Showing papers on "Birefringence published in 2016"
TL;DR: In this article, a review examines the known deep UV NLO materials with respect to their crystal structure, band gap, SHG efficiency, laser damage threshold, and birefringence.
Abstract: Deep ultraviolet (absorption edge 6.2 eV) nonlinear optical (NLO) materials are of current interest owing to their technological applications and materials design challenges. Technologically, the materials are used in laser systems, atto-second pulse generation, semiconductor manufacturing, and photolithography. Designing and synthesizing a deep UV NLO material requires crystallographic non-centrosymmetry, a wide UV transparency range, a large second-harmonic generating coefficient (dij > 0.39 pm/V), moderate birefringence (Δn ∼ 0.07), chemical stability and resistance to laser damage, and ease in the growth of large high-quality single crystals. This review examines the known deep UV NLO materials with respect to their crystal structure, band gap, SHG efficiency, laser damage threshold, and birefringence. Finally, future directions with respect to new deep UV NLO materials are discussed.
431 citations
TL;DR: The substantial reduction of light scattering multiplicity at tissue optical clearing that leads to a lesser influence of scattering on the measured intrinsic polarization properties of the tissue and allows for more precise quantification of these properties is demonstrated.
Abstract: This tutorial-review introduces the fundamentals of polarized light interaction with biological tissues and presents some of the recent key polarization optical methods that have made possible the quantitative studies essential for biomedical diagnostics. Tissue structures and the corresponding models showing linear and circular birefringence, dichroism, and chirality are analyzed. As the basis for a quantitative description of the interaction of polarized light with tissues, the theory of polarization transfer in a random medium is used. This theory employs the modified transfer equation for Stokes parameters to predict the polarization properties of single- and multiple-scattered optical fields. The near-order of scatterers in tissues is accounted for to provide an adequate description of tissue polarization properties. Biomedical diagnostic techniques based on polarized light detection, including polarization imaging and spectroscopy, amplitude and intensity light scattering matrix measurements, and polarization-sensitive optical coherence tomography are described. Examples of biomedical applications of these techniques for early diagnostics of cataracts, detection of precancer, and prediction of skin disease are presented. The substantial reduction of light scattering multiplicity at tissue optical clearing that leads to a lesser influence of scattering on the measured intrinsic polarization properties of the tissue and allows for more precise quantification of these properties is demonstrated.
231 citations
TL;DR: In this article, it was shown that the reduced crystal symmetry of ReS2 leads to anisotropic optical properties that persist from the bulk down to the monolayer limit.
Abstract: Rhenium disulfide (ReS2), a layered group VII transition metal dichalcogenide, has been studied by optical spectroscopy. We demonstrate that the reduced crystal symmetry, as compared to the molybdenum and tungsten dichalcogenides, leads to anisotropic optical properties that persist from the bulk down to the monolayer limit. We find that the direct optical gap blueshifts from 1.47 eV in the bulk to 1.61 eV in the monolayer limit. In the ultrathin limit, we observe polarization-dependent absorption and polarized emission from the band-edge optical transitions. We thus establish ultrathin ReS2 as a birefringent material with strongly polarized direct optical transitions that vary in energy and orientation with sample thickness.
225 citations
TL;DR: The PVLAS experiment as discussed by the authors employed a sensitive polarimeter based on a high finesse Fabry-Perot cavity, and the data were analyzed taking into account the intrinsic birefringence of the dielectric mirrors of the cavity.
Abstract: Vacuum magnetic birefringence was predicted long time ago and is still lacking a direct experimental confirmation. Several experimental efforts are striving to reach this goal, and the sequence of results promises a success in the next few years. This measurement generally is accompanied by the search for hypothetical light particles that couple to two photons. The PVLAS experiment employs a sensitive polarimeter based on a high finesse Fabry–Perot cavity. In this paper we report on the latest experimental results of this experiment. The data are analysed taking into account the intrinsic birefringence of the dielectric mirrors of the cavity. Besides a new limit on the vacuum magnetic birefringence, the measurements also allow the model-independent exclusion of new regions in the parameter space of axion-like and milli-charged particles. In particular, these last limits hold also for all types of neutrinos, resulting in a laboratory limit on their charge.
