Broadband Radiation Research Articles

Multidimensional high-temperature field measurement method for flame based on near infrared radiation spectrum of water vapor

The temperature distribution is an important parameter in the design and optimization of :various engine types, and this paper begins by analyzing the temperature sensitivities of different waveband combinations. Based on an evaluation of the effects of self-absorption phenomena on the temperature measurement accuracy, a multidimensional temperature measurement method based on the integrated ratio of the intensity of the water vapor broadband radiation spectra is proposed. To address the multidimensional temperature inversion problem, the practicality of a nonlinear reconstruction approach is evaluated, and a multiple linear iteration algorithm is proposed. In addition, validation experiments are performed in the form of methane/oxygen laminar flame temperature measurements in combination with parallel rotating beam spectroscopy, and the results show that the method exhibits temperature measurement resolution of 10 K. The method proposed and developed in this work provides a new experimental concept for high-temperature measurements and is expected to provide reliable multidimensional temperature data for combustion chambers and plumes in engine ground experiments.

Development of a compact NDIR CO2 gas sensor for harsh environments

We present a compact non-dispersive infrared (NDIR) gas sensor that can be adopted in high-moisture and low temperature environments. The NDIR gas sensor was formed by a Micro-Electro-Mechanical Systems (MEMS) emitter working at 400℃ with a broadband radiation spectrum, a MEMS thermopile detector that integrated with an optical filter, a compact gas-cell with high optical coupling efficiency of ∼ 48 %, and an imbedded miniature thermostat. The footprint of the sensor is 24 mm × 8 mm × 8 mm (length × width × height), making it just 30 % of the size of conventional NDIR gas sensors with equivalent detection resolution, and its heat capability is around 7.29 J/K that promise a cost-effective thermoregulation. The sensor is capable of accurately detecting gas, such as Carbon dioxide, within a concentration range spanning from 0.5 % to 20 % with a reading error of ± 0.1 % covering a working temperature from −20 ℃ to 60 ℃. Because the sensor is equipped with a waterproof breathable film at the air hole, the sensor can operate in the humidity range from 0 to 100 %RH, and the maximum detection reading error is 0.1 % ± 230 ppm. It is possible to be applied for both low temperature environments (e.g. food preservation cabinet, biological cabinet, etc) and high-moisture environments (e.g. greenhouse, humidity chambers, etc.).

Application of Photonic Crystals for the Generation of Broadband Radiation at the LINAC-200 Linear Accelerator

Application of Photonic Crystals for the Generation of Broadband Radiation at the LINAC-200 Linear Accelerator

Surface Degradation of Thin-Layer Al/MgF2 Mirrors under Exposure to Powerful VUV Radiation.

Thin-layer Al/MgF2 coatings are currently used for extraterrestrial far-UV astronomy as the primary and secondary mirrors of telescopes (such as "Spektr-UF"). Successful Hubble far-UV measurements have been performed thanks to MgF2 on Al mirror coatings. Damage of such thin-layer coatings has been previously studied under exposure to high-energy electrons/protons fluxes and in low Earth orbit environments. Meanwhile, there is an interest to test the stability of such mirrors under the impact of extreme radiation fluxes from pulsed plasma thrusters as a simulation of emergency onboard situations and other applications. In the present studies, the high current and compressed plasma jets were generated by a laboratory plasma thruster prototype and operated as effective emitters of high brightness (with an integral overall wavelength radiation flux of >1 MW/cm2) and broadband radiation. The spectrum rearrangement and hard-photon cut-off at energy above Ec were implemented by selection of a background gas in the discharge chamber. The discharges in air (Ec ≈ 6 eV), argon (Ec ≈ 15 eV) and neon (Ec ≈ 21 eV) were studied. X-ray diffraction and reflectometry, electron and atomic force microscopy, and IR and visible spectroscopy were used for coating characterization and estimation of degradation degree. In the case of the discharges in air with photon energies of E < 6 eV, only individual nanocracks were found and property changes were negligible. In the case of inert gases, the energy fraction was ≈50% in the VUV range. As found for inert background gases, an emission of such hard photons with energies higher than the MgF2 band gap energy of ≈10.8 eV caused a drastic light-induced ablation and degradation of the irradiated coatings. The upward trend of degradation with an increasing of the maximum photon energies was detected. The obtained data on the surface destruction are useful for the design of methods for coating stability tests and an understanding of the consequences of emergencies onboard space research stations.

