Jian Fu

Microfiber knot dye laser based on the evanescent-wave-coupled gain

The authors demonstrate a composite microring laser formed by immersing a silica microfiber knot in a rhodamine 6G dye solution. When the dye molecules are evanescently pumped by 532nm wavelength laser pulses guided along a 350μm diameter knot, lasing oscillation occurs inside the evanescently coupled closed-ring microcavity with a linewidth of about 0.06nm. Laser emission around 570 and 580nm wavelengths, which is evanescently coupled back into the microfiber, is observed with a threshold of about 9.2μJ∕pulse. The use of the microfiber knot cavity suggests a convenient and efficient approach to both pumping and collection of the evanescent-wave-coupled dye laser.

Nanowires/microfiber hybrid structure multicolor laser

We demonstrate a compact hybrid structure red-green-ultraviolet three-color laser consisting of three distinct semiconductor nanowires (CdSe, CdS and ZnO) attached to a silica microfiber, which is pumped by 355 nm wavelength laser pulses. The exciting of the nanowires and the collection of the photoluminescence (PL) are implemented by means of evanescent coupling through the same silica microfiber. When pump energy higher than 1.3 microJ, three spatially and spectrally distinct lasing groups can be measured at the same output port simultaneously. The approach can be extended to other materials to produce hybrid lasers that cover ultraviolet to near infrared spectral regions.

Modeling bending losses of optical nanofibers or nanowires

Bending losses of nanofibers or nanowires with circular 90 degrees bends are simulated using a three-dimensional finite-difference time-domain (3D-FDTD) method. Dependences of bending losses on wavelength and polarization of guided light are investigated, as well as the diameters, refractive indices, and bending radii of nanowires. The acceptable bending losses (approximately 1 dB/90 degrees) predicted in glass, polymer, and semiconductor nanowires with bending radii down to micrometer level may offer valuable references for assembling highly compact photonic integrated circuits or devices with optical nanowires.

Modeling endface output patterns of optical micro/nanofibers

Endface output patterns of micro/nanofibers (MNFs) are simulated using a Three-Dimension Finite-Difference Time-Domain (3D-FDTD) method. The intensity distribution and beam widths of near- or farfield output patterns of freestanding silica and tellurite MNFs with flat, angled, spherical and tapered endfaces in air and/or water are obtained. It shows that, for a subwavelength-diameter MNF, highly confined output beam can be obtained in the near field, and the beam width can be tuned by the ratio of fiber diameter and light wavelength with a minimum width smaller than the wavelength. Meanwhile, MNFs with shaped endfaces behave differently from standard fibers in reflection, redirection and focus of light beam at the endfaces. These results may offer valuable references for practical evaluation and application of terminated MNFs with wavelength- or subwavelength-scale endfaces.

Subwavelength focusing by a micro/nanofiber array

Utilizing the diffraction property of the output light from micro/nano fibers (MNFs), a new scheme based on a MNF array to focus optical beams with subwavelength resolution in the far field is proposed. By using the three-dimensional finite-difference beam-propagation method (FD-BPM), we investigated the diffraction interference effect of the MNFs during light propagation. The numerical stimulation demonstrates a focusing spot with FWHM of 0.43 lambda at a distance of 9 lambda from the output endface of MNF array. The effects of the characteristics of the array, the MNFs, and the input optical field on focusing are analyzed in detail.

Fabrication of micro-axicons using direct-laser writing

A novel direct-laser writing fabrication process for micro-axicons is demonstrated. A fiber-axicon-generated Bessel beam was utilized to write on UV-curable optical epoxy to form new axicons and axicon arrays, and geometrical parameters of the replicated epoxy axicons were analyzed in terms of both apex angle and proximity of the writing axicons. The shape of the fabricated axicons was demonstrated to be controllable through laser exposure, proximity, and apex angle of the source axicon, and the fabricated axicons are capable of generating a quality Bessel beam with an excellent focusing performance.

Subwavelength focusing of light by a tapered microtube

We propose a mechanism for subwavelength focusing at optical frequencies based on the use of a tapered microtube fabricated from a glass capillary tube. Using coherent illumination at 671 nm and a near-field scanning optical microscope probe which was mounted on a three-axis piezonanopositioning stage, a sequence of two-dimensional intensity profiles were obtained. Our experimental results reveal the smallest focal spot with a near diffraction-limited full width at half-maximum of ∼435 nm (0.65λ) at a distance of ∼1.47 μm (2.2λ) from the output end face of microtube. The experimental results are in agreement with our numerical simulation.