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We investigate the phase behaviours of Pluronic L62 in aqueous solution in the presence of aerosol-OT (AOT) molecules by small angle neutron scattering (SANS). The presence of AOT significantly changes the micellization phenomenon of L62 micelles in aqueous solution, including their critical micelle temperature (CMT), global size, and asphericity. The origin of these observations is attributed to the complexation between the neutral L62 surfactants and the ionic AOT molecules, which additionally provides charge to the mixed micelles: we analyse the data and extract meaningful information using the Ornstein–Zernike integral formalism. As a result, we observe that the co-micellization of L62 and AOT is very stable across a wide temperature range.

The electrochemical reduction of water to produce molecular hydrogen is a potential strategy for generating clean, renewable energy. However, the development of an efficient and durable catalyst remains a significant challenge. Inspired by nature, we herein describe a three-dimensional, porous hybrid catalyst fabricated by using a biomimetic approach, in which a small amount of ruthenium (Ru) nanoparticles is uniformly dispersed onto a carbonized collagen scaffold. Coordination between Ru nanoparticles and the active sites of collagen promotes the electrochemical formation of molecular hydrogen by altering the electronic state of Ru and regulating the free energy of hydrogen chemisorption. The prepared catalyst exhibits superior activity for hydrogen evolution, with an overpotential and Tafel slope comparable to those of a Pt/C catalyst. This work provides a novel perspective on the fabrication of high-efficiency electrochemical catalysts.

Finding the optimal parameters in laser processing applications can be time-consuming given the large parameter space and various sources of error. This problem is exacerbated by day-to-day variation in laser beam characteristics and a large variety of materials that need to be processed. The ideal laser processing system should be “smart,” meaning that it can sense changes in the environment, make proper adjustment, and predict parameters for new materials. As a step toward this goal, we propose a method to efficiently extract the areas of a large number of laser-induced damages in situ using an automated data acquisition system that can control laser parameters, motorized stage movement, image capturing/processing, and feature extraction. The damage areas are extracted and compared with direct measurements. Damage areas are fed into an artificial neural network (ANN) for prediction. Various ANN structures and training functions are tested to create the optimal ANN for prediction. ANN predictions were found to be capable enough to accurately model and optimize the laser processing parameters that were investigated. With the capability of collecting a large amount of usable data in a short period of time, this acquisition system can be used to train sophisticated ANNs for complicated tasks such as quality control and failure prediction.

We report on the experimental and theoretical study of direct laser writing on dielectrics with reduced feature size by using temporally and spatially separated pulses. Features on the front surface of fused silica within the overlapping area of two laser pulses are obtained by tuning the delay time between the two pulses. The observed dependence of feature position on delays longer than the free-carrier lifetime indicates an ionization pathway initiated by self-trapped excitons. The sensitivity of feature size with the increase of laser intensity is studied by simulating the free-carrier density distribution for different temporal and spatial separation of two laser pulses. It is found that the sensitivity is influenced by the spatial and temporal separation of the two pulses.

We report on the experimental and theoretical studies of ultrafast laser-induced optical breakdown on the surface of fused silica to elucidate the mechanism of damage formation and sub-optical-cycle dynamics in material processing using single and a burst of two femtosecond laser pulses. Ionization pathways, including photo-ionization (PI) and avalanche ionization (AI), are investigated by using single-beam and double-beam laser damage threshold measurements, which are used to analyze electron dynamics and extract the avalanche coefficient. The relationship between damage size and laser fluence is interpreted as a result of a combination of PI and AI. Electrical field rather than laser intensity is the fundamental influential factor in PI, and AI is found to play a significant role in creating the free electron density needed for optical breakdown. These findings are verified by a double-pulse delay-scan experiment where two cross-polarized pulses are used to induce damage with delay within a few optical cycles. Variation of the damage diameter is observed within one optical cycle, which is explained by the periodic change of polarization in the combined electric field. This finding shows the potential of controlling laser induced damage by tuning the temporal overlap of a burst of ultrashort laser pulses.

Cadmium chalcogenide colloidal quantum wells or nanoplatelets (NPLs), a class of new materials with atomically precise thickness and quantum confinement energy, have shown great potential in optoelectronic applications. Short exciton lifetimes in two-dimensional (2D) NPLs can be improved by the formation of type II heterostructures, whose properties depend critically on the mechanism of exciton transport. Herein, we report a study of room-temperature exciton in-plane transport mechanisms in type-II CdSe/CdTe core/crown (CC) colloidal NPL heterostructures with the same CdSe core and different CdTe crown sizes. Photoluminescence excitation measurements show unity quantum efficiency for transporting excitons created at the crown to the CdSe/CdTe interface (to form lower-energy charge-transfer excitons). At near band edge excitation, the crown-to-core transport time increases with crown size (from 2.7 to 5.6 ps), and this size-dependent transport can be modeled well by 2D diffusion of thermalized excitons in ...

Pursuing ever-smaller feature size in laser-based lithography is a research topic of vital importance to keep this technique competitive with other micro-/nano-fabrication methods. Features smaller than the diffraction-limited spot size can be obtained by “thresholding”, which utilizes the deterministic nature of damage threshold with ultrashort laser pulses and is achieved by precisely tuning pulse energies so that only the central portion of the focal spot produces permanent modification. In this paper, we examine the formulation commonly used to describe thresholding and show that the relationship between feature size (r) and laser fluence (F) is invariant with respect to the nature of laser absorption. Verified by our experiments performed on metal, semiconductor, and dielectric samples, such invariance is used to predict the smallest feature size that can be achieved for different materials in a real-world system.

