An ultrafast light gate

A device has been built which is capable of gating light on and off on the picosecond time scale. The gate is built in much the same fashion as traditional Kerr cells, the difference lying in the use of powerful optical pulses rather than electrical pulses to induce a birefringence in various liquids. In the experiment, powerful light pulses from a mode-locked Nd:glass laser, ∼5 psec in duration (wavelength 1.06µ), are used to induce a birefringence in a number of liquids including CSµ, nitrobenzene, chlorobenzene, dichloroethane and others. The transmission of the gate as a function of time is probed by means of 0.53µ green light pulses derived from the 1.06µ pulses by second harmonic generation. With CS2the transmission curve has a width of 8 psec; the peak transmission is 10% and the extinction ratio is 200. With nitrobenzene the curve has an exponentially falling tail on one side. The associated decay time is 32± 6 psec. Since 32 psec is a time characteristic of molecular reorientation times, this observation provides direct support for the long held belief that orientational effects dominate light induced refractive index changes in nitrobenzene.