David P. West

Crystallization‐resistant photorefractive polymer composite with high diffraction efficiency and reproducibility

We report on a high optical quality, long‐life guest‐host photorefractive polymer composite which exhibits a 60% device steady‐state diffraction efficiency and 120 cm−1 two‐beam coupling gain, well in excess of its absorption of 3.5 cm−1, at a wavelength of 676 nm. In contrast to alternative composite materials of comparable high dye content, this performance has been achieved without unacceptably compromising device lifetime or ease of fabrication and storage. This is an efficient, easily prepared and comparatively stable polymer composite with device lifetimes under storage conditions already exceeding eight months and no indication of sample degradation with repeated holographic read–write cycles.

CdSe/CdS core/shell quantum dots as sensitizer of a photorefractive polymer composite

Abstract CdSe/CdS core/shell, tri-n-octylphosphine oxide passivated, quantum dots are used to sensitize a photorefractive polymer composite. The composite also consists of poly(N-vinylcarbazole) as the nominally charge transporting matrix and an electro-optic chromophore. The efficacy of sensitization and consequent photorefractive performance is investigated using transmission spectroscopy and ellipsometry, two-beam coupling and degenerate four-wave mixing experiments. The photorefractive nature of the photo-induced grating is confirmed by the observation of asymmetric two-beam coupling. Four-wave mixing reveals record diffraction efficiencies for a nano-particle-sensitized photorefractive polymer at the field levels applied (1.3% at 70 V.μm−1). A recently developed analytical technique is used to extract space-charge field rise time values from degenerate four-wave mixing transients. In turn, analysis of the dependence of the rise time on applied field is used to determine the zero-field charge dissociation efficiency to be 3.6 × 10−5 ± 0.5 × 10−5. It is further shown that the magnitude of this parameter accounts for most of the difference in photorefractive response rate between the present material and a similar C60 sensitized composite.

Photorefractive performance of a CdSe/ZnS core/shell nanoparticle-sensitized polymer

We report the photorefractive performance of a polymer composite sensitized by CdSe/ZnS core/shell nanoparticles, and also comprising poly(N-vinylcarbazole) and an electro-optic chromophore. The nanoparticles are characterized by absorption and photoluminescence spectroscopy, elemental analysis, transmission electron microscopy, and powder x-ray diffraction. The electro-optic response of the composite is measured independently of the photorefractive effect by transmission ellipsometry. An asymmetric two-beam coupling gain of 30.6+/-0.4 cm(-1) is obtained, confirming photorefractivity. Degenerate four-wave mixing is used to assess photorefractive performance and, at a poling field of 70 V microm(-1), yields a diffraction efficiency of 4.21%+/-0.03%, a holographic contrast of 3.05 x 10(-4)+/-1 x 10(-6), a space-charge rise time of 25+/-2 s, and a sensitivity of 4.7 x 10(-5)+/-4 x 10(-6) cm3 J(-1). These results constitute a significant improvement on the performance of previous nanoparticle-sensitized photorefractive polymer composites.

Second-harmonic generation from thick all-polymeric Langmuir–Blodgett films prepared using polyurethanes

A polyurethane containing 4-dialkylamino-4′-nitroazobenzene (Disperse Red) chromophores has been used as the ‘active’ layers in alternating Y-type Langmuir-Blodgett films for second-harmonic generation from light of wavelength 1.06 µm. The choice of ‘passive’ polymer greatly affects the intensity of the second-harmonic light, I2ω, obtained. With the best ‘passive’ polymer investigated, the square root of I2ω increases in proportion to the number of bilayers in the film up to a thickness of ca. 75 bilayers. The value of I2ω decreases significantly over a period of months when the films are stored at 20 °C.