191 citations
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TL;DR: In this paper, the equivalence between normalized Stokes parameters and diffraction contrasts in appropriately designed phase-gradient birefringent metasurfaces was brought to the fore.
Abstract: Measuring light's state of polarization is an inherently difficult problem, since the phase information between orthogonal polarization states is typically lost in the detection process. In this work, we bring to the fore the equivalence between normalized Stokes parameters and diffraction contrasts in appropriately designed phase-gradient birefringent metasurfaces and introduce a concept of all-polarization birefringent metagratings. The metagrating, which consists of three interweaved metasurfaces, allows one to easily analyze an arbitrary state of light polarization by conducting simultaneous (i.e., parallel) measurements of the correspondent diffraction intensities that reveal immediately the Stokes parameters of the polarization state under examination. Based on plasmonic metasurfaces operating in reflection at the wavelength of 800 nm, we design and realize phase-gradient birefringent metasurfaces and the correspondent metagrating, while experimental characterization of the fabricated components convincingly demonstrates the expected functionalities. We foresee the use of the metagrating in compact polarimetric setups at any frequency regime of interest.
159 citations
TL;DR: The Raman spectra were measured in dependence on the polarization direction for backscattering on three crystallographic planes of β-Ga2O3 and modelled these dependencies using a modified Raman tensor formalism which takes birefringence into account.
Abstract: The Raman spectrum and particularly the Raman scattering intensities of monoclinic β-Ga2O3 are investigated by experiment and theory. The low symmetry of β-Ga2O3 results in a complex dependence of the Raman intensity for the individual phonon modes on the scattering geometry which is additionally affected by birefringence. We measured the Raman spectra in dependence on the polarization direction for backscattering on three crystallographic planes of β-Ga2O3 and modelled these dependencies using a modified Raman tensor formalism which takes birefringence into account. The spectral position of all 15 Raman active phonon modes and the Raman tensor elements of 13 modes were determined and are compared to results from ab-initio calculations.
144 citations
TL;DR: In this paper, the authors describe the attractive properties of these materials, along with the unique capabilities and novel crystal growth and processing that continue to provide record-breaking conversion efficiencies and output powers in the mid-infrared.
Abstract: Advances in growth of the birefringent crystals ZnGeP2 and CdSiP2, as well as all-epitaxial processing of orientation-patterned semiconductors GaAs (OP-GaAs) and GaP (OP-GaP), are extending solid-state laser output deep into the mid-infrared. These materials exhibit the highest nonlinear coefficients and broadest infrared transparency ranges among all practical nonlinear optical crystals. In this review paper we describe the attractive properties of these materials, along with the unique capabilities and novel crystal growth and processing that continue to provide record-breaking conversion efficiencies and output powers in the mid-infrared.
143 citations
20 Oct 2016
TL;DR: In this article, the spin-Hall effect of light has been shown to be a transverse spin-dependent shift of the paraxial light beam transmitted through the plate, which is a novel example of the spin Hall effect of energy.
Abstract: The linear birefringence of uniaxial crystal plates has been known since the 17th century, and it is widely used in numerous optical setups and devices. Here we demonstrate, both theoretically and experimentally, the fine lateral circular birefringence of such crystal plates. We show that this effect is a novel example of the spin-Hall effect of light, i.e., a transverse spin-dependent shift of the paraxial light beam transmitted through the plate. The well-known linear birefringence and the new circular birefringence form an interesting analogy with the Goos–Hanchen and Imbert–Fedorov beam shifts that appear in the light reflection at a dielectric interface. We report experimental observation of the effect in a remarkably simple system of a tilted half-wave plate and polarizers using polarimetric and quantum-weak-measurement techniques for beam-shift measurements. In view of much recent interest in spin–orbit interaction phenomena, our results could find applications in modern polarization optics and nanophotonics.
