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This study optimized gold nanoparticle concentration to improve the spectral emission of AlO molecular bands in nanoparticle-enhanced laser-induced Al plasmas.

This study investigated the effect of laser polarization on the molecular bands of femtosecond laser-induced breakdown spectroscopy (LIBS). Results from AlO molecules indicated that the intensities of molecular emission bands under circular polarization in femtosecond LIBS were higher than those under linear polarization. The trend became more evident as laser energy increased. The vibration temperatures at different waveplate angles were also calculated to understand further the effect of laser polarization on the molecular emission signal. Finally, similar research on CN molecules for PMMA targets was carried out. The results showed that the emission intensity of the CN molecule was almost consistent with that of the AlO molecule. The study provided a method to improve the molecular emission signal in femtosecond LIBS with circular laser polarization.

This study combined Tesla coil discharge with laser-induced breakdown spectroscopy to improve the spectral intensity of Cu plasmas. The spectral intensity can be increased by about 40 times using Tesla coil discharge.

In this study, the influence of spark discharge on Al(I) and AlO emission intensities in laser-induced breakdown spectroscopy (LIBS) is investigated experimentally. A femtosecond pulsed laser is used to excite an aluminum sample and produces aluminum plasma between the sample surface and a discharge electrode. The plasma triggers high-voltage spark discharge (SD), enhancing the spectral intensity of the aluminum plasma. The emission of Al atomic lines and AlO bands in the case of the SD is stronger than that in the case using femtosecond LIBS alone. And the spectral intensities of Al(I) lines increase continuously as the discharge voltage increases from 0 kV to 2.0 kV, while AlO molecular emission first increases and then almost remains unchanged. Finally, the vibration temperature of the AlO radical is calculated by fitted AlO emission data, and the vibration temperature of the AlO radical hardly changes with the increase of the discharge voltage. The results indicate that the atomic spectral intensity is sensitive to the discharge voltage, but not the molecular band emission in femtosecond LIBS.

The current research confirms that substrate temperature plays a dominant role in improving the detection sensitivity of surface-enhanced laser-induced breakdown spectroscopy (SE-LIBS) technology. In the experiment, SE-LIBS is used to detect trace heavy metal elements in aqueous solutions. The influence of the substrate temperature on the spectral intensity of Pb and Cr is discussed. Calibration curves are constructed for Pb and Cr at different substrate temperatures. The corresponding limit of detection (LoD) is calculated. The LoDs of Pb and Cr decrease from 31.7 ng mL−1 and 4.6 ng mL−1 to 8.0 ng mL−1 and 1.2 ng mL−1 as the substrate temperature increases from 25 °C to 200 °C. Increasing the substrate temperature can further improve the analytical ability and detection sensitivity of SE-LIBS for detecting heavy metal elements. The study provides a feasible scheme for the detection of heavy metal elements in wastewater.

This study used laser-induced breakdown spectroscopy (LIBS) combined with spark discharge (SD) and an electro-deposition method to analyze trace heavy metal Pb in an aqueous solution. First, the electro-deposition method was used to enrich the trace heavy metal Pb element in an aqueous solution on the surface of a high-purity Al plate with an enrichment voltage of 8 V and an enrichment time of 10 min, thereby overcoming the problem of low sensitivity and accuracy in the direct detection of water samples by LIBS. Second, SD-LIBS was used to detect the Pb element on the surface of the Al plate to enhance the emission intensity of Pb(I). Finally, the calibration curves of Pb(I) at 405.78 nm were plotted for different discharge voltages (0, 2, and 4 kV), and it was found that at higher discharge voltage, the slope (S) of the calibration curve increased and the limit of detection (LOD) decreased. The S values were 0.0403 (0 kV), 0.1643 (2 kV), and 0.2853 (4 kV), and the LODs were 53.5 (0 kV), 13.1 (2 kV), and 7.5 (4 kV) ng mL−1. Therefore, the combination of the electro-deposition method and the SD-LIBS technique can realize higher sensitivity detection of trace heavy metals in aqueous solutions.

