Evanescent Coupling Research Articles

A Novel Comb-Like Nonreciprocal Evanescent Wave Filter Based on the 1-D Ternary Magnetized Plasma Photonic Crystals

In this article, the magnetized plasma in 1-D ternary periodic photonic crystals is utilized as a kind of single negative material whose real part of permittivity is negative. Some discrete resonant peaks with high transmittance can be observed in the transmission band due to the evanescent wave coupling between plasma layers and dielectric layers. The nonreciprocal effect can be achieved by injecting counter-propagating TM waves into the proposed structure whose time-reversal symmetry and space-reversal symmetry are broken. The transmission band of nonreciprocal evanescent waves can be modulated dynamically due to the tuning of the period number, the plasma frequency, the plasma cyclotron frequency, the filling factor of plasma, and the incident angle. The simulation results indicate that the proposed structure can achieve 100% transmission and strong nonreciprocity, which can supply multichannel unidirectional filters with theoretical references.

Probing Optical Losses and Dispersion of Fully Guided Waves through Critical Evanescent Coupling

Probing Optical Losses and Dispersion of Fully Guided Waves through Critical Evanescent Coupling

Towards integrated photonic interposers for processing octave-spanning microresonator frequency combs

Microcombs—optical frequency combs generated in microresonators—have advanced tremendously in the past decade, and are advantageous for applications in frequency metrology, navigation, spectroscopy, telecommunications, and microwave photonics. Crucially, microcombs promise fully integrated miniaturized optical systems with unprecedented reductions in cost, size, weight, and power. However, the use of bulk free-space and fiber-optic components to process microcombs has restricted form factors to the table-top. Taking microcomb-based optical frequency synthesis around 1550 nm as our target application, here, we address this challenge by proposing an integrated photonics interposer architecture to replace discrete components by collecting, routing, and interfacing octave-wide microcomb-based optical signals between photonic chiplets and heterogeneously integrated devices. Experimentally, we confirm the requisite performance of the individual passive elements of the proposed interposer—octave-wide dichroics, multimode interferometers, and tunable ring filters, and implement the octave-spanning spectral filtering of a microcomb, central to the interposer, using silicon nitride photonics. Moreover, we show that the thick silicon nitride needed for bright dissipative Kerr soliton generation can be integrated with the comparatively thin silicon nitride interposer layer through octave-bandwidth adiabatic evanescent coupling, indicating a path towards future system-level consolidation. Finally, we numerically confirm the feasibility of operating the proposed interposer synthesizer as a fully assembled system. Our interposer architecture addresses the immediate need for on-chip microcomb processing to successfully miniaturize microcomb systems and can be readily adapted to other metrology-grade applications based on optical atomic clocks and high-precision navigation and spectroscopy.

Polarization conversion in a polariton three-waveguide coupler

We propose an approach to control of polarization of propagating light in a waveguide geometry. We suggest using a three-waveguide coupler etched in a planar semiconductor microcavity with embedded quantum wells. The structure allows converting a linear polarization of polaritons, photons hybridized with quantum well excitons, excited by a resonant optical pump, to circular polarization, herewith opposite circularly-polarized components are spaced apart from each other in different waveguides. The mutual effect of the evanescent coupling of waveguides and the TE-TM splitting of guided modes are in the basis of the predicted effect.

Inverse asymmetrical ridge taper polarization splitter–rotatorcovering optical fiber communication band from O to U

A novel on-chip ultra-broadband polarization splitter–rotator (PSR) based on an inverse asymmetrical ridge taper coupler is proposed by using evanescent coupling between two straight waveguides with gradual ridge widths. The two ridge-widths decrease inversely to realize mode hybridization between fundamental transverse magnetic (TM0) mode and fundamental transverse electric (TE0) mode, which effectively expands the bandwidth. The structural parameters are optimized by utilizing the frequency-domain finite-difference and eigenmode expansion method. The results show that efficient polarization splitting and rotating are simultaneously achieved with a broad operating bandwidth from 1250 nm to 1750 nm and a high polarization conversion efficiency of more than 95%. The insertion loss is below 0.05 dB and 0.2 dB over the bandwidth for TE0 and TM0, respectively. To the best of our knowledge, the PSR achieves the broadest bandwidth up to 500 nm (covering all the optical fiber communication O-, E-, S-, C-, L-, and U-bands) for the first time.

