Parallel Polarization Research Articles

Impact of light polarization on laser speckle contrast imaging with a custom phantom for microvascular flow

Laser speckle contrast imaging (LSCI) is a non-invasive, powerful, and cost-effective imaging technique that has seen widespread adoption across various medical fields, particularly for blood flow imaging. While LSCI provides physicians with valuable insights into changes or occlusions in blood flow, the technique is susceptible to various factors and parameters that can impact measurement sensitivity and signal-to-noise ratio (SNR). These include the scattering of light, which can affect the quality and reliability of the LSCI data. The polarization of light holds significant promise to enhance the performance of LSCI. In this study, we employed polarization manipulation of light to investigate its impact on the performance of LSCI for measuring flow. Focusing on the application of LSCI in microcirculation within capillaries, we examined the effect of polarization control on the technique's flow measurement capabilities using a custom-designed phantom system. This phantom consisted of three tubes with inner diameters of 1.1 mm, 1.6 mm, and 2.8 mm, embedded in a polydimethylsiloxane (PDMS) matrix with optical properties similar to biological tissue. By manipulating the polarization of both the incident and reflected light, alternating between parallel and perpendicular states, we compared the performance of our LSCI system in detecting flow for different tube diameters and depths within the phantom. Our study revealed that while depth is a critical parameter influencing flow detection using LSCI, employing perpendicular polarization (between incident and reflected light) resulted in the lowest measurement error and highest SNR compared to parallel polarization and the absence of polarization control.

Ultrafast terahertz conductivity in epitaxial graphene nanoribbons: an interplay between photoexcited and secondary hot carriers

Abstract Optical pump-terahertz probe spectroscopy has been used to investigate ultrafast photo-induced charge carrier transport in 3.4 µm wide graphene ribbons upon scaling the optical pump intensity. For low pump fluences, the deposited pump energy is rapidly redistributed through carrier-carrier scattering, producing secondary hot carriers: the picosecond THz photoconductivity then acquires a negative sign and scales linearly with an increasing pump fluence. At higher fluences, there are not enough equilibrium carriers able to accept the deposited energy, directly generated (excess) carriers start to contribute significantly to the photoconductivity with a positive sign leading to its saturation behavior. This leads to a non-monotonic variation of the carrier mobility and plasmonic resonance frequency as a function of the pump fluence and, at high fluences, to a balance between a decreasing carrier scattering time and an increasing Drude weight. In addition, a weak carrier localization observed for the polarization parallel to the ribbons at low pump fluences is progressively lifted upon increasing the pump fluence as a result of the rise of initial carrier temperature.

Ferroelectric Smectic C Liquid Crystal Phase with Spontaneous Polarization in the Direction of the Director.

In the previous study, the existence of an unidentified ferroelectric smectic phase is demonstrated in the low-temperature region of the ferroelectric smectic A phase, where the layer spacing decreases with decreasing temperature. In the present study, the phase is identified by taking 2D X-ray diffraction images of a magnetically oriented sample while allowing it to rotate and constructed a 3D reciprocal space with the sample rotation angle as the third axis for the whole picture of the reciprocal lattice vectors originating from the smectic structure. Consequently, circular diffraction images are obtained when the reciprocal lattice vectors are evenly distributed on the conical surface at a certain inclination angle in the reciprocal space. This result provides clear evidence that the phase in question is smectic C. The polarization properties also showed that the observed smectic C phase has spontaneous polarization in the direction parallel to the director and is identified as ferroelectric smectic C. These results provide a new type of classification for liquid crystalline phases that has been established over many years and is a significant contribution to the basic science of soft matter research.