114 citations
TL;DR: In this article, the vector-coupled nonlinear Schrodinger equation is applied to describe the propagation of Thirring optical solitons in birefringent fibers with Kerr law nonlinearity, detuning, intermodal dispersion and spatiotemporal dispersion.
Abstract: The vector-coupled nonlinear Schrodinger equation, which can be applied to describe the propagation of Thirring optical solitons in birefringent fibers with Kerr law nonlinearity, detuning, intermodal dispersion and spatiotemporal dispersion, has been studied analytically. By means of the complex envelope function ansatz, exact Thirring bright-dark combosolitons are reported, and the properties of these solitons are discussed.
99 citations
95 citations
TL;DR: A double-wavelength metasurface based on polarization dependent dielectric meta-atoms that control the phases of two orthogonal polarizations independently is proposed and demonstrated.
Abstract: Metasurfaces are ultrathin optical structures that manipulate optical wavefronts. Most metasurface devices which deflect light are designed for operation at a single wavelength, and their function changes as the wavelength is varied. Here we propose and demonstrate a double-wavelength metasurface based on polarization dependent dielectric meta-atoms that control the phases of two orthogonal polarizations independently. Using this platform, we design lenses that focus light at 915 and 780 nm with perpendicular linear polarizations to the same focal distance. Lenses with numerical apertures up to 0.7 and efficiencies from 65% to above 90% are demonstrated. In addition to the high efficiency and numerical aperture, an important feature of this technique is that the two operation wavelengths can be chosen to be arbitrarily close. These characteristics make these lenses especially attractive for fluorescence microscopy applications.
TL;DR: An opportunity of producing the widest variety of "free-form" 4G optical components with arbitrary spatial patterns of the optical anisotropy axis orientation with the aid of a digital spatial light polarization converter (DSLPC).
Abstract: Geometrical phase or the fourth generation (4G) optics enables realization of optical components (lenses, prisms, gratings, spiral phase plates, etc.) by patterning the optical axis orientation in the plane of thin anisotropic films. Such components exhibit near 100% diffraction efficiency over a broadband of wavelengths. The films are obtained by coating liquid crystalline (LC) materials over substrates with patterned alignment conditions. Photo-anisotropic materials are used for producing desired alignment conditions at the substrate surface. We present and discuss here an opportunity of producing the widest variety of "free-form" 4G optical components with arbitrary spatial patterns of the optical anisotropy axis orientation with the aid of a digital spatial light polarization converter (DSLPC). The DSLPC is based on a reflective, high resolution spatial light modulator (SLM) combined with an "ad hoc" optical setup. The most attractive feature of the use of a DSLPC for photoalignment of nanometer thin photo-anisotropic coatings is that the orientation of the alignment layer, and therefore of the fabricated LC or LC polymer (LCP) components can be specified on a pixel-by-pixel basis with high spatial resolution. By varying the optical magnification or de-magnification the spatial resolution of the photoaligned layer can be adjusted to an optimum for each application. With a simple "click" it is possible to record different optical components as well as arbitrary patterns ranging from lenses to invisible labels and other transparent labels that reveal different images depending on the side from which they are viewed.
TL;DR: A compact optical fiber magnetic field sensor based on the principle of the Sagnac interferometer is proposed, which is attractive due to its compact size, low cost, and immunity to electromagnetic interference beyond what conventional magnetic field sensors can offer.
Abstract: A compact optical fiber magnetic field sensor based on the principle of the Sagnac interferometer is proposed. Different from the conventional ones, a ferrofluid-filled high-birefringence photonic crystal fiber (HB-PCF) is inserted into the Sagnac as a magnetic field sensing element. The refractive index of the ferrofluid filled in the cladding air holes of the HB-PCF will change with respect to the applied magnetic field, and subsequently, the birefringence of the HB-PCF will change, which will affect the shifts of the output interference spectrum in Sagnac. Experiments are carried out to verify the simulation model and the results indicate that the interference spectrum exhibits a red shift with the increment in the magnetic field intensity. The sensitivity of the proposed sensor is up to 0.073 nm/mT for a magnetic field intensity ranging from 10 to 40 mT, while the resolution is 0.001 mT. The proposed magnetic field sensor is attractive due to its compact size, low cost, and immunity to electromagnetic interference beyond what conventional magnetic field sensors can offer.
