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Metal (such as Cr, Ti, etc.) adhesion layers, which are generally used to prevent nanostructures from falling off during electron beam lithography processes, will introduce interface damping, decrease the near-field enhancement, and shorten the dephasing time of localized surface plasmons (LSP). Maintaining metal adhesion layers while alleviating the induced interface damping in nanostructures is crucial for high-performance sensing, surface-enhanced Raman scattering elements, plasmon-based photocathodes, and plasmon-mediated catalysis. Here, we experimentally demonstrated that the mitigation of interface damping of metal adhesion layers can be achieved through the coupling between the bright and dark plasmonic modes of gold nanorods. We attribute the mitigation to stronger confinement across the plasmon energy, which effectively reduces the proportion of plasmon energy injected into the Cr adhesive layers. Compared to weak coupling, the non-radiative damping of plasmonic modes 1 and 2 is reduced by approximately 74% and 85%, respectively, under strong coupling conditions. The experimental results are supported by finite-difference time-domain simulations and are well explained by the calculated interaction potential for different gap sizes. This research will further benefit applications where low interface damping is required, such as the construction of low-threshold nanolasers and ultrasensitive sensing systems. [ABSTRACT FROM AUTHOR]

Revealing the coaction effect of radiative and non-radiative damping on the lifetime of the localized surface plasmon resonance (LSPR) mode is a prerequisite for the applications of LSPR. Here, we systematically investigated the coaction effect of radiative and non-radiative damping on the lifetime of the super-radiant and sub-radiant LSPR modes of gold nanorods using time-resolved photoemission electron microscopy (TR-PEEM). The results show that the lifetime of the LSPR mode depends on the length of the gold nanorod, and the different variation behavior of an LSPR mode lifetime exists between the super-radiative mode and the sub-radiative one with the increase of nanorod length (volume). Surprisingly, it is found that the lifetime of the super-radiant LSPR mode can be comparable to or even longer than that of the sub-radiant LSPR mode, instead of the usual claim that a sub-radiant LSPR mode has a longer life than the super-radiant mode. Those TR-PEEM experimental results are supported by finite-difference time-domain simulations and are well explained by the coaction effect with the calculation of the radiative and non-radiative damping rate with the increase of the nanorod volume. We believe that this study is beneficial to build a low-threshold nano-laser and ultrasensitive molecular spectroscopy system. [ABSTRACT FROM AUTHOR]

To enable organic light-emitting diodes (OLEDs) to be rolled and folded, we need a flexible encapsulation layer that can protect organic materials and metal electrodes that are susceptible to moisture and oxygen. Thin films that encapsulate organic electronic devices need to have excellent mechanical properties to prevent cracks during bending. Using plasma-enhanced chemical vapor deposition (PECVD) and other techniques, we fabricated high-density, stress gradient sandwich-structured films and studied the residual stress of deposited films on encapsulating films and their effect on delamination and cracking. We found that by adjusting the H2/N2 gas ratio and optimizing the Si:N:H ratio of PECVD SiNx:H films, denser, more etch-resistant, higher compressive stress and lower hydrogen content films can be deposited, thereby enable better flexible thin film encapsulation (TFE). We also deposited an inorganic/organic/inorganic sandwich structure film and utilized stress gradient changes to relieve the tensile stress on the outer film during bending. After standardized testing, the OLED with the stress gradient encapsulation structure has no dark spots after bending 10 000 times (bending radius 2 mm). This technique can be used in flexible TFE for various organic devices, showing promising applications in bendable and wearable products. [ABSTRACT FROM AUTHOR]

