Evanescent Field Coupling Research Articles

Generation of mode-locked pulses assisted by reduced graphene oxide/cerium oxide (rGO/CeO2) embedded in arc-shaped fiber

Abstract This study demonstrates the synthesis of a reduced graphene oxide/cerium oxide (rGO/CeO2) composite and its successful use as a saturable absorber (SA) in ultrafast photonics. The rGO/CeO2 composite was synthesized using a facile hydrothermal approach, and an arc-shaped fiber was fabricated using a polishing technique. The arc-shaped fiber employed indirect evanescent field coupling, allowing interaction between the composite material and the laser, which facilitated the generation of mode-locked pulses. An arc-shaped fiber drop-casted with rGO/CeO2 was connected to a 2.0 µm laser source, achieving a modulation depth of 50.16%. The emitted laser pulses have a duration of 1.776 ps, an operational wavelength of 1904 nm, and a spectral bandwidth (FWHM) of 2.76 nm. The pulse energy was 89.36 pJ with a high signal-to-noise ratio (SNR) of 63 dB. This study introduces rGO/CeO2 embedded in an arc-shaped fiber as an effective SA, exhibiting favorable nonlinear optical absorption in the near 2.0 µm wavelength region, making it suitable for applications in ultrafast photonics.

A miniaturized tunable optical attenuator with ultrawide bandwidth based on evanescent-field coupling between nanofibers

We propose a novel miniaturized optical attenuator based on the evanescent-field coupling between two nanofibers. Benefiting from a wavelength-dependent waveguiding property of the coupled structure, a tunable attenuation with maximum extinction ratio of ~ 20 dB is demonstrated with an ultrawide optical bandwidth up to 0.7 μm in experiment. The wavelength-depended waveguiding properties of both one single nanofiber and coupled nanofibers are investigated in both theory and simulation. Using an adiabatic coupling structure, an attenuation range from − 0.16 dB to − 18.46 dB is obtained within a spectrum from 1.2 μm to 1.7 μm experimentally. Moreover, the simulated results indicate that, the attenuation only shows a slight change of ~ 0.1 dB with a lateral misalignment of 0.5 μm between the two nanofibers, indicating a high tolerance of this attenuator to the lateral misalignment. Considering the wide bandwidth as well as the ultracompact structure, this attenuator shows high potential in applications such as all-fiber optical sensing and communicating.

Broadband mode-division (de)multiplexer using nanorod-assisted multimode subwavelength gratings

—Subwavelength grating (SWG) allows flexible tuning of the dispersion of waveguides, enabling wider bandwidths for directional coupler-based devices. However, its light field constraint capability for high-order modes is so weak that the mode scattering loss from sidewalls is high, which impedes low-loss transmission and efficient energy coupling from adjacent waveguides. In this paper, we propose nanorod-assisted multimode subwavelength grating (NSWG) waveguides that support stable high-order modes transmission like strip waveguides, and maintain refractive index tunability similar to SWGs. We fully analyze the mode transmission and coupling characteristics of NSWG waveguides for modes TE1, TE2, and TE3. Results show that such waveguides can hold high transmission fidelity and low losses for high-order modes (<0.33 dB/cm, 0.58 dB/cm, and 1.0 dB/cm for TE1, TE2, and TE3). Arbitrary high-order modes among NSWGs can also be excited efficiently by evanescent field coupling. As proofs of concept, we also demonstrate a three-channel mode-division multiplexer with broad bandwidth (136 nm/135 nm/100 nm), low insertion loss (0.29 dB/0.85 dB/0.62 dB), and crosstalk (-21.4 dB/-20.4 dB/-18.0 dB) for TE0/TE1/TE2. And the device has a compact footprint of 5 × 61 μm. At 1.55 μm, all three modes satisfy IL < 0.19 dB and CT < -20.0 dB. The proposed waveguide structures may provide an alternative building block for broadband and compact multimode on-chip directional coupler-based devices.

Wavelength-tunable broadband lasers based on nanomaterials.

Nanomaterials are widely used in the fields of sensors, optoelectronics, biophotonics and ultrafast photonics due to their excellent mechanical, thermal, optical, electrical and magnetic properties. Particularly, owing to their nonlinear optical properties, fast response time and broadband operation, nanomaterials are ideal saturable absorption materials in ultrafast photonics, which contribute to the improvement of laser performance. Therefore, nanomaterials are of great importance to applications in wavelength-tunable broadband pulsed lasers. Herein, we review the integration and applications of nanomaterials in wavelength-tunable broadband ultrafast photonics. Firstly, the two integration methods, which are direct coupling and evanescent field coupling, and their characteristics are introduced. Secondly, the applications of nanomaterials in wavelength-tunable broadband lasers are summarized. Finally, the development of nanomaterials and broadband tunable lasers is reviewed and discussed.

