R. W. Hellwarth

Generation of time-reversed wave fronts by nonlinear refraction*

We describe a nonlinear method for generating, nearly instantaneously, a time-reversed replica of any monochromatic-beam wave pattern. The method employs the interaction of the incident beam, of arbitrary wave front, with counter-propagating plane “pump” waves in a homogeneous, transparent, nonlinear medium. Media are shown to exist in which time-reversed waves can be generated with high efficiency using available laser pump sources.

Phase-conjugating mirror with continuous-wave gain

We demonstrate a phase-conjugating mirror that has a continuous-wave power reflectivity much greater than unity (gain ~100). This mirror uses nonresonant degenerate four-wave mixing in a single crystal of barium titanate (BaTiO3). With our mirror we have (1) observed cw self-oscillation in an optical resonator formed by this mirror and a normal mirror, (2) demonstrated a cw oscillator that, in spite of phase-distorting material placed inside the resonator, will always emit a TEM00 mode, and (3) demonstrated an optical image amplifier. This mirror will work at any visible wavelength and with weak (milliwatt or weaker) pump beams.

Hole-electron competition in photorefractive gratings

A band-conduction model for the photorefractive effect is derived in which simultaneous hole and electron conduction is taken into account. The anomalous behavior of nearly compensated BaTiO3 crystals in beam-coupling experiments is explained.

Giant Optical Pulsations from Ruby

Giant pulses of optical maser radiation have been produced which are several orders of magnitude larger than the commonly observed spontaneous pulses. The pulses were produced by varying the effective reflectivity of the reflecting surfaces at the ends of the ruby rod through a Kerr‐cell switching technique. The measured pulse characteristics are found to be in agreement with theoretical predictions.

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.

Infrared‐to‐optical image conversion by Bragg reflection from thermally induced index gratings

We have observed efficient reproduction at visible wavelengths of 1.06‐μ images. We employed a phase‐matched four‐wave mixing process in which three waves at ν, ν, and ω mix to generate a fourth wave at ω, with no resonant conditions in either frequency. The image conversion was seen with each of the 14 liquids tried as a nonlinear medium and also with a glass. Thermal index changes were the dominant mechanism.

Optical measurement of the photorefractive parameters of Bi12SiO20

We employ an optical technique requiring no electrodes to measure three parameters characterizing photorefractive traps in nominally undoped n‐type bismuth silicate at 515‐ and 488‐nm wavelengths. These parameters are (1) an effective density of active electron trap sites (∼1016 cm−3), (2) the contribution αpcm−1 of the photorefractive excitations to the total absorption coefficient (∼3.8 cm−1 or 54% at 488 nm and 1.4 cm−1 or 86% at 515 nm), and (3) the average distance over which an optically excited electron moves in the absence of electric fields before being retrapped (∼4 μm). With these parameters (and the crystal properties) standard models give predictions for all photorefractive effects, with or without applied electric fields and crystal motion, provided that there is negligible change in the density of occupied traps during an electron recombination time (∼microseconds). Our technique employs measurement of the light‐induced exponential decays of spatial ‘‘gratings’’ of trapped charges as a func...

Picosecond nonlinear optical response of three rugged polyquinoxaline‐based aromatic conjugated ladder‐polymer thin films

We have fabricated the first optical quality polymer films of aromatic ladder polymers having an unbroken conjugated backbone, and find third‐order nonlinear susceptibilities two orders of magnitude larger than CS2. The nonlinearities respond in less than 6 ps. Although we find their space symmetry to agree with that expected for a nonresonant electronic nonlinearity, the mechanism may well depend on lattice motion. The polymers are stable and rugged at room temperature. We also develop and analyze a simple form of degenerate four‐wave mixing for the measurements, finding expressions for the effects of uncertainties in the spatial and temporal pulse profiles.

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.

Theory of phase conjugation by stimulated scattering in a waveguide

We consider the backward optical wave stimulated by a multimode, monochromatic, incident optical wave in a waveguide filled with a transparent nonlinear medium, when the incident wave is negligibly perturbed by the nonlinear processes. We derive the conditions on guide length, area, mode number, and Stokes shift in order that a given high percentage of the power in the backscattered field be the “phase conjugate” of the incident field, i.e., be proportional to its complex conjugate in the entrance plane of the waveguide.