In the past 20 years, we have reached a broad understanding of many light-driven phenomena in nanoscale systems. The temporal dynamics of the excited states are instead quite challenging to explore, and, at the same time, crucial to study for understanding the origin of fundamental physical and chemical processes. In this review, we examine the current state and prospects of ultrafast phenomena driven by plasmons both from a fundamental and applied point of view. This research area is referred to as ultrafast plasmonics and represents an outstanding playground to tailor and control fast optical and electronic processes at the nanoscale, such as ultrafast optical switching, single photon emission, and strong coupling interactions to tailor photochemical reactions. Here, we provide an overview of the field and describe the methodologies to monitor and control nanoscale phenomena with plasmons at ultrafast timescales in terms of both modeling and experimental characterization. Various directions are showcased, among others recent advances in ultrafast plasmon-driven chemistry and multi-functional plasmonics, in which charge, spin, and lattice degrees of freedom are exploited to provide active control of the optical and electronic properties of nanoscale materials. As the focus shifts to the development of practical devices, such as all-optical transistors, we also emphasize new materials and applications in ultrafast plasmonics and highlight recent development in the relativistic realm. The latter is a promising research field with potential applications in fusion research or particle and light sources providing properties such as attosecond duration. [ABSTRACT FROM AUTHOR]

Various types of micro-/nanostructures can be fabricated by femtosecond laser irradiation on metal surfaces. It has received extensive attention and research because of its potential application value. In this paper, the effect of laser focusing position on the micro-/nanostructure formation on the surface of a titanium alloy (TC4) was systematically researched. The experimental results show that, in the process of femtosecond laser focusing, a spatial distribution of laser energy with a narrower beam center and a ring around the beam is formed in front of the geometric focus and the distribution of laser energy changes along the laser transmission direction. This property can affect the morphology of the micro-/nanostructure on the surface of irradiated materials. It is believed that the new spatial distribution of laser fluence caused by focusing is due to the nonlinear process of plasma defocus and laser self-focusing caused by multi-photon or tunnel ionization in front of the target by focusing the femtosecond laser. The discovery of the nonlinear characteristics of the focused femtosecond laser and the application of the unique energy spatial distribution to the surface morphology of the fabricated surface put forward a new idea for the direct fabrication of micro-/nanostructures via femtosecond laser, which has certain significance for the development of micro-/nanofabrication technology of femtosecond lasers. [ABSTRACT FROM AUTHOR]

The use of any solid material as a fuel and the discharge stability make a laser triggered discharge-produced-plasma (LT-DPP) an attractive light source. Density, size, uniformity, and so on of a fuel control the pinch process by a Lorentz force, and understanding the plasma dynamics in the initial stage is the most important in the LT-DPP study. This work reports how a material for a pinch is supplied and how a laser-produced-plasma (LPP) generated on the electrode behaves before the main discharge current. Our study revealed that the material of a pinch plasma is mainly supplied by laser ablation of a cathode not by current heating of the electrodes. The delay time of a discharge is determined by the velocity of an LPP generated on the cathode, and the traveling velocity of about 1 × 105 m/s of LPP is determined by two factors: the pressure of LPP and acceleration by the electric field. Before the main discharge current starts to flow, two X-ray pulses are generated. The first X-ray pulse is generated on the cathode where a laser is irradiated. The second X-ray pulse is generated on the anode when the LPP generated on the cathode arrives, and the arrival of the LPP triggers the main discharge current. The material for the pinch is not the LPP, but supplied by laser ablation. A faintly bright region appears between electrodes several tens of ns after the start of the main discharge current and this region is slowly imploded and a pinch plasma is formed at the current peak. [ABSTRACT FROM AUTHOR]

Modulating the length and position of plasma filament is desired for many filament-based applications. We proposed a novel method to manipulate the properties of filament along the propagation direction by introducing asymmetric focusing with a wedge plate in the forced focusing region. By simply varying the inclination degree of a wedge and the distance between the wedge and lens, a plasma filament can break into two sub-filaments along the propagation direction, and the separation between sub-filaments can be controlled. The experiment results are in agreement with the results of simulation by the ray-tracing method. Furthermore, the effect of the wedge angle and rotation angle on the control of the filament is studied. It is found that the separation of the sub-filaments and the azimuth angle of the filament around the original optical axis can be controlled by the wedge angle and rotation angle of the wedge, respectively. When combined with a phase plate, the control method also shows the ability to modulate the multi-filament array with wavefront modification. Our research provides a simple and flexible method for 3-D control of femtosecond laser filaments for practical applications. [ABSTRACT FROM AUTHOR]