Two-photon 3D printing diaphragm-integrated ring waveguide coupler for ultrasound detection.

We demonstrate a diaphragm-integrated ring waveguide coupler fabricated by the two-photon direct laser wring technique as an ultrasonic sensor, which is integrated on an optical fiber tip. The device consists of a micro-ring waveguide with a diameter of 5 µm functionalized as an optical fiber tip light reflection mirror and a straight waveguide connecting a diaphragm. The evanescent field coupling can be realized between the two waveguides, and the coupling efficiency can be changed due to the variation of the coupling gap induced by ultrasound. Accordingly, the light reflection can be changed. Based on the plate vibration theory, the vibration frequency can be changed through optimizing the diaphragm size. The experiments show that the device exhibits a high sensitivity and low noise equivalent acoustic signal level of 1.07 mPa/Hz1/2 at 100 kHz, which has great potential in various acoustic wave sensing applications.

Surface-enhanced Raman spectroscopy long-range detection investigation based on the alternating coupling of a surface plasmonic array and dielectric waveguide.

Surface-enhanced Raman spectroscopy (SERS) is widely used to detect low-concentration samples in biology, medicine, etc. We design and theoretically investigate a SERS sensor with a surface plasmonic array coupled alternately with a dielectric waveguide. The effect of the incident angle on the coupling efficiency of an evanescent field is systematically studied. The results show that the maximum evanescent field coupling efficiency can be obtained at an incident angle of 66°. The proposed SERS sensor has a transmission length of 1.027cm and a high enhancement performance with an enhancement factor of 1.574×104 at a wavelength of 633nm. The integration of this SERS sensor with a metal array and a dielectric waveguide prevents the direct illumination of the sample molecules by the excited light. Furthermore, the long-range nondestructive detection of the SERS signals of the low-concentration sample molecules can be achieved.

Spider Silk as a Flexible Light Waveguide for Temperature Sensing

We propose an approach to utilize the natural spider dragline silk (SDS) as a flexible light waveguide integrated into an optic device. SDS is utilized as an air-clad biocompatible light waveguide in the fabricated multimode interferometer. Based on the evanescent field coupling theory, the light propagating in the microfiber can be coupled into SDS, and then coupled to another microfiber connected with a spectrometer. The difference in the effective refractive index of the modes propagating in the silk fiber changes in different external environments, thus the output multimode interference spectrum shifts with the change of environment. Accordingly, the multimode interferometer is encapsulated in polydimethylsiloxane (PDMS), a kind of flexible and temperature-sensitive material, which makes the interferometer have competitive temperature sensitivity. With the increase in temperature, the refractive index of PDMS decreases, and thus the output multimode interference spectrum blue shifts. The interferometer shows repeatability, reversibility and stability in temperature sensing, with a sensitivity of −1.15 nm/°C. The presented interferometer proves that the SDS is a natural independent light guide with good mechanical properties to be applied in the fabrication of integrated optic devices.

Ultrahigh Quality Microlasers from Controlled Self‐Assembly of Ultrathin Colloidal Semiconductor Quantum Wells

AbstractColloidal quantum wells (CQWs) have emerged as a promising class of gain material in various optical feedback configurations. This is due to their unique excitonic features arising from their 1D quantum confinement. However, existing methods for integrating CQW onto microresonators will cause low laser quality due to uneven CQW coating. To overcome this, the use of liquid‐interface kinetically driven self‐assembly is proposed to coat ultrathin, close‐packed layers of colloidal CdSe/Cd1−xZnxS core/shell CQWs between 7 and 14 nm onto the surface of silica microsphere cavities. The fabricated CQW‐whispering‐gallery‐mode microlasers possess a commendable high quality (Q) factor of 13 000 at room temperature. Stable single‐mode lasing output is demonstrated through evanescent field coupling between a CQW‐coated microsphere and a thin uncoated microfiber in a 2D‐3D microcavity configuration. These promising results highlight the suitability of the liquid‐interface kinetically driven self‐assembly method for realizing ultrathin CQW‐coated microlasers and its high compatibility for integrating colloidal nanocrystals onto complex 3D microstructures for future miniaturized colloidal optoelectronic and photonic applications.