Time-resolved optical Kerr fluorescence system was used to investigate time-resolved red-shift of coumarin 153 in different solvent mixtures. The mixtures included four mole fractions of phenol-cyclohexane solvents (0, 0.013, 0.08, and 0.3), and anisole-cyclohexane solvents with the mole fraction of 0.3. The measured time-resolved fluorescence showed that, in the solvent mixtures containing phenol, the time-dependent frequency shift accelerated with the increase in the mole fraction of phenol-cyclohexane mixtures. However, the time-dependent red-shift in the fluorescence was not observed in the anisole-cyclohexane mixture, the solvent polarity could not influence the spectral Stokes shift compared with phenol. The results indicated that coumarin 153 formed an excited hydrogen bond with phenol, and the excited hydrogen bond was strengthened with an increase in the mole fraction of phenol. And, these processes also suggested that the increase in the phenol ratio improves a large number of hydrogen bond formed between phenol and carbonyl group of coumarin 153, the charge distribution will be faster towards lower the free energy of the system due to the stronger dipole moment. Therefore, the corresponding solvation response in phenol-cyclohexane mixtures with higher mole fractions decays very rapidly.

Two-dimensional (2D) materials have become a worldwide hot topic due to their fascinating properties, including high carrier mobility, tunable bandgap, ultra-broadband optical absorption and response. The versatility of 2D materials enable it hold great potential to achieve high performance Terahertz (THz) optoelectronic devices. However, the THz radiation, range from infrared to microwave, known as the THz gap, much less investigated than that of other electromagnetic wave. Motivated by this lack of knowledge, we reviewed the recent advances of research into 2D materials based THz optoelectronic devices. Firstly, we introduced the background and motivation of this review. Then, the suitable 2D material candidates are exhibited, followed by a comprehensive review of their applications in THz generation devices, modulator, THz shielding, and photodetectors. Finally, the challenges and further development directions are concluded. We believe that some milestone investigations of 2D materials based THz optoelectronic devices will emerge soon, which will bring about great industrial revelations in 2D materials-based nanodevice commercialization.

A combined freezing, soaking, and centrifugal desalination (FSCD) process was employed to desalt seawater taken from Bohai Bay. The seawater was frozen into two kinds of sea water ice having different texture and size using either commercial refrigerator or experimental setup. For both kinds of ice samples, the influences of soaking-related parameters on the desalination effect were studied. For most ice samples treated with FSCD process, the salt removal efficiencies are higher than 90%. The purity of ice product increases as increasing soaking time and at higher initial soaking liquid temperature, while the ice yield rate decreases. At the same ice yield rate, the salt removal efficiency for ice flakes is higher than that for crushed ice samples, whereas when the raw seawater of 27 °C is used as soaking liquid, FSCD process is not feasible due to too lower ice yield rate.

In the femtosecond laser field with linear polarization, electrons experience alternative acceleration and deceleration during each optical cycle of the pulse. However, a circularly polarized femtosecond laser can continuously accelerate free electrons. The electrons will get higher energy compared with linear polarization. In femtosecond laser ablation of metals, the light absorption mainly is due to free electrons within metals. The spectral emission of plasma generated with a circularly polarized femtosecond laser will differ from that generated with a linearly polarized femtosecond laser. This study investigated femtosecond laser-induced breakdown spectroscopy (LIBS) of brass. The spectral intensity with circular polarization was higher than that with linear polarization. The results demonstrate the capability of a circularly polarized laser to facilitate signal improvement in femtosecond LIBS.

Amphiphilic peptides modified by molecular design can self-assemble into specific nanostructures with interesting applications in the fields of biomedicine and biotechnology. Lysyl oxidase (LO) is ubiquitous in human serum. However, enzymatic self-assembly of amphiphilic peptides remains a challenge for lipid-soluble drug delivery under the induction of LO. Here, we designed a positively charged amphiphilic peptide, A6K2, that could stably self-assemble to form nanovesicles. The lysine in the peptide molecule could be covalently cross-linked under enzyme catalysis, and the major transition was from random coil to β-sheet secondary structures, eventually leading to the destruction of the peptide nanovesicles. The lipid-soluble antitumour drug doxorubicin (DOX) as a model drug could be loaded into the hydrophobic core of the nanovesicles formed by the amphiphilic peptide A6K2, even though DOX was not covalently linked to the peptide monomer. The amount of DOX-encapsulated A6K2 nanovesicles in human hepatocellular carcinoma BEL-7402 cells was significantly higher than that in human liver L02 cells, indicating excellent selectivity. The amphiphilic peptide A6K2 inhibited tumour cell growth and had low cytotoxicity to mammalian cells, and it showed antibacterial activity against G+ and G- bacteria. These advantages make enzymatic self-assembling A6K2 nanovesicles of great interest in drug delivery for biomedical applications.

