End Facet Research Articles

Microrefractometer Based on Off-Center Polymer Caps Bonded Onto Optical Fiber Tips

In this study, we report on a microscopic refractometer built with off-center polymer spherical caps bonded on the end facet of a standard single-mode fiber (SMF). To achieve such microcaps, sub nanoliter amounts of UV-curable polymer were deposited onto the tip of SMFs that were cleaved with small angles. The working mechanism of our devices is explained as the interference of two Gaussian beams. High-contrast (visibility) interference patterns were achieved by optimizing the size of the polymer microcaps. The microrefractometers introduced here can operate over a broad wavelength range from 800 to 1600 nm approximately. The measuring refractive index range of our devices goes from 1 up to the index of the polymer. The refractive index sensitivity is high (∼10−4) over the whole measuring range. Temperature and refractive index cause two distinct effects on the interference patterns. Thus, our microrefractometers can be temperature self-compensated. Owing to the microscopic size of the refractometers, they can be ideal candidates to monitor refractive index in small spaces such as microfluidic channels. Other microsensors can be developed with suitable polymers or other materials that can be bonded on optical fiber tips.

Opto-mechanical lab-on-fibre seismic sensors detected the Norcia earthquake

We have designed and developed lab-on-fibre seismic sensors containing a micro-opto-mechanical cavity on the fibre tip. The mechanical cavity is designed as a double cantilever suspended on the fibre end facet and connected to a proof mass to tune its response. Ground acceleration leads to displacement of the cavity length, which in turn can be remotely detected using an interferometric interrogation technique. After the sensors characterization, an experimental validation was conducted at the Italian National Institute of Geophysics and Volcanology (INGV), which is responsible for seismic surveillance over the Italian country. The fabricated sensors have been continuously used for long periods to demonstrate their effectiveness as seismic accelerometer sensors. During the tests, fibre optic seismic accelerometers clearly detected the seismic sequence that culminated in the severe Mw6.5 Norcia earthquake that struck central Italy on October 30, 2016. The seismic data provided by the optical sensors were analysed by specialists at the INGV. The wave traces were compared with state-of-the-art traditional sensors typically incorporated into the INGV seismic networks. The comparison verifies the high fidelity of the optical sensors in seismic wave detection, indicating their suitability for a novel class of seismic sensors to be employed in practical scenarios.

Polarization nondegenerate fiber Fabry-Perot cavities with large tunable splittings

We demonstrate a type of microcavity with large tunable splitting of polarization modes. This polarization nondegenerate cavity consists of two ellipsoidal concave mirrors with controllable eccentricity by CO2 laser machining on fiber end facets. The experiment shows that the cavities can combine the advantages of high finesse above 104 and large tunable polarization mode splitting to the GHz range. As the splitting of the cavity can be finely controlled to match atom hyperfine levels or optomechanics phonons, it will blaze a way in experiments on cavity quantum electrodynamics and cavity optomechanics.

Compact refractive index sensor based on an S-tapered fiber probe

We have demonstrated a refractive index (RI) sensor based on an S-tapered fiber probe (STFP) with a silver mirror on its end facet. The fiber probe has a compact size of about 1 mm, making it proper for sensing in narrow or limited space. The reflection spectra of the STFPs with different structure lengths have been analyzed and simulated. Its RI sensitivity in the range of 1.332 ~1.387 reaches 268.8 nm/RI unit, which is 9 times higher than that of the existing reflective fiber-taper-based RI sensor. Furthermore, the thermal response experimental results show that the STFP is temperature insensitive.

Ultrasound detection at fiber end-facets with surface plasmon resonance cavities.

Ultrasound detection is performed by measuring laser reflection off a surface plasmon resonance cavity which is integrated at a single-mode fiber end facet. It shows a total noise (or laser RIN) equivalent pressure of 9.7KPa (or 5.2KPa) over 0-20MHz, a 6-dB angular detection range as large as 70° near 10MHz, a detection bandwidth larger than 125MHz, and a stable performance for over 20min without feedback in ambient conditions. Its small form factor, fiber-optic integration, almost omnidirectional responsivity and large bandwidth are favorable for in vivo applications and high resolution imaging.