On the Consistency of Ice Cloud Optical Models for Spaceborne Remote Sensing Applications and Broadband Radiative Transfer Simulations

AbstractAqua satellite Moderate Resolution Imaging Spectroradiometer (MODIS) 1‐km observations are collocated with Clouds and the Earth's Radiant Energy System (CERES) fields of view taken during July 2008 afternoon satellite passes over the equatorial western Pacific Ocean. Radiation simulations are compared with collocated CERES observations to better understand the sensitivity of computed fluxes to two ice cloud broadband radiation parameterization schemes and inferred ice cloud characteristics. In particular, the radiation computational schemes and ice cloud property retrievals are based on two respective ice particle models, the MODIS Collection 6 (MC6) aggregate model and a more microphysically consistent two‐habit model (THM). The simulation results show that both MC6 and THM overestimate the shortwave (SW) and longwave (LW) cloud radiative effects at the top of the atmosphere, as compared to the CERES observations; the difference between the MC6 and THM‐based ice cloud retrievals is too small to compensate for the differences between the two model‐based radiation schemes. Therefore, the present finding suggests that broadband radiative simulations are more sensitive to the radiation parameterization scheme than to the input cloud properties retrieved using the corresponding ice cloud particle optical property model.

Improved Numerical Modeling of Terahertz Wave Propagation in Epoxy Coating with the Finite-Difference Time-Domain Method

Electromagnetic theoretical analysis was usually used to investigate the pulsed terahertz wave interaction with the medium. For epoxy coatings, the material dispersion of the coating was often simplified or ignored in the traditional propagation model. It is difficult to elaborate on the propagation mechanism and to differentiate the coating deterioration as serving time increases. An improved propagation model based on the finite-difference time domain was established to characterize the propagation in the epoxy protective coating under broad-band terahertz radiation. Either an intact or defective coating structure was investigated, and the simulated results were compared with the experimental test. The dissipation mechanism was considered in the proposed model. The results indicated that the terahertz reflections varied with the change in optical and dielectric properties caused by coating aging, which influences the intrinsic impedance of the medium. Moreover, it could well elucidate the propagation mechanism of pulsed terahertz waves in rusted, defective coating structures.

Characteristic Mode Inspired Broadband Circularly Polarized Folded Transmitarray Antenna

The circularly polarized (CP) antennas with high gains have gained growing interest due to their significant advantages, but the existing works usually suffer from narrow bandwidths. In this work, a novel circularly polarized folded transmitarray antenna (CPFTA) inspired by the characteristic mode analysis (CMA) method is proposed to achieve a broadband and high-gain CP radiation. The proposed CPFTA consists of a top-layer multifunctional metasurface (MTS), a bottom-layer polarization conversion metasurface (PCM) integrated with a linearly polarized (LP) microstrip feed source. Enabled by the CMA, the unit cell etched with an asymmetric U-slot of the MTS is optimized to generate two pairs of CP modes to broaden the bandwidth of the CPFTA. Moreover, a modified broadband E-shaped LP microstrip antenna is integrated with the PCM, resulting in a compact profile. A CPFTA prototype is fabricated and characterized to verify the effectiveness of the design. The measured results show that the fabricated antenna could obtain an impedance bandwidth of 31.7%, a 3 dB axial ratio (AR) bandwidth of 36.5%, and a 3 dB gain bandwidth of 30.1% with a peak gain of 22.35 dBic at 9.55 GHz.

