Ultrafast Recovery Time Research Articles

Ultrafast recovery time and broadband saturable absorption properties of black phosphorus suspension

As a new type of two-dimensional crystal material, black phosphorus (BP) exhibits excellent electronics and optical performance. Herein, we focus on carrier relaxation dynamics and nonlinear optical properties of BP suspension. Atomic force microscopy, transmission electron microscopy, and optical transmission spectrum are employed to characterize the structure and linear optical properties of the BP. Additionally, pump-probe experiments at wavelength of 1550 nm were carried out to study the carrier dynamics in BP suspension, and ultrafast recovery time was observed (τs = 24 ± 2 fs). Furthermore, we demonstrate the saturable absorption signals by open aperture Z-scan experiments at wavelengths of 1550 nm, 532 nm, and 680 nm. The results indicate that BP has broadband saturable absorption properties and the nonlinear absorption coefficients were determined to be β2 = −0.20 ± 0.08 × 10−3 cm/GW (532 nm), β2 = −0.12 ± 0.05 × 10−3 cm/GW (680 nm), and β2 = −0.15 ± 0.09 × 10−3 cm/GW (1550 nm).

Preparation and Gas Sensing Properties of Hierarchical Flower-Shaped Bi2WO6

Hierarchical flower-shaped Bi2WO6 was obtained by a simple hydrothermal method. Morphology and structure of the Bi2WO6 were characterised by single electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and N2 adsorption techniques. Gas sensing properties of the Bi2WO6 sensor were investigated by a static gas-sensing system. The results show the as-synthesised flower-shaped product is pure orthorhombic Bi2WO6, which is composed of nanosheets with ~10–20 nm in thickness and hundreds of nanometres in planar size. At this optimal operating temperature of 300°C, the Bi2WO6 sensor exhibits ultra-fast response (1-2 s) and fast recovery time (6–12 s) towards ethanol detection, and high selectivity to other gases such as methanol, benzene, dichloromethane, and hexane.

Self-powered ultrafast broadband photodetector based on p-n heterojunctions of CuO/Si nanowire array.

A new self-powered broadband photodetector was fabricated by coating an n-silicon nanowire (n-Si NW) array with a layer of p-cupric oxide (CuO) nanoflakes through a new simple solution synthesis method. The p-n heterojunction shows excellent rectification characteristics in the dark and distinctive photovoltaic behavior under broadband light illumination. The photoresponse of the detector at zero bias voltage shows that this self-powered photodetector is highly sensitive to visible and near-infrared light illuminations, with excellent stability and reproducibility. Ultrafast response rise and recovery times of 60 and 80 μs, respectively, are shown by the CuO based nanophotodetector. In addition, the broadband photodetector can also provide a rapid binary response, with current changing from positive to negative upon illumination under a small bias. The binary response arises from the photovoltaic behavior and the low turn-on voltage of the CuO/Si NW device. These properties make the CuO/Si NW broadband photodetector suitable for applications that require high response speeds and self-sufficient functionality.

Ultrahigh performance humidity sensor based on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite film

A ultrahigh performance humidity sensor based on graphene oxide (GO)/poly(diallyldimethylammonium chloride) (PDDA) nanocomposite film was reported in this paper. The multilayered film of GO/PDDA was fabricated on a polyimide substrate using layer-by-layer self-assembly technique. The structures of the self-assembled films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The humidity sensing behaviors of the film sensor were investigated at room temperature over a wide range of 11–97% relative humidity. Unprecedented response of up to 265,640% was demonstrated for the presented sensor when exposed to varying relative humidity levels, which is better than that of the best conventional humidity sensor. Furthermore, the presented sensor exhibited ultrafast response and recovery times capable of monitoring human breath. Moreover, the possible humidity sensing mechanism of the proposed sensor was discussed by using complex impedance spectra and bode diagrams. This measurement results observed highlight the layer-by-layer self-assembled graphene oxide/polyelectrolyte film is a candidate material for constructing humidity sensors.

Wavelength-versatile graphene-gold film saturable absorber mirror for ultra-broadband mode-locking of bulk lasers.

An ultra-broadband graphene-gold film saturable absorber mirror (GG-SAM) with a spectral coverage exceeding 1300 nm is experimentally demonstrated for mode-locking of bulk solid-state lasers. Owing to the p-type doping effect caused by graphene-gold film interaction, the graphene on gold-film substrate shows a remarkably lower light absorption relative to pristine graphene, which is very helpful to achieve continuous-wave mode-locking in low-gain bulk lasers. Using the GG-SAM sample, stable mode-locking is realized in a Yb:YCOB bulk laser near 1 μm, a Tm:CLNGG bulk laser near 2 μm and a Cr:ZnSe bulk laser near 2.4 μm. The saturable absorption is characterised at an intermediate wavelength of 1.56 μm by pump-probe measurements. The as-fabricated GG-SAM with ultra-broad bandwidth, ultrafast recovery time, low absorption, and low cost has great potential as a universal saturable absorber mirror for mode-locking of various bulk lasers with unprecedented spectral coverage.

