Optical activity in sum-frequency vibrational spectra has been observed for the first time in chiral liquids. The electric-dipole allowed chiral element of the nonlinear susceptibility appears to be 3 orders of magnitude smaller than typical allowed achiral elements. This is partly because the observed chirality requires a breakdown of the Born-Oppenheimer approximation.

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We have measured steady-state photoabsorption spectra of a soluble trans-polyacetylene.We have found that photoexcitation of the soluble polyacetylene transforms only into neutral solitons in the temperature range 85 -300 K, with photoabsorption signals associated with charged solitons not observed. We consider the features of soliton excitations in comparison with other known types of polyacetylene.

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We have developed quite a simple and original lock-in photodetection system that works at a fixed radio frequency. The system includes a photodetector, a frequency down converter and a standard low-frequency lock-in amplifier. The resonant implementation of the radio-frequency attachment provides high gain and high selectivity, which have enabled us to attain the shot-noise-limited sensitivity to relative optical power change δP/P≃4×10−8 Hz−1/2 at a detection frequency of 6.2 MHz in measurements with a cw mode-locked Nd:yttrium aluminum garnet laser. We have realized the precise position-sensitive photodetection for laser pointing noise measurements and picosecond photoacoustic spectroscopy of solids.

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