Unintended electromagnetic radiation from Starlink satellites detected with LOFAR between 110 and 188 MHz

We report on observations of 68 satellites belonging to the SpaceX Starlink constellation with the LOFAR radio telescope. Radiation associated with Starlink satellites was detected at observing frequencies between 110 and 188 MHz, which is well below the 10.7– 12.7 GHz radio frequencies used for the downlink communication signals. A combination of broad-band features, covering the entire observed bandwidth, as well as narrow-band (bandwidth &lt; 12.2 kHz) emission at frequencies of 125, 135, 143.05, 150, and 175 MHz, was observed. The presence and properties of both the narrow- and broad-band features vary between satellites at different orbital altitudes, indicating possible differences between the operational state of, or the hardware used in, these satellites. While the narrowband detections at 143.05 MHz can be attributed to reflections of radar signals from the French GRAVES Space Surveillance Radar, the signal properties of the broad- and narrow-band features at the other frequencies suggest that this radiation is intrinsic to the Starlink satellites and it is seen for 47 out of the 68 Starlink satellites that were observed. We observed spectral power flux densities vary from 0.1 to 10 Jy for broad-band radiation, to 10 to 500 Jy for some of the narrow-band radiation, equivalent to electric field strengths of up to 49 dB [µ V m−1] (as measured at a 10 m distance from the satellites, with a measurement bandwidth of 120 kHz). In addition, we present equivalent power flux density simulations of the full Starlink phase 1 constellation, as well as other satellite constellations, for one frequency band allocated to radio astronomy by the International Telecommunication Union (ITU). With these, we calculate the maximum radiation level that each satellite constellation would need to have to comply with regulatory limits for intended emissions in that band. However, these limits do not apply if the radiation is unintended, that is to say if it does not originate from intentionally radiated signals for radio communication or other purposes. We discuss the results in light of the (absence of) regulations covering these types of unintended electromagnetic radiation and the possible consequences for astronomical radio observations.

Laser-induced nitrogen oxide fluorescence from nitro compounds by 222 nm laser

This paper presents the study results of the single-frequency interaction of KrCl laser broadband radiation with nitro compound vapors in the atmosphere. The optical method of a substance spectroscopic analysis including photofragmentation of a complex R–NO2 molecule followed by laser-induced fluorescence of vibrationally excited fragments of NO X2Π, v''>0 (PF-LIF) was studied in the experiments. It has been shown that R–NO2 molecules exposed to λ = 222 nm FWHM 0.3 nm KrCl laser broadband radiation undergo photofragmentation followed by resonant excitation of several electronic-vibrational levels of NO X2Π, v'' (2,4). At the same time, the concentration of fragmented NO X2Π, v'' (2,4) increases due to the secondary PF-LIF NO2 process during the nanosecond laser pulse.

Broadband directional thermal radiator with flexible intensity–directivity tunability in the whole visible spectrum

Controlling spectral and directional distributions of thermal radiation plays an important role in designing functional structures for thermal management. As a wideband phenomenon, thermal radiation is supposed to be manipulated within broad wave ranges for the case of practical thermal applications. However, currently, it is still challenging to constrain broadband radiation into wanted directions in a controllable manner. In this work, based on light-trapping effects mediated by periodic germanium strips on a silver substrate, we design a thermal radiator with broadband directional (BBD) emissivity in the whole visible spectrum. The radiator is free from intricate nanofabrication and can achieve low-dispersive directional emissivity within a continuous wave range of 0.4–0.8 μm. In addition, the proposed radiator exhibits flexible tunability on the BBD performance and emission intensity, making it an outstanding candidate for functional surfaces in thermal energy management.

Airborne observations of the surface cloud radiative effect during different seasons over sea ice and open ocean in the Fram Strait