In this paper, an Nd:YAG laser was used to ablate an aluminum target and produce laser-induced plasma. A heating stage and two parallel plates were used to heat the target and confine the plasma. As determined by measuring the spectral emission, the combination of spatial confinement and an increased target temperature had stronger enhancement effects compared with the cases of spatial confinement or an increased target temperature alone. In addition, the effect of increasing the target temperature and spatial confinement on plasma expansion dynamics was investigated by time-resolved plume morphology. By capturing the images of the plasma plume during its expansion, it was easier to understand the enhanced mechanism of plasma emission. When the target was heated, the plasma plume expanded faster and became narrower. Moreover, when the spatial confinement was combined with the increased target temperature, the reflected shockwave confined the expansion of the plasma plume in the lateral direction, which led to further compression of plasma in the lateral direction and further expansion of the plasma plume in the axial direction compared with the case without spatial confinement.

To improve the emission intensity of laser-induced breakdown spectroscopy (LIBS), laser-induced plasmas with high temperatures are beneficial to the sensitivity of LIBS. However, the plasmas in typical LIBS also have high densities as the plasma temperature is high. High plasma density will increase line width and self-absorption, resulting in lower spectral resolution. The spectral resolution is very important for LIBS. Higher spectral resolution will improve the accuracy of element determination. To improve the LIBS resolution, the plasma should have both low density and high temperature. In this study, the influence of distance between focusing lens and target surface on femtosecond LIBS was investigated in air. The results indicated that the intensity of spectral lines depended strongly on the distance between the focusing lens and target surface. With the increase of the distance, the line intensity firstly rose, dropped, rose, and then dropped. This could be attributed to Kerr self-focusing effect and plasma defocusing effect during femtosecond laser pulse propagation in air. In addition, plasma temperature and electron density over the distance between the focusing lens and target surface were also calculated by using Boltzmann plot and Stark broadening. The change in the plasma temperature was not consistent with that in the electron density. When the spectral intensity was strongest, the plasma temperature was highest, while the electron density was lowest. Therefore, by changing the distance between the focusing lens and target surface, high-temperature and low-density plasma with strong spectral intensity can be obtained in femtosecond LIBS. This plasma with high temperature and low density can be used to improve the LIBS sensitivity. We hope that our research will be beneficial to improve LIBS resolution and optimize LIBS emission intensity in future femtosecond LIBS.

It is well known that Bessel beams have non-diffractive characteristics, which can be generated by Gaussian beams focused by an ideal axicon. In general, the length of filament generated by Bessel beams is longer than that by Gaussian beams and the electron density in the filament generated by Bessel beams is more uniform. This paper experimentally studied the propagation distance-resolved characteristics of copper plasma emission induced by axicon-focused femtosecond laser filamentation in the air. The evolution of the spectral intensity, plasma temperature, and electron density with the filament propagation path was obtained. The experiment results showed that when the base angle of the axicon was 5.0°, the spectral intensity along with the filament propagation path was more stable than that the base angle of the axicon was 0.5°. The changes in the plasma temperature and electron density along the filament propagation path were consistent with the change in the spectral intensity. This work provides a demonstration for the applications of filament-induced breakdown spectroscopy (FIBS), such as long-distance detection.

This paper used femtosecond Gaussian and vortex beams to ablate a Cu target and generate Cu plasmas. A Gaussian beam pulse is shaped into a vortex beam by a vortex wave plate (topological charge m = 1). The spectral intensity of Cu plasmas produced by the two lasers was measured, finding that the spectra with vortex beam were stronger than that with Gaussian beam. The spectral intensity was doubled by simply changing the Gaussian beam into the vortex beam. In addition, the position for maximum Cu atomic line emission along with the laser path was closer to the position of focusing-lens with increasing laser energy. Finally, the Boltzmann plot calculated the plasma temperature, finding that the plasma temperature with the vortex beam was also higher than that with the Gaussian beam. The results indicated that vortex beams could improve the spectral intensity of the femtosecond laser-induced plasmas.

