Ultrafast Time Research Articles

Passively Q-Switched Thulium-Holmium Doped Fibre Laser with Electrochemical Exfoliation Graphene in Chitin Based Passive Saturable Absorber

Recent years saw low-dimensional carbon nanostructures like graphene as excellent saturable absorber (SA) for its unique electric and optical properties. Graphene exhibits ultrafast recovery times and high damage thresholds and can be manufactured by relatively simple methods. In this work, Graphene was synthesized using the electrochemical exfoliation (ECE) method. Chitin biopolymer was used to develop graphene-based SA as a more environmentally friendly alternative to conventional synthetic polymers. The fabricated passive SA was then integrated into Thulium-Holmium-doped fibre laser (THDFL) in ring cavity for pulsed laser generation via Q-switching. The optical and physical properties of the graphene-based SA were characterized using Raman spectroscopy, field emission scanning electron microscopy (FESEM) and energy-dispersive x-ray spectroscopy (EDS). Q-switched pulse was successfully demonstrated using the fabricated graphene SAs. ECE graphene-chitin SA generated a Q-switched pulse region at 2 micro-metre with the repetition rate and shortest pulse width of 67.78 kHZ and 10.03 micro-second, respectively. The signal-to-noise ratio (SNR) values obtained was 40 dB indicating high stability of the pulse laser-generated. This study demonstrated the potential of graphene embedded in chitin biopolymer as a sustainable and environmentally friendly SA for a wide range of applications, particularly for pulsed fibre lasers.

New ultrafast low-cost polyurethane based plastic scintillator

The study of plastic scintillators with polyurethane (PU) matrix based on cycloaliphatic isocyanate 4,4′-methylenebis (cyclohexyl isocyanate) (H12MDI) and 2,5-diphenyloxazole (PPO) as a luminescence activator is presented. A number of samples with different PPO content amounting up to 20 wt% were synthesized and their optical and luminescence characteristics were investigated. Pure H12MDI – PU based polymer exhibits low optical density in the spectral region 350–500 nm characteristic of the PPO emission band. The PU sample loaded with 5 wt% PPO demonstrated a light yield of 1088 photons/MeV and ultrafast rise and decay time of few picoseconds and few nanoseconds, respectively. The estimated coincidence time resolution (CTR) value for a PU sample is 1.4 times smaller than that known for a commercial scintillator BC-422 available for ultrafast timing applications.

Ultrafast control over chiral sum-frequency generation.

We introduce an ultrafast all-optical approach for efficient chiral recognition that relies on the interference between two low-order nonlinear processes that are ubiquitous in nonlinear optics: sum-frequency generation and third-harmonic generation. In contrast to traditional sum-frequency generation, our approach encodes the medium's handedness in the intensity of the emitted harmonic signal, rather than in its phase, and it enables full control over the enantiosensitive response. We show how, by sculpting the sub-optical-cycle oscillations of the driving laser field, we can force one molecular enantiomer to emit bright light while its mirror twin remains dark, thus reaching the ultimate efficiency limit of chiral sensitivity via low-order nonlinear light-matter interactions. Our work paves the way for ultrafast and highly efficient imaging and control of the chiral electronic clouds of chiral molecules using lasers with moderate intensities, in all states of matter: from gases to liquids to solids, with molecular specificity and on ultrafast time scales.

Large lateral photovoltaic effect with ultrafast optical relaxation time in SnS2/n-Si junctions.

A large lateral photovoltaic effect (LPE) with a fast optical response time is necessary to develop high-performance position-sensitive detectors. In this paper, we report an LPE with a high self-powered position sensitivity and ultrafast optical relaxation time in S n S 2/n-S i junctions prepared using pulsed laser deposition. A large built-in electric field was generated at the S n S 2/S i interface, which resulted in a large LPE with a positional sensitivity of up to 116mV/mm. Furthermore, the measurement circuit with multiple parallel resistors had a strong influence on the ultrafast optical response time of the LPE and the fastest optical relaxation time observed was ∼0.44µs. Our results suggest that the S n S 2/S i junction would be a promising candidate for a wide range of optoelectronic device applications.

