption peaks are created which can be used as multiwindow sensor and PIT. The analytical results are in good agreement with the obtained results of coupled mode theory (CMT) method.

Pump phase modulation is an important method for signal group velocity control. By controlling the phase difference between pump and signal, the fast and slow light can be changed easily, and the degree of signal delay or advancement can be greatly improved. In this letter, the fast and slow light of a Gaussian signal with pump phase modulation is studied for the first time. The Gaussian pulse signal is similar to the actual network transmission signal, so it is significant to investigate it. Unlike a sinusoidal signal, the duty ratio of a Gaussian signal can be adjusted, leading to the difference in pump phase modulation of the Gaussian signal. The experimental and simulation results show that the smaller the duty ratio of the Gaussian signal is, the smaller the impact of the pump phase adjustment on the fractional delay and advancement will be. Moreover, it is also found that at a certain duty ratio, two signal peaks appear within a certain adjustment range of phase difference.

Fizeau’s experiment, inspiring Einstein’s special theory of relativity, reveals a small dragging effect acting on light inside a moving medium. Dispersion can enhance such light drag according to Lorentz’s predication. Here fast- and slow-light-enhanced light drag is demonstrated experimentally in a moving optical microcavity through stimulated Brillouin scattering induced transparency and absorption. The strong dispersion provides an enhancement factor up to ~104, greatly reducing the system size down to the micrometer range. These results may offer a unique platform for a compact, integrated solution to motion sensing and ultrafast signal processing applications. In accordance with the relativistic addition of velocities, the Fizeau experiment shows the dragging of light inside a moving medium which can be enhanced in dispersive medium. Here a strong dispersion, induced by stimulated Brillouin scattering, is shown to significantly enhance the light drag effect in a moving optical microcavity.

Structural slow light is the dispersion engineering process by which the group velocity of light can be drastically reduced in a periodic waveguide structure. Enabling large group delay and enhancing the light-matter interaction on a subwavelength scale, on-chip slow light is of great interest in a vast array of fields such as non-linear optic, sensing, laser physics, telecommunication and computing. In this work, we experimentally demonstrate, for the first time, slow light in subwavelength grating waveguides on the silicon-on-insulator platform. We present a comprehensive numerical study in analytical modelling and 3-D FDTD. Multiple waveguides variations were fabricated using an electron-beam lithography process. Figures of merit such as group index, bandwidth and loss-per-delay are examined in both theory and experiment. A maximum measured group index of 47.74 with a loss-per-delay of 103.37 dB/ns has been achieved near the wavelength of 1550 nm. A broad bandwidth of 8.82 nm was measured, in which the group index remains larger than 10. We also show that the region of slow light operation can be shifted over a large wavelength span by controlling a single design parameter.

We investigate the properties of the optical output fields in a spinning optomechanical system under the condition of optomechanically induced transparency (OMIT), where the cavity is optically driven by a strong pump field and a weak probe field and the mechanical resonator is driven by a coherent phonon pump. When the driven frequency of the phonon pump equals the frequency difference of the pump and probe fields, we show an enhancement OMIT, where the probe transmission can exceed unity via controlling the amplitude and phase of the phonon pump. Furthermore, the phase dispersion of the transmitted probe field is modified with manipulating the spinning direction of the resonator, which leads to a tunable delayed probe light transmission, due to the clockwise and counterclockwise optical fields in the resonator undergo the different Sagnac effect. Combining Sagnac effect, we demonstrate that the large positive or negative group delay of the output probe field can be achieved by adjusting the phase and amplitude of the coherent phonon pump, which can realize a tunable conversion between the slow and fast light effect with manipulating the spinning direction of the resonator, power of the pump field as well as the amplitude and phase of the phonon pump.

