High current, high energy proton beams accelerated by a sub-nanosecond laser

The sub-nanosecond laser system available at PALS facility in Prague has been used in order to produce MeV proton beams with typical current density approaching 1 A/cm² at few tens of centimeters from the target surface. In spite of the relatively long pulse duration (0.3 ns) and low intensity (∼10¹⁶ W/cm²), far away from the forefront laser facilities used for advanced proton beam acceleration in the recent years (from tens of femtoseconds to few picoseconds), the obtained results are promising both in terms of maximum proton energy and fast proton current. Real-time diagnostics systems, mainly in time-of-flight (TOF) configuration, have been used in order to estimate maximum and peak energy of the plasma fast proton component, peak current density, total number of fast protons and conversion efficiency of laser energy into accelerated fast proton total energy. Optimization of the maximum attainable proton energy and current has been carried out by irradiating targets of different composition as well as varying the laser energy and the focal spot diameter. Experimental results, as well as possible applications in material science and nuclear physics, are discussed and compared with literature data.     

Low energy proton beams from laser-generated plasma

Low energy proton beams are produced by ns pulsed Nd:Yag laser operating in repetition rate at intensities of the order of 10¹⁰ W/cm². Laser pulses interacting with thick and thin solid hydrogenated targets, placed in high vacuum, produce non-equilibrium plasmas with ion emission in backward and forward directions along the normal to the target surface.The ion emission is analyzed with time-of-flight techniques by using ion collectors and ion deflecting spectrometers. The spectra analysis permits the evaluation of the plasma temperature, density, proton energy and current, ion energy and charge state distributions.Special targets, based on thin polymers coupled to metals or to conducting nanostructures, induce high electric field in the plasma and confer high kinetic energy to the protons, up to about 200 eV.By using a post-acceleration system, placed along the target normal direction, it is possible to accelerate further the protons up to the energy depending on the applied acceleration voltage. The maximum proton energy of 30 keV, the current density of about10 nA/cm² and the beam quality can be improved, as discussed.     

Room temperature deposited vanadium oxide thin films for uncooled infrared detectors

We demonstrate the room temperature deposition of vanadium oxide thin films by pulsed laser deposition (PLD) technique for application as the thermal sensing layer in uncooled infrared (IR) detectors. The films exhibit temperature coefficient of resistance (TCR) of 2.8%/K implies promising application in uncooled IR detectors. A 2-D array of 10-element test microbolometer is fabricated without thermal isolation structure. The IR response of the microbolometer is measured in the spectral range 8-13 μm. The detectivity and the responsivity are determined as ∼6×10⁵ cm Hz^1/2/W and 36 V/W, respectively, at 10 Hz of the chopper frequency with 50 μA bias current for a thermal conductance G∼10-3 W/K between the thermal sensing layer and the substrate. By extrapolating with the data of a typical thermally isolated microbolometer (G∼10⁻⁷ W/K), the projected responsivity is found to be around 10⁴ V/W, which well compares with the reported values.     

Effect of hydrogen plasma treatment on the surface morphology, microstructure and electronic transport properties of nc-Si:H

Hydrogenated nanocrystalline silicon (nc-Si:H) films, deposited by reactive radio-frequency sputtering with 33% hydrogen dilution in argon at 200 °C, were treated with low-power hydrogen plasma at room temperature at various power densities (0.1-0.5 W/cm²) and durations (10 s-10 min). Plasma treatment reduced the surface root mean square roughness and increased the average grain size. This was attributed to the mass transport of Si atoms on the surface by surface and grain boundary diffusion. Plasma treatment under low power density (0.1 W/cm²) for short duration (10 s) caused a significant enhancement of crystalline volume fraction and electrical conductivity, compared to as-deposited film. While higher power (0.5 W/cm²) hydrogen plasma treatment for longer durations (up to 10 min) caused moderate improvement in crystalline fraction and electrical properties; however, the magnitude of improvement is not significant compared to low-power (0.1 W/cm²)/short-duration (10 s) plasma exposure. The results indicate that low-power hydrogen plasma treatment at room temperature can be an effective tool to improve the structural and electrical properties of nc-Si:H.     

