Laser energy density

Solution Phase Studies. Freshly prepared solutions were used for all experiments. All solution phase studies were carried out using samples which had been thoroughly de-gassed using a minimum of three freeze-pump-thaw cycles for each sample, lire final pressure above the sample was in all cases less than 5 x 1(H mBar. Laser energy densities of less than 2.5 mJ cm-2 were used such that sample degradation was kept to a minimum. 

Laser energy density 5-1.3 Jem 2 depending on lens position... 

The technique consists of firing a pulsed excimer laser at a stoichiometric pellet of the material to be deposited and under suitable conditions of laser energy density, oxygen partial pressure, substrate tenperature and deposition angle, high quality films are deposited. What is remarkable about the process is the stoichiometric deposition of films adhieved by this technique. The composition of the pellet is closely reproduced in the films. A schematic of the deposition system is shown in fig. 4. The deposition and annealing parameters are shewn in table 3. 

Figure 19. Pulsed excimer laser (248 nm, 30 ns) etching of high thin films (a) etch rate versus laser energy density on a linear graph (b) same data on a semi log graph cind the slope of the curve is the inverse absorption length of the light in the film (ref. 36).

It is suggested that the degree of polymerization in composites synthesized by laser-electrochemical polymerization could be higher than that of composites synthesized by pure electrochemical polymerization. The conductivity of the composites varies with laser energy density by one order of magnitude. On the other hand, for a fixed laser energy of 7 mJ cm, the conductivity of the composites is not very dependent on the ratio of the monomers. 

Pedraza et al. [197] used various media and laser energy density to nanostructure... 

FIGURE 47.34 Laser energy density is measured from points on the distribution curve where the beam s intensity is 1/e (13.5 percent) of peak intensity or between points A1 and A2 in this plot. 

Gonzalo, J., Afonso, C.N., and Perrtere, f. (1996) Influence of laser energy density on the plasma expansion dynamics and film stoichiometry during laser ablation of BiSrCaCuO. J. Appl Phys., 79, 8042. 

The small cross-sectional area covered by a laser light beam coupled with the energy density in the beam leads to power levels reaching from milliwatts to many hundreds of kilowatts per square meter. 

A typical example might involve use of a krypton fluoride excimer laser operating at 249 nm with a pulse duration around 100 nanoseconds and a pulse repetition rate which can be varied up to 200 Hz. For metal deposition, energy densities in the range from 0.1 to 1 J/cm per pulse are typical. 

Lasers act as sources and sometimes as amplifiers of coherent k—uv radiation. Excitation in lasers is provided by external particle or photon pump sources. The high energy densities requked to create inverted populations often involve plasma formation. Certain plasmas, eg, cadmium, are produced by small electric discharges, which act as laser sources and amplifiers (77). Efforts that were dkected to the improvement of the energy conversion efficiencies at longer wavelengths and the demonstration of an x-ray laser in plasma media were successful (78). 

The energy densities of laser beams which are conventionally used in the production of thin films is about 10 — 10 Jcm s and a typical subsU ate in the semiconductor industry is a material having a low drermal conductivity, and drerefore dre radiation which is absorbed by dre substrate is retained near to dre surface. Table 2.8 shows dre relevant physical properties of some typical substrate materials, which can be used in dre solution of Fourier s equation given above as a first approximation to dre real situation. 

Figure 5 CL spectra of uHrahigh vacuunn-cleaved CdS before and after in situ deposition of 50 A of Cu, and after in situ laser annealing using an energy density of 0.1J /cm. The electron-beam voltage is 2 kV. ...

The layout of the experimental set-up is shown in Figure 8-3. The laser source was a Ti sapphire laser system with chirped pulse amplification, which provided 140 fs pulses at 780 nm and 700 pJ energy at a repetition rate of 1 kHz. The excitation pulses at 390 nm were generated by the second harmonic of the fundamental beam in a 1-nun-thick LiB305 crystal. The pump beam was focused to a spot size of 80 pm and the excitation energy density was between 0.3 and 12 ntJ/crn2 per pulse. Pump-... 

Here Q(t) denotes the heat input per unit volume accumulated up to time t, Cp is the specific heat per unit mass at constant pressure, Cv the specific heat per unit mass at constant volume, c is the sound velocity, oCp the coefficient of isobaric thermal expansion, and pg the equilibrium density. (4) The heat input Q(t) is the laser energy released by the absorbing molecule per unit volume. If the excitation is in the visible spectral range, the evolution of Q(t) follows the rhythm of the different chemically driven relaxation processes through which energy is... 

Indeed, most of the applications of laser-plasmas rely on the efficient production of energetic electrons driven by the interaction of ultraintense laser pulses with plasmas created from solids or gases. In fact, in these interaction conditions, laser energy is efficiently transferred to electrons generating a population of so-called fast or hot electrons. The process of fast electron generation often takes place near the critical density (the density at which the laser frequency iv0 equals the local plasma frequency wpe) surface [8, 9]... 

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