TL;DR: In this article, a new beryllium-free fluoride carbonate Ca2Na3(CO3)3F was successfully synthesized through molecular engineering design, and large single crystals were grown by spontaneous crystallization with molten fluxes.
Abstract: It is a great challenge to explore deep-ultraviolet (deep-UV) nonlinear optical (NLO) materials that can achieve a subtle balance among large nonlinear coefficients, moderate birefringence, and deep-ultraviolet (UV) transparency. A new beryllium-free fluoride carbonate Ca2Na3(CO3)3F was successfully synthesized through molecular engineering design, and large single crystals were grown by spontaneous crystallization with molten fluxes. The substitution of NLO-active [BO3] groups for [CO3] groups resulted in an optimal balance among the SHG coefficient, birefringence, and UV transparency. Via comparison of these two iso-structural compounds, the second-harmonic generation coefficients and birefringence of Ca2Na3(CO3)3F have been greatly improved. Remarkably, Ca2Na3(CO3)3F exhibited a wide transparent region with a deep-UV absorption edge at 190 nm. These results demonstrated Ca2Na3(CO3)3F is a promising NLO material in the UV or deep-UV region.
TL;DR: In this paper, a side-polished birefringent photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR) was proposed.
Abstract: We propose a highly sensitive side-polished birefringent photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR). The polished surface of the proposed structure is coated with indium tin oxide (ITO) to excite plasmon and the analytes can be placed on the flat surface easily instead of filling the voids. The birefringent nature of the structure helps in coupling more fields to the ITO-dielectric interface. With the optimum thickness of 110 nm of ITO, the structure shows a maximum wavelength sensitivity of 17000 nm/RIU with a resolution of 5.8 × 10−6 RIU. Further this also showed an amplitude sensitivity of 74 RIU−1 along with a resolution of 1.35 × 10−5 RIU. Moreover, the effect of bending on this low loss structure is also analyzed.
TL;DR: Some crucial design parameters such as power fraction, bending loss, and dispersion for usability in the THz regime are focused on.
Abstract: A polarization-maintaining porous-core spiral photonic crystal fiber is proposed for efficient transmission of polarization-maintaining terahertz (THz) waves. The finite element method with perfectly matched layer boundary conditions is used to characterize the guiding properties. We demonstrate that by creating artificial asymmetry in the porous core, an ultrahigh birefringence of 0.0483 can be obtained at the operating frequency of 1.0 THz. Moreover, a low effective material loss of 0.085 cm−1 and very small confinement loss of 1.91×10−3 dB/cm are achieved for the y-polarization mode with optimal design parameters. This article also focuses on some crucial design parameters such as power fraction, bending loss, and dispersion for usability in the THz regime.
TL;DR: Lenses were recorded in liquid crystal polymer layers only a few micrometers thick, using laser-based photoalignment and UV photopolymerization, which allowed focusing and defocusing laser beams depending on the sign of the circularity of laser beam polarization.
Abstract: We report on lenses that operate over the visible wavelength band from 450 nm to beyond 700 nm, and other lenses that operate over a wide region in the near-infrared from 650 nm to beyond 1000 nm. Lenses were recorded in liquid crystal polymer layers only a few micrometers thick, using laser-based photoalignment and UV photopolymerization. Waveplate lenses allowed focusing and defocusing laser beams depending on the sign of the circularity of laser beam polarization. Diffraction efficiency of recorded waveplate lenses was up to 90% and contrast ratio was up to 500:1.
TL;DR: In this article, the authors present results of calculations and numerical simulations for a scenario to measure this effect using multi-MeV photons generated in the collision of electrons with a laser pulse, and find that the birefringence of the vacuum should be measurable using experimental parameters attainable in the near future.
