Penetration Depth Of Evanescent Wave Research Articles

Design and performance of a plastic optical fiber leakage sensor

In this article design and operation of a plastic optical fiber (POF) sensor based on the unconventional light leakage from one fiber to another one causing intensity modulation are presented. The dominant loss mechanism was found to be the evanescent loss and based on this phenomenon the optical fiber sensor was designed. The penetration depth of evanescent wave as a function of different refractive index of cladding was calculated. Operation of this sensor was tested as a liquid level sensor for different liquids and the experimental results are compared. The dry (air interface layer) and wet output (liquid interface layer) signals for this probe were measured for a series of measurements and important factors concerning sensor operation are described. The precision of measured values, reproducibility of the results (1.35% error), and the stability of sensing operation as a function of time at different launching powers are also reported (0.85% error). The reported results for this design are promising and verify the successful operation of such a device as a liquid level probe and also as an on/off level switch.

Minimization of detection volume by surface-plasmon-coupled emission

We report theoretical predictions and experimental observations of the reduced detection volume with the use of surface-plasmon-coupled emission (SPCE). The effective fluorescence volume (detection volume) in SPCE experiments depends on two near-field factors: the depth of evanescent wave excitation and a distance-dependent coupling of excited fluorophores to the surface plasmons. With direct excitation of the sample (reverse Kretschmann excitation) the detection volume is restricted only by the distance-dependent coupling of the excitation to the surface plasmons. However, with the excitation through the glass prism at surface plasmon resonance angle (Kretschmann configuration), the detection volume is a product of evanescent wave penetration depth and distance-dependent coupling. In addition, the detection volume is further reduced by a metal quenching of excited fluorophores at a close proximity (below 10 nm). The height of the detected volume size is 40–70 nm, depending on the orientation of the excited dipoles. We show that, by using the Kretschmann configuration in a microscope with a high-numerical-aperture objective (1.45) together with confocal detection, the detection volume can be reduced to 1–2 attoL. The strong dependence of the coupling to the surface plasmons on the orientation of excited dipoles can be used to study the small conformational changes of macromolecules.

Fluorescence correlation spectroscopy in surface plasmon coupled emission microscope

Study of dynamics of single molecules by Fluorescence Correlation Spectroscopy (FCS) requires that the rate of photon detection per molecule be high, that the background be low, and that there be a large change in fluorescent signal associated with change in a position of a molecule. FCS applied to microscopic Surface Plasmon Coupled Emission (SPCE) suggests a powerful method to meet those requirements. In this method, the observational volume is made shallow by placing a sample on a thin metal film and illuminating it with the laser beam at Surface Plasmon Resonance (SPR) angle through high numerical aperture objective. The illuminating light excites surface plasmons in the metal film that produce an evanescent wave on the aqueous side of the interface. The thickness of the detection volume is a product of evanescent wave penetration depth and distance-dependent fluorescence coupling to surface plasmons. It is further reduced by a metal quenching of excited fluorophores at a close proximity (below 10 nm) to a surface. The fluorescent light is emitted through the metal film only at an SPCE angle. Objective collects emitted light, and a confocal aperture inserted in its conjugate image plane reduces lateral dimensions of the detection volume to a fraction of a micrometer. By using diffusion of fluorescent microspheres, we show that SPCE-FCS is an efficient method to measure molecular diffusion and that on gold surface the height of the detection volume is ~35 nm.

Subnanometric measurements of evanescent wave penetration depth using total internal reflection microscopy combined with fluorescent correlation spectroscopy

In total internal reflection microscopy (TIRM), quantitative interpretation of results often requires a precise knowledge of the penetration depth of the evanescent wave. Standard TIRM practice is to calculate this depth from the microscope’s geometry, but this can introduce significant errors. We show how to calibrate the penetration depth of an evanescent wave in TIRM. An evanescent wave is obtained by illuminating a surface at an incident angle greater than the critical angle. Its penetration depth generally depends on the wavelength and the incident angle of the illumination, and on the indices of refraction on either side of the reflecting suface, but cannot be larger than the field of view. By introducing a fluorescent species (such as fluorescein) and measuring its diffusion time, it is possible to measure very precisely the penetration depth of the evanescent wave.

Effect of taper geometries and launch angle on evanescent wave penetration depth in optical fibers

A large penetration depth of an evanescent wave is the key to success for developing an ultra high-resolution fiber-based evanescent wave biosensor. Tapering the fiber and launching light at an angle has the potential of increasing the penetration depth of evanescent wave manifolds. The effects of tapering, launch angle and taper length of the fiber have been explored in detail using a ray-tracing model to calculate the highest possible penetration depth of the evanescent field. Evanescent wave penetration depths of the order of the size of living cells have been achieved by optimizing the parameters relating geometry of tapered fibers.

