Ultrafast Signal Research Articles

Improved Ion Transport in Hydrogel-Based Nanofluidics for Osmotic Energy Conversion.

In nature, ultrafast signal transfer based on ion transport, which is the foundation of biological processes, commonly works in a hydrogel–water mixed mechanism. Inspired by organisms’ hydrogel-based system, we introduce hydrogel into nanofluidics to prepare a hydrogel hybrid membrane. The introduction of a space charged hydrogel improves the ion selectivity evidently. Also, a power generator based on the hydrogel hybrid membrane shows an excellent energy conversion property; a maximum power density up to 11.72 W/m2 is achieved at a 500-fold salinity gradient. Furthermore, the membrane shows excellent mechanical properties. These values are achievable, which indicates our membrane’s huge potential applications in osmotic energy conversion.

Recent Progress in Optoelectronic Synapses for Artificial Visual‐Perception System

The human visual system undertakes most of the information perceiving tasks, and nearly 80% of the perceived information is obtained via the visual system. The basic functions of the human visual system can be emulated by neuromorphic visual‐perception systems in the light region from UV to near‐IR. An optoelectronic synapse served as the basic unit of the neuromorphic visual‐perception system is required to combine photosensing function and synaptic element. In addition, optoelectronic synapses demonstrate the prospective advantage of large bandwidth, ultrafast signal transmission, and low electrical energy loss, which is expected in the photonic signal‐triggered computing. This work reviews recent progress in the optoelectrical synapses. Device architectures and working mechanisms are discussed. The applications in the artificial visual‐perception system for image memorization and pattern recognition are reviewed. The main challenges and opportunities of optoelectrical synapses are also presented.

Visualizing conical intersection passages via vibronic coherence maps generated by stimulated ultrafast X-ray Raman signals

The rates and outcomes of virtually all photophysical and photochemical processes are determined by conical intersections. These are regions of degeneracy between electronic states on the nuclear landscape of molecules where electrons and nuclei evolve on comparable timescales and thus become strongly coupled, enabling radiationless relaxation channels upon optical excitation. Due to their ultrafast nature and vast complexity, monitoring conical intersections experimentally is an open challenge. We present a simulation study on the ultrafast photorelaxation of uracil, based on a quantum description of the nuclei. We demonstrate an additional window into conical intersections obtained by recording the transient wavepacket coherence during this passage with an X-ray free-electron laser pulse. Two major findings are reported. First, we find that the vibronic coherence at the conical intersection lives for several hundred femtoseconds and can be measured during this entire time. Second, the time-dependent energy-splitting landscape of the participating vibrational and electronic states is directly extracted from Wigner spectrograms of the signal. These offer a physical picture of the quantum conical intersection pathways through visualizing their transient vibronic coherence distributions. The path of a nuclear wavepacket in the vicinity of the conical intersection is directly mapped by the proposed experiment.

Temporal Imaging for Ultrafast Spectral-Temporal Optical Signal Processing and Characterization

Temporal imaging is one of the most promising all-optical signal processing techniques that enable single-shot ultrafast signal characterization in both spectral and temporal domains. However, conventional temporal imaging systems have inherent constraints such as limited record length, narrow spectral observation range and complex system configuration, which prevent their potential to be fully unleashed. In this article, we review the recent efforts in developing advanced temporal imaging systems with much enhanced performance. Their powerful applications are demonstrated by real-time single-shot characterization of Kerr soliton generation in microresonator, noise-like pulses in mode-locked fiber lasers, broadband frequency combs and the dynamic waveform in ultrafast optical tomography. In addition, potential future developments of this technique in several important directions are discussed.

Highly nonlinear trion-polaritons in a monolayer semiconductor

Highly nonlinear optical materials with strong effective photon-photon interactions are required for ultrafast and quantum optical signal processing circuitry. Here we report strong Kerr-like nonlinearities by employing efficient optical transitions of charged excitons (trions) observed in semiconducting transition metal dichalcogenides (TMDCs). By hybridising trions in monolayer MoSe2 at low electron densities with a microcavity mode, we realise trion-polaritons exhibiting significant energy shifts at small photon fluxes due to phase space filling. We find the ratio of trion- to neutral exciton–polariton interaction strength is in the range from 10 to 100 in TMDC materials and that trion-polariton nonlinearity is comparable to that in other polariton systems. The results are in good agreement with a theory accounting for the composite nature of excitons and trions and deviation of their statistics from that of ideal bosons and fermions. Our findings open a way to scalable quantum optics applications with TMDCs.

Fast- and slow-light-enhanced light drag in a moving microcavity

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.

Functional all-optical logic gates for true time-domain signal processing in nonlinear photonic crystal waveguides.

