Broadband Reflection Spectrum Research Articles

Characterizing the Broadband Reflection Spectrum of MAXI J1803-298 during Its 2021 Outburst with NuSTAR and NICER

MAXI J1803-298 is a transient black hole candidate discovered in 2021 May during an outburst that lasted several months. Multiple X-ray observations reveal recurring “dipping” intervals in several of its light curves, particularly during the hard/intermediate states, with a typical recurrence period of ∼7 hr. We report analysis of four NuSTAR observations of the source, supplemented with NICER data where available, over the duration of the outburst evolution covering the hard, intermediate, and the soft states. Reflection spectroscopy reveals the black hole to be rapidly spinning (a * = 0.990 ± 0.001) with a near edge-on viewing angle (i = 70° ± 1°). Additionally, we show that the light-curve dips are caused by photoelectric absorption from a moderately ionized absorber whose origin is not fully understood, although it is likely linked to material from the companion star impacting the outer edges of the accretion disk. We further detect absorption lines in some of the spectra, potentially associated with Fe xxv and Fe xxvi, indicative of disk winds with moderate to extreme velocities. During the intermediate state and just before transitioning into the soft state, the source showed a sudden flux increase, which we found to be dominated by soft disk photons and consistent with the filling of the inner accretion disk, at the onset of state transition. In the soft state, we show that models of disk self-irradiation provide a better fit and a preferred explanation to the broadband reflection spectrum, consistent with previous studies of other accreting sources.

Collective and Quasi-Local Modes in the Optical Spectra of YB6 and YbB6 Hexaborides with Jahn–Teller Structural Instability

The broad-band reflection spectra of YB6 and YbB6 hexaborides with Jahn–Teller instability of the boron cage have been measured at room temperature. An optical conductivity analysis has revealed, along with the Drude electronic components, heavily overdamped collective modes, which are notable in YB6 for high dielectric contributions, Δε = 2000–5700. The fraction of nonequilibrium charge carriers in YB6, which is at the boundary of structural instability in the hexaboride family, reaches 85–90%, whereas this fraction in doped YbB6 semiconductor is not higher than 25%. It has been shown that unlike the predictions of the topological Kondo insulator model, the surface “metallization” in Yb2+B6 crystals can be explained by additional doping of a surface layer with Yb3+ ions.

Design of vanadium-dioxide-based resonant structures for tunable optical response.

Phase change materials are suitable for tunable photonic devices where the optical response can be altered under external stimuli, such as heat, an electrical or an optical signal. In this scenario, we performed numerical simulations to study the optical properties of a flat unpatterned resonant structure and a grating, both coated with a thin film of vanadium dioxide (VO2). Our results suggest that it is possible to modulate broadband and narrowband reflectance spectra of the resonant structures in the visible to near-infrared range by more than 40 % when the VO2 undergoes an insulator-to-metal phase transition. Resonant devices with a tunable spectral response may find application in sensors, filters, absorbers, and detectors.

Long-wavelength optical properties of the Ca0.33Sr0.33Ba0.33F2 solid solution single crystals

The optical characteristics of Ca0.33Sr0.33Ba0.33F2 (CSBF) single crystal of a ternary solid solution of calcium, strontium and barium fluorides were studied in order to create disordered optical media. Characteristics of the single crystals of individual fluorides CaF2, SrF2, BaF2 and binary solid solutions Ca0.6Sr0.4F2 and Ba0.67Sr0.33F2 were obtained. Broadband reflection and transmission spectra of the said crystals were recorded in the frequency range of 3–5000 cm−1, and the values of the complex refractive indexes and dielectric constants were calculated. The generalized four parameter model were used for calculation of absorption band parameters. For the CSBF crystal, a significant broadening of the band was observed in the absorption region at optical phonon modes along with an increased absorption in the region below the optical phonon frequencies, which is typical for disordered dielectric media.

Single-peak fiber Bragg gratings in suspended-core optical fibers.

Femtosecond laser inscribed fiber Bragg gratings in pure-silica suspended-core optical fibers have previously been demonstrated as a promising platform for high temperature sensing. However, the density of gratings that could be written on a single fiber was limited by undesired reflections associated with higher order modes in these high numerical aperture fibers. This resulted in a complex, broadband reflection spectrum with limited multiplexing capability. In this work we utilize modifications to the fine structure of the suspended core optical fibers to fine tune the relative confinement loss of the optical fiber modes, thus reducing the contribution from such higher order modes. The effects of these changes on mode propagation are modeled, giving a range of fibers with different confinement loss properties which can be tailored to the specific length scale of a desired application. We achieve single-peak reflections from individual fiber Bragg gratings, significantly improving performance for multipoint sensing and demonstrate this technique by writing 20 gratings onto a single fiber.

