[Show abstract][Hide abstract] ABSTRACT: Combined effects of polarization, split gap, and rod width on the resonance hybridization and near field properties of strongly coupled gold dimer-rod nanosystem are comparatively investigated in the light of the constituent nanostructures. By aligning polarization of the incident light parallel to the long axis of the nanorod, introducing small split gaps to the dimer walls, and varying width of the nanorod, we have simultaneously achieved resonance mode coupling, huge near field enhancement, and prolonged plasmon lifetime. As a result of strong coupling between the nanostructures and due to an intense confinement of near fields at the split and dimer-rod gaps, the extinction
spectrum of the coupled nanosystem shows an increase in intensity and blueshift in wavelength. Consequently, the near field lifespan of the split-nanosystem is prolonged in contrast to the constituent nanostructures and unsplit-nanosystem. On the other hand, for polarization of the light perpendicular to the long axis of the nanorod, the effect of split gap on the optical responses of the coupled nanosystem is found to be insignificant compared to the parallel polarization. These findings and such geometries suggest that coupling an array of metallic split-ring dimer with long nanorod can resolve the huge radiative loss problem of plasmonic waveguide. In addition, the Fano-like resonances and immense near field enhancements at the split and dimer-rod gaps imply the potentials of the nanosystem for practical applications in localized surface plasmon resonance spectroscopy and sensing.
[Show abstract][Hide abstract] ABSTRACT: In this work, we investigated the plasma morphology induced by a Nd:YAG laser with the aim of improving the understanding of the formation and dynamics of the plasma in two cases, with and without a magnetic field. Single laser pulse production of a plasma in the absence and presence of a magnetic field was performed with an aluminum target in air. A fast photography technique was employed to obtain information about the expansion dynamics and confinement of the aluminum plasma in each case. The generation of the laser plasma was allowed to expand at two locations with different magnetic field strengths, which correspond to the strength 0.58 T in the center of two magnetic poles and 0.83 T at a distance of 4 mm from the upper pole (N). The plume showed lateral confinement at longer delays when the target was placed at the center of the two poles. When the target was placed at a distance of 4 mm from the upper pole it was observed that the plume was divided into two lobes at the initial stage and traveled towards the center of the magnetic field with further elapse of time.
No preview · Article · Aug 2015 · Plasma Science and Technology
[Show abstract][Hide abstract] ABSTRACT: Metals in nature exhibit a mediocre wettability and a high optical reflectance from the visible region to the infrared. This Letter reports that, by formation of nano- and microscale structures via a simple raster scanning of a focused femtosecond laser pulse without any further treatment, structured aluminum and nickel surfaces exhibit combined features of superhydrophobicity with a contact angle of 155.5°, and a high optical absorption with a reflectivity of several percent over a broad spectral range (0.2-2.5 μm). Thus, a multifunctional structured metal surface that integrates superhydrophobicity and a high broadband absorptivity has been easily realized by one-step femtosecond laser processing.
No preview · Article · Jun 2015 · Chinese Optics Letters
[Show abstract][Hide abstract] ABSTRACT: A virtual reconfigurable plasma Vee antenna consisting of a set of laser plasma filaments produced by femtosecond laser pulses in air is investigated in this paper. The calculation results show that radiation pattern becomes more complex and gain shows initially rapid rise but gradually saturate as the leg length increases, but the pattern and gain are not seriously affected by the plasma conductivity; particularly, the gain of the Vee antenna with plasma conductivity σ = 100S/m can reach about 80% of that of a copper antenna. Radiation efficiency of the antenna has shown a strong dependence on radius of the antenna leg, and an efficiency of 65%, considered to have a proper performance, can be obtained with the channel radius of about 10 mm. Apex angle variation can lead to significant change of the radiation pattern and influence the gain; the best apex angle corresponding to maximal gain and good directivity for the third resonance antenna leg length is found to be at 74° at 600 MHz and σ = 100 S/m. The calculation has shown that at terawatt laser power level, the plasma channel conductivity is close to that of conventional plasma antenna, and peak gain of the Vee antenna is more than 8 dB with a good directivity. In addition, the radiation pattern of special Vee antennas with apex angle 180°-dipole antennas, for first and third resonance leg lengths, is compared and underneath physics of the difference is given. The laser-induced plasma channel antenna is especially suitable for achieving good directivity and gain, which has advantage over conventional plasma antenna with gas discharge tube or metal antenna.
No preview · Article · Jun 2015 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: Features of the asymmetric nanocross including extinction spectrum, local electric field intensity, and temporal response of the local electric field under ultrashort laser illumination are investigated in this paper. It is found that, due to the simultaneous excitation of local electric fields in the arms that are perpendicular and parallel to the laser polarization direction of the asymmetric nanocross, extinction spectrum exhibits multiple resonant peaks and the position of the peaks can be tuned by changing the lengths of the arms. Simulation results disclose that there is a strong connection between optical response of the parallel and perpendicular arms. Moreover, temporal response of electric field in arms of the asymmetric nanocross shows that oscillations in the parallel arms start earlier than that of the perpendicular arms, and they are in phase when one of the parallel arms resonantly excited, which further reflects the relationship between the parallel and perpendicular arms. Therefore, we demonstrate that the perpendicular arm excitation is attributed to that of the nonresonant parallel arm in the asymmetric structure which cannot keep the overall electric neutrality of the nanostructure, and thus, perpendicular arms are activated to maintain this balance.
