Vincent W. Chen

A Visible-Light-Active Heterojunction with Enhanced Photocatalytic Hydrogen Generation

A visible-light-active carbon nitride (CN)/strontium pyroniobate (SNO) heterojunction photocatalyst was fabricated by deposition of CN over hydrothermally synthesized SNO nanoplates by a simple thermal decomposition process. The microscopic study revealed that nanosheets of CN were anchored to the surface of SNO resulting in an intimate contact between the two semiconductors. Diffuse reflectance UV/Vis spectra show that the resulting CN/SNO heterojunction possesses intense absorption in the visible region. The structural and spectral properties endowed the CN/SNO heterojunction with remarkably enhanced photocatalytic activity. Specifically, the photocatalytic hydrogen evolution rate per mole of CN was found to be 11 times higher for the CN/SNO composite compared to pristine CN. The results clearly show that the composite photocatalyst not only extends the light absorption range of SNO but also restricts photogenerated charge-carrier recombination, resulting in significant enhancement in photocatalytic activity compared to pristine CN. The relative band positions of the composite allow the photogenerated electrons in the conduction band of CN to migrate to that of SNO. This kind of charge migration and separation leads to the reduction in the overall recombination rate of photogenerated charge carriers, which is regarded as one of the key factors for the enhanced activity. A plausible mechanism for the enhanced photocatalytic activity of the heterostructured composite is proposed based on observed activity, photoluminescence, time-resolved fluorescence emission decay, electrochemical impedance spectroscopy, and band position calculations.

Three-dimensional organic microlasers with low lasing thresholds fabricated by multiphoton and UV lithography

Cuboid-shaped organic microcavities containing a pyrromethene laser dye and supported upon a photonic crystal have been investigated as an approach to reducing the lasing threshold of the cavities. Multiphoton lithography facilitated fabrication of the cuboid cavities directly on the substrate or on the decoupling structure, while similar structures were fabricated on the substrate by UV lithography for comparison. Significant reduction of the lasing threshold by a factor of ~30 has been observed for cavities supported by the photonic crystal relative to those fabricated on the substrate. The lasing mode spectra of the cuboid microresonators provide strong evidence showing that the lasing modes are localized in the horizontal plane, with the shape of an inscribed diamond.

Adhesion Enhancements and Surface-Enhanced Raman Scattering Activity of Ag and Ag@SiO2 Nanoparticle Decorated Ragweed Pollen Microparticle Sensor

A simple solution processed layer-by-layer approach was used to immobilize metal nanoparticles (NPs) on the surface of ragweed pollen exine to obtain multifunctional particles with significant surface-enhanced Raman scattering (SERS), two-photon excited fluorescence, and enhanced adhesion properties. The rugged pollen exine was functionalized with an amine terminated silane and then treated with Ag or Ag@SiO2 NPs that were electrostatically attached to the exterior of the pollen by incubation in an NP solution of the appropriate pH. Nanoparticle agglomeration on the pollen gives rise to broadband near infrared (NIR) (785-1064 nm) plasmonic activity, and strong SERS signals from benzenedithiol deposited on NP-pollen composite particles were observed. In addition to SERS activity, the AgNP coating provides a twofold increase in the adhesive properties of the RW pollen exine on a silicon substrate, leading to a robust, adhesive, broadband NIR excitable SERS microparticle.

Nonlinear optical components for all-optical probabilistic graphical model

The probabilistic graphical models (PGMs) are tools that are used to compute probability distributions over large and complex interacting variables. They have applications in social networks, speech recognition, artificial intelligence, machine learning, and many more areas. Here, we present an all-optical implementation of a PGM through the sum-product message passing algorithm (SPMPA) governed by a wavelength multiplexing architecture. As a proof-of-concept, we demonstrate the use of optics to solve a two node graphical model governed by SPMPA and successfully map the message passing algorithm onto photonics operations. The essential mathematical functions required for this algorithm, including multiplication and division, are implemented using nonlinear optics in thin film materials. The multiplication and division are demonstrated through a logarithm-summation-exponentiation operation and a pump-probe saturation process, respectively. The fundamental bottlenecks for the scalability of the presented scheme are discussed as well.To circumvent the limitations of electronic computers, moving to hybrid optical-electronic or all-optical devices may be useful. Here, Babaeian et al. present an all-optical implementation of the probabilistic graphical model using nonlinear optics in thin films to implement mathematical functions.

Conformal coating of tailored photonic crystals fabricated using multiphoton lithography

Titania sol-gel and silver electro-less deposition processes were utilized to apply conformal coatings to multiphoton lithographically fabricated polymer photonic crystal templates, resulting in enhanced refractive index and stop band reflectivity.

Fast and efficient analysis and design of three-dimensional photonic crystal structures for functional dispersive devices

We show that the propagation of optical beams inside three-dimensional photonic crystals can be efficiently described by an approximate scalar diffraction model. The main advantage of this model is that it reduces the memory and computation time for direct simulations of three-dimensional photonic crystals by converting the problem into an equivalent problem for which simple geometrical optics can be applied. Then we use this model for efficient design of photonic crystal dispersive devices such as demultiplexers and microspectrometers. Finally, experimental verification of the results for the polymer woodpile structures will be given.

Fabrication of tailored photonic crystals using multiphoton lithography

Multiphoton lithography methods have been used to fabricate tailored photonic crystal structures with controllable stop bands that are effective templates for the formation of high contrast photonic crystals by post-fabrication processing. Metallization of such structures produces photonic crystals with a high reflectance in the infrared spectrum.

Advances in Two-Photon 3D Microfabrication

The development of two-photon materials for the fabrication of features with 80 nm resolution in 3D microfabrication and the fabrication of a range of photonic crystals with mid-IR stop bands will be discussed.

Two-photon 3D microfabrication with polymer, metal nanocomposite and hybrid materials

The development of two-photon materials that can undergo efficient photochemical processes, radical or acid generation and metal reduction, and their use in 3D microfabrication of polymeric, metallic, and titania-organic hybrid materials will be discussed.