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PR Hightlights (Vol.9, Iss. 2): 简单直接新方法增强光线纳米聚焦——光子晶体光纤耦合金属纳米天线

2021-04-08

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简单直接新方法增强光线纳米聚焦——光子晶体光纤耦合金属纳米天线

 

光纤由于具有长距离传输低损耗、应用灵活等优点,从而成为使用范围最为广泛的光波导。其在诸如光纤生物传感器、化学传感器、光纤激光器以及光纤内近场成像等一系列应用中,起到照明和收集光线的作用。

随着人们对纳米量级光子器件以及量子通讯需求的日益提升,亟需一种可以将光纤中传输的光线(光子模式)聚焦至纳米尺度的方法。表面等离激元(SPP)波导在光线沿金属-介电表面传播时,能够控制并约束光束在纳米尺度,但其光学损耗过大,因此传播距离仅有微米级。因此,如何将低损耗光子波导模式耦合至高约束SPP模式,反之亦然,是纳米尺度光纤光学实际应用中必须要解决的问题。

此类光纤纳米聚焦器件的研发难点在于:如何实现模场大小不同、模场分布不同的光子模式和等离子体模式之间的相位匹配(光子模式在微米量级,而等离子体模式在纳米量级,且通常沿径向偏振);此外,目前已有的将金属纳米线等纳米量级的等离子体纳米波导集成到微米量级光纤的方法,都需要工序繁多、精度要求高的复杂加工过程。

针对以上研究现状,贝勒大学张臻蓉副教授和加州大学欧文分校Howard Lee 副教授合作,将研究重点放在了一种简单且直接的方法上,可高效地将光纤光子模式耦合、聚焦到纳米量级等离子体模式(图1(a))。该装置是一个电子束诱导蒸发技术在光子晶体光纤(PCF)端面上制作的针状等离子体纳米天线(图1(b)),其相关的制作流程、实验表征、理论仿真发表于Photonics Research2021年第2期上(Khant Minn, Blake Birmingham, Brian Ko, Ho Wai Howard Lee, Zhenrong Zhang. Interfacing photonic crystal fiber with a metallic nanoantenna for enhanced light nanofocusing[J]. Photonics Research, 2021, 9(2): 02000252)。

在贝勒大学 Khant Minn发明的实验平台上,PCF的基模沿光纤传播,通过端射耦合,耦合至铂金纳米天线上的SPP模式中。这种直接耦合的方式放宽了光子模式与等离子体模式严格的相位匹配要求,在宽带宽范围内实现耦合的同时,还降低了传输损耗。耦合后的SPP模式随后沿着天线向锥形端传播,并汇聚在纳米量级的尖端上,产生一个高度增强且受限的场。在聚焦离子束扫描电子显微镜(FIB-SEM)腔内,仅需一步就可直接将纳米量级的金属尖端刻写到光纤上,进而实现对等离子体天线位置和大小的精确操控。

图1 光子晶体光纤(PCF)-纳米天线混合探针。(a)器件原理图及金属纳米线波导上等离子体模式的强度分布仿真。(b)电子束诱导沉积过程示意图;PCF及纳米线制备的SEM图像

该合作研究组提出的光纤-等离子体探针的单步制作方法,除了可以实现设计出的具有纳米量级分辨率3D天线的量产,还能够以高精度实现任意配置的光纤-天线耦合。比如在加工过程中,在金属纳米天线底部开一个金薄膜矩形孔,从而将光纤中常见线性偏振通过非对称耦合和聚焦,耦合至尖端径向偏振纳米聚焦等离激元中。该装置对输入偏振的选择性可在散射光的侧面成像中观察到,证实了聚焦SPP模式的等离子体特性。

贝勒大学的张臻蓉副教授和加州大学欧文分校Howard Lee副教授认为,光纤-等离子体探针将为遥感、近场光谱仪、单光子激发、量子传感器、纳米级光刻以及Lab-on-fiber集成器件等新型光纤设备和应用的研发奠定基础。下一步工作将放在优化耦合和聚焦效率,以及将光纤探针集成到扫描探针显微镜中,以用于纳米级化学成像技术如基于光纤的针尖增强拉曼光谱、荧光光谱等。

 

Interfacing photonic crystal fiber with a metallic nanoantenna for enhanced light nanofocusing