[Show abstract][Hide abstract] ABSTRACT: Evolution of a plasma plume from an Al target ablated with a nanosecond
1064 and 355 nm laser respectively under oblique incidence in air is
studied using the time-resolved shadowgraph imaging technique. The
characteristics of plasma plume expansion with different focusing
conditions (focal point on, ahead of and after the target surface) are
experimentally investigated. Experimental results show that the
evolution of the plasma plume is strongly influenced by air breakdown
which occurs prior to the laser beam reaching the target. Without the
occurrence of air breakdown, the temporal evolution of the Al plasma
plume with both UV and IR ablation laser wavelengths shows the plume
expansion with an ellipsoid-shaped plume front travelling mainly against
the incoming laser beam due to the formation of a laser-supported
detonation wave at the initial stage of laser ablation, and then the
shape of the plume front turns into a sphere. Experimental results also
show that a higher portion of the laser pulse energy reaches the target
surface at UV laser wavelength than that of an IR laser due to the
higher penetrating ability of the UV laser wavelength to the plasma.
No preview · Article · Dec 2013 · Journal of Physics D Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Microwave guiding along double parallel lines of femtosecond-laser-generated plasma filament has been demonstrated over a distance of about 8 cm in air, corresponding to a maximum microwave signal intensity enhancement more than sixfold the free-space propagation. It is shown that the operating frequency and the line electric width influence the propagation coefficient of microwaves propagating along this transmission line. Based on channeling microwaves along this line and by measuring and comparing the propagated microwave signals, the basic parameters of laser-generated plasma filament, namely, its electron density and conductivity, are obtained.
No preview · Article · Jul 2013 · Physical Review E
[Show abstract][Hide abstract] ABSTRACT: Aluminum alloy was analysed by using femtosecond laser-induced breakdown spectroscopy under argon, air and helium environments at pressures ranging from 1 to 80 kPa. The results reveal that both spectra intensity and lines detection are significantly influenced by the ambient conditions. In all ambient gases, as the pressure increases the emitted light initially increases, attains its maximum intensity and then decreases with further increase in pressure. It is also observed that some lines are well detected at low pressures in argon while they are absent at the same pressures in helium. In addition, plasma parameters such as electron densities and electron temperature have been investigated at different pressures in the three gases. Hotter and denser plasma has been observed in argon than that in air and helium. Furthermore, it is noted that plasma parameters at relative low pressures of argon (1 kPa) are similar to those obtained at relative high pressures of helium (80 kPa). The optimum conditions for the use of argon and helium as ambient gases have been determined. In fact, argon provides the best environment of femtosecond laser-induced breakdown spectroscopy only at relative low pressures while helium constitutes a good environment only at relative high pressures.
No preview · Article · Jun 2013 · Journal of Physics D Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Potential impacts of lightning-induced plasma on cloud ice formation and precipitation have been a subject of debate for decades. Here, we report on the interaction of laser-generated plasma channels with water and ice clouds observed in a large cloud simulation chamber. Under the conditions of a typical storm cloud, in which ice and supercooled water coexist, no direct influence of the plasma channels on ice formation or precipitation processes could be detected. Under conditions typical for thin cirrus ice clouds, however, the plasma channels induced a surprisingly strong effect of ice multiplication. Within a few minutes, the laser action led to a strong enhancement of the total ice particle number density in the chamber by up to a factor of 100, even though only a 10(-9) fraction of the chamber volume was exposed to the plasma channels. The newly formed ice particles quickly reduced the water vapor pressure to ice saturation, thereby increasing the cloud optical thickness by up to three orders of magnitude. A model relying on the complete vaporization of ice particles in the laser filament and the condensation of the resulting water vapor on plasma ions reproduces our experimental findings. This surprising effect might open new perspectives for remote sensing of water vapor and ice in the upper troposphere.
Full-text · Article · Jun 2013 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · May 2013 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Characteristics of shock wave as well as its evolution of aluminum
plasma produced by nanosecond YAG laser is investigated by time-resolved
optical shadowgraph images. Experimental results show that shock wave is
strongly influenced by the laser parameters and target arrangement.
Shock waves from aluminum plasma and air plasma are observed
simultaneously by shadowgraphs when the distance from lens to target
surface (DLTS) is longer than the lens focal length, and a narrow bright
"line" is observed in the region where shock waves from Al plasma and
air plasma meet. The longitudinal expansion velocity of shock wave from
Al plasma is largely influenced by DLTS and laser intensity as well, and
it increases with laser intensity at the early stage of plasma expansion
and reach to a maximum of 8.1×104 m/s.