Polarization-sensitive mechanically-tunable microwave filter using metallic photonic crystals

A tunable, polarization-sensitive microwave filter based on a reconfigurable dual-layered two-dimensional metallic photonic crystal is presented. The filter consists of a metallic plate with periodic holes and metal pillars which are anchored in a substrate material. The diameter of the pillars is smaller than that of the holes and the lateral position of the pillars with respect to the holes is tunable. We study the transmission of the device for linearly polarized electromagnetic radiation, which is polarized parallel and perpendicular to the displacement of the movable membrane, respectively. The tuning range spans from 20.18 GHz to 26.58 GHz for perpendicular polarization and from 20.18 GHz to 18.42 GHz for parallel polarization. The 3 dB operation bandwidth varies between 3.82 GHz and 6.44 GHz for perpendicular polarization and between 3.42 GHz and 3.82 GHz for parallel polarization. Experimental data are in good agreement with finite element method (FEM) simulations. The proposed structure is scalable to other frequency ranges such as terahertz frequencies.

Exploring the optical properties of naphdiyne sheet: First-principles study

Naphdiyne sheet is a two-dimensional carbon-based structure composed of naphthyl rings and acetylenic linkages. The optical characteristics of naphdiyne sheets are investigated using density functional theory. The results showed that this sheet is suitable for energy storage systems due to its high dielectric constant. The dielectric constant of naphdiyne is higher than that of graphene. The refractive index, absorption, reflection, and transmission coefficients are calculated based on the dielectric function. A notable optical absorption is observed across a wide energy range for parallel polarization. The transparency of this material is evident in its reflection and transmission constants, particularly in high-energy regions. The findings suggest that the naphdiyne sheets hold promise for use in nanoelectronics and optoelectronics.

First principles study of BN triphenylene-graphdiyne monolayer and bilayer structures with varying C-chain lengths: insights into optical behavior

First-principles calculations engaging density functional theory (DFT) are employed to systematically study the optical characteristics of monolayer and bilayer boron nitride (BN) triphenylene-graphdiyne (Tp-BNyne) structures featuring varying lengths of C-chains. The thermal stability of Tp-BNyne structures at temperatures up to 1000 K is verified. The weak van der Waals interactions due to the small binding energies and significant interlayer distances maintain the cohesion between the layers. The investigation revealed that all Tp-BNyne structures under examination exhibit semiconductor behavior with a band gap in the range of 0.97–2.74 eV. The bilayer configurations demonstrated a narrower energy band gap in comparison to the monolayer ones. Increasing the length of C-chains leads to a reduction in the energy band gap. Delving into the optical behavior of Tp-BNyne structures under photon incidence with parallel and perpendicular polarizations, a distinct anisotropy in the optical characteristics of Tp-BNyne is revealed. The static dielectric constant increases and the optical band gap decreases with increasing C-chain length. The absorption coefficients of monolayer and bilayer Tp-BNyne structures, on the order of 107/m, demonstrate that these sheets can effectively absorb light in the visible and ultraviolet regions. These findings present Tp-BNyne sheets as promising candidates for use in photovoltaic devices to convert sunlight into electrical current, as well as for designing optical devices for ultraviolet protection. Additionally, Tp-BNyne structures are transparent materials, especially in the high-energy range.

A power spectrum approach to the search for axion-like particles from resolved galaxy clusters using CMB as a backlight

