Imaging of Silicon Naphthalocyanine in Toluene
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111 Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
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In addition to
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transmission data, the
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light scattered in three
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as a function of the pulse
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directions, 50, 90, and 130 from the forward direction, was measured
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energy using 1-cm2 area silicon detectors.
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response of the system is distributed
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demands a knowledge of the local laser intensity.
Since the longitudinal spherical aberration induced by the cell is dramatic, we begin with measurements of the in situ beam profile. In addition, both the concurrent and delayed shadowgraphs are used to infer the local beam diameter and crude features of the beam profile at each pulse energy. The shadowgraph data can also be used to ascertain how much energy is deposited at a given location, but the information on the system energetics was not analyzed in time for this report. SiNc fluoresces weakly in the spectral band of silicon detectors, therefore most of the non-shadowgraph image data pertains to scattering. At this time all fluences given are approximate. Most of the discussion will apply to a 2-mm cell with a transmission of 72%. Spherical Aberration A flat sample placed in a converging beam is an inherent source of spherical aberration. The outer rays refract more than the paraxial rays. This effect increases inversely with the square of the f-number of the focusing optics. In a medium with refractive index, n, the aberration increases as n2 - 1. As toluene has an index of 1.5 it presents a rather bad case. One consequence is that the diameter of the beam focus can be substantially greater than that measured in air. Longitudinal spherical aberration is much more severe; the majority of the energy in the incoming beam is contained within a profile which necks down rapidly to a narrow pencil (Figure 2). In all images presented in this paper the length of the horizontal axis is 1.3 mm. The beam waist itself elongates enormously in an asymmetric manner, with most of the elongation toward the focusing optic. This accentuates the progression of the most active region toward the input window. The aberration induced by the cell will be modeled analytically in future work. In this experiment the effective f-number for most of the beam energy is 7 rather than the value of 3.6 which would apply were the cell not present. The focal volume increased to a diameter of 13 ýim and a length of 300 pim. The outer diameter of the beam waist observed in the shadowgraphs increases as the pulse energy is ramped up.
112
Low-Energy Response A departure from the initial transmission is first detected at 20 nJ, corresponding to a beam waist fluence of about 15 mJ/cm 2. The new trend in the limiting curve remains perfectly linear out t
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