An optical micro/nanofiber (MNF) is a quasi-one-dimensional free-standing optical waveguide with a diameter close to or less than the vacuum wavelength of light. Combining the tiny geometry with high-refractive-index contrast between the core and the surrounding, the MNF exhibits favorable optical properties such as tight optical confinement, strong evanescent field, and large-diameter-dependent waveguide dispersion. Meanwhile, as a quasi-one-dimensional structure with extraordinarily high geometric and structural uniformity, the MNF also has low optical loss and high mechanical strength, making it favorable for manipulating light on the micro/nanoscale with high flexibility. Over the past two decades, optical MNFs, typically being operated in single mode, have been emerging as a miniaturized fiber-optic platform for both scientific research and technological applications. In this paper, we aim to provide a comprehensive overview of the representative advances in optical MNFs in recent years. Starting from the basic structures and fabrication techniques of the optical MNFs, we highlight linear and nonlinear optical and mechanical properties of the MNFs. Then, we introduce typical applications of optical MNFs from near-field optics, passive optical components, optical sensors, and optomechanics to fiber lasers and atom optics. Finally, we give a brief summary of the current status of MNF optics and technology, and provide an outlook into future challenges and opportunities.
We propose to generate a sub-nanometer-confined optical field in a nanoslit waveguiding mode in a coupled nanowire pair (CNP). We show that, when a conventional waveguide mode with a proper polarization is evanescently coupled into a properly designed CNP with a central nanoslit, it can be efficiently channeled into a high-purity nanoslit mode within a waveguiding length <10 μm. The CNP can be either freestanding or on-chip by using a tapered fiber or planar waveguide for input-coupling, with a coupling efficiency up to 95%. Within the slit region, the output diffraction-limited nanoslit mode offers an extremely confined optical field (∼0.3 nm × 3.3 nm) with a peak-to-background ratio higher than 25 dB and can be operated within a 200-nm bandwidth. The group velocity dispersion of the nanoslit mode for ultrafast pulsed operation is also briefly investigated. Compared with the previous lasing configuration, the waveguiding scheme demonstrated here is not only simple and straightforward in structural design but is also much flexible and versatile in operation. Therefore, the waveguiding scheme we show here may offer an efficient and flexible platform for exploring light–matter interactions beyond the nanometer scale, and developing optical technologies ranging from superresolution nanoscopy and atom/molecule manipulation to ultra-sensitivity detection.
Optical spectral broadening induced by self-phase modulation (SPM) in single CdTe nanowires is measured. A significant spectral broadening of about 10 nm is observed using ps near-infrared (NIR) pulses with coupled peak power of a dozen W. Benefiting from the large effective nonlinearity and refractive index of these nanowires, the necessary propagating length goes down to several hundred μms. A relatively large nonlinear-index coefficient (n2) more than 1×10-17 m2 /W is obtained from transmission spectra experimentally within measured spectral region, which suggest great possibility for these nanowires in developing ultracompact nonlinear optical devices.
We reported a simple, robust, and highly sensitive temperature sensor using intrinsic Mach-Zehnder interferometer formed by means of bending a tapered microfiber, embedded in polydimethylsiloxane. The outer temperature perturbations modulate the refractive index of the polymer through thermo-optic and thermal expansion effects of the polymer. This leads to a phase difference between interfering guided modes through the bent-microfiber, which ultimately results prominent wavelength shift in the transmission spectrum. The sensor exhibits a linear temperature response with a sensitivity as high as -6.25nm/°C over the temperature range from 24° to 40 °C. The sensitivity of the sensor increases as wavelength increases.
We demonstrate three microfluidic chip based microfiber/nanofiber sensors for ultra-sensitive absorption, fluorescence, and femtoliter-scale sensing, respectively. The sensors shown here may open up new opportunities for ultra-sensitive biosensing and single molecule analysis.
KEYWORDS: Silver, Near field optics, Near field scanning optical microscopy, Surface plasmons, Near field, Plasmonics, Light scattering, Magnesium fluoride, Integrated optics, Nanowires
We demonstrate surface plasmon (SP) excitation in Ag nanowires directly coupled with a microfiber in the presence of a
MgF2 substrate. With scanning near-field optical microscopy, evident light output from the Ag nanowire and the
evanescent field of the coupling structure are observed in the near-field optical image. The tip enhancement of a Ag
nanowire is also analyzed from the optical intensity graph. Results presented in this work suggest a simple SP excitation
approach for plasmonic and photonic circuits with high compactness.
We experimentally demonstrate silver nanowire based plasmonic devices at optical communication wavelength 1.55 μm.
The plasmon propagation loss in a 300 nm diameter silver nanowire is measured to be 0.3 dB/μm. Two types of
plasmonic functional devices based on the coupling between two silver nanowires, nano-couplers and nano-splitters, are
realized.
Optical silica nanowires fabricated using a taper-drawing approach exhibit extraordinary uniformity, making them suitable for low-loss optical wave guiding. Air-clad optical nanowires can be used as subwavelength-diameter single-mode waveguides from the ultraviolet to the near-infrared spectral range. Using these nanowires as building blocks we assembled photonic devices that are much smaller than comparable existing devices, indicating the great potential for developing micro- and nanoscale photonic devices for future applications in a variety of fields such as optical communication, optical sensing and high-density optical integration.
Subwavelength-diameter silica wires fabricated using a taper-drawing approach exhibit excellent diameter uniformity and atomic-level smoothness, making them suitable for low-loss optical wave guiding from the UV to the near-infrared. Such air-clad silica wires can be used as single-mode waveguides; depending on wavelength and wire diameter, they either tightly confine the optical fields or leave a certain amount of guided energy outside the wire in the form of evanescent waves. Using these wire waveguides as building blocks we assembled microscale optical components such as linear waveguides, waveguide bends and branch couplers on a low-index, non-dissipative silica aerogel substrate. These components are much smaller than comparable existing devices and have low optical loss, indicating that the wire-assembly technique presented here has great potential for developing microphotonics devices for future applications in a variety of fields such as optical communication, optical sensing and high-density optical integration.
Based on the exact solution of Maxwell’s equations and numerical calculations, we have investigated the basic theoretical properties of metal-coated silica nanowires. Modal profile, field and power distributions, and conditions for surface plasmon excitation are studied. It shows that a thin layer of metal coat can influence the distribution of the electromagnetic field and enhance confinement ability of light power inside silica core due to its negative dielectric function, which may be favorable for reducing the size of wire waveguide for microphotonic applications.
We propose to use bent silica wires with nanometric diameters to guide light as optical waveguide bend. We bend silica wires with scanning tunneling microscope probes under an optical microscope, and wire bends with bending radius smaller than 5 μm are obtained. Light from a He-Ne laser is launched into and guided through the wire bends, measured bending loss of a single bend is on the order of 1 dB. Brief introductions to the optical wave guiding and elastic bending properties of silica wires are also provided. Comparing with waveguide bends based on photonic bandgap structures, the waveguide bends from silica nanometric wires show advantages of simple structure, small overall size, easy fabrication and wide useful spectral range, which make them potentially useful in the miniaturization of photonic devices.
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