Microwave Spectral Region Research Articles

Shrinking a gradient-index-lens antenna system with a spaceplate

The miniaturization of optical systems is an ongoing challenge across the electromagnetic spectrum. While the thickness of optical elements themselves can be reduced using advances in metamaterials, it is the voids between these elements—which are necessary parts of an optical system—that occupy most of the volume. Recently, a novel optical element coined a “spaceplate” has been proposed, which replaces a region of free space with a thinner optical element that emulates the free-space optical response function—thus having the potential to substantially shrink the volume of optical systems. While there have been a few proof-of-principle demonstrations of spaceplates, they have not yet been deployed in a real-world optical system. In this work, we use a bespoke-designed spaceplate to reduce the length of a gradient-index- (GRIN) lens microwave antenna. Our antenna is designed to operate at 23.5 GHz and the incorporation of a nonlocal metamaterial spaceplate enables the distance between the antenna feed and the GRIN lens to be reduced by almost a factor of 2. We find that the radiation patterns from a conventional and space-squeezed antenna are very similar, with a very low cross-polarization, and only a minor increase in the side-lobe levels when introducing the spaceplate. Our work represents a demonstration of a spaceplate integrated into a real-world optical system operating in microwave spectral region, highlighting the potential for this concept to reduce the physical size of systems in applications including imaging, spectroscopy, radar, and communications. Published by the American Physical Society 2024

Space squeezing optics: Performance limits and implementation at microwave frequencies

Optical systems often largely consist of empty space as diffraction effects that occur through free-space propagation can be crucial for their function. Contracting these voids offers a path to the miniaturization of a wide range of optical devices. Recently, a new optical element—coined “spaceplate”—has been proposed, which is capable of emulating the effects of diffraction over a specified propagation distance using a thinner non-local metamaterial [Reshef et al., Nat. Commun. 12, 3512 (2021)]. The compression factor of such an element is given by the ratio of the length of free-space that is replaced to the thickness of the spaceplate itself. In this work, we test a prototype spaceplate in the microwave spectral region (20–23 GHz)—the first such demonstration designed to operate in ambient air. Our device consists of a Fabry–Pérot cavity formed from two reflective metasurfaces with a compression factor that can be tuned by varying the size of perforations within each layer. Using a pair of directive horn antennas, we measure a space compression factor of up to ∼6 over a numerical aperture (NA) of 0.34 and a fractional bandwidth of 6%. We also investigate the fundamental trade-offs that exist between the compression factor, transmission efficiency, NA, and bandwidth of this single resonator spaceplate design and highlight that it can reach arbitrarily high compression factors by restricting its NA and bandwidth.

Bulk hydrometeor optical properties for microwave and sub-millimetre radiative transfer in RTTOV-SCATT v13.0

Abstract. Satellite observations of radiation in the microwave and sub-millimetre spectral regions (broadly from 1 to 1000 GHz) can have strong sensitivity to cloud and precipitation particles in the atmosphere. These particles (known as hydrometeors) scatter, absorb, and emit radiation according to their mass, composition, shape, internal structure, and orientation. Hence, microwave and sub-millimetre observations have applications including weather forecasting, geophysical retrievals and model validation. To simulate these observations requires a scattering-capable radiative transfer model and an estimate of the bulk optical properties of the hydrometeors. This article describes the module used to integrate single-particle optical properties over a particle size distribution (PSD) to provide bulk optical properties for the Radiative Transfer for TOVS microwave and sub-millimetre scattering code, RTTOV-SCATT, a widely used fast model. Bulk optical properties can be derived from a range of particle models including Mie spheres (liquid and frozen) and non-spherical ice habits from the Liu and Atmospheric Radiative Transfer Simulator (ARTS) databases, which include pristine crystals, aggregates, and hail. The effects of different PSD and particle options on simulated brightness temperatures are explored, based on an analytical two-stream solution for a homogeneous cloud slab. The hydrometeor scattering “spectrum” below 1000 GHz is described, along with its sensitivities to particle composition (liquid or ice), size and shape. The optical behaviour of frozen particles changes in the frequencies above 200 GHz, moving towards an optically thick and emission-dominated regime more familiar from the infrared. This region is little explored but will soon be covered by the Ice Cloud Imager (ICI).

