Optical Evanescent Field Research Articles

Ultra-sensitive DNA detection with single-base mismatch resolution using a tapered thin-diameter photonic crystal fiber interferometer

A tapered thin-diameter photonic crystal fiber interferometer (TTD-PCFI) biosensor is proposed and demonstrated for DNA hybridization detection with an ultra-low limit of detection (LoD) and high specificity. The TTD-PCFI is fabricated by arc-discharged melting and adiabatic tapering, with a lower loss and stronger evanescent optical field. The fabricated TTD-PCFI with a diameter of 8.63 µm achieves a high refractive index sensitivity of 3842.5 nm/RIU. DNA hybridization detection of concentration ranging from 1 fM to 1 pM is achieved, with a sensitivity of 632 pm/lg(fM). The measured LoD is as low as 410 aM. Single-base mismatch detection of 25-base target single-stranded DNA is also achieved. The proposed label-free DNA biosensor has the advantages of the ultra-low LoD, high specificity, and real-time detection, which has great application prospects in the fields of disease diagnosis, microbial detection, and environmental science.

Hybrid Fibers with Subwavelength-Scale Liquid Core for Highly Sensitive Sensing and Enhanced Nonlinearity.

Interest grows in designing silicon-on-insulator slot waveguides to trap optical fields in subwavelength-scale slots and developing their optofluidic devices. However, it is worth noting that the inherent limitations of the waveguide structures may result in high optical losses and short optical paths, which challenge the device's performance in optofluidics. Incorporating the planar silicon-based slot waveguide concept into a silica-based hollow-core fiber can provide a perfect solution to realize an efficient optofluidic waveguide. Here, we propose a subwavelength-scale liquid-core hybrid fiber (LCHF), where the core is filled with carbon disulfide and surrounded by a silicon ring in a silica background. The waveguide properties and the Stimulated Raman Scattering (SRS) effect in the LCHF are investigated. The fraction of power inside the core of 56.3% allows for improved sensitivity in optical sensing, while the modal Raman gain of 23.60 m-1·W-1 is two times larger than that generated around a nanofiber with the interaction between the evanescent optical field and the surrounding Raman media benzene-methanol, which enables a significant low-threshold SRS effect. Moreover, this in-fiber structure features compactness, robustness, flexibility, ease of implementation in both trace sample consumption and reasonable liquid filling duration, as well as compatibility with optical fiber systems. The detailed analyses of the properties and utilizations of the LCHF suggest a promising in-fiber optofluidic platform, which provides a novel insight into optofluidic devices, optical sensing, nonlinear optics, etc.

Three-dimensional polarization effects in optical tunneling.

We consider the three-dimensional (3D) polarimetric properties of an evanescent optical field excited in the gap of a double-prism system by a random plane wave. The analysis covers the case of frustrated total internal reflection (FTIR), i.e.,optical tunneling, and relies on the characteristic decomposition of the 3×3 polarization matrix. We find in particular that, for any incident partially polarized plane wave, the evanescent field inside the gap is necessarily in a nonregular, genuine 3D polarization state. We also show that the 3D polarimetric properties of the field at the second boundary are sensitive to the changes of the gap width and that the relevant effects occur for the smaller widths when the angle of incidence of the plane wave becomes larger. The results of this work uncover new aspects of the polarimetric structure of genuine 3D evanescent fields and may find applications in near-field optics and surface nanophotonics.

Seven-layer analysis model of an optical waveguide excitation fluorescence microscopy.

In this paper, an optical waveguide evanescent field fluorescence microscopy is studied. Based on Maxwell's equation, a seven-layer theoretical analysis model is developed for the evaluation of an optical waveguide excitation fluorescence microscopy. The optical waveguide excitation fluorescence microscopy structure is systematically and comprehensively analysed at the wavelengths of 488, 532 and 646nm for fluorescent dyes. The analysis results provide some useful suggestions, which will be beneficial to the research of an optical waveguide evanescent field fluorescence microscopy.

Selective fiber optic TFBG-assisted biosensors featuring functional coatings

The paper reports on the results of the development of tilted fiber Bragg grating-assisted selective biosensors. The main sensing principle is based on the dependence of an optical evanescent field parameters on the composition of the medium under study. Sensors feature functional coatings of antibodies against protein molecules common in blood composition – fibrinogen and D-dimer. The work explores and clearly demonstrates the selective sensitivity of sensors to specific proteins by the example of simultaneous measurements using a pair of sensors functionalized with different antibodies.

Optical manipulation of matter waves.

