Nd-laser Pulses Research Articles

Tuning optical properties of noble metal nanoparticle-composed glasses by laser radiation

Noble metal nanoparticle composed glasses attract significant attention due to the unique optical properties that they express in the near UV and visible spectral range. These are related to the high values of the extinction cross section and nonlinear optical characteristics. In this work we study the ability of laser irradiation to induce modification of the optical properties of borosilicate glasses that contain gold nanoparticles. The process is investigated by application of laser pulses of nanosecond Nd:YAG system on glasses that consist of nanoparticles with different size and shape. The results show that at certain conditions the glass optical properties can be modified as a change of the nanoparticles plasmon resonance wavelength is observed. The influence of the laser fluence and pulse number on this effect is studied. Two fluence regimes are defined: (i) at low fluences, close to the optical properties modification threshold the increase of the laser fluence results in a blue shift of the resonance wavelength; (ii) further increase of the laser fluences induces a red shift. Similar behavior is observed by changing the number of the applied pulses. Here after application of several thousand laser pulses additional, third regime of blue shift is realized. Theoretical models based on multiparticle Mie scattering theory and heat conduction equation are applied to explain the observed modifications. On their basis and performed analyses can be concluded that the induced optical properties variations are related to modification of the nanoparticles size and shape by melting, fragmentation and coalescence. The obtained results indicate an ability of nanoparticle size and shape modifications with a high spatial resolution in 3D and can be used for fabrication of integrated optical systems.

Effects of nanoparticles size and concentration and laser power on nonlinear optical properties of Au and Au–CdSe nanocrystals

Au and Au–CdSe nanoparticles (NPs) have been synthesized by organometallic pyrolysis method. Nano-crystals (NCs) structure was confirmed using high resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD). Nonlinear optical absorption is investigated by Z-scan technique using nanosecond laser pulses of second harmonic Nd:YAG. Intensity-dependence of nonlinear absorption on both nano-size and concentrations is reported. These are interesting findings which can be used to fabricate optical limiting and optical switching devices from NPs and hybrid systems.

Synthesis, structural, optical and electrical properties of metal nanoparticle–rare earth ion dispersed in polymer film

Cu-nanoparticles have been prepared by ablating a copper target submerged in benzene with laser pulses of Nd:YAG (wavelength: 355, 532 nm and 1,064 nm). Colloidal nanoparticles have been characterized by UV–Vis spectroscopy and transmission electron microscopy. The obtained radius for the nanoparticles prepared using 1,064 nm irradiation lies in the range 15–30 nm, with absorption peak at 572 nm. Luminescence properties of Tb3+ ions in the presence and absence of Cu-nanoparticles have been investigated using 355 nm excitation. An enhancement in luminescence of Tb3+ by local field effect causing increase in lifetime of 5D4 level of Tb3+ ion has been observed. Frequency and temperature-dependent conductivity of Tb3+ doped PVA thin films with and without Cu-nanoparticles have been measured in the frequency range 20 Hz–1 MHz and in the temperature range 318–338 K (well below its melting temperature). Real part of the conductivity spectra has been explained in terms of power law. The electrical properties of the thin films show a decrease in dc conductivity on incorporation of the Cu-nanoparticles.

Synthesis and Characterization of Au-Zn Nanoalloy by Laser Irradiation in Liquid Media

Synthesis and Characterization of Au–Zn Nanoalloy by Laser Irradiation in Liquid Media F. Hajiesmaeilbaigia, M. Fazeli Jadidib and A. Motamedia,∗ aLaser & Optics Research School, NSTRI, P.O. Box 11365, 8486 Tehran, Iran bDepartment of Science, Faculty of Physics, University of Zanjan, Zanjan, Iran Alloy nanoparticles due to their possibility of regulation, the region of absorption peak and consequently their optical and electrical properties, have a specific significance. In this research, nanosecond laser pulses of Nd:YAG laser with 532 nm wavelength that is close to absorption peak of gold and zinc nanoparticles, were used for synthesis of alloy. Atomic absorption spectrum UV–Visible, X-ray diffraction analysis and transmission electron microscopy images were utilized for characterization as well. For synthesis of Au–Zn alloy nanoparticles, zinc and gold nanoparticles were prepared separately by laser ablation method and then the mixture of their colloidal solutions with specified ratio was exposed to laser radiation. Thereby, gold and zinc nanoparticles were combined by absorbing of laser beam and gaining of required energy for melting, and Au–Zn alloy nanoparticles were formed. Wavelength of 532 nm is more effective in the formation of alloy than 1064 nm because of being close to wavelength of surface plasmon resonance of gold and zinc nanoparticles. Increase of concentration of gold nanoparticles compared to zinc nanoparticles in the mixed solution causes that gold nanoparticles absorb the most of the energy of laser and then larger gold particles are created, so alloy nanoparticles are not formed. The best time duration for irradiation and accomplishment of alloy formation is 30 min.