Axions or axion-like particles (ALPs) are hypothetical particles predicted by beyond standard model theories, which make one of the dark matter candidates. These particles can convert into photons and vice-versa in the presence of a magnetic field, with a probability decided by its coupling strength gaγ . One of the ways to detect these particles is by using the Cosmic Microwave Background (CMB) as a backlight. As the CMB photons pass through a galaxy cluster, they can get converted into ALPs in the mass range 10-15 eV to 10-11 eV through resonant conversion in the presence of cluster magnetic fields. This leads to a polarized spectral distortion (α-distortion) in the CMB as the photon polarization parallel to the magnetic field in the galaxy cluster is involved in the conversion. The fluctuations in the magnetic field and electron density in a galaxy cluster lead to spatially varying α-distortion around the cluster, with a power spectrum that is different from the lensed E-mode and B-mode CMB polarization power spectrum for the standard model of cosmology. By measuring the difference in the polarization power spectrum around a galaxy cluster from the all-sky signal, one can find new α-distortion in the sky. For the resolved galaxy clusters, if the redshift, electron density, and magnetic field profiles of the cluster can be constrained using optical, X-ray, and radio observations, one can measure the coupling strength gaγ from the ALP power spectrum. The contamination from CMB and galactic foregrounds such as synchrotron and dust can be mitigated by using multiple frequency bands by leveraging on the difference in the spectral shape of the signal from foregrounds. Using the ILC technique to clean the foregrounds, we show that the new power spectrum-based approach of the resolved galaxy clusters from upcoming CMB experiments such as Simons Observatory and CMB-S4 can detect (or put constraints) on the ALP-photon coupling strength of gaγ < 5.2 × 10-12 GeV-1 and gaγ < 3.6 × 10-12 GeV-1 at 95% C.I. respectively for ALPs of masses 10-13 eV or for smaller gaγ for lighter ALP masses.

Scattering Elimination in 2D IR Immune from Detector Artifacts.

Highly scattering samples, such as polymer droplets or solid-state powders, are difficult to study via coherent two-dimensional infrared (2D IR) spectroscopy. Previously, researchers have employed (quasi-) phase cycling, local-oscillator chopping, and polarization control to reduce scattering, but the latter method poses a limit on polarization-dependent measurements. Here, we present a method for Scattering Elimination Immune from Detector Artifacts (SEIFDA) in pump-probe 2D IR experiments. Our method extends the negative probe delay method of removing scattering from pump-probe spectroscopy to 2D experiments. SEIFDA works well for all polarizations when combined with the optimized noise reduction scheme to remove additive and multiplicative noise. We demonstrate that our method can be employed with any polarization scheme and reliably lowers the scattering at parallel polarization to comparable levels to the conventional 8-frame phase cycling with probe chopping (8FPCPC) at perpendicular polarization. Our system can acquire artifact free spectra in parallel polarization when the signal intensity is as little as 5% of the intensity of the interference between the pump pulses scattered into the detector. It reduces the time required to characterize the scattering term by at least 50% over 8FPCPC. Through detailed analysis of detector nonlinearity, we show that the performance of 8FPCPC can be improved by incorporating nonlinear correction factors, but it is still worse than that of SEIFDA. Application of SEIFDA to study the encapsulation of Nile red in polymer droplets demonstrates that this method will be very useful for probing highly scattering systems.

The effects of thickness, polarization, and strain on vibrational modes of 2D Fe3GeTe2

In this study, we investigated the effects of thickness, light polarization, and strain on the Raman spectra of two-dimensional (2D) Fe3GeTe2 (FGT) crystals synthesized via chemical vapor transport. The crystals are thoroughly characterized using a combination of microscopic, diffraction, and spectroscopic techniques. Particularly, a systematic angle-resolved polarized Raman spectroscopy study reveals a clear polarization dependence of the Raman intensity in both parallel and crossed polarization directions, with the Ag1 mode completely disappearing in the crossed polarization direction. The angle-dependent intensity of both the Ag1 and E2g2 modes in parallel polarization and the intensity of the E2g2mode in the crossed polarization remain constant at all angles. These findings align with predictions from Raman tensor analysis, providing compelling evidence for the unambiguous assignment of the Ag1 and E2g2 modes to specific peaks observed in the Raman spectrum of FGT, resolving existing confusion in the literature regarding their assignment. Furthermore, we examined the effect of strain on the Raman spectrum of 2D FGT in-situ using a bending device. Our study, conducted on a monolayer to few-layer 2D FGT deposited onto polyethylene terephthalate and subjected to outward (inward), i.e., tensile (compressive) bending, demonstrates appreciable downshifting (upshifting) of the Raman peak position of both Ag1 and E2g2 modes. These findings are particularly significant given that strain engineering represents an effective approach to modulate the magnetic properties of FGT and other 2D magnetic materials.