We present a conceptual study on the realization of functional and easily scalable all-optical NOT, AND and NAND logic gates using bandgap solitons in coupled photonic crystal waveguides. The underlying structure consists of a planar air-hole type photonic crystal with a hexagonal lattice of air holes in crystalline silicon (c-Si) as the nonlinear background material. The remaining logical operations can be performed using combinations of these three logic gates. A unique feature of the proposed working scheme is that it operates in the true time-domain, enabling temporal solitons to maintain a stable pulse envelope during each logical operation. Hence, multiple concatenated all-optical logic gates can be easily realized, paving the way to multiple-input all-optical logic gates for ultrafast full-optical digital signal processing. In the suggested setup, there is no need to amplify the output signal after each operation, which can be directly used as a new input signal for another logical operation. The feasibility and efficiency of the proposed logic gates as well as their scalability is demonstrated using our original rigorous theoretical formalism together with full-wave computational electromagnetics.

Microwave Photonic Radars

As the only method for all-weather, all-time and long-distance target detection and recognition, radar has been intensively studied since it was invented, and is considered as an essential sensor for future intelligent society. In the past few decades, great efforts were devoted to improving radar's functionality, precision, and response time, of which the key is to generate, control and process a wideband signal with high speed. Thanks to the broad bandwidth, flat response, low loss transmission, multidimensional multiplexing, ultrafast analog signal processing and electromagnetic interference immunity provided by modern photonics, implementation of the radar in the optical domain can achieve better performance in terms of resolution, coverage, and speed which would be difficult (if not impossible) to implement using traditional, even state-of-the-art electronics. In this tutorial, we overview the distinct features of microwave photonics and some key microwave photonic technologies that are currently known to be attractive for radars. System architectures and their performance that may interest the radar society are emphasized. Emerging technologies in this area and possible future research directions are discussed.

On-chip terahertz modulation and emission with integrated graphene junctions

The efficient modulation and control of ultrafast signals on-chip is of central importance in terahertz (THz) communications and a promising route toward sub-diffraction limit THz spectroscopy. Two-dimensional (2D) materials may provide a platform for these endeavors. We explore this potential, integrating high-quality graphene p–n junctions within two types of planar transmission line circuits to modulate and emit picosecond pulses. In a coplanar strip line geometry, we demonstrate the electrical modulation of THz signal transmission by 95%. In a Goubau waveguide geometry, we achieve complete gate-tunable control over THz emission from a photoexcited graphene junction. These studies inform the development of on-chip signal manipulation and highlight prospects for 2D materials in THz applications.

Adiabatic frequency shifting in epsilon-near-zero materials: the role of group velocity

The conversion of a photon’s frequency has long been a key application area of nonlinear optics. It has been discussed how a slow temporal variation of a material’s refractive index can lead to the adiabatic frequency shift (AFS) of a pulse spectrum. Such a rigid spectral change has relevant technological implications, for example, in ultrafast signal processing. Here, we investigate the AFS process in epsilon-near-zero (ENZ) materials and show that the frequency shift can be achieved in a shorter length if operating in the vicinity of R e { ε r } = 0 . We also predict that, if the refractive index is induced by an intense optical pulse, the frequency shift is more efficient for a pump at the ENZ wavelength. Remarkably, we show that these effects are a consequence of the slow propagation speed of pulses at the ENZ wavelength. Our theoretical predictions are validated by experiments obtained for the AFS of optical pulses incident upon aluminum zinc oxide thin films at ENZ. Our results indicate that transparent metal oxides operating near the ENZ point are good candidates for future frequency conversion schemes.

Synthetic photonic lattice for single-shot reconstruction of frequency combs

We formulate theoretically and demonstrate experimentally an all-optical method for reconstruction of the amplitude, phase, and coherence of frequency combs from a single-shot measurement of the spectral intensity. Our approach exploits synthetic frequency lattices with pump-induced spectral short- and long-range couplings between different signal components across a broad bandwidth of hundreds of GHz in a single nonlinear fiber. When combined with ultra-fast signal conversion techniques, this approach has the potential to provide real-time measurement of pulse-to-pulse variations in the spectral phase and coherence properties of exotic light sources.

Manipulation of the dielectric properties of narrow-band gap material by an ultrashort laser pulse

The control of dielectric responses of narrow-band gap materials by an ultrashort laser pulse is investigated theoretically using time-dependent density functional theory, taking gallium phosphide as a concrete example. We demonstrate that, by using properly selected laser parameters an ultrashort laser pulse can induce reversible change of the electric and optical properties. The anisotropy of the transient permittivity of the material throughout the pulse duration can be enhanced so strongly that the dispersion of electromagnetic waves in the material becomes hyperbolic in some frequency interval with increasing laser energy. We describe the evolution of the transient permittivity throughout the duration of the laser pulse, pointing out to a fundamental possibility for applications of ultrafast all-optical signal processing and optical-field effect devices.

