Ultracompact photonic coupling splitters twisted by PTT nanowires

Biophotonic probes for bio-detection and imaging

The rapid development of biophotonics and biomedical sciences makes a high demand on photonic structures to be interfaced with biological systems that are capable of manipulating light at small scales for sensitive detection of biological signals and precise imaging of cellular structures. However, conventional photonic structures based on artificial materials (either inorganic or toxic organic) inevitably show incompatibility and invasiveness when interfacing with biological systems. The design of biophotonic probes from the abundant natural materials, particularly biological entities such as virus, cells and tissues, with the capability of multifunctional light manipulation at target sites greatly increases the biocompatibility and minimizes the invasiveness to biological microenvironment. In this review, advances in biophotonic probes for bio-detection and imaging are reviewed. We emphatically and systematically describe biological entities-based photonic probes that offer appropriate optical properties, biocompatibility, and biodegradability with different optical functions from light generation, to light transportation and light modulation. Three representative biophotonic probes, i.e., biological lasers, cell-based biophotonic waveguides and bio-microlenses, are reviewed with applications for bio-detection and imaging. Finally, perspectives on future opportunities and potential improvements of biophotonic probes are also provided.

Air Stable Organic-Inorganic Perovskite Nanocrystals@Polymer Nanofibers and Waveguide Lasing

Organic-inorganic hybrid perovskites have been considered as promising gain materials for lasing. Despite previous reports of lasing from nanocrystals, thin films and single crystals, the stability of perovskite lasers has been a challenge for its practical applications. Herein, a scalable strategy to prepare ultrastable perovskite@polymer hybrid fibers by employing a facile emulsion electrospinning approach is demonstrated. During the electrospinning process, polymethyl methacrylate (PMMA) first solidifies into an outer shell layer. Meanwhile, emulsion drops containing poly(vinylidene fluoride) (PVDF) and perovskite precursor are pushed inward and evolve into perovskite nanocrystals covered by PVDF. The PMMA with smooth surface benefits the light transport and the water-resistant PVDF blocks the moisture. The methylammonium lead bromide perovskite-embedded fibers can emit intensive light after storage in humid ambient environment (relative humidity >60%) or even in water. Amplified spontaneous emissions from the fibers network and waveguide lasing from chopped single fiber is demonstrated.

Reduced graphene oxide wrapped on microfiber and its light-control-light characteristics

Reduced graphene oxide (rGO) sheet wrapped on the tapered region of microfiber is demonstrated to enhance the interaction between rGO and strong evanescent field of optical fiber. The 405 nm and 980 nm lasers are employed to illuminate the rGO to investigate the response characteristics of the optical transmitted power (λ = 1550 nm) in the MF. The transmitted optical power of the MF with rGO changes with ~1.7 dB relative variation when the violet light is ranging from 0 mW to 12 mW (~0.21dB/mW) in the outside-pumped experiment. And in the inside-pumped experiment, the change of the 980 nm laser power from 0 mW to 156.5 mW makes ~6 dB relative variation power of the transmitted optical powers of the MF with rGO. These results indicate the optical transmitted power of the MF with wrapped rGO can be manipulated by the 405 and 980 nm light (order of mW), which signifies the device can potentially be applied as all optically and versatilely controllable devices.

Mode tailoring in subwavelength-dimensional semiconductor micro/nanowaveguides by coupling optical microfibers

Benefitted from large fraction of evanescent wave and high endface reflectivity, we have realized mode tailoring in subwavelength-dimensional semiconductor micro/nanowaveguides (MN-WGs) by coupling optical silica microfibers. By investigating the reflection spectra, it was found that the microfiber tips could offer effective reflection and can been used to continuously and reversibly tune the interference wavelengths by changing the contact points with the MN-WGs. The measured extinction ratio in the interference patterns was as high as ~10 dB. In addition, tunable free spectral range of photoluminescence emissions and humidity sensing were also demonstrated. Its advantages of non-destructively tuning, simple fabrication, easy interrogation, and remote monitoring, offer great possible prospects for developing miniature tunable lasers, sensors, and biological endoscopy.

Hybrid optical fiber add-drop filter based on wavelength dependent light coupling between micro/nano fiber ring and side-polished fiber

In this paper, we report our experimental study on directly coupling a micro/nano fiber (MNOF) ring with a side-polished fiber(SPF). As a result of the study, the behavior of an add-drop filter was observed. The demonstrated add-drop filter explored the wavelength dependence of light coupling between a MNOF ring and a SPF. The characteristics of the filter and its performance dependence on the MNOF ring diameter were investigated experimentally. The investigation resulted in an empirically obtained ring diameter that showed relatively good filter performance. Since light coupling between a (MNOF) and a conventional single mode fiber has remained a challenge in the photonic integration community, the present study may provide an alternative way to couple light between a MNOF device and a conventional single mode fiber based device or system. The hybridization approach that uses a SPF as a platform to integrate a MNOF device may enable the realization of other all-fiber optical hybrid devices.

