Unpolarized Light Research Articles

Study of a coronagraphic mask using evanescent waves.

The evanescent wave coronagraph (EvWaCo) is a specific kind of band-limited coronagraph using the frustrated total internal reflection phenomenon to produce the coronagraphic effect (removing starlight from the image plane in order to make the stellar environment detectable). In this paper, we present a theoretical and experimental study of the EvWaCo coronagraphic mask. First, we calculate the theoretical transmission and we show that this mask is partially achromatic. Then, we present the experimental results obtained in unpolarized light at the wavelength λ≈900 nm and relative spectral bandwidth Δλ/λ≈6%. In particular, we show that the coronagraph provides a contrast down to a few 10-6 at an angular distance of about ten Airy radii.

Making unpolarized light sensitive to polarization-sensitive devices

Here we propose a novel scheme in which depolarized or unpolarized light can respond to polarization-dependent optical elements such as an input-polarization-dependent wavelength-selective filter. An optical apparatus for realizing the proposed scheme consists of a polarization beam splitter, two Faraday rotators, and four waveplates. By harnessing the polarization decomposition/superposition property of a polarization-diversity loop and the irreciprocity of optical Faraday rotation, we could make unpolarized light selectively excite one of the two principal axes of a polarization-dependent element like polarized one while maintaining the unpolarized nature. The principle of operation of this apparatus was theoretically analyzed through the investigation of the polarization variation of light passing through the optical elements comprising it. Then, its polarization-selective feature was experimentally verified by employing a long-period fiber grating inscribed on polarization-maintaining fiber, which had polarization-dependent spectral dips with different resonance wavelengths, as a polarization-dependent element in the polarization-diversity loop. One of the two spectrally separated resonances of the grating could be freely chosen through the proper control of the waveplates in the apparatus.

A time to polarize the unpolarized.

Optics Wavelength, intensity, and polarization are several parameters that define a light field. Light from most natural and many artificial light sources is unpolarized. However, an instantaneous snapshot of a propagating light field shows that it may be completely polarized. Shevchenko et al. propose and demonstrate experimentally that there is a finite window of time for that snapshot in which the unpolarized light is polarized. Using a Michelson interferometer with polarization- sensitive components combined with two-photon absorption detectors, they show for two different unpolarized light sources that the polarization time window varies from 10 to 100 fs. The effect of a finite polarization time could be exploited in a number of applications such as communication and sensing. Optica 4 , 64 (2017).

High-Sensitivity Polarization-Independent Biochemical Sensor Based on Silicon-on-Insulator Cross-Slot Waveguide

Slot waveguides reported so far are strongly polarization dependent. The concept of a novel cross-slot waveguide (WG) is presented here, which contains both vertical and horizontal slots and supports the power enhancement in the slot region for both the quasi-TE and quasi-TM polarizations. This WG can be fabricated by exploiting well developed CMOS technology. For the unpolarized operating light, the cross-slot WG is optimized to have the maximum total slot confinement factors for the two polarizations. As for a polarization-independent design, confinement in slot for each polarization can reach 39.4% when the silicon core width and height are set to around 223 and 216 nm, respectively. It is shown here that a biochemical sensor employing a cross-slot-WG exhibits a significantly better sensitivity, compared to either a vertical-slot-WG or a horizontal-slot-WG-based biochemical sensor. As an example, when both the vertical slot width and horizontal slot height are set to 100 nm, its sensitivity is almost double compared with a simple vertical or horizontal slot with 100 nm gap.

A novel filtering structure allowing polarization-sensitive devices to respond to unpolarized light

A novel filtering structure allowing polarization-sensitive devices to respond to unpolarized light

Fully solution-processed, transparent organic power-generating polarizer

We fabricate transparent organic power-generating polarizer by all solution process. Based on the conventional indium–tin-oxide-coated glass as the bottom cathode, the subsequent layers are prepared by a combination of solution processing methods. Sprayed silver nanowires film serves as the top anode and can transmit greater than 80% of the visible light with sheet resistance of 16 Ω/□. By adopting the quasi-bilayer structure for the photoactive layer composed of rubbed polymer donors to produce anisotropic optical property underneath fullerene acceptors, the finished device demonstrates a power conversion efficiency of 1.36% with unpolarized light, a dichroic ratio of 3.2, and a high short circuit current ratio of 2.6 with polarized light. Our proposed fabrication procedures of devices take into account not only the cost-effective production, but also the flexibility of devices for applying in flexible, scalable circuits to advance the development of future technology.