Development of an adjustable Kirkpatrick-Baez microscope for laser driven x-ray sources.

A prototype of a highly adjustable Kirkpatrick-Baez (KB) microscope has been designed, built, and tested in a number of laser driven x-ray experiments using the high power (200 TW) VEGA-2 laser system of the Spanish Centre for Pulsed Lasers (CLPU). The presented KB version consists of two, perpendicularly mounted, 500 μm thick silicon wafers, coated with a layer of platinum, a few tens of nanometers thick. Unlike the usual millimeter thick glass substrate, this design allows for a larger bending flexibility and large adjustment range. According to simulations, this KB microscope offers broadband multikiloelectron volt reflection spectra (1 eV-20 keV), allowing more spectral tunability than conventional Bragg crystals. In addition to be vacuum compatible, this prototype is characterized by a relatively small size (21 cm × 31 cm × 27 cm) and permits remote control and modification both of the radii of curvature (down to 10 m) and of the grazing incidence angle (up to 60 mrad). A few examples of focusing performance tests and experimental results are discussed.

Fiber-integrated refractive index sensor based on a diced Fabry-Perot micro-resonator.

We report on a fiber-integrated refractive index sensor based on a Fabry-Perot micro-resonator fabricated using simple diamond blade dicing of a single-mode step-index fiber. The performance of the device has been tested for the refractive index measurements of sucrose solutions as well as in air. The device shows a sensitivity of 1160 nm/RIU (refractive index unit) at a wavelength of 1.55 μm and a temperature cross-sensitivity of less than 10-7 RIU/°C. Based on evaluation of the broadband reflection spectra, refractive index steps of 10-5 of the solutions were accurately measured. The conducted coating of the resonator sidewalls with layers of a high-index material with real-time reflection spectrum monitoring could help to significantly improve the sensor performance.

Photoluminescence of terbium nitrate hexahydrate incorporated into pores of opal photonic crystals

We have measured photoluminescence (PL) and broadband reflection spectra of the surface of an opal photonic crystal (OPC) filled with terbium nitrate hexahydrate (TNH). The excitation sources used were a halogen lamp, lasers with emission wavelengths of 266 and 337 nm, and semiconductor light-emitting diodes (369 and 385 nm). It has been shown that resonance excitation of the TNH (Tb(NO3)3 · 6H2O) filled OPC by pulsed laser light at a wavelength of 266 nm gives rise to superluminescence: an increase in the relative intensity of the 545-nm PL band, corresponding to the Tb3+ 5 D 4–7 F 5 transition. The efficiency of pumping the upper laser level of the OPC increases due to the increase in the density of photon states (Purcell effect) near the bandgap edge in the OPC. The lack of clear superluminescence under excitation by the other excitation sources is related to specific features of the absorption of incident light by the rare-earth ion and to the low output power of the cw light sources.

SOI-based high performance multi-subpart profile grating mirror

In this study, a SOI-based high performance multi-subpart profile grating mirror (MPGM) is proposed and fabricated. It is shown that with the multi-subpart profile and strongly modulated thin grating layer, the MPGM experimentally demonstrated an ultra broadband reflection spectrum from 1.21 to1.62µm, very high reflectivity (R>97%), and low sensitivity to incident angle at the range of −12.3°<θ<+13.1°.

Broadening of effective photonic band gaps in biological chiral structures: From intrinsic narrow band gaps to broad band reflection spectra

Under normal illumination with non-polarized light, reflection spectra of the cuticle of golden-like and red Chrysina aurigans scarabs show a structured broad band of left-handed circularly polarized light. The polarization of the reflected light is attributed to a Bouligand-type left-handed chiral structure found through the scarab's cuticle. By considering these twisted structures as one-dimensional photonic crystals, a novel approach is developed from the dispersion relation of circularly polarized electromagnetic waves traveling through chiral media, to show how the broad band characterizing these spectra arises from an intrinsic narrow photonic band gap whose spectral position moves through visible and near-infrared wavelengths.