The polarization of optical fields is a crucial degree of freedom in the all-optical analogue of electromagnetically induced transparency (EIT). However, the physical origins of EIT and polarization induced phenomena have not been well distinguished, which can lead to confusion in associated applications such as slow light and optical/quantum storage. Here we study the polarization effects in various optical EIT systems. We find that a polarization mismatch between whispering gallery modes in two indirectly coupled resonators can induce a narrow transparency window in the transmission spectrum resembling the EIT lineshape. However, such polarization induced transparency (PIT) is distinct from EIT: it originates from strong polarization rotation effects and shows unidirectional feature. The coexistence of PIT and EIT provides new routes for the manipulation of light flow in optical resonator systems., 8 pages, 4 figures, 57 references. The published version can be found via ULR: https://www.pnas.org/content/118/3/e2012982118

We present a hybrid gyromagnetic photonic crystal (GPC) waveguide composed of different GPC waveguide segments possessing various cylinder radii and waveguide widths but biased by a uniform external magnetic field. We demonstrate in frequency and time domains that based on the strong coupling of two counter-propagating topologically protected one-way edge states, the intriguing slow light rainbow trapping (SLRT) of electromagnetic (EM) waves can be achieved, that is, EM waves of different frequencies can be slowed down and trapped at different positions without cross talk and overlap. More importantly, due to the existence of one-way edge states, external EM waves can be non-reciprocally coupled to the SLRT waveguide channel, although the incident position of the EM wave is far away from the waveguide channel. Besides, the frequency range of the slow light states can also be easily regulated by tuning the intensity of an external magnetic field, which is very beneficial to solve the contradiction between slow light and broad bandwidth. Our results can be applied to the design of high-performance photonic devices, such as an optical buffer, optical switch, and optical filter.

A two axes fiber optical gyroscope comprising a two loops structure (TLS) using a 3 × 3 coupler is presented. The length of the two loops is 2 m and 1.8 m. When one of the two axes is vertical to another, a dual-axis gyroscope comes into being. The phase difference between the counter-propagating waves in TLS is calculated and discussed. The sensitivity can be improved by adding a bias phase, but only if the phase difference is a monotonic function of the angular rate, with the best sensitivity obtained by adding a phase bias of ±3π/2.

The framework for designing and fabricating a slow-light waveguide structure with conical Swiss roll metamaterial core at Terahertz frequencies has been carried out in this article. In the earliest work, theoretical backgrounds based on Maxwell’s equations have been developed for anisotropic single-negative permeability slab waveguides and anisotropic metamaterial slab waveguides. Subsequently, simulation results fulfilled by the MATLAB programming tool verify extremely low group velocities in the aforementioned slab waveguides in the Terahertz regime frequency. A volumetric conical Swiss roll metamaterial has been proposed as a practical achievement for slow-light waveguides. Dispersion characteristics of the electromagnetic waves in the proposed conical Swiss roll metamaterial have been investigated using the CST simulation tool in Terahertz frequencies. Furthermore, a 2-D dispersion diagram fulfilled by CST and MATLAB validates highly electromagnetic field concentration as well as the presence of backward waves in the conical Swiss roll configuration.

Fano-like lineshapes in whispering-gallery modes (WGMs) microresonators are of critical for many actual applications, such as high-sensitivity sensors, slow light, and optical switches. In this paper, we theoretically and experimentally demonstrate dynamic transmission lineshapes, including Lorentzian lineshapes, Fano-like lineshapes and gain-like lineshapes, with a simple system, where a surface nanoscale axial photonic (SNAP) microbottle is coupled to the transition of a tapered fiber by carefully choosing tapered fiber diameters. Controlled and robust coupling with a clean and almost equidistant spectrum for different axial modes is achieved while maintaining contact between the resonator and the taper. Our device offers five similar dynamic transmission lineshapes arranged in order simultaneously, demonstrating stable tuning and a high number of potential degrees of freedom in contrast to other coupling systems for single- or double-coupled microresonators. By using coupled-mode theory, these transmission lineshapes are fitted to explain these experiment observations. We also explore a tunable transmission spectra obtained by increasing the powers of the input laser. Our approach hold unique potential in sensitivity-enhanced sensing, quantum information processing, and all-optical switching.

A plane monochromatic wave propagates in vacuum at the velocity c. However, wave packets limited in space and time are used to transmit energy and information. Here it has been shown based on the wave approach that the on-axis part of the pulsed beams propagates in free space at a variable speed, exhibiting both subluminal and superluminal behaviours in the region close to the source, and their velocity approaches the value of c with distance. Although the pulse can travel over small distances faster than the speed of light in vacuum, the average on-axis velocity, which is estimated by the arrival time of the pulse at distances z ≫ ld (ld is the Rayleigh diffraction range) and z > cτ (τ is the pulse width) is less than c. The total pulsed beam propagates at a constant subluminal velocity over the whole distance. The mutual influence of the spatial distribution of radiation and the temporal shape of the pulse during nonparaxial propagation in vacuum is studied. It is found that the decrease in the width of the incident beam and the increase in the central wavelength of the pulse lead to a decrease in the propagation velocity of the wave packet.