Abstract: Real photon-photon scattering is a long-predicted phenomenon that is being searched for in experiment in the form of a birefringent vacuum at optical and x-ray frequencies. We present results of calculations and numerical simulations for a scenario to measure this effect using multi-MeV photons generated in the collision of electrons with a laser pulse. We find that the birefringence of the vacuum should be measurable using experimental parameters attainable in the near future.
TL;DR: In this article, the numerical design optimization of a new kind of relatively simple porous-core photonic crystal fiber (PCF) for terahertz (THz) waveguiding was reported.
Abstract: We report on the numerical design optimization of a new kind of relatively simple porous-core photonic crystal fiber (PCF) for terahertz (THz) waveguiding. A novel twist is introduced in the regular hexagonal PCF by including a diamond-shaped porous-core inside the hexagonal cladding. The numerical results obtained from an efficient finite-element method, which confirms a high birefringence of the order $10^{\mathrm {-2}}$ and low effective material loss of 0.07 cm $^{\mathrm {-1}}$ at 0.7-THz operating frequency. The proposed PCF is anticipated to be useful in polarization sensitive THz appliances.
TL;DR: In this paper, a dual-hole unit-based porous-core hexagonal photonic crystal fiber (H-PCF) with low loss and high birefringence in terahertz regime was proposed.
Abstract: We present a new kind of dual-hole unit-based porous-core hexagonal photonic crystal fiber (H-PCF) with low loss and high birefringence in terahertz regime. The proposed fiber offers simultaneously high birefringence and low effective material loss (EML) in the frequency range of 0.5–0.85 THz with single-mode operation. An air-hole pair is used inside the core instead of elliptical shaped air holes to introduce asymmetry for attaining high birefringence; where the birefringence can be enhanced by rotating the dual-hole unit axis of orientation. The proposed H-PCF provides a birefringence of $\sim 0.033$ and an EML of 0.43 dB/cm at an operating frequency of 0.85 THz.
TL;DR: A polarizing birefringent NANF design that can achieve orthogonal polarization loss ratios as large as 30dB over the C-band while eliminating any undesirable polarization coupling effect thereby resulting in a single polarization output in a hollow core fiber regardless of the input polarization state is proposed.
Abstract: We systematically study different approaches to introduce high birefringence and high polarization extinction ratio in hollow core antiresonant fibers. Having shown the ineffectiveness of elliptical cores to induce large birefringence in hollow core fibers, we focus on designing and optimizing polarization maintaining Hollow Core Nested Antiresonant Nodeless Fibers (HC-NANF). In a first approach, we create and exploit anti-crossings with glass modes at different wavelengths for the two polarizations. We show that suitable low loss high birefringence regions can be obtained by appropriately modifying the thickness of tubes along one direction while leaving the tubes in the orthogonal direction unchanged and in antiresonance. Using this concept, we propose a new birefringent NANF design providing low loss (~40dB/km) and high birefringence (>10−4) over a record bandwidth of ~550nm, and discuss how bandwidth can be traded off to further reduce the loss to a few dB/km. Finally, we propose a polarization mode-stripping technique in the birefringent NANF. As a demonstration, we propose a polarizing birefringent NANF design that can achieve orthogonal polarization loss ratios as large as 30dB over the C-band while eliminating any undesirable polarization coupling effect thereby resulting in a single polarization output in a hollow core fiber regardless of the input polarization state.
TL;DR: In this paper, a fine lateral circular birefringence of uniaxial crystal plates is demonstrated, both theoretically and experimentally, for the spin-Hall effect of light, i.e., a transverse spin-dependent shift of the paraxial light beam transmitted through the plate.