Bismuth Telluride nanocrystal: broadband nonlinear response and its application in ultrafast photonics

We come up with a hybrid liquid exfoliation method to prepare bismuth telluride nanocrystals efficiently and cost-effectively. The nonlinear transmittance of the nanocrystals has been characterized with Z-scan technique, which can manifest its broadband saturable absorption behavior experimentally. The as-fabricated nanocrystals were integrated onto fiber end facet to form a fiber compatible nonlinear absorption device with optical deposition method, which was then used to modulate the fiber laser with different cavity configurations to deliver pulsed laser successfully. The noise-like pulse and dissipative soliton have been obtained with wavelength centered at 1562 nm and 1068 nm, respectively. These results confirm the effectiveness of the hybrid liquid exfoliation method to prepare bismuth telluride into nanocrystals, and the broadband nonlinear optical response and ultrafast photonics application potential of the nanocrystals.

Ionic self-assembly of bundles of ultralong SC/MB nanobelts with enhanced electrocatalytic activity for detection of ascorbic acid

The formation of bundles of ultralong nanobelts by an anion biological surfactant sodium cholate (SC) and a cationic dye (MB) through ionic self-assembly approach was obtained. The nanobelts possess smooth surfaces, flat end facets and solid internal structure. This one dimensional self-assembly is dominated by the π–π stacking interactions between MB molecules in concert with the hydrogen bonding between SC molecules. The shape and length of the bundles of SC/MB nanobelts could be easily controlled by changing the SC concentration and the aging temperature. Moreover, the electrocatalytic properties of the SC/MB nanobelts modified electrode were also investigated and the results indicated that the bundles of ultralong SC/MB nanobelts exhibited efficient electrocatalytic activity towards l-ascorbic acid (AA) in phosphate buffer solution (pH = 7.0). In addition, the electrostatic interaction between AA and MB also facilitates the electrons transfer on the surface of the GCE and promotes the oxidation of AA. The present work provides an alternative way to design and fabricate the ultralong 1D nanobelt structures with tunable sizes using small organic molecules. This system may also open up a way for the design and development of optical and electronic devices in the potential bio-applications and electrocatalyst for fuel cells.

4?×?20 GHz silica-based AWG hybrid integrated receiver optical sub-assemblies

Both the 4×20 GHz coarse wavelength division multiplexing and LAN-WDM receiver optical sub-assemblies (ROSAs) were developed. The ROSA package was hybrid integrated with a planar lightwave circuit arrayed waveguide grating (AWG) with 2% refractive index difference and a four-channel top-illuminated positive-intrinsic-negative photodetector (PD) array. The output waveguides of the AWG were designed in a multimode structure to provide flat-top optical spectra, and their end facet was angle-polished to form a total internal reflection interface to realize vertical coupling with a PD array. The maximum responsivity of ROSA was about 0.4 A/W, and its 3 dB bandwidth of frequency response was up to 20 GHz for each transmission lane. The hybrid integrated ROSA would be a cost-effective and easy-assembling solution for 100 GbE data center interconnections.

Facile fabrication of porous ZnS nanostructures with a controlled amount of S vacancies for enhanced photocatalytic performances.

ZnS nanostructures of barbell-shaped porous and hollow nanoplates with a controlled amount of S vacancies have been facilely fabricated via the hydrothermal treatment of ZnS(en)0.5 (en = ethylenediamine) nanoplates. The amount of S vacancies as well as the morphologies of ZnS nanostructures have been controlled by adjusting the hydrolysis time; the layered structure of ZnS(en)0.5 nanoplates decomposes to yield discrete ZnS nanoparticles at two end facets of template nanoplates, producing barbell-shaped porous and hollow ZnS nanoplates with abundant S vacancies finally. The photocatalytic activity of ZnS nanostructures prepared via hydrolysis for 4 h is 8.2 times higher than that of commercial ZnS. The photocatalytic activity of ZnS nanostructures increases with the increase of emission at 390 nm arising from sulfur vacancies, suggesting that the high photocatalytic efficiency of ZnS nanostructures results mainly from the high amount of sulfur vacancies. Surface defects such as sulfur vacancies can trap photogenerated electrons to block the recombination of charges, enhancing the photocatalytic efficiency of the as-prepared ZnS nanostructures. It has also been found that both ˙OH and ˙O2- act as the major reactive species in the photocatalytic decomposition of rhodamine B via our prepared ZnS nanostructures. Barbell-shaped porous and hollow ZnS nanoplates are suggested to have great applicability to photocatalysts in waste-water treatment.