In this paper, a theoretical analysis of a flat-top microring resonator (FTMRR) is undertaken. The intensity and group delay responses of the FTMRR are derived from a transfer matrix formalism, which shows that flat-top resonance is possible in this single-ring cavity by stimulating and manipulating slow light in the counterclockwise (CCW) mode and fast light in the clockwise (CW) mode. The analytical solutions required for achieving the flat-top responses are obtained by assuming that the slope around the resonant wavelength equals zero. In addition, an improvement in the bandwidth of the FTMRR (in comparison with the MRR), the tunable and reconfigurable optical responses for filtering/switching, and the small variation in intensity during the tuning of the flat-top group delay observed in the previous experiments are all proved through theoretical analysis.

Active manipulation of group delay of electromagnetic waves is highly desirable for telecommunications and information processing. However, most control means of the group delay currently demonstrated experimentally in the artificial metamaterials require different external excitations necessitating complex setups. Here, electrical manipulation of resonance strength of electromagnetically induced reflectance (EIR) are theoretically and numerically demonstrated in a hybrid metal-graphene complementary metamaterial for actively tunable slow light applications. Combining a monolayer graphene with a metal-based complementary metamaterial composed of a dipole slot and a quadrupole slot, a sharp EIR window in the complementary structure is excited due to strong coupling between two resonators. As shifting the Fermi energy of graphene implanted under the dark mode resonator via electrostatic doping, the EIR window can realize an on-to-off switch, and the modulation depth of 73.3% for the EIR resonance strength is achieved with actively switchable group delay of 6.44 ps. Further investigations show that the active modulation can be attributed to the change in the damping rate of the dark mode caused by the tunable conductivity of graphene. This work provides an alternative way for innovative design and realization of dynamically controllable terahertz slow light devices, which may show potential applications in future terahertz wireless communications.

The interplay between order and disorder in photonic lattices opens up a wide range of novel optical scattering mechanisms, resonances, and applications that can be obscured by typical ordered design approaches to photonics. Striking examples include Anderson localization, random lasers, and visible light scattering in biophotonic structures such as butterfly wings. In this work, we present a profound example of light localization in photonic crystal waveguides by introducing long-range correlated disorder. Using a rigorous three-dimensional Bloch mode expansion technique, we demonstrate how inter-hole correlations have a negative contribution to the total out-of-plane radiative losses, leading to a pronounced enhancement of the quality factor, $Q$, and $Q/V$ cavity figures of merit in the long-range correlation regime. Subsequently, the intensity fluctuations of the system are shown to globally increase with the correlation length, highlighting the non-trivial role of long-range disorder on the underlying scattering mechanisms. We also explore the possibility of creating ultra-high quality cavity modes via inter-hole correlations, which have various functionalities in chip-based nonlinear optics and waveguide cavity-quantum electrodynamics.

Refractive index fronts propagating in waveguides are special spatiotemporal perturbations. The interaction of light with such fronts can be described in terms of an indirect transition where the frequency and wavenumber of a guided mode both are changed. In recent years, front-induced transitions have been used in dispersion-engineered waveguides for frequency conversion, optical delays, and bandwidth and pulse duration manipulation. These concepts have originated from different research areas of photonics, such as nonlinear fibre optics, slow-light waveguides, plasma physics, moving media and relativistic effects. Here, we discuss these concepts, providing a unifying theoretical description and highlight the potential of this exciting research field for light manipulation in guided optics., Gaafar, M.A., Baba, T., Eich, M. et al. Front-induced transitions. Nat. Photonics 13, 737–748 (2019) doi:10.1038/s41566-019-0511-6