Transparent Depolarizing Organic and Inorganic Films for Optics and Sensors.

Fabrication of novel organic and inorganic depolarizing films derived from quasinematic cellulose nanocrystal (CNC) organization is demonstrated. These films convert linearly polarized and circularly polarized light into unpolarized light over the entire visible region. Patterning of the quasinematic CNCs on top of a chiral nematic film gives latent images that are revealed only upon observation through the circularly polarizing filters.

Polarization time of unpolarized light

Light produced by most natural and artificial sources is unpolarized or partially polarized. At any instant of time, however, such random light can be regarded as fully polarized, but the polarization state may vary drastically within short time intervals. This rate of change is another attribute that separates one unpolarized beam from another. Here, we study such polarization dynamics and, for the first time to our knowledge, measure the characteristic time, called the polarization time, in which the instantaneous polarization state stays essentially unaltered. The technique employs a polarization-sensitive Michelson interferometer and two-photon absorption detection valid for electromagnetic light, yielding superior femtosecond time resolution. We analyze two unpolarized light sources: amplified spontaneous emission from a fiber amplifier and a dual-wavelength laser source. The characterization of polarization dynamics can have significant applications in optical sensing, polarimetry, telecommunication, and astronomy, as well as in quantum and atom optics.

Reflectance of micron-sized dust particles retrieved with the Umov law

The maximum positive polarization Pmax that initially unpolarized light acquires when scattered from a particulate surface inversely correlates with its geometric albedo A. In the literature, this phenomenon is known as the Umov law. We investigate the Umov law in application to single-scattering submicron and micron-sized agglomerated debris particles, model particles that have highly irregular morphology. We find that if the complex refractive index m is constrained to Re(m)=1.4–1.7 and Im(m)=0–0.15, model particles of a given size distribution have a linear inverse correlation between log(Pmax) and log(A). This correlation resembles what is measured in particulate surfaces, suggesting a similar mechanism governing the Umov law in both systems. We parameterize the dependence of log(A) on log(Pmax) of single-scattering particles and analyze the airborne polarimetric measurements of atmospheric aerosols reported by Dolgos & Martins in [1]. We conclude that Pmax ≈ 50% measured by Dolgos & Martins corresponds to very dark aerosols having geometric albedo A=0.019 ± 0.005.

Organic photodiodes from homochiral l-proline derived squaraine compounds with strong circular dichroism.

We suggest and explore a novel route towards organic photodetectors sensitive to the circular polarization state of light. For this, we insert fullerene-blended thin films of homochiral squaraine compounds acting as a highly circular dichroic active layer into conventional bulk hetero-junction photodiodes. Initially, we discuss steady-state characterization of photodiodes with unpolarized light. The homochiral, l-proline derived squaraine compounds are obtained via a chiral pool synthesis in sizable quantities. The aggregation behavior of the two compounds with varying side chain length is complex. They exhibit H-type spectral signatures only in colloidal solution, and both H- and J-type features with large splitting in neat and fullerene-blended thin films. We probe strong excitonic circular dichroism for both aggregate species, showing the most competitive dissymmetry factors up to -0.055 in fullerene-blended thin films. We vary the blend ratio and layer thickness of such active layers in the photodiodes. The device performance is in all cases limited by a low fill factor which is accompanied by a voltage-dependent photocurrent. Current-voltage measurements show light intensity dependent characteristics, which are S-shaped, contrary to our expectations, only for thin active layers independent of the blend ratio. The external quantum efficiency is in some cases extraordinarily high, exceeding 100 percent in the blue spectral range under modest reverse bias voltages for thin, fullerene-rich devices. However, the most promising are the devices with thick, donor-rich layers defined by a spectral overlap of the strongest photocurrent response and the maximum circular dichroism within the green spectral range. Thus, we demonstrate the feasibility of combining photodiode functionality and strong circular dichroism as intrinsic material properties.