High-speed and high-power germanium photodetector with a lateral silicon nitride waveguide

Up to now, the light coupling schemes of germanium-on-silicon photodetectors (Ge-on-Si PDs) could be divided into three main categories: (1) vertical (or normal-incidence) illumination, which can be from the top or back of the wafer/chip, and waveguide-integrated coupling including (2) butt coupling and (3) evanescent coupling. In evanescent coupling the input waveguide can be positioned on top, at the bottom, or lateral to the absorber. Here, to the best of our knowledge, we propose the first concept of Ge-on-Si PD with double lateral silicon nitride ( Si 3 N 4 ) waveguides, which can serve as a novel waveguide-integrated coupling configuration: double lateral coupling. The Ge-on-Si PD with double lateral Si 3 N 4 waveguides features uniform optical field distribution in the Ge region, which is very beneficial to improving the operation speed for high input power. The proposed Ge-on-Si PD is comprehensively characterized by static and dynamic measurements. The typical internal responsivity is evaluated to be 0.52 A/W at an input power of 25 mW. The equivalent circuit model and theoretical 3 dB opto-electrical (OE) bandwidth investigation of Ge-on-Si PD with lateral coupling are implemented. Based on the small-signal (S21) radio-frequency measurements, under 4 mA photocurrent, a 60 GHz bandwidth operating at − 3 V bias voltage is demonstrated. When the photocurrent is up to 12 mA, the 3 dB OE bandwidth still has 36 GHz. With 1 mA photocurrent, the 70, 80, 90, and 100 Gbit/s non-return-to-zero (NRZ) and 100, 120, 140, and 150 Gbit/s four-level pulse amplitude modulation clear openings of eye diagrams are experimentally obtained without utilizing any offline digital signal processing at the receiver side. In order to verify the high-power handling performance in high-speed data transmission, we investigate the eye diagram variations with the increase of photocurrents. The clear open electrical eye diagrams of 60 Gbit/s NRZ under 20 mA photocurrent are also obtained. Overall, the proposed lateral Si 3 N 4 waveguide structure is flexibly extendable to a light coupling configuration of PDs, which makes it very attractive for developing high-performance silicon photonic integrated circuits in the future.

Tunable plasmonic filter based on parallel bulk Dirac semimetals at terahertz frequencies.

A plasmonic bandpass filter based on parallel bulk Dirac semimetals (BDSs) is proposed and numerically investigated using the finite-difference time-domain method. The proposed filter is realized by the evanescent coupling between the resonator and waveguide, and Fabry-Parot resonant theory is used to analyze its realization mechanism. The performance of the filter can be tuned by changing the coupling distance, length of the resonator, and Fermi levels of the BDSs. We further simulate a plasmonic broadband filter using coupling mode splitting by locating two identical resonators along the waveguide direction. The pass band of the proposed broadband filter can be tuned by adjusting the coupling distances between the resonators and waveguide.

SPR Signal Amplification Based on Dynamic Field Enhancement at the Sensor Surface

A new method of SPR signal amplification based on dynamic field enhancement at the sensor surface to detect C-reactive protein (CRP) is proposed. The gold nanoparticles-detection antibody (AuNPs-dAb) conjugates, as the dynamical agent, were applied to enhance the biosensor sensitivity. The mechanism of SPR sensitivity enhancement is due to the strong evanescent field coupling between the localized SPR (LSPR) of AuNPs and SPR of gold-silver alloy film which proved by COMSOL Multiphysics and MATLAB simulation. The biosensor sensitivity got with sandwich immunoassay (cAb/CRP/AuNPs-dAb) is 5.56-fold of the structure without AuNPs (cAb/CRP/dAb). Moreover, concentration of AuAg-SPR biosensor in CRP detection was lower for an order of magnitude than that of the conventional gold film cladding SPR (Au-SPR) biosensor, according to its direct reaction with the immobilized cAb. The dynamic field enhancement based AuAg-SPR biosensors have high sensitivity in detecting trace biomarker of CRP, and the improvement can also be applied to the detection of other biomarkers in low concentration sensitively.

