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1. Periodic Poling of X-Cut Thin-Film Lithium Niobate: The Route to Submicrometer Periods

   https://doi.org/10.1109/IFCS-ISAF41089.2020.9234870  

   Rusing, M.; Roeper, M.; Amber, Z.; Kirbus, B.; Eng, L.M.; Zhao, J.; Mookherjea, S. (July 2020, 2020 Joint Conference of the IEEE International Frequency Control Symposium and International Symposium on Applications of Ferroelectrics (IFCS-ISAF))

   null (Ed.)

   Ultra-short poling periods of 1 μm and below in lithium niobate will allow nonlinear optical devices with operation to the UV regime or narrow-band counter-propagating single-photon generation. However, fabrication of such periods in bulk Lithium Niobate penetrating the complete modal areas of waveguides has been challenging. In this work, we demonstrate the fabrication of periodic domain grids with submicrometer periodicity in 300 nm x-cut thin-film lithium niobate. The poling was achieved through application of a single, shaped electrical pulse and electrodes fabricated with a combination of electron-and direct laser-writing lithography. The poling results were investigated with piezo-response force microscopy and second-harmonic microcopy and indicate the poled domains to penetrate fully across the complete film thickness. This will enable the fabrication of novel nonlinear optical devices combining the high efficiency of thin films with ultra-short domain periods. 

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2. Nanosecond Diffuse Reflectance Spectroscopy for In-Situ Analysis of Electron Back-Recombination and Dye Regeneration in Fully Functional, Highly Efficient, Opaque Dye-Sensitized Solar Cell Devices

   https://doi.org/10.1021/acs.jpcc.3c05382  

   Ghadiri, Elham; Moser, Jacques-E. (December 2023, The Journal of Physical Chemistry C)

   Professor Gregory Hartland (Ed.)

   An improved optical design for nanosecond diffuse reflectance (DR) spectroscopy is presented. The in-situ analysis of the electron back-reaction and dye regeneration processes in efficient opaque dye-sensitized solar cell devices (DSCs) was scrutinized for the first time using nanosecond DR spectroscopy. The efficient DSC device is based on an opaque TiO2 double-layer film comprising 400 nm light-scattering particles and 20 nm optically transparent particles. Transmission-based laser techniques are not suitable for studying these or other devices by using the opaque morphologies of TiO2 films. However, time-resolved DR flash photolysis enables the exploration of photophysical processes in a broad variety of opaque or highly light-absorbing and light-scattering materials. We experimentally verified the three important components of DR-based spectroscopy: optical configuration, sample condition, and theoretical quantitative optical models. The large optical angle for diffusive light enables efficient light collection and measurement at a relatively low power. We tested the steady-state and time-resolved concentration dependence of the Kubelka−Munk theory for the quantitative analysis of time-resolved results and observed that the dynamics of electron back-reactions are strongly affected by the morphological parameters of the TiO2 films. With a lifetime of 50 μs, the kinetics of electron back-recombination in the device’s photoanode, which is manufactured with 400 nm TiO2 particles and 20 nm TiO2 particles, are 2 orders of magnitude faster than what has been reported to date for 20 nm particles (1 ms). In contrast to electron back-recombination, the dye regeneration process is not influenced by the TiO2 film morphology. 

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3. Ultra-fast Thermoreflectance Imaging for Electronic, Optoelectronic, and Thermal Devices

   https://doi.org/10.1109/BCICTS45179.2019.8972732  

   Bahk, Je-Hyeong; Shakouri, Ali (January 2020, 2019 IEEE BiCMOS and Compound semiconductor Integrated Circuits and Technology Symposium (BCICTS))

   We review the recent advances in thermal characterization of micro/nanoscale electronic, optoelectronic, thermal devices based on thermoreflectance imaging. Thermoreflectance imaging is a non-invasive optical technique that can visualize surface thermal response of devices and integrated circuits (IC). Recent advances of the technique have enabled high-resolution, ultra-fast transient thermal imaging with 800 ps temporal resolution. Using visible or UV illumination, spatial resolution of about 200-250 nm can be achieved. Many IC substrates, e.g. Si, GaAs, are transparent to near IR illumination in 1-1.5 μm wavelength range. Through-substrate thermal imaging of flip-chip bonded ICs with micron spatial resolution has been demonstrated. We provide key examples of various devices characterized by the technique such as CMOS ICs, GaN HEMT, nanowire transistors, thin-film solar cells, and micro-thermal cloaking devices. In addition to the validation of electrothermal models, material and fabrication defects can be identified. Finally we discuss the advantages/limitations, and perspective of thermoreflectance imaging technique. 

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4. Supramolecular Azopolymers for Dynamic Surface Microstructures Using Digital Polarization Optics

   https://doi.org/10.1002/adom.202202245  

   Strobelt, Jonas; Van_Soelen, Matthew; Abourahma, Heba; McGee, David_J (March 2023, Advanced Optical Materials)

   Abstract Thin film supramolecular azopolymers support the all‐optical generation of dynamic surface microstructures. Using a spatial light modulator (SLM) illuminated at 488 nm, structured polarized light drives surface waves of sinusoidal profile with periods 700 nm–5 µm at speeds up to 1 µm s⁻¹. Multiple regions on the film surface within the SLM focal plane can be independently set into motion, each with unique period, speed, amplitude, and propagation direction. The underlying mechanism is the photomechanical response of the azopolymer, which is more commonly exploited for the fabrication of static surface microstructures. Hydrogen‐bonded systems such as the supramolecular system described here are particularly advantageous due to their facile fabrication from commercially available components. In addition to applications in dynamic diffractive optics, this programmable system for optical surface waves is well‐suited for studies in nanoparticle manipulation, as well as in bioengineering as a reconfigurable surface template for directed cell growth. 

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5. Acid Exfoliation of Imine‐linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films

   https://doi.org/10.1002/anie.201913975  

   Burke, David W.; Sun, Chao; Castano, Ioannina; Flanders, Nathan C.; Evans, Austin M.; Vitaku, Edon; McLeod, David C.; Lambeth, Robert H.; Chen, Lin X.; Gianneschi, Nathan C.; et al (February 2020, Angewandte Chemie International Edition)

   Abstract Covalent organic frameworks (COFs) are highly modular porous crystalline polymers that are of interest for applications such as charge‐storage devices, nanofiltration membranes, and optoelectronic devices. COFs are typically synthesized as microcrystalline powders, which limits their performance in these applications, and their limited solubility precludes large‐scale processing into more useful morphologies and devices. We report a general, scalable method to exfoliate two‐dimensional imine‐linked COF powders by temporarily protonating their linkages. The resulting suspensions were cast into continuous crystalline COF films up to 10 cm in diameter, with thicknesses ranging from 50 nm to 20 μm depending on the suspension composition, concentration, and casting protocol. Furthermore, we demonstrate that the film fabrication process proceeds through a partial depolymerization/repolymerization mechanism, providing mechanically robust films that can be easily separated from their substrates. 

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