P. Laporte

Comparison of heat-affected zones due to nanosecond and femtosecond laser pulses using transmission electronic microscopy

This letter presents a method aimed at quantifying the dimensions of the heat-affected zone (HAZ), produced during nanosecond and femtosecond laser–matter interactions. According to this method, 0.1 μm thick Al samples were microdrilled and observed by a transmission electronic microscopy technique. The holes were produced at laser fluences above the ablation threshold in both nanosecond and femtosecond regimes (i.e., 5 and 2u2009J/cm2, respectively). The grain size in the samples was observed near the microholes. The main conclusion is that a 40 μm wide HAZ is induced by the nanosecond pulses, whereas the femtosecond regime does not produce any observable HAZ. It turns out that the width of the femtosecond HAZ is less than 2 μm, which is our observation limit.

Femtosecond pulsed laser deposition of diamond-like carbon thin films for tribological applications

Abstract Pulsed laser ablation is a well-known technique used for thin film deposition of hard and wear resistant diamond-like carbon (DLC) films. Most of the previous studies were carried out by using pulse duration in the nanosecond range. Compared to conventional nanosecond laser ablation, femtosecond laser allows the production of high energy (up to a few keV) ions in the plasma, which may strongly affect the structure and properties of the deposited films. The present study was achieved by ablating graphite targets with femtosecond (10−15 s range) laser pulses. DLC films were deposited under vacuum onto (1xa00xa00) p-type silicon substrates at room temperature. The laser pulse energy was 1.5 mJ at a repetition rate of 1 kHz. The fluence (or energy density) range was between 1 and 6 J/cm2. The nature and mechanical properties of the films are characterized by X-ray absorption near-edge spectroscopy and nanoindentation techniques. The tribological behavior of the films are also investigated in a pin-on-plate configuration. Correlations between the structure of the films and some of their properties are highlighted, depending on the deposition conditions. Discussion is focused on the comparison between present results obtained using the femtosecond mode, with previously published results related to DLC films deposited using the nanosecond mode.

Study of plasma expansion induced by femtosecond pulsed laser ablation and deposition of diamond-like carbon films

Abstract Diamond-like carbon (DLC) films were deposited in high vacuum conditions at room temperature, by ablating graphite targets with femtosecond laser pulses. The structure of the films deposited onto silicon substrates were characterized by Raman spectroscopy and X-ray absorption near edge spectroscopy (XANES). Films exhibit unusual structure, Raman spectra showing the presence of nanocrystalline diamond in amorphous matrix. Plasma plume was imaged by a gated ICCD camera in the UV-Visible range. The behavior of the plume shape as well as the kinetic energy of the particles are investigated. The behavior of the expansion dynamics of the plume and the properties of thin films are studied in order to determine the optimal growth conditions for femtosecond pulsed laser deposition of DLC films.

Determination of NADH in the rat brain during sleep-wake states with an optic fibre sensor and time-resolved fluorescence procedures

The present paper reports a nanosecond time-resolved fluorescence derived from the cortex and the area of the periaqueductal gray including the nucleus raphe dorsalis (PAG-nRD) in unanaesthetized freely moving rats. The measurements were acquired through a single optic fibre transmitting a subnanosecond nitrogen laser pulse (337 nm, 15 Hz) and collecting the brain fluorescence occurring at 460 nm which might depend on mitochondrial NADH (reduced form of nicotinamide adenine dinucleotide). The fluorometric method was combined with polygraphic recordings, and this procedure allowed us to define, for the first time, variations of the 460 nm signal occurring throughout the sleep-wake cycle. In the PAG-nRD, the signal exhibited moderate heterogeneous variation in amplitude during slow-wave as compared to the waking state. Constant increases were observed during paradoxical sleep as compared to the waking state. For this state of sleep the magnitude of the variations depended on the optic fibre location. In the cortex and during either slow-wave sleep or paradoxical sleep, the signal presented moderate increases which were significant during paradoxical sleep. The magnitude of the redox variations observed either in the PAG-nRD or in the cortex might be ascribed to the oxidative energy balance which is related to sleep states.

Diamond-like carbon deposited by femtosecond pulsed-laser ablation: evidence of nanocrystalline diamond

Pulsed laser ablation is a well-known technique used for thin film deposition, extending from oxydes to hard and wear resistant Diamond-Line Carbon (DLC) films. Most of the previous studies devoted to DLC thin films elaboration have used pulsed duration in the nanosecond range. The present study concerns femtosecond (10-15 s range) laser ablation of a graphite target for the elaboration of Diamond-Like Carbon. Compared to conventional nanosecond laser ablation, femtosecond laser pulses allow the production of high energy (up to a few keV) ions in the plasma, which may strongly affect the structure and properties of the deposited films. DLC films have been deposited under vacuum onto (100) p-type silicon substrates at room temperature, by ablating graphite targets with femtosecond laser pulses. The nature and properties of the film have been characterized by various techniques, including Raman, XPS and AFM. Discussion will be focused on the comparison between present results obtained using femtosecond laser pulses, with previously published results related to DLC films deposited using nanosecond laser pulses. Especially, Raman spectra of DLC films obtained by nanosecond laser ablation always show the two well-known D and G bands (located respectively at around 1350 cm-1 and 1550 cm-1), whereas some DLC films obtained when using femtosecond laser pulses exhibit an intense peak at 1140 cm-1, which may be attributed to nanocrystalline diamond.

