Fajian Zhang

Nonlinear differential ultrafast laser absorption spectroscopy observation of charge carrier dynamics of CdSxSe1−x nanocrystal doped glasses

Based on femtosecond transient absorption spectroscopy, the charge carrier dynamics of CdSxSe1−x nanocrystal doped glasses was investigated by analyzing the nonlinear differential transient absorption spectra. Time constants were extracted, which are different from the literature results using similar samples: they are pump laser intensity-dependent and decrease with increasing pump laser intensity, whereas risetime constants are pump laser intensity-independent and high-order exciton recombination can be identified from the spectra. The threshold pump intensity was determined, which is lower than the literature result. Reasons for high-order recombination were analyzed, and broad size, state distribution and high pump intensity are found as key factors. Auger recombination may play a key role in the multi-exciton recombination. Theoretical calculations and analysis support this conclusion. A schematic model for electron transport and the formation of triexcitons in our samples are proposed to explain these results.

Ultrathin film actuators fabricated by layer-by-layer molecular self-assembly

Polymer/metal cluster nanocomposite thin films as new types of electroactive polymer (EAP) material have been synthesized by the layer-by-layer (LBL) electrostatic self-assembly process. A self-assembled 11-bilayer poly-S-119/Pt nanocluster film, with a thickness of approximately 30 nm and a Pt content of 0.49 at.%, exhibited an actuation response similar to that of conventional ionic polymer-metal composite films for low applied voltage in air, but nonlinear bending behaviour in response to a slightly higher applied voltage. The film has a uniform nanostructure with the Pt nanoclusters dispersed within the multiple molecular layers, as confirmed by atomic force microscopy. The self-assembled electroactive films may have different actuation mechanisms from those of traditional piezoelectric materials, and from other EAP materials, due to the quite different LBL laminated structure. Further studies concerning the principles governing the novel processing of the films and applications are underway.

Linear electro-optic tensor ratio determination and quadratic electro-optic modulation of electrostatically self-assembled CdSe quantum dot films

A new class of CdSe quantum dot-doped polymer films are synthesized using electrostatic layer-by-layer self-assembly processing technique. Transmission electron spectroscopy and electron beam diffraction results show that the diameter of the CdSe quantum dots is 2–3 nm and that the CdSe clusters possess a hexagonal structure. X-ray photoelectron spectroscopy allows determination of the concentration of the CdSe quantum dots within the resulting films. Linear electro–optic (Pockels) and quadratic electro–optic (Kerr) effect behavior of the films are investigated by using Mach–Zenhder interferometric and ellipsometric techniques, respectively. Linear electro–optic tensor ratios r333/r113 of 4.3±0.2 were determined. Theoretical calculations of the quadratic electro–optic coefficients are proposed for the conventional ellipsometric technique. The orientational enhancement effect originating from the permanent dipole moment and induced dipole moment of the CdSe clusters is also discussed. From the analysis of ...

Electro-optic property measurements of electrostatically self-assembled ultrathin films

A new class of ultrathin electro-optic films have been developed by the electrostatically self-assembled monolayer process. Using a simple ellipsometric technique, the electro-optic coefficients were measured as a function of the frequency of the applied electric field. The maximum of the coefficients is 600 pm/V. From these measurements, we deduce the orientational and the electronic contributions to the electro-optic modulation. At modulating frequencies lower than 20 Hz, the orientation dominates the electro-optic modulation process. At frequencies higher than 800 Hz, electrons are responsible for electro-optic behavior.

Piezoelectric ultrathin polymer films synthesized by electrostatic self-assembly processing

Ultrathin piezoelectric composite films composed of poly(sodium 4-styrenesolfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) were synthesized using the electrostatic self-assembly (ESA) process. The ESA-processed PSS/PDDA films have a layer-by-layer laminated structure, and exhibit a piezoelectric response directly without electric field poling. The measured piezoelectric coefficient d33 = 6.0 pC N-1. The molecular self-assembly process plays a very important role in molecular alignment, resulting in a net macroscopic polarization of the layer-by-layer structured ultrathin film, although the process is quite different from that used to form conventional piezoelectric materials. Further study concerning the principles governing the novel ESA processing of piezoelectric and other functional films is on-going.