Luminous effects like the emission of fluorescence, supercontinuum and terahertz are of great importance in the applications of femtosecond filamentation, which is however short in duration. By carrying out the experiment of femtosecond filamentation in gas flow, we observe the plasma jet induced by femtosecond filament for the first time, indicating that luminous effects still exist long after the termination of femtosecond filamentation. The temporal–spatial measurement of the luminous spectra shows that in air the luminous effect comes from the supercontinuum generated by oxygen, which lasts about 50 μ s; while in the pure nitrogen, it comes from the nitrogen fluorescence emission, which lasts much longer, reaching several hundred microseconds. Both of them are much longer than the plasma longevity in femtosecond filament in air. This work reveals the “long-lived” luminous effects after femtosecond filamentation and will be helpful to the understanding of femtosecond filamentation in air.

MoS2 nanodots are emerging as promising semiconductor materials for optoelectronic devices. However, most of the recent attention is focused on the fabrication of MoS2 nanodots, and the survey for exciton dynamics of MoS2 nanodots remains less explored. Herein, we use femtosecond transient absorption spectroscopy to investigate the carrier dynamics of MoS2 nanodots. Our results show that defect-assisted carrier recombination processes are well consistent with the observed dynamics. The photo-excited carriers are captured by defects with at least two different capture rates via Auger scattering. Four processes are deemed to take part in the carrier relaxation. After photoexcitation, carrier cooling occurs instantly within ~0.5 ps. Then most of carriers are fast captured by the defects, and the corresponding time constant increases from ~4.9 ps to ~9.2 ps with increasing pump fluence, which may be interpreted by saturation of the defect states. Next a small quantity of carriers is captured by the other kinds of defects with a relatively slow carrier capture time within ~65 ps. Finally, the remaining small fraction of carriers relaxes via direct interband electron-hole recombination within ~1 ns. Our results may lead to deep insight into the fundamentals of carrier dynamics in MoS2 nanodots, paving the way for their further applications.

Expansion dynamics of a laser-induced plasma plume, with spatial confinement, for various distances between the target surface and focal point were studied by the fast photography technique. A silicon wafer was ablated to induce the plasma with a Nd:YAG laser in an atmospheric environment. The expansion dynamics of the plasma plume depended on the distance between the target surface and focal point. In addition, spatially confined time-resolved images showed the different structures of the plasma plumes at different distances between the target surface and focal point. By analyzing the plume images, the optimal distance for emission enhancement was found to be approximately 6 mm away from the geometrical focus using a 10 cm focal length lens. This optimized distance resulted in the strongest compression ratio of the plasma plume by the reflected shock wave. Furthermore, the duration of the interaction between the reflected shock wave and the plasma plume was also prolonged.

We measure the backward fluorescence spectra generated by the femtosecond filament transmitting through the butane/air flame, and study the fluorescence emission from combustion intermediates (CN, CH and C2 radicals), air (mainly N2 and N2+). It is found that the fluorescence emission from combustion intermediates, N2 and N2+ shows difference when the femtosecond filament transmits through different parts of the butane/air flame, and we attempt to analyze it in this paper. This study demonstrates that the filament-induced fluorescence technique can be utilized to sense the combustion intermediates.

Femtosecond laser-induced breakdown spectroscopy (LIBS) of Ge at different initial sample temperatures is investigated. The spectral intensity of the Ge(I) 422.66 nm line is shown to decrease with increasing temperature. And, higher spectral emission intensity with an 800 nm laser wavelength is observed in comparison to a 400 nm laser wavelength. In order to understand this result, the transient relative reflectivity of a semiconductor Ge sample at different temperatures is measured by an ultrafast time-resolved pump-probe experiment. It is found that the change in the transient reflectivity of Ge at higher temperature is less than that at lower temperature, which may be the main reason for the decrease of spectral emission intensity.

A dual-wavelength femtosecond double-pulse laser is used to induce the Cu plasma spectroscopy in air. The laser wavelengths are a fundamental wavelength (800 nm) and a second harmonic wavelength (400 nm) from Ti:sapphire laser. The inter-pulse delay of double-pulse is from −300 ps to 160 ps. The observed spectral intensity is dependent on the inter-pulse delay of the dual-wavelength femtosecond double-pulse. We analyze the characteristics of the plasma temperature and the electron number density on the inter-pulse delay of double-pulse with two different wavelengths. For 800 nm + 400 nm, the spectral emission enhancement is based on more material ablation. For 400 nm + 800 nm, the enhanced mechanism is plasma reheating effect. This study will provide a better way to understand the mechanism of femtosecond double-pulse LIBS.

