Phase-conjugate signal, temporal

Fig. 7. Setup for the degenerate four wave mixing experiments. The input beam is split in three beams. The beam splitter BS3 deflects a part of one of the pump beams to a power meter, which detects laser power fluctuations. The delay line with the retro reflector R adjusts the temporal overlap of the two pump beams coming from the front side on the sample. The long delay line with retro reflector R2 is moved to probe the temporal behavior of the nonlinearity in the sample. The phase conjugated signal beam propagates from the sample back to BSj and is then deflected through a stack of attenuation filters on a second power meter. An iris in front of the power meter increases the signal to noise ratio by removing scattered light...

Figure 6 shows the temporal variation of the phase conjugate signal beam with time for DRGMMA polymer thin film with 1pm thickness. In contrary to... 

Fig. 6. Temporal variation of phase conjugate signal from a DRGMMA thin film at 1.064 pm.

Fig. 9. Temporal variation of phase conjugate signal at 0.532 pm from a DRGMMA thin film at with probe beam at the same wavelength and pump beams at 1.064 pm. I ON...

Time-Resolved Measurements. The time-dependence of the third-order nonlinearity is an important characteristic for device applications and may also indicate the nature of the process causing the nonlinearity (24), The temporal behavior of the phase conjugate signal was monitored by delaying the probe beam with respect to the pump. 

Fig. 8. Temporal scans of the phase conjugated output beam in degenerate four wave mixing experiments. On the left side, the nonlinearity is instantaneous with the DFWM signal being the autocorrelation of the laser pulse. On the right side, the DFWM signal does not vanish after the writing beams have left the sample. Two photon absorption has induced a grating of excited and ground state molecules which is read by the delayed beam and decays slowly...

In the backward-wave geometry, the fundamental laser beam is usually split into forward, backward and pump beam. It is necessary to use delay lines in the case where the pulses are very short in order to assure a good temporal overlap between the three input beams. The output phase-conjugated intensity is maximized by changing the path length of one of the pump beams. The maximum value for the DFWM signal is measured for both sample (fample) and a reference (fe/erence)-In the case of non-absorbing medium, they are simply related [24] ... 

This concept is in essence a chromatographic effect similar to that observed in gas chromatography (GC), with the conjugated polymer film acting as the stationary phase. It is possible that like in GC and other candidate technologies for explosive detection, these responses could be empirically standardized for expected analytes of interest and sensory devices caHbrated to deconvolute temporal quenching signals to determine which analytes are present. This would further enhance the selectivity of what is already a very selective sensor for TNT and related compounds. 

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