Pulse electrochemical synaptic transistor for supersensitive and ultrafast biosensors

AbstractHigh sensitivity and fast response are the figures of merit for benchmarking commercial sensors. Due to the advantages of intrinsic signal amplification, bionic ability, and mechanical flexibility, electrochemical transistors (ECTs) have recently gained increasing popularity in constructing various sensors. In the current work, we have proposed a pulse‐driven synaptic ECT for supersensitive and ultrafast biosensors. By pulsing the presynaptic input (drain bias, VD) and setting the modulation potential (gate bias) near transconductance intersection (VG,i), the synaptic ECT‐based pH sensor can achieve a record high sensitivity up to 124 mV pH−1 (almost twice the Nernstian limit, 59.2 mV pH−1) and an ultrafast response time as low as 8.75 ms (7169 times faster than the potentiostatic sensors, 62.73 s). The proposed synaptic sensing strategy can effectively eliminate the transconductance fluctuation issue during the calibration process of the pH sensor and significantly reduce power consumption. Besides, the most sensitive working point at VG,i has been elaborately figured out through a series of detailed mathematical derivations, which is of great significance to provide higher sensitivity with quasi‐nonfluctuating amplification capability. The proposed electrochemical synaptic transistor paired with an optimized operating gate offers a new paradigm for standardizing and commercializing high‐performance biosensors.image

Flexible pressure sensor enhanced by polydimethylsiloxane and microstructured conductive networks with positive resistance-pressure response and wide working range

Improving the performance of flexible pressure sensors has received increasing attention owing to their numerous applications in advanced technologies. However, many approaches that yield exceptional performance often involve complex manufacturing processes and high costs, which are not conducive to the large-scale production and application of flexible pressure sensors. Therefore, this paper presents a simple and efficient method, combining a pre-strain strategy with polydimethylsiloxane infiltration, for manufacturing flexible pressure sensors with a wide working range, enhanced sensitivity, and good stability. Sensors fabricated using this method exhibit an improved sensitivity of 0.0331 kPa−1 within the broad working range of 0.11–1250 kPa, ultra-fast response time of 8 ms, and good stability as well as repeatability over 2500 cycles of 15% strain loading. The sensing mechanism and resistance model of the sensor were investigated, and a sensing model based on the tunneling effect and resistance model was developed. The coefficient of determination between the model and the experimental data exceeded 0.99, thereby verifying its validity. Finally, a sensor array was tested to locate the pressure and recognize the object shape within a large pressure range, demonstrating its potential applications in engineering fields with high-pressure requirements. This study provides a reference for the simple and cost-effective manufacturing of advanced flexible sensors and offers valuable insights into the large-scale production of flexible sensors.

Imaging and Controlling Ultrafast Electron Pulses Emitted from Plasmonic Nanostructures.

We experimentally study photoemission from gold nanodisk arrays using space-, time-, and energy-resolved photoemission electron microscopy. When excited by a plasmonic resonant infrared (IR) laser pulse, plasmonic hotspots are generated owing to local surface plasmon resonance. Photoelectrons emitted from each plasmonic hotspot form a nanoscale and ultrashort electron pulse. When the system is excited by an extreme ultraviolet (EUV) laser pulse, a uniformly distributed photoelectron cloud is formed across the sample surface. When excited by the IR and EUV laser pulses together, both the photoemission image and kinetic energy vary significantly for the IR laser-generated electrons depending on the time delay between the two laser pulses. These observations are well explained by the Coulomb interaction with the EUV laser-generated electron cloud. Our study offers a feasible approach to manipulate the energy of electron pulse emitted from a plasmonic nanostructure on an ultrafast time scale.