Integration of celestial compass cues in the central complex of the locust brain.

Many insects rely on celestial compass cues such as the polarization pattern of the sky for spatial orientation. In the desert locust, the central complex (CX) houses multiple sets of neurons, sensitive to the oscillation plane of polarized light and thus probably acts as an internal polarization compass. We investigated whether other sky compass cues like direct sunlight or the chromatic gradient of the sky might contribute to this compass. We recorded from polarization-sensitive CX neurons while an unpolarized green or ultraviolet light spot was moved around the head of the animal. All types of neuron that were sensitive to the plane of polarization (E-vector) above the animal also responded to the unpolarized light spots in an azimuth-dependent way. The tuning to the unpolarized light spots was independent of wavelength, suggesting that the neurons encode solar azimuth based on direct sunlight and not on the sky chromatic gradient. Two cell types represented the natural 90 deg relationship between solar azimuth and zenithal E-vector orientation, providing evidence to suggest that solar azimuth information supports the internal polarization compass. Most neurons showed advances in their tuning to the E-vector and the unpolarized light spots dependent on rotation direction, consistent with anticipatory signaling. The amplitude of responses and its variability were dependent on the level of background firing, possibly indicating different internal states. The integration of polarization and solar azimuth information strongly suggests that besides the polarization pattern of the sky, direct sunlight might be an important cue for sky compass navigation in the locust.

Exposing different in-depth pitches in the cuticle of the scarab beetle Cotinis mutabilis

The cuticle of the scarab beetle Cotinis mutabilis reflects left-handed polarized light indicating the presence of a helicoidal structure. Different in-depth pitches in the cuticle are corroborated by optical microscopy images of the cuticle which originally is yellowish or reddish but becomes greenish after gently scratching its top side. Using the Mueller-matrix formalism the degree of polarization (total and circular) of reflected light is determined for unpolarized incident light. The effects of the finite thickness of the cuticle on the broadening and strength of the selective Bragg reflection are discussed on the basis of dispersion relations for optical modes in helicoidal structures and simulated spectra.

Modified analytic expression for the single-scattering phase function

In electromagnetic radiative transfer calculation, the accuracy and the computation time are usually determined by the representation of single-scattering phase function. Accurate calculation is time consuming even for spherical particle, thus, an analytic representation is commonly adopted to approximate the exact phase function and then accelerate the calculation. Most widely used single-scattering phase functions are the Henyey-Greenstein phase function and modified Henyey-Greenstein phase function (Henyey-Greenstein*). Although the Henyey-Greenstein phase function and the Henyey-Greenstein* phase function can represent the forward-scattering peak of Mie-scattering phase function well, they fail to reproduce the backscattering behavior, limiting the accuracy of the calculation. In order to better fit exact calculations and simulate the backward-scattering peak, we develop a new analytic expression based on the fundamental theory of electromagnetic scattering and radiation transmission. This phase function is an algebraic expression with one single free parameter (asymmetry factor), and can be expanded into Legendre polynomials. The new phase function converges to the Rayleigh phase function when the asymmetry factor approximates to 0, and it can approach to the Henyey-Greenstein phase function as the asymmetry factor is about 1. We compare the Henyey-Greenstein phase function, the Henyey-Greenstein* phase function, and the new phase function for different asymmetry factors, and find that the new phase function provides a more realistic description for the unpolarized light scattering from small particles. Furthermore, the calculated value for the ratio of the scattering intensity at 90 degree to that in the backward direction is more reasonable. We also investigate the effectiveness by approximating the scattering from polydispersed particles through comparing the new phase function, the Henyey-Greenstein* phase function, and the Mie-scattering phase function for three types of Derimendjian's polydispersions. Results show that the new phase function fits the Mie-scattering phase function much better than the Henyey-Greenstein* phase function. For the new phase function, the root-mean-square error is small for 73.3% data. By contrast, only 26.7% data fit the Mie-scattering phase function well for the Henyey-Greenstein* phase function. Similarly, the effectiveness of new function is most significant when calculating the ratio of the scattering intensity at 90 degree to that in the backward direction. In summary, the new Henyey-Greenstein* phase function provides a more accurate calculation for the scattering intensity in the backward direction, and is conducive to electromagnetic radiative transfer calculation. Furthermore, because the proposed phase function has the same basic form as the Heny-Greenstein phase function, reformatting radiative transfer model in terms of the new phase function should require relatively little effort.

