Threshold power

Perrin et al. [245] have shown that the increase in deposition rate is more pronounced above a certain threshold power density. This power density is close to the lowest power density in the experiments shown here. The deposition rate is probably the main parameter causing the observed difference between the low-power and high-power material and the deterioration of layers at higher power levels. This confirms the known observation that in general, the quality of material deposited in the y -regime is worse than in the a-regime [245]. 

The efficiency of the 2P-induced polymerization process in SR9008 using q.2 as an initiator has been compared to that using conventional radical photoinitiators, including benzil, benzophenone, bis(M,W-dimethylamino)benzil, 4,4 -bis(J f,Ar-dimethylamino)benzophenone, c3, and c4. The threshold power for... 

In this context, the threshold power is the minimum power necessary to obtain a well formed structure which is not deformed or washed away during the developing step. As the 2P-excitation rate does... 

Fig. 17 SEM images of structures fabricated in a triacrylate resin at threshold powers using a 730 nm excitation of r.l and b 520 nm excitation of r.3. The lower panels show the dimensions, transversal to the beam propagation direction, of single lines of the strnc-tures for each excitation wavelength. Reproduced with permission from [179], 2007, Optical Society of America...

The lower the threshold power, the easier it is to achieve high conversion efficiency of the noisy pulses and abberated beams which are typical of high-power lasers. Materials with threshold powers an order of magnitude lower than KDP could achieve conversion efficiencies closer to 90% for beam of similar quality to Nova. This has motivated us to undertake a broad and systematic search for such crystals. 

The transparency requirement quoted above immediately sets a limit on the maximum size of the nonlinearity we can expect to find in such materials. Data for known nonlinear crystals implies that it is unlikely to find crystals with phase-matchable d coefficients much greater than 2 pm/V if their UV edge is shorter than 250 nm. Although the size of the d coefficients is bounded by the transparency requirement, threshold powers substantially smaller than those of KDP can still be... 

From this information, we have estimated(25) that between 0.5 and 1% of the chiral organic salts formed from amino acids and alpha-hydroxy acids have lower threshold powers than KDP for doubling or tripling 1.05 pm light. The probability P of finding such a crystal in a random sample of N crystals from the population of chiral organic salts is given by... 

We have investigated the phasematching properties of many, but not all, of the crystals given in Table 2 using the direct measurement technique. In general, we have found that, as expected, most of the crystals do not have threshold powers lower than KDP for generating 0.527 or 0.351 pm light. Most often this occurs... 

Figure 6. Sample sizes required to assure that a low threshold power crystal would be found among various crystal types.

L-arginine acetate (LAAc) is a low threshold power THG crystal which has emerged from our survey. The loci for type ISHG and type I THG of 1.064 pm... 

The photophysical properties of terphenyl-based acyclic (22a) and cyclic hemispherands (22c, 22d, see fig. 27) have been investigated in organic solvents and in KBr pellets (1 wt%) with the purpose of introducing them later in optical amplifiers (Sloofif et al., 1998). Absorption cross sections for the 1.54 pm emission, which is quite broad with fwhh = 70 nm, amount to 0.62, 1.1, and 0.93 x 10 20 cm2 for [Er(22a)], [Er(22c)] and [Er(22d)], respectively. The best photoluminescence intensities are obtained with the cyclic ligands. The optical gain of the complexes doped into a polymer channel waveguide is on the order of 1.7 dB cm-1, while the threshold power is as low as 1.4 mW. 

However, real optically pumped lasing was observed both for GalnN/GaN and for GaN/AlGaN structures. The mirrors were formed either by cleaving or by dry etching. The threshold power densities were of the order of a few hundred kW/cm2 [20-22], In fact, even optically pumped second-... 

The laser action of a glass or crystal depends on the laser peak cross-section and the threshold power for lasing. The peak cross-section a is given by [116]... 

Rare earth doped lasers of Nd(III), and Ho(III), and Er(III) are available and in use [117]. The threshold powers have been calculated for these ions in various glasses and are as follows. 

The threshold powers can be lowered by transferring energy from other ions. The effect of added ions on threshold power is obvious from the data given below. 

The results are shown in Fig. 3.5. According to Fig. 3.5b, mechanochemical yield per unit of consumed energy appeared to be a constant only starting from some threshold power per 1 g of the mixture under treatment. This threshold is about 5 W/g. The existence of threshold effect in solids is connected with fragile-viscous transition the latter occurs only when a definite level of energy input to a solid is exceeded. 

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