White-light supercontinuum from a femtosecond laser filamentation is essential for many applications due to its broadening spectrum and remote sensing ability. We propose to enhance the supercontinuum intensity by presetting the gaseous lattice in the path of femtosecond laser filamentation. Our results show that the introduction of a gaseous lattice can increase the spectral intensity in both visible and infrared wings, up to about ten times over the no lattice case in a gas medium. We find that the gaseous lattice dramatically affects the time-domain evolutions of the optical intensity and the electron density during the nonlinear propagation of the pulse, thereby resulting in spectral intensity enhancement. The results provide a new way to enhance the supercontinuum generation in the ultra-intense nonlinear process by using a gaseous pre-lattice with a special spatial refractive index profile. [ABSTRACT FROM AUTHOR]

In applying a laser-triggered discharge-produced plasma (LT-DPP) as a light source, the most important issues are the supply of fuel and the suppression of a large-scale nonuniformity called the zippering effect. This paper reports pinch formation in LT-DPP under various electrode separations from 4 mm to 10 mm, which provides information for suppressing the zippering effect. Spherically expanding fuel ablated from the cathode becomes visible when the discharge current becomes large. By 100 ns after the laser trigger, the laser-ablated fuel extends 5 mm from the cathode. The width of the fuel reduces gradually with the increase of the current and forms a pinch at the current peak at 200 ns for the case of a 5mm electrode separation. When the electrode separation is larger, neck-like fuel distribution is observed and the pinched portion propagates toward the anode. The condition for reducing the zippering effect is discussed. [ABSTRACT FROM AUTHOR]

The authors propose that high catalysis of TiO2 nanoparticles (NPs) originates in the generation of excited states in the conduction band, which realize a long lifetime of hot electrons. This paper confirms their claim by observing individual NPs in the femtosecond laser pump-and-probe technique using photoemission-electron-microscopy. The low slope of the power dependence of the brightness of NPs is attributed to thermionic electron emission from an excited state below the vacuum level. An intermediate excited state is required to explain the asymmetrical delay dependence of the brightness enhancement, and the energy position is assigned to 0.78 eV above the conduction band minimum from the laser wavelength for the longest decay time of the brightness enhancement. A lifetime as long as 4 ps for hot electrons of 0.8 eV kinetic energy is observed when resonantly excited by a 760 nm light. The observed lifetime in TiO2 NPs is more than 10 times longer than those in previous papers using single crystals, which supports the authors' claim. [ABSTRACT FROM AUTHOR]

This study reports that thermionic emission can dominate over multi-photon ionization in gold nanoparticles under femtosecond laser irradiation. The brightness of individual particles observed with photoemission electron microscopy increased steeply for laser power above 200 mW, which is reproduced well by the Richardson–Dushman equation. The temperature estimated by fitting the equation is about 1 eV. This high temperature suggests that the laser is absorbed by defects. The difference in brightness among the nanoparticles as large as a few orders of magnitude is attributed to variation in the defect density. The particles having a similar temperature despite exhibiting largely varying brightness levels can be explained by slow energy transfer from the defects to the ambient electron gas. Discussions for interpreting the observed results suggest that thermionic emission can be a useful diagnostic tool for characterizing nanoparticles. [ABSTRACT FROM AUTHOR]