Terahertz dependence on the laser phase and wavelength in two-color circularly polarized laser fields

Two-color circularly polarized laser fields with the same helicity (CP-S) are an appealing type of driving field that is essential for generating strong terahertz (THz) radiation. In this work, we theoretically investigate how the phase delay between two components in a CP-S field and their wavelength affect THz radiation yield with a photocurrent model. Simulations show that, as the laser wavelength increases, the THz yield becomes more and more phase dependent, which is maximized when the relative phase is 0 and minimized when the relative phase is π. Such a phase dependence is completely different from the commonly used two-color linearly polarized laser pulse with parallel polarizations (LP-P). In addition, in the CP-S field, the THz radiation energy may exhibit a wavelength scaling λ α (λ is the fundamental wavelength) with a bigger exponent than in the LP-P field at longer wavelengths. Our findings have important implications for the generation of powerful THz radiation via femtosecond laser interaction with gasses.

Selection rules in the excitation of the divacancy and the nitrogen-vacancy pair in 4 H - and 6 H -SiC

In this study, we address selection rules with respect to the polarization of the optical excitation of two color centers in 4H−SiC and 6H−SiC with potential for applications in quantum technology, the divacancy and the nitrogen-vacancy pair. We show that the photoluminescence of the axial configurations of higher symmetry (C3v) than the basal ones (C1h) can be canceled using any excitation (resonant or nonresonant) with polarization parallel to the crystal axis (EL||c). The polarization selection rules are determined using group-theoretical analysis and simple physical arguments showing that phonon-assisted absorption with EL||c is prohibited despite being formally allowed by group theory. A comparison with the selection rules for the silicon vacancy, another defect with C3v symmetry, is also carried out. Using the selection rules, we demonstrate selective excitation of only one basal divacancy configuration in 4H−SiC, the PL3 line, and discuss the higher contrast and increased Debye-Waller factor in the selectively excited spectrum. Published by the American Physical Society 2024

Multispectral Depolarization Based Living-Skin Detection: A New Measurement Principle.

Camera-based photoplethysmographic imaging enabled the segmentation of living-skin tissues in a video, but it has inherent limitations to be used in real-life applications such as video health monitoring and face anti-spoofing. Inspired by the use of polarization for improving vital signs monitoring (i.e. specular reflection removal), we observed that skin tissues have an attractive property of wavelength-dependent depolarization due to its multi-layer structure containing different absorbing chromophores, i.e. polarized light photons with longer wavelengths (R) have deeper skin penetrability and thus experience thorougher depolarization than those with shorter wavelengths (G and B). Thus we proposed a novel dual-polarization setup and an elegant algorithm (named "MSD") that exploits the nature of multispectral depolarization of skin tissues to detect living-skin pixels, which only requires two images sampled at the parallel and cross polarizations to estimate the characteristic chromaticity changes (R/G) caused by tissue depolarization. Our proposal was verified in both the laboratory and hospital settings (ICU and NICU) focused on anti-spoofing and patient skin segmentation. The clinical experiments in ICU also indicate the potential of MSD for skin perfusion analysis, which may lead to a new diagnostic imaging approach in the future.

Tuning optical properties of palgraphyne under biaxial mechanical strain: A density functional theory investigation

Palgraphyne is a two-dimensional material of carbon atoms with sp2 and sp hybridization. The influence of the biaxial mechanical strain on its optical properties is studied based on density functional theory calculations. It is observed that palgraphyne exhibits anisotropic behavior for photons with parallel and perpendicular polarizations. The dielectric constant of palgraphyne is higher than graphene. It has been found that biaxial tensile and compressive strains lead to an increase and decrease in the dielectric constant, respectively. The absorption spectra of palgraphyne sheet across the infrared to ultraviolet regions suggest that it has the potential to be a valuable material for utilization as absorber. Additionally, the reflection and transmission constants indicate that the sheet displays high transparency specially in the ultraviolet region. The peaks of optical quantities including dielectric constant, absorption coefficient, optical coefficient, reflection and transmission constants are moved to lower and higher energies under tensile and compressive strains, respectively. The anticipated results could make palgraphyne a candidate for optoelectronic applications.

