Nanosecond four-wave mixing and holography in BSO crystals

Diffraction response of photorefractive polymers over nine orders of magnitude of pulse duration

The development of a single mode fiber-based pulsed laser with variable pulse duration, energy, and repetition rate has enabled the characterization of photorefractive polymer (PRP) in a previously inaccessible regime located between millisecond and microsecond single pulse illumination. With the addition of CW and nanosecond pulse lasers, four wave mixing measurements covering 9 orders of magnitudes in pulse duration are reported. Reciprocity failure of the diffraction efficiency according to the pulse duration for a constant energy density is observed and attributed to multiple excitation, transport and trapping events of the charge carriers. However, for pulses shorter than 30 μs, the efficiency reaches a plateau where an increase in energy density no longer affects the efficiency. This plateau is due to the saturation of the charge generation at high peak power given the limited number of sensitizer sites. The same behavior is observed in two different types of devices composed of the same material but with or without a buffer layer covering one electrode, which confirm the origin of these mechanisms. This new type of measurement is especially important to optimize PRP for applications using short pulse duration.

Analog time-reversed ultrasonically encoded light focusing inside scattering media with a 33,000× optical power gain

Recent breakthrough in wavefront engineering shows great promises in controlling light propagation inside scattering media. At present, the digital approaches enjoy high gain, while their speeds are slow because of high data throughputs. In contrast, the analog approaches are intrinsically fast but suffer from poor efficiencies and small gains. Further improvements in both speed and gain are necessary to advance the existing technologies toward real-world applications. Here, we report analog time-reversal of acousto-optically tagged photons with a flux amplification of over 33,000 times (45 dB) at a target location inside scattering media. Such a substantial power gain enhancement is achieved when the temporal width of the time-reversed photon packet is squeezed below the carrier-recombination-limited hologram decay time in a photorefractive crystal. Despite a focusing energy gain below unity, the unprecedented power gain is expected to enable new optical imaging, sensing, manipulation and treatment applications.

Phase-coupling-induced ultraslow light propagation in solids at room temperature

We show that the group velocities of light pulses can be decelerated dramatically by the use of a dispersive phase-coupling effect through a wave mixing process. We have observed experimentally such a phase-coupling-induced ultraslow light propagation with a group velocity as low as 0.05 m/s in a photorefractive Bi12SiO20 crystal at room temperature. Moreover, the ultraslow light is amplified in the Bi12SiO20 crystal because of the unidirectional energy transfer from a coupling beam to the ultraslow light. This technique to produce ultraslow light propagation is valid for all nonlinear wave mixing processes with a dispersive phase-coupling effect.

Numerical modeling of photorefractive response to short-pulsed illumination

Numerical modeling of the nonlinear response of non ferroelectric photorefractive crystals to short-duration laser pulses is demonstrated. The model is based on the band-transport equations for a single-level, single-charge carrier system. The capabilities of the numerical model surpass the capabilities of linearized theories. The accuracy of the model is demonstrated by comparison with published diffraction efficiency data. The growth of the harmonics of a unity-modulation sinusoidal grating and the response of Bi(12)SiO(20) to multiple Q-switched pulses are two examples that demonstrate how the numerical model surpasses the linearized theory. Comparison with experiment dictates that a sinusoidal grating be assumed, but the model is general enough to accommodate arbitrary patterns of incident energy density.

Charge shielding effects of photoexcited mobile carriers on occupied deep trapping centers in BSO

Evidence is presented to support the theory that photoexcited charge carriers screen the field due to the trapped charges that form the phase grating in photorefractive materials. The diffraction intensity due to the grating and the decay time constants of the trapped charges are influenced by this screening. Electricfield modulation of these effects is also present. The effects have been measured and compared to the expected generation rates of free and trapped charges.

Picosecond photorefractive beam coupling in GaAs

We report the first observation to our knowledge of the photorefractive effect on picosecond time scales. Photorefractive beam coupling in GaAs with picosecond, 1.06-microm pulses is observed owing to charge separation between electrons and the ionized defect EL2(+) at low fluences and to separation between free electrons and holes created by two-photon interband absorption for high fluences. The accompanying processes of linear absorption, two-photon absorption, and transient energy transfer are also observed.

Collinear Bragg diffraction in photorefractive Bi(12)SiO(20)

We demonstrate the possibility of collinear Bragg diffraction in photorefractive Bi(12)SiO(20) (BSO) crystals of two incident writing and readout beams of different wavelengths. Two grating wave vectors K(1) and K(2) are created by interference of independent and collinear beams at wavelengths lambda(1) and lambda(2). In the photorefractive medium the nonlinearities of the photoinduced space-charge field generate the new grating wave vectors K(NL) = mK(1) +/- nK(2). Bragg diffraction is thus achieved at a readout wavelength lambda(R) such as 1/lambda(R) = (m/lambda(l)) +/- (n/lambda(2)). The proposed method permits quasi-nondestructive readout of a phase-volume grating in BSO at lambda(R) = 840 nm (semiconductor laser) while recording is done by interference of two incident and collinear beams at lambda(1) = 632.8 (He-Ne laser) and lambda(2) = 514.5 (Ar laser).