Controlling the non-linear optical properties of MgO by tailoring the electronic structure

The study of the non-linear response of matter to high electric fields has recently encompassed harmonic generation in solids at near-infrared (NIR) driving wavelengths. Interest has been driven by the prospect of ultrafast signal processing and all-optical mapping of electron wave-functions in solids. Engineering solid-state band structures to control the non-linear process has already been highlighted theoretically. Here, we show experimentally for the first time that second harmonic generation (SHG) can be enhanced by doping crystals of magnesium oxide (MgO) with chromium (Cr) atoms. We show that the degree of enhancement depends non-linearly on dopant concentration. The SHG efficiency is shown to increase when Cr dopants are introduced into pure MgO. A physical picture of the effect of Cr dopants is aided by density functional theory (DFT) calculations of the electronic structure for pure and doped samples. This work shows an unambiguous enhancement of the SHG efficiency by modifying the electronic structure. The observed effects are consistent with an electronic structure that facilitates the surface induced SHG and demonstrates a minimal angular dependence. This work highlights the potential of manipulating the electronic structure of solids to control or test theories of their non-linear optical response.

Design of a Miniaturized Broadband Silicon Hybrid Plasmonic Temporal Integrator for Ultrafast Optical Signal Processing

A fractional-order silicon hybrid plasmonic temporal integrator with considerably wide full-width at half- maximum (FWHM) bandwidth of ∼1 THz, higher than the previously reported photonic temporal integrators, and noticeably compact footprint of ∼5 μm × 3 μm is proposed. The presented plasmonic temporal integrator is implemented based on a double-ring add-drop resonator structure with ring radii of 1.5 μm. The 3-D finite-difference time-domain (3D-FDTD) simulation results show that the fractional-order of the integrator is 0.64 and it is capable of processing the ultrashort pulses with the duration of 450 fs. The integration time window is estimated as 520 fs, while the achieved energy efficiency is as high as −1.92 dB. The compact size and wide bandwidth of the proposed integrator makes it suitable for ultrafast optical signal processing applications. As an example, the functionality of the designed integrator for solving ordinary differential equations (ODEs) is investigated.

Adaptive pre-shaping for ultrashort pulse control during propagation in AZO/ZnO multilayered metamaterial at the epsilon-near-zero spectral point

A novel approach to the application of an adaptive pre-shaping algorithm for ultrashort pulse distortion compensation during the propagation in AZO/ZnO multilayered metamaterials (thickness 300-700 nm) at the epsilon-near-zero spectral point is investigated. We show that using the Broyden-Fletcher-Goldfarb-Shanno algorithm to minimize the residual between frequency-resolved optical gating traces of the distorted output pulse and the zero phase pulse of 100 fs duration can yield increased output pulse field strength and a central frequency shift towards the epsilon-near-zero spectral point, which can be of future use for applications in ultrafast communication, signal processing, and super resolution imaging.

Third-order nonlinear optical susceptibility of crystalline oxide yttria-stabilized zirconia

Nonlinear all-optical technology is an ultimate route for the next-generation ultrafast signal processing of optical communication systems. New nonlinear functionalities need to be implemented in photonics, and complex oxides are considered as promising candidates due to their wide panel of attributes. In this context, yttria-stabilized zirconia (YSZ) stands out, thanks to its ability to be epitaxially grown on silicon, adapting the lattice for the crystalline oxide family of materials. We report, for the first time to the best of our knowledge, a detailed theoretical and experimental study about the third-order nonlinear susceptibility in crystalline YSZ. Via self-phase modulation-induced broadening and considering the in-plane orientation of YSZ, we experimentally obtained an effective Kerr coefficient of n ^ 2 YSZ = 4.0 ± 2 × 10 − 19 m 2 · W − 1 in an 8% (mole fraction) YSZ waveguide. In agreement with the theoretically predicted n ^ 2 YSZ = 1.3 × 10 − 19 m 2 · W − 1 , the third-order nonlinear coefficient of YSZ is comparable with the one of silicon nitride, which is already being used in nonlinear optics. These promising results are a new step toward the implementation of functional oxides for nonlinear optical applications.