Theoretical and experimental study of modulation transfer spectroscopy for non-cycling transitions of Rb D1 lines

Modulation transfer spectroscopy (MTS) of the D1 transitions of 85Rb and 87Rb atoms with only non-cycling transitions have been investigated theoretically and experimentally using the density matrix equation for four different polarization configurations: linear orthogonal (lin⊥lin), linear parallel (lin ∥ lin), circular orthogonal (σ+−σ−), and circular parallel (σ+−σ+). MTS signals for the Fg=3→Fe=2 transition of 85Rb atom and Fg=2→Fe=1 transition of the 87Rb atom among D1 transition lines were found to be stronger than those for the other transitions. The two non-cycling transitions have strong MTS signals because of the branching ratios and populations of the relevant ground states. The observed line shapes of the in-phase and quadrature components of MTS signals agree well with the theoretical shapes obtained by solving the density matrix equation.

Modelling of electromagnetic wave propagation at free space-human skin interface

The proliferation of the ambient environment with many wireless communication infrastructures has constituted a source of concern, at least from health point of view. Hence, there is a need for the investigation of electromagnetic wave interaction with human tissue. This paper models electromagnetic wave propagation at the free-space-human tissue interface, in an attempt to understand how electromagnetic waves interact with the human skin. Maxwell’s equations were used to derive the governing equations of propagating electromagnetic waves in free space while the solution of the boundary-value problem arising from the incident electromagnetic wave at the free space-human tissue interface was found through the use of appropriate boundary conditions. The solution led to the emergence of four Fresnel equations, a pair each for perpendicular and parallel polarizations of the incident wave, at the boundary. Through the use of the Fresnel equations obtained, components of the incident electromagnetic wave power at the boundary can be quantified. Typical profiles of the normalized reflected and absorbed powers at the free space-human skin interface at 28, 60, and 73 GHz were computed, using measured values of permittivity of human tissue. It is found that the total absorption of the power of the incident wave by the human skin occurs when the angle of incidence is between 690 and 770 for the three frequencies used. This is irrespective of the polarization of the incident wave. Information on such distinct angles of incidence for total absorption of power does not appear to exist in the literature as they represent new, previously unreported facts in the existing body of research. Furthermore, computed profiles of reflected power versus incidence angle displayed here, for perpendicular polarization, demonstrate strong alignments with established literature, thus reinforcing the credibility of the results presented in this paper.

Polarized QED cascades over pulsar polar caps

ABSTRACT The formation of e± plasmas within pulsar magnetospheres through quantum electrodynamics (QED) cascades in vacuum gaps is widely acknowledged. This paper aims to investigate the effect of photon polarization during the QED cascade occurring over the polar cap of a pulsar. We employ a Monte Carlo-based QED algorithm that accurately accounts for both spin and polarization effects during photon emission and pair production in both single-particle and particle-in-cell (PIC) simulations. Our findings reveal distinctive properties in the photon polarization of curvature radiation (CR) and synchrotron radiation (SR). CR photons exhibit high linear polarization parallel to the plane of the curved magnetic field lines, whereas SR photons, on average, demonstrate weak polarization. As the QED cascade progresses, SR photons gradually dominate over CR photons, thus reducing the average degree of photon polarization. Additionally, our study highlights an intriguing observation: the polarization of CR photons enhances e± pair production by approximately 5 per cent, in contrast to the inhibition observed in laser–plasma interactions. Our self-consistent QED-PIC simulations in the corotating frame reproduce the essential results obtained from single-particle simulations.

