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    Fiber Optics and Optical Communications

Jun Ma   Yang He   Xue Bai   Li-Peng Sun   Kai Chen   Kyunghwan Oh   Bai-Ou Guan  

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Interaction of acoustic waves and microbubbles occurs in numerous biomedical applications including ultrasound imaging, drug delivery, lithotripsy treatment, and cell manipulation, wherein the acoustically driven microbubbles routinely act as active microscale oscillators or actuators. In contrast, microbubbles were utilized here as passive receivers to detect broadband ultrasound waves in aqueous environments. The microbubble was photothermally generated on a microstructured optical fiber (MOF) tip, forming a flexible Fabry–Pérot cavity whose gas–water interface was sensitive to ultrasound waves. The MOF severed as both a low-loss waveguide and a compact light condenser, allowing high-efficiency generation and stabilization of ultrasmall microbubbles. Integrated with all-fiber interferometry, a 10 μm diameter microbubble exhibited a low noise-equivalent pressure level of ～3.4mPa/Hz1/2 and a broad bandwidth of ～0.8MHz, capable of detecting weak ultrasounds emitted from red blood cells irradiated by pulsed laser light. With advantages of high sensitivity, compact size, and low cost, the microbubble-based ultrasound sensor has great potential in biomedical imaging and sensing applications.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1558-1565      

Kang-Jie Lim   Samuel Kai-Wen Seah   Joash Yong’En Ye   Wendy Weiying Lim   Chu-Perng Seah   Yunn-Boon Tan   Suting Tan   Huiting Lim   Raghuraman Sidharthan   Arumugam Rajendra Prasadh   Chen-Jian Chang   Seongwoo Yoo   Song-Liang Chua  

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A short absorption length ytterbium (Yb)-doped large-mode area (LMA) fiber is presented as a step forward to mitigate the stern problem of nonlinear scatterings in a tandem pumping scheme adopted for high-power fiber laser. The short absorption length was realized by incorporating high Yb concentration in the fiber core. Furthermore, by replacing the inherent silica cladding with a Ge-doped cladding, we were able to obtain low core numerical aperture (NA) and negate the detrimental effect of index-raising by high Yb concentrations. This overcomes the long-standing limitation in step-index Yb-doped fibers (YDFs) where high cladding absorption inevitably results in high NA, thus hampering single-mode operation. We report an LMA (～575μm2) YDF with NA of 0.04 and absorption of 27 dB/m at 976 nm—both traits promote power scaling of single-mode tandem pumped fiber lasers. To our knowledge, this is the highest cladding absorption attained in a low-NA step-index fiber to date. An all-fiber tandem-pumped amplifier was built using only ～14m of the YDF. The amplifier delivered a near-Gaussian beam (M2～1.27) at 836 W output power (pump power limited) with a high slope efficiency of ～83%. Thanks to the short length and the tandem pumping, no indication of limiting factors such as stimulated Raman scattering, photodarkening, and transverse mode instability was observed.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1599-1604      

    Lasers and Laser Optics

Yi Zhou   Yu-Xuan Ren   Jiawei Shi   Kenneth K. Y. Wong  

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Soliton explosions, among the most exotic dynamics, have been extensively studied on parameter invariant stationary solitons. However, the explosion dynamics are still largely unexplored in breathing dissipative solitons as a dynamic solution to many nonlinear systems. Here, we report on the first observation of a breathing dissipative soliton explosion in a net-normal-dispersion bidirectional ultrafast fiber laser. The breathing soliton explosions could be stimulated by the soliton buildup process or alteration of polarization settings. Transient breathing soliton pairs with intensive repulsion that is sensitive to initial conditions can also be triggered by multiple soliton explosions in the soliton buildup process instead of being triggered by varying polarization settings. The high behavior similarity also exists in the breathing soliton buildup and explosion process owing to the common gain and loss modulation. In addition, dissipative rogue waves were detected in the breathing soliton explosion, and the collision of breathing soliton significantly enhanced the amplitude of rogue waves, which is characteristic of the breathing solitons in a bidirectional fiber laser. These results shed new insights into complex dissipative soliton dynamics.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1566-1572      