Compact Biocompatible Fiber Optic Temperature Microprobe Using DNA-Based Biopolymer

A highly compact biocompatible microprobe type fiber optic temperature sensor was experimentally demonstrated utilizing an inherently high thermo-optic coefficient of DNA biopolymer. The sensor was based on an all-fiber multimode interferometer (MMI) along a coreless silica fiber (CSF) spliced to an end of a single mode fiber. Au film was deposited the CSF end facet to provide a double path for MMI and it also worked as a probe terminal. The circumferential area of CSF was coated with DNA-cetyltrimethylammonium chloride (CTMA) thin solid film, which served as a temperature sensing head. We experimentally investigated thermo-optical properties of DNA-CTMA thin solid films to find its large negative thermo-optical coefficient −4.15 × 10−4/°C in the temperature range from 20 to 70 °C. DNA-CTMA coated fiber optic probe was immersed in a water bath to simulate the bio compatible environment whose temperature was varied in the range from 30 to 70 °C. The proposed sensor showed a high-temperature sensitivity of −0.22 nm/°C in the spectral shifts, and 0.085 dB/°C in the reflected optical power changes. The proposed probe can be readily applied in various types of in vivo point of care temperature monitoring.

INVITED] Nanofabrication of phase-shifted Bragg gratings on the end facet of multimode fiber towards development of optical filters and sensors

This work describes the process of nanofabrication of phase-shifted Bragg gratings on the end facet of a multimode optical fiber with a pulsed DC sputtering system based on a single target. Several structures have been explored as a function of parameters such as the number of layers or the phase-shift. The experimental results, corroborated with simulations based on plane-wave propagation in a stack of homogeneous layers, indicate that the phase-shift can be controlled with a high degree of accuracy. The device could be used both in communications, as a filter, or in the sensors domain. As an example of application, a humidity sensor with wavelength shifts of 12 nm in the range of 30 to 90% relative humidity (200 pm/% relative humidity) is presented.

Iodide-Switched Deposition for the Synthesis of Segmented Pd-Au-Pd Nanorods: Crystal Facet Matters.

Segmented metallic nanorods with well-defined shapes and controllable components play an important role on the systematic investigation of their shape-dependent catalytic, electric, and plasmonic properties of metal nanostructures. Unfortunately, the shape and composition of segmented nanorods are difficult to be precisely controlled via colloidal methods. Here, we reported the growth of Pd-Au-Pd bimetallic heterostructures by using Au 5-fold twinned bipyramids (BPs) as seeds, with KI as a structure-directing reagent. Through a series of control experiments we revealed that two parameters were identified as critical factors for the growth of segmented Pd-Au-Pd nanorods. First, 5-fold twinned Au BPs with low-index end facets and high-index side facets function as a unique template for directed growth. Second, iodide can switch the deposition of Pd on the Au BPs. A high concentration of iodide is believed to block the high-index facets of the Au BPs and lower the reaction kinetics to promote the selective growth of two Pd segments on the Au BPs. As a result, uniformed segmented Pd-Au-Pd nanorods were obtained. The segmented nanorods exhibit intense extinction in the near-IR range and could be a potential candidate for plasmon-based biological applications such as thermal therapy.

Investigating surface effects of GaN nanowires using confocal microscopy at below-band gap excitation

Abstract

Robust Cesium Lead Halide Perovskite Microcubes for Frequency Upconversion Lasing

AbstractHalide perovskite nanomaterials have recently attracted a lot of attention in the nanoscale laser research field, especially two‐photon pumped lasing in halide perovskite nanomaterials has been considered as an ideal alternative strategy to achieve frequency upconversion. However, the poor stability of current organic–inorganic lead halide perovskite materials hinder their further practical applications. Herein, facile solution‐processed cesium lead halide perovskite CsPbX3 (X = Br, I, or Cl) microcubes with low‐threshold lasing, high quality, enhanced stability, and excellent wavelength tunability are reported. These as‐prepared CsPbX3 microcubes display excellent structure stability under ambient conditions for several months and they are found to be more robust than their organic–inorganic counterparts. The smooth end facets and wavelength‐comparable dimensions make these microcubes promising for high‐quality laser cavities in three dimensions. Fabry–Perot lasing is demonstrated in CsPbX3 microcubes, the process of which is investigated by dynamic emission. In addition, tunable amplified spontaneous emission is achieved with low threshold under both one‐ and two‐photon excitation, which can maintain a stable emission for over 10 hours under continuous intense laser shots in ambient atmosphere. The findings suggest that solution‐processed all‐inorganic perovskite microcubes can be used as excellent gain medium for frequency upconversion lasers, which would offer a new platform for nonlinear photoelectric devices.

