Ultrafast Absorption Research Articles

2.3 µm nanosecond passive Q-switching of an LD-pumped Tm:YLF laser using gold nanorods as a saturable absorber

Developing new saturable absorbers for use in the mid-infrared region has practical significance for short-pulsed lasers and related scientific and industrial applications. The performance of gold nanorods (GNRs) as saturable absorbers at novel mid-infrared wavelengths needs to be evaluated even though these benefit from ultrafast nonlinear responses and broadband saturable absorption. Passive Q-switching of an LD-pumped 2.3 µm Tm:YLF laser using GNRs was successfully realized in this study. Pulses with an 843 ns pulse width and a 6.67 kHz repetition rate were achieved using this Q-switched laser. Results showed that GNRs provide promising passive Q-switches for 2.3 µm Tm-doped lasers.

Power of Aerogel Platforms to Explore Mesoscale Transport in Catalysis.

We describe the opportunity to deploy aerogels-an ultraporous nanoarchitecture with co-continuous networks of meso/macropores and covalently bonded nanoparticulates-as a platform to address the nature of the electronic, ionic, and mass transport that underlies catalytic activity. As a test case, we fabricated Au||TiO2 junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO2 network (Au "in" TiO2, AuIN-TiO2 aerogel) or deposited onto preformed TiO2 aerogel (Au "on" TiO2, AuON/TiO2 aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but AuIN-TiO2 aerogels impose a greater degree of Au contact with TiO2 particles than does the AuON/TiO2 form. Both aerogel variants enable transport of electrons over micrometer-scale distances across the TiO2 network to Au||TiO2 junctions, as evidenced by electron paramagnetic resonance (EPR) and ultrafast visible pump-IR probe time-resolved absorption spectroscopy. The siting of gold nanoparticles in the TiO2 network more effectively disperses trapped electrons. Density functional theory (DFT) calculations find that increased physical contact between Au and TiO2, induced by oxygen vacancies, produces increased hybridization of midgap states and quenches unpaired trapped electrons. We assign the apparent differences in electron-transport capabilities to a combination of the relatively better-wired Au||TiO2 junctions in AuIN-TiO2 aerogels, which have a greater capacity to dilute accumulated charge over a larger interfacial surface area, with an enhanced ability to discharge the accumulated electrons via catalytic reduction of adsorbed O2 to O2- at the interface. Solid-state 1H nuclear magnetic resonance experiments show that proton spin-lattice relaxation times and possibly proton diffusion are strongly coupled to Au||TiO2 interfacial design, likely through spin coupling of protons to unpaired electrons trapped at the TiO2 network. Taken together, our results show that Au||TiO2 interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in catalytic aerogel architectures.

Synthesis, structure and ultrafast nonlinear absorption properties of ZnO-time/MoS2 films

In this paper, ZnO, MoS2 and ZnO-time/MoS2 films with excellent nonlinear absorption (NA) properties were successfully prepared by single/two-step magnetron sputtering. The morphology, structure and NA property of the samples were studied. The scanning electron microscope results show that cluster-like MoS2 have different morphology. This phenomenon can be attributed to the fact that the multi-directional growth of the ZnO base films causes the original growth orientation of MoS2 to be changed. The NA characteristics under different excitation intensities are studied by Z-Scan technology. The material exhibits the reverse saturation absorption characteristic caused by two-photon absorption. Furthermore, the order of magnitude of NA coefficient for samples was determined to be 10−6 cm/W. The ZnO-time/MoS2 films display excellent NA characteristics, which are desirable for field of optical switch, optical limiting, optical communication and integrated optics.

Palladium diselenide as a direct absorption saturable absorber for ultrafast mode-locked operations: from all anomalous dispersion to all normal dispersion

Abstract Layered transition metal dichalcogenides with excellent nonlinear absorption properties have shown remarkable performance in acting as ultrafast photonics devices. In our work, palladium diselenide (PdSe2) nanosheets with competitive advantages of wide tunable bandgap, unique puckered pentagonal structure and excellent air stability are prepared by the liquid-phase exfoliation method. Its ultrafast absorption performance was verified by demonstrating conventional and dissipative soliton operations within Er-doped fiber lasers. The minimum pulse width of the conventional soliton was 1.19 ps. Meanwhile, dissipative soliton with a 46.67 mW output power, 35.37 nm spectrum width, 14.92 ps pulse width and 2.86 nJ pulse energy was also generated successfully. Our enhanced experiment results present the excellent absorption performance of PdSe2 and highlight the capacity of PdSe2 in acting as ultrafast photonics devices.

