Two dimensional (2D) graphene and transition metal dichalcogenides are an emerging class of extremely interesting materials showing unique physical properties, such as large third-order optical nonlinearity, offering potential applications in optical limiting. Here we report the optical limiting properties of Titanium disufide (TiS<sub>2</sub>), and Graphene sheets measured using open aperture and photo-acoustic z-scan techniques. Our best results were observed in TiS<sub>2</sub> Sheets, yielding an optical limiting of 77% at an irradiance of 0.713 GW/cm<sup>2</sup>with 2PA and 3PA absorption coefficient to be 80cm/GW and 2000 cm<sup>3</sup>/GW<sup>2</sup> respectively.Also, TiS<sub>2</sub> sheets show improved shelf life and stability upon irradiation with higher laser powers. This demonstrates the feasibility of using them as a potential candidate for optical limiting applications.
The complexity of photorefractive polymers arises from multiple contributions to the photo-induced index grating. Analysis of the time dynamics of the two-beam coupling signal is used to extract information about the charge species responsible for the grating formation. It has been shown in a commonly used photorefractive polymer at moderate applied electric fields, the primary charge carriers (holes) establish an initial grating which, however, are followed by a subsequent competing grating (electrons) that decreases the two-beam coupling efficiency. We show by upon using higher applied bias fields, gain enhancement can be achieved by eliminating the electron grating contribution and returning to hole gratings only.
There is a substantial interest in finding materials with high nonlinear optical (NLO) properties of materials because of its attractive applications in optical limiting for safety protections. In an effort to develop highly performing optical limiting materials, recently we have found that fluorination of graphene oxides leads to improvement in their NLO properties.
Photorefractive composites derived from conducting polymers offer the advantage of dynamically recording holograms
without the need for processing of any kind. Thus, they are the material of choice for many cutting edge applications,
such as updatable three-dimensional (3D) displays and 3D telepresence. Using photorefractive polymers, 3D images or
holograms can be seen with the unassisted eye and are very similar to how humans see the actual environment
surrounding them. Absence of a large-area and dynamically updatable holographic recording medium has prevented
realization of the concept. The development of a novel nonlinear optical chromophore doped photoconductive polymer
composite as the recording medium for a refreshable holographic display is discussed. Further improvements in the
polymer composites could bring applications in telemedicine, advertising, updatable 3D maps and entertainment.
Nonlinear optical transmission in materials has several applications including laser mode-locking, pulse shaping, optical
bistability, optical switching, and optical power limiting. Organic molecules suitable for functionalization have been
extensively investigated for their third order nonlinearities. We have measured the optical nonlinearity of different novel
organic and composite systems including nanocomposite polymer films of Au, Ag and Pt, Organic ionic crystals
(pyridinium and quinolinium salts), Au-alkanethiol clusters, thiophene based polymers, and Schiff base complexes, using
the z-scan and degenerate four wave mixing techniques, employing laser pulses of nanosecond and femtosecond
durations respectively. Most of these materials are found to be efficient optical power limiters under our excitation
conditions, and their nonlinear extinction coefficients have been calculated. Enhancement in the optical nonlinearity was
sometimes obtained even by mixing of two organic media. From degenerate four wave mixing experiments, the third
order nonlinear susceptibility χ<sup>(3)</sup> and figure of merit χ<sup>(3)</sup>/α values also have been determined for some of these
materials. The above experiments conducted in a large number of organic and composite materials unravel the potential
of these media for diverse photonic applications.