Three-dimensional coherent diffraction snapshot imaging using extreme-ultraviolet radiation from a free electron laser

The possibility of obtaining a three-dimensional (3D) representation of a single object with sub-µm resolution is crucial in many fields, from material science to clinical diagnostics. This is typically achieved through tomography, which combines multiple 2D images of the same object captured with different orientations. However, this serial imaging method prevents single-shot acquisition in imaging experiments, a mandatory feature for the development of time-resolved 3D imaging. In the present experiment, we report on an approach for extracting information on the 3D structure of an object by using extreme-ultraviolet (EUV) radiation. In this method, two EUV pulses hit an isolated 3D object simultaneously from different directions, generating independent coherent diffraction patterns, resulting in two distinct bidimensional projections after phase retrieval. These views are then used, in combination with a ray tracing algorithm, to retrieve information on the 3D structure. This EUV stereoscopic imaging approach, similar to the natural process of binocular vision, provides sub-µm spatial resolution and single-shot capability. Moreover, ultrafast time resolution and spectroscopy can be readily implemented, and a further extension to x-ray wavelengths can be envisioned as well.

Magnetic Field Sensor Based on Magnetic Optical Surface Plasmon Resonance

Surface plasmon resonance with high sensitivity is widely used in molecular dynamics detection. Herein, a magneto‐optic surface plasmon resonance (MOSPR) sensor based on Au/Co/Au thin film for detecting magnetic fields is developed. The strong coupling between the magneto‐optical waveguide mode and the surface plasmon resonance mode is analyzed and the linear relationship between the reflectivity of the MOSPR sensor and the magnetic field is established. The response, response–recovery time, and dispersion relationship for different magnetic fields are also numerically simulated. Additionally, experimental data on the MOSPR sensor's response to magnetic fields at room temperature (25 °C) are obtained. During the experiment, a differential signal in the light path is applied to reduce the noise generated by the environment. The results demonstrate that the MOSPR sensor exhibits high response, ultrafast response time, and long‐term stability. The study provides a promising approach for detecting magnetic fields with high sensitivity and accuracy.

Theoretical insights into ultrafast excited state proton transfer coupled with twisted intramolecular charge transfer mechanism in 7-(2ʹ-pyridyl)indole: Effect of hydrogen bonding

The ultrafast proton transfer (PT) process in the excited state of 7-(2′-pyridyl)indole (abbreviated as 7PyIn) has been elucidated by on-the-fly dynamics simulations using the algebraic-diagrammatic construction scheme of second order (ADC(2)) method under the resolution of identity (RI) approximation. The tautomers of 7PyIn alone and associated with an acetonitrile molecule in the 7PyIn-CH3CN complex are most likely found after the excited state intramolecular proton transfer (ESIntraPT) within an ultrafast time scale between 40 and 60 fs. While a methanol molecule in the complex of 7PyIn-CH3OH could establish both ESIntraPT and excited-state intermolecular proton transfer (ESInterPT). ESInterPT of 7PyIn-CH3OH possibly occurs through a concerted process via the intermolecular hydrogen bonding bridge of the methanol molecule almost twice faster than that of ESIntraPT. This slower circumstance of the ESIntraPT process found in 7PyIn-CH3OH may be caused by the competition between formations of the intramolecular and intermolecular hydrogen bonds between 7PyIn and methanol. In addition, after the accomplishment of excited state proton transfer (ESPT), the initiation of conical intersection could be taking place by the structural twist of 7PyIn. The geometries, energies, proton probabilities, and provided mechanisms via the dynamics simulations of 7PyIn and its solvent clusters exhibiting ESIntraPT and ESInterPT are explored to provide the dynamic pictures of the ESPT processes at the molecular level.

Rapid Detection and Selective Extraction of Au(III) from Electronic Waste Using an Oxime Functionalized MOF-on-MOF Heterostructure.

Gold is not only a precious resource for many industries but also a global contaminant as a result of the discharge of gold-containing waste. Designing appropriate materials for the detection and extraction of gold is of great significance for the economy and environment. However, most reports only focus on sensing or adsorption for gold because of the difficulty of combining two functions in a single material. Herein, an oxime-functionalized MOF-on-MOF heterostructure (MOF-808@ZIF-90-XE) for the simultaneous detection and recovery of Au3+ is first demonstrated. The constructed hybrid material exhibits good stability, ultra-fast response time (3.6 s) as well as excellent sensitivity for the detection of Au3+ . Experimental characterizations and theoretical calculations suggest that dynamic quenching and competitive adsorption may be possible sensing mechanisms. In addition, MOF-808@ZIF-90-XE shows outstanding selectivity and admirable adsorption capacity (1575mgg-1 ) for the capture of Au3+ owing to high surface area and abundant active adsorption sites. This paper provides a new strategy by designing a hybrid MOF-on-MOF heterostructure for the detection and extraction of gold.