 

Optical fiber is the most broadly used optical waveguide for transmitting light because of its low loss transmission even over long distances and its flexibility, which has been used for illumination and collection of light in various applications including optical fiber biosensors and chemical sensors, fiber lasers, and in-fiber near-field imaging. With the increasing demand for nanoscale photonic devices and quantum communication, an efficient way is needed to focus the light transmitted by optical fiber (the photonic mode) down to nanoscale-confined light. Surface plasmon polariton (SPP) waveguides can control and confine light in the nanometer scale as the light is propagating along the metal-dielectric interface. However, the travel distance of SPP is in the micrometer scale due to the high optical loss. Thus, efficient coupling of low-loss photonic waveguide modes to the highly-confined SPP mode and vice versa is necessary for practical nanoscale fiber optics. The development of these optical fiber nanofocusing devices has been challenging as it requires the phase matching of photonic mode and plasmonic mode that exhibit different mode sizes (microscale vs nanoscale) and mode profiles (with the plasmonic mode being naturally radially polarized). Additionally, current attempts to integrate a nanometer-sized plasmonic nano-waveguide, e.g. a metal nanowire, onto a micrometer-sized fiber have required a challenging fabrication process with multiple procedures and precise alignments.

The collaborative research between Prof. Zhenrong Zhang's group at Baylor University and Prof. Howard Lee's group at the University of California, Irvine in the United States focuses on a simple and straightforward method of efficient coupling and focusing of the optical fiber photonic mode to the nanoscale plasmonic mode (Fig. 1a). The device involved is a needle-like plasmonic nano-antenna fabricated on the end facet of a photonic crystal fiber (PCF) by an electron beam induced evaporation technique (Fig. 1b). The fabrication, experimental characterization, and theoretical simulation of the device are published in Photonics Research Vol. 9, No. 2, 2021 (Khant Minn, Blake Birmingham, Brian Ko, Ho Wai Howard Lee, Zhenrong Zhang. Interfacing photonic crystal fiber with a metallic nanoantenna for enhanced light nanofocusing[J]. Photonics Research, 2021, 9(2): 02000252).

In this platform, introduced by Khant Minn from Baylor University, the fundamental core mode of the PCF propagates through the fiber and couples to the SPP mode on the platinum nano-antenna via the end-fire coupling. Such direct coupling relaxes the challenging phase-matching requirement of the photonic mode and the plasmonic mode, enables broadband coupling, and reduces propagation losses. The coupled SPP's then propagate along the antenna toward the tapered end where they converge to produce a highly enhanced and confined field at the nano-scale apex. The precise control of the position and size of the plasmonic antenna is achieved by directly writing a nanoscale metallic tip onto the fiber in a single step inside a focused-ion and scanning electron microscope (FIB-SEM) chamber.

Fig. 1. The photonic crystal fiber (PCF)-nanoantenna hybrid probe. (a) Schematics of the device and simulated intensity profile of plasmonic mode on metallic nanowire waveguide. (b) Schematics of the electron beam induced deposition process and SEM image of PCF and fabricated nanowire

This work presented a single-step fabrication of a fiber-plasmonic probe which allows for any fiber-antenna coupling configuration to be realized with high precision, in addition to the high yield of the designer 3D antenna with nanometer resolution. For example, during fabrication, a gold thin film rectangular aperture was opened at the base of the metal nanoantenna to enable the asymmetrical coupling and focusing of the commonly-accessible lineally-polarized light from the optical fiber to the radially-polarized nano-focusing plasmons at the tip apex. Input polarization selectiveness of the device observed in the side images of the scattered light demonstrated the plasmonic nature of the focused SPP.

Prof. Zhenrong Zhang from Baylor University and Prof. Howard Lee from the University of California, Irvine believe that the fiber-plasmonic probe is a promising platform for the development of novel optical fiber devices and applications such as remote sensing and nearfield spectroscopes, single photon excitation and quantum sensors, nano-scale optical lithography, and lab-on-fiber devices.

Future work will focus on the optimization of the coupling and focusing efficiencies and the integration of these fiber probes in scanning probe microscopes for nanoscale chemical imaging such as fiber-based tip-enhanced Raman and fluorescence spectroscopies.