High-efficiency fluorescent polyimides based on locally excited triarylamine-containing dianhydride moieties

Three fluorescent polyimides (PIs) were readily synthesized from the polycondensation of triarylamine-based tetracarboxylic dianhydride monomers with aliphatic diamine. The PL intensity of solid films and nanofibers revealed high PL quantum yields of up to 32% and 35%, respectively.

Micro- and submicrostructuring thin polymer films with two and three-beam single pulse laser interference lithography

In this work we report the application of two and three-beam single pulse laser interference lithography to thin polymer films of poly(trimethylene terephthalate) (PTT). By irradiating the sample surface with temporary and spatially overlapped single pulses from two or three coherent beams and changing the angles of incidence, we have accomplished the fabrication of large-area polymer micro and submicrogratings as well as submicrometric cavities arranged in a hexagonal lattice. The characterization of the structures in real space by atomic force microscopy (AFM) and scanning electron microscopy (SEM) has allowed us to determine the formation mechanism of the microgratings to be based on different ablation regimes depending on the local fluence. Moreover, complementary characterization of the submicrometric cavities in reciprocal space by grazing incidence small-angle X-ray scattering (GISAXS) confirms the existence of large areas where two-dimensional order is present. The experiments presented in this work demonstrate the suitability of single pulse laser interference lithography for micro and submicrostructuring polymer films, opening up new possibilities for patterning and paving the way for potential applications where polymer structures are involved.

Light propagation in conjugated polymer nanowires decoupled from a substrate

Light-emitting conjugated polymer nanowires are vertically grown and remotely manipulated into a freestanding straight or curved structure in three-dimension. This approach enabled us to eliminate substrate coupling, a critical issue in nanowire photonics in the past decade. We for the first time accomplished characterization of propagation and bending losses of nanowires completely decoupled from a substrate.

Photonic nanowires: from subwavelength waveguides to optical sensors

Nanowires are one-dimensional (1D) nanostructures with comparatively large aspect ratios, which can be useful in manipulating electrons, photons, plasmons, phonons, and atoms for numerous technologies. Among various nanostructures for low-dimensional photonics, the 1D nanowire is of great importance owing to its ability to route tightly confined light fields in single-mode with lowest space and material requirements, minimized optical path, and high mechanical flexibilities. In recent years, nanowire photonics have increasingly been attracting scientists' interests for both fundamental studies and technological applications because 1D nanowires have more favorable properties than many other structures, such as 0D quantum dots (QDs) and 2D films. As subwavelength waveguides, free-standing nanowires fabricated by either chemical growth or physical drawing techniques surpass nanowaveguides fabricated by almost all other means in terms of sidewall smoothness and diameter uniformity. This conveys their low waveguiding losses. With high index contrast (typically higher than 0.5) between the core and the surrounding or with surface plasmon resonance, a nanowire can guide light with tight optical confinement. For example, the effective mode area is less than λ(2)/10 for a dielectric nanowire or less than λ(2)/100 for a metal nanowire, where λ is the vacuum wavelength of the light. As we increase the wavelength-to-diameter ratio (WDR) of a nanowire, we can enlarge the fractional power of the evanescent fields in the guiding modes to over 80% while maintaining a small effective mode area, which may enable highly localized near-field interaction between the guided fields and the surrounding media. These favorable properties have opened great opportunities for optical sensing on the single-nanowire scale. However, several questions arise with ongoing research. With a deep-subwavelength cross-section, how can we efficiently couple light into a single nanowire? How can we fabricate a nanowire with low optical loss? How can we activate a passive nanowire for optical sensing? And lastly, how can we adapt mature optical measurement technology onto a nanowire? In this Account, we highlight our initial attempts to address the above-mentioned challenges. First, we introduce the fabrication and functionalization of low-loss photonic nanowires. We show that nanowires fabricated by either top-down physical drawing (e.g., for amorphous nanowires) or bottom-up chemical growth (e.g., for crystalline nanowires) can yield excellent geometric and structural uniformities with surface roughness down to atomic level and minimize the scattering loss for subwavelength optical or plasmonic waveguiding. Then, relying on a near-field fiber-probe micromanipulation, we demonstrate optical launching of single nanowires by evanescent coupling, with coupling efficiency up to 90% for dielectric nanowires and 80% for plasmonic nanowires. Third, we discuss the waveguiding properties of nanowires and emphasize their outstanding capability of waveguiding tightly confined optical fields with high fractional evanescent fields. In addition, we briefly show a balance between the loss, confinement, and bandwidth in a waveguiding nanowire. Fourthly, we present promising approaches to single-nanowire optical sensors. By measuring optical absorption or spectral transmission of a nanowire and activating nanowires with sensitive dopants, we demonstrate a single-nanowire optical sensor with high sensitivity, fast response, and low optical power. This may lead to a novel platform for optical sensing at nanoscale. Finally, we conclude with an outlook for future challenges in the light manipulation and sensing applications of photonic nanowires.