Ultrafast Absorption of Polysulfides through Electrostatic Confinement by Protonated Molecules for Highly Efficient Li-S Batteries.

The lithium-sulfur battery is a promising high-energy-density storage system, which suffers from severe capacity fading due to the "shuttle effect" and low Coulombic efficiency caused by the dissolution of lithium polysulfides. At the molecular level, suppressing the shuttle effect has been greatly required for high-performance Li-S batteries. Herein, we propose a new strategy by utilizing a protonated organic absorbent (N1,N4-bis(pyridine-3-ylmethyl)butane-1,4-diammonium nitrate ([H2PBD]2+·(NO3)22-) for ultrafast absorption of polysulfides through electrostatic attractions and for fixing the polysulfides in the cathode by hydrogen-bond interactions. A lithium-sulfur battery cathode based on a commercial carbon black (CB) and an absorbent (10%) with high sulfur content (70%) exhibits a low capacity decay of 0.099% per cycle over 400 cycles at a rate of 0.5C along with 91% Coulombic efficiency. This strategy and the finding of an electrostatic absorbent offer a new alternative insight into designing cheaper lithium-sulfur batteries for their practical application in the future.

Investigation of ultrafast multi-photon absorption and optical limiting response in a pyrazine-based N-heteroacene

The ultrafast optical nonlinearity of a pyrazine-based N-heteroacene (Py-0-TQ) is systematically investigated via Z-scan, optical limiting and transient absorption spectrum. Reverse saturable absorption at visible wavelengths is observed. Further study on excited-state kinetics indicates that the strong nonlinear optical (NLO) absorption originates from the cooperating effect between different NLO mechanisms including excited-state absorption (ESA) and two-photon absorption (TPA). Structure-property relation is analyzed, result indicates that the central π-system in Py-0-TQ plays a key role in the cooperating effect via providing a broadband, long-lived ESA and an enhanced TPA as trigger. The experimental and theoretical studies suggest that pyrazine-based N-heteroacene molecules with proper design may have great potential in NLO applications.

Ultrafast broadband reverse saturation absorption in Al-doped InSe thin films for optical limiting at visible/near-infrared

Abstract Metal-doped InSe has received much attention due to the ultrafast response and strong nonlinearity at high laser fluence. Herein, Al-doped InSe with different doping contents (1.5 W, 2.0 W, and 2.5 W) and pure InSe thin films are fabricated using a magnetron sputtering technology to systematically study their nonlinear absorption behavior at visible/near-infrared. Al-doped InSe (2.5 W) thin film exhibits ultrafast carrier absorption (

Ultrafast saturable absorption of large-diameter single-walled carbon nanotubes for passive mode locking in the mid-infrared.

We study the saturable absorption properties of single-walled carbon nanotubes (SWCNTs) with a large diameter of 2.2 nm and the corresponding exciton resonance at a wavelength of 2.4 µm. At resonant excitation, a large modulation depth of approximately 30 % and a small saturation fluence of a few tens of µJ/cm2 are evaluated. The temporal response is characterized by an instantaneous rise and a subpicosecond recovery. We also utilize the SWCNTs to realize sub-50 fs, self-start mode locking in a Cr:ZnS laser, revealing that the film thickness is an important parameter that affects the possible pulse energy and duration. The results prove that semiconductor SWCNTs with tailored diameters exceeding 2 nm are useful for passive mode locking in the mid-infrared range.

Leveraging excited-state coherence for synthetic control of ultrafast dynamics.

