Optical micro/nanofiber (MNF) is a kind of quasi one-dimensional (1D) free-standing optical waveguide with a diameter close to or less than the vacuum wavelength of the transmitted light. As shown in Fig. 1, the earliest report of glass MNFs with diameters around 1 μm could be traced back to 1887 when Boys fabricated glass threads by drawing molten minerals at a high speed. Owing to excellent elasticity and small resilience of these threads, they were used as elastic springs or suspension wire for measuring a very small force or torsion. Since the optical waveguiding theory had not yet been developed at that time, those threads were not used for optical waveguiding.
In 1910, Hondros and Debye reported the first theoretical model for waveguiding light along a dielectric cylinder. They showed that electromagnetic waves could be confined and propagated in a lossless subwavelength-diameter dielectric cylindrical waveguide. Later, these waveguiding modes in cylindrical waveguides were named as "surface waves" and received increasing attention. In 1966, Kao and Hockham proposed the possibility of developing low-loss glass fibers, opening up the era of optical fiber communication, as well as low-loss fiber optics and technology. From then on, taper drawing standard silica fibers became a routine approach to fabricating optical MNFs. Relying on the surface waves (i.e., waveguided evanescent fields) of these MNFs, a variety of photonic applications were developed including optical filters, couplers, evanescent field amplification, sensors and supercontinuum generation, while the diameter of the MNFs used or assumed were mostly larger than the vacuum wavelength of the guided light.
In 2003, Tong and Mazur experimentally demonstrated that subwavelength-diameter silica nanofibers taper drawn from silica fibers could be used for low-loss optical waveguiding, opening an opportunity for guiding light in MNFs with smaller sizes and stronger "surface waves". With the flourishing of light field manipulation and nanotechnology in the early 21st century, high-quality optical MNFs as low dimensional optical waveguide structures for ultra-low-loss waveguiding on the wavelength or subwavelength scale, have attracted extensive research interest in the field of nanophotonics and enabled a variety of applications.
Fig.1. Roadmap of the development of optical MNFs.
Recently, a team of scientists led by Prof. Limin Tong and Xin Guo at Zhejiang University was invited by the Co-Editors-in-Chief to contribute a comprehensive review paper entitled "Optical microfiber or nanofiber: a miniature fiber-optic platform for nanophotonics", which was published on the third issue of Photonics Insights. (Jianbin Zhang, Hubiao Fang, Pan Wang, Wei Fang, Lei Zhang, Xin Guo and Limin Tong. Optical microfiber or nanofiber: a miniature fiber-optic platform for nanophotonics[J]. Photonics Insights, 2024, 3(1): R02).
In this paper, they first give an overview of the history of optical MNFs. Then, they introduce the basic structure, fabrication and characterization techniques of the MNFs. Next, they highlight linear and nonlinear optical and mechanical properties of the MNFs. Finally, they summarize typical applications of optical MNFs and provide an outlook into future challenges and opportunities.
Benefitting from the rapid development of glass fiber technology in the past decades, high-temperature drawing is the main fabrication technique of glass MNFs. Currently, the frequently used heating methods are flame heating, electric heating and laser heating. Taking the silica glass as an example, when the heating temperature rises to the softening temperature of a silica fiber (> 1100 ℃), one can draw the fiber into a MNF via a high-precision fabrication system (Fig. 2). Generally, both ends of as-fabricated MNFs are connected to standard fibers through conical transition regions, which is conducive to practical applications.
Fig. 2. (a) Structural diagram of a biconical optical MNF. (b) Typical flame-heated taper-drawing fabrication system.
Structural characterization of glass MNFs is typically carried out with optical or electron microscopes. Figure 3(a) shows a typical bright-field optical microscopic image of a 550-nm-diameter silica MNF with a uniform diameter. The scanning electron microscope (SEM) images of optical MNFs in Figs. 3(b)-3(e) manifest extraordinary surface smoothness, perfect circular cross-section and outstanding mechanical strength. Figure 3(f) gives a transmission electron microscope (TEM) image of a 330-nm-diameter MNF, revealing an ultra-low surface roughness (< 0.2 nm). As research into MNF optics advances, in-situ diameter measurement of optical MNFs has gained growing attention and a number of novel optical methods have been reported in recent years including optical diffraction imaging, near-field probing, nonlinear phase matching, stress-strain analysis, Rayleigh scattered light imaging and short-time Fourier transform analysis on the modal evolution, with a spatial resolution from 15 nm to 40 pm.
