Total Laser Fluence Research Articles

Study on removal of aluminum alloy oxide film by continuous-nanosecond combined laser with different pulse delays

In this paper, the thermal stresses arising from thermal expansion and the adsorption forces between the oxide film and the substrate are calculated from the one-dimensional heat conduction equation. Comparing the magnitude of the two, the laser cleaning conditions are obtained, and relevant models such as plasma shock wave pressure are established. The plasma plume, surface morphology, and elemental composition of aluminium alloy oxide film removed by continuous-nanosecond combined laser with different nanosecond pulse delays were experimentally investigated. At total laser fluence of 3630.47 J/cm2 (continuous laser fluence of 3628.73 J/cm2 and nanosecond laser fluence of 1.74 J/cm2), the removal effect is better with the increase of pulse delay, and the best removal effect is achieved when the pulse delay is 600 ms. Based on the experimental results, the mechanism of oxide film removal by combined lasers with different pulse delays was discussed, and at least two mechanisms of thermal stress against adsorption and plasma shock wave breaking the oxide layer were present to act.

Impact of electron correlation on the light-induced demagnetization of elemental ferromagnetic metals

The local spin-density approximation (LSDA) is known to describe poorly the electronic structure of 3d transition metals, yet most density-functional-based ab-initio studies of ultra-fast demagnetization rely on it. One way to account for Coulomb correlations among the localized d electrons and go beyond LSDA is to include the effective correlation energy (or Hubbard) U. By doing so, we show here that electronic correlations lead to sizable changes of the laser-induced demagnetization of iron, cobalt, and nickel. We study how the various laser parameters, such as pulse duration or intensity, change the magnetization dynamics. It turns out that the total laser fluence is not suitable to quantify how much a laser pulse demagnetizes a material, as changes in pulse duration and shape influence significantly the outcome. The findings are traced back to the electronic structure of the material, and explained based on phase space for optical transitions.Graphic abstract

Regular Nanowire Formation on Fe-Based Metal Glass by Manipulation of Surface Waves

We report the formation of a sole long nanowire structure and the regular nanowire arrays inside a groove on the surface of Fe-based metallic glass upon irradiation of two temporally delayed femtosecond lasers with the identical linear polarization parallel and perpendicular to the groove, respectively. The regular structure formation can be well observed within the delay time of 20 ps for a given total laser fluence of F = 30 mJ/cm2 and within a total laser fluence range of F = 30–42 mJ/cm2 for a given delay time of 5 ps. The structural features, including the unit width and distribution period, are measured on a one-hundred nanometer scale, much less than the incident laser wavelength of 800 nm. The degree of structure regularity sharply contrasts with traditional observations. To comprehensively understand such phenomena, we propose a new physical model by considering the spin angular momentum of surface plasmon and its enhanced inhomogeneous magnetization for the ferromagnetic metal. Therefore, an intensive TE polarized magnetic surface wave is excited to result in the nanometer-scaled energy fringes and the ablative troughs. The theory is further verified by the observation of nanowire structure disappearance at the larger time delays of two laser pulses.

Ultrafast laser ablation of copper by GHz bursts

The ablation of copper, using a 10 GHz burst ultrafast laser with a wavelength of 1030 nm, a pulse duration of 1 ps, a variable total laser fluence, and a number of sub-pulses per burst, is investigated theoretically. A two-temperature model with an extended Lorentz–Drude model for dynamic optical properties is used to simulate the melting and ablation process. Due to the heat accumulation from the preceding pulses, multipulse laser ablation could be advantageous over a single pulse. Moreover, the ablation performance can be maximized by properly selecting the pulse number, separation time, and energy in a laser burst. The numerical result shows that the present prediction is in fairly agreement with existing experimental result. Under the same total laser fluence of 32 J/cm2, a 10 GHz burst laser with an optimized 128 sub-pulse can significantly enhance the ablation depth, 4.2 times that a single pulse does. It is found that the optimized ablation depth is a linear function of the total fluence of ultrafast laser bursts.

Drilling of Copper Using a Dual-Pulse Femtosecond Laser

The drilling of copper using a dual-pulse femtosecond laser with wavelength of 800 nm, pulse duration of 120 fs and a variable pulse separation time (0.1–150 ps) is investigated theoretically. A one-dimensional two-temperature model with temperature-dependent material properties is considered, including dynamic optical properties and the thermal-physical properties. Rapid phase change and phase explosion models are incorporated to simulate the material ablation process. Numerical results show that under the same total laser fluence of 4 J/cm2, a dual-pulse femtosecond laser with a pulse separation time of 30–150 ps can increase the ablation depth, compared to the single pulse. The optimum pulse separation time is 85 ps. It is also demonstrated that a dual pulse with a suitable pulse separation time for different laser fluences can enhance the ablation rate by about 1.6 times.

Ultrafast near-field enhancement dynamics in a resonance-mismatched nanorod excited by temporally shaped femtosecond laser double pulses

The spatial–temporal dynamics of the near e-field enhancement in a resonance-mismatched nanorod irradiated by the femtosecond laser double pulses is theoretically investigated. A model combining the non-equilibrium electron excitation dynamics and the near field scattering was built for well exploring ultrafast dynamics of the e-field enhancement around the nanorod. It is revealed that the average enhancement factor can be evidently improved with increasing the double pulses separation. The energy ratio of the temporally shaped double pulses with respect to the highest enhancement factor shows an evident decrease with the increasing the total laser fluence. In addition, the most evident promotion of average enhancement factor from 51.2 to 78.8 can be acquired under laser fluence of 0.7Fth, which could be attributed to the temperature modulation of electron states excited by the temporally shaped double pulses. The study provides the base for understanding ultrafast plasmon dynamics for advancing the applications such as fs super-resolution fabrication, fs near field imaging and the generation of ultrashort extreme-ultraviolet pulses.