Asymptotic evolution of speckle patterns to synthesize non-homogeneous string beams

We analyze the evolution of the speckle pattern subjected to a compression showing that the spatial evolution of the probability density function satisfies a non-linear diffusion equation. Its asymptotic solution corresponds to a non-homogeneous wave with string-shape. During the compression, an irradiance interaction is generated between the speckles. The interaction satisfies a type-logistic equation. The electric field components associated with the speckles present a random behavior, however, during the compression process, the transversal components cancel each other, and the resulting light presents a state of polarization parallel to the propagation coordinate, justifying the non-homogeneous behavior. Computer simulations are presented.

Transmission characteristics of linearly polarized light in altostratus help explain “UV-sky-pol” paradox

The perception of skylight polarization in the ultraviolet (UV) by some species is surprising, because the degree of polarization (DoP) of light from the clear sky is considerably lower in the UV than in the visible spectral range. This is the so-called “ultraviolet paradox of the perception of skylight polarization (UV-sky-pol paradox)”. To explain the “UV-sky-pol” paradox, we analyzed the polarization retention characteristics of parallel polarized light transmits in altostratus at multiple wavelengths. According to the polarization sensitive bands of flies, honeybee, scarab beetles and spider, the study wavelength was set at 350 nm, and the wavelength of control group was set at 400, 450, 500, 550 and 600 nm, respectively. Then, we used the polarized light Monte Carlo method to simulate the forward transmission of 100 000 parallel polarization photons in altostratus. Calculation results show that the polarization retention characteristics are excellent at the 350 nm wavelength. Finally, we analyzed the transmission characteristics of parallel polarized light passes through a droplet at 0°–15° forward scattering angle. The analysis results show that there have been significant polarization retention channels in the UV band around 350 nm. This study can help to elucidate the “UV-sky-pol” paradox.

From Feast to Famine: A Systematic Study of Accretion onto Oblique Pulsars with 3D GRMHD Simulations

Disk-fed accretion onto neutron stars can power a wide range of astrophysical sources ranging from X-ray binaries, to accretion-powered millisecond pulsars, ultraluminous X-ray sources, and gamma-ray bursts. A crucial parameter controlling the gas–magnetosphere interaction is the strength of the stellar dipole. In addition, coherent X-ray pulsations in many neutron star systems indicate that the star's dipole moment is oblique relative to its rotation axis. Therefore, it is critical to systematically explore the 2D parameter space of the star's magnetic field strength and obliquity, which is what this work does, for the first time, in the framework of 3D general-relativistic magnetohydrodynamics. If the accretion disk carries its own vertical magnetic field, this introduces an additional factor: the relative polarity of the disk and stellar magnetic fields. We find that depending on the strength of the stellar dipole and the star–disk relative polarity, the neutron star's jet power can either increase or decrease with increasing obliquity. For weak dipole strength (equivalently, high accretion rate), the parallel polarity results in a positive correlation between jet power and obliquity, whereas the antiparallel orientation displays the opposite trend. For stronger dipoles, the relative-polarity effect disappears, and jet power always decreases with increasing obliquity. The influence of the relative polarity gradually disappears as obliquity increases. Highly oblique pulsars tend to have an increased magnetospheric radius, a lower mass accretion rate, and enter the propeller regime at lower magnetic moments than aligned stars.

Orthogonally and linearly polarized green emission from a semipolar InGaN based microcavity

Abstract Polarized light has promising applications in biological inspections, displays, and precise measurements. Direct emission of polarized light from a semiconductor device is highly desired in order to reduce the size and energy-consumption of the whole system. In this study, we demonstrate a semipolar GaN-based microcavity light-emitting diode (MCLED) that could simultaneously produce green light with perpendicular and parallel polarizations to the c*-axis. Orthogonally polarized emission with a narrow linewidth (∼0.2 nm) arises from the valence band splitting and birefringent nature of the semipolar GaN material, as well as the mode selection of the resonant cavity. By modulating the cavity length, the device is capable of switching between single- and multi-mode emission spectra. We believe that the approach of employing a cavity structure and semipolar GaN can be extended to produce orthogonally and linearly polarized blue, red, and violet light by adjusting the material compositions.