Yuwei Zhao   Jintao Fan   Youjian Song   Uwe Morgner   Minglie Hu  

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Internal motions in femtosecond soliton molecules provide insight into universal collective dynamics in various nonlinear systems. Here we introduce an orbital-angular-momentum (OAM)-resolved method that maps the relative phase motion within a femtosecond soliton molecule into the rotational movement of the interferometric beam profile of two optical vortices. By this means, long-term relative phase evolutions of doublet and triplet soliton molecules generated in an all-polarization-maintaining mode-locked Er-fiber laser are revealed. This simple and practical OAM-resolved method represents a promising way to directly visualize the complex phase dynamics in a diversity of multisoliton structures.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1580-1585      

    Nanophotonics and Photonic Crystals

Zhexin Zhao   Dylan S. Black   R. Joel England   Tyler W. Hughes   Yu Miao   Olav Solgaard   Robert L. Byer   Shanhui Fan  

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To be useful for most scientific and medical applications, compact particle accelerators will require much higher average current than enabled by current architectures. For this purpose, we propose a photonic crystal architecture for a dielectric laser accelerator, referred to as a multi-input multi-output silicon accelerator (MIMOSA), that enables simultaneous acceleration of multiple electron beams, increasing the total electron throughput by at least 1 order of magnitude. To achieve this, we show that the photonic crystal must support a mode at the Γ point in reciprocal space, with a normalized frequency equal to the normalized speed of the phase-matched electron. We show that the figure of merit of the MIMOSA can be inferred from the eigenmodes of the corresponding infinitely periodic structure, which provides a powerful approach to design such devices. Additionally, we extend the MIMOSA architecture to electron deflectors and other electron manipulation functionalities. These additional functionalities, combined with the increased electron throughput of these devices, permit all-optical on-chip manipulation of electron beams in a fully integrated architecture compatible with current fabrication technologies, which opens the way to unconventional electron beam shaping, imaging, and radiation generation.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1586-1598      

Luocheng Huang   James Whitehead   Shane Colburn   Arka Majumdar  

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Metasurface optics have demonstrated vast potential for implementing traditional optical components in an ultracompact and lightweight form factor. Metasurfaces, however, suffer from severe chromatic aberrations, posing serious limitations on their practical use. Existing approaches for circumventing this involving dispersion engineering are limited to small apertures and often entail multiple scatterers per unit cell with small feature sizes. Here, we present an alternative technique to mitigate chromatic aberration and demonstrate high-quality, full-color imaging using extended depth of focus (EDOF) metalenses and computational reconstruction. Previous EDOF metalenses have relied on cubic phase masks, where the image quality suffers from asymmetric artefacts. Here we demonstrate the use of rotationally symmetric masks, including logarithmic-aspherical, and shifted axicon masks, to mitigate this problem. Our work will inspire further development in achromatic metalenses beyond dispersion engineering and hybrid optical–digital metasurface systems.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1613-1623      

    Optical and Photonic Materials

Qionghua Mo   Tongchao Shi   Wensi Cai   Shuangyi Zhao   Dongdong Yan   Juan Du   Zhigang Zang  

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All-inorganic cesium lead bromide (CsPbBr3) perovskite quantum dots (QDs) with excellent optical properties have been regarded as good gain materials for amplified spontaneous emission (ASE). However, the poor stability as the results of the high sensitivity to heat and moisture limits their further applications. Here, we report a facile one-pot approach to synthesize CsPbBr3@SiO2 QDs at room temperature. Due to the effective defects passivation using SiO2, as-prepared CsPbBr3@SiO2 QDs present an enhanced photoluminescence quantum yield (PLQY) and chemical stability. The PLQY of CsPbBr3@SiO2 QDs reaches 71.6% which is higher than 46% in pure CsPbBr3 QDs. The PL intensity of CsPbBr3@SiO2 QDs maintains 84% while remaining 24% in pure CsPbBr3 after 80 min heating at 60°C. The ASE performance of the films is also studied under a two-photon-pumped laser. Compared with the films using pure CsPbBr3 QDs, those with as-prepared CsPbBr3@SiO2 QDs exhibit a reduced threshold of ASE. The work suggests that room-temperature-synthesized SiO2-coated perovskites QDs are promising candidates for laser devices.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1605-1612      