Analysis of power transfer between two multi-core fibers with long-period gratings

Here, we numerically propose and demonstrate a technique to couple light between two multi-core fibers (MCFs) using long-period gratings (LPGs). The light is coupled from one core of the input MCF to all cores of the output MCF. For that, an LPG is inscribed in the input core of the input MCF and identical LPGs are inscribed in all cores of the output MCF. First, the light is launched into the input core of the MCF and the optical power is transferred to the cladding due to the LPG inscribed in that core. The optical power in the cladding is then transferred to the other MCF cladding by evanescent field coupling. The optical power in the cladding of the output MCF is distributed by all its cores due to the identical LPGs inscribed in them. We optimized the LPGs period, their lengths and offset distance to increase the power transfer at 1480 nm. We achieved a power transfer of 92% of the input power, distributed by all MCF cores, in 10.6 cm of length. We also studied the power transfer sensitivity to the LPGs period.

Direct Observation of Self-Hybridized Exciton-Polaritons and Their Valley Polarizations in a Bare WS2 Layer.

The strong excitonic properties of transition metal dichalcogenides (TMD) have led to the successful demonstration of exciton-polaritons (EPs) in various optical cavity structures. Recently, self-hybridized EPs have been discovered in a bare TMD layer, but experimental investigation is still lacking because of their nonradiative nature. Herein, the direct observation of self-hybridized EPs in a bare multilayer WS2 via the evanescent field coupling technique is reported. Because of the thickness-dependent Rabi splitting energy, the dispersion curves of the EPs change sensitively with sample thickness. Moreover, continuous tuning of EP dispersion curves is demonstrated by controlling the excitation laser power. Lastly, it is observed that guided EPs retain valley polarization up to 0.2 at room temperature, representing a valley-preserved strong coupling regime. It is believed that the high tunability and valley polarization properties of the guided EPs in bare TMD layers can facilitate new nanophotonic and valleytronic applications.

A Universal Pick‐and‐Place Assembly for Nanowires

With the introduction of techniques to grow highly functional nanowires of exotic materials and demonstrations of their potential in new applications, techniques for depositing nanowires on functional platforms have been an area of active interest. However, difficulties in handling individual nanowires with high accuracy and reliability have so far been a limiting factor in large-scale integration of high-quality nanowires. Here, a technique is demonstrated to transfer single nanowires reliably on virtually any platform, under ambient conditions. Functional nanowires of InP, AlGaAs, and GeTe on various patterned structures such as electrodes, nanophotonic devices, and even ultrathin transmission electron microscopy (TEM) membranes are transferred. It is shown that the versatility of this technique further enables to perform on-chip nano-optomechanical measurements of an InP nanowire for the first time via evanescent field coupling. Thus, this technique facilitates effortless integration of single nanowires into applications that werepreviously seen as cumbersome or even impractical, spanning a wide range from TEM studies to in situ electrical, optical, and mechanical characterization.

Interlayer Slope Waveguide Coupler for Multilayer Chalcogenide Photonics

The interlayer coupler is one of the critical building blocks for optical interconnect based on multilayer photonic integration to realize light coupling between stacked optical waveguides. However, commonly used coupling strategies, such as evanescent field coupling, usually require a close distance, which could cause undesired interlayer crosstalk. This work presents a novel interlayer slope waveguide coupler based on a multilayer chalcogenide glass photonic platform, enabling light to be directly guided from one layer to another with a large interlayer gap (1 µm), a small footprint (6 × 1 × 0.8 µm3), low propagation loss (0.2 dB at 1520 nm), low device processing temperature, and a high bandwidth, similar to that in a straight waveguide. The proposed interlayer slope waveguide coupler could further promote the development of advanced multilayer integration in 3D optical communications systems.

Ultralow-Threshold and High-Quality Whispering-Gallery-Mode Lasing from Colloidal Core/Hybrid-Shell Quantum Wells.