The mixture of graphene oxide (GO) and dye molecules may provide some new applications due to unique electronic, optical, and structural properties. Methylene blue (MB), a typical anionic dye, can attach on GO via π-π stacking and electrostatic interaction, and the molecule removal process on GO has been observed. However, it remains unclear about the ultrafast carrier dynamics and the internal energy transfer pathways of the system which is composed of GO and MB. We have employed ultrafast optical pump-probe spectroscopy to investigate the excited dynamics of the GO-MB system dispersed in water by exciting the samples at 400 nm pump pulse. The pristine MB and GO dynamics are also analyzed in tandem for a direct comparison. Utilizing the global analysis to fit the measured signal via a sequential model, five lifetimes are acquired: (0.61±0.01) ps, (3.52±0.04) ps, (14.1±0.3) ps, (84±2) ps, and (3.66±0.08) ns. The ultrafast dynamics corresponding to these lifetimes was analyzed and the new relaxation processes were found in the GO-MB system, compared with the pristine MB. The results reveal that the functionalization of GO can alter the known decay pathways of MB via the energy transfer from GO to MB in system, the increased intermediate state, and the promoted energy transfer from triplet state MB to ground state oxygen molecules dissolved in aqueous sample.

In this paper, we experimentally investigate femtosecond laser-induced nitrogen fluorescence emission at different air pressures (0.1 Pa–100 kPa). The variation of nitrogen molecule ( N 2) and molecular nitrogen ion ( N 2 +) characteristic fluorescence spectral lines' intensity as a function of air pressure is observed. When the air pressure is between 10 Pa and 100 kPa, the intensity of fluorescence signals from N 2 + and N 2 shows opposite variation as a function of air pressure. We deduce that dissociative recombination is the main pathway for the production of the C 3 ∏ u state of N 2. This research helps elucidate the physical emission mechanism of femtosecond laser-induced nitrogen fluorescence emission.

Spectral broadening is an important nonlinear effect during the interaction of intense femtosecond laser pulses with air. In this paper, we experimentally study the dependence of spectral broadening on pressure and pulse energy. It is found that increasing both pressure and pulse energy can enhance the spectral intensity, while only increasing pressure leads to obvious blue shift of spectra, and the pulse energy has little effect on it. To get a better insight of the mechanism of spectral broadening, the numerical simulation relating the instantaneous frequency to the time dependent pulse intensity and time dependent plasma density in air is carried out. The results indicate that the plasma generation induced self-phase modulation which is related to the pressure plays an important role in blue shift of spectrum. Besides the transmission of the laser pulse is measured and decreases with pressure. It proves that the energy transfer of the laser pulse is promoted by pressure. This study will be helpful to understand a deeper physical mechanism of femtosecond filament induced supercontinnum generation in air.

In this study, a femtosecond laser was focused to ablate brass target and generate plasma emission in air. The influence of lens to sample distance (LTSD) on spectral emission of brass plasma under linearly and circularly polarized pulses with different pulse energies was investigated. The results indicated that the position with the strongest spectral emission moved toward focusing lens with increasing the energy. At the same laser energy, the line emission under circularly polarized pulse was stronger compared with linearly polarized pulse for different LTSDs. Next, electron temperature and density of the plasma were obtained with Cu (I) lines, indicating that the electron temperature and density under circularly polarized pulse were higher compared to that under linearly polarized pulse. Therefore, changing the laser polarization is a simple and effective way to improve the spectral emission intensity of femtosecond laser-induced plasma.

Temporally shaped femtosecond laser pulse is used to generate the air plasma channel. The length of plasma channel is optimized by a genetic algorithm. Compared with the transform-limited pulse, the temporally shaped femtosecond laser produced by the spatial light modulator with the genetic algorithm can lead to a significant increase in length and brightness of plasma channel in atmosphere. In particular, the length of the plasma channel produced by the optimized shaped pulse can be extended by 50%. This method can be especially advantageous in the context of femtosecond laser-induced plasma channel.