Bioinspired Luminescent HOF-Based Foam as Ultrafast and Ultrasensitive Pressure and Acoustic Bimodal Sensor for Human-Machine Interactive Object and Information Recognition.

Bionic sensors have extensively served smart robots, medical equipment, and flexible wearable devices. The luminescent pressure-acoustic bimodal sensor can be treated as a remarkable, multifunctional, integrated bionic device. Here, a blue-emitting hydrogen-bonded organic framework (HOF-TTA) as luminogen combines with melamine foam (MF), generating the flexible and elastic HOF-TTA@MF (1 and 2) as a pressure-auditory bimodal sensor. In the luminescent pressure sensing process, 1 has excellent maximum sensitivity (132.02kPa-1 ), low minimum detection limit (0.0 1333Pa), fast response time (20ms), high precision and great recyclability. 2 as a luminescent auditory sensor exhibits the highest response to the 520Hz sound at 255-1453Hz. In the process of sensing sound at 520Hz, 2 possesses high sensitivity (1 648 441.3cpsPa-1 cm-2 ), low detection limit (0.36dB) and ultrafast response time (10ms) within 11.47-91.77dB. The sensing mechanisms toward pressure and auditory are analyzed in detail by finite element simulation. Furthermore, 1 and 2, as a human-machine interactive bimodal sensor, can recognize nine different objects and word information of "Health", "Phone", and "TongJi" with high accuracy and strong robustness. This work provides a facile fabricated method of luminescent HOF-based pressure-auditory bimodal sensors and endows them with new recognition functions and dimensions.

Relaxor ferroelectric ceramics with excellent energy storage density obtained from BT-based ceramics

Lead-free energy storage ceramic capacitors which have high-power density and ultrafast discharge time are widely used in electronic systems. However, lead-free energy storage ceramic materials still suffer from low energy storage density and poor stability. In this study, the synergistic optimization of polarization and electric field breakdown strength (Eb) is realized by doping with highly polarized Bi0.5Na0.5TiO3 (Ps > 42 μC/cm2) in BaTiO3-based ceramics, thereby optimizing the energy storage density. 0.15BNT ceramics have a large ΔP (Pmax-Pr) of 29 μC/cm2 under Eb of 525 kV/cm, resulting in excellent total energy storage density (Wtotal = 5.4 J/cm3), large recoverable energy storage density (Wrec = 4.8 J/cm3) and high energy storage efficiency (η = 89%). In particular, 0.15BNT ceramics also exhibit a high-power density (PD = 80 MW/cm3), a large discharge density (Wdis = 1.9 J/cm3), and an ultrafast discharge time (t0.9 = 37 ns). In addition, 0.15BNT ceramics also show excellent frequency stability (1–120 Hz) and temperature stability (25–200 °C). These excellent properties establish that 0.15BNT ceramics are promising material candidates for applications.

Photonic Crystal Based Ultrafast and Highly Sensitive Refractive Index Sensor

In this paper, a novel approach is proposed to detect the concentration of virus with different refractive indices in saliva using two-dimensional photonic crystal (PC) structures. The proposed 18×17 hexagonal PC structure consists of air holes in a silicon slab with silicon-on-insulator (SOI) techniques. The plane wave expansion (PWE) method is used to find the photonic band gap (PBG). The mid-gap wavelength of PBG increases linearly with a refractive index of infiltrated samples in air holes. The change in PBG size increases linearly by an average slope of 4.45 with respect to the change in refractive index concentration. The proposed sensor exhibits a maximum sensitivity of 1206.4 nm/RIU for 100% concentration of virus and minimum sensitivity of 950.4 nm/RIU for 0% concentration of virus with an ultra-fast average response time of 10.6 fs to test the samples. The present work can be useful for sensing variation in bio-molecular RI and other infection’s presence in various biological targets at an early stage.