Propagation of Nd-laser pulses through crystalline silicon wafers

Propagation of pulses from an Nd:YAG laser (wavelength, 1.064 μm; pulse duration, 270 ns; pulse energy, 225 μJ) through crystalline silicon wafers is studied experimentally. Mathematical modelling of the process is performed: the heat conduction equation is solved numerically, the temperature dependences of the absorption and refraction of a substance, as well as generation of nonequilibrium carriers by radiation are taken into account. The constructed model satisfactorily explains the experimentally observed intensity oscillations of transmitted radiation.

Aperiodic x-ray multilayer mirrors and their application in plasma spectroscopy

We discuss the potentialities of aperiodic multilayer structures for the reflection of XUV radiation, including attooptical applications. Pilot samples of aperiodic normal-incidence broadband focusing multilayer mirrors are employed in a stigmatic transmission grating spectrograph with an operating range of 12.5–30 nm.This spectrograph was used to study (i) the XUV spectra arising from the charge exchange of laser-produced plasma ions with rare-gas (Xe, Kr, He) jet atoms, (ii) a debris-free Xe-jet plasma XUV source driven by Nd-laser pulses.

Plasma and X-UV source characteristics for Al targets heated by 40 ns Nd-laser pulses

The total laser energy (3–32 J), temporal history (40 ns full width at half maximum) and focal energy distribution (3×1013 W/cm2 peak, 1.5×1013 W/cm2 average) for λ=1.06 μm Nd:glass laser interactions with planar Al targets were measured. Laser-produced plasma emissions within the 1–3 keV x-ray range were recorded, with limited measurements in the 10–1000 eV range. Primary emphasis was given to obtaining the dependence of kilovolt x-ray emission on irradiance (varied by altering the energy on target or the lens-target spacing). A maximum efficiency for the conversion of incident laser energy to radiation above 1.5 keV was 0.25% into 2π sr. Integrated x-ray energy radiated over this range of photon energy was found to increase as the 3.2 power of laser energy at best focus. X-ray emission decreased when a prepulse preceded the main pulse by a controlled amount exceeding 0.2 μs but <5 μs, with a minimum emission at 0.5 μs separation. Plasma temperatures were estimated from x-ray line ratios and continuum slopes: a value near 230 eV was obtained from free-bound continuum slope for 3×1013 W/cm2 peak irradiance. Temperatures decreased at lower irradiances, as inferred from x-ray line ratios.

The plasma beat-wave acceleration experiment at Ecole Polytechnique

We present an experiment for demonstrating the principle of plasma beat-wave acceleration. The beating of two Nd-laser pulses creates a relativistic plasma wave in a deuterium plasma. Electrons at an energy of 3 MeV are injected into the plasma. We observe several hundred electrons accelerated up to 3.7 MeV. This paper is mainly devoted to the description of the experimental apparatus. In the design of the apparatus, we gave particular attention to efficient electron injection and to background noise suppression. We present also some preliminary results of electron acceleration experiments.

Pulsation of harmonic and K alpha emission from laser-produced plasmas.

We report simultaneous time-resolved measurements of second-harmonic and K\ensuremath{\alpha} emission from layered, planar targets irradiated by 180 ps, Nd-laser pulses focused to an intensity of (1--3)\ifmmode\times\else\texttimes\fi{}${10}^{16}$ W/${\mathrm{cm}}^{2}$. We observe that pulsations of the second-harmonic emission appear to coincide with pulsations of the x-ray fluorescence with each burst lasting about 20 ps and separated by 30--50 ps. The x-ray pulsations indicate that fast electrons are produced in short bursts which can modulate the effective ``temperature'' in the quarter critical region thereby affecting emission from the two-plasmon decay instability. We suggest these pulsations appear because the laser intensity reaching the critical surface is modulated by a chaotic time-dependent Brillouin reflection from the underdense plasma. Some implications of such a process are discussed.