Abstract. This study analyses the cloud radiative effect (CRE) obtained from near-surface observations of three airborne campaigns in the Arctic north-west of Svalbard: Airborne measurements of radiative and turbulent FLUXes of energy and momentum in the Arctic boundary layer (AFLUX, March/April 2019), Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD, May/June 2017), and Multidisciplinary drifting Observatory for the Study of Arctic Climate – Airborne observations in the Central Arctic (MOSAiC-ACA, August/September 2020). The surface CRE quantifies the potential of clouds to modify the radiative energy budget at the surface and is calculated by combining broadband radiation measurements during low-level flight sections in mostly cloudy conditions with radiative transfer simulations of cloud-free conditions. The significance of surface albedo changes due to the presence of clouds is demonstrated, and this effect is considered in the cloud-free simulations. The observations are discussed with respect to differences of the CRE between sea ice and open-ocean surfaces and between the seasonally different campaigns. The results indicate that the CRE depends on cloud, illumination, surface, and thermodynamic properties. The solar and thermal-infrared (TIR) components of the CRE, CREsol and CRETIR, are analysed separately, as well as combined for the study of the total CRE (CREtot). The inter-campaign differences of CREsol are dominated by the seasonal cycle of the solar zenith angle, with the strongest cooling effect in summer. The lower surface albedo causes a stronger solar cooling effect over open ocean than over sea ice, which amounts to −259 W m−2 (−108 W m−2) and −65 W m−2 (−17 W m−2), respectively, during summer (spring). Independent of campaign and surface type, CRETIR is only weakly variable and shows values around 75 W m−2. In total, clouds show a negative CREtot over open ocean during all campaigns. In contrast, over sea ice, the positive CREtot suggests a warming effect of clouds at the surface, which neutralizes during mid-summer. Given the seasonal cycle of the sea ice distribution, these results imply that clouds in the Fram Strait region cool the surface during the sea ice minimum in late summer, while they warm the surface during the sea ice maximum in spring.

Surface current engineering enabled broadband monopole patch antenna with low profile

Antennas with broadband radiation and small-size are highly required in wireless communication system. The trade-off relation between bandwidth and size became a difficulty when it comes to miniaturized antenna design. A method of realizing antenna miniaturization and bandwidth enhancement at the same time is needed. Monopole antennas are the most common used one in the devices of Internet of Things, Internet of Vehicles, and wireless access points et al. In this paper, a wideband low-profile top-loaded patch monopole antenna based on surface current engineering is proposed. The topology combines a center-fed circular patch with inductive vias. The patch is designed with patterns, which consists of four concentric annular rings and some strips in radial direction. In this way, three resonances are generated, and the pattern is optimized to bring three resonances close to each other. The number of the strips is specially designed to bring three resonances close to each other. By loading the inductive vias on the outermost rings, connecting to the ground, the resonance at low frequencies can be enhanced, as a consequence of the optimized impedance matching. The field distribution is analyzed to verify the operation principle. A prototype is fabricated, and the measured results agree well with the simulated ones. The relative −10 dB bandwidth is 69% and the omnidirectional radiation pattern is stable in the operating band. The antenna achieves a rather wideband radiation while maintaining a low profile. The method of antenna miniaturization and bandwidth enhancement based on surface current engineering paves the way for antenna design in low-profile ultra-wideband applications.

All-optical image classification through unknown random diffusers using a single-pixel diffractive network

Classification of an object behind a random and unknown scattering medium sets a challenging task for computational imaging and machine vision fields. Recent deep learning-based approaches demonstrated the classification of objects using diffuser-distorted patterns collected by an image sensor. These methods demand relatively large-scale computing using deep neural networks running on digital computers. Here, we present an all-optical processor to directly classify unknown objects through unknown, random phase diffusers using broadband illumination detected with a single pixel. A set of transmissive diffractive layers, optimized using deep learning, forms a physical network that all-optically maps the spatial information of an input object behind a random diffuser into the power spectrum of the output light detected through a single pixel at the output plane of the diffractive network. We numerically demonstrated the accuracy of this framework using broadband radiation to classify unknown handwritten digits through random new diffusers, never used during the training phase, and achieved a blind testing accuracy of 87.74 ± 1.12%. We also experimentally validated our single-pixel broadband diffractive network by classifying handwritten digits “0” and “1” through a random diffuser using terahertz waves and a 3D-printed diffractive network. This single-pixel all-optical object classification system through random diffusers is based on passive diffractive layers that process broadband input light and can operate at any part of the electromagnetic spectrum by simply scaling the diffractive features proportional to the wavelength range of interest. These results have various potential applications in, e.g., biomedical imaging, security, robotics, and autonomous driving.

