Showing papers on "Total internal reflection published in 2021"

Overview of refractive index sensors comprising photonic crystal fibers based on the surface plasmon resonance effect [Invited]

Abstract: Optical fibers have been widely applied to telecommunication, imaging, lasers, and sensing. Among the different types of fibers, photonic crystal fibers (PCFs), also called microstructured optical fibers, characterized by air holes arranged along the length of fibers have experienced tremendous advance due to their unique advantages. They are regarded as a desirable platform to excite surface plasmon resonance (SPR) because of easy realization of phase matching conditions between the fundamental core mode and the plasmonic mode, which plays a critical role in miniaturization and integration of SPR sensors. In this mini-review, the current status of PCF sensors based on SPR is summarized. The theory of SPR is discussed, and simulation methods for PCF-SPR sensors are described. The important parameters including the refractive index detection range, resonance wavelength, and spectral sensitivity responsible for the sensing properties of PCF-SPR sensors are reviewed. The fabrication and the comparison of performances are also illustrated, and, finally, the challenges and future perspectives are outlined.

Electrically Tunable Singular Phase and Goos–Hänchen Shifts in Phase‐Change‐Material‐Based Thin‐Film Coatings as Optical Absorbers

Abstract: The change of the phase of light under the evolution of a nanomaterial with time is a promising new research direction. A phenomenon directly related to the sudden phase change of light is the Goos-Hanchen (G-H) shift, which describes the lateral beam displacement of the reflected light from the interface of two media when the angles of incidence are close to the total internal reflection angle or Brewster angle. Here, an innovative design of lithography-free nanophotonic cavities to realize electrically tunable G-H shifts at the singular phase of light in the visible wavelengths is reported. Reversible electrical tuning of phase and G-H shifts is experimentally demonstrated using a microheater integrated optical cavity consisting of a dielectric film on an absorbing substrate through a Joule heating mechanism. In particular, an enhanced G-H shift of 110 times of the operating wavelength at the Brewster angle of the thin-film cavity is reported. More importantly, electrically tunable G-H shifts are demonstrated by exploiting the significant tunable phase change that occurs at the Brewster angles, due to the small temperature-induced refractive index changes of the dielectric film. Realizing efficient electrically tunable G-H shifts with miniaturized heaters will extend the research scope of the G-H shift phenomenon and its applications.

Goos-Hänchen shifts of Gaussian beams reflected from surfaces coated with cross-anisotropic metasurfaces

Abstract: The Goos-Hanchen (GH) shift of a Gaussian beam reflected from surfaces coated with a cross-anisotropic metasurface (CM) is theoretically investigated. It is revealed that, the CM coating has influence on the spatial GH shift as well as the angular one, only at the total internal reflection condition, where the incident angle θ is above critical angle θ c . Moreover, near θ c , a positive or negative larger peak spatial GH shift occurs while a pair of positive and negative peaks angular GH shifts appear, compared to the case of no coating, which are induced by the CM coating. Therefore, our results may provide a feasible method to obtain a larger GH shift, and open new possibilities for applications of the CMs.

Mueller matrix ellipsometer based on discrete-angle rotating Fresnel rhomb compensators.

Abstract: A spectroscopic Mueller matrix ellipsometer based on two rotating Fresnel rhomb compensators with a nearly achromatic response and optimal retardance is described. In this instrument, the compensators rotate in a discrete manner instead of continuously rotating, and this allows for a well-conditioned measurement even for low intensity samples. Moreover, in this configuration, the exposure time of the CCD detector can be varied within orders of magnitude without interfering with the dynamics of the compensator rotation. An optimization algorithm determines the optimal set of discrete angles that allows the determination of the Mueller matrix in the presence of noise. The calibration of the instrument is discussed, and examples of experimentally determined Mueller matrices are provided.