We introduce a simple, fast, and non-intrusive experimental method to obtain the basic parameters of femtosecond laser-generated plasma filament. The method is based on the channeling of microwaves along both a plasma filament and a well-defined conducting wire. By comparing the detected microwaves that propagate along the plasma filament and a copper wire with known conductivity and spatial dimension, the basic parameters of the plasma filament can be easily obtained. As a result of the possibility of channeling microwave radiation along the plasma filament, we were then able to obtain the plasma density distribution along the filament length. [ABSTRACT FROM AUTHOR]

A plasma column waveguide formed by a bundle of closely spaced plasma filaments induced by the propagation of ultrafast laser pulses in air and revived by a longer infrared laser pulse is shown to support microwave radiation. We consider values of both the plasma electron density and microwave frequency for which the refractive index of plasma is lower than the refractive index of air; therefore, a leaky plasma waveguide can be realized in extremely high frequency band. The guiding mechanism does not require high conductance of the plasma and can be easily excited by using commercial femtosecond laser sources. A theoretical study of leaky mode characteristics of isotropic and homogeneous plasma column waveguides is investigated with several values of plasma and waveguide structure parameters. The microwave transmission loss was found to be mainly caused by the microwave leakage through the air-plasma interface and is weakly dependent on the plasma absorption. In spite of losses of microwaves caused by leakage and plasma absorption, it is shown to be much lower than both that accompanying to surface waves attaching to single conducting plasma wire and the free space propagation over distances in the order of the filament length, which opens exciting perspectives for short distance point to point wireless transmission of pulsed-modulated microwaves. [ABSTRACT FROM AUTHOR]

Microwave guiding along single plasma filament generated through the propagation of femtosecond (fs) laser pulses in air has been demonstrated over a distance of about 6.5 cm, corresponding to a microwave signal intensity enhancement of more than 3-fold over free space propagation. The current propagation distance along the fs laser filament is in agreement with the calculations and limited by the relatively high resistance of the single plasma filament. Using a single fs laser filament to channel microwave radiation considerably alleviate requirements to the power of fs laser pulses compared to the case of the circular filaments waveguide. In addition, it can be used as a simple and non-intrusive method to obtain the basic parameters of laser-generated plasma filament. [ABSTRACT FROM AUTHOR]

We have explored here a hollow cylindrical laser plasma multifilament waveguide with discontinuous finite thickness cladding, in which the separation between individual filaments is in the range of several millimeters and the waveguide cladding thickness is in the order of the microwave penetration depth. Such parameters give a closer representation of a realistic laser filament waveguide sustained by a long stable propagation of femtosecond (fs) laser pulses. We report how the waveguide losses depend on structural parameters like normalized plasma filament spacing, filament to filament distance or pitch, normal spatial frequency, and radius of the plasma filament. We found that for typical plasma parameters, the proposed waveguide can support guided modes of microwaves in extremely high frequency even with a cladding consisting of only one ring of plasma filaments. The loss of the microwave radiation is mainly caused by tunneling through the discontinuous finite cladding, i.e., confinement loss, and is weakly dependent on the plasma absorption. In addition, the analysis indicates that the propagation loss is fairly large compared with the loss of a plasma waveguide with a continuous infinite thickness cladding, while they are comparable when using a cladding contains more than one ring. Compared to free space propagation, this waveguide still presents a superior microwave transmission to some distance in the order of the filamentation length; thus, the laser plasma filaments waveguide may be a potential channel for transporting pulsed-modulated microwaves if ensuring a long and stable propagation of fs laser pulses. [ABSTRACT FROM AUTHOR]

Nano- and microscale structures on a material surface formed by femtosecond laser processing have greatly changed optical characteristics, wettability, as well as other properties of the material. In this work, we report the formation of nano- and microscale structures on a spherical Al surface with femtosecond laser filament, and find that the filament-processed surface has a strong light-trapping ability from UV to IR (0.2-2.5 μm). Our result demonstrates that this method can be used to process a spherical surface without the complexity of a 4-axis sample control, and in principle, it is applicable to any non-planar sample. [ABSTRACT FROM AUTHOR]