Opal-matrix nanocomposites containing metallic nanoparticles

A method has been proposed for the fabrication of three-dimensional photonic crystals based on ordered opal matrices (OMs), with pores containing iron group metal (M = Ni, Co, Fe) nanoparticles. The core of the method is the reduction of salts and oxides of these metals with supercritical isopropanol. The phase composition of OM/M composites depends on the composition of the starting salts (nitrates or chlorides): the use of ferric chloride leads to the formation of nanoparticles of solid solutions based on nickel metal, α-cobalt, or β-cobalt (Ni-Fe and Co-Fe systems) in opal pores; with the corresponding nitrates, we obtain OM/NiCo (cubic solid solution), OM/Fe, OM/Ni3Fe, OM/NiFe, OM/CoFe, and OM/NiCoFe nanocomposites. We have measured broadband reflection spectra of the (111) surface of the photonic crystals using a fiber-optic technique for taking reflection spectra. The intensity peak in the reflection (band gap) spectrum of the OM/M (M = Fe, Co, Ni) nanocomposites is shown to be shifted to longer wavelengths.

In vitro terahertz monitoring of muscle tissue dehydration under the action of hyperosmotic agents

Dehydration of muscle tissue in vitro under the action of biologically compatible hyperosmotic agents is studied using a laser terahertz spectrometer in the frequency range from 0.25 to . Broadband terahertz absorption and reflection spectra of the bovine skeletal muscle tissue were obtained under the action of glycerol, polyethylene glycol with the molecular weight 600 (PEG-600), and propylene glycol. The presented results are proposed for application in developing the methods of image contrast enhancement and increasing the depth of biological tissue probing with terahertz radiation.

NuSTARANDXMM-NEWTONOBSERVATIONS OF NGC 1365: EXTREME ABSORPTION VARIABILITY AND A CONSTANT INNER ACCRETION DISK

We present a spectral analysis of four coordinated NuSTAR+XMM-Newton observations of the Seyfert galaxy NGC 1365. These exhibit an extreme level of spectral variability, which is primarily due to variable line-of-sight absorption, revealing relatively unobscured states in this source for the first time. Despite the diverse range of absorption states, each of the observations displays the same characteristic signatures of relativistic reflection from the inner accretion disk. Through time-resolved spectroscopy we find that the strength of the relativistic iron line and the Compton reflection hump relative to the intrinsic continuum are well correlated, as expected if they are two aspects of the same broadband reflection spectrum. We apply self-consistent disk reflection models to these time-resolved spectra in order to constrain the inner disk parameters, allowing for variable, partially covering absorption to account for the vastly different absorption states observed. Each of the four observations is treated independently to test the consistency of the results obtained for the black hole spin and the disk inclination, which should not vary on observable timescales. We find both the spin and the inclination determined from the reflection spectrum to be consistent, confirming NGC 1365 hosts a rapidly rotating black hole; in all cases the dimensionless spin parameter is constrained to be a* > 0.97 (at 90% statistical confidence or better).

Continuous Pitch Stabilization of Cholesteric Liquid Crystals and Display Applications

We report a mechanism of the continuous pitch variation of the cholestric liquid crystal (CLC) with reactive mesogen based on the phase separation model and the 4 × 4 matrix analysis. From the density variation of the polymerized RM with chirality, the corresponding pitch variation was directly estimated and the reflectance was calculated using the 4 × 4 matrix analysis. The calculated result quantitatively coincided with the experimental result with broadband reflection spectrum. Also, we demonstrated a simple transflective LC display using the CLC white reflector with broadband reflection spectrum covering entire visible range.

Reflection spectra of etched FBGs under the influence of axial contraction and stress-induced index change

We present a new theoretical model for the broadband reflection spectra of etched FBGs which includes the effects of axial contraction and stress-induced index change. The reflection spectra of the etched FBGs with several different taper profiles are simulated based on the proposed model. In our observation, decaying exponential profile produces a broadband reflection spectrum with good uniformity over the range of 1540-1560 nm. An etched FBG with similar taper profile is fabricated and the experimental result shows good agreement with the theoretical model.

Design and tolerance analysis of photonic crystal slabs with ultrahigh reflection

A design strategy is proposed to optimize the structure of photonic crystal (PC) slabs in order to achieve high reflection on its interface. A broad band (Δλ/λ > 19%) and ultrahigh (>95%) reflection spectrum can be realized for one-dimensional (1D) and two-dimensional (2D) PC slabs. Tolerance analysis of design parameters reveals distinct merits for different PC structures: as to maintain high reflection, 1D PC slabs show better structural tolerance, while 2D PC ones have a much greater tolerance for the material parameter. Such a difference may provide a criterion for the design of PCs-assist mirror structure in optoelectronic devices.