We investigate the dielectric and electrical properties of sol-gel/DNA-CTMA blends, with particular interest
in capacitor applications in energy storage. Methacryloyloxypropyltrimethoxysilane (MAPTMS) was the solgel
precursor, and DNA-CTMA was blended in to the resulting sol-gel at various weight percentages. The
blends were tested for their dielectric properties and dielectric breakdown strength; the 5% DNA blend was
found to be optimal with a dielectric constant in the range of 7.5, while the breakdown strength was greater
than 800 V/μm for 1 μm films and about 500 V/μm for 5μm films. Hybrid sol-gel/DNA-CTMA/barium
titanate nanoparticle composites were also formulated and their dielectric properties measured. While a high
dielectric constant was achieved (38), this came at the expense of a significantly reduced breakdown voltage
(160V/μm). We discuss these results as well as other aspects of the dielectric and electrical properties of
We introduce a simple yet efficient approach for nanoimprinting sub-50
nm dimensions starting from a low molecular weight plasticized polymer melt. This
technique enabled us to successfully imprint versatile large area nanopatterns with
high degrees of fidelity and rational control over the residual layers. The key
advantage is its reliability in printing versatile nanostructures and nanophotonic
devices doped with organic dyes owing to its low processing temperature. Since
nanopatterns can be fabricated easily at low costs, this approach offers an easy
pathway for achieving excellent nanoimprinted structures for a variety of photonic,
electronic and biological research and applications.
The electron transporting molecule tris(8-hydroxyquinoline) aluminum (Alq<sub>3</sub>) was introduced into a photorefractive
composite in a low density to study the effects of electron traps on the performance. Compared to a control sample, Alq<sub>3</sub>
samples exhibited higher dielectric strength, over-modulation at reduced voltage, and increased writing speed. Transient
measurements indicated grating revelation via decay of a competing grating. The dynamics are consistent with a bipolar
charge transport model. Overall, Alq<sub>3</sub> improves the sensitivity, trapping, and breakdown voltage without significant
losses in absorption or phase stability.
The very first demonstration of our refreshable holographic display based on photorefractive polymer was published in
Nature early 2008<sup>1</sup>. Based on the unique properties of a new organic photorefractive material and the holographic
stereography technique, this display addressed a gap between large static holograms printed in permanent media
(photopolymers) and small real time holographic systems like the MIT holovideo. Applications range from medical
imaging to refreshable maps and advertisement. Here we are presenting several technical solutions for improving the
performance parameters of the initial display from an optical point of view. Full color holograms can be generated
thanks to angular multiplexing, the recording time can be reduced from minutes to seconds with a pulsed laser, and full
parallax hologram can be recorded in a reasonable time thanks to parallel writing. We also discuss the future of such a
display and the possibility of video rate.
Two-beam coupling (TBC) in a photorefractive polymer using transmission and reflection geometries is
investigated. With drift (due to an applied electric field) and diffusion, a linearized analysis suggests a phase shift
between the intensity grating and the induced refractive index grating different from the ideal value of 90 degrees,
which is supported by experimental results using a transmission grating geometry. In a self-pumped reflection
grating geometry, which is also experimentally studied, the phase shift can be closer to 90 degrees due to a shorter
grating period. Absorption and absorption gratings during TBC is also experimentally investigated.
Infiltration of planar 2D silicon photonic crystals with nanocomposites using a
simple melt processing technique is presented. The nanocomposites that were developed by
evenly dispersing functionalized TiO<sub>2</sub> nanoparticles into a photoconducting polymer exhibit
high optical quality and tunable refractive index. The infiltrated photonic crystals show
tuning of the photonic band-gap that is controllable by the adjustment of the nanoparticle
loading level. These results may be useful in the development of tunable photonic devices,
hybrid light emitting diodes and photovoltaics.
We have investigated photorefractive (PR) properties of a polymer composite with low glass-transition temperature (T<sub>g</sub>) in a symmetric reflection geometry. A diffraction efficiency of more than 30% is observed in 105μm thick devices. In low T<sub>g</sub> photorefractive polymers, poling of the nonlinear optical chromophores at room temperature leads to birefringence in the material. The birefringence will alter the Bragg condition, as the propagation vectors for object and reference beams as well as the readout angle are influenced. We observed the Bragg-mismatch effect that caused a reduction in diffraction efficiency as the external field is increased. We have varied the angle of readout beam slightly at each bias field to get the highest efficiency.
