Origin of high propagation loss in electrospun polymer nanofibers

Abstract

We evaluate optical propagation loss (α) in electrospun poly(methyl methacrylate) (PMMA) nanofibers with different wavelength (λ) and determine the origin of the loss. Aligned single electrospun nanofibers composed of PMMA and a small amount of an organic dye are fabricated with an average diameter of approximately 640 nm. After cladding seven fiber samples, α is evaluated to be 26–62 dB cm−1 at wavelengths 590−680 nm. Moreover, α depended linearly on λ−4, and from the fitting functions we determined the ratio of the following two possible losses for α: loss at the interface between the fiber-core and cladding because of non-uniformity within the fibers (αun), and loss because of excess light scattering in the fibers resulting from density inhomogeneity of PMMA (αsc). For the fibers, αun is evaluated to be 6.9–22 dB cm−1, which represents 19%–50% of α at λ of 650 nm with α ∼ αun + αsc. Thus, we conclude that the high α in these fibers originates from both their poor uniformity and density inhomogeneity. Furthermore, a quantitative investigation of uniformity in the individual fibers revealed that the root mean square roughness ranges from 5.5 nm to 9.0 nm and the theoretical value of αun was ∼1 dB cm−1 showing reasonable agreement with experimental data. These findings hold for low-loss polymer nanofiber waveguides, which have high aspect ratio and fine patterning even in three dimensions.

Waveguiding properties in dye-doped submicron poly(N-vinylcarbazole) fibers

“…The estimate a values are on the same order of magnitude as those previously reported for optically active polymer submicron fibers, a = 1.0 × 10 −2 −2.0 × 10 −1 µm −1 (corresponding to propagation losses of 4.3 × 10 −2 −8.7 × 10 −1 dB·µm −1 ). The estimated a decreased with increasing λ for Fibers A and B because the excess light scattering was suppressed in the fiber as λ increased, which has been previously observed in submicron polymer fibers and explained by Debye theory . The estimated a from Fibers C, D, and E did not decrease monotonically with increasing λ .…”

“…Propagation loss in light‐emissive submicron polymer fibers has previously been evaluated by measuring the guided light intensity with different propagation lengths. This requires scanning the excitation light spot or detection area along the fibers using waveguiding excitation with tapered silica fibers or direct excitation/detection . Several groups reported temporal photobleaching of light‐emitting materials embedded in the fibers that was caused by the excitation light irradiation.…”

Amorphous Electrically Actuating Submicron Fiber Waveguides

“…Each aligned electrospun PDLLA/R6G fiber is clad in CYTOP, and the loss coefficient of the guiding light in each fiber is evaluated, indicating the attenuation of guided FL emitted from the doped R6G molecules with various lengths ( L ) between their fiber ends and their points of excitation. Details on the loss coefficient measurement procedure are given briefly in the Experimental Section and elsewhere . Figure a shows the FL intensities after guiding through PDLLA/R6G fibers with different values of L .…”

“…The reabsorption coefficients (α ab ) of the doped R6G molecules for guided light are also evaluated with respect to η for the fiber waveguides. A detailed procedure for evaluating α ab is outlined both in the Experimental Section and elsewhere . The inset in Figure shows transmittance of the R6G solution at various solution thicknesses ( t ), where R6G is dissolved in DMF with the same concentration as in the PDLLA/R6G fibers (2 × 10 −3 mol L −1 ).…”

“…Fluorescent microscopy images were captured using a fluorescence microscope (IX‐51, Olympus) through a sharp‐cut filter (BA590, Olympus) that transmitted light with wavelength greater than 590 nm, where the whole imaged area was irradiated by excitation light with a wavelength between 510 and 550 nm, filtered with an interference filter (BP510‐550, Olympus). Details of the loss coefficient measurement procedure for the guiding light in each fiber are provided elsewhere . Briefly, a collimated 532 nm laser (LCM‐T‐111, Laser‐Export Co. Ltd.) beam with a 440 µm radius irradiated each CYTOP‐clad PDLLA/R6G fiber in a direction perpendicular to the fiber axis at various lengths ( L ) between the fiber end and the point of irradiation.…”