Abstract: The linear birefringence of uniaxial crystal plates is known since the 17th century, and it is widely used in numerous optical setups and devices. Here we demonstrate, both theoretically and experimentally, a fine lateral circular birefringence of such crystal plates. This effect is a novel example of the spin-Hall effect of light, i.e., a transverse spin-dependent shift of the paraxial light beam transmitted through the plate. The well-known linear birefringence and the new circular birefringence form an interesting analogy with the Goos-Hanchen and Imbert-Fedorov beam shifts that appear in the light reflection at a dielectric interface. We report the experimental observation of the effect in a remarkably simple system of a tilted half-wave plate and polarizers using polarimetric and quantum-weak-measurement techniques for the beam-shift measurements. In view of great recent interest in spin-orbit interaction phenomena, our results could find applications in modern polarization optics and nano-photonics.
TL;DR: A novel class of geometric phase elements-"Bragg-Berry" optical elements based on a helicity-preserving circular Bragg-reflection phenomenon are reported on, which allow the efficient spin-orbit tailoring of light fields without the need to fulfill any condition on birefringent phase retardation.
Abstract: We report on highly reflective spin-orbit geometric phase optical elements based on a helicity-preserving circular Bragg-reflection phenomenon. First, we present a dynamical geometric phase experiment using a flat chiral Bragg mirror. Then, we show that shaping such a geometric phase allows the efficient spin-orbit tailoring of light fields without the need to fulfill any condition on birefringent phase retardation, in contrast to the case of transmission spin-orbit optical elements. This is illustrated by optical vortex generation from chiral liquid crystal droplets in the Bragg regime that unveils spin-orbit consequences of the droplet's curvature. Our results thus introduce a novel class of geometric phase elements-"Bragg-Berry" optical elements.
TL;DR: It is shown that in the limit of averaging over a sufficiently large scattering depth, the observed Raman intensities converge and can be described by an effective Raman tensor given here.
Abstract: We present a formalism for calculating the Raman scattering intensity dependent on the polarization configuration for optically anisotropic crystals. It can be applied to crystals of arbitrary orientation and crystal symmetry measured in normal incidence backscattering geometry. The classical Raman tensor formalism cannot be used for optically anisotropic materials due to birefringence causing the polarization within the crystal to be depth dependent. We show that in the limit of averaging over a sufficiently large scattering depth, the observed Raman intensities converge and can be described by an effective Raman tensor given here. Full agreement with experimental results for uniaxial and biaxial crystals is demonstrated.
TL;DR: In this paper, a Mach-Zehnder interferometric hydrophone is demonstrated using polarization-maintaining photonic-crystal-fiber (PM-PCF), spliced between two single-mode-fibers, operated at 1550 nm source.
Abstract: Low frequency under-water acoustic signal detections are challenging, especially for marine applications. A Mach-Zehnder interferometric hydrophone is demonstrated using polarization-maintaining photonic-crystal-fiber (PM-PCF), spliced between two single-mode-fibers, operated at 1550 nm source. These data are compared with standard hydrophone, single-mode and multimode fiber. The PM-PCF sensor shows the highest response with a power shift (2.32 dBm) and a wavelength shift (392.8 pm) at 200 Hz. High birefringence values and the effect of the imparted acoustic pressure on this fiber, introducing the difference between the fast and slow axis changes, owing to the phase change in the propagation waves, demonstrate the strain-optic properties of the sensor.
TL;DR: In this article, the reflection bandwidth of a cholesteric liquid crystal (CLC) is given by Δλ = ΔnP, where the birefringence Δn is Δn−−−no.
