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1. Mechanical tunability in one-dimensional photonic crystals fabricated by direct laser writing

   https://doi.org/10.1117/12.2658707  

   Stinson, Victoria P.; Shuchi, Nuren Z.; McLamb, Micheal; Boreman, Glenn D.; Hofmann, Tino (March 2023, SPIE OPTO)

   Piyawattanametha, Wibool; Park, Yong-Hwa; Zappe, Hans (Ed.)

   Recently, two-photon polymerization has been successfully employed to fabricate high-contrast one-dimensional photonic crystals. Using this approach, photonic bandgap reflectivities over 90% have been demonstrated in the infrared spectral range. As a result of this success, modifications to the design are being explored which allow additional tunability of the photonic bandgap. In this paper, a one-dimensional photonic crystal fabricated by two-photon polymerization which has been modified to include mechanical flexures is evaluated. Experimental findings suggest these structures allow mechanically induced spectral shifting of the entire photonic bandgap. These results support the use of one-dimensional photonic crystals fabricated by two-photon polymerization for opto-mechanical applications. 

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2. Photonic Crystals Fabricated by Two-Photon Polymerization with Mechanical Defects

   https://doi.org/10.3390/opt4020021  

   Stinson, Victoria Paige; Shuchi, Nuren; Louisos, Dustin; McLamb, Micheal; Boreman, Glenn D.; Hofmann, Tino (June 2023, Optics)

   One-dimensional photonic crystals have been used in sensing applications for decades, due to their ability to induce highly reflective photonic bandgaps. In this study, one-dimensional photonic crystals with alternating low- and high-density layers were fabricated from a single photosensitive polymer (IP-Dip) by two-photon polymerization. The photonic crystals were modified to include a central defect layer with different elastic properties compared to the surrounding layers, for the first time. It was observed that the defect mode resonance can be controlled by compressive force. Very good agreement was found between the experimentally measured spectra and the model data. The mechanical properties of the flexure design used in the defect layer were calculated. The calculated spring constant is of similar magnitude to those reported for microsprings fabricated on this scale using two-photon polymerization. The results of this study demonstrate the successful control of a defect resonance in one-dimensional photonic crystals fabricated by two-photon polymerization by mechanical stimuli, for the first time. Such a structure could have applications in fields, such as micro-robotics, and in micro-opto–electro–mechanical systems (MOEMSs), where optical sensing of mechanical fluctuations is desired. 

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3. Parametric study of 1D plasma photonic crystals with smooth and discontinuous density profiles

   https://doi.org/10.1063/5.0143827  

   Thomas, W. R.; Shumlak, U. (June 2023, Physics of Plasmas)

   Plasma photonic crystals (PPCs) have the potential to significantly expand the capabilities of current millimeter wave technologies by providing high speed (microsecond time scale) control of energy transmission characteristics in the GHz through low THz range. Furthermore, plasma-based devices can be used in higher power applications than their solid-state counterparts without experiencing significant changes in function or incurring damage. Plasmas with periodic variations in density can be created externally, or result naturally from instabilities or self-organization. Due to plasma's diffuse nature, PPCs cannot support rapid changes in density. Despite this fact, most theoretical work in PPCs is based on solid-state photonic crystal methods and assumes constant material properties with abrupt changes at material interfaces. In this work, a linear model is derived for a one-dimensional cold-plasma photonic crystal with an arbitrary density profile. The model is validated against a discontinuous Galerkin method numerical solution of the same device configuration. Bandgap maps are then created from derived group velocity data to elucidate the operating regime of a theoretical PPC device. The bandgap maps are compared for one-dimensional PPCs with both smooth and discontinuous density profiles. This study finds that bandgap behavior is strongly correlated with the density profile Fourier content and that density profile shapes can be engineered to produce specific transmission characteristics. 

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4. Tailored thioxanthone‐based photoinitiators for two‐photon‐controllable polymerization and nanolithographic printing

   https://doi.org/10.1002/polb.24891  

   Chi, Teng; Somers, Paul; Wilcox, Daniel A.; Schuman, Ashley J.; Iyer, Vasudevan; Le, Ran; Gengler, Jamie; Ferdinandus, Manuel; Liebig, Carl; Pan, Liang; et al (October 2019, Journal of Polymer Science Part B: Polymer Physics)

   ABSTRACT Printing of high‐resolution three‐dimensional nanostructures utilizing two‐photon polymerization has gained significant attention recently. In particular, isopropyl thioxanthone (ITX) has been implemented as a photoinitiator due to its capability of initiating and depleting polymerization on demand, but new photoinitiating materials are still needed in order to reduce the power requirements for the high‐throughput creation of 3D structures. To address this point, a suite of new thioxanthone‐based photoinitiators were synthesized and characterized. Then two‐photon polymerization was performed using the most promising photoinitiating molecule. Importantly, one of the initiators, 2,7‐bis[(4‐(dimethylamino)phenyl ethynyl)‐9H‐thioxanthen‐9‐one] (BDAPT), showed a fivefold improvement in the writing threshold over the commonly used ITX molecule. To elucidate the fundamental mechanism, the excitation and inhibition behavior of the BDAPT molecule were evaluated using density functional theory (DFT) calculations, low‐temperature phosphorescence spectroscopy, ultra‐fast transient absorption spectroscopy, and the two‐photon Z‐scan spectroscopic technique. The improved polymerization threshold of this new photoinitiator presents a clear pathway for the modification of photoinitiators in 3D nanoprinting. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2019,57, 1462–1475 

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5. Enhancing functionalities of atomically thin semiconductors with plasmonic nanostructures

   https://doi.org/10.1515/nanoph-2018-0185  

   Cotrufo, Michele; Sun, Liuyang; Choi, Junho; Alù, Andrea; Li, Xiaoqin (February 2019, Nanophotonics)

   Abstract Atomically thin, two-dimensional, transition-metal dichalcogenide (TMD) monolayers have recently emerged as a versatile platform for optoelectronics. Their appeal stems from a tunable direct bandgap in the visible and near-infrared regions, the ability to enable strong coupling to light, and the unique opportunity to address the valley degree of freedom over atomically thin layers. Additionally, monolayer TMDs can host defect-bound localized excitons that behave as single-photon emitters, opening exciting avenues for highly integrated 2D quantum photonic circuitry. By introducing plasmonic nanostructures and metasurfaces, one may effectively enhance light harvesting, direct valley-polarized emission, and route valley index. This review article focuses on these critical aspects to develop integrated photonic and valleytronic applications by exploiting exciton–plasmon coupling over a new hybrid material platform. 

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