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1. On-chip optical levitation with a metalens in vacuum

   https://doi.org/10.1364/OPTICA.438410  

   Shen, Kunhong; Duan, Yao; Ju, Peng; Xu, Zhujing; Chen, Xi; Zhang, Lidan; Ahn, Jonghoon; Ni, Xingjie; Li, Tongcang (October 2021, Optica)

   Optical levitation of dielectric particles in vacuum is a powerful technique for precision measurements, testing fundamental physics, and quantum information science. Conventional optical tweezers require bulky optical components for trapping and detection. Here, we design and fabricate an ultrathin dielectric metalens with a high numerical aperture of 0.88 at 1064 nm in vacuum. It consists of 500-nm-thick silicon nano-antennas, which are compatible with an ultrahigh vacuum. We demonstrate optical levitation of nanoparticles in vacuum with a single metalens. The trapping frequency can be tuned by changing the laser power and polarization. We also transfer a levitated nanoparticle between two separated optical tweezers. Optical levitation with an ultrathin metalens in vacuum provides opportunities for a wide range of applications including on-chip sensing. Such metalenses will also be useful for trapping ultracold atoms and molecules. 

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2. High-speed tunable optical absorber based on a coupled photonic crystal slab and monolayer graphene structure

   https://doi.org/10.1364/OE.476763  

   Pan, Mingsen; Liu, Aaron; Liu, Zhonghe; Zhou, Weidong (January 2022, Optics Express)

   Reconfigurable metasurfaces have been pursued intensively in recent years for the ability to modulate the light after fabrication. However, the optical performances of these devices are limited by the efficiency, actuation response speed and mechanical control for reconfigurability. In this paper, we propose a fast tunable optical absorber based on the critical coupling of resonance mode to absorptive medium and the plasma dispersion effect of free carriers in semiconductor. The tunable absorber structure includes a single-layer or bi-layer silicon photonic crystal slab (PCS) to induce a high-Q optical resonance, a monolayer graphene as the absorption material, and bottom reflector to remove transmission. By modulating the refractive index of PCS via the plasma dispersion of the free carrier, the critical coupling condition is shifted in spectrum, and the device acquires tuning capability between perfect absorption and total reflection of the incident monochromatic light beam. Simulation results show that, with silicon index change of 0.015, the tunable absorption of light can achieve the reflection/absorption switching, and full range of reflection phase control is feasible in the over coupling region. The proposed reconfigurable structure has potential applications in remote sensing, free-space communications, LiDAR, and imaging. 

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3. Teeth Mold Point Cloud Completion Via Data Augmentation and Hybrid RL-GAN

   https://doi.org/10.1115/1.4056566  

   Toscano, Juan Diego; Zuniga-Navarrete, Christian; Siu, Wilson David; Segura, Luis Javier; Sun, Hongyue (August 2023, Journal of Computing and Information Science in Engineering)

   Abstract Teeth scans are essential for many applications in orthodontics, where the teeth structures are virtualized to facilitate the design and fabrication of the prosthetic piece. Nevertheless, due to the limitations caused by factors such as viewing angles, occlusions, and sensor resolution, the 3D scanned point clouds (PCs) could be noisy or incomplete. Hence, there is a critical need to enhance the quality of the teeth PCs to ensure a suitable dental treatment. Toward this end, we propose a systematic framework including a two-step data augmentation (DA) technique to augment the limited teeth PCs and a hybrid deep learning (DL) method to complete the incomplete PCs. For the two-step DA, we first mirror and combine the PCs based on the bilateral symmetry of the human teeth and then augment the PCs based on an iterative generative adversarial network (GAN). Two filters are designed to avoid the outlier and duplicated PCs during the DA. For the hybrid DL, we first use a deep autoencoder (AE) to represent the PCs. Then, we propose a hybrid approach that selects the best completion to the teeth PCs from AE and a reinforcement learning (RL) agent-controlled GAN. Ablation study is performed to analyze each component’s contribution. We compared our method with other benchmark methods including point cloud network (PCN), cascaded refinement network (CRN), and variational relational point completion network (VRC-Net), and demonstrated that the proposed framework is suitable for completing teeth PCs with good accuracy over different scenarios. 

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4. Near-infrared metalens for high-resolution and deep focus optical coherence tomography

   https://doi.org/10.1063/5.0131170  

   Haerinia, Mohammad; Zheng, Bowen; Hutchins, Aaron; Tang, Hong; Li, Hang; Dong, Yunxi; Guo, Wei; Zhang, Hualiang (December 2022, AIP Advances)

   High-resolution endoscopic optical imaging is a crucial technique in biological imaging to examine the inside organs. There is a trade-off between lateral resolution and depth of focus in such applications. Traditional Optical Coherence Tomography provides an increased depth range but falls short of desired resolution. The combination of both higher resolution and larger imaging depth of focus of metalens can improve the clinical utility of endoscopic optical imaging. In this work, we designed, analyzed, and fabricated a 500 µm diameter metalens operating at 1300 nm to achieve high resolution and large imaging depth of focus, therefore, addressing this need. The full width at half maximum and depth of focus for the proposed metalens are 3.10 and 286 µm, respectively. 

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5. Green Synthesis of UV‐Reactive Polycarbonates from Levoglucosenone and 5‐Hydroxymethyl Furfural

   https://doi.org/10.1002/marc.202300483  

   Kayishaer, Aihemaiti; Annatelli, Mattia; Hansom, Chloe M; Mouterde, Louis M.; Peru, Aurélien A.; Aricò, Fabio; Allais, Florent; Fadlallah, Sami (January 2024, Macromolecular Rapid Communications)

   Abstract This study focuses on the synthesis of fully renewable polycarbonates (PCs) starting from cellulose‐based platform molecules levoglucosenone (LGO) and 2,5‐bis(hydroxymethyl)furan (BHMF). These unique bio‐based PCs are obtained through the reaction of a citronellol‐containing triol (Triol‐citro) derived from LGO, with a dimethyl carbonate derivative of BHMF (BHMF‐DC). Solvent‐free polymerizations are targeted to minimize waste generation and promote an eco‐friendly approach with a favorable environmental factor (E‐factor). The choice of metal catalyst during polymerization significantly influences the polymer properties, resulting in high molecular weight (up to 755 kDa) when Na₂CO₃is employed as an inexpensive catalyst. Characterization using nuclear magnetic resonance confirms the successful incorporation of the furan ring and the retention of the terminal double bond of the citronellol pendant chain. Furthermore, under UV irradiation, the presence of both citronellol and furanic moieties induces singular structural changes, triggering the formation of three distinct structures within the polymer network, a phenomenon herein occurs for the first time in this type of polymer. These findings pave the way to new functional materials prepared from renewable monomers with tunable properties. 

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