In this paper, we theoretically analyze the slow-light π-phase-shifted fiber Bragg grating (π-FBG) and its applications for single and multipoint/quasi-distributed sensing. Coupled-mode theory (CMT) and transfer matrix method (TMM) are used to establish the numerical modeling of slow-light π-FBG. The impact of slow-light FBG parameters, such as grating length (L), index change (Δn), and loss coefficient (α) on the spectral properties of π-FBG along with strain and thermal sensitivities are presented. Simulation results show that for the optimum grating parameters L = 50 mm, Δn = 1.5×10−4, and α = 0.10 m-1, the proposed slow-light π-FBG is characterized with a peak transmissivity of 0.424, the maximum delay of 31.95 ns, strain sensitivity of 8.380 μe-1, and temperature sensitivity of 91.064 °C-1. The strain and temperature sensitivity of proposed slow-light π-FBG is the highest as compared to the slow-light sensitivity of apodized FBGs reported in the literature. The proposed grating have the overall full-width at half maximum (FWHM) of 0.2245 nm, and the FWHM of the Bragg wavelength peak transmissivity is of 0.0798 pm. The optimized slow-light π-FBG is used for quasi-distributed sensing applications. For the five-stage strain quasi-distributed sensing network, a high strain dynamic range of value 1469 μe is obtained for sensors wavelength spacing as small as 2 nm. In the case of temperature of quasi-distributed sensing network, the obtained dynamic range is of 133 °C. For measurement system with a sufficiently wide spectral range, the π-FBGs wavelength grid can be broadened which results in substantial increase of dynamic range of the system.

Electromagnetically induced transparency like effect, which can form a sharp transmission window within a broad spectrum, has been widely studied due to its potential in many optical devices. In this work, unlike traditional couple-ring resonator structure, we propose a new scheme to achieve electromagnetically induced transparency using only one hole-ring resonator. Thus, the proposed scheme can greatly miniaturize the device. At the wavelength of about 1550 nm, the structure can form multiple transparency windows and can behave as a refractive index sensor with high resolution, the figure of merit can reach larger than 530. The proposed scheme can pave the way for many practical applications such as optical sensors, optical switches and slow light devices.

Slow-light performance based on photonic-crystal coupled-cavity waveguide (PC-CCW) is investigated with low-dispersion pulse transmission. A chain of periodically arranged specific defect cavities of air holes are created along the axis of the waveguide and each of the cavities looks like an irregular hexagon with arrow-like left and right sides to form the PC-CCW. By changing the radius and position of certain defects, slow light modes are achieved. Through optimization, we obtained a highest compromise value for an operating bandwidth of 12.9 nm with an average group index of 70.75 and a normalized delay-bandwidth product (NDBP) of 0.5873. This value of NDBP with low group velocity dispersion is higher than that in other structures based on waveguides or coupled cavities reported previously. The arrow-like shape of the cavity sides is shown to be perfect for reducing the transmission loss of the waveguide. The proposed structure with high NDBP is beneficial for buffering applications and slow light devices.

We theoretically investigate force-dependent induced transparency effect of an output field in an atom-assisted optomechanical system where a mechanical resonator is driven by a time-dependent force. It is shown that there exists an additional transparency window besides a standard optomechanical induced transparency window due to the Jaynes–Cummings coupling between a two-level atom and cavity. We find that the transmission amplitude of optomechanical induced transparency effect is largely suppressed or enhanced by adjusting the amplitude and phase of the external force. Benefiting from the atom and the external force, the effect of tunable fast and slow light can be realizable, which may be helpful for quantum information storage processing.

We demonstrate a classical analogue of electromagnetically induced transparency (EIT) metamaterial that consisting of anomalous split resonance ring and cross-shaped wire with simulations and experiments. The transmission spectrum is tunable with the incident angles varies from 0° to 60° in different modes. Under normal incident wave, the metamaterial leads to polarization insensitive EIT-like effect. The distributions of surface current at peaks and dips of transmission have been analyzed. Furthermore, the group index presents maximum at the transparency peak, which verifies the slow light effect. The proposed metamaterial could provide an avenue for various applications, such as refractive index sensor and microwave switch.

We reveal that non-Hermitian lossy couplings in an inter-cavity light transfer process are crucial for an optimum light transfer, unlike the prevailed belief. Our results turn out the fact that the light transfer can have multiple maxima following the increased inter-cavity distance. To validate this finding both in the weak and strong coupling regimes, we demonstrate our claim in the vicinity of the so-called exceptional point. We believe our results can contribute to realizing coupled-optical-cavity-based devices which is functional with an ultra-efficient light transfer, especially when the device scale is as small as the operation wavelength.