PAMTRA 1.0: the Passive and Active Microwave radiative TRAnsfer tool for simulating radiometer and radar measurements of the cloudy atmosphere

Abstract. Forward models are a key tool to generate synthetic observations given knowledge of the atmospheric state. In this way, they are an integral part of inversion algorithms that aim to retrieve geophysical variables from observations or in data assimilation. Their application for the exploitation of the full information content of remote sensing observations becomes increasingly important when these are used to evaluate the performance of cloud-resolving models (CRMs). Herein, CRM profiles or fields provide the input to the forward model whose simulation results are subsequently compared to the observations. This paper introduces the freely available comprehensive microwave forward model PAMTRA (Passive and Active Microwave TRAnsfer), demonstrates its capabilities to simulate passive and active measurements across the microwave spectral region for upward- and downward-looking geometries, and illustrates how the forward simulations can be used to evaluate CRMs and to interpret measurements to improve our understanding of cloud processes. PAMTRA is unique as it treats passive and active radiative transfer (RT) in a consistent way with the passive forward model providing upwelling and downwelling polarized brightness temperatures and radiances for arbitrary observation angles. The active part is capable of simulating the full radar Doppler spectrum and its moments. PAMTRA is designed to be flexible with respect to instrument specifications and interfaces to many different formats of input and output, especially CRMs, spanning the range from bin-resolved microphysical output to one- and two-moment schemes, and to in situ measured hydrometeor properties. A specific highlight is the incorporation of the self-similar Rayleigh–Gans approximation (SSRGA) for both active and passive applications, which becomes especially important for the investigation of frozen hydrometeors.

Identification of a vibrationally excited level in methyl formate through microwave and far-infrared spectroscopy

Methyl formate (HCOOCH3) is an important interstellar molecule. More than 1000 rotational transitions including those from the ground state, torsionally excited states, and of its isotopologues have been observed towards several astrophysical sources. The laboratory spectra of methyl formate in the microwave spectral region contain many unassigned transitions and many of them are likely to be due to rotational transitions in the low-lying excited states. We report the laboratory identification of new rotational transitions in the COC deformation (ν12) excited state. The identification was made possible by combining the microwave data with rotation–vibration spectra taken in the far-infrared region at the Canadian Light Source synchrotron.

(Invited) Enhancing Reliability of Gas Sensors with New Design Rules for Sensing Materials and Transducers

Contemporary gas monitoring scenarios for industrial safety, environmental surveillance, medical diagnostics, personal wellness, and other applications demand sensors with higher accuracy, enhanced stability, and often lower power; all in unobtrusive formats and at low cost [1-3]. Unfortunately, available sensors based on traditional detection principles often have not only inadequate accuracy and stability but also have relatively high power demands, pushing the limits of existing detection concepts where we are reaching their fundamental performance limits. These limitations of available sensors drive the innovative designs of new generation of sensors. We are focusing on development of new principles of gas sensing based on multiparameter signal excitation and detection resulting in a new generation of gas sensors based on the multivariable response principles. Design criteria of these individual sensors involve a sensing material with multi-response mechanisms to different gases and a multivariable transducer with independent outputs to recognize these different gas responses. In our talk, we will discuss our different sensor types that operate over the electromagnetic spectrum ranging from the radio-frequency to microwave and to optical regions. We will discuss new performance capabilities of the developed sensors using two broad examples of our recent developments. In the first example, we will discuss our multivariable sensors in the radio-frequency and microwave spectral regions [4-6]. We are implementing impedance spectroscopy and diverse types of sensing materials and transducers with the goals of improving gas-selectivity of our sensors and rejection of interferences. Examples of our sensing materials in these spectral regions include conjugated and dielectric polymers, ligand-functionalized metal nanoparticles, metal oxides, and carbon allotropes. Examples of our transducers include resonant and non-resonant structures. In the second example, we will discuss our multivariable sensors in the optical spectral region [7-11]. We are implementing our bio-inspired three-dimensional nanomaterials with the visible-light reflectance or transmittance measurements with the goals of improving gas-selectivity of our sensors and enhancing sensor stability. Examples of our sensing materials operating in the optical spectral region include polymeric and inorganic nanostructures functionalized based on our design rules for multi-gas detection using individual sensors. Our developments resulted in sensors with previously unavailable performance characteristics in wearable, stationary, airborne and other formats where our multivariable sensors independently quantify up to four individual gases in complex mixtures, reject interferences with up to 2,000,000-fold overloading in concentrations over the analytes, and enhance sensor-response stability. Such performance characteristics are attractive when selectivity advantages of classic gas chromatography, ion mobility, and mass spectrometry instruments are canceled by requirements for no consumables, low power, low cost, and unobtrusive form factors for Internet of Things, Industrial Internet, and other applications. We will conclude with a perspective for future needs in fundamental and applied aspects of gas sensing and with the 2030 roadmap for ubiquitous gas monitoring.[1] Bogue, R. Towards the trillion sensors market. Sensor Rev. 34, 137-142 (2014).[2] Alexander, M., Bernhart, W., Rossbach, C. & Nölling, K. Smart Strategies for Smart Sensors. (Roland Berger GMBH, 2017).[3] Potyrailo, R. A. Ubiquitous wearable and disposable chem-bio sensors: markets demands and innovative technology solutions. (Sensors Expo & Conference, San Jose, CA, June 26-28, 2018).[4] Potyrailo, R. A., Surman, C., Nagraj, N. N. & Burns, A. Materials and Transducers Toward Selective Wireless Gas Sensing. Chem. Rev. 111, 7315–7354 (2011).[5] Potyrailo, R. A. Multivariable sensors for ubiquitous monitoring of gases in the era of Internet of Things and Industrial Internet. Chem. Rev. 116, 11877–11923 (2016).[6] Potyrailo, R. A. Toward high value sensing: monolayer-protected metal nanoparticles in multivariable gas and vapor sensors. Chem. Soc. Rev. 46, 5311-5346 (2017).[7] Potyrailo, R. A. et al. Morpho butterfly wing scales demonstrate highly selective vapour response. Nature Photonics 1, 123-128 (2007).[8] Potyrailo, R. A. et al. Discovery of the surface polarity gradient on iridescent Morpho butterfly scales reveals a mechanism of their selective vapor response. Proc. Natl. Acad. Sci. U.S.A. 110, 15567–15572 (2013).[9] Potyrailo, R. A. et al. Towards outperforming conventional sensor arrays with fabricated individual photonic vapour sensors inspired by Morpho butterflies. Nature Commun. 6, 7959 (2015).[10] Potyrailo, R. A., Karker, N., Carpenter, M. A. & Minnick, A. Multivariable bio-inspired photonic sensors for non-condensable gases. J. Opt. 20, 024006 (2018).[11] Potyrailo, R. A. et al. Multi-gas sensors for enhanced reliability of SOFC operation. ECS Transactions 91, 319-328 (2019).