In this paper, we present a study on the effect of inter-pulse delay using femtosecond double-pulse laser-induced breakdown spectroscopy in a collinear geometry. The temporal evolution of spectral intensity is performed for the lines of Fe I 423.60 nm, Fe I 425.08 nm and Fe I 427.18 nm. It is found that, by selecting appropriate inter-pulse delay, the signal enhancement can be significantly increased compared with the single-pulse case. A three-fold enhancement in the current experiment is obtained. The plasma temperature and electron density are also investigated based on the theory of Boltzmann plot and Stark broadening. We attribute the main mechanism for emission enhancement to the plasma re-heating effect.

Our experiments show that initial energy and humidity strongly influences the water vapor fluorescence induced by ultrashort intense femtosecond laser pulses. It is found that the fluorescence signal can be enhanced by both increasing the humidity in the case of fixed energy and increasing the pulse energy in the case of fixed humidity. In addition, water vapor fluorescence emission in the linear polarization is stronger than that in the circular polarization due to the higher dissociation efficiency of linearly polarized pulses. The mechanism of water vapor fluorescence emission during femtosecond filamentation is discussed based on the analysis of these phenomena.

The laser polarization state has a great influence on the plasma fluorescence emission during femtoseond filamentation in air. For the spectral lines from N2, in the case of focusing lens with longer focal length (f=100 cm), due to the impact excitation, circular polarization leads to stronger fluorescence emission when the laser energy is higher than the ‘energy threshold’ (2.0 mJ). As a lens with shorter focal length (f=40 cm) is used, a similar phenomenon can be observed, however, the ‘energy threshold’ is much lower, which is lower than 0.8 mJ. For the lines from N 2 + , especially for the 391 nm one, their emission is stronger in the linear polarization state. The mechanism of plasma fluorescence emission during femtosecond filamentation is discussed based on the analysis of these phenomena, which will be helpful to the remote sensing and spectrum analysis.

Using a transient absorption (TA) setup switching between the single and double photon pump, a reasonable model is proposed to interpret the quasi-bounded vibration motions and dissociation-ionization of CS 2 based on the 3-photon absorption (3PA) around 800 nm. Besides global fitting, mathematical tools including fast Fourier transform (FFT), continuous (Morlet) wavelet transform (CWT), and principal content analysis (PCA) are successively applied to reveal the corresponding ultrafast dynamics. Inter-system crossing is demonstrated to be the key why dissociation or even ionization is able to take place at low intensity. As a result, plasma generation, filamentation and “re-absorption” reported in Ref. Zhang et al. (2019) can be interpreted with this model.

In laser-induced breakdown spectroscopy (LIBS), spark-discharge (SP) is an effective auxiliary method to improve spectral emission intensity, and can effectively affect plasma temperature and electron density. In traditional LIBS technology, nanosecond laser is usually used as excitation source. Compared with nanosecond laser, femtosecond (fs) pulse laser has many advantages in the interaction between laser and matter. The SP can also enhance the emission intensity of fs-LIBS. However, the discussion on plasma temperature and electron density of SP assisted fs-LIBS is rare. In this study, we measured time-resolved spectra of fs-LIBS with the SP. By using the Boltzmann plot and the Stark broadening, we calculated time-resolved plasma temperatures and electron densities for different laser energies at different discharge voltages. We hope that this study can provide a better understanding for SP assisted fs-LIBS.

Laser intensity has a great influence on the excitation, ionization and other dynamical processes of atoms and molecules. During the femtosecond laser filamentation, the different values of intensity along the propagation direction will have a great influence on the spatial distribution of the products generated by these processes, and that of the fluorescence emitted by these products. In this paper, by measuring the spatial distribution of fluorescence induced by femtosecond laser filament in air, we find that along the propagation direction, the fluorescence signal from N2 appears in front of that from N 2 + . Based on the experimental observations, we discuss the mechanism for the formation of N2 ( C 3 Π u ), and arrive at the conclusion that in the case of shorter focal length, the intersystem crossing process plays a main part in the formation of the C 3 Π u state of N2.