Experimental observation of self-imaging in SMF-28 optical fibers

Abstract: Spatial self-imaging, consisting of the periodic replication of the optical transverse beam profile along the propagation direction, can be achieved in guided wave systems when all excited modes interfere in phase. We exploited material defects photoluminescence for directly visualizing self-imaging in a few-mode, nominal singlemode SMF-28 optical fiber. Visible luminescence was excited by intense femtosecond infrared pulses via multiphoton absorption processes. Our method permits us to determine the mode propagation constants and the cutoff wavelength of transverse fiber modes.

Tunable propagation of surface plasmon-phonon polaritons in graphene-hBN metamaterials

Abstract: We present a detailed study on the tunable propagation derived from the coupled surface plasmon–phonon polaritons (SPPPs) with the effective medium theory (EMT) in graphene-hBN metamaterials (MMs). Four reststrahlen bands (RBs) are observed, two of which mainly come from the hBN and the others originate from the effect of the graphene. The RBs frequency windows can be adjusted by the chemical potential and the filling ratio. An epsilon-near-zero-and-pole( e NZP )hyperbolic metamaterial (HMM) is detected at the precise frequency, chemical potential and filling ratio where the HMM undergoes a completely inversion of anisotropy. We derive the relative dispersion relation and demonstrate that the propagation of SPPP modes can be regulated by modifying the chemical potential. In addition, the tunability of the graphene-hBN MMs also can be improved by changing the thicknesses of the hBN and the number of graphene sheets. The energy-flux density in the graphene-hBN MM seriously deviates from its wave vector and can be localized at a certain depth. Besides, the ghost SPPP modes with the oscillation-attenuation character are observed at some special conditions through checking the distribution of electric fields. The attenuation total reflection (ATR) measurement is established to examine these SPPP modes. The numerical results show that the observation of each SPPP modes requires different conditions dictated by material parameters and the polarization direction of the incident light.

Attenuated Total Reflection at THz Wavelengths: Prospective Use of Total Internal Reflection and Polariscopy

Abstract: Capabilities of the attenuated total reflection (ATR) at THz wavelengths for increased sub-surface depth characterisation of (bio-)materials are presented. The penetration depth of a THz evanescent wave in biological samples is dependent on the wavelength and temperature and can reach 0.1–0.5 mm depth, due to the strong refractive index change ∼0.4 of the ice-water transition; this is quite significant and important when studying biological samples. Technical challenges are discussed when using ATR for uneven, heterogeneous, high refractive index samples with the possibility of frustrated total internal reflection (a breakdown of the ATR reflection mode into transmission mode). Local field enhancements at the interface are discussed with numerical/analytical examples. Maxwell’s scaling is used to model the behaviour of absorber–scatterer inside the materials at the interface with the ATR prism for realistic complex refractive indices of bio-materials. The modality of ATR with a polarisation analysis is proposed, and its principle is illustrated, opening an invitation for its experimental validation. The sensitivity of the polarised ATR mode to the refractive index between the sample and ATR prism is numerically modelled and experimentally verified for background (air) spectra. The design principles of polarisation active optical elements and spectral filters are outlined. The results and proposed concepts are based on experimental conditions at the THz beamline of the Australian Synchrotron.

Critical angle reflection imaging for quantification of molecular interactions on glass surface.

Abstract: Quantification of molecular interactions on a surface is typically achieved via label-free techniques such as surface plasmon resonance (SPR). The sensitivity of SPR originates from the characteristic that the SPR angle is sensitive to the surface refractive index change. Analogously, in another interfacial optical phenomenon, total internal reflection, the critical angle is also refractive index dependent. Therefore, surface refractive index change can also be quantified by measuring the reflectivity near the critical angle. Based on this concept, we develop a method called critical angle reflection (CAR) imaging to quantify molecular interactions on glass surface. CAR imaging can be performed on SPR imaging setups. Through a side-by-side comparison, we show that CAR is capable of most molecular interaction measurements that SPR performs, including proteins, nucleic acids and cell-based detections. In addition, we show that CAR can detect small molecule bindings and intracellular signals beyond SPR sensing range. CAR exhibits several distinct characteristics, including tunable sensitivity and dynamic range, deeper vertical sensing range, fluorescence compatibility, broader wavelength and polarization of light selection, and glass surface chemistry. We anticipate CAR can expand SPR′s capability in small molecule detection, whole cell-based detection, simultaneous fluorescence imaging, and broader conjugation chemistry. Here, the authors present a method for quantifying molecular interactions on a glass surface, based on measuring surface refractive index changes via the reflectivity near the critical angle. They demonstrate tunable sensitivity and dynamic range, deep vertical sensing range, also for intracellular signals.