Hybrid III-V diamond photonic platform for quantum nodes based on neutral silicon vacancy centers in diamond.

Integrating atomic quantum memories based on color centers in diamond with on-chip photonic devices would enable entanglement distribution over long distances. However, efforts towards integration have been challenging because color centers can be highly sensitive to their environment, and their properties degrade in nanofabricated structures. Here, we describe a heterogeneously integrated, on-chip, III-V diamond platform designed for neutral silicon vacancy (SiV0) centers in diamond that circumvents the need for etching the diamond substrate. Through evanescent coupling to SiV0 centers near the surface of diamond, the platform will enable Purcell enhancement of SiV0 emission and efficient frequency conversion to the telecommunication C-band. The proposed structures can be realized with readily available fabrication techniques.

An Ultra-High-Q Lithium Niobate Microresonator Integrated with a Silicon Nitride Waveguide in the Vertical Configuration for Evanescent Light Coupling.

We demonstrate the hybrid integration of a lithium niobate microring resonator with a silicon nitride waveguide in the vertical configuration to achieve efficient light coupling. The microring resonator is fabricated on a lithium niobate on insulator (LNOI) substrate using photolithography assisted chemo-mechanical etching (PLACE). A fused silica cladding layer is deposited on the LNOI ring resonator. The silicon nitride waveguide is further produced on the fused silica cladding layer by first fabricating a trench in the fused silica while using focused ion beam (FIB) etching for facilitating the evanescent coupling, followed by the formation of the silicon nitride waveguide on the bottom of the trench. The FIB etching ensures the required high positioning accuracy between the waveguide and ring resonator. We achieve Q-factors as high as 1.4 × 107 with the vertically integrated device.

Formaldehyde sensor with enhanced performance using microsphere resonator-coupled ZnO nanorods coated glass

An enhanced formaldehyde (CH2O) sensor incorporating microsphere resonator and zinc oxide (ZnO) nanorods coated glass is reported. The microsphere resonator with a diameter of 234 µm and the ZnO nanorods coated glass surface was fabricated using the hydrothermal synthesis method for 12 h. A significant response to formaldehyde concentration levels ranging from 0 ppm to 0.18 ppm was observed. Sensitivity improves by a factor of 1.84 and 1.33 in terms of output power and wavelength shift, respectively, compared to straight microfiber laid on the ZnO coated glass substrate. It produces exceptionally good resolution with 0.0031 ppm as compared to its counterpart with 0.0058 ppm. The proposed formaldehyde sensor utilizes a high energy density behaviour of a microsphere resonator for evanescent field coupling with the surrounding analyte. It also possesses a robust chemisorption process on the ZnO nanorods surface to enhance the sensitivity towards formaldehyde vapour. Besides improving performance, it exhibits a simple approach that reduced the complexity to handle the microfiber during the synthesis process. This proposed technique has shown excellent experiment results for formaldehyde sensing applications.

Propagative Oscillations in Codirectional Polariton Waveguide Couplers.

We report on novel exciton-polariton routing devices created to study and purposely guide light-matter particles in their condensate phase. In a codirectional coupling device, two waveguides are connected by a partially etched section that facilitates tunable coupling of the adjacent channels. This evanescent coupling of the two macroscopic wave functions in each waveguide reveals itself in real space oscillations of the condensate. This Josephson-like oscillation has only been observed in coupled polariton traps so far. Here, we report on a similar coupling behavior in a controllable, propagative waveguide-based design. By controlling the gap width, channel length, or propagation energy, the exit port of the polariton flow can be chosen. This codirectional polariton device is a passive and scalable coupler element that can serve in compact, next generation logic architectures.