A stable and highly-sensitive flexible gas sensor based on Ceria (CeO2) nano-cube decorated rGO nanosheets for selective detection of NO2 at room temperature

Sensing and monitoring nitrogen dioxide (NO2) gas is essential, due to its adverse effect on humans and the environment. However, conventional NO2 gas-sensing materials have limitations like elevated temperature operations, lower response, and low selectivity. Herewith, ceria nano-cube decorated reduced graphene oxide nanosheets (CeO2@rGO) were synthesized by a hydrothermal process, and as-synthesized products were characterized by various methods, viz. XRD, FE-SEM, FTIR, Raman spectrometer, BET, and IV characteristics. Afterwards, CeO2@rGO flexible gas sensor was fabricated by drop-cast technique, which is utilized for real-time monitoring of NO2 at room temperature (RT) (24ºC). Notably, the CeO2@rGO-based sensor upholds a significantly high response of 52.84 % towards 70 ppm of NO2, with an ultrafast response and recovery time of 7 s and 58 s respectively, with the LOD of 1 ppm. In addition, the CeO2@rGO flexible gas sensor exhibited long-term stability up to 30 days, solid repeatability, and stability in high relative humidity (RH) conditions. The marvelous sensing characteristics of the CeO2@rGO were attributed to the synergetic effect between CeO2 and rGO. The mechanism of sensing response was exhaustively interpreted by the “surface electron depletion layer mechanism”. Certainly, the scientific investigation conducted in the present study will undeniably advance the possible uses of detecting NO2 at RT.

Realizing ultrahigh energy-storage density in Ca0.5Sr0.5TiO3-based linear ceramics over broad temperature range

In the realm of energy storage, there is an exigent need for dielectric materials that exhibit high energy storage density (Wrec) and efficiency (η) over wide temperature ranges. Linear dielectrics exhibit superior breakdown strength (Eb) compared to ferroelectrics, yet their utility is restricted by low polarization. Here, an ultrahigh Wrec up to 7.92 J/cm3 and η ≈ 94.3% are realized in (Ca0.5Sr0.5)1-1.5xSmxTiO3 ceramic with ultrafast discharge time (t0.9 = 20 ns) which is one of the best energy storage performances recorded linear ceramics. By employing the charge balance method, the production of oxygen vacancies (VO··) can be prevented, leading to improved insulation. The creation of A-site vacancies serves to enhance the breakdown field strength. Furthermore, the incorporation of Sm3+ effectively limits the thermal activation of oxygen vacancies at elevated temperatures, ultimately resulting in an increase in bulk resistivity. The results also unveil the outstanding stability across a wide range of temperatures (20 °C–140 °C) and frequencies (1 Hz–500 Hz). Consequently, this study provides significant insights into enhancing Eb and attaining high-energy storage capacitors.

Hydrogen-Bonding Induced Crosslinked Polymer Network for Highly Stable Electrochromic Device and a Construction Strategy for Black-Bilayer Electrochromic Film.

This work presents a new strategy to achieve highly stable electrochromic devices and bilayer film construction. A novel solution-processable electrochromic polymer P1-Boc with quinacridone as the conjugated backbone and t-Boc as N-substituted non-conjugated solubilizing groups is designed. Thermal annealing of P1-Boc film results in the cleavage of t-Boc groups and the formation of N─H⋯O═C hydrogen-bonding crosslinked network, which changes its intrinsic solubility characteristics into a solvent-resistant P1 film. This film retains the electrochemical behavior and spectroelectrochemistry properties of the original P1-Boc film. Intriguingly, the electrochromic device based on the P1 film exhibits an ultrafast switching time (0.56/0.80 s at 523nm) and robust electrochromic stability (retaining 88.4% of the initial optical contrast after 100000 cycles). The observed cycle lifetime is one of the highest reported for all-organic electrochromic devices. In addition, a black-transparent bilayer electrochromic film P1/P2 is developed in which the use of the solvent-resistant P1 film as the bottom layer avoids interface erosion of the solution-processable polymer in a multilayer stacking.