Object-dependent spatial resolution of the reflection-mode terahertz solid immersion microscopy.

Abstract: Terahertz (THz) solid immersion microscopy is a novel promising THz imaging modality that overcomes the Abbe diffraction limit. In our prior work, an original reflection-mode THz solid immersion microscope system with the resolution of 0.15λ (in free space) was demonstrated and used for imaging of soft biological tissues. In this paper, a numerical analysis, using the finite-difference time-domain technique, and an experimental study, using a set of objects with distinct refractive indexes, were performed in order to uncover, for the first time, the object-dependent spatial resolution of the THz solid immersion microscopy. Our findings revealed that the system resolution remains strongly sub-wavelength 0.15–0.4λ for the wide range of sample refractive indices n = 1.0–5.0 and absorption coefficients α = 0–400 cm−1 (by power). Considering these findings, two distinct regimes of the THz solid immersion microscopy were identified. First is the total internal reflection regime that takes place when the sample refractive index is relatively low, while the sub-wavelength resolution is enabled by both the evanescent and ordinary reflected waves at the interface between a high-refractive-index material and an imaged object. Second is the ordinary reflection regime that occurs when the sample refractive index is high enough, so that there is no more total internal reflection at the interface, while only the ordinary reflected waves inside a high-refractive-index material are responsible for the sub-wavelength resolution. The resultant conclusions are general and can be applied for analysis of solid immersion lenses operating in other spectral ranges, such as visible and infrared, given linear nature of the Maxwell’s equations.

On the Complete Radiation Pattern of a Vertical Hertzian Dipole Above a Low-Loss Ground

Abstract: The complete radiation field pattern of a vertical Hertzian dipole antenna on or above a lossless or low-loss dielectric half-space is studied using a rigorous Sommerfeld formalism. The reflected fields in the air above the interface and the subsurface fields transmitted into the dielectric are computed by numerical integration of the Sommerfeld integrals. Furthermore, to facilitate the physical interpretation of these results, a detailed asymptotic saddle-point integration method analysis is presented, which includes terms that vary in magnitude with the second power of the inverse distance from the dipole. It is shown that the second-order field constituents are dominant at the interface, where the first-order geometrical fields vanish. These second-order terms comprise an evanescent wave propagating along the interface in the upper half-space and a lateral wave, also known as the head wave, which propagates in the subsurface along the direction of the critical angle. The two waves only exist between two cones whose half-angles are equal to the critical angle, and their interference with the geometrical-optics fields determines the radiation pattern for elevation angles near the horizon. The far zone surface fields on either side of the interface comprise two second order waves that propagate along the interface, one with the phase velocity in the air, and the other with the phase velocity in the dielectric. Away from the interface, the leading field components vary with the first power of the inverse distance, which explains the sharp dip in the field pattern at the interface—a phenomenon known as the interface pattern extinction. Another distinctive phenomenon, observed in the subsurface field pattern, is the rippling that occurs in the angular range between the critical angle cone and the interface. The asymptotic analysis has shown that this pattern scalloping results from the interference of the lateral wave with the geometrical-optics spherical wave.