Abstract: Cholesteric liquid crystals (CLCs) are known to exhibit selective reflection of incident radiation due to their periodic helical structure, which makes them promising candidates for a myriad of different photonic applications. At normal incidence, CLCs reflect circularly polarized incident light of the same handedness as the cholesteric helix and of wavelength λ between noP and neP, where no and ne are the ordinary and extraordinary refractive indices, respectively, of the locally uniaxial structure, and P is the pitch of the helix. Thus, the reflection bandwidth Δλ is given by Δλ = ΔnP, where the birefringence Δn = ne − no. Within the bandwidth, right-circularly polarized light is reflected by a right-handed helix, whereas left-circularly polarized light is transmitted. Outside the bandwidth, both polarization states are transmitted. Therefore, Δλ depends on Δn. Moreover, Δn is typically limited to 0.3–0.4 for colorless organic compounds, and Δλ is often
Abstract: Spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) offer a high potential to overcome several limitations of conventional purely charged-based laser devices. Presumably, the highest potential of spin-VCSELs lies in their ultrafast spin and polarization dynamics, which can be significantly faster than the intensity dynamics in conventional devices. Here, we experimentally demonstrate polarization oscillations in spin-VCSELs with frequencies up to 44 GHz. The results show that the oscillation frequency mainly depends on the cavity birefringence, which can be tuned by applying mechanical strain to the VCSEL structure. A tuning range of about 34 GHz is demonstrated. By measuring the polarization oscillation frequency and the birefringence governed mode splitting as a function of the applied strain simultaneously, we are able to investigate the correlation between birefringence and polarization oscillations in detail. The experimental findings are compared to numerical calculations based on ...
TL;DR: The abnormal angle-dependent polarized Raman scattering of the Ag modes in thin BP crystal, which deviates from the normal Raman selection rules, is successfully interpreted by the theoretical model based on birefringence.
Abstract: The incident and scattered light engaged in the Raman scattering process of low symmetry crystals always suffer from the birefringence-induced depolarization. Therefore, for anisotropic crystals, the classical Raman selection rules should be corrected by taking the birefringence effect into consideration. The appearance of the 2D anisotropic materials provides an excellent platform to explore the birefringence-directed Raman selection rules, due to its controllable thickness at the nanoscale that greatly simplifies the situation comparing with bulk materials. Herein, a theoretical and experimental investigation on the birefringence-directed Raman selection rules in the anisotropic black phosphorus (BP) crystals is presented. The abnormal angle-dependent polarized Raman scattering of the Ag modes in thin BP crystal, which deviates from the normal Raman selection rules, is successfully interpreted by the theoretical model based on birefringence. It is further confirmed by the examination of different Raman modes using different laser lines and BP samples of different thicknesses.
TL;DR: It is reported that twisting the periodically structured “space” within a coreless photonic crystal fiber creates a helical channel where guided modes can form despite the absence of any discernible core structure.
Abstract: A century ago, Einstein proposed that gravitational forces were the result of the curvature of space-time and predicted that light rays would deflect when passing a massive celestial object. We report that twisting the periodically structured “space” within a coreless photonic crystal fiber creates a helical channel where guided modes can form despite the absence of any discernible core structure. Using a Hamiltonian optics analysis, we show that the light rays follow closed spiral or oscillatory paths within the helical channel, in close analogy with the geodesics of motion in a two-dimensional gravitational field. The mode diameter shrinks, and its refractive index rises, as the twist rate increases. The birefringence, orbital angular momentum, and dispersion of these unusual modes are explored.
TL;DR: This Letter suggests a novel kind of porous-core photonic crystal fiber (PCF) for efficient transportation of polarization maintaining (PM) terahertz (THz) waves and introduces an asymmetry in both the pores and the porous-cladding of the structure to achieve an ultra-high birefringence.
Abstract: In this Letter, we suggest a novel kind of porous-core photonic crystal fiber (PCF) (to the best of our knowledge) for efficient transportation of polarization maintaining (PM) terahertz (THz) waves. We introduce an asymmetry in both the porous-core and the porous-cladding of the structure to achieve an ultra-high birefringence. Besides, only circular air holes have been used to represent the structure, which makes the fiber remarkably simple. The transmission characteristics have been numerically examined based on an efficient finite element method (FEM). The numerical results confirm a high birefringence of ∼0.045 and a very low effective absorption loss of 0.08 cm−1 for optimal design parameters at 1 THz. We have also thoroughly investigated some important modal properties such as bending loss, power fraction, dispersion, and fabrication possibilities to completely analyze the structure’s usability in a multitude of THz appliances. Moreover, physical insights of the proposed fiber have also been discussed.