Observation of whispering-gallery and directional resonant laser emission in ellipsoidal microcavities

Whispering-gallery resonant laser emission is reported in CdSe-tagged polystyrene microcavities. A clear intensity threshold for laser emission is observed. Using a recently developed asymptotic expansion derived from Mie theory, we have theoretically identified the experimentally observed discrete resonant peaks and assigned them to different quantum states. In fluorescent carboxylate-modified ellipsoidal polystyrene microcavities, we have observed that whispering-gallery mode and directional laser emission can be excited simultaneously. To understand our experimental results, we extend the asymptotic expansion from spherical to ellipsoidal microcavities to explain the whispering-gallery mode behavior and develop a simple ray-optics model to interpret the directional emission. The obtained numerical data agree well with experimental observation.

Investigation of the Photoresist Pattern Profile Contrast Improvement in Interference Lithography Technique Using 488-nm Laser

The interference lithography technique at 488 nm is explored theoretically and experimentally, and the effect of photoresist pattern profile contrast improvement is presented. In order to produce high contrast photoresist patterns using interference lithography, the system setup and process have to be optimized strictly, and process optimization can be facilitated by simulation. In the proposed simulation method, the absorption coefficient of photoresist varying with wavelength is considered by using photoresists with lower absorption coefficients, or, for the same photoresist, using laser sources with longer wavelengths. The visibility of aerial fringe patterns of the photoresist can be improved greatly. However, after developing, the contrast of photoresist patterns was not improved. The reason is that the photo sensitivity and etching rate V of photoresist decrease at 488 nm. This offsets the effect of lower absorption coefficients even though a 488-nm argon ion laser source is useable for some photoresists. This opens up a new window for the interference lithography technique.

Nanometer-scale resonant cavities

Cavity enhanced directional resonance has been experimentally observed in single optically-trapped polystyrene particles, which have a size range from 9 μm down to several hundred nm. The higher resonance peaks correspond to longitudinal modes of the directional laser oscillation in a deformed spherical resonator. The lower peaks are attributed to whispering-gallery modes. For a particle with a diameter of approximately 9 μm, the longitudinal mode numbers of the resonator that is responsible for the observed sharp emission peaks are identified using a ray optics model. It is suggested that both directional and whispering-gallery modes can exist in nanometer-scale size resonators. The potential use of such resonant optical cavities in fiber-based optical sensors for chemical diagnostics is suggested. Considerable additional work is required to completely model and measure the observation of effects briefly summarized here.

Synthesis of piezoelectric thin films by molecular self-assembly

Peizoelectric ultrathin composite film comprised of PSS and PDDA was synthesized using the electrostatic self-assembly (ESA) process. The ESA processed PSS/PDDA film is a layer- by-layer laminated structure, which exhibits piezoelectric response directly, with a piezoelectric coefficient d33 = 6.0 pC/N and without poling treatment. It is assumed that the self-assembly process may play a role in molecular alignment resulting in net polarization in the layer-by- layer structured ultrathin film, a process quite different form that used for form conventional piezoelectric films.

Giant Magnetoresistance of Electrostatic Self-Assembled Fe 3 O 4 Nanocluster and Polymer Thin-Films

Giant magnetoresistance (GMR) as large as 25% at 25°C has been observed for multilayer ultrathin films of iron oxide (Fe 3 O 4 ) nanoclusters and polyimide molecules alternately adsorbed onto single crystal silicon and quartz substrates using a novel self-assembly technique. This process involves the alternate dipping of a substrate into an aqueous solution of anionic polyimide precursor (polyamic salt, PAA) followed by dipping it into an aqueous solution of cationic polydiallyldimethylammonium chloride (PDDA)-coated Fe 3 O 4 , nanoparticles. The regular formation of alternating monolayers is verified by UV-vis spectroscopy and contact angle measurements. Vibrating sample magnetometry indicates the formation of ultrasoft films.