Optical Pump Terahertz Probe (OPTP) and Time Resolved Terahertz Spectroscopy (TRTS) of emerging solar materials

Photoconductivity is the crucial benchmark to assess the potential of any emerging material for future solar applications. Many optical techniques, like transient absorption and photoluminescence, explore bound electron states and provide indirect access to photoconductivity. Direct current (DC) measurements under solar simulation determine the total performance of a novel solar device. While this technique has a clear appeal, it involves electrical contacts, causing contact resistance, which impacts the measured conductivity. Furthermore, DC measurements do not provide any insight into ultrafast effects and the photophysics defining a novel material. Terahertz (THz) spectroscopy presents a contact-free technique to measure photoconductivity on a sub-ps time scale. These measurements can be performed on as-synthesized sample materials, including powders. The ultrafast time resolution informs us of trapping dynamics and reveals what physical processes limit the carrier lifetime in a novel material. Additionally, complex conductivity can be measured at THz frequencies. THz-conductivity and photoconductivity shed light on scattering effects, providing a road map toward minimizing these effects. However, THz spectroscopy is less intuitive than widely used DC measurements, and the interpretation of THz-results is more challenging. This tutorial aims to familiarize the reader with the main THz techniques used to explore emerging materials. We will illustrate how carrier lifetimes can be extracted from optical pump THz probe measurements. We will guide the reader through the process of extracting accurate photoconductivities from time resolved THz spectroscopy measurements and present the most commonly used models to describe the underlying physics. We will then discuss the difference between sample and material parameters and highlight potential pitfalls. The tutorial concludes with a perspective view on the ever evolving field of optical pump-THz probe spectroscopy of emerging materials.

Design of All-Optical Subtractors Utilized with Plasmonic Ring Resonators for Optical Computing

In this paper, a novel plasmonic all-optical half-subtractor and full-subtractor are designed for optical computing. The structure of plasmonic subtractors consists of a metal–insulator–metal (MIM) waveguide and rectangular ring resonators covered by a graphene layer. Due to the nonlinear optical properties of graphene, the states of the plasmonic resonators can be controlled by the pump intensity of a pump beam focused on the graphene layer. The resonators can work as all-optical switches with an ultra-fast response time to constitute optical logic devices according to the directed logic mechanism. A finite-difference time-domain method is utilized to numerically investigate the transmission of the output signals which represent the results of subtraction operations. Simulation results obtained indicate that the proposed plasmonic devices have the ability to implement half-subtraction and full-subtraction with a small feature size and fast response time, and provide a new concept and method for the design and realization of optical computing devices.

Excited state electronic structure and dynamics in diblock π-conjugated oligomers

The excited state electronic structure and ultrafast energy transfer in two diblock oligomers F3-TBT and F3-mP-TBT was explored (F3 = tri-(9,9-dioctylfluorene), TBT = 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole, mP = meta-phenylene). These diblock oligomers feature strongly coupled, π-conjugated F3 and TBT segments that have different bandgaps. F3-TBT is a fully π-conjugated chromophore in which the two conjugated segments are directly linked. By contrast, in F3-mP-TBT the chromophores are separated by a meta-linked phenylene unit. The objective of this study was to discern whether the Franck-Condon excited state produced by selective excitation of the F3 moiety in fully conjugated F3-TBT gives rise to a localized or fully delocalized excited state. The oligomers’ excited state dynamics were probed by steady-state UV–visible absorption, fluorescence and excitation wavelength dependent ultrafast time resolved transient absorption (TA) spectroscopy. Density functional theory (DFT) calculations were applied to provide insight concerning the electronic structure of the oligomers. The diblock oligomers feature distinct absorption bands due to transitions mainly localized on the F3 (high energy, 3.5 eV) and TBT (low energy, 2.75 eV) segments. Ultrafast TA spectroscopy with excitation corresponding to the low energy TBT unit reveals that the initial excitation is localized on TBT and does not vary with time. By contrast, in both oligomers excitation corresponding to high energy F3 produces an excited state that evolves rapidly (200–300 fs) into the lower energy state that is localized on the TBT chromophore. The ultrafast process is attributed to relaxation of an excited state that is predominantly localized on F3 into a lower energy state that is localized on TBT. The observation of an F3-localized excited state is unexpected for F3-TBT, given that the molecule is fully π-conjugated.