Cavity optomechanics with photonic bound states in the continuum

We propose a versatile, free-space cavity optomechanics platform built from two photonic crystal membranes, one of which is freely suspended, and designed to form a microcavity less than one wavelength long. This cavity features a series of photonic bound states in the continuum that, in principle, trap light forever and can be favorably used together with evanescent coupling for realizing various types of optomechanical couplings, such as linear or quadratic coupling of either dispersive or dissipative type, by tuning the photonic crystal patterning and cavity length. Crucially, this platform allows for a quantum cooperativity exceeding unity in the ultrastrong single-photon coupling regime, surpassing the performance of conventional Fabry-Perot-based cavity optomechanical devices in the non-resolved sideband regime. This conceptually novel platform allows for exploring new regimes of the optomechanical interaction, in particular in the framework of pulsed and single-photon optomechanics.

Radiative rate inhibition in fluorescent nanodiamond-microcavity system

Fluorescent nanodiamonds (NDs) are hard, nontoxic, have high melting point and possess physical and chemical stability. In this paper, the NDs with nitrogen vacancy (NV) centres have been studied with steady-state and time-resolved photoluminescence spectroscopy. The broad fluorescence spectrum spans in the visible-near IR region (from 500 nm to 900 nm) with presence of prominent characteristic peaks of neutral and negatively charged NVs. The fluorescence decay of bulk NV centres shows multi-exponential decay with different lifetimes at two excitation wavelengths. When brought in contact with the polystyrene microspheres, the NV centres exhibit whispering gallery modes (WGMs) built upon the background spectrum. The fluorescence decay of NV centres in presence of the microcavity gives evidence of inhibition of the decay rate. Simulations from the finite element method show evanescent field coupling. The results have been explained in terms of the Purcell's effect as well as the slow light that results due to the interference of the ballistic and circulating lights in the microcavity.

Magnetic field sensor based on evanescent wave coupling effect of photonic crystal slab microcavity

In this study, a novel magnetic field sensor (MFS) based on 2D photonic crystal slab (2D PCS) microcavity is proposed. Magnetic fluid film (MFF) covers the surface of PCS and is coupled to high-Q microcavity by evanescent field. The slight disturbance of the refractive index of MFF can cause a larger response of the microcavity optical local field. The magnetic field calibration is realized by measuring the output power of W1 waveguide. The finite difference time domain (FDTD) method was used to analyze and optimize the sensing characteristics of MFS. The simulation results show that the PCS magnetic sensor has good linearity and sensitivity. When the MFF with a thickness of L = 0.4 μm completely covers the surface of PCS, the normalized sensitivity of W1 type waveguide-single microcavity coupling sensor structure can reach 2.52/T. Compared with the traditional photonic crystal magnetic sensor, the PCS magnetic sensor structure designed in this paper can adopt the existing MEMS processing technology and is easy to fabricate.

A Fiber-Attached Coupler for Transmission Bandpass Whispering Gallery Mode Resonator

A fiber attached coupler for transmission bandpass whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The coupler fabricated by femtosecond laser ablation consists of a pair of curved waveguides with a curvature radius of 50 μm, by which the WGM field is excited and collected through evanescent field coupling, respectively. As a proof of concept, a polymer microsphere functioning as the WGM cavity is embedded into a femtosecond laser ablated hole between the two curved waveguides. A passband with a full width of 0.15 nm at half maximum is observed when the microsphere is surrounded by air, verifying the feasibility of the proposed coupler for constructing a WGM cavity-based transmission narrow bandpass optical fiber filter. Benefitting from the relatively high thermo-optical coefficient, the passband of a packaged transmission bandpass WGM resonator can be thermo-optically tuned with a sensitivity of 446.6 pm/°C.

Silicon integrated multi-mode ring resonator

Abstract Ring resonator is an essential element in silicon integrated circuit, it is widely used as filter, wavelength multiplexer and switch in single-mode operation regime. As the rapid development of mode division multiplexing (MDM) technique, ring resonator that can process multi-mode signals simultaneously and uniformly is highly desired. However, the severe modal dispersion makes identical transmission for different modes very hard. In this paper, by breaking through the limitation of conventional multi-mode manipulation design with evanescent coupling or mode interference, we propose and demonstrate a multi-mode ring resonator (MMRR) inspired by the free space geometric optics. Arbitrary number of supporting modes can be achieved by simply widening the waveguide width. For proof-of-concept demonstration, an MMRR supporting four modes is fabricated with uniform transmittance. Furthermore, architecture of cascaded four MMRRs are also demonstrated experimentally.

