Liangmin 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.

High-Performance Photovoltaic Behavior of Oriented Purple Membrane Polymer Composite Films

The photovoltaic behavior of films in which bacteriorhodopsin molecules are embedded in a polyvinyl alcohol matrix has been investigated by using both pulsed laser excitation and regular light illumination. Response times as short as milliseconds, photocurrents as great as 120 micro A/cm(2), and photovoltages as large as 3.8 V have been obtained. A theoretical model has been developed and used to extract several physical parameters and fit the experimental results. Some important intrinsic parameters have been obtained. Theoretical results indicate that the average displacement of the excited protons is on the order of several tens of microns. Other curve fits show that photocurrent and photovoltage increase linearly with external field, but increase exponentially with flash power. These theoretical models and results can be extended to other kinds of photoactive polymeric materials.

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.

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.

Electrostatically self-assembled electro-optic materials and devices

Thin film electro-optic materials have been synthesized by a novel electrostatic self-assembly (ESA) method. This method allows the molecular-level, layer-by-layer formation of multilayer thin and thick films of alternating anionic and cationic molecules and other materials. We have found that during the adsorption of dipolar molecules from solution to form a single molecular layer, the dipoles align themselves. In a multilayered material, this leads to multiple functionalities that require a noncentrosymmetric molecular structure, such as active optical properties and piezoelectric behavior. Such properties are usually achieved in other materials by electric field poling. In this paper, we describe the precursor molecular chemistries that we have developed to make electro-optic thin films by this method, how the films are formed, the resulting molecular orientation within the film, and measured electro- optic coefficients to date.

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.

Reproducibility of electrostatically self-assembled electro-optic thin films

Thin film electro-optic materials have been synthesized by a novel electrostatic self-assembly (ESA) method using both manual and automated processes. This paper discusses the reproducibility of such ESA-formed electro-optic thin films formed by both methods. Multiple films were fabricated based on the same layer-by-layer molecular design. They were evaluated using UV-vis spectroscopy and multiwavelength ellipsometry to demonstrate linear growth with the addition of layers, and to measure the thickness of the formed film. Their electro-optic coefficients have also been measured using Mach-Zehnder and Teng and Man approaches. Variations in the properties, including absorption, thickness and electro-optic coefficients, for all of the test samples are reported. We analyze possible causes for such variations, which include time-dependent differences in solution chemistry and differences between manual and automated fabrication processes. We further suggest approaches to both the solution chemistry and thin film design that may be used to limit the effect of such variations on the performance of electro-optic devices.

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.

Electrostatically self-assembled electro-optic thin films

Thin film electro-optic materials have been synthesized by a novel electrostatic self-assembly (ESA) method. This wet chemistry synthesis method allows the molecular-level, layer-by-layer formation of multilayer thin and thick films of alternating anionic and cationic molecules and other materials. We have found that during the adsorption of dipolar molecules from solution to form a single molecular layer, the dipoles align themselves. In a multilayered material, this leads to multiple functionalities that require a noncentrosymmetric molecular structure such as active optical properties and piezoelectric behavior. Such properties are usually achieved in other materials by electric field poling. In this paper, we describe the precursor molecular chemistries that we have developed to make electro-optic thin films by this method, how the films are formed, the resulting molecular orientation within the film, and measured by electro-optic coefficients to date. We also describe how the ESA process precursor chemistry may be modified to allow the incorporation of noble metal nanoclusters to form flexible thin films with electrical conductivity on the order of that bulk metals. Such conducting films have been used to form electrode layers on prototype electro-optic devices.