Design-specific control over excited-state dynamics is necessary for any application seeking to convert light into chemical potential. Such control is especially desirable in iron(II)-based chromophores, which are an Earth-abundant option for a wide range of photo-induced electron-transfer applications including solar energy conversion1 and catalysis2. However, the sub-200-femtosecond lifetimes of the redox-active metal-to-ligand charge transfer (MLCT) excited states typically encountered in these compounds have largely precluded their widespread use3. Here we show that the MLCT lifetime of an iron(II) complex can be manipulated using information from excited-state quantum coherences as a guide to implementing synthetic modifications that can disrupt the reaction coordinate associated with MLCT decay. We developed a structurally tunable molecular platform in which vibronic coherences-that is, coherences reflecting a coupling of vibrational and electronic degrees of freedom-were observed in ultrafast time-resolved absorption measurements after MLCT excitation of the molecule. Following visualization of the vibrational modes associated with these coherences, we synthetically modified an iron(II) chromophore to interfere with these specific atomic motions. The redesigned compound exhibits a MLCT lifetime that is more than a factor of 20 longer than that of the parent compound, indicating that the structural modification at least partially decoupled these degrees of freedom from the population dynamics associated with the electronic-state evolution of the system. These results demonstrate that using excited-state coherence data may be used to tailor ultrafast excited-state dynamics through targeted synthetic design.

Standing growth mechanism and ultrafast nonlinear absorption properties of WS2 films

Tungsten disulfide (WS2) films were successfully prepared by magnetron sputtering. The structure, morphology and standing growth mechanism of WS2 films under different sputtering conditions (sputtering time and substrate temperature) were studied. The scanning electron microscopy (SEM) showed that the sputtering time reached 1.5 h or the substrate temperature reached 250 °C, the morphology of WS2 reveal a “sickle” texture on the surface. The standing growth mechanism was demonstrated by SEM results. X-ray diffraction (XRD) patterns showed that only (101) diffraction peak existed in WS2 films. The nonlinear absorption in WS2 films were characterized by Z-scan technique in the femtosecond regimes at 800 nm. The results showed that the WS2 films have two-photon absorption process, and the transition from saturable absorption to reverse saturable absorption was observed. The nonlinear absorption coefficients of the WS2 films at different sputtering time were 6.386 × 10−10 (for 2 h), 9.704 × 10−10 (for 1.5 h) and −9.759 × 10−10 m/W (for 1 h), respectively. The nonlinear absorption coefficients of the WS2 films at different substrate temperature were −6.275 × 10−10 (for 250 °C), 5.430 × 10−9 (for 300 °C) and 3.899 × 10−9 m/W (for 350 °C), respectively. The standing WS2 films have excellent nonlinear optical performance and simple synthesis route, which provide a good prospect for the application of two-dimensional materials in nonlinear optics.

Ultrashort XUV pulse absorption spectroscopy of partially oxidized cobalt nanoparticles

High-order harmonic generation (HHG) based transient extreme ultraviolet (XUV) absorption spectroscopy is an emerging technique to trace photoinduced charge carrier dynamics in condensed phase materials with femtosecond and even attosecond temporal resolution and elemental specificity. However, its application to nanoparticulate samples that are relevant, for example, for novel photocatalytic light harvesting concepts, has been limited. This is in part due to the challenge to produce residual-free samples on ultrathin, XUV-transparent substrates as well as a widespread understanding that sparsely distributed nanoparticles do not provide sufficient contrast for XUV absorption measurements. Here, we present static XUV absorption spectra of partially oxidized Co nanowire-structures with diameters of approximately 4.5 nm and lengths between 10 and 40 nm, recorded with an ultrashort pulse HHG light source. Nanoparticles are synthesized by the agglomeration of Co atoms inside superfluid helium droplets, followed by surface deposition and oxidation in ambient air. The method is uniquely suited for residual-free synthesis of transition metal nanowires and their deposition on ultrathin substrates. Analysis by high-resolution transmission electron microscopy reveals the formation of CoO nanowires with regions of unoxidized Co in their interior. The nanoparticle samples are investigated in an HHG-driven ultrafast XUV absorption setup. Despite the low surface coverage of only 23%, the recorded spectrum exhibits a distinct absorption feature at the Co M2,3(2p) edge near 60 eV with a peak height of about 40 mOD. The results support the feasibility of table-top ultrafast transient XUV absorption studies of photoinduced dynamics in transition metal oxide nanoparticles with sub-monolayer surface coverage.