Atomic-level modeling of a silicon nanofilm irradiated by ultrashort-pulsed laser bursts

Atomic-level simulations of a silicon nanofilm irradiated by ultrashort-pulsed laser bursts were performed using the molecular dynamics coupled with an ultrafast energy transport model. The effects of pulse number in a laser burst and separation time between pulses on the thermomechanical response of the irradiated film were investigated. The numerical results showed that for laser bursts with an identical total energy, the more the pulses or the longer the pulse separation time, the lower the carrier temperature and number density. The same trend was also observed for the lattice temperature at the total laser fluence 0.1 J/cm2 but 0.18 J/cm2, as a result of the dynamical change in optical properties. It was also found that the induced thermal stress wave was weakened by increasing either pulse separation time or pulse number in a laser burst.

Ultrafast solid–liquid–vapor phase change in a thin gold film irradiated by multiple femtosecond laser pulses

Phase change processes including melting, vaporization and resolidification during the multiple femtosecond laser pulses irradiations on a thin gold film are investigated numerically. A two-temperature model is coupled with the phase interface track methods to describe the temperature variation and the solid–liquid and liquid–vapor interface evolutions. The relationship between the maximum vaporization temperature and melting depth, ablation depth are analyzed and compared with that of the single pulse irradiation. It is found that for two pulses irradiation, if deeper melting depth is desirable, the second pulse should be launched as soon as the melting depth induced by the first pulse reaches to the maximum. In comparison to a single pulse irradiation resulting in the same lattice temperature, this two-pulse approach leads to a greater melting depth. For laser irradiation with more than two consecutive pulses, if the total laser fluence remains a constant, larger pulse number and longer separation times between pulses leads to a smaller melting depth. However, the melting depth is much deeper compared to that by induced by a single pulse that causes the same lattice temperature.

Tritium locked in silica using 248nm KrF laser irradiation

In this Letter we report on selectively occluding tritium in a silica film on a silicon substrate using a combination of high-pressure tritium loading and 248nm KrF laser irradiation. Sixty percent of tritium dissolved in the silica film was bonded by laser irradiation. The concentration of the bonded tritium was proportional to the total laser fluence. Tritium effusion experiments indicated that the laser-locked tritium existed stably in the glass matrix up to 400°C. In this work we point a way to a safe and simple approach for the integration of on-chip radioisotope micropower sources for micromechanical and microelectronic applications.

Laser ablation of polymers using 395 nm and 790 nm femtosecond lasers

We compared a Ti:sapphire fs laser (790 nm) with a second harmonics (395 nm) fs laser, and then mixed them for ablating polyethylene (PE). Compared to the 790 nm fs laser, the 395 nm fs laser harmonics could etch PE faster. However, isolated carbon was formed on the ablated surface, in addition to C=O and C=C-H bonds. When we mixed a faint beam of the 395 nm fs laser harmonics with the 790 nm fs laser, the etching depth became even deeper. Moreover, the chemical composition of the ablated surface remained unchanged. At a total laser fluence of 80 mJ/cm2, the most suitable laser fluences for the 395 nm fs laser harmonics and the 790 nm fs laser were found to be approximately 2 and 78 mJ/cm2 respectively.

Microstructure and tribology of laser mixed Fe/Ti/C multilayered films on AISI 304 stainless steel

An excimer laser was used to mix Fe/Ti/C multilayered films on 304 stainless steel substrates. The samples were processed at both 1.1 and 1.7 J/cm2 with the number of pulses at each position varied between 1 and 10. Composition, microstructure, phase evolution, and tribological properties were observed to correlate with total laser fluence. Increases in Fe concentration from substrate interdiffusion and loss of C content were observed with increasing total laser fluence. The best tribological properties were observed in films possessing a combination of an amorphous or Fe3C phase plus fine grain TiC following processing at low and intermediate total laser fluence. At higher total fluences a combination of α-Fe, Fe3C, and fine-grain TiC was observed along with degradation in the wear and friction properties. Under optimum laser processing conditions the modified surface had a friction coefficient under dry sliding conditions reduced by a factor of 2 relative to uncoated 304 stainless steel and was significantly more wear resistant. These improvements in wear and friction appear to be related to the reduced chemical reactivity of the amorphous and carbide phases and to the influence of microstructure on improved mechanical properties.

Highly selective and efficient multiphoton dissociation of polyatomic molecules in multiple-frequency IR-laser fields

An ir multiphoton dissociation (MPD) process in multiple-frequency ir-laser fields has been experimentally realized. A selectivity ofS=104 was obtained in separating13C/12C isotopes upon multiple-frequency ir multiphoton dissociation (MFMPD) of the CF2HCl molecule, the dissociation yield13β for the13CF2HCl molecule amounting to around 1%. A yield of13β was reached at a selectivity ofS=102, and the total laser fluence required for the process was reduced. A new mechanism—“sticking” of molecules on the lower discrete vibrational levels—responsible for the low MPD yields observed for some molecules is discussed and a technique to eliminate it and thus maximize the dissociation yields is proposed. Ways to improve the selectivity by MFMPD are analyzed and a simple method for obtaining from a single TEA CO2 laser a multiple-frequency radiation suitable for experimental realization of MFMPD is suggested.