Broadband Spintronic Terahertz Source with Peak Electric Fields Exceeding 1.5 MV/cm

Spintronic terahertz emitters (STEs) are desirable broadband terahertz sources, but their limited signal strength has hindered practical application. By optimizing the photonic and thermal environment, the authors present an STE that could overcome this obstacle. Benchmarking against the state-of-the-art terahertz emitters based on optical rectification, this STE delivers strong terahertz pulses with comparable peak electric field and fluence, and offers additional features such as broadband radiation, easy alignment, and rotation of the terahertz polarization plane without power loss. This work will open up a promising pathway to nonlinear terahertz spectroscopy with spintronic sources.

Monitor Calibrator as an Alternative to Spectrofluorimeter: Determination of Quinine in Beverages and Medicinal Preparations

A possibility of using a monitor calibrator for the determination of luminescent compounds is shown on an example of quinine. The determination is based on the irradiation of a sample with broadband radiation in the visible and near UV spectral regions from a built-in source exciting phosphor molecules and the simultaneous registration of the radiation incident on the detector. Measurement conditions are selected. Quinine can be determined in the range of 60−750 µM, the limit of detection is 20 µM. The determination is not affected by common inorganic ions, as well as sweeteners and acidity regulators present in many beverages. The developed method of determination is applicable to the analysis of carbonated drinks and drugs. Compared to a traditional spectrofluorimeter, monitor calibrator is characterized by compactness, mobility, ability of detecting luminescence in cuvettes of various sizes and shapes, and lower cost.

Optical Characterization of Thin Films by Surface Plasmon Resonance Spectroscopy Using an Acousto-Optic Tunable Filter

The paper presents the application of the acousto-optic tunable filter (AOTF) in surface plasmon resonance (SPR) spectroscopy to measure the optical thickness of thin dielectric coatings. The technique presented uses combined angular and spectral interrogation modes to obtain the reflection coefficient under the condition of SPR. Surface electromagnetic waves were excited in the Kretschmann geometry, with the AOTF serving as a monochromator and polarizer of light from a white broadband radiation source. The experiments highlighted the high sensitivity of the method and the lower amount of noise in the resonance curves compared with the laser light source. This optical technique can be implemented for nondestructive testing in the production of thin films in not only the visible, but also the infrared and terahertz ranges.

Broadband electromagnetic emission via mode conversion mediated by stimulated Raman scattering in inhomogeneous plasma

Electromagnetic emission via linear mode conversion from electron plasma waves (EPWs) excited by stimulated Raman scattering (SRS) of an incident laser pulse in inhomogeneous plasma is investigated theoretically and numerically. It is found that the mode conversion can occur naturally in underdense plasma region below the quarter critical density provided that EPWs are generated due to the development of backward SRS when the laser pulse is incident at certain angle with the plasma density gradient. The produced radiation may cover a broad frequency range up to half of the incident laser frequency. The dependence of the radiation conversion efficiency on the laser intensity, incident angle, laser pulse duration, plasma density scale length, and initial electron temperature is analyzed based on one-dimensional particle-in-cell simulation. In two-dimensional geometry, due to the development of sideward SRS, it is found that the mode conversion to occur even at normal incidence of the laser pulse. The radiation frequency, bandwidth, duration, and amplitude can be well controlled by the laser and plasma parameters, suggesting that it may provide a new source of tunable broadband radiation as well as a diagnosis of the development of SRS.

Low RCS, Broadband Polarization Conversion Metasurface Antenna Based on Reactance Loading

A reactance-loaded polarization conversion metasurface (PCMS) is presented and exploited to realize a metasurface antenna (MTA) with both low radar cross section (RCS) and broadband radiation. Distinct from the conventional low-RCS PCMS, a metallic surface with etched open and closed slits is integrated into a PCMS consisting of double L-shaped particle pairs. Due to the edge capacitance induced by open slits and the surface inductance imposed by closed slits, the PCMS is endowed with reactance features that act as a perturbation to modulate the near-field interactions, thereby expanding the polarization conversion bandwidth. Besides, the reactance-loaded metallic surface, which has the same period as L-shaped particle pairs, combined with the grounded substrate, constitutes a broadband metasurface radiator. With the help of characteristic mode analysis, a linearly polarized aperture-coupled PCMS antenna is proposed, which effectively stimulates two resonant modes with broadside radiation to help bring about a broad operating bandwidth of 45.52%. Subsequently, the checkerboard-layout PCMS array is coupled to a sequentially feeding network through apertures on the ground plane, implementing a circularly polarized MTA with a broad 3 dB axial ratio bandwidth of 20.11%. Relying on phase cancellation and outstanding polarization conversion behaviors, an ultrawideband RCS reduction bandwidth of 7–39 GHz (139.13%) is also available.