Low-resistance p-type ohmic contacts for high-power InGaAs/GaAs-980 nm CW semiconductor lasers

In this paper, the optimization of ohmic contacts for semiconductor lasers based on InGaAs/GaAs/GaAlAs layers is reported. Transmission electron microscopy (TEM) and electrical methods were used to study extensively the Pt/Ti/Pt/Au metallization system. The contact fabrication technology was optimized towards achieving the lowest electrical resistance. The technological control and optimization concerned the contact annealing temperature and thickness of metallic layers that form the contact. The average specific contact resistance was below 5×10⁻⁶ Ω cm² (with the record value of 8×10⁻⁷ Ω cm²) for the 10 nm Pt/20 nm Ti/30 nm Pt/150 nm Au system. The presented system was used in fabrication of continuous wave (CW) operated laser diodes. The chips mounted on passively cooled copper block achieved optical powers over 1 W, threshold current density values of 140-160 A/cm² and differential efficiencies above 1 W/A. The value of the characteristic temperature T₀ for discussed lasers varied in the range of 180-200 K.     

Investigations for InAs/GaAs multilayered quantum-dot structure treated by high energy proton irradiation

This paper focuses on the high energy proton irradiation effect of InAs/GaAs multilayers quantum-dot (QD) wafer and photodetector. With high energy proton path simulation, the releases of proton energy and trap distribution in QD multilayers are predicted well. Treated by 1 and 3 MeV protons, all protons almost penetrate the multilayers of QD structures and stop deeply in GaAs substrate. InAs QD multilayer structures/Infrared photodetector have been irradiated by protons with different energies (1 and 3 MeV) and doses (1 × 10⁹∼ 1 × 10¹³ protons/cm²). The photoluminescence (PL) and photoresponsivity (PR) spectrum of samples were measured and discussed with as grown and post irradiation.     

Magnetic field effect on electron emission from plasma

The influence of external magnetic field on the significant parameters of electrons from laser induced plasma (LIP) is investigated. A Q-switched Nd:YAG laser (1064 nm, 9-14 ns, 10 mJ, 1.1 MW) is focused on annealed, 4N pure (99.99%) Silver target (2 × 2 × 0.2 cm³) for production of plasma under vacuum ∼10⁻³ torr. Temperature, density and energy measurements for electrons were made by self fabricated Langmuir probe both in the absence and presence of external magnetic field (∼1.2 T) at different positions. The signals are recorded on 200 MHz UTT 2202 digital storage oscilloscope. The results thus obtained reveal decrease in electron temperature, energy and density in presence of external magnetic field. Confinement of plasma is also observed.     

Properties of amorphous Si thin film anodes prepared by pulsed laser deposition

Amorphous Si (a-Si) thin film anodes were prepared by pulsed laser deposition (PLD) at room temperature. Structures and properties of the thin films were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and electrochemical measurements. Galvanostatic charge/discharge tests of half cells using lithium counter electrode were conducted at a constant current density of 100 μA/cm² in different voltage windows. Cyclic voltammetry (CV) was obtained between 0 and 1.5 V at various scan rates from 0.1 to 2 mV/s. The apparent diffusion coefficient (D_Li) calculated from the CV measurements was about ∼10⁻¹³ cm²/s. The Si thin film anode was also successfully coupled with LiCoO₂ thin film cathode. The a-Si/LiCoO₂ full cell showed stable cycle performance between 1 and 4 V.     