Laser-induced breakdown spectroscopy (LIBS) is an important technique which is widely used to analyze element composition. In order to improve the sensitivity of LIBS, much effort has been made to enhance the spectral intensity of LIBS by proposing a number of methods. In addition, we find that laser polarization has great influence on the emission intensity of femtosecond LIBS. By comparing the emission intensity of femtosecond LIBS in the circular polarization with that in the linear polarization, the spectral intensity in the case of circular polarization is stronger than that in the case of linear polarization. Moreover, this phenomenon is more obvious as laser energy increases. The polarization plays an important role in LIBS signal intensity. Based on the observation, the enhanced mechanism of the laser polarization for the spectral intensity is discussed in this paper, which will be helpful in spectral analysis and component analysis.

This study was conducted to investigate spatial confinement effects in laser-induced Cu plasma via optical emission spectroscopy. Two sequential enhancement events of Cu atomic emission lines were observed. This phenomenon was attributed to the compressed plasma by the reflected shockwave facilitating a highly condensed plasma core area with high plasma temperature and a dense population of excited atoms.

In this paper, we propose and demonstrate a study of Fe plasma using collinear dual-wavelength femtosecond double-pulse laser-induced breakdown spectroscopy (LIBS) with a fundamental wavelength (800 nm) and a second harmonic wavelength (400 nm) from Ti:sapphire laser. By varying the time separation of the dual-wavelength femtosecond double-pulse, the experimental results clearly show the signal enhancement up to a factor of 10 and more than 10 times, in comparison with it at 0 ps time separation. The electron temperature and electron density are analyzed as the basic parameters of plasma properties, and they are respectively based on the theory of Boltzmann plot and Stark broadening. It proves that dual-wavelength femtosecond double-pulse LIBS is excellent for enhancing the emission intensity of the signal.

We use a computational model to study the ablation mechanism of metal target irradiated by femtosecond pulse laser. It is confirmed that the Coulomb explosion can occur during femtosecond laser ablation of metal. The influence of thermal ablation and Coulomb explosion on the ablation depth is respectively investigated. Comparing the calculated results with the experimental ones, we find that the theoretical results which consider the thermal ablation only agree well with the experimental ones at high laser fluence, and those which take the Coulomb explosion into account fit well with the experimental ones at lower laser fluence, which exactly explains the ablation mechanism. In contrast with the previous theoretical results which only consider the thermal ablation, our theoretical simulation describes the ablation mechanism straightforward by making comparison of ablation depth, and provides a more reasonable explanation that fits with the actual ablation process.

Charge carrier mobility is one of the most significant properties for organic semiconductors. In this work, the electronic structures and charge transport properties of 9,10-bis((E)-2-(pyrid-n-yl)vinyl) (n=2, 3, 4) anthracene (BP2VA, BP3VA and BP4VA) were investigated via the analysis of the molecular geometry, the reorganization energy, the frontier orbital and density of state, as well as the electronic coupling and the charge mobility. The results indicated that the linkage between 9,10-divinyl anthracene unit and pyridine (ortho-, meta- and para-) influenced not only the intra-molecular conformation (i.e., the reorganization energies), but also the intermolecular interaction (i.e., transfer integrals), and finally the charge mobility of the molecules. It is also found that: (1) The calculated charge mobilties of holes are dozens of times higher than those of electrons for the three molecules. (2) The charge mobilities of hole and electron of the three molecules display the trend: μBP4VA>μBP2VA>μBP3VA, and the hole mobility of BP4VA is as high as ~1 cm2/(V·s).

The charge transport properties of three crystalline polymorphs (α, β and γ) of 9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene (BP2VA) were investigated at the first-principle level using Density Functional Theory (DFT) method based on Marcus theory, as well as the hybrid quantum mechanical and molecular mechanical (QM/MM) method. The polymorphism dependent charge transport property was explored by the calculation and analysis of the reorganization energy, the transfer integral, as well as the charge carrier mobilities of three different crystalline polymorphs. The results show that (1) β-BP2VA has the highest charge carrier mobility (for hole is 0.988 cm2 V−1 s−1 and for electron is 0.053 cm2 V−1 s−1) among the three crystalline polymorphs. (2) Comparison of the reorganization energy between individual molecule in gas-phase and embedded molecule when considering the steric effect of surrounding molecules suggests that the reorganization energy is dependent on intermolecular interaction for BP2VA. The calculated charge carrier mobilities are strongly dependent on the crystalline polymorphs. The enlightenment to us is that during the designing of optoelectronic materials with high charge mobility, we should pay attention to the molecular packing mode in the bulk material besides the molecular structure itself.