An optical modulator based on an engineered silicon-indium tin oxide (Si-ITO) structure is proposed with a tunable group delay. A large group delay is reported by slowing down the light in a Si-ITO grating embedded rib structure. Optical modulation and a tunable group delay are realized by utilizing the electrically tunable permittivity of ITO in the engineered waveguide. The extinction ratio over 8 dB for a 10 µm long device and the modulation efficiency around 12 V-µm are reported for a wide wavelength from 1530 to 1570 nm. The resulting modulation efficiency and the extinction ratio show a significant improvement as compared to conventional modulators based on rib waveguides. We also report around 82 psec electrical tuning in the group delay for a wide wavelength range. This concept is promising in view of realizing tunable delay lines, along with slow light modulators with a reduced device footprint and low energy dissipation.

In this paper, we apply the scatterers of cylindrical rings to a two-dimensional photonic crystals (PCs) slab. The effects of the number of layers, the thickness, the index, and the height of the cylindrical layers on the photonic band gaps (PBGs) of such slab with different lattice arrangements are studied. It turns out that our new structure helps to obtain a large range of the PBGs. The maximum bandwidth is obtained with the value of 0.1497 (2πc/a). The PBGs are moved to the lower frequencies with the augment of thickness, refractive index, and height. The choice of height, refractive index, and thickness is a trade-off, and adding the number of dielectric layers is not always positively correlated with the area of PBGs. In addition, in the W1 waveguide with a triangular lattice layout, we obtain a slow light of 0.026×c. Compared with the square lattice, the triangular lattice is more suitable for slowing down the speed of light.

We investigate a theoretical model for a dynamic Moir\'e grating which is capable of producing slow and stopped light with improved performance when compared with a static Moir\'e grating. A Moir\'e grating superimposes two grating periods which creates a narrow slow light resonance between two band gaps. A Moir\'e grating can be made dynamic by varying its coupling strength in time. By increasing the coupling strength the reduction in group velocity in the slow light resonance can be improved by many orders of magnitude while still maintaining the wide bandwidth of the initial, weak grating. We show that for a pulse propagating through the grating this is a consequence of altering the pulse spectrum and therefore the grating can also perform bandwidth modulation. Finally we present a possible realization of the system via an electro-optic grating by applying a quasi-static electric field to a poled $\chi^{(2)}$ nonlinear medium., Comment: 9 pages, 14 figures

Light pulses entering an elongated bottle microresonator (BMR) from a transversely oriented input-output waveguide (microfiber) slowly propagate along the BMR length and bounce between turning points at its constricting edges. To avoid insertion losses and processing errors, a pulse should completely transfer from the waveguide into the BMR and, after being processed, completely return back into the waveguide. For this purpose, the waveguide and BMR should be impedance matched along the pulse bandwidth. Here we show how to enhance the impedance matched bandwidth by optimization of the BMR effective radius variation in a small vicinity of the input-output waveguide., 5 pages, 5 figures

Slow light is the phenomenon of a large reduction in group velocity, normally due to a material resonance. Slow light causes spatial compression and consequently large field enhancement which has been suggested as a method for increasing nonlinear optical effects. Here we investigate the use of a Moiré grating for enhancing second-harmonic generation (SHG) in a χ(2) medium.&more...

Light detection and ranging (LiDAR) is drawing attention along with the topic of autonomous vehicles, but conventional LiDAR is large and expensive due to its mechanical scanner. Therefore, compact nonmechanical solid-state LiDAR, exploiting flash illumination, optical phased array, and slow light grating have been actively developed. This presentation reviews the status of the mechanical and nonmechanical LiDAR development and introduces the challenge to realize a solid-state one using Si photonic crystal slow light waveguide.