    Research Articles

Yang Li   Chao Liang   Gaopeng Wang   Jielei Li   Shi Chen   Shihe Yang   Guichuan Xing   Hui Pan  

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Modifying the surface of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) with toluene during the high-speed spin-coating process of dimethylformamide considerably improves the wettability and morphology of PTAA and results in improvement of the crystallinity and absorption of perovskite film. The hole mobility and ohm contact have also been improved accordingly. Combined with these improved parameters, inverted perovskite solar cells with high efficiency of 19.13% and long-term stability could be achieved, which are much better than those with untreated PTAA. Importantly, our devices can keep 88.4% of the initial power conversion efficiency after 30 days of storage in ambient air.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.A39-49      

Michal Baranowski   Paulina Plochocka   Rui Su   Laurent Legrand   Thierry Barisien   Frederick Bernardot   Qihura Xiong   Christophe Testelin   Maria Chamarro  

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High magnetic field spectroscopy has been performed on lead chloride-based perovskite, a material that attracts significant interest for photovoltaic and photonic applications within the past decades. Optical properties being mainly driven by the exciton states, we have measured the fundamental parameters, such as the exciton binding energy, effective mass, and dielectric constant. Among the inorganic halide perovskites, CsPbCl3 owns the largest exciton binding energy and effective mass. This blue emitting compound has also been compared with lower band gap energy perovskites and other semiconducting phases, showing comparable band gap dependences for binding energy and Bohr radius.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.A50-55      

    Silicon Photonics

Aditya Malik   Joel Guo   Minh A. Tran   Geza Kurczveil   Di Liang   John E. Bowers  

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Heterogeneously integrated lasers in the O-band are a key component in realizing low-power optical interconnects for data centers and high-performance computing. Quantum-dot-based materials have been particularly appealing for light generation due to their ultralow lasing thresholds, small linewidth enhancement factor, and low sensitivity to reflections. Here, we present widely tunable quantum-dot lasers heterogeneously integrated on silicon-on-insulator substrate. The tuning mechanism is based on Vernier dual-ring geometry, and a 47 nm tuning range with 52 dB side-mode suppression ratio is observed. These parameters show an increase to 52 nm and 58 dB, respectively, when an additional wavelength filter in the form of a Mach–Zehnder interferometer is added to the cavity. The Lorentzian linewidth of the lasers is measured as low as 5.3 kHz.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1551-1557      

    Surface Optics and Plasmonics

Li Long   Jianfeng Chen   Huakang Yu   Zhi-Yuan Li  

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Tip-enhanced Raman spectroscopy (TERS) offers a powerful means to enhance the Raman scattering signal of a molecule as the localized surface plasmonic resonance will induce a significant local electric field enhancement in the nanoscale hot spot located within the nanogap of the TERS system. In this work, we theoretically show that this nanoscale hot spot can also serve as powerful optical tweezers to tightly trap a molecule. We calculate and analyze the local electric field and field gradient distribution of this nanogap plasmon hot spot. Due to the highly localized electric field, a three-dimensional optical trap can form at the hot spot. Moreover, the optical energy density and optical force acting on a molecule can be greatly enhanced to a level far exceeding the conventional single laser beam optical tweezers. Calculations show that for a single H2TBPP organic molecule, which is modeled as a spherical molecule with a radius of rm=1nm, a dielectric coefficient ε=3, and a polarizability α=4.5×10-38C·m2/V, the stiffness of the hot-spot trap can reach a high value of about 2pN/[(W/cm2)·m] and 40pN/[(W/cm2)·m] in the direction perpendicular and parallel to the TERS tip axis, which is far larger than the stiffness of single-beam tweezers, ～0.4pN/[(W/cm2)·m]. This hard-stiffness will enable the molecules to be stably captured in the plasmon hot spot. Our results indicate that TERS can become a promising tool of optical tweezers for trapping a microscopic object like molecules while implementing Raman spectroscopic imaging and analysis at the same time.

       PDF全文               HTML全文 Photonics Research, 2020年第8卷第10期 pp.1573-1579