Pseudopotential hole–particle formalism for excitations in xenon molecules and clusters. I. Theory, atomic and molecular tests

A one-electron pseudopotential hole–particle formalism is implemented to investigate excitations in xenon molecules and clusters. Within this framework, averaged relativistic electron-Xe and electron-Xe+ pseudopotentials are determined to incorporate the excited particle contributions. A consistent hybrid scheme for spin–orbit coupling is developed, involving an atoms-in-molecules type approximation for the hole and a pseudopotential operator for the particle. The reliability of the one-electron pseudopotential scheme is first checked on the atomic spectrum of xenon and the transferability to high excited states is demonstrated. The molecular behavior of the formalism is also investigated by determining the potential energy curves of the lowest excimer states of Xe2*. The spectroscopic constants (De, ωe, and ωexe, respectively) are found to be 4173, 108, and 1.17 cm−1 for state (1)0u−(6su200a3P2), 4197, 109, and 1.11 cm−1 for state (1)1u(6su200a3P2), and 4250, 107, and 1.14 cm−1 for state (1)0u+(6su200a3P2).

Fiber-Optic Time-Resolved Fluorescence Sensor for in Vitro Serotonin Determination

At pH 7 and with the excitation at wavelengths above 315 nm, previously unreported fluorescence of 5-HT (5-hydroxytryptamine) is observed. Two fluorescence bands were observed for 5-HT; the first emits at around 390 nm with an associated lifetime near 1 ns, and the other (well known) emits at 340 nm with an associated lifetime of 2.7 ns. With both static and time-resolved fluorescences, the spectral and temporal effects of the excitation wavelength were studied between 285 and 340 nm. With these basic spectroscopic properties as a starting point, a fiber-optic chemical sensor (FOCS) was developed in order to measure 5-HT with a single-fiber configuration, nitrogen laser excitation, and fast digitizing techniques. Temporal effects including fluorescence of the optical fiber were studied and compared with measurements both directly in cuvette and through the fiber-optic sensor. Less than thirty seconds are required for each measurement. A detection limit of 5-HT is reached in the range of 5 μM. Our system, with an improved sensitivity, could therefore be a possible and convenient “tool” for in vivo determination of 5-HT.

TEMPERATURE EFFECT UP TO 1100 K ON THE XE2* EXCIMER LUMINESCENCE USING TWO-PHOTON EXCITATION

The temperature effect on the Xe2* excimer emission near 172 nm was studied up to 1100 K. As the temperature is increased, the line shape becomes asymmetric, its width increases and its maximum shifts to the blue. The results were simulated using Franck–Condon calculations. Good agreement between the experimental data and the derived profiles was obtained using a Morse potential for the excited state. The deduced parameters for this potential are De=0.51±0.01u2009eV, re=3.06±0.05u2009A and β=0.85±0.05a0−1. Comparisons were made with available data.

Transient visible spectroscopy from (1)0u−, (1)1u and (1)0u+ states of Xe2∗ in the range 500–600 nm

Abstract Transient absorption spectroscopy in the region λ=500–600 nm on relaxed Xe 2 ∗ excimers has been performed using the laser-reduced fluorescence technique. The observed transitions are assigned as originating from the (1)0u−/6s 3 P2, (1)1u/6s 3 P2 states and also from the (1)0u+/6s 3 P1 state. The analysis and interpretation of the spectrum is carried out using specially developed pseudopotential calculations. Making use of the detailed assigment of the transitions with respect to both initial and final states, an experimental value for the (1)0u−–(1)0u+ energy separation at the excimer equilibrium distance is proposed in the range 417–440 cm−1.

High UV average power at 15 kHz by frequency doubling of a copper HyBrID vapor laser in β-barium borate

Abstract Second harmonic generation of a copper HyBrID laser oscillator was performed at 15 kHz. 5.1 W UV (255 nm) total average power at 15 kHz were generated in two β-barium borate (BBO) crystals by frequency doubling of two 511 nm beams from a single unpolarized HyBrID copper vapor laser. The beam quality factors of the UV emission are found to be M X 2 =30±2 and M Y 2 =24±2. These two UV beams, combined at the focal point of a focusing objective, may enable high repetition rate UV laser processing.