Gouy shift and temporal reshaping of focused single-cycle electromagnetic pulses.

We discuss exact solutions of Maxwell's equations that describe the evolution of single-cycle electromagnetic pulses. The solutions are applied to recent observations of the diffraction transformation of terahertz pulses. In particular, we elucidate the role of the Gouy shift in the temporal reshaping and polarity reversals of single-cycle terahertz pulses.

Optical correlator that uses cesium vapor.

We obtain the correlation pattern between two amplitude-modulated input images of 36 and 280 random black and white pixels, using the standard degenerate four-wave-mixing geometry of 852-nm optical beams in a 1-mm-thick cesium-vapor cell. The total optical input power is 3.2 mW to obtain 0.4 nW of power in the correlation pattern. We verified that the buildup time of the four-wave-mixing signal is ~30 ns. Using the random patterns, we analyze the performance of the cesium optical correlator in terms of the number of pixels that can be processed and the number of photons per pixel used. We show how to scale our experimental results for the efficiency of the cesium correlator to images containing a larger number of pixels.

Measurement of the complex polarizability of electron traps in Bi(12)SiO(20) by a moving-grating technique.

We measure the optical two-beam coupling gain in three nominally undoped cubic n-type Bi(12)SiO(20) crystals as a function of the beam frequency difference Omega and the beam intensity. The crystal geometry is chosen so that the beam coupling through the ordinary photorefractive effect is absent. From the dependence of the gain on Omega the full complex polarizability difference alpha(fe) between a full and an empty deep trap at the wavelength of 515 am is deduced to be (-1.8 - 4.6i +/- 0.7 +/- 1.1i) x 10(-39) F m(2) in SI units or (-1.6 - 4.1i +/- 0.6 +/- 1.0i) x 10(-23) cm(3) in Gaussian units. This suggests that the hole photoexcitation cross section sigma(h) is larger than that for an electron, sigma(e). Our data are consistent with the electron and hole parameters deduced from extensive previous measurements (in one of the crystals) analyzed with the standard electron-hole competition equations. This consistency requires that the average density of full traps be at least 20 times larger than the average density N(A) of empty traps and that sigma(h) be (2.4 +/- 0.8) x 10(-17) cm(2), while N(A) is (1.4 +/- 0.4) x 10(16) cm(-3) and sigma(e) is less than ~6 x 10(-8) cm(2). This is to our knowledge the first determination of these parameters in a sillenite crystal.

Research on nonlinear optical materials: an assessment.

The seven papers making up this assessment are based on the Workshop on Nonlinear Optical Materials held in April 1986.

Optical phase conjugation by backscattering in barium titanate.

We demonstrate optical-beam phase conjugation by the process of two-beam coupling in photorefractive barium titanate. The incident, image-bearing beam causes exponential gain for counterpropagating waves, which are fed by noise and emerge with a power of the order of 10% of the incident beam and phase conjugate to it. This is expected from the calculated plane-wave gain plus the analogy to the theory of phase conjugation of complex wave fronts by stimulated Brillouin backscattering. We conjugate beams at either 515 or 488 nm at between 10- and 50-mW power, and find, as expected, no frequency shift (<1 Hz) in the process.

Observation of intensity-induced nonreciprocity in a fiber-optic gyroscope.

Nonreciprocal phase shift has been predicted and observed in a fiber-optic gyroscope based on a Sagnac interferometer when the intensities of the counterpropagating beams are unequal. The magnitude of this intensity-induced phase shift is 1.4 rad/W power difference. At 1-microW power difference, this is equivalent to a rotation rate of 0.2 degrees /h in our 200-m-long fiber with a core radius of 2.25 microm that is wound around a spool of 19-cm diameter. The data are consistent with theoretical predictions based on four-wave mixing in the quartz fiber.

Photorefractive-index gratings formed by nanosecond optical pulses in BaTiO(3).

We write and read a refractive-index grating in a photorefractive crystal BaTiO(3) with a single 20-nsec laser pulse at 532 nm. The grating formed is erasable with similar pulses. Diffraction efficiency decreases exponentially with the cumulative erasing light energy for 20-nsec pulses at intensity levels of 5-30 MW/cm(2). Optical energies required to write and erase gratings with 20-nsec pulses are about an order of magnitude larger than for millisecond-to-to-second-long pulses at 515 nm, even though the grating is still formed by a one-photon process.

Spatial-diffusion measurements in impurity-doped solids by degenerate four-wave mixing.

A method is introduced that measures the spatial-migration rate of electronic excitation in condensed media over distances of the order of 0.1 microm. Two volume holographic gratings of widely differing modulation periods are simultaneously produced, using a phase-conjugate wave geometry of degenerate four-wave mixing. Spatial migration results in a reduced scattering efficiency of one grating and is observed as a polarization rotation of the backward-going output wave. Upper limits are placed on the diffusion constants in pink ruby and Nd(3+)-doped silicate glass.