Polar asymmetry of photoionization by a field with 〈 E3〉 ≠ 0. Theory and experiment

We predicted and calculated the polar asymmetry of the electron angular distribution, due to the interference between one-photon ionization by a field of frequency 2ω and two-photon ionization by a field of frequency ω. The effect is observed experimentally at simultaneous illumination of a photomultiplier by picosecond Nd-laser pulses (λ = 1.06 μm) and its second harmonic radiation (λ = 0.53 μm).

Pulsation of 1 omega 0 and 2 omega 0 emission from laser-produced plasmas. I. Experiment.

We report measurements on time-resolved spectra of the 2${\ensuremath{\omega}}_{0}$ (and, indirectly, 1${\ensuremath{\omega}}_{0}$) emission from plasmas produced by focusing 70--400-psec duration Nd-laser pulses to intensities above \ensuremath{\approxeq}${10}^{14}$ W/${\mathrm{cm}}^{2}$ onto planar targets. We observe repeated bursts of the emission lasting only 10--20 psec and typically separated by 30--50 psec. During these bursts the emitted frequency was observed to sweep rapidly across the wavelength band (\ensuremath{\approxeq}50 A\r{} wide), indicating the operation of processes that modulate the emission via phase. The results support our previous work [Dragila, Maddever, and Luther-Davies, Phys. Rev. A 36, 5292 (1987)], where we concluded that such phase modulation must occur in order to explain the observation that time-integrated spectra obtained in the same conditions were modulated.

Phase formation and redistribution of Xe implanted in the Ni-Si system after fast melting and solidification

The behaviour of Xe implanted at the Ni-Si interface and irradiated with Nd-laser pulses is studied in details and compared with Xe implantation into NiSi2 and into pure Si. Ion beam mixing followed by laser irradiation is able to form good quality epitaxial NiSi2 layer on Si.

Velocity measurements by X-ray backlighting of plane A1 targets irradiated with frequency-tripled Nd-laser pulses

The hydrodynamic behavior of thin A1 disk targets irradiated with a 1 ns frequency-tripled Nd-laser pulse is experimentally studied at irradiances ranging from 10 13 W cm -2 to 5 x 10 14 W cm -2 and compared to numerical simulations from a one-dimensional lagrangian hydrodynamic code. Velocity measurements as a function of target thickness — ranging from 0.75 μm to 25 μm — are performed by means of time and space resolved X-ray backlighting. Comparison is done with free surface velocity measurements deduced from visible interferometry and streak shadowgraphy, and with dense target velocities inferred from the double-foil technique. Results are discussed in terms of preheat effects, beam inhomogeneities and energy smoothing.

Characteristics of ablation plasma from planar, laser-driven targets

The momentum, energy, and velocity characteristics of plasma ablating from planar targets irradiated by long Nd-laser pulses (4 ns,<1014 W/cm2) are measured and the dependence of ablation parameters upon absorbed irradiance is determined. Large laser spots are used in these experiments so that the results are not sensitive to boundary effects.

Absorption in metallic targets irradiated by 50 ps laser pulses at intensities below 1012W/cm2

The energy reflected from plane metallic targets irradiated by 50 ps Nd-laser pulses, in the intensity range 1010-1012 W/cm2, has been measured. The authors find that for different conditions of incident energy and spot size, the target absorption is well correlated with incident intensity. The presence of radiation diffused out of the collecting lens angle when the spot size is very small can be accounted for by a simple geometrical interpretation. The experimental results at intensities below 1011 W/cm2 are in good agreement with a model in which absorption takes place through the high collision rate of the degenerate electron gas at the solid density; the validity of this model is confirmed by the measured blue-shift of the reflected radiation.

Laser-plasma interaction and ablative acceleration of thin foils at 1012–1015 W/cm2

The interaction physics and hydrodynamic motion of thin-foil targets irradiated by long, low-flux Nd-laser pulses (3 nsec, 1012–1015 W/cm2) are studied experimentally and compared with theoretical models. Laser light absorption is high (80%–90%) and thin-foil targets are accelerated up to 107 cm/sec with good (20%) hydrodynamic efficiency in the 1012–1013 W/cm2 range. These results agree with a simple rocket ablation model. Details of thermal heat flow, both axially (related to ablation depth) and laterally (related to beam uniformity requirements), are also presented.

Picosecond laser plasma creation in the presence of high electrostatic field on the surface of metals

Abstract Plasma was created by picosecond Nd-laser pulses on metal surfaces. In contrast to usual experiments the plasma was created in the presence of strong outer electrostatic field. In addition to strong ion and electron emission, very intense X-ray radiation was also observed, the intensity and energy of which depended to a very great extent both on the value and the polarity of the outer electrostatic field.