Ultra-narrow optical spectral features have broad applications in spectroscopy, slow light, and sensing. Features approaching sub-MHz, or equivalently, Q-factors approaching 1 billion and beyond, are challenging to obtain in solid-state systems, ultimately limited by loss. We present a new paradigm to achieve tunable sub-MHz 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 fiber, 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

Transparent materials do not absorb light but have profound influence on the phase evolution of transmitted radiation. One consequence is chromatic dispersion, i.e., light of different frequencies travels at different velocities, causing ultrashort laser pulses to elongate in time while propagating. Here we experimentally demonstrate ultrathin nanostructured coatings that resolve this challenge: we tailor the dispersion of silicon nanopillar arrays such that they temporally reshape pulses upon transmission using slow light effects and act as ultrashort laser pulse compressors. The coatings induce anomalous group delay dispersion in the visible to near-infrared spectral region around 800 nm wavelength over an 80 nm bandwidth. We characterize the arrays’ performance in the spectral domain via white light interferometry and directly demonstrate the temporal compression of femtosecond laser pulses. Applying these coatings to conventional optics renders them ultrashort pulse compatible and suitable for a wide range of applications., Controlling the dispersion of femtosecond light pulses remains a key challenge for their application. Here, the authors report dispersion-engineered transmissive nanocoatings for ultrashort laser pulse compression in the vis-NIR spectral region.

Controlling the light propagation velocity with aim to slow it down and hence creating the group delay of transmitted light using fiber Bragg gratings is possible by carefully adjusting their parameters. For the past decade or so they have been considered a prospective candidate for optical buffers in transparent optical network elements as well as for the sensitivity and resolution improving approach in sensing solutions, which has initiated an increased focus on their research. Since the group time delay depends on various FBG parameters, numerical investigation accurately modelling underlying physics of the slow light mechanism in FBGs still presents valuable subject. In this paper the influence of several FBG parameters is numerically investigated, specifically the refractive index modulation depth, the grating length and other modelling aspects on spectral characteristics of group delays representing structural slow light in uniform FBGs.

Controlling pulse dispersion in temporal and spectral domains are imperative for modern applications of ultrashort pulses in optical communications, opto-electronics and imaging. In this work we present the detailed study of ultrashort pulse dispersion in a hyperbolic metamaterial. This highly anisotropic structure comprised of plasmonic nanorods promises the flexibility in dispersion engineering beyond the capabilities of classical materials. We show experimentally and theoretically that the delicate balance between local and nonlocal effects play a crucial role in the optical response of the system leading to controlled switching between “superluminal” and stopped light propagation regimes of electromagnetic waves. We compare the effective medium simulations with full vectorial numerical analysis combining split-step Fourier method and finite difference time domain algorithm. The experimental studies were performed with 150 fs pulses in 560-740 nm spectral range. Using the high-resolution spectral interferometry, we were able to retrieve up to third-order dispersion terms of the metamaterial, confirming the theoretical predictions of pulse dynamics, and validate the strength of the effect. The dispersive properties of plasmonic nanorod metamaterials may play a crucial role in advanced dispersion compensation devices in telecommunication applications, supersensitive tuneable interferometers and slow light buffers in all optical systems.

We determine the fundamental limit of microresonator field uniformity. It can be achieved in a specially designed microresonator, called a bat microresonator, fabricated at the optical fiber surface. We show that the relative nonuniformity of an eigenmode amplitude along the axial length L of an ideal bat microresonator cannot be smaller than 1 3 π 2 n 4 λ − 4 Q − 2 L 4 , where n , λ , and Q are its refractive index, eigenmode wavelength, and Q -factor, respectively. For a silica microresonator with Q = 10 8 , this eigenmode has axial speed ∼ 10 − 4 c , where c is the speed of light in vacuum, and its nonuniformity along length L = 100 \unicode{x00B5} m at wavelength λ = 1.5 µ m is ∼ 10 − 7 . For a realistic fiber with diameter 100 µm and surface roughness 0.2 nm, the smallest eigenmode nonuniformity is ∼ 0.0003 . As an application, we consider a bat microresonator evanescently coupled to high Q -factor silica microspheres, which serves as a reference supporting ultraprecise straight-line translation.