Polymers were one of the first materials to be processed by ultrafast lasers. However, the nature of absorption for near-infrared laser beams is not fully understood, and therefore it remains challenging to process polymeric materials with high energy efficiency. In this study, the pulse-to-pulse evolution of optical properties (reflectance, transmittance, and absorptance) of polypropylene (PP), which is an important polymeric material widely used in many industrial applications, is determined by performing time-resolved measurements for a wide range of pulse energies. The goal is to differentiate between linear and nonlinear absorption in different laser-matter interaction regimes and select the processing condition that yields the highest energy efficiency. The experiment is performed by recording the reflection and transmission of each laser pulse in an ellipsoidal mirror-based setup, which enables the collection of scattering reflection with nearly full coverage. Absorption is calculated from the experimental data, and a model consisting of linear and nonlinear absorption is used to analyze the results. It is found that PP undergoes a dramatic morphological change from pulse to pulse, which is accompanied by changes in optical properties, that is, the tuning of the laser condition to fully utilize the laser energy. Their results could help increase energy efficiency in ultrashort-pulsed laser processing of polymers toward the high-throughput operation.

Two-dimensional (2D) nanomaterials have attracted much interest as promising candidates for next generation energy storage, electronics and catalytic applications, due to their unique topology and electronic structure. However, the development of an operable and versatile solution processing technique for simple fabrication of high-quality 2D nanosheets continues to present a significant challenge. Here, an ultrathin indium sulfide nanosheet is synthesized via a simple solution-phase reaction involving two steps: i) thermal decomposition of precursors to form a crystal nucleus; ii) growth of 2D nanosheets in a mixture of amino ligand and alkane solvent. The alkane, matching the amino ligand structure in terms of geometry, could assemble with the ligand to form a soft template and thus induce a 2D arrangement of the nucleus. Moreover, the chemical inertness of the alkane facilitated rotation of crystal seeds, resulting in anisotropic growth of the nanosheet. We further demonstrated that the geometrically matched alkane-assisted solution processing reaction could be applied to synthesize various 2D metal chalcogenides. Functionally, the acquired indium sulfide nanosheets, which possess high photoelectric activity, were introduced for fabrication of photoelectrodes capable of efficient photoelectrochemical water splitting. Our work opens up a new perspective toward the construction and potential applications for various 2D nanomaterials.

Material processing by ultrafast lasers is an attractive technology for high-precision fabrication, such as cutting, drilling, and surface modification, of a wide range of material, including dielectrics, semiconductor, metals, and polymer composites. However, it is still challenging to apply ultrafast laser processing in many applications because some key processes, such as absorption and heat accumulation, are not fully understood, especially for polymeric materials, which have a low melting temperature and, therefore, are more vulnerable to thermal damage. In this study, an analytical solution to a transient, two-dimensional thermal model is derived using Duhamel's theorem and Hankel’s transform method to understand the effect of laser parameters during ultrafast laser interactions with polypropylene (PP), which is a material widely used in many industrial applications. To correlate with theoretical calculation, laser cutting experiments are carried out on PP sheets. This study found that the total energy absorbed in the material and the laser intensity are two important factors to estimate the laser processing performance. In addition, time-resolved measurements are performed by using fast photodiodes and an oscilloscope to understand the dynamics of ultrafast laser interactions during the laser cutting process. Transmitted and reflected signals are monitored and analyzed to extract information on nonlinearity and the absorption coefficient.

The authors report on the theoretical and experimental studies of laser-induced optical breakdown on the surface of fused silica to elucidate the influence of time delay and spatial separation between two ultrashort pulses on the position and size of the modification. Carriers involved in the damage formation including free electrons in the conduction band and self-trapped excitons (STEs) are investigated. The relationship between damage morphology and time delay shows that the seeding carriers (free electrons and STEs) generated from the first pulse are found to play a significant role for the second pulse—which is temporally and spatially separated from the first pulse—in creating the critical electron density needed for an optical breakdown. Consequently, processing outcomes, such as accuracy (position of the hole) and resolution (size of the hole), depend on the interplay of various laser-induced physics that can be tailored for specific goals. As a demonstration, laser lithography with resolution below the diffraction limit is achieved by exploiting multipulse induced physics. This work is a step toward repeatable laser processing of dielectrics beyond the diffraction limit and provides insights into ultrafast laser-matter interaction under the condition of an extremely high pulse repetition rate.

We demonstrate a method of laser ablation with reduced feature size by using a pair of ultrashort pulses that are partially overlapped in space. By tuning the delay between the two pulses, features within the overlapping area are obtained on the surface of fused silica. The observed dependence of the feature position on delays longer than the free-carrier lifetime indicates an ionization pathway initiated by self-trapped excitons. This method could be used to enhance the resolution of laser-based lithography.

We utilized femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (CARS) to study the ultrafast vibrational dynamics in distilled water at room temperature. The CARS signals from the broad OH-stretching modes between 3100 cm−1 and 3700 cm−1 were obtained and analyzed. The dephasing times of four Raman modes in water were detected and compared.

Spectrally dispersed femtosecond time-resolved coherent anti-Stokes Raman spectroscopy is utilized to study the ultrafast vibrational dynamics in rhodamine B dye in solution at room temperature. The anti-Stokes intensities are monitored as a function of time and wavenumber. By simply changing the timing of laser pulses, the vibrational dynamics of excited Raman transitions in rhodamine B molecules can be tracked and detected.

The ultrafast molecular vibrational dynamics in PMMA sheets is studied by femtosecond time-resolved coherent anti-Stokes Raman spectroscopy at room temperature. The C – H stretch modes at 2870 cm-1 and 3008 cm-1 in PMMA sheets are excited and detected. The coherence relaxation times and beat wavenumbers of the Raman modes are obtained.