AUTOMATED QUANTIFICATION OF RENAL FIBROSIS WITH SIRIUS RED: IS A ROUTINE METHOD OF THE KIDNEY BIOPSY EVALUATION?

The Aim of our study was to determine the intra- and interobserver variation of the automated quantitative evaluation if cortical interstitial fibrosis performed on kidney biopsy samples stained with Sirius red compared to conventional approaches.
 Materials and methods. The range of fibrosis was evaluated on 15 renal biopsies obtained from adult patients with minimal change nephropathy, focal segmental glomerulosclerosis and membranous nephropathy. Different methods of fibrosis measurement were used: the automated quantification of Sirius red stained areas with and without polarization, the computerized quantification of the fibrotic interstitium on trichrome-stained samples and the visual assessment offibrotic tissues using quantitative and semiquantitative approaches on trkhrome and PAMS-stained slides. The intra- and interobserver variation of these methods was performed.
 Results. The quantitative evaluation of interstitial fibrosis with Sirius red stain under both polarized and unpolarized light demonstrated nearly perfect inter- and intraobserver reproducibility (ICC=0,85-0,99). Whereas, visual methods had relatively high variance between observations (ICC=0,48-0,66).
 Conclusions. The automated quantification of cortical interstitial fibrosis as performed on renal biopsy samples stained with Sirius red has been shown to be a highly accurate approach that could be used as a routine method of the kidney biopsy evaluation.

High-Efficiency Nanowire Solar Cells with Omnidirectionally Enhanced Absorption Due to Self-Aligned Indium-Tin-Oxide Mie Scatterers.

Photovoltaic cells based on arrays of semiconductor nanowires promise efficiencies comparable or even better than their planar counterparts with much less material. One reason for the high efficiencies is their large absorption cross section, but until recently the photocurrent has been limited to less than 70% of the theoretical maximum. Here we enhance the absorption in indium phosphide (InP) nanowire solar cells by employing broadband forward scattering of self-aligned nanoparticles on top of the transparent top contact layer. This results in a nanowire solar cell with a photovoltaic conversion efficiency of 17.8% and a short-circuit current of 29.3 mA/cm2 under 1 sun illumination, which is the highest reported so far for nanowire solar cells and among the highest reported for III-V solar cells. We also measure the angle-dependent photocurrent, using time-reversed Fourier microscopy, and demonstrate a broadband and omnidirectional absorption enhancement for unpolarized light up to 60° with a wavelength average of 12% due to Mie scattering. These results unambiguously demonstrate the potential of semiconductor nanowires as nanostructures for the next generation of photovoltaic devices.

Optical polarization of high-energy BL Lacertae objects

We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope. We wish to understand if there are differences in the BL Lac objects that are detected with the current-generation TeV instruments compared to those that have not yet been detected. The mean polarization fraction of the TeV-detected BL Lacs is 5% while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host galaxy is accounted for. The TeV sources show somewhat lower fractional polarization variability amplitudes than the non-TeV sources. Also the fraction of sources with a smaller spread in the Q/I - U/I -plane and a clumped distribution of points away from the origin, possibly indicating a preferred polarization angle, is larger in the TeV than in the non-TeV sources. These differences between TeV and non-TeV samples seems to arise from differences between intermediate and high spectral peaking sources instead of the TeV detection. When the EVPA variations are studied, the rate of EVPA change is similar in both samples. We detect significant EVPA rotations in both TeV and non-TeV sources, showing that rotations can occur in high spectral peaking BL Lac objects when the monitoring cadence is dense enough. Our simulations show that we cannot exclude a random walk origin for these rotations. These results indicate that there are no intrinsic differences in the polarization properties of the TeV-detected and non-TeV-detected high-energy BL Lac objects. This suggests that the polarization properties are not directly related to the TeV-detection, but instead the TeV loudness is connected to the general flaring activity, redshift, and the synchrotron peak location. (Abridged)