Light is used to steer the motion of atoms in free space, enabling cooling and trapping of matter waves through ponderomotive forces and Doppler-mediated photon scattering. Likewise, light interaction with free electrons has recently emerged as a versatile approach to modulate the electron wave function for applications in ultrafast electron microscopy. Here, we combine these two worlds, theoretically demonstrating that matter waves can be optically manipulated via inelastic interactions with optical fields. This allows us to modulate the translational part of the wave function and produce temporally and spatially compressed atomic beam pulses. We realize such modulation through stimulated photon absorption and emission by atoms traversing phase-matching evanescent optical fields generated upon light scattering by a nanostructure and via stimulated Compton scattering in free space without any assistance from material media. Our results support optical manipulation of matter waves as a powerful tool for microscopy, spectroscopy, and exploration of fundamental phenomena associated with light-atom interactions.

Light scattering in TIRF microscopy: A theoretical study of the limits to surface selectivity

In TIRF microscopy, the sample resides near a surface in an evanescent optical field that, ideally, decreases in intensity with distance from the surface in a pure exponential fashion. In practice, multiple surfaces and imperfections in the optical system and refractive index (RI) inhomogeneities in the sample (often living cells) produce propagating scattered light that degrades the exponential purity. RI inhomogeneities cannot easily be avoided. How severe is the consequent optical degradation? Starting from Maxwell’s equations, we derive a first-order perturbative approximation of the electric field strength of light scattered by sample RI inhomogeneities of several types under coherent evanescent field illumination. The approximation provides an expression for the scattering field of any arbitrary RI inhomogeneity pattern. The scattering is not all propagating; some is evanescent and remains near the scattering centers. The results presented here are only a first-order approximation, and they ignore multiple scattering and reflections off the total internal reflection (TIR) surface. For simplicity, we assume that the RI variations in the z direction are insignificant within the depth of the evanescent field and consider only scattering of excitation light, not fluorescence emission light. The general conclusion of most significance from this study is that TIR scattering from a sample with RI variations typical of those on a cell culture alters the effective thickness of the illumination to only ∼50% greater than it would be without scattering. The qualitative surface selectivity of TIR fluorescence is largely retained even in the presence of scattering. Quantitatively, however, scattering will cause a deviation from the incident exponential decay at shorter distances, adding a slower decaying background. Calculations that assume a pure exponential decay will be approximations, and scattering should be taken into account. TIR scattering is only slightly dependent on polarization but is strongly reduced for the highest accessible incidence angles.

A First Study of the Kinetics of Metal Ion Adsorption at Solid/Liquid Interface using Evanescent Wave-based Optical Microfiber

Although the use of chelating agent (CA)-functionalized optical microfiber (OM) for heavy metal ion sensing has been receiving tremendous research interest, a thorough study of the kinetics and adsorption of metal ions process which occurs at the solid/liquid interface has yet to be investigated. Kinetics and adsorption studies of metal ions, M2+ (i.e. Cd2+, Pb2+, and Zn2+) were carried out using CA-functionalized OM. The optically measured metal adsorption efficiency is interrelated to the properties of the chelating ligand, metal ion, and the molecular structure of the CA-M2+ complex that include the coordination number, bond length, and bond angle parameters. The kinetics adsorption chemistry of M2+ onto the CA-functionalized OM is well-fitted to the pseudo-first-order kinetic model. Moreover, this work also investigated the effects of the resultant optical evanescent field strength and the CA-M2+ structure towards the adsorption efficiency of OM with reduced tapered waist diameter. To the best of our knowledge, this is the first reported study to find this correlation in the metal adsorption efficiency at solid phase using an OM. The denouement of this study unveils a new physical chemistry insight of metal ion adsorption at the solid/liquid interface.

Monitoring Water Contamination in Jet Fuel Using Silica-Based Bragg Gratings

This work quantifies water contamination in jet fuel (Jet A-1), using silica-based Bragg gratings. The optical sensor geometry exposes the evanescent optical field of a guided mode to enable refractometery. Quantitative analysis is made in addition to the observation of spectral features consistent with the emulsification of water droplets and Stokes’ settling. The measurements are observed for cooling and heating cycles between ranges of 22 °C and −60 °C. The maximum spectral sensitivity for water contamination was 2.4 pm/ppm-v, with a resolution of <5 ppm-v.

A Split-Pole-Gapped NFT Write Head Design for Transition Curvature Reduction in Heat-Assisted Magnetic Recording

A write head design consisting of a split pole and a gapped near-field transducer is presented. Through simulation, we found our design can produce a maximum effective cross-track recording field gradient of 300 Oe/nm, and a maximum difference of 3.6 kOe between the center and ±20 nm away in the cross-track direction in the recording media near the magnetic gap (MG). The tradeoff between improved cross-track magnetic gradient and loss of optical efficiency due to parasitic absorption by the pole is studied as a function of the thickness of a low optical-index separator between the MG and the optical gap (OG). The separator is tapered to bring the cross-track field gradient close to the OG near the air bearing surface, while limiting the total amount of interaction of the evanescent optical field with the pole. With this tapered low-index separator, we are able to preserve the recording field cross-track gradient and amplitude, while reducing the parasitic optical absorption in the recording head due to pole proximity.