In this paper we theoretically investigate electron emission during femtosecond laser ablation of single-layer metal (copper) and double-layer structures. The double-layer structure is composed of a surface layer (copper) and a substrate layer (gold or chromium). The calculated results indicate that the double-layer structure brings a change to the electron emission from the copper surface. Compared with the ablation of a single-layer, a double-layer structure may be helpful to decrease the relaxation time of the electron temperature, and optimize the electron emission by diminishing the tailing phenomenon under the same absorbed laser fluence. With the increase of the absorbed laser fluence, the effect of optimization becomes significant. This study provides a way to optimize the electron emission which can be beneficial to generate laser induced ultrafast electron pulse sources.

We have optimized a femtosecond time-resolved system by the optical Kerr-gate (OKG) method to investigate characteristics of various Kerr media. Optical Kerr gating is widely used in ultrafast measurements ranging from pulse characterization to spectroscopy and microscopy. We compared the efficiencies and the temporal responses of three Kerr media CS2, benzene and GGG. We have demonstrated that benzene has fast response and low efficiency while CS2 has high efficiency and slow response. Benzene as a Kerr medium shows the most befitting characteristics in the three media.

In this paper, we present a study on the influence of interpulse delay in laser-induced silicon plasma with femtosecond double-pulse, and two subpulses have different laser energies. The meansured optical emission line collected by a lens is the Si (I) at 390.55 nm. The range of double-pulse interpulse delay is from -150 ps to 150 ps. Unlike the femtosecond double pulses with two same energies, the combination of low + high energies can enhance the spectral emission intensity, while the combination of high + low energies probably reduces the spectral line intensity compared with single-pulse femtosecond laser. The results indicate that the interpulse delay is very important for laser-induced breakdown spectroscopy with femtosecond double-pulse to improve the optical emission intensity.

The role of inter-pulse delay time in laser-induced Cu plasma with femtosecond and nanosecond double-pulse irradiation had been investigated by optical emission spectroscopy. The experimental results showed that a significant enhancement effect in the atomic line emission was observed by changing the inter-pulse delay time. The spectral intensity of Cu plasma produced by a femtosecond + nanosecond double-pulse was higher than that produced by a nanosecond + femtosecond dual-pulse. That is to say, the shorter laser pulse (femtosecond) was efficient in coupling to the sample and the longer laser pulse (nanosecond) was suitable for re-exciting the produced plasma. In addition, the corresponding plasma temperature and electron density were also calculated. The plasma temperature in the case of femtosecond + nanosecond was lower than that in the case of nanosecond + femtosecond, while the electron density in the case of femtosecond + nanosecond was higher than that in the case of nanosecond + femtosecond. This study is helpful for the research on sample detection using femtosecond and nanosecond double-pulse laser-induced breakdown spectroscopy.

Microwave technology was applied in two combined freezing-based processes to desalinate raw seawater. The two processes employed freezing, microwaving and centrifugal desalination (FMCD) process and freezing, microwaving, gravity-induced and centrifugal desalination (FMGCD) process respectively. The removal efficiencies of salt, TDS (total dissolved solids), Cl−, Ca2+ and Mg2+, the ice yield rate as well as the duration time of the two desalination processes were studied. The results were compared with that of our previous study using combined freezing, gravity-induced and centrifugal desalination (FGCD) process. The study showed that microwave treatment can greatly accelerate the desalination in the two combined processes. By applying a two-minute microwave treatment, the desalination effect of FMGCD process is similar to or better than that of FGCD process. Taking TDS and Cl− concentration as reference indicators, the ice product quality of FMGCD process can meet the drinking water standard requirement when the brine reject proportion is >54%.

In this paper, we explored the influence of distance between sample surface and focal point on the expansion dynamics of laser-induced plasma under different sample temperatures in air. A Nd:YAG nanosecond pulse laser was used to ablate silicon and produce plasma plume. Using fast photography technology, we captured the images of the plasma plumes at different focusing positions and sample temperatures. During the expansion of plasma plume, with the decrease of the distance between focal point and sample surface, the emission intensity of the plasma increased first, and then dropped. In addition, the increase in the sample temperature could improve the expansion velocity of the plasma plume. Moreover, higher laser energy and sample temperatures could enhance the emission intensity and increase the plasma lifetime.