Melt pool evolution on inclined NV E690 steel plates during laser direct metal deposition

Abstract: The melt pool evolution during the laser direct metal deposition (DMD) is intricate. However, most of the research focused on a horizontal substrate, and only a little literature has reported the studies on an inclined substrate with a non-vertically irradiated laser beam. This investigation aims to investigate the melt pool evolution on a substrate with different inclination angles. A paraxial high-speed camera and a coaxial industrial camera were applied to obtain the instantaneous images of the melt pool. Besides, the image processing technique was implemented to extract the coaxial melt pool area. The results showed that several small melt droplets were generated in front of the melt pool on a horizontal substrate; however, a similar phenomenon was not observed on an inclined substrate. Besides, the melt droplets dynamics were driven by the surface tension, resisted by the inertia, and the spreading process was much faster than the solidification process. When the substrate was inclined, the laser energy distribution on the substrate was demonstrated. The “critical angle” was defined as the maximum inclination angle when the single track without any humps. In our experiments, the critical angle existed between 36° and 38°, and a periodic appearance of the humping formation was observed at the inclination angle of 40°. Our obtained results revealed the melt droplets dynamics on a horizontal substrate, the range of the critical angle, the laser energy distribution on an inclined substrate, and the humping formation mechanism.

Super-resolution imaging for sub-IR frequencies based on total internal reflection

Abstract: For measurements designed to accurately determine layer thickness, there is a natural trade-off between sensitivity to optical thickness and lateral resolution due to the angular ray distribution required for a focused beam. We demonstrate a near-field imaging approach that enables subwavelength lateral resolution in images with contrast dependent on optical thickness. We illuminate a sample in a total internal reflection geometry, with a photoactivated spatial modulator in the near field, which allows optical thickness images to be computationally reconstructed in a few seconds. We demonstrate our approach at 140 GHz (wavelength 2.15 mm), where images are normally severely limited in spatial resolution, and demonstrate mapping of optical thickness variation in inhomogeneous biological tissues.

Reflection and transmission of elastic waves through nonlocal piezoelectric plates sandwiched in two solid half-spaces

Abstract: A nonlocal stress expansion Legendre orthogonal polynomial method is proposed to study the reflection and transmission of elastic waves through piezoelectric nanoplates sandwiched in two solid half-spaces. The proposed method avoids employing the relationship between local stress and nonlocal stress to construct boundary conditions. The reflection and transmission of plane waves and out-plane waves are studied separately with considering the electrical open circuit boundary. The convergence of presented method is discussed. The correctness of presented method is verified by comparing with the published data of piezoelectric nanoplates immersed in water, which can be considered as a simplified situation sandwiched in two solid half spaces. In addition, the nonlocal effect on the wave mode conversion, stress and electric displacement distribution, and piezoelectricity are discussed. Results show that the nonlocal effect reconstructs the wave mode conversion, and enhances the influence of the piezoelectricity on the critical angle.

Magnetic field and Fermi energy modulated quantized Imbert–Fedorov shifts in graphene

Abstract: We theoretically investigate the spatial Imbert–Fedorov (SIF) shifts of a light beam reflected from a graphene–substrate system in the presence of an externally applied magnetic field. We impinge a monochromatic light beam of finite width on the surface of a graphene–substrate system and investigate the reflection and transmission coefficients of the beam. We find that the Fermi energy modulated quantized transverse shifts can be achieved in the graphene–substrate system for incident angles in the vicinity of the Brewster angle and frequencies in the terahertz regime. In the case of partial reflection of the light beam, IF shifts acquire moderate magnitudes, while for the case of total internal reflection in the quantum Hall regime, our results show giant negative and positive SIF shifts. Furthermore, we demonstrate that the Brewster angle changes with changing magnetic field and Fermi energy. Our findings are important from the point of view of tuning the IF shifts with magnetic field and Fermi energy conveniently and effectively, which is required to develop new tunable photonic devices in the terahertz regime.

Direct measurement of the scattering coefficient

Abstract: In general, the measurement of the main three optical properties (µa, µs and g) in turbid media requires a very precise measurement of the total transmission (TT), the total reflection (TR) and the collimated transmission (CT). Furthermore, an inverse algorithm such as inverse adding doubling or inverse Monte-Carlo-simulations is required for the reconstruction of the optical properties. Despite many available methods, the error free measurement of the scattering coefficient or the g-factor still remains challenging. In this study, we present a way to directly calculate the scattering coefficient from the total and collimated transmission. To allow this, it can be shown that TTCT is proportional to eμs⋅d for a wide range of optical properties if the sample is thick enough. Moreover, a set-up is developed and validated to measure the collimated transmission precisely.