Deep level transient spectroscopy on charge traps in high-k ZrO2

The deep trap properties of high-dielectric-constant (k) ZrO₂ thin films were examined by deep level transient spectroscopy (DLTS). The hole traps of a ZrO₂ dielectric deposited by sputtering were investigated in a MOS structure over the temperature range, 375 K-525 K. The potential depth, cross section and concentration of hole traps were estimated to be ∼ 2.5 eV, ∼ 1.8 × 10^− 16 cm² and ∼ 1.0 × 10¹⁶ cm^− 3, respectively. DLTS of ZrO₂ dielectrics can be used to examine the threshold voltage shift (?V_th) during the operation of SONOS-type flash memory devices, which employ high-k materials.     

Some studies on highly transparent wide band gap indium molybdenum oxide thin films rf sputtered at room temperature

Transparent wide band gap indium molybdenum oxide (IMO) thin films were rf sputtered on glass substrates at room temperature. The films were studied as a function of sputtering power (ranging 40-180 W) and sputtering time (ranging 2.5-20 min). The film thickness was varied in the range 50-400 nm. The as-deposited films were characterized by their structural (XRD), morphological (AFM), electrical (Hall Effect measurements) and optical (visible-NIR spectroscopy) properties. XRD studies revealed that the films are amorphous for the sputtering power ≤ 100 W and the deposition time ≤ 5 min, and the rest are polycrystalline with a strong reflection from (222) plane showing a preferential orientation. A minimum bulk resistivity of 2.65 × 10^− 3 Ω cm and a maximum carrier concentration of 4.16 × 10²⁰ cm^− 3 are obtained for the crystalline films sputtered at 180 W (10 min). Whereas a maximum mobility (19.5  cm² V^− 1 s^− 1) and average visible transmittance (∼ 85%) are obtained for the amorphous films sputtered at 80 W and 100 W respectively for 10 min. A minimum transmittance (∼ 18%) was obtained for the crystalline films sputtered at 180 W (∼ 305 nm thick). The optical band gap was found varying between 3.75 and 3.90 eV for various sputtering parameters. The obtained results are analyzed and corroborated with the structure of the films.     

Effect of impurities in charge-exchange carbon foil on foil thickness reduction

Carbon thin foils are commonly used as a charge stripping material in particle accelerators. Depending on the original foil thickness, changes in thickness during beam irradiation vary: thin foils (∼10 μg/cm²) thicken by build-up, medium thickness foils (∼15 μg/cm²) remain unchanged, and thick foils (∼20 μg/cm²) become thinner. The thickness reduction differs even under identical manufacturing processes and conditions.The factor causing foil thinning is unknown. On the basis of the low sputtering rate of carbon, it can be said that impurities contained in the foil cause foil thinning.Carbon foils contain impurities such as water. These impurities dissociate and combine with carbon and then evaporate. Taking this into consideration, we examined the gas composition during beam irradiation, to determine which impurity causes foil thinning. As a result, we found that oxygen contained in the foil plays a role in foil thinning.     

Quasi-phase matching of third harmonic generation in corrugated discharge capillary

Quasi-phase matching third harmonic of Ti:sapphire laser was measured by propagation through corrugated discharge capillaries. The 1 cm long capillary with periodical varying inner radius was used with corrugation frequency of 200 μm. The longitudinal plasma density was periodically varied by a discharge current ablating the inner capillary walls, altering the refractive index accordingly. Average plasma density measured was 10¹⁸ cm⁻³ at minimum corrugation radius. Peak laser intensity was 10¹⁰ W/cm² allowing enough pondermotive energy to the electron for high harmonic generation.     

Silicon oxide synthesized using an atmospheric pressure microplasma jet from a tetraethoxysilane and oxygen mixture

Local deposition of SiO_x was studied using an atmospheric pressure very-high-frequency (VHF) inductive coupling microplasma jet (AP-MPJ) from a tetraethoxysilane ((Si(OC₂H₅)₄), TEOS) and oxygen mixture. The SiO_x obtained showed the dielectric constant of 3.8 with a low leakage current of the order of ∼ 10^− 6 A ·cm^− 2 up to 8 MV ·cm^− 1. Bottom-gated sputtered-ZnO thin-film transistors with a AP-MPJ SiO_x as a gated dielectric layer exhibited a relatively high field-effect mobility of 24 cm² V^− 1 s^− 1, a threshold voltage of 14 V and an on/off current ratio of ∼ 10⁴, a performance comparable to that of thermal silicon dioxide. The TFT performance was also obtained for the top-gated ZnO-TFTs with a field-effect mobility of 1.4 cm² ·V^− 1 s^− 1, a threshold voltage of − 1.9 V, and an on/off current ratio of ∼ 10³.     