Static refractive index engineering of a singlewalled carbon nanotube through co-doping: A theoretical study

This paper presents a theoretical study of pristine single walled carbon nanotube (SWCNT) and SWCNT co- doped with combinations of Aluminum (Al), Phosphorous (P) and Nitrogen (N) along with their dielectric function spectra and calculated refractive indices. Dielectric function spectra of a pristine SWCNT and the SWCNT co-doped with Aluminum (Al) & Phosphorus (P) and another one co-doped with Al, P and Nitrogen (N) have been calculated using density functional theory implemented through the Cambridge sequential total energy package (CASTEP). The spectra show the variation of real and imaginary part of the dielectric function with frequency of the incident light. Polarized and unpolarized light as well as light through polycrystalline media has been considered. The value of static refractive index has been calculated using these curves. A substantial increase in the value of the static refractive index is observed when a pristine SWCNT is co-doped with Al and P but if in addition N atom is also introduced, a reduction in the value of static refractive index is observed though it does not fall lower than that of the pristine SWCNT. Thus, we propose that co-doping with atoms of different combinations of elements can be used as a novel and effective tool to manipulate the dielectric function and refractive index of SWCNTs and can pave the way to effective designing of various sensitive optical devices using SWCNTs for a variety of technological applications.

Holographic waveguide display with a combined-grating in-coupler.

Volume holographic gratings are widely used as couplers in eyewear waveguide display systems, but they show a relative lower TM polarized energy compared to transverse-electric (TE) incidence. In this paper, we propose a novel holographic waveguide display system with a combined-grating as the in-coupler. When used as an in-coupler for a holographic waveguide display system, a subwavelength metal grating is designed onto the volume holographic grating to increase the total diffraction efficiency of the coupling gratings. Theoretical calculations show that this design increases the diffraction efficiency by 16.4% for TM polarization, 4.3% for TE mode, and 10.0% for unpolarized light, compared to a single volume holographic grating. Calculations also show that the use of this design as an in-coupler for a holographic waveguide system increases the luminance efficiency for these three modes by 26.8%, 9.0%, and 15.6%, respectively.

Classical spectral ghost ellipsometry.

We introduce a novel ghost reflection ellipsometer for a spectral characterization of homogeneous thin films and interfaces. The device makes use of a uniform, spatially incoherent, unpolarized light source with Gaussian statistics and of the detection of intensity correlations. Unlike traditional ellipsometers, no source or detector calibration and reference sample are needed. The method is also insensitive to instrumentation errors. The ellipsometer that we present here is a classical analog of a quantum twin-photon arrangement discussed earlier in the literature. However, the classical configuration is easier to implement and use, because entangled photon pairs are not needed and appropriate light sources and detectors are readily available.

The copepod Calanus spp. (Calanidae) is repelled by polarized light.

Both attraction and repulsion from linearly polarized light have been observed in zooplankton. A dichotomous choice experiment, consisting of plankton light traps deployed in natural waters at a depth of 30 m that projected either polarized or unpolarized light of the same intensity, was used to test the hypothesis that the North Atlantic copepod, Calanus spp., is linearly polarotactic. In addition, the transparency of these copepods, as they might be seen by polarization insensitive vs. sensitive visual systems, was measured. Calanus spp. exhibited negative polarotaxis with a preference ratio of 1.9:1. Their transparency decreased from 80% to 20% to 30% in the unpolarized, partially polarized, and electric (e-) vector orientation domains respectively - that is, these copepods would appear opaque and conspicuous to a polarization-sensitive viewer looking at them under conditions rich in polarized light. Since the only difference between the two plankton traps was the polarization cue, we conclude that Calanus spp. are polarization sensitive and exhibit negative polarotaxis at low light intensities (albeit well within the sensitivity range reported for copepods). We hypothesize that Calanus spp. can use polarization vision to reduce their risk of predation by polarization-sensitive predators and suggest that this be tested in future experiments.