Research Topics

 

Experimental Methods

 

 

The Optical Heterodyne Detected Optical Kerr Effect Method

 

 

 

 

The OHD-OKE experimental method is illustrated schematically in figure 23.  These are non-resonant experiments.  The output of a 5 kHz Ti:Sapphire regen is used for the two pulses required in the experiment.  The pump pulse is brought into the sample with a 45° polarization.  The molecules have anisotropic polarizabilities.  The pump pulse induces a dipole in the molecules that is not along the E-field direction because of the polarizability anisotropy.  The induce dipole interactions with the pump E-field and exerts a torque on the molecule.  The torque causes a very slight net alignment of the molecules along the E-field direction.  This alignment at t = 0 causes the sample to be optically anisotropic and therefore birefringent.  The probe pulse is brought in horizontally (0°), which is 45° relative to the pump.  In the absence of the quarter wave plate (see figure 23) it passes through crossed polarizers.  Without the pump pulse, the sample is isotropic, and the probe is extinguished by the analyzer polarizer (90°) after the sample.  With the pump pulse, the sample is birefringent, and the probe becomes slightly elliptically polarized.  The small component of the elliptically polarized light that is vertically polarized (90°) passes through the analyzer polarizer and is the signal in an experiment that is not heterodyne detected.  As the molecules in the sample randomize their orientations, the birefringence decays, and therefore the signal decays.

 

In the heterodyne detected experiment, the probe pulse passes through the first polarizer and then through a quarter wave plate set at an angle (a few degrees) to make the probe slightly elliptically polarized.  Thus the probe has a component of its E-field in the direction of the signal that is produced by the sample’s birefringence induced by the pump.  This is the local oscillator for heterodyne detection.  The signal E-field, S, combines with the local oscillator E-field (L), and the absolute values squared of the sum is detected, |L + S|2 = L2 + 2LS(t) + S2.  The cross term 2LS(t) is detected.  L2 is time independent, and because L is much bigger than S, the S2 term is negligible. Heterodyne detection increases the signal because the cross term is measured, and it enables phase cycling methods that further improve the signal-to-noise ratios.  The experiment measures time derivative of the polarizability-polarizability correlation function (orientational relaxation).  According to linear response theory, the recovery from a small perturbation is caused by equilibrium fluctuations (fluctuation dissipation theorem).  Measurement of the recovery from the pump perturbation of the orientations measures equilibrium orientational fluctuations.

 

To cover the very wide range of times, 100 fs to tens of microseconds, four schemes are used.  For very short time, a 60 fs pulses are used and a stepper motor delay line records the data to 30 ps.  The pulse is then stretched to 2 ps.  This gives better signal at longer times.  Measurements are again made with the stepper motor delay line to 600 ps.  The pulses are then stretched to 50 ps.  A different delay line that can measure to 25 ns is used.  Finally with the 50 ps pump pulse, a CW diode laser is used as the probe and a 1 ns digitizer records the data.  In each case, the data from one time range overlaps with that from the next.  Only a multiplicative factor is used to match the different data sets where they overlap.  The data sets then form one continuous set of data like that shown in figure 18 in the section Complex Liquids.

 

 

 

Optical Heterodyne Detected Optical Kerr Effect Method Recent Publications

 

282.     “Orientational Dynamics of the Glass Forming Liquid, Dibutylphthalate: Time Domain Experiments and Comparison to Mode Coupling Theory,” David D. Brace, S. D. Gottke, H. Cang, and M. D. Fayer, J. Chem. Phys. 116, 1598-1606 (2002).

 

 

 

 

 >> Next: UV-Vis Transient Absorption and Fluorescence

[ Top | Previous: Polarization Selective Pump-Probe ]

 

[ Home | Research Overview | Group Members | Quantum Mechanics Book | Publications ]