The association between anti-insulin aspart antibodies and the pharmacokinetic and pharmacodynamic characteristics of fast-acting insulin aspart in children and adolescents with type 1 diabetes.

BackgroundFast‐acting insulin aspart (faster aspart) is a novel formulation of insulin aspart (IAsp) ensuring ultrafast absorption and effect.AimTo compare the pharmacokinetics between faster aspart and IAsp, based on free or total IAsp measurement, and investigate the association between anti‐IAsp antibodies and faster aspart and IAsp pharmacological properties in children and adolescents with type 1 diabetes (T1D).MethodsIn a randomized, two‐period crossover trial, 12 children, 16 adolescents, and 15 adults (6‐11, 12‐17, and 18‐64 years) received 0.2 U/kg double‐blindsingle‐dose subcutaneous faster aspart or IAsp followed by a standardized liquid meal test.ResultsAcross age groups, the pharmacokinetic profile was left‐shifted including greater early exposure for faster aspart vs IAsp irrespective of free or total IAsp assay. Onset of appearance occurred 2.4 to 5.0 minutes (free) or 1.8 to 3.0 minutes (total) earlier for faster aspart vs IAsp (P < .05). Treatment ratios (faster aspart/IAsp) for 0 to 30 minutes IAsp exposure were 1.60 to 2.11 and 1.62 to 1.96, respectively (children, free: P = .062; otherwise P < .05). The ratio of free/total IAsp for overall exposure (AUCIAsp,0‐t) was negatively associated with anti‐IAsp antibody level across age. Pooling with a previous similar trial showed no clear association between anti‐IAsp antibodies and meal test 1‐ or 2‐hour postprandial glucose increment independent of age and insulin treatment (R 2 ≤ .070; P ≥ .17).ConclusionsIn children and adolescents with T1D, faster aspart provides ultrafast pharmacokinetics irrespective of free or total IAsp assay. Elevated anti‐IAsp antibodies are associated with higher total IAsp concentration, but do not impact faster aspart and IAsp glucose‐lowering effect.

Ultrafast strong-field absorption in gapped graphene

We study theoretically the strong-field absorption of an ultrafast optical pulse by a gapped graphene monolayer. At low field amplitudes, the absorbance in the pristine graphene is equal to the universal value of $2.3$ percent. Although the ultrafast optical absorption for low field amplitudes is independent of the polarization, linear or circular, of the applied optical pulse, for high field amplitudes, the absorption strongly depends on the pulse polarization. For a linearly polarized pulse, the optical absorbance is saturated at the value of $\approx 1.4$ percent for the pulse's amplitude of $\geq 0.4~\mathrm{V/\AA}$, but no such saturation is observed for a circularly polarized pulse. For the gapped graphene, the absorption of a linearly polarized pulse has a weak dependence on the bandgap, while for a circularly polarized pulse, the absorption is very sensitive to the bandgap. %Opening a bandgap in graphene by placing in on, for example, SiC substrate strongly modify the ultrafast absorption at small field amplitudes.

Effect of preheated, delayed annealing process on the ultrafast carriers dynamics of perovskite films using ultrafast absorption spectroscopy

Abstract Organic halogenated perovskite material, as an optical material with great development potential, shows excellent application value in the field of solar cells. Solar cells based on organometal halide perovskites possess photoelectricity efficiencies exceeding 25.2%, however, they suffer from oxidization and degradation. In this paper, we present a fabrication technique based on the preheating treatment and delayed annealing to obtain high quality perovskite films. Using the femtosecond transient absorption (TA) spectroscopy, we investigate the transport and lifetime of internal carriers of perovskite films prepared by the preheating treatment and delayed annealing process. The experimental results show that the perovskite film obtained by the preheated treatment is easier to control the carrier recombination rate than the perovskite film obtained by the conventional method. Furthermore, perovskite film with delayed annealing 1 h has a shorter transport time of carriers from the perovskite layer to the hole transport layer, effectively avoiding the direct recombination of carriers. The novel method put forward in this work enables the stabilization and high efficiency of perovskite solar cells.