We report numerically electromagnetic-induced transparency (EIT) and Fano resonances in simple plasmonic metasurfaces consist of gold nanobars arranged in Pi, H and four shaped fashion. The bright and dark elements in the metasurfaces are responsible for the emergence of EIT and Fano effects in the transmission spectrum. The concept of symmetry breaking is also introduced by incorporating multiple cavities in the metasurface, which relaxes the dipole coupling selection rules resulting in a mixture of dipole and higher order modes that interact and engenders EIT and Fano modes simultaneously in a nanostructure. Furthermore, the EIT and Fano resonances experience a significant red-shift by increasing the refractive index of the background medium due to which high sensitivity of around 574 nmRIU -1 , figure of merit of 32, and contrast ratio of 41% are realized. Moreover, the effective group index of the proposed metasurface is retrieved and is observed to be very high around the steep asymmetric Fano line shape and within the EIT window, signifying its potential use in slow light applications.

We investigate the optical response of the probe output field in a three-mode cavity optomechanical system, in which both the optical modes are coupled to the same mechanical oscillator. The optomechanically induced transparency and amplification effects can be achieved by controlling the optomechanical interaction in the auxiliary optical mode. With the tunneling interaction between two optical modes, we not only observe the Fano and optical absorption effects in the red-detuned regime but also find two controllable singular points representing the considerable optical amplification effect in the blue-detuned regime and analyze the conversion between fast light and slow light. These optical properties of the probe output field may benefit forward achieving the potential applications in coherent control of laser pulse and optical storage.

Funding: Hellenic Foundation for Research and Innovation (HFRI), General Secretariat for Research and Technology (GSRT) (No. 1819) (KLT). The time-bandwidth limit is a mathematical tenet that affects all reciprocal resonators, stating that the product of the spectral bandwidth that can couple into a resonant system and its characteristic energy decay time is always equal to 1. Here, we develop an analytical and numerical model to show that introducing nonreciprocal coupling to a generalized resonator changes the power balance between the reflected and intracavity fields, which consequently overcomes the time-bandwidth limit of the resonant system. By performing a full evaluation of the time-bandwidth product (TBP) of the modeled resonator, we show that it represents a measure of the increased delay imparted to a light wave, with respect to what the bandwidth of the reciprocal resonant structure would allow to the same amount of in-coupled power. No longer restricted to the value 1, we show that the TBP can instead be used as a figure of merit of the improvement in intracavity power enhancement due to the nonreciprocal coupling. Postprint

Slow and fast light require precise tailoring of the refractive index of a medium. We present the first experimental demonstration of such control inside plasma, reporting group velocity of light between 0.12c and -0.34c.

Optical fibers play a key role in many different fields of science and technology. In particular, fibers with a diameter of several micrometers are intensively used in photonics. For these applications, it is often important to precisely know and control the fiber radius. Here, we demonstrate a novel technique to determine the local radius variation of a 30-micrometer diameter silica fiber with sub-\AA ngstr\"om precision with axial resolution of several tens of micrometers over a fiber length of more than half a millimeter. Our method relies on taking an image of the fiber's whispering-gallery modes (WGMs). In these WGMs, the speed of light propagating along the fiber axis is strongly reduced. This enables us to determine the fiber radius with a significantly enhanced precision, far beyond the diffraction limit. By exciting different axial modes, we verify the precision and reproducibility of our method and demonstrate that we can achieve a precision better than 0.3 \AA. The method can be generalized to other experimental situations where slow light occurs and, thus, has a large range of potential applications in the realm of precision metrology and optical sensing., Comment: 9 pages, 6 figures

We show that stable slow-light dark solitons with finite continuous-wave background can be generated in a Λ -type atomic system via electromagnetically induced transparency (EIT). We also show that such dark solitons can be stored and retrieved with high efficiency and fidelity. Moreover, an optical routing of them can be realized via EIT in the system with a double- Λ -type level configuration.

Usually, the pump light in lasers should perform fast light owing to operating in the absorption band. In this study, we observe and demonstrate anomalous slowdown of the pump light in a Raman fiber laser. Experiments show that the pump light can be slowed down to sub-nanoseconds at a repetition rate of 50–500 MHz. Theoretical analysis shows that the hole-burning effect is formed at the Raman gain spectrum in the saturation regime, which imposes on the pump light by normal dispersion. Consequently, the pump light experiences an unusual slow light effect rather than the fast light effect after absorption. We believe it has promising potentials in the improvement of ultrashort pulse generation, and may have significant influence on improving the conversion efficiency in pulse-pumped laser systems.

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.