Nanostructuring thin polymer films with optical near fields

In the present work, we report on the application of optical near fields to nanostructuring of poly(trimethylene terephthalate) (PTT) thin films. By exposure to a single ultraviolet nanosecond laser pulse, the spatial intensity modulation of the near-field distribution created by a silica microsphere is imprinted into the films. Setting different angles of incidence of the laser, elliptical or circular periodic ring patterns can be produced with periods as small as half the laser wavelength used. These highly complex patterns show optical and topographical contrast and can be characterized by optical microscopy (OM) and atomic force microscopy (AFM). We demonstrate the key role of the laser wavelength and coherence length in achieving smooth, extended patterns in PTT by using excimer laser (193 nm) and Nd:YAG laser (266 nm) pulses. Reference experiments performed in Ge2Sb2Te5 (GST) demonstrate that nanopatterning in PTT is triggered by ablation as opposed to GST, in which nanopatterning originates from laser-induced phase change, accompanied by a small topographical contrast. The experiments presented in this work demonstrate the suitability of optical near fields for structuring polymer films, opening up new possibilities for nanopatterning and paving the way for potential applications where optical near fields and polymer nanostructures are involved.

Wavelength-converted wave-guiding in dye-doped polymer nanofibers

Nanoscale wavelength-converted optical components are promising components for communication and optical information processing in integrated photonic system. In this work, we report a facile strategy for realizing continuously tunable wavelength-converted wave-guiding in dye-doped nanofibers. The nanofibers with diameters of 200–800 nm have an absorption coefficient of about 80 cm⁻¹ and a self-absorption coefficient of about 30 cm⁻¹, and exhibit relatively high PL efficiency and high photobleaching resistance under an optical pump. By launching the pump light into the nanofibers, the excited light in the nanofibers got self-absorption and reemitted at a longer wavelength, resulting in a gradual wavelength conversion during propagation. On the basis of this wavelength-converted wave-guiding, nanoscale wavelength-converted splitters were demonstrated by assembling the nanofibers into crossed structures. We believe that the dye-doped nanofibers would bring new exciting opportunities in developing new wavelength-converted optical components for nanophotonic device integration.

Crystallization under one-dimensional confinement in alumina nanopores of poly(trimethylene terephthalate) and its composites with single wall carbon nanotubes

We report the preparation of semicrystalline polymer nanorods of PTT and of its nanocomposites with SWCNTs by infiltration of the molten polymer into disordered anodic alumina membranes. An accurate study of the crystalline orientation of these systems has been accomplished by means of X-ray microdiffraction. While polymer residual film exhibits isotropic character, edge-on lamellae are formed upon approaching the polymer/membrane interface. This effect might be due to the elongational flow that takes place in the molten state as polymer chains infiltrate the AAO membrane. At the interface, edge-on and flat-on crystalline lamellae coexist as a consequence of the strong interaction between the polymer and the AAO surface. Inside the nanopores, the confined environment induces a kinetic selection of polymer crystals which only allows the growth of crystalline lamellae with its a-axis parallel to that of the pore. In the case of PTT/SWCNT nanocomposites, this effect, in conjunction with the strong interaction between polymer and AAO surface, seems to prevail over the templating effect of the carbon nanotubes and a similar orientation to that of the neat PTT case is observed.

Wavelength-converted/selective waveguiding based on composition-graded semiconductor nanowires

Compact wavelength-sensitive optical components are desirable for optical information processing and communication in photonic integrated system. In this work, optical waveguiding along single composition-graded CdS(x)Se(1-x) nanowires were systematically investigated. Under a focused laser excitation, the excited light can be guided passively along the bandgap-increased direction of the nanowire, keeping the photonic energy of the guided light almost unchanged during the whole propagation. In comparison, the excited light is guided actively through incessantly repeated band-to-band reabsorption and re-emitting processes along the bandgap-decreased direction, resulting in a gradual wavelength conversion during propagation. On the basis of this wavelength-converted waveguiding, a concept of nanoscale wavelength splitter is demonstrated by assembling a graded nanowire with several composition-uniform nanowires into branched nanowire structure. Our study indicates that composition-graded semiconductor nanowires would open new exciting opportunities in developing new wavelength-sensitive optical components for integrated nanophotonic devices.