Design of on-chip hybrid plasmonic Mach-Zehnder interferometer for temperature and concentration detection of chemical solution

We report a compact lab-on-chip design of a Mach-Zehnder interferometer (MZI) incorporating a novel metal strip loaded horizontal slot hybrid plasmonic waveguide (HSHPW) in the sensing arm and a dielectric horizontal slot (DHS) waveguide in the reference arm. The HSHPW is optimized to confine a high ∼60% and ∼82% evanescent optical field in the low index dielectric slot and an active sensing region, respectively which enhance the device sensitivity with a comparative lower propagation loss than a typical plasmonic waveguide. We report here a single MZI configuration which not only exhibits an excellent temperature sensitivity of 243.9 pm/°C but also liquid concentration sensitivity of 437.3 nm/RIU for a 40 μm long HSHPW. To mitigate loss arising from each section such as butt coupling and plasmonic modal losses, the HSHPW has been optimized by incorporating an asymmetric power splitter which shows a considerable improvement in the fringe visibility and device insertion loss. Thus, the proposed single MZI design shows an excellent response to the temperature and liquid concentration sensing with a maximum total loss and extinction ratio of 2.56 dB and>25 dB, respectively. A much simpler CMOS friendly compact design is also found to have a great robustness to the fabrication tolerances.

Soliton molecules in a fiber laser based on optic evanescent field interaction with WS2

Passively mode-locked fiber laser serves as an ideal playground for exploring the dynamics of dissipative solitons. Recently, two-dimensional materials have attracted growing interests for their excellent optical properties in the research field of ultrafast optics. Here, we report an observation of soliton molecules in a passively mode-locked fiber laser based on optic evanescent field interaction with tungsten disulfide (WS2), which contributes to the study of multi-soliton complexes. Particularly, the WS2 saturable absorber (SA) is fabricated by optically depositing the few-layer WS2 nanosheets onto microfiber. In the experiment, stable mode-locking operations are realized, which manifest the effective performance of the WS2 SA. Harmonic mode-locking phenomena are also observed, and these separate solitons facilitate the generation of multi-soliton complexes. Furthermore, due to soliton–soliton interaction, wandering particle-like solitons can be bound together to produce soliton molecules and bunch of soliton molecules. These results enrich both the exploration of multi-soliton complexes and potential industrial applications.

Three-Dimensional Measurement of the Helicity-Dependent Forces on a Mie Particle.

Recently, it was shown that a Mie particle in an evanescent field ought to experience optical forces that depend on the helicity of the totally internally reflected beam. As yet, a direct measurement of such helicity-dependent forces has been elusive, as the widely differing force magnitudes in the three spatial dimensions place stringent demands on a measurement's sensitivity and range. In this study, we report the simultaneous measurement of all components of this polarization-dependent optical force by using a 3D force spectroscopy technique with femtonewton sensitivity. The vector force fields are compared quantitatively with our theoretical calculations as the polarization state of the incident light is varied and show excellent agreement. By plotting the 3D motion of the Mie particle in response to the switched force field, we offer visual evidence of the effect of spin momentum on the Poynting vector of an evanescent optical field.

LED-based interference-reflection microscopy combined with optical tweezers for quantitative three-dimensional microtubule imaging.

Optical tweezers combined with various microscopy techniques are a versatile tool for single-molecule force spectroscopy. However, some combinations may compromise measurements. Here, we combined optical tweezers with total-internal-reflection-fluorescence (TIRF) and interference-reflection microscopy (IRM). Using a light-emitting diode (LED) for IRM illumination, we show that single microtubules can be imaged with high contrast. Furthermore, we converted the IRM interference pattern of an upward bent microtubule to its three-dimensional (3D) profile calibrated against the optical tweezers and evanescent TIRF field. In general, LED-based IRM is a powerful method for high-contrast 3D microscopy.

On-Chip Spiral Waveguides for Ultrasensitive and Rapid Detection of Nanoscale Objects.