The realization of efficient on-chip microlasers with scalable fabrication, ultralow threshold, and stable single-frequency operation is always desired for a wide range of miniaturized photonic systems. Herein, an effective way to fabricate nanostructures- whispering-gallery-mode (WGM) lasers by drop-casting CdSe/CdS@Cd1- x Znx S core/buffer-shell@graded-shell nanoplatelets (NPLs) dispersion onto silica microspheres is presented. Benefiting from the excellent gain properties from the interface engineered core/hybrid shell NPLs and high-quality factor WGM resonator from excellent optical field confinement, the proposed room-temperature NPLs-WGM microlasers show a record-low lasing threshold of 3.26 µJ cm-2 under nanosecond laser pumping among all colloidal NPLs-based lasing demonstrations. The presence of sharp discrete transverse electric- and magnetic-mode spikes, the inversely proportional dependence of the free spectra range on microsphere sizes and the polarization anisotropy of laser output represent the first direct experimental evidence for NPLs-WGM lasing nature, which is verified theoretically by the computed electric-field distribution inside the microcavity. Remarkably, a stable single-mode lasing output with an ultralow lasing threshold of 3.84 µJ cm-2 is achieved by the Vernier effect through evanescent field coupling. The results highlight the significance of interface engineering on the optimization of gain properties of heterostructured nanomaterials and shed light on developing future miniaturized tunable coherent light sources.

Passive near-field optical scanning imaging based on semiconductor nanowire/tapered microfiber probe

&lt;sec&gt;In this paper, we propose a passive near-field scanning imaging system by using the structure of cadmium sulfide (CdS) nanowire/tapered microfiber probe, which combines the near-field scanning structure and the nanowire/microfiber coupling technology. In the passive near-field scanning imaging system, a passive nanoprobe is adopted to detect the intensity change of the reflected light field on the sample surface, which not only retains the advantage of the nanoprobe for the strong restriction of the reflected light on the sample surface, but also reduces the interference of strong excitation light during detection. Through the high efficiently evanescent field coupling between the CdS nanowire and the tapered microfiber, the collected light signal is transmitted to the photodetector in the far field, and finally the imaging of the target sample morphology can be realized.&lt;/sec&gt;&lt;sec&gt;At first, the light field model of the nanowire/tapered microfiber probe structure is verified by the finite element analysis method. The calculated collection efficiency from the sample to the probe is about 4.65‰ and the transmission efficiency from the nanowire to the tapered microfiber is about 74.47%. The collection efficiency is improved by an order of magnitude compared with traditional metal-coated near-field probe. In the experiments, a scanning step of 20 nm and a probe-sample distance of 230 nm are selected. The nanowire/tapered microfiber probe and traditional tapered fiber probe are both used to measure the widths of different CdSe nanoribbons samples, and the atomic force microscopy measurement is used as the benchmark to calculate their measurement error, which is increased about 3 times. By changing the angle &lt;i&gt;θ&lt;/i&gt; between the probe and the sample, it is found that the resolution obtained using the designed nanowire/microfiber probe is always higher than only using the tapered microfiber probe. Comparing with the tapered microfiber probe scheme, the measurement error is reduced to a value less than 7.2%.&lt;/sec&gt;&lt;sec&gt;In addition, compared with the active luminescence probe scheme, this passive near-field scanning scheme reduces the preparation complexity of the optical probe and the detection structure complexity of the optical system. The large microscopic illumination area can avoid the influence of the small laser spot size on imaging, and the imaging range is determined only by the travel distance of the linear stage. Therefore, our work may provide an attractive approach for developing new near-field scanning microscopy systems in the future.&lt;/sec&gt;

Enhanced Evanescent Field Coupling of Smart Particles in Tubular Optical Microcavity for Sensing Application

AbstractWhispering‐gallery mode (WGM) optical microcavity with enhanced evanescent field is one of the most perfect systems to study light–matter interaction. Unique sensing devices and potential applications with excellent performance can be expected by integrating smart particles into the microcavity, such as small mode volume and high sensitivity. Here, it is demonstrated that rolled‐up tubular optical microcavity decorated with palladium (Pd) nanoparticles (NPs) is capable for hydrogen (H2) sensing. Such microtube with subwavelength wall thickness exhibits enhanced sensing performance, e.g., detection limit of 1000 parts per million (ppm) and sensitivity in the order of 10−5 nm ppm−1, due to strong coupling in the evanescent field. Experiments and simulations are compared and guided the optimization for high figures of merit. The approach proves a unique route to couple smart particles with rolled‐up WGM microcavity for gas sensing, which have great application potentials in the field of nanophotonics and smart sensors.