Photonic crystals have now started to make the transition from basic to applied research, with new
materials systems and device results being published on a frequent basis. While a number of
photonic crystals have been made using organic materials, the lack of high index organic materials
has impeded their development. We have investigated several novel high index organic systems for
use in both 2-D and 3-D photonic crystals. 2-D photonic crystal templates were made by a rapid
multibeam interference method in the photoresist SU-8, using 532nm laser radiation. These samples,
typically on glass, were then infiltrated by a number of methods including from solution and melt, as
well through chemical vapor deposition. Solutions of a titanium precursor with a cured refractive
index of 2.1 at 633nm were infiltrated and cured in the SU-8 structure, with the infiltrant deposited by
both by spin coating and casting. The resulting structure was shown to preserve the six-fold
symmetry of the initial photonic crystal and subsequent firing at high temperature effectively
removed the SU-8 template. We have also explored the infiltration of nanoamorphous carbon into
the photonic crystals using chemical vapor deposition. This material, which is essentially a
carbon-silicon ceramic, has exceptional infrared optical properties with a refractive index > 2 for
wavelengths beyond 2 μm. The SU-8 polymer template has been shown to survive the CVD
deposition process and the resulting infiltrated structure also preserves the initial PC symmetry. A
series of metal-like PCs with a full range of properties is enabled by the ability to dope the
nanoamorphous carbon with metals that possess exceptional refractive indices in the infrared regions
of interest. We have also investigated the potential for nonlinear optical devices based upon
azobenzene copolymer infiltrated silicon PCs and demonstrate the excellent properties of this material
with respect to all-optical effects.
We report the photorefractive properties of tetraphenyldiaminobiphenyl (TPD) based polymer composites
that have been developed for single pulse laser operation at 532 nm. With an optimized composite, we
demonstrate more than 50% diffraction efficiency using 4 mJ/cm<sup>2</sup> single shot writing and 633 nm
continuous wave (cw) beam reading. The present devices showed a 300 μs fast response time. This
reveals the potential for these polymer devices in applications which require fast writing and erasure. Since
the writing pulse-width is in nanosecond time scale, the recording is totally insensitive to vibrations. These
devices can also be used as a stepping stone to realize all-color holography since they are sensitive to both
green (532nm) and red (633nm) wavelengths. The holograms can be written with either of these two
wavelengths and can be read by the same wavelength or the other wavelength with high diffraction
efficiency. This demonstrates that these devices have the advantage of performing two-color holography, a
step closer to a dynamic full-color holographic recording medium.
We propose and demonstrate a novel technique for efficient local fixing of photorefractive polymer hologram using a
laser beam. In the new technique, a CO<sub>2</sub> laser beam is used to heat the sample and a local hologram can be fixed easily.
By using glass and sapphire with particular thickness as the substrates for the photorefractive device, the hologram can
be fixed efficiently and at much faster speed. The fixation efficiency can be greater than 80% and the hologram can be
fixed in a few seconds. This technique is critical for dynamic holographic 3D display and holographic data storage.
Organic-inorganic hybrid sol-gel materials have attracted increasing attention in recent years as low-cost, rugged materials for integrated optical devices such as optical couplers, splitters, and electro-optic modulators. These materials can be easily processed by spin-coating, wet-etching photolithography, and low-temperature baking. Precise control of waveguide core-cladding refractive indices produces well-confined low-loss propagation and good matching of the absolute refractive index to that of fused silica results in low optical coupling loss to optical fiber. The increased thermal and mechanical stability of these materials, relative to optical polymers, results in numerous packaging options and improved reliability. However organic-inorganic hybrid sol-gel materials have not yet been often used as host of active dopants such as erbium (III) ions for 1550nm optical amplification. This limitation owes primarily to matrix and chelate dominated nonradiative relaxation processes, as high phonon energy OH and OH-like oscillators can bridge off the energy from the excited erbium (III) ions at very high rates. Different strategies have been proposed to protect erbium (III) ions from matrix and chelate quenching, including host and ligand fluorination, and inorganic microstructure shielding. Here we report on our work of encapsulating erbium (III) ions in transparent, refractive index matched, and highly re-dispersible lanthanum phosphate nanoparticles and the work of examining the optical properties of these nanoparticles as active dopants in organic-inorganic hybrid sol-gels adopting 2-methacryloxypropyl trimethoxysilane (MAPTMS) as a precursor. 980nm laser pumped photoluminescence at 1535nm was obtained from solid bulk samples of 300mg La.<sub>99</sub>Er.<sub>01</sub>PO<sub>4</sub> nanoparticles doped in 1mL hybrid sol-gel. Thick bulk samples of this composition exhibited exceptional clarity and little trace of nanoparticle scattering effects. The lifetime of the nanoparticle doped hybrid sol-gel composite was measured to be 220μs, indicating an intermediate relaxation rate between that of an erbium organic complex and annealed erbium doped glass. La.<sub>99</sub>Er.<sub>01</sub>PO<sub>4</sub> nanoparticle doped hybrid sol-gel films were also prepared and the refractive index was measured to be 1.4966 at 1550nm, which is very close to that of optical fiber and provides a suitable index difference from an undoped and metal oxide tuned sol-gel at 1.4870 to comprise an efficient single-mode waveguide system.