Transparent amorphous conductive Cd-In-Sb-O thin films for flexible devices

Transparent conductive amorphous Cd-In-Sb-O thin films were deposited on a flexible polyethylene naphthalate film by rf magnetron sputtering at room temperature. The large Hall mobility of ∼26 cm² V⁻¹ s⁻¹ was observed on the films with carrier density >10²⁰ cm⁻³. The carrier density varied from the order of 10²⁰ to 10¹⁷ cm⁻³ with increasing the oxygen partial pressure. The Hall mobility reached up to ∼17 cm² V⁻¹ s⁻¹, even at carrier density of ∼10¹⁷ cm⁻³. Flexible transparent filed-effect transistor was also fabricated using the Cd-In-Sb-O thin films as a channel layer and the device performance was investigated. The device exhibited a field-effect mobility of ∼0.45 cm² V⁻¹ s⁻¹ and an on-off ratio of ∼10² at room temperature.     

Structural, optical, and nonlinear optical absorption/refraction studies of the manganese nanoparticles prepared by laser ablation in ethanol

The structural, optical, and nonlinear optical properties of the manganese nanoparticles prepared by laser ablation in various liquids were investigated using the 532 and 1064 nm, 50 ps laser pulses. The TEM and spectral measurements showed temporal dynamics of size distribution of Mn nanoparticles in solutions. The nonlinear absorption (β = 2 × 10⁻¹⁰ and 4 × 10⁻¹¹ cm W⁻¹) and positive nonlinear refraction (γ = 8 × 10⁻¹⁵ and 2 × 10⁻¹⁴ cm² W⁻¹) of picosecond radiation were observed in the Mn colloidal suspensions using the 1064 and 532 nm radiation, respectively     

Spectroscopic properties and energy transfer process in Er doped ZrF4-based fluoride glass for 2.7 μm laser materials

Intense 2.7 μm emission from Er³⁺ doped in a new type of ZrF₄-based fluoride glass is reported. 2.7 μm emission characteristics and energy transfer process upon excitation of a conventional 980 nm laser diode are investigated. Based on the absorption spectra, the Judd-Ofelt parameters and radiative properties were calculated and compared with those of other glass hosts. The prepared glass possesses higher predicted spontaneous transition probability (29.04 s⁻¹) along with larger calculated emission cross section (9.16 × 10⁻²¹ cm²). Besides, the energy transfer coefficient of laser upper level (⁴I_11/2) can reach as high as 6.56 × 10⁻³⁹ cm⁶/s. Hence, these results indicate that this Er³⁺ doped ZrF₄-based fluoride glass has potential applications in 2.7 μm laser.     

Concentration profiles of Zn ions implanted with 60 keV for nanoparticle formation in silica glass

Surface recession due to sputtering under low-energy and high-fluence heavy-ion implantation makes shallower and broader depth profile of implanted ions than those calculated by conventional ion-range simulation-codes such as SRIM. Depth profiles of Zn atoms in silica glasses (SiO₂) implanted with Zn⁺ ions of 60 keV up to 1.0×10¹⁷ ions/cm² were evaluated using both experimental methods as Rutherford backscattering spectrometry (RBS), sputtering depth-profiling by X-ray photoelectron spectroscopy (XPS), and an advanced numerical simulation code TRIDYN, which includes the sputtering loss effects. The TRIDYN code predicts the shallowing of the projectile range from ∼46 to ∼27 nm with increasing the fluence up to 1×10¹⁷ ions/cm², and very high-concentration (∼20 at%) of Zn atoms close to the surface. However, RBS and XPS results exclude such high concentration close to the surface. These results suggest remarkable redistribution of Zn atoms from the nearer surface to the deeper region during the implantation. In fact, Zn-atom concentration near the surface and that near the projectile range are, respectively, lower and higher than those by the SRIM code predictions.     