Indian Remote Sensing Sensor System: Current and Future Perspective

Over last four decades, a vibrant Remote Sensing programme has seen the development of space-based and airborne remote sensing sensor systems for various science and application studies. The Indian Space Programme, is applications driven with major objective towards national development. A series of earth observation systems have been launched in both polar and geo synchronous orbits starting from Bhaskara, the first experimental Earth Observation satellite in 1979 to Cartosat-2S successfully launched in 2017. A large number of active and passive sensors are observing in optical and microwave spectral regions, providing valuable data at different spatial resolutions for land ocean and atmospheric applications, as per application requirements. Satellite Observations during natural and human-induced hazards have become crucial for protecting the global environment, reducing disaster losses, and achieving sustainable development. Today, India has the world’s largest constellation of remote sensing satellites in operation which is being extensively used for resource management and integrated planning for national development. The paper discusses the evolution and functioning of both Optical and Microwave Earth Observations Remote Sensing Sensors developed in India.

Investigation on water content in fresco mock-ups in the microwave and near-IR spectral regions

Water diffusion inside masonry is responsible for the majority of the decay phenomena observed in wall paintings and frescos. Thus, the diagnostics of moisture and water content and their monitoring represent a key issue. In order to preserve the integrity of surfaces of artistic interest, investigations by means of non-destructive techniques (NDT) are preferred over others. The aim of this research is to determine methodologies to quantify the moisture content (MC) of frescos by means of the integrated use of two non-invasive techniques, namely fiber optic reflectance spectroscopy (FORS) in the near-IR region and evanescent field dielectrometry (EFD) in the microwave range. The FORS technique has been employed in order to assess the amount of water adsorbed from the surface by means of an analysis of the reflectance spectra in the Vis–NIR (350-2200 nm) range. This technique investigates the electronic and vibrational transitions that are characteristic of each compound and enables their identification. The water content is evaluated on the basis of the 1920 nm and 1450 nm absorption bands. The EFD system consists of a resonant probe connected to a network analyzer. The resonance frequency of the cavity under different moisture-content conditions of frescos is in the 1.0–1.5 GHz range. The device makes it possible to compute, in real time, the MC from a measurement of the transmission coefficient (amplitude versus frequency) through the probe. Fresco mock-ups have been prepared in collaboration with the Opificio delle Pietre Dure in order to recreate most of the possible chromatic shades obtained by mixing iron oxides and hydroxide-based pigments. Measurements were performed by employing both techniques on fresco models after wet-dry cycles obtained by means of poultices with a known water content. The results obtained with these two techniques were compared, and cross relationships between the EFD and FORS data were defined.

