U.V Induced Visible Band Excited State Absorption in Liquid Crystal Media.
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nonlinear absorption to be studied. EXPERIMENTAL To measure the excited state dynamics in liquid crystal media a pump/probe experiment was set-up as shown in figure 1. An ultraviolet laser was focused into the sample inducing an excited state population, (via linear absorption), which was examined using a cw laser focused through the same region. A nitrogen laser operating at a wavelength of 337nm excited the sample and two cw lasers (argon-ion at 488nm or diode laser at 680nm) measured the excited state relaxation. The nitrogen laser was operated at a repetition rate of 1Hz, had a pulse-length of 3ns and an average energy per pulse of 5/J. This laser which had a rectangular spatial profile was focused using a 5cm focal length lens to 1/e2 diameter 100lim by 80jtm (in air). The liquid crystal sample was contained in a 2mm path-length cell housed in a temperature controlled oven (Mettler FP82). The optical density of 2mm of liquid crystal at 337nm is 3.4 and the cell
windows transmitted approximately 75% of the incident energy. The small fraction of energy transmitted by the liquid crystal was measured by detector D2 and used to trigger the T,, LX SAMPLE experiment. I
UV FILTER
-.
LUt Ff
Li
NITROGEN
[~j
LASER
L2L2
:10cm L3 :20cm
LINE FILTERr--
Figure 1. Experimental set-up used to measure the liquid crystal excited state dynamics. The cw beams were focused through the "active" region using a 10cm focal length lens to 1/e2 radii of 25pm (488nm) and 35/Am (680nm) respectively. The pump and probe beams crossed at an angle of 370. After passing through the sample the cw beam was reimaged onto a fast silicon photodiode (rise time 3ns) using a 20cm focal length lens. The active area of the detector 2 was 1mm 2 and the beams were focused to 0.03mm on the detector. Tight focusing was the beam size at the detector did not would change effects that refractive important to ensure that contribute to the signal, only nonlinear absorption effects were examined. A line filter appropriate to the cw wavelength was used to block the fluorescence from the sample. The signal was recorded on a digitising oscilloscope (HP-54111D), which averaged 50 pulses before transferring the data to a micro-computer. The detector/digitiser background signal was also measured with the probe beam blocked and subtracted from the signal in order to improve the signal to noise ratio. RESULTS The three liquid crystal materials used in this study were all cyanobiphenyls but with different alkyl chain lengths. CB1 (4-cyano-4-1-lalkylbiphenyl) is a high melting solid (TK_ I= 109'C), with a virtual nematic to isotropic phase transition at 45°C. This material can be supercooled below TKI and measurements were made at temperatures down to 80'C. CB5 (4-
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cyano-4-5-alkylbiphenyl) has a nematic to isotropic phase transition, TN.I at 35.5°C and the third material 8CB (4-cyano-4-5-alkylbiphenyl) has a transition to the isotropic phase at 40'C. All the measurements described in this report were performed with the materials in their isotropic phase. E
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