Broadband Dielectric Measurements on Highly Scattering Materials

Dielectric properties of highly scattering dyed corn meal have been studied in broadband microwave and millimeter-wave frequency ranges from 3.95 to 12.4 GHz and from 34 to 120 GHz. Complex dielectric permittivity has been obtained using a free-space quasi-optical spectroscopy technique and a standard waveguide transmission/reflection (T/R) calibration method. The broadband transmittance and reflectance spectra have been recorded to yield the refractive and absorption index and calculate the real and imaginary parts of dielectric permittivity. The density dependence of complex permittivity of highly scattering dyed corn meal is presented. Different scattering mechanisms and the scattering model for a corn meal material have been discussed. Detailed analysis of measurement uncertainties, including instrumentation errors, to manifest the accuracy of both techniques, has also been performed.

A Multilayer-Based High-Performance Multisubpart Profile Grating Reflector

In this letter, a multilayer-based high-performance subwavelength multisubpart profile grating reflector (MPGR) is proposed and fabricated. The properties of the reflector are investigated by rigorous coupled-wave analysis for multilayered grating structures. It is shown that with the properly configured profile and strongly modulated grating layer in transverse-electric polarized wave, the MPGR experimentally demonstrates a broadband reflection spectrum from 1.56 to 1.8 ¿m, very high reflectivity (>97%), and good angular insensitivity of about 27.8°.

Comet 67P/Churyumov-Gerasimenko at a large heliocentric distance

Aims. The Jupiter family comet 67P/Churyumov-Gerasimenko (67P/C-G) is the target comet of ESA's ROSETTA mission. A detailed portrait of this comet has been drawn from observations around perihelion, but what needs to be enhanced is the description of the comet's behaviour at large heliocentric distance. It is not only important for planning the rendezvous of the ROSETTA spacecraft with the comet, but will also provide valuable information on basic physical properties of the nucleus and the possible distant activity of JFCs. Methods. We performed broad-band imaging and longslit spectroscopy of 67P/C-G in the visible wavelength range in June 2004, May 2006, and August 2006, when the comet was at the heliocentric distance of r ≥ 4.9 AU. The observations were performed at the ESO VLT with the FORS2 instrument. Results. The comet appears point-like at all observing epochs, indicating that no significant coma is present around the nucleus. From our 67P/C-G images, we determined a synodic rotational period of the nucleus of 12.7047 ± 0.0011 h and a linear phase function coefficient β = 0.076 ± 0.003 mag/°. We estimated the large-to-small axis ratio a/b ≥ 1.45 ± 0.09 and, assuming an albedo of 0.04, an effective nucleus radius of 2.38 ± 0.04 km, which corresponds to the mean absolute magnitude H = 15.35 ± 0.04 mag. No colour variation with the rotational phase is found, based on broadband colour indices and reflectance spectra of the nucleus. A trail-like structure of heavy grains is clearly detected in June 2004, displaying a surface brightness of ∼28 mag/ 2 in R the filter. For the first time we also determined visible colours of the trail-like structure: (V - R) = 0.95 ± 0.14 mag/ 2 and (R - 1) = 0.39 ± 0.14 mag/ 2 . A trail-like structure is also barely visible in the May 2006 images, but a quantitative study was not possible.

Comparative psychophysics of bumblebee and honeybee colour discrimination and object detection

Bumblebee (Bombus terrestris) discrimination of targets with broadband reflectance spectra was tested using simultaneous viewing conditions, enabling an accurate determination of the perceptual limit of colour discrimination excluding confounds from memory coding (experiment 1). The level of colour discrimination in bumblebees, and honeybees (Apis mellifera) (based upon previous observations), exceeds predictions of models considering receptor noise in the honeybee. Bumblebee and honeybee photoreceptors are similar in spectral shape and spacing, but bumblebees exhibit significantly poorer colour discrimination in behavioural tests, suggesting possible differences in spatial or temporal signal processing. Detection of stimuli in a Y-maze was evaluated for bumblebees (experiment 2) and honeybees (experiment 3). Honeybees detected stimuli containing both green-receptor-contrast and colour contrast at a visual angle of approximately 5 degrees , whilst stimuli that contained only colour contrast were only detected at a visual angle of 15 degrees . Bumblebees were able to detect these stimuli at a visual angle of 2.3 degrees and 2.7 degrees , respectively. A comparison of the experiments suggests a tradeoff between colour discrimination and colour detection in these two species, limited by the need to pool colour signals to overcome receptor noise. We discuss the colour processing differences and possible adaptations to specific ecological habitats.