Fig. 3. Structural characterization of silica MNFs. (a) Optical microscope image of a 550-nm-diameter silica MNF. SEM images of (b) self-supporting bundle of MNFs assembled with silica MNFs with diameters of 140, 510, and 30 nm, (c) 790-nm diameter silica MNF with a smooth surface, (d) 400-nm-diameter tellurite glass MNF with a circular cross-section, and (e) 360-nm-diameter silica MNF with a bending radius of 3 μm. (f) TEM image of the surface of a 330-nm-diameter silica MNF. Inset: electron diffraction pattern of the MNF. [Nature 426, 816 (2003); Opt. Express 14, 82 (2006); Front. Optoelectron. China 3, 54 (2010)]
Generally, as the fiber diameter decreases to the subwavelength scale, the MNF exhibits fascinating optical properties including a strong evanescent field (Fig. 4), tight optical confinement, surface field enhancement, and diameter/wavelength-dependent large waveguide dispersion, making it favorable for manipulating light on the micro/nanoscale with high flexibility. As a basic waveguide structure, the optical losses of the MNFs are mostly concerned, such as scattering loss, bending loss and optical absorption. With the improvement of the fabrication and characterization techniques, the measured waveguiding losses of as-fabricated MNFs, especially of subwavelength-diameter MNFs, have been effectively reduced over the last 20 years (Fig. 5). Recently, 10-W-level continuous-wave optical waveguiding in silica MNFs has been demonstrated, showing great potential in the interaction of high-power light field and matters on the nanoscale.
Fig. 4. Poynting vectors of the fundamental mode in silica MNFs with different diameters at 633-nm wavelength in 3D view (upper row) and 2D view (lower row). [Nanophotonics 2, 407 (2013)]
Fig. 5. Typical optical losses of glass MNFs over the last 20 years.
On the other hand, owing to the nearly perfect structural uniformity, optical MNFs exhibit a high tensile strength (e.g., higher than 10 GPa) and excellent elasticity, which
enable robust and flexible manipulation of freestanding MNFs in various surroundings. These favorable mechanical properties make the MNFs ideal for constructing novel nanophotonic devices and exploring related applications.
Based on the above optical and mechanical properties, optical MNF has been emerging as a miniaturized fiber-optic platform for studying light-matter interaction on the micro/nanoscale and developing related photonic technologies.
• Firstly, optical MNFs have the special advantages of low-loss waveguiding with tightly confined high-fraction evanescent fields, which makes them favorable for compact and high-efficiency near-field optical coupling with external structures on the micro/nanoscale. Assisted with micro/nanomanipulation, a variety of MNF-based coupling structures, including 1D optical waveguides, 2D materials, and 3D micro-cavities, have been demonstrated.
• Secondly, the tight optical confinement and surface enhancement are highly favorable for enhancing nonlinear optical effects in the MNF or coupled materials.
• Thirdly, the waveguide dispersion of optical MNFs is strongly dependent on the diameter or wavelength, offering a compact, flexible, low-loss, and fiber-compatible scheme for dispersion management in nonlinear optics, pulse compression and fiber lasers.
• Fourthly, the large field gradient in the vicinity of the MNF surface has been exploited for manipulating micro/nanoparticles and cold atoms. Finally, owing to their small mass, optical MNFs can exhibit a sensitive optomechanical response to the momentum change of the waveguided light fields, which has been adopted for studying optoacoustic interactions and optomechanical technology.
So far, a variety of MNF optics and technologies have been reported ranging from near-field optics, optical sensing, nonlinear optics, fiber lasers, to atom optics and optomechanics (Fig. 6), which are of great significance for frontier research in multidisciplinary fields.
Fig. 6. Overall description of optical MNFs in terms of characteristics and applications.
To conclude, this review article provides a comprehensive overview of the history and representative advances in optical MNFs in recent years. As a unique one-dimensional cylinder with a highly symmetric structure and nearly perfect surface quality and diameter uniformity, glass MNF can offer extraordinarily low waveguiding loss (e.g., 0.03 dB/m at 780-nm wavelength in silica MNFs), nearly 100% power in evanescent waves, high waveguided power density (> 20 W∕μm2 for silica MNF at 1550-nm wavelength), and a mechanical strength approaching the theoretical limit. Based on their excellent optical and mechanical properties, optical MNFs have exhibited unique advantages and broad application prospects.
Looking into the future, the authors propose to explore the fundamental limits of MNF optics. By further improving and optimizing the geometry of optical MNFs, higher surface quality and diameter uniformity of the MNFs can be achieved, making them more suitable for generating and manipulating extreme light fields. Besides, it is critical to expand available MNF material systems and improve the fabrication technology of optical MNFs with different materials for expanding the operation spectral range and real applications. Finally, technological improvement and innovation in the fabrication, characterization and applications of novel optical MNFs are highly desired for developing MNF optics and technological applications.