Field-independent grating formation rate in a photorefractive polymer composite sensitized by CdSe quantum dots

The rate of grating formation in a photorefractive polymer composite sensitized by CdSe quantum dots is found to be independent of applied field, in contrast to similar composites sensitized by other types of nanoparticles. The photorefractive polymer composite under study consists of poly(N-vinylcarbazole) as the nominal charge transporting matrix, an electro-optic dye, and CdSe quantum dots passivated by tri-n-octylphosphine oxide. Both the field-independent grating formation rate and the lower diffraction efficiency compared to other nanoparticle-sensitized composites are attributed to low initial trap density, i.e., a reduced stability of ionized quantum dots within the composite in the dark.

Efficient second-harmonic generation from all-polymeric Langmuir-Blodgett “AB” films containing up to 600 layers

Abstract Alternating Langmuir-Blodgett films were prepared for second harmonic (SH) generation from an “active” polymer containing a hemicyanine chromophore and various “passive” polymers. With the best “passive” polymer, films containing up to 300 active layers (600 layers in total with a total thickness of 1.5 μm) had the intensity of the SH signal produced from incident light of wavelength 1.064 μm proportional to the square of the number of active layers. The intensity of the SH signal was unchanged after the film had been left in the dark at 20 °C for 8 months. Over a period of 4 days the intensity of the SH signal was stable at 60 °C, but unstable at 80 °C.

Full-field coherence-gated holographic imaging through scattering media using a photorefractive polymer composite device

We report full-field, retroreflective holographic imaging through turbid media using a photorefractive polymer composite as a coherence gate. A four-wave mixing geometry was used to record and reconstruct two-dimensional images of test objects through 6.5 scattering mean-free-paths in real-time. Images with a transverse spatial resolution better than 42μm were acquired in a few seconds using a 6.3mW helium neon laser at 633nm. The photorefractive devices used are based on a poly (N-vinylcarbazole) (PVK):2,4,7-trinitro-9-fluorenone dimalenitrile (TNFDM) charge transport network, doped with the electro-optic chromophore 1-(2′-ethylhexyloxy)-2,5-dimethyl-4-(4″-nitrophenylazo)benzene (EHDNPB).

Photorefractive holographic contrast enhancement via increased birefringence in polymer composites containing electro-optic chromophores with different alkyl substituents

The birefringence of a photorefractive polymer composite in response to an applied electric field is used to predict the holographic index contrast attainable. Predictions have been obtained for three different polymer composites based on the charge transfer complex matrix poly(N-vinyl carbazole)/2,4,7-trinitro-9-fluorenone, doped with electro-optic azo chromophores differing only in their alkyl substitutions. A comparison with experimental data indicates that unexpected large variations in the holographic contrast obtainable from different chromophores are, in most cases, accounted for by a corresponding variation in electric field-induced birefringence. This variation may be due to a combination of the chromophore number density and the composite viscosity. Where the birefringence does not correlate with holographic index contrast, a prediction based on a trap density limited space-charge field fits the data well.

Quasi-phase matched third harmonic generation

Quasi-phase matched third harmonic generation has been demonstrated using a simple silica structure of six modulation periods. Like the quadratic quasi-phase matched structures based on lithium niobate, this structure is limited in performance primarily by the tolerance necessary in the thickness of the individual layers in the structure. With suitable development such a device might become a practical and inexpensive alternative source of blue or near ultraviolet radiation based on common pulsed near infra-red solid state lasers.

Photorefractive performance of polymer composite sensitized by CdSe nanoparticles passivated by 1-hexadecylamine

The performance of a photorefractive polymer composite sensitized by 1-hexadecylamine capped CdSe nanoparticles is reported. The polymer composite also comprises the charge transporting matrix poly(N-vinylcarbazole) and the electro-optic chromophore 1-(2-ethylhexyloxy)-2,5-dimethyl-4-(4-nitrophenylazo) benzene. At an applied field of 70 V μ m−1 two beam coupling gain of 13.2 cm−1 was observed, confirming the photorefractive nature of the induced grating. At the same field, a holographic contrast of 9.12×10−4±6×10−6, a photorefractive sensitivity of 5.1×10−4 ±0.2×10−4 cm3 J−1 and a space-charge field rise time of 13±1 s were obtained.