Experimental Study of the Influence of the Transmission Function of Acousto-Optic Tuneable Filter on the Characteristics of the Interference Pattern in Off-Axis Digital Holography

Acousto-optic tuneable filters (AOTFs) are used in digital holography (DH) to obtain interference images at different wavelengths, which expands the possibilities of studying technical and biological objects. However, the width of the spectral bands selected by AOTF is quite large, which can lead to a decrease in the size of the high-contrast region of the interference pattern in off-axis DH schemes and reduce the quality of the obtained holograms. In this work, the effect of the geometry of acousto-optic (AO) interaction and the power of the driving signal on the spectral transmission function of the AOTF, the visibility of the interference pattern, and the width of the effective field of view (FOV) is experimentally studied. For this, a setup with a broadband radiation source, Mach-Zehnder interferometer, and a spectrometer was used. The interference patterns were recorded and the transmission spectra were measured for several values of the angle of incidence on the entrance face of the AOTF in the frequency tuning range corresponding to the visible spectrum. We evaluated the dependence of the coherence length on the rotation angle of the AOTF and the central transmission wavelength and compared the results obtained by the interference method and those calculated from the spectra measured by the spectrometer. It is shown that when the AOTF is rotated through the angles from 5° to –15° relative to the wide-aperture AO interaction geometry, it is possible to increase the coherence length and the width of the effective FOV by a factor of 2.5. It was established that the width of the FOV can significantly decrease with an increase in the driving power. The results obtained can be used to certify the AOTF, optimize the characteristics of DH setups with AOTF, and determine the optimal parameters of their operation.

Oscillations of convective flow around a continuous optical discharge in high-pressure xenon

Laser-sustained plasma or continuous optical discharge (COD), predicted and first obtained in pioneering works initiated and inspired by Professor Raizer et al (1970 JETP Lett. 11 120–3; 1970 JETP Lett. 11 302–4; 1972 Sov. Phys.—JETP 34 763–9), is still a unique method of sustaining localized dense plasma in the laboratory and some special devices such as high-brightness broadband radiation sources [6–10]. Although the parameters determining the COD state can be stabilized, in practice, COD tends to regular oscillations. As stated in the paper, the oscillations may be attributed to the regular pulsing of thermal gravitational convective plumes around the COD. Application problems arising from instability of this type is pulsing of the emission from promising radiation sources utilizing COD plasma. This paper presents a detailed analysis of the experimental data on COD convective plume radii and oscillation frequencies depending on the plasma-forming gas pressure. The analysis leads to a similarity relation for the oscillation frequency. The similarity relation turns out to be common for optical discharges and laminar flames under conditions of prevailing buoyancy forces. This indicates the hydrodynamic nature of the instability considered, regardless of whether the energy input method is in the form of absorbed laser radiation or a chemical reaction. To support the idea of a hydrodynamic origin of the pulsations, a numerical simulation of a convective plume from a concentrated heat source was carried out. The thermal dissipation power of the heat source was equal to that of the COD. The results of numerical simulations demonstrate good agreement with those of the experiments. The dynamic temperature, density and gas velocity distributions obtained show that the direct cause of the oscillations is the dynamics of the toroidal vortices developed in the convection plume. The pulsing frequency equal to the frequency of the vortex formation may be increased under incoming additional forced gas flow that may also suppress the convective oscillations. The results obtained may be useful for studying optical discharges, and improving the parameters of high-brightness broadband laser-plasma radiation sources based on them.