Temporal Imaging Using Dispersive Gradient-Index Time Lenses

We study the temporal imaging with a gradient-index (GRIN) time lens in which both dispersion and time-varying quadratic phase modulation are introduced. The lens is built by using cross-phase modulation between a signal and a parabolic pump pulse in the nonlinear optical fiber with group velocity dispersion. A general theoretical analysis of temporal imaging by the GRIN time lens is performed in detail. Compared to thin lens in previous studies, the introduction of dispersion, benefits periodic self-imaging of both time waveform and frequency spectrum for optical pulses. Interestingly, the temporal and spectral profiles of the input can be exchanged with each other periodically in the GRIN time lens. By combining the GRIN time lens with dispersive elements on both sides, the temporal waveforms of incident optical pulses can be stretched and compressed. For an input signal composed of multiple pulses experiencing different dispersions, the temporal depth imaging of the signal is realized by scanning the timing of the pump pulse applied on the lens. The study may find great applications in ultrafast optical signal processing and measurement.

Measurement of ultra-fast signal progression related to face processing by 7T fMRI.

Given that the brain is a dynamic system, the temporal characteristics of brain function are important. Previous functional magnetic resonance imaging (fMRI) studies have attempted to overcome the limitations of temporal resolution to investigate dynamic states of brain activity. However, finding an fMRI method with sufficient temporal resolution to keep up with the progress of neuronal signals in the brain is challenging. This study aimed to detect between‐hemisphere signal progression, occurring on a timescale of tens of milliseconds, in the ventral brain regions involved in face processing. To this end, we devised an inter‐stimulus interval (ISI) stimulation scheme and used a 7T MRI system to obtain fMRI signals with a high signal‐to‐noise ratio. We conducted two experiments: one to measure signal suppression depending on the ISI and another to measure the relationship between the amount of suppression and the ISI. These two experiments enabled us to measure the signal transfer time from a brain region in the ventral visual stream to its counterpart in the opposite hemisphere through the corpus callosum. These findings demonstrate the feasibility of using fMRI to measure ultra‐fast signals (tens of milliseconds) and could facilitate the elucidation of further aspects of dynamic brain function.

Large scale indium tin oxide (ITO) one dimensional gratings for ultrafast signal modulation in the visible spectral region.

Indium tin oxide (ITO) is a heavily doped semiconductor with a plasmonic response in the near infrared region. When exposed to light, the distribution of conduction band electron induces a change in the real and imaginary parts of the dielectric permittivity. The coupling of the electromagnetic waves with the electrons in the conduction band of metallic nanostructures with ultrashort light pulses results in a nonlinear plasmonic response. Such optical modulation occurring on ultrafast time scales, e.g. picosecond response times, can be exploited and used to create integrated optical components with terahertz modulation speed. Here, we present a photophysical study on a one dimensional ITO grating, realized using a femtosecond micromachining process, a very industrially accessible technology. The geometries, dimensions and pitch of the various gratings analyzed are obtained by means of direct ablation in a controlled atmosphere of a homogeneous thin layer of ITO deposited on a glass substrate. The pitch has been selected in order to obtain a higher order of the photonic band gap in the visible spectral region. Femtosecond micromachining technology guarantees precision, repeatability and extreme manufacturing flexibility. By means of ultrafast pump-probe spectroscopy, we characterize both the plasmon and inter-band temporal dynamics. We observe a large optical nonlinearity of the ITO grating in the visible range, where the photonic band gap occurs, when pumped at the surface plasmon resonance in the near infrared (1500 nm) region. All together, we show the possibility of all-optical signal modulation with heavily doped semiconductors in their transparency window with a picosecond response time through the formation of ITO grating structures.

Temperature-responsive resistance sensitivity controlled by L-ascorbic acid and silane co-functionalization in flame-retardant GO network for efficient fire early-warning response

Frequent fire disasters have not only caused massive casualties, irreparable property and priceless artefacts loss, but also create severe environmental damage. Therefore, it is imperative, but also challenging, to obtain excellent fire resistance and efficient early-warning alarm response in precombustion of combustible materials. In this work, a green flame-retardant graphene oxide (GO) based paper/coating is designed and constructed via using a facile 3-methacryloxypropyltrimethoxysilane and L-ascorbic acid (LAA) co-functionalization strategy. The optimized co-modified GO network shows good structure stability and improves the flame retardancy of combustible materials. Further, such GO network can provide an ultrafast flame detection signal of only ~1 s and ideal fire early warning responses (e.g. a low responsive temperature of ~120 °C and an extremely rapid responsive time of ~7 s at 300 °C) in precombustion which are among the best performances of the state-of-the-art fire alarm sensors. Moreover, the transformation of silane molecules into a compact protective layer on sheets and the resistance transition of insulating GO network into conductive reduced GO path promoted by LAA molecules are clarified. This work provides a novel strategy and paradigm to achieving excellent flame resistance and ideal fire early warning response of GO network for fire safety and prevention applications.