Ultrasensitive and rapid detection of nano-objects is crucial in both fundamental studies and practical applications. Optical sensors using evanescent fields in microcavities, plasmonic resonators, and nanofibers allow label-free detection down to single molecules, but practical applications are severely hindered by long response time and device reproducibility. Here, an on-chip dense waveguide sensor to monitor single unlabeled nanoparticles in a strong optical evanescent field is demonstrated. The spiral nanowaveguide design enables two orders of magnitude enhancement in sensing area compared to a straight waveguide, significantly improving the particle capture ability and shortening the target analysis time. In addition, the measurement noise is suppressed to a level of 10-4 in the transmitted power, pushing the detection limit of single particles down to the size of 100 nm. The waveguide sensor on the silicon-on-isolator platform can be fabricated reproducibly by the conventional semiconductor processing and compatible with surface functionalization chemistries and microfluidics, which could lead to widespread use for sensing in environmental monitoring and human health.

Optically sizing single atmospheric particulates with a 10-nm resolution using a strong evanescent field

Although an accurate evaluation of the distribution of ultrafine particulate matter in air is of utmost significance to public health, the usually used PM2.5 index fails to provide size distribution information. Here we demonstrate a low-profile and cavity-free size spectrometer for probing fine and ultrafine particulate matter by using the enhanced particle-perturbed scattering in strong optical evanescent fields of a nanofiber array. The unprecedented size resolution reaches 10 nm for detecting single 100-nm-diameter nanoparticles by employing uniform nanofibers and controlling the polarizations of the probe light. This size spectrometry was tested and used to retrieve the size distribution of particulate matter in the air of Beijing, yielding mass concentrations of nanoparticles, as a secondary exercise, consistent with the officially released data. This nanofiber-array probe shows potential for the full monitoring of air pollution and for studying early-stage haze evolution and can be further extended to explore nanoparticle interactions.

Methane absorption spectroscopy on a silicon photonic chip

Infrared tunable diode-laser absorption spectroscopy (IR-TDLAS) is an enabling technology for trace-gas detection, with applications ranging from air-quality monitoring to medical diagnostics. However, such sensors typically utilize discrete optical components that pose practical cost limits for large-scale network deployments. Here, we leverage silicon photonics technology to demonstrate IR-TDLAS on an integrated CMOS-compatible platform for methane (CH4) spectroscopy. Using near-IR (1650 nm) light from a distributed-feedback laser and an uncooled InGaAs detector, the evanescent optical field of a high-index contrast nanoscale silicon waveguide is used to probe ambient CH4, yielding Gaussian-noise-limited sub-100 parts-per-million by volume detection limits. Our results demonstrate the feasibility of chip-scale photonic integration for realizing compact, cost-effective, and versatile gas sensors capable of tackling diverse energy and environmental challenges, such as natural-gas leak quantification and localization for fugitive-emissions monitoring.

Linearly decayed evanescent optical field in planar refractive index well

Evanescent optical field with linearly decaying profile is theoretically analyzed at the critical angle of incidence in a planar structure of one dimensional refractive index well (RIW). The linearity of the evanescent field is due to the presence of the second refractive index barrier, which also shifts the position of total internal reflection (TIR) away from the critical angle. The decaying rate is determined by the refractive indices of the two barriers, as well as the width of the well. With this linearly decayed evanescent field (LDEF), various profiles across the well, for example uniform one, can be formed via appropriate combination of the LDEFs, which can promote new applications in fields of material analysis and sensing in the molecular scale.

Evanescent vortex: Optical subwavelength spanner

Conventional optical spanners based on free-space focused vortex beams are very difficult to manipulate subwavelength objects due to the diffraction limit, while optical subwavelength spanners are not explored. Evanescent wave is one potential tool to realize subwavelength trapping. By combining vortex with evanescent field, we find that the evanescent vortex can function as an optical subwavelength spanner. We investigate the factors that will affect the generation/function of this subwavelength spanner, including numerical aperture and topological charge. Further, by calculating the optical force and potential on the illuminated objects, we have demonstrated that the evanescent optical vortex field is able to trap 200 nm polystyrene spherical particles and to rotate them around the ring-shaped field at the same time, making it a subwavelength optical spanner. This mechanism can be used as a tool to study the behaviour of very small objects in physics and biology.

Optically bound colloidal lattices in evanescent optical fields.

In this Letter, we demonstrate the formation of a stable two-dimensional lattice of colloidal particles in the interference pattern formed by four evanescent optical fields at a dielectric interface. The microspheres are observed to form a two-dimensional square lattice with lattice vectors inclined relative to the beam propagation directions. We use digital video microscopy and particle tracking to measure the Brownian motion of particles bound in the lattice, and use this to characterize fluctuations in the local ordering of particles using the bond orientational order parameter, the probability distribution of which is shown to be a chi-squared distribution. An explanation for the form of this distribution is presented in terms of fluctuations of the modes of a ring of particles connected by springs.