Four-wave mixing spectroscopy of gas-surface scattering

A detailed theory of four-wave mixing (FWM) at a gas-solid interface is developed. Geometry of excitation, where two electromagnetic waves propagate perpendicularly to the interface and the third one is evanescent traveling along it, is considered. Assuming that the evanescent wave penetration depth into the gas is much less than the mean free path of gas molecules, it is shown that FWM spectroscopy allows to distinguish between the contributions from gas molecules moving to the surface, desorbed from it and directly scattered into the gas. It is concluded that by scanning the wave frequencies across the molecular transitions it is possible to determine the parameters of the scattering kernel for direct gas-surface scattering and the velocity-dependent sticking probability. A method based on this principle could open opportunity for studying adsorption, desorption and gas-surface scattering dynamics not only under thermodynamically nonequilibrium conditions but as well as in an equilibrium gas phase.

Ray optics calculation of the radiation forces exerted on a dielectric sphere in an evanescent field

A ray optics approach was used to calculate the forces and the torque exerted on a dielectric sphere in the evanescent field produced by a linearly polarized Gaussian beam. The particle was assumed to be immersed in a dielectric fluid next to a solid dielectric plate with the evanescent field produced at the solid-fluid interface. Comparisons with calculations performed by use of more rigorous electromagnetic wave theory show that the ray optics results agree to within a factor of 2 even for particle radii as small as twice the incident wavelength. Calculation of the forces for conditions typical of a total internal reflection microscopy experiment show that the evanescent field has a negligible effect on either the net forces exerted on the particle or the particle motion (i.e., rotation or translation parallel to the interface). By our modifying the parameters of the experiment, however--namely, the incident beam power, radius of incident beam, and evanescent wave penetration depth--forces that are comparable with the net particle weight and capable of translating the particle several micrometers per second, as well as rotating the particle several revolutions per second, can be produced. The ability to micromanipulate a particle in this fashion could offer useful applications for studying particle and surface interactions.

Spontaneous Reconfiguration of Adsorbed Lysozyme Layers Observed by Total Internal Reflection Fluorescence with a pH-Sensitive Fluorophore

By conjugating proteins with a common pH-sensitive fluorescent label, fluorescein isothiocyanate (FITC), and controlling the ionic strength, we provide a means to decrease the characteristic length scale of the total internal reflection fluorescence (TIRF) technique by two orders of magnitude. The usual characteristic length scale for TIRF is an optical length, specifically the evanescent wave penetration depth (on the order of 100 nm). In our experiments the penetration depth is replaced by the Debye screening length as the characteristic length scale. This is readily controlled to match the dimensions of an adsorbed protein layer (on the order of 1 nm). We achieve this length scale reduction by coupling the well-known pH-sensitivity of fluorescence emission by FITC-labeled proteins with the variation of electrostatic potential near a negatively charged surface. Using this fine-resolution TIRF capability in combination with scanning angle reflectometry, we find that lateral repulsions induce a dramatic reconfiguration of adsorbed lysozyme layers on negatively charged silica surfaces. This occurs as the surface concentration approaches the jamming limit for random sequential adsorption. The reconfiguration evidently optimizes electrostatic interactions in the adsorbed layer and decreases the effective excluded area per lysozyme. The decrease in effective excluded area allows adsorption to continue beyond the jamming limit to ultimately attain a hexagonal close packed monolayer of horizontally oriented lysozyme molecules. The adsorption kinetics switch abruptly from being transport-limited to surface-limited after the reconfiguration.

An evanescent wave biosensor. II. Fluorescent signal acquisition from tapered fiber optic probes

A biosensor was developed using antibodies, fluorescence and the evanescent wave to detect antigen binding at the surface of an optical fiber. Cladding was removed from the core along the distal end of a step-index optical fiber, and recognition antibodies were immobilized on the declad core to form the probe sensing region. Immersing the declad probe in aqueous solution creates a V-number mismatch between the immersed probe and the clad fiber. Probes created with reduced sensing region radius exhibited improved response by decreasing the V-number mismatch. Tapering the radius of this region has further improved probe response. Ray tracing analysis of the tapered probe demonstrated that the evanescent wave penetration depth increases along the length of the taper. Experiments correlating position of refraction along the taper with launch angle at the proximal end were realized in the ray tracing model. An evanescent wave immunoassay was performed with a series of the tapered fiber probes, each tapered from the fiber core radius (100 microns) to different end radii. An end radius of 29 microns was found to produce maximal signal from the tapered probe. Factors leading to the determination of the optimized probe are discussed.