Mode-locking of 2 μm Tm,Ho:YAG laser with GaInAs and GaSb-based SESAMs

We have investigated passive mode-locking of Tm,Ho:YAG lasers with GaInAs- and GaSb-based semiconductor saturable absorber mirrors (SESAMs). With a GaInAs-based SESAM, stable dual-wavelength mode-locking operation was achieved at 2091 nm and 2097 nm, generating pulses with duration of 56.9 ps and a maximum output power of 285 mW. By using the GaSb-based SESAMs, we could generate mode-locked pulses as short as 21.3 ps at 2091 nm with a maximum output power of 63 mW. We attribute the shorter pulse duration obtained with the GaSb SESAMs to the ultrafast recovery time of the absorption and higher nonlinearity compared to standard GaInAs SESAMs.

Well vertically aligned ZnO nanowire arrays with an ultra-fast recovery time for UV photodetector

Well-crystallized ZnO nanowire arrays were grown on GaN/sapphire by one-step chemical vapor deposition under control of the fabrication pressure of 1000–2500 Pa and the best-aligned arrays were obtained at 1000 Pa. A photoluminescence study shows a red shift with nanowire diameter increase. Under 365-nm UV irradiation of 0.3 mW/cm2, the photoresponse study of the best ZnO arrays shows an ultra-fast tri-exponential rise with three constants of 0.148, 0.064 and 0.613 s, and a bi-exponential decay behavior with two recovery constants of 30 and 270 ms. The ZnO/GaN heterojunction barriers could be responsible for the ultra-fast tri-exponential rise and bi-exponential decay behavior.

Room temperature photo-induced phase transitions of VO2 nanodevices

We have successfully fabricated VO2 nanodevices using a photo-induced phase transition mechanism under an ultraviolet-light (λ = 365 nm, I = 7 μW cm−2). On the obtained VO2 nanowires, the photo-induced insulator-to-metal (IM) phase transition occurred from monoclinic (insulator or semiconductor) to tetragonal (metal) structures at room temperature. The photocurrent-to-dark current ratio of the nanodevice was 285 with ultra-fast response and recovery times of 1.3 and 4.5 ms, respectively, at a UV light intensity of 7 μW cm−2 . As the UV light intensity increased to 2.33 mW cm−2, a maximum photocurrent-to-dark current ratio of 719 was observed. A high-resolution transmission electron microscopy (HRTEM) image indicates that the nanowires grew along the [100] axis as a single crystal. The VO2 nanowires were then fabricated into field-effect transistor (FET) devices. High electron mobilities of 29 and 7.93 cm2 V−1 S−1 were obtained with UV on (2.33 mW cm−2) and off, respectively. The electrical properties of the photo-induced IM phase transitions of the VO2 nanowires were investigated.

Atomic‐Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers

AbstractThe optical conductance of monolayer graphene is defined solely by the fine structure constant, α = $e^2 /\hbar c$ (where e is the electron charge, $\hbar $ is Dirac's constant and c is the speed of light). The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero‐gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, use of atomic layer graphene as saturable absorber in a mode‐locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band is demonstrated. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the graphene thickness. These results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth, and wideband tunability.

Ultra-Fast Gain Recovery and Compression Due to Auger-Assisted Relaxation in Quantum Dot Semiconductor Optical Amplifiers

The ultra-fast gain dynamics in quantum-dot semiconductor optical amplifiers (QD-SOAs) have been studied for different types of Auger-assisted relaxation processes. The ultra-fast gain recovery time and gain compression are studied for p-type doped and un-doped QD-SOAs using rate equation model. Our calculations show that the ultra-fast gain dynamics is governed by electron-electron Auger-assisted process for un-doped QD-SOA and by electron-hole Auger-assisted process for p-type doped (NA=1.25times1018 cm-3) QD-SOA. We find that the ultra-fast gain recovery time for un-doped QD-SOA is comparable with that of p-type doped QD-SOA when both electron hole and electron-electron processes present in the active region. We find that the percentage of ultra-fast gain compression in un-doped QD-SOA is limited to ~ 72%. While for p-type doped (NA=1.25times1018 cm-3) QD-SOA, we find that the percentage of ultra-fast gain compression increases as the applied current increases where it can reach >95% at very high applied current.

Highly Sensitive and Stable Humidity Nanosensors Based on LiCl Doped TiO2 Electrospun Nanofibers

A new type of humidity nanosensor based on LiCl-doped TiO2 nanofibers with poly(vinyl pyrrolidone) (PVP) nanofibers as sacrificial template has been fabricated through electrospinning and calcination. The sensor exhibited excellent sensing characteristics, such as ultrafast response and recovery times, good reproducibility, linearity, and environmental stability, which are of importance for applications in humidity monitoring and control.