Nitro End Groups: Remarkable Vibrational Reporters for Charge Transfer in the Excited States of Oligo(p-phenyleneethynylene)-Bridged Donor–Acceptor Dyads

The D-π-A dyads [(n-Hex)2N(1,4-C6H4)]CC[(1,4-C6H4)NO2] (1) and [(n-Hex)2N(1,4-C6H4)]CC(1,4-C6H4)CC[(1,4-C6H4)NO2] (2) have been studied by ultrafast time-resolved infrared absorption spectroscopy. After excitation into their singlet charge-transfer (1CT) state, a fast decay (ca. 6 ps for 1 and 1 ps for 2) of the initially populated singlet state into a ground state (major pathway) and a longer lived excited state (possibly the triplet state) is observed. The nitro and alkyne groups were used as vibrational reporters to probe the changes induced by the charge-transfer process. For the first time, we confirm experimentally that these changes are consistent with expectations based on the traditional valence-bond representations of the CT states of these push-pull chromophores. An almost identical charge transfer takes place in the two dyads, despite π-bridges of different lengths between the donor and acceptor groups. Complementary DFT calculations support the experimental assignments and have helped clarify the photophysical behavior of 1 and 2.

The pump fluence and wavelength-dependent ultrafast carrier dynamics and optical nonlinear absorption in black phosphorus nanosheets

Abstract Pump fluence and probe wavelength-dependent ultrafast carrier dynamics and optical nonlinear absorption in black phosphorus nanosheets are investigated by transient absorption spectroscopy and open-aperture Z scan techniques. The decay time becomes longer with larger wavelengths under pump wavelengths of both 400 nm and 800 nm excitation. For 800 nm excitation, pump fluence-dependent lifetime shows complex behaviors, which might be due to the competition between the linear absorption and two photon absorption. For 400 nm excitation, an additional decaying channel is observed at a larger pump fluence, which is explained by an effective subband structure. In open-aperture Z scan measurements, strong saturation absorption is observed in the visible region over a broad band from 450 nm to 700 nm. The saturation intensity shows an increasing trend with increase in the wavelength. Also, the saturation intensities under different pulse widths and solvents are discussed in detail. Our results show that black phosphorus nanosheets have great potential in future ultrathin optoelectronic devices.

Ghost Polarization Communication

There is an increasing and demanding interest in the realization of secure communication schemes. Here, we conceive and realize an approach for a message-encoding scheme between two parties, Alice and Bob, by exploiting the infinite number of polarization states of unpolarized thermal light on the Poincar\'e sphere together with their correlation properties to camouflage and recover a message, thus realizing ghost polarization communication. At first, we investigate the second-order correlation coefficient ${g}^{(2)}(\ensuremath{\tau})$ of classical unpolarized broadband amplified-spontaneous-emission light emitted by an erbium-doped fiber amplifier at 1550 nm by manipulating its instantaneous polarization state utilizing various polarization optics in the beam paths of a Hanbury-Brown and Twiss like ghost polarimetry setup using ultra-fast two-photon absorption in a photomultiplier tube. The observed polarization state modifications are in excellent agreement with our developed model based on the Stokes vector dynamics and a Glauber protocol for ${g}^{(2)}(\ensuremath{\tau})$. On the basis of these results we then proceed towards the realization of a message-encoding scheme. By changing the instantaneous polarization state using a half-wave plate, Alice encodes and subsequently transmits a message. The camouflaged message can be recovered uniquely by Bob measuring the second-order correlations of the modified instantaneous polarization state with his reference beam. By using the agreed keypad of the communication scheme, he is able to retrieve the bit values of the encoded message. Finally, we address real-world implementation of this first proof-of-principle demonstration and discuss its security issues. This scheme demonstrates a method of establishing a secure communication link between two parties directly on the physical layer based on polarization correlations of classical light. It is expected that by the realization of this correlated photon modality not only avenues for ghost modalities, in general, are opened but also insight into polarization, even more than 175 years after Stokes will be achieved.

Phonon-Mediated and Weakly Size-Dependent Electron and Hole Cooling in CsPbBr3 Nanocrystals Revealed by Atomistic Simulations and Ultrafast Spectroscopy.