Topological two-dimensional Su–Schrieffer–Heeger analog acoustic networks: Total reflection at corners and corner induced modes

Abstract: In this work, we investigate some aspects of an acoustic analog of the two-dimensional Su–Schrieffer–Heeger model. The system is composed of alternating cross-sectional tubes connected in a square network, which in the limit of narrow tubes is described by a discrete model coinciding with the two-dimensional Su–Schrieffer–Heeger model. This model is known to host topological edge waves, and we develop a scattering theory to analyze how these waves scatter on edge structure changes. We show that these edge waves undergo a perfect reflection when scattering on a corner, incidentally leading to a new way of constructing corner modes. It is shown that reflection is high for a broad class of edge changes such as steps or defects. We then study the consequences of this high reflectivity on finite networks. Globally, it appears that each straight part of the edges, separated by corners or defects, hosts localized edge modes isolated from their neighborhood.

Unveiling the coupling of single metallic nanoparticles to whispering-gallery microcavities.

Abstract: Whispering gallery mode resonators (WGMRs) trap light over many optical periods using total internal reflection. Thereby, they host multiple narrowband circulating modes that find applications in quantum electrodynamics, optomechanics, sensing and lasing. The spherical symmetry and low field leakage of dielectric microspheres make it difficult to probe their high-quality optical modes using far-field radiation. However, local field enhancement from metallic nanoparticles (MNPs) placed at the edge of the resonators can interface the optical far-field and the bounded cavity modes. In this work, we study the interaction between whispering gallery modes and the MNP surface plasmons with nanometric spatial resolution by using electron-beam spectroscopies in a scanning transmission electron microscope. We show that gallery modes are induced over the spectral range of the dipolar plasmons of the nanoparticle. Additionally, we explore the dependence of the transverse electric and transverse magnetic polarization of the induced gallery mode on the induced dipole moment of the MNP. Our study demonstrates a viable mechanism to effectively excite high-quality-factor whispering gallery modes and holds potential for applications in optical sensing and light manipulation.

Negative refraction mediated by bound states in the continuum

Abstract: Negative refraction might occur at the interface between a two-dimensional photonic crystal (PhC) slab and a homogeneous medium, where the guiding of the electromagnetic wave along the third dimension is governed by total internal reflection. Herein, we report on the observation of negative refraction in the PhC slab where the vertical guiding is enabled by a bound state in the continuum and essentially beyond the light cone. Such abnormal refraction and guiding mechanism are based on the synchronous crafting of spatial dispersion and the radiative lifetime of Bloch modes within the radiative continuum. Microwave experiments are provided to further validate the numerical proposal in an all-dielectric PhC platform. It is envisioned that the negative refraction observed beyond the light cone might facilitate the development of optical devices in integrated optics, such as couplers, multiplexers, and demultiplexers.

The refractive index of human blood measured at the visible spectral region by single-fiber reflectance spectroscopy

Abstract: The refractive index is an essential biophysical parameter used in many diagnostic and therapeutic biomedical applications. In the present study, the refractive index of control and anemic blood was measured using the total internal reflection fiber optics technique. For control, the refractive index measured by the indicated method was significantly higher than anemic blood over the wavelengths in the visible spectral region used in this study. Strong linear correlations between refractive index and hemoglobin concentration were obtained for control and anemic blood. These findings could enhance the use of the refractive index in many applications of blood analysis and hematology.