We describe the material characteristics and photorefractive properties of novel tetraphenyldiaminobiphenyl (TPD) based polymer composites that were developed for operation wavelengths up to 1 micron. With an optimized composite, we demonstrated more than 50% external diffraction efficiency coupled with a fast response time of about 35 ms at 980 nm. In addition to this high performing composite, we have developed a composite with high two beam coupling gain (300 cm<sup>-1</sup>). To accomplish these attractive photorefractive properties in the near-infrared, we explored the chemical flexibility of the guest-host approach. We employed a new dye with enhanced near-infrared absorption to extend the sensitivity into this long wavelength range. Styrene-based chromophores were utilized to enable high refractive index modulation. We explored ellipsometry as well as photo-conductivity measurements to optimize the composition of the composites. In addition to the composites that contain a single chromophore species, we also analyzed samples prepared with a mixture of chromophores. Our studies reveal the potential of this new polymer-composite family to extend the operation wavelength of the photorefractive materials to even longer wavelengths. Attractive photorefractive properties coupled with long wavelength sensitivity make these materials potential candidates for imaging and communication applications.
We report on the trapping mechanisms in bis-triarylamine (PATPD) based polymer composites. Although exceptional stability under continuous operation has been reported in PATPD-based composites, a small degradation of the response time in photorefractive devices under continuous operation has been found when improved styrene-based
chromophores, with high figure-of-merit, are used. The accumulation of relatively large densities (~10<sup>17</sup> cm<sup>-3</sup>)
of filled traps is observed even though to first approximation the transport manifold has the lowest ionization potential of all the moieties in the composite, so no apparent deep trapping sites are to be present. The results of spectroscopic studies where the formation of chromophore aggregates is explored and correlated with the formation of hole-trapping sites that dominate the temporal evolution of the photogenerated current density and C<sub>60</sub> anion accumulation after several minutes of continuous operation will be presented and compared with numerical simulations considering a two-trapping site model in materials containing the chromophore DBDC.
We report on the photorefractive properties of two polymer composites that utilize a new bis-triarylamine side-chain polymer matrix. Correctly locating the frontier orbitals of the new transport manifold with respect to the HOMO levels of chromophores, allows stable continuous operation over exposure levels of more that 4 kJ/cm<sup>2</sup> when samples are electrically biased at 57 V/μm. This operational stability is combined with video-rate compatible grating build-up times and a dynamic range that allows index modulations of 3 x 10<sup>-3</sup> and gain coefficients on the order of 100 cm<sup>-1</sup> at moderate fields. The thermal stability of one of the composites reported is excellent, showing no signs of phase separation even after one week at 60°C. A comparison with the stability of composites where the new matrix was replaced by PVK is also presented.
A novel approach for the detection of nitrogen dioxide gas is described. This optical fiber based sensor (FOS) works on the principal of evanescent wave (EW) absorption phenomenon. EWs at the uncladded portion of a multimode fiber is utilized for the senor development by replacing this region with a coating of Metallophthalocyanine, which is thermally deposited at a reduced pressure. MPcs are very sensitive to NO<SUB>2</SUB> gas and there is a change in the EW absorption in the NO<SUB>2</SUB> environment. Compared to other gas sensing devices, this is highly sensitive technique. The attraction of this FOS is its simple architecture and the easiness to implement.