Ultrashort pulse-generation by saturable absorber mirrors based on polymer-embedded carbon nanotubes

We demonstrate passive mode locking of solid-state lasers by saturable absorbers based on carbon nanotubes (CNT). These novel absorbers are fabricated by spin-coating a polymer doped with CNTs onto commercial dielectric laser-mirrors. We obtain broadband artificial saturable absorber mirrors with ultrafast recovery times without the use of epitaxial growth techniques and the well-established spin-coating process allows the fabrication of devices based on a large variety of substrate materials. First results on passive mode locking of Nd:glass and Er/Yb:glass lasers are discussed. In the case of Er/Yb:glass we report the to our knowledge shortest pulse generated in a self-starting configuration based on Er/Yb:bulk-glass: 68 fs (45 fs Fourier-limit) at 1570 nm wavelength at a pulse-repetition rate of 85 MHz.

Ultrafast Fiber Pulsed Lasers Incorporating Carbon Nanotubes

We present the first passively mode-locked fiber lasers based on a novel saturable absorber incorporating carbon nanotubes (SAINT). This device offers several key advantages such as: ultrafast recovery time (<1 ps), high-optical damage threshold, mechanical and environmental robustness, chemical stability, and the ability to operate in transmission, reflection, and bidirectional modes. Moreover, the fabrication cost and complexity of SAINT devices are potentially lower than that of conventional semiconductor saturable absorber mirror devices. Therefore, it is expected that SAINT will greatly impact future pulsed laser design and development.

Radiationless Relaxation in a Synthetic Analogue of the Green Fluorescent Protein Chromophore

The fluorescence and ultrafast ground-state recovery times of the isolated chromophore of the green fluorescent protein have been studied in basic alcohol solutions. The fluorescence quantum yield increases more than 103 times between 295 and 77 K. The major part of the increase occurs in the supercooled liquid range, and continues below the glass transition. The ground-state recovery at 295 K is essentially (95%) complete in under 5 ps, is nonexponential, and only weakly dependent on solvent viscosity. These results are inconsistent with a viscosity-controlled radiationless process involving large scale intramolecular reorganization. If intramolecular motion is involved it must be of small scale. Alternative mechanisms are discussed. A thermally activated radiationless decay process is consistent with the present data, but the mechanism is unclear. For either mechanism the high quantum yield in the intact protein must arise through protein−chromophore interactions which effectively suppress the radiationless channel.

Nonlinearity and recovery time of 1.55 µmintersubbandabsorption in InGaAs/AlAs/AlAsSb coupled quantum wells

Values for the nonlinearity and recovery time of 1.5 /spl mu/m intersubband absorption have been experimentally obtained for the first time by using InGaAs/AlAs/AlAsSb coupled quantum wells. The third-order susceptibility (/spl chi//sup (3)/) has been evaluated to be 5.8/spl times/10/sup -17/m/sup 2//V/sup 2/. An ultrafast recovery time (/spl tau/) of /spl sim/685 fs has been obtained while keeping the figure of merit (/spl chi//sup (3)///spl alpha//spl tau/) as large as those of interband transitions.

Intersubband transitions and ultrafast all-optical modulation using multiple InGaAs-AlAsSb-InP coupled double-quantum-well structures

InP-based InGaAs-AlAs-AlAsSb coupled double-quantum-well structures have been optimized using wave-function engineering techniques to achieve near-infrared intersubband (ISB) transitions. Intersubband transitions at communication wavelengths of 1.3 and 1.55 /spl mu/m can be achieved in both near-symmetric and asymmetric coupled quantum-well structures by tailoring the well width, the central barrier width, and the carrier population in the conduction subband states by controlling either the doping level or the carrier temperature. We demonstrate ultrafast all-optical modulation of interband-resonant light at 1.3 /spl mu/m using intersubband-resonant light pulses at 1.55 /spl mu/m. An ultrafast absorption recovery time of 1.3 ps has been observed at 1.3 /spl mu/m, which can be reduced to 800 fs by probing at a higher energy above the Fermi level in the conduction band.

Ultrafast characteristics of InGaP-InGaAlP laser amplifiers

We characterize a visible, 670 nm, diode laser amplifier with respect to parameters of interest in short pulse generation and amplification. With a single pulse in the amplifier, we measure the differential gain and saturation energy of the amplifier, and changes in the optical spectrum of a pulse traveling through the amplifier. We also measure the ultrafast gain dynamics using a pump and probe technique. We find the ultrafast gain recovery time due to carrier heating is 400 fs/spl plusmn/30 fs. Our results differ quantitatively from those reported for InGaAsP and AlGaAs amplifiers.

Ultrafast recovery time in a strained InGaAs-AlAs p-i-n modulator

We have performed sub-picosecond resolution measurements of the optical recovery of InGaAs-AlAs multiple quantum well p-i-n structures that exhibit strong excitonic features, a clear quantum confined Stark effect and low photocurrent. The recovery times, measured as a function of applied electrical bias and optical excitation density, are much shorter than that predicted by carrier thermionic emission rates from the quantum wells. An observed ultrafast recovery time of 28 ps makes this material system promising as a fast modulator with low thermal dissipation or as a saturable absorber for semiconductor and fiber laser mode locking. >