Temporal reflection and refraction of optical pulses inside a dispersive medium: an analytic approach

Abstract: We develop an analytic approach for reflection of light at a temporal boundary inside a dispersive medium and derive frequency-dependent expressions for the reflection and transmission coefficients Using the analytic results, we study the temporal reflection of an optical pulse and show that our results agree fully with a numerical approach used earlier Our approach provides approximate analytic expressions for the electric fields of the reflected and transmitted pulses Whereas the width of the transmitted pulse is modified, the reflected pulse is a mirrored version of the incident pulse When a part of the incident spectrum lies in the region of total internal reflection, both the reflected and transmitted pulses are distorted considerably

Highly efficient second-harmonic generation in an electro-optic chiral material using total-internal-reflection-based optical rotation quasi-phase-matching technique

Abstract: The present work introduces the concept of a total-internal-reflection (TIR)-based optical rotation, quasi-phase-matching (QPM) technique for generating highly efficient second-harmonic optical output in a rectangular slab of magnesium oxide-doped lithium niobate crystal coated with a thin layer of yttrium oxide. Combinational effects of TIR optical rotation QPM and fractional QPM techniques are experienced at certain bounce points inside the slab, thereby enhancing the conversion efficiency. In this analysis, the thin-film is used for controlling the phase-shifts accompanying the propagation of p- and s-polarized light at the slab-film interface during TIR. A conversion efficiency of 32%, corresponding to a second-harmonic wavelength of 532 nm has been observed using computer-aided simulation. Optical losses such as surface roughness, absorption, and interference effect due to the nonlinear law of reflection have also been incorporated.

Multifunctional Waterborne Acoustic Metagratings: From Extraordinary Transmission to Total and Abnormal Reflection

Abstract: Wavelength-thick multifunctional metagratings for waterborne sound are highly desirable in various application scenarios, such as human health monitoring and ocean exploration. In this article, we present a class of metagratings with which distinct and switchable wave manipulation functionalities, such as extraordinary transmission, total reflection, and abnormal reflection can be achieved through a single acoustic grating. Simultaneous and high-efficiency control over both reflected and transmitted waves is achieved through a systematic design approach in which wave diffraction theory, effective medium theory, and intelligent optimization algorithms are concurrently utilized. Switching between different functionalities can be achieved simply by changing the operation frequency, or by changing the incident angles. Robust and perfect wave-front manipulation effects are obtained over a wide range of incident angles and operation frequencies. Our work not only provides a design paradigm of planar acoustic devices for multiple manipulation purposes, but also presents a feasible solution for the development of integrated acoustic devices for distinct underwater applications.

Planar photonic chips with tailored angular transmission for high-contrast-imaging devices.

Abstract: A limitation of standard brightfield microscopy is its low contrast images, especially for thin specimens of weak absorption, and biological species with refractive indices very close in value to that of their surroundings. We demonstrate, using a planar photonic chip with tailored angular transmission as the sample substrate, a standard brightfield microscopy can provide both darkfield and total internal reflection (TIR) microscopy images with one experimental configuration. The image contrast is enhanced without altering the specimens and the microscope configurations. This planar chip consists of several multilayer sections with designed photonic band gaps and a central region with dielectric nanoparticles, which does not require top-down nanofabrication and can be fabricated in a larger scale. The photonic chip eliminates the need for a bulky condenser or special objective to realize darkfield or TIR illumination. Thus, it can work as a miniaturized high-contrast-imaging device for the developments of versatile and compact microscopes. The authors design a planar photonic chip with several multilayers of photonic band gaps and a region of dielectric nanoparticles for tailored angular transmission. They use it as sample substrate for high-contrast darkfield and total internal reflection microscopy on a conventional microscope.