Stimulus-responsive light coupling and modulation with nanofiber waveguide junctions

We report a systematic study of light coupling at junctions of overlapping SnO(2) nanofiber waveguides (WGs) as a function of gap separation and guided wavelength. The junctions were assembled on silica substrates using micromanipulation techniques and the gap separation was controlled by depositing thin self-assembled polyelectrolyte coatings at the fiber junctions. We demonstrate that the coupling efficiency is strongly dependent on the gap separation, showing strong fluctuations (0.1 dB/nm) in the power transfer when the separation between nanofibers changes by as little as 2 nm. Experimental results correlate well with numerical simulations using three-dimensional finite-difference time-domain techniques. To demonstrate the feasibility of using coupled nanofiber WGs to modulate light, we encased the junctions in an environment-responsive matrix and exposed the junctions to gaseous vapor. The nanofiber junctions show an ~95% (or ~80%) modulation of the guided 450 nm (or 510 nm) light upon interaction with the gaseous molecules. The results reveal a unique nanofiber-based sensing scheme that does not require a change in the refractive index to detect stimuli, suggesting these structures could play important roles in localized sensing devices including force-based measurements or novel chemically induced light modulators.

Optical properties of quantum-dot-decorated polymer nanofibers

We report a one-step process for decorating poly(trimethylene terephthalate) (PTT) nanofibers with CdSe/ZnS core/shell quantum dots (QDs). Using the QD-decorated PTT nanofibers with diameters of 400-800 nm as active subwavelength waveguides, their high photostability for 630 nm red light and low absorption coefficient down to 2.6 cm( - 1) were characterized by both evanescent waveguiding excitation and irradiation excitation. Compared with the irradiation excitation, a 200 times enhancement was obtained from the active subwavelength waveguides under the evanescent waveguiding excitation.

All-optical full-color displays using polymer nanofibers

We report a number of crossed nanofiber structures for full-color micro/nanodisplays, which were formed by assembling flexible poly(trimethylene terephthalate) (PTT) nanofibers under an optical microscope with the assistance of micromanipulators. The color pixels of the displays consist of micro/nanometer sized color spots in a radius of 300-1500 nm, which were realized through crossed junctions of the PTT nanofibers. The colors of the spots were tuned by changing the power ratios of the launched red, green, and blue lights. We further present a new way to develop white light illumination by combination of red, green, and blue lights with assembly techniques and low production costs.

Tunable refractive index sensor with ultracompact structure twisted by poly(trimethylene terephthalate) nanowires

We report a tunable refractive index sensor with ultracompact structure in a 2 x 2 poly(trimethylene terephthalate) (PTT) nanowire coupling splitter assembled by twisting two flexible PTT nanowires. The sensor consists of two input branches, a twisted coupling region, and two output branches. The changes of optical power caused due to variations in the surrounding medium around the twisted coupling region were measured in the output branches. The highest measured sensitivity of the sensor is 26.96 mW/RIU (refractive index unit) and the maximum detection limit on refractive index change is 1.85 x 10(-7). The average tunabilities observed are 1.2 mW per RIU per twisted turn and 1.8 mW per RIU per 5 degrees branching angle change.

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.

Wavelength, cross-angle, and core-diameter dependence of coupling efficiency in nanowire evanescent wave coupling

We report a systematic study on the dependence of nanowire evanescent wave coupling efficiency on wavelength, cross angle, and core diameter. An 800 nm diameter silica taper was used for light launching. Three poly(trimethylene terephthalate) nanowires with 640 nm, 790 nm, and 950 nm diameters were used for light transmission. Five wavelengths (visible, 473, 532, 650 nm; near-IR, 1,310 and 1,550 nm) were used as transmission signals. The dependence of coupling efficiency was investigated at cross angles from 90 degrees to 0 degrees with a 5 degrees changing step. It is observed that the coupling efficiency is higher for longer wavelength of light, smaller cross angle, and smaller nanowire diameter.

Low-power all-optical switch based on the bend effect of a nm fiber taper driven by outgoing light

We demonstrate a very simple all-optical switch (AOS) based on the bend effect of a nm fiber taper driven by outgoing light. The AOS operates at relative low control power compared with those of using nonlinear effect. In our configuration, we find that the turnoff and turnon time of the AOS are about 500 ms and 760 ms, respectively. The optimized extinction ratio of about 15 db under 32 mW control light at 980 nm is achieved. This AOS effect will be very useful for creation of different interesting functional devices.