We combine state-of-the-art ultrafast photoluminescence and absorption spectroscopy and nonadiabatic molecular dynamics simulations to investigate charge-carrier cooling in CsPbBr3 nanocrystals over a very broad size regime, from 0.8 to 12 nm. Contrary to the prevailing notion that polaron formation slows down charge-carrier cooling in lead-halide perovskites, no suppression of carrier cooling is observed in CsPbBr3 nanocrystals except for a slow cooling (over ∼10 ps) of “warm” electrons in the vicinity (within ∼0.1 eV) of the conduction band edge. At higher excess energies, electrons and holes cool with similar rates, on the order of 1 eV ps–1 carrier–1, increasing weakly with size. Our ab initio simulations suggest that cooling proceeds via fast phonon-mediated intraband transitions driven by strong and size-dependent electron–phonon coupling. The presented experimental and computational methods yield the spectrum of involved phonons and may guide the development of devices utilizing hot charge carriers.

Radical Formation in the Photoactivated Adenylate Cyclase OaPAC Revealed by Ultrafast Spectroscopy

Adenylate cyclases (AC) form a class of enzymes which are involved in the conversion of ATP to cAMP, playing an important role in many regulatory processes as the cAMP produced is involved in signalling pathways controlled by cAMP binding proteins. There is a very special subgroup of AC enzymes called photoactivated adenylate cyclases (PAC) where the ATP to cAMP conversion is controlled by light. Interestingly the photoregulation is controlled by BLUF (Blue light sensing using FAD) domains which are responsible for light regulation in many processes like phototoaxis, photophobia etc. Light regulation of BLUF domains is governed by the reorganization of the hydrogen bond network around the flavin and it is reflected in a ∼10 nm red shift of the flavin absorption peak. The first discovered photo-activated adenylate cyclase was found in a unicellular flagellate Euglenagracilis where the flagellar movement is regulated via a photoactivated AC. Another photoactivated adenylate cyclase called bPAC was discovered in the sulfide-oxidizing bacterium Beggiatoa sp. where the light regulation is solved by the BLUF domain located at the N-terminal part of the protein, the cAMP conversion is accomplished by a class IIIb type adenylyl cyclase at the C-terminal. Another promising PAC was discovered recently in the photosynthetic cyanobacterium Oscillatoriaacuminata which shows substantial activation (20-fold increase in the cAMP level in light compared to the dark) upon light illumination. OaPAC is a 366-aa residue long homodimer having a class III adenylate cyclase domain at the C-terminal and a BLUF domain at the N-terminal. In this work we characterized the primary photochemistry of OaPAC by the means of ultrafast (femtosecond to nanosecond) spectroscopy. Ultrafast transient visible and transient infrared absorption spectroscopy reveals that neutral flavin radical is formed after the blue light absorption.

Fused Silica with Embedded 2D‐Like Ag Nanoparticle Monolayer: Tunable Saturable Absorbers by Interparticle Spacing Manipulation

AbstractSaturable absorbers are key elements for generation of ultrafast laser pulses. The commercially available semiconductor saturable absorber mirrors are wavelength sensitive and with complex fabrication process and high cost. Fused silica is one of the basic materials for various optical applications. Plasmonic nanoparticles can be used to efficiently modulate the optical properties of dielectric materials owing to the enhanced field effects based on localized surface plasmonic resonance. Ion implantation is a direct technique to synthesize plasmonic nanoparticles inside dielectric materials, which can simultaneously tailor the optical nonlinearity of substrates significantly. In this work, Ag ion implantation into fused silica with tunable interparticle spacing is used to generate broadband optical nonlinearities and to endow silica with ultrafast saturable absorption property. This ion implantation forms a 2D‐like Ag nanoparticle monolayer buried inside fused silica wafer. The fused silica with embedded 2D‐like Ag nanoparticle monolayer is further applied as a new saturable absorber to generate ultrafast laser pulses with pulse duration of 27 ps and repetition rate of 6.5 GHz through the passive mode‐locking process. This work opens up a new route to develop low‐cost, highly stable saturable absorbers, and offers the possibility to build novel silica‐based photonic systems through nanoparticle spacing manipulation.