Three-dimensional single molecule localization close to the coverslip: a comparison of methods exploiting supercritical angle fluorescence.

The precise spatial localization of single molecules in three dimensions is an important basis for single molecule localization microscopy (SMLM) and tracking. At distances up to a few hundred nanometers from the coverslip, evanescent wave coupling into the glass, also known as supercritical angle fluorescence (SAF), can strongly improve the axial precision, thus facilitating almost isotropic localization performance. Specific detection systems, introduced as Supercritical angle localization microscopy (SALM) or Direct optical nanoscopy with axially localized detection (DONALD), have been developed to exploit SAF in modified two-channel imaging schemes. Recently, our group has shown that off-focus microscopy, i.e., imaging at an intentional slight defocus, can perform equally well, but uses only a single detection arm. Here we compare SALM, off-focus imaging and the most commonly used 3D SMLM techniques, namely cylindrical lens and biplane imaging, regarding 3D localization in close proximity to the coverslip. We show that all methods gain from SAF, which leaves a high detection NA as the only major key requirement to unlock the SAF benefit. We find parameter settings for cylindrical lens and biplane imaging for highest z-precision. Further, we compare the methods in view of robustness to aberrations, fixed dipole emission and double-emitter events. We show that biplane imaging provides the best overall performance and support our findings by DNA-PAINT experiments on DNA-nanoruler samples. Our study sheds light on the effects of SAF for SMLM and is helpful for researchers who plan to employ localization-based 3D nanoscopy close to the coverslip.

Low dark current and high bandwidth evanescent wave coupled PIN photodetector array for 400 Gbit/s receiving system

Compared with surface and edge incident photodetectors, evanescent coupling photodetector (ECPD) has high bandwidth and high quantum efficiency, so it has a broad application prospect in the field of high-speed optical communication. The evanescent wave coupled photodetector is composed of a diluted waveguide, a single-mode ridge waveguide and a PIN photodiode. By directional evanescent wave coupling, the coupling efficiency of the incident light from the fiber to the absorption core of the photodetector is improved. In this paper, the structure design, experimental preparation and test results of an indium phosphorus based evanescent wave coupled photodetector array are introduced in detail. The test results show that the dark current of the evanescent wave coupled photodetector array is as low as 215 pA and 1.23 pA under –3 and 0 V bias, respectively. When the active area is 5 μm × 20 μm, the device still has a high responsivity of 0.5 A/W (without antireflection film). The high frequency performance of the detector is tested. The bandwidth of each detector is more than 25 GHz, and the total bandwidth is more than 400 GHz. Any optical device can be integrated. The detector array can be applied to the WDM receiving system of 400 Gbit/s and coherent receiving system of 200 Gbit/s.

Enhanced absorption per unit mass for infrared arrays using subwavelength metal–dielectric structures

The absorption-to-mass ratio of the infrared arrays is enhanced to ∼ 1.33 − 7.33 times larger than the previously reported structures by incorporating two design characteristics: first, the coupling of evanescent fields in the air gaps around pixels to create effectively larger pixel sizes and, second, the use of guided-mode resonance (GMR) within the subwavelength metal–dielectric gratings. The bilayer T i − S i 3 N 4 gratings achieve broadband long-wave infrared (LWIR, λ ∼ 8 − 12 µ m ) absorption by the combined effects of free carrier absorption by the thin Ti films and vibrational phonon absorption by the thick S i 3 N 4 films. In the presence of GMR, this broadband absorption can be enormously enhanced even with low fill factor subwavelength grating cells. Further, the spacing and design of the cells can be modified to form a pixel array structure that couples the light falling in the air gaps via evanescent field coupling. Calculations are performed using the finite difference time domain technique. Excellent broadband absorption is observed for the optimized arrays, yielding maximum absorption of 90% across the LWIR and an average absorption per unit mass (absorption/mass) per pixel of 3.45 × 10 13 k g − 1 .