Fourier Transform Techniques for Measuring Absorption of Transient Species Inoptical Limiting Materials
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Mat. Res. Soc. Symp. Proc. Vol. 597 ©2000 Materials Research Society
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Figure 1. PIA measurements with a step scan interferometer and phase sensitive
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the required position and held while the laser is modulated with the mechanical chopper. The signal from the silicon diode detector is fed through a lock-in amplifier that yields the difference value caused by photo-induced absorption. This signal is fed back into the interferometer and digitized. The mirror is then stepped to a new position and the process is repeated. In this way, an interferogram of the difference signal is created. It is then Fourier transformed, paying special attention to the phase correction method used, to yield the (IpumpedIunpumpnd) spectrum. Phase correction is a concern due to the possibility of both positive and negative going signals in the spectrums as well as the fact that the position of maximum intensity in the difference interferogram may not be at the actual center of the 9 interferogram . For this reason, the "Mertz-Signed" method of phase correctionn° is used and the position of the centerburst is chosen from a dc-interferogram collected under similar experimental conditions. Step-scan mode also permits the acquisition of fully time resolved transient absorption spectra (Figure 2). In this experiment, the white light and a pulsed laser propagate quasi co-linearly through the sample. The white light is then passed through the interferometer as in the previous experiment. The mirror is held fixed at each position while the laser is pulsed and the changing white light signal is digitized for multiple time slices by a transient recorder. A matrix of white light intensity as a function of mirror position and time relative to the laser pulse is then acquired. This data is then Fourier transformed across all mirror positions for each individual time resulting in time (those taken before the laser has become incident on the resolved spectra. The spectra corresponding to Iunpemped sample) are then subtracted from the spectra corresponding to IMpe to yield the difference spectrum. The interferometers can also be operated in the standard continuous, or "rapid scan" mode. Because of the throughput advantage, spectra can be acquired in short times compared to scanning dispersive instruments. Due to this fact, time sampled spectral series ranging from seconds to minutes can be produced using dual spectrum
difference methods. This time scale is too slow for modulation and phase sensitive techniques yet too fast for a standard scanning instrument. Step Scan Fourier Transform Spectroscopy The mirror isstepped to aposition and heldthere while atime dqpatdent signal triggered and digitized with a transient ro-order. This process is repeated at every mirror position until a full matrix of intensity asa function of miror position andtime is recorded.
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