Optimizing the Sensing Efficiency of Plasmonic Based Gas Sensor

Abstract: This paper investigates the behavior of the surface plasmon polaritons (SPPs) on dielectric-metal interface using Ag thin film on glass substrate. The Kretschman configuration, which is sensitive to the change in the local environment adjacent to Ag thin film, has been modeled using COMSOL Multiphysics, RF module. The graphical presentation for the change in excitation spectra of SPPs on the interface has been analyzed by adjusting the incident angle greater than critical angle of glass while keeping the thickness of Ag thin film constant. The cross-sectional view reveals that the maximum amplitude of electric field occurs at 43° incidence. In order to study the behavior of resonance dip at varying refractive index (from 1.00 to 1.01), the reflection spectra for transverse magnetic (TM) mode of incident light has been extracted using far-field analysis. To further explore the sensitivity and resolution of the device, nm/RIU is collected by using the change in the wavelength (nm) of SPPs for minimum reflection. The remarkably maximum sensitivity of the device has been calculated as 23,000 nm/RI and Q value calculated for Ag-based sensing is 13.

Localized surface plasmon resonance inflection points for improved detection of chemisorption of 1-alkanethiols under total internal reflection scattering microscopy.

Abstract: Plasmonic gold nanoparticles are widely used in localized surface plasmon resonance (LSPR) sensing. When target molecules adsorb to the nanoparticles, they induce a shift in the LSPR scattering spectrum. In conventional LSPR sensing, this shift is monitored at the maximum of the LSPR scattering peak. Herein, we describe the sensitivity of detecting chemisorption of 1-alkanethiols with different chain lengths (1-butanethiol and 1-haxanethiol) on single gold nanorods (AuNRs) of fixed diameter (25 nm) and three different aspect ratios under a total internal reflection scattering microscope. For single AuNRs of all sizes, the inflection point (IF) at the long-wavelength side (or low-energy side) of the LSPR scattering peak showed higher detection sensitivity than the traditionally used peak maximum. The improved sensitivity can be ascribed to the shape change of the LSPR peak when the local refractive index is increased by chemisorption. Our results demonstrate the usefulness of tracking the curvature shapes by monitoring the homogeneous LSPR IF at the red side of the scattering spectrum of single AuNRs.

Waveguide display device

Abstract: An optical waveguide includes an input diffractive optical element arranged for being aligned with an optical projector for diffracting the light beam therefrom, a waveguide substrate arranged for reflecting the light beam diffracted by the input diffractive optical element by means of total internal reflection, and an output diffractive optical element coupled at said waveguide substrate for partially diffracting the light beam as a diffracted light and partially transmitting the light beam as a transmitted light during the total internal reflection of the light beam within the waveguide substrate. The diffracted light is diffracted by the output diffractive optical element and is projected out of the waveguide substrate toward the user eye. The transmitted light is continuously transmitted and reflected within the waveguide substrate by the total internal reflection until the transmitted light is totally diffracted out of the waveguide substrate, so as to complete an exit pupil expansion.

Refractive index sensor for sensing high refractive index bioliquids at the THz frequency

Abstract: In this paper, a highly sensitive and low loss refractive index (RI) biosensor for high RI bio-analytes detection, such as for cholesterol, nicotine, and bacillus bacteria, is proposed. A novel, to the best of our knowledge, decagonal structure with porous core photonic crystal fiber (PC-PCF) is introduced. The porous core consists of a rectangular sensing hole. The analytes to be sensed are considered in liquid form and infiltrated into the sensing holes, which makes the sensing process simpler and more straightforward. Cladding of the PC-PCF consists of multilayer circular air holes arranged in a decagonal pattern. For durability and stability of the sensor, TOPAS is used as the fiber material. A perfectly matched layer is used for boundary conditions. The correlation among optical power, material, and structural properties is analyzed by the finite element method. The sensing performance of the designed sensor is observed at THz frequency (1.4–3.8 THz). The results under high RI of the analytes (1.52–1.55) are as follows: maximum sensitivity of 98.31% for x polarization and 98.26% for y polarization, very low confinement loss of 1.5×10−14dB/m, narrow effective mode area of 1.92×10−7m2, minimum effective material loss of 0.000164cm−1, and very low waveguide dispersion of 0.002±0.05 ps/THz/cm. In addition, the effect of variation of structural parameters on sensor performance is also analyzed. The proposed PC-PCF-based biosensor can be very useful for sensing higher RI biochemical analytes.