No preview · Article · May 2013 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: We theoretically investigated the electromagnetic wave (EMW) transmission along two parallel wires of laser plasma filaments produced by the filamentation of ultrafast laser pulses in air. Many factors, such as wire diameter and separation, electron density, and operating frequency are shown to influence the propagation loss. By taking into consideration the radiation and transmission effects of the wires, the calculations of the two parallel filament wires reasonably agrees with that of the standard commercial twin-lead wire. Specifically, the optimum separation of the two wires is determined for a given frequency and an effective electron density of the wires. When compared with free-space propagation, transmission enhancement of tens dB is obtained using optimized wire configurations. Thus, the two plasma wires may be a potential channel for point to point directed delivery of EM energy or communication of pulsed-modulated EM radiation.
No preview · Article · May 2013 · Applied Physics B
[Show abstract][Hide abstract] ABSTRACT: Deterministic wavelength-dependent multifilamentation is controlled in fused silica by adjusting the diffraction pattern generated by a loosely focusing 2D periodic lens array. By simply translating the sample along the propagation axis the number and distribution of filaments can be controlled and are in agreement with the results of linear diffraction simulations. The loose focusing geometry allows for long filaments whose distribution is conserved along their propagation inside the sample. The effect of incident energy and polarization on filament number is also studied. Laser filamentation controlled by a microlens array could be a promising method for easy and fast 3D track writing in transparent materials.
[Show abstract][Hide abstract] ABSTRACT: A hollow cylindrical plasma waveguide, which cladding consists of a large number of a chaotically distributed plasma filaments induced by the propagation of femtosecond (fs) laser pulses in air, is shown to support guided modes of pulsed infrared (IR) laser radiation. Taking into account the discontinuity and the finiteness of the waveguide cladding, the loss coefficient loss of the laser radiation is calculated for different spatial configurations. We report how the waveguide loss depends on its structural parameters like normalized plasma diameter, distance between filaments, core-radius, cladding's thickness, and filaments' electron density. For typical plasma parameters, the loss of the fs plasma waveguide is found to be lower than that of freely propagating IR laser beams to distances in the order of the filamentation length. This fact allows the delivery of collimated pulsed laser light over long distances in atmospheric air, which is necessary for optical-based remote sensing and the detection of chemical and biological agents.
No preview · Article · Jan 2013 · Optics Communications
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Jan 2013 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: We investigated the possibility of Sommerfeld surface waves to propagate along a conducting plasma channel produced by the filamentation of ultrafast laser pulses in air. Using the approximation of a homogenous cylindrical wire of laser plasma filaments, the phase velocity and the propagation loss of different wire configurations are calculated. The phase velocity of the propagating wave proved to be close to the speed of laser pulses, which makes attaching to such instantaneous plasma channel feasible over distances in the order of the filament length. Wire diameter, electron density and operating frequency are appearing to influence the attaching distances and propagation loss. The attenuation of the propagating wave along the plasma wire appears to be lower than that of free space over some distances in the order of the filamentation length, which opens exciting perspectives for short distance point to point wireless transmission of pulsed-modulated microwaves.
No preview · Article · Sep 2012 · Applied Physics B
[Show abstract][Hide abstract] ABSTRACT: The effects of ambient pressures on the intensity of spectral emission, electron temperature and density of femtosecond laser produced plasma have been investigated. For this purpose, Al targets were ablated by employing a Ti: Sapphire laser system (50fs, 800nm) under various filling pressures of argon gas. The optical emission spectroscopy of Al plasma has been studied using the Laser Induced Breakdown Spectroscopy (LIBS) system. The results indicate that the pressure of the ambient atmosphere is one of the controlling factors of the plasma plume characteristics. The measurements reveal also that some lines which are nearly unresolved at high pressures become well resolved at low pressures.
[Show abstract][Hide abstract] ABSTRACT: A microwave waveguide that consists of a set of laser plasma filaments produced in air by the propagation of femtosecond laser pulses is investigated according to the hollow conducting waveguide concept. The conductivity, skin depth of the electromagnetic waves in this plasma waveguide and the energy required to excite such a waveguide are calculated for different possible configurations. A hollow conducting plasma waveguide is shown to support guided modes of electromagnetic radiation from millimetre to centimetre wavelength range. Our calculations show that, under the concept of conducting waveguide, it is more suitable to use the TE01 mode rather than TE11 to achieve an extended attenuation length. The attenuation length of the low-loss mode TE01 is shown to be dependent on the geometry of the plasma waveguides, the operating frequency and the plasma effective electron density. The effect of the plasma wall density spread on TE01 propagation is evaluated. Using the hollow conducting plasma waveguide operating in TE01 mode, an enhancement of microwave transmission over both free space propagation and dielectric plasma waveguide is obtained.
No preview · Article · Jul 2012 · Journal of Physics D Applied Physics