A room temperature all-optical sensor based on two-dimensional SnS2 for highly sensitive and reversible NO2 sensing

Fiber-optic gas sensors have been considered a low-cost, effective, and robust approach for monitoring nitrogen dioxide (NO2) gas which is a major toxic gaseous pollutant. The integration of functional nanoscale materials provides additional dimensions for realizing ultra-sensitive and selective NO2 detection, however, the trade-off is the need for sophisticated photonic structures or external non-optical peripherals (e.g. electrical heaters). In this work, we demonstrate the development of a room temperature, all-optical, and high-performance NO2 sensor based on a simple D-shaped optical fiber incorporated with ultra-thin two-dimensional (2D) tin disulfide (SnS2). A visible light source at 473 nm is used to power the optical fiber, and at the same time excite the 2D SnS2 layer via the evanescent field, to generate extra charge carriers. Upon exposure to NO2 at room temperature, the physisorbed gas molecules induce charge exchange with the 2D SnS2. This significantly re-distributes the photo-excited charge carriers in the ultra-thin material, therefore manipulating the corresponding optical absorption and scattering. As a result, the optical output power intensity varies as the sensor output through the evanescent field coupling. This all-optical sensor demonstrates an optical power variation of up to 7 µW upon the exposure of NO2 gas at a low concentration of 50 ppb. This response is fully reversible with an extremely low limit of detection (LOD) of 0.464 ppb. We consider that this work provides a feasible and simple solution to realize high-performance optical gas sensors without the integration of external non-optical peripherals for effective monitoring of environmentally hazardous gases.

Mode-locked thulium/holmium co-doped fiber laser using WTe2-covered tapered fiber

In this work, a WTe2 coated tapered fiber was used as a saturable absorber (SA) to induce mode-locking in a thulium/holmium doped fiber (THDF) laser through evanescent field interaction. The WTe2 was drop-casted onto the tapered fiber to form an SA which had a modulation depth of 18.08%, saturation intensity of 0.14 MW/cm2, and non-saturable absorption of 81.92%. Mode-locked pulses were obtained at a wavelength of 1909.81 nm with a pulse width of 1.77 ps, and a 3 dB bandwidth of 2.2 nm. It has a signal-to-noise ratio of 52 dB and a repetition rate of 11.54 MHz at a pump power of 436.4 mW. The maximum output power, pulse energy, and peak power obtained were 3.51 mW, 303.9 pJ, and 0.17 kW respectively. The use of evanescent field coupling in the tapered fiber with the WTe2 nanoparticles allows the realization of high-power mode-locking applications.

Chiral Photonic Circuits for Deterministic Spin Transfer

AbstractChiral quantum optics has attracted considerable interest in the field of quantum information science. Exploiting the spin‐polarization properties of quantum emitters and engineering rational photonic nanostructures has made it possible to transform information from spin to path encoding. Here, compact chiral photonic circuits with deterministic circularly polarized chiral routing and beamsplitting are demonstrated using two laterally adjacent waveguides coupled with quantum dots. Chiral routing arises from the electromagnetic field chirality in waveguide, and beamsplitting is obtained via the evanescent field coupling. The spin‐ and position‐dependent directional spontaneous emission are achieved by spatially selective micro‐photoluminescence measurements, with a chiral contrast of up to 0.84 in the chiral photonic circuits. This makes a significant advancement for broadening the application scenarios of chiral quantum optics and developing scalable quantum photonic networks.

740-watt level optical tap coupler using side-polished large-mode-area double clad fibers for a high power fiber laser.

We fabricated a fiber-optic directional coupler based on evanescent field coupling between side-polished large mode area (LMA) double clad fibers (DCFs) for a high power fiber laser. The tapping ratio of the fabricated coupler was measured to be - 32 dB. The fundamental mode coupled in a core of the lower side-polished fiber (SPF) was transferred to the upper SPF without clad-mode coupling. Two SPFs were directly faced to increase an optical handling power up to 740 W. The tapping ratio of the coupler was constantly maintained at the applied laser output. The beam quality of the laser including the fabricated coupler was maintained to be 1.22, without mode distortion by the coupler.

A Whispering-Gallery Mode Microsphere Resonator Based on Optical Fiber With an Open Microcavity

A whispering-gallery mode microsphere resonator based on optical fiber with an open microcavity is demonstrated, which is created by use of femtosecond laser micromachining and fusion splicing techniques. When a microsphere is placed into the open microcavity, whispering-gallery mode can be excited through evanescent field coupling. The device can be constructed in either single-mode fiber or multimode fiber, with the insertion loss of ~13.2 and ~21.0 dB, and the quality factor of 4.81X10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and 2 X10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> respectively. Because of its features of compact structure, low cost, easy fabrication, convenient operation and high durability, the device is expected to have many photonics applications.