High-intensity laser-driven proton acceleration: influence of pulse contrast

Proton acceleration from the interaction of ultra-short laser pulses with thin foil targets at intensities greater than 10(18) W cm(-2) is discussed. An overview of the physical processes giving rise to the generation of protons with multi-MeV energies, in well defined beams with excellent spatial quality, is presented. Specifically, the discussion centres on the influence of laser pulse contrast on the spatial and energy distributions of accelerated proton beams. Results from an ongoing experimental investigation of proton acceleration using the 10 Hz multi-terawatt Ti:sapphire laser (35f s, 35 TW) at the Lund Laser Centre are discussed. It is demonstrated that a window of amplified spontaneous emission (ASE) conditions exist, for which the direction of proton emission is sensitive to the ASE-pedestal preceding the peak of the laser pulse, and that by significantly improving the temporal contrast, using plasma mirrors, efficient proton acceleration is observed from target foils with thickness less than 50 nm.     

Electrons emission from laser induced metallic plasmas

Localized behavior of laser induced metallic plasmas has been investigated using Langmuir probe as an electrostatic diagnostic tool. A Q-switched Nd:YAG pulsed laser (1064 nm, 12 ns, 1.1 MW) is tightly focused on metal targets (Cu, Zn, Cd, Ag, Pt and Au) having dimensions 2 × 2 × 0.2 cm³ under vacuum ∼10⁻³ torr. The varying biasing voltages are applied to Langmuir probe. The electric signals are recorded on two channel 200 MHz digital storage oscilloscope (UNI T - UTT 2202). A comparison shows strong dependence of electron parameters (temperature, density, Debye length, plasma frequency and number of particles in Debye sphere) in plasmas on target materials' properties (atomic number, surface binding energy etc.). The maximum values for electron density (8.08502 × 10¹⁷ m⁻³), Debye length (8.07066 × 10⁻⁴ m), plasma frequency (5.19627 × 10¹⁰ Hz) are found for silver and copper metals, respectively, where as the electron temperature shows variation in this trend i.e. maximum value (1.36581 × 10⁶ K) for cadmium and minimum value is for gold (1.0008 × 10⁵ K). the maximum value of number of particles in Debye length at +15 V (1.47 × 10²⁶) for Pt and that minimum for Ag (2.1355 × 10⁷).     

Structural and optical properties of tellurite thin film glasses deposited by pulsed laser deposition

Tellurite (TeO₂-TiO₂-Nb₂O₅) thin film glasses have been produced by pulsed laser deposition at room temperature at laser energy densities in the range of 0.8-1.5 J/cm² and oxygen pressures in the range of 3-11 Pa. The oxygen concentration in the films increases with laser energy density to reach values very close to that of the bulk glass at 1.5 J/cm², while films prepared at 1.5 J/cm² and pressures above 5 Pa show oxygen concentration in excess of 10% comparing to the glass. X-ray photoelectron spectroscopy shows the presence of elementary Te in films deposited at O₂ pressures ≤ 5 Pa that is not detected at higher pressures, while analysis of Raman spectra of the samples suggests a progressive substitution of TeO₃ trigonal pyramids by TeO₄ trigonal bipyramids in the films when increasing their oxygen content. Spectroscopic ellipsometry analysis combined with Cauchy and effective medium modeling demonstrates the influence of these compositional and structural modifications on the optical response of the films. Since the oxygen content determines their optical response through the structural modifications induced in the films, those can be effectively controlled by tuning the deposition conditions, and films having large n (2.08) and reduced k (< 10^− 4) at 1.5 μm have been produced using the optimum deposition conditions.