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures.

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

High-Sensitivity Gas-Pressure Sensor Based on Fiber-Tip PVC Diaphragm Fabry–Pérot Interferometer

We demonstrate a novel polyvinyl chloride (PVC) diaphragm-based fiber-tip Fabry–Perot interferometer for gas-pressure measurements with ultrahigh sensitivity. The PVC diaphragm has been coated to the end facet of a well-cut standard single-mode fiber by use of a plastic welder. An ultrahigh-pressure sensitivity of ∼65.5 nm/MPa at 1565 nm and a low-temperature cross sensitivity of ∼–5.5 kPa/°C have been experimentally demonstrated. The proposed sensor has advantages of high pressure sensitivity, miniature size, low cost, and easy fabrication.

Humidity Sensor Based on Bragg Gratings Developed on the End Facet of an Optical Fiber by Sputtering of One Single Material.

The refractive index of sputtered indium oxide nanocoatings has been altered just by changing the sputtering parameters, such as pressure. These induced changes have been exploited for the generation of a grating on the end facet of an optical fiber towards the development of wavelength-modulated optical fiber humidity sensors. A theoretical analysis has also been performed in order to study the different parameters involved in the fabrication of this optical structure and how they would affect the sensitivity of these devices. Experimental and theoretical results are in good agreement. A sensitivity of 150 pm/%RH was obtained for relative humidity changes from 20% to 60%. This kind of humidity sensors shows a maximum hysteresis of 1.3% relative humidity.

Compact silicon-based optrode with integrated laser diode chips, SU-8 waveguides and platinum electrodes for optogenetic applications

We report on a compact optrode, i.e. a MEMS-based, invasive, bidirectional neural interface allowing to control neural activity using light while neural signals are recorded nearby. The optrode consists of a silicon (Si) base carrying two pairs of bare laser diodes (LDs) emitting at 650 nm and of two 8 mm-long, 250 µm-wide and down to 50 µm-thick shanks extending from the base. Each LD is efficiently coupled to one of four 15 or 20 µm-wide and 13 µm-high SU-8 waveguides (WGs) running in pairs along the shanks. In addition, each shank comprises four 20 µm-diameter platinum electrodes for neural recording near the WG end facets. After encapsulation of the LDs with a Si cover chip blocking stray light and protecting the LDs from the harsh environment to which the probe is destined, the compact base measures only 4 × 4 × 0.43 mm3. The time averaged radiant emittance at the WG end facet is 96.9 mW mm−2 for an LD current of 35 mA at a duty cycle of 5%. The absolute electrode impedance at 1 kHz is 1.54 ± 0.06 MΩ. Using infrared thermography, the temperature increase of the probe during LD operation was determined to be about 1 K under neuroscientifically relevant operating conditions.

Second-order distributed-feedback surface plasmon resonator for single-mode fiber end-facet biosensing

Integrating surface plasmon resonance (SPR) devices upon single-mode fiber (SMF) end facets renders label-free biosensing systems that have a dip-and-read configuration, high compatibility with fiber-optic techniques, and in vivo monitoring capability, which however meets the challenge to match the performance of free-space counterparts. We report a second-order distributed feedback (DFB) SPR cavity on an SMF end facet and its application in protein interaction analysis. In our device, a periodic array of nanoslits in a gold film is used to couple fiber guided lightwaves to surface plasmon polaritons (SPPs) with its first order spatial Fourier component, while the second order spatial Fourier component provides DFB to SPP propagation and produces an SPP bandgap. A phase shift section in the DFB structure introduces an SPR defect state within the SPP bandgap, whose mode profile is optimized to match that of the SMF to achieve a reasonable coupling efficiency. We report an experimental refractive index sensitivity of 628 nm RIU−1, a figure-of-merit of 80 RIU−1, and a limit of detection of 7 × 10−6 RIU. The measurement of the real-time interaction between human immunoglobulin G molecules and their antibodies is demonstrated.