Single-scattering properties of ice particles in the microwave regime: Temperature effect on the ice refractive index with implications in remote sensing

An ice crystal single-scattering property database is developed in the microwave spectral region (1 to 874GHz) to provide the scattering, absorption, and polarization properties of 12 ice crystal habits (10-plate aggregate, 5-plate aggregate, 8-column aggregate, solid hexagonal column, hollow hexagonal column, hexagonal plate, solid bullet rosette, hollow bullet rosette, droxtal, oblate spheroid, prolate spheroid, and sphere) with particle maximum dimensions from 2µm to 10mm. For each habit, four temperatures (160, 200, 230, and 270K) are selected to account for temperature dependence of the ice refractive index. The microphysical and scattering properties include projected area, volume, extinction efficiency, single-scattering albedo, asymmetry factor, and six independent nonzero phase matrix elements (i.e. P11, P12, P22, P33, P43 and P44). The scattering properties are computed by the Invariant Imbedding T-Matrix (II-TM) method and the Improved Geometric Optics Method (IGOM). The computation results show that the temperature dependence of the ice single-scattering properties in the microwave region is significant, particularly at high frequencies. Potential active and passive remote sensing applications of the database are illustrated through radar reflectivity and radiative transfer calculations. For cloud radar applications, ignoring temperature dependence has little effect on ice water content measurements. For passive microwave remote sensing, ignoring temperature dependence may lead to brightness temperature biases up to 5K in the case of a large ice water path.

Pressure broadening calculations for OH in collisions with argon: Rotational, vibrational, and electronic transitions

Collisional parameters describing both the pressure-induced broadening and shifting of isolated lines in the spectrum of the hydroxyl radical in collisions with argon have been determined through quantum scattering calculations using accurate potential energy surfaces describing the OH(X2Π, A2Σ+)–Ar interactions. These calculations have been carried for pure rotational, vibrational, and electronic transitions. The calculated pressure broadening coefficients are in good agreement with the available measurements in the microwave, infrared, and ultraviolet spectral regions. Computed pressure broadening coefficients as a function of temperature are reported for these three types of transitions.

The water vapour continuum in near-infrared windows – Current understanding and prospects for its inclusion in spectroscopic databases

Spectroscopic catalogues, such as GEISA and HITRAN, do not yet include information on the water vapour continuum that pervades visible, infrared and microwave spectral regions. This is partly because, in some spectral regions, there are rather few laboratory measurements in conditions close to those in the Earth’s atmosphere; hence understanding of the characteristics of the continuum absorption is still emerging. This is particularly so in the near-infrared and visible, where there has been renewed interest and activity in recent years. In this paper we present a critical review focusing on recent laboratory measurements in two near-infrared window regions (centred on 4700 and 6300cm−1) and include reference to the window centred on 2600cm−1 where more measurements have been reported. The rather few available measurements, have used Fourier transform spectroscopy (FTS), cavity ring down spectroscopy, optical-feedback – cavity enhanced laser spectroscopy and, in very narrow regions, calorimetric interferometry. These systems have different advantages and disadvantages. Fourier Transform Spectroscopy can measure the continuum across both these and neighbouring windows; by contrast, the cavity laser techniques are limited to fewer wavenumbers, but have a much higher inherent sensitivity. The available results present a diverse view of the characteristics of continuum absorption, with differences in continuum strength exceeding a factor of 10 in the cores of these windows. In individual windows, the temperature dependence of the water vapour self-continuum differs significantly in the few sets of measurements that allow an analysis. The available data also indicate that the temperature dependence differs significantly between different near-infrared windows. These pioneering measurements provide an impetus for further measurements. Improvements and/or extensions in existing techniques would aid progress to a full characterisation of the continuum – as an example, we report pilot measurements of the water vapour self-continuum using a supercontinuum laser source coupled to an FTS. Such improvements, as well as additional measurements and analyses in other laboratories, would enable the inclusion of the water vapour continuum in future spectroscopic databases, and therefore allow for a more reliable forward modelling of the radiative properties of the atmosphere. It would also allow a more confident assessment of different theoretical descriptions of the underlying cause or causes of continuum absorption.

Electromagnetic and magnetic vector potential bio-information and water

This work developed over the past 40 years starting from dielectric measurements on enzymes and the subsequent finding that the measurements were affected by electric, magnetic, electromagnetic fields and quantum fields. A request for help in the diagnosis and therapy of chemically sensitive patients who had become sensitive to their electromagnetic environment came in 1982. The same symptoms could be provoked by a chemical or a frequency challenge and this led to an appreciation of the synergy between chemical and frequency environmental sensitivities. Experimental cooperation with theoretical physicist Herbert Fröhlich FRS and others led to an understanding of the physics of coherent water in living systems and a mechanism for the memory of water for coherent frequencies. In a coherent system there are interacting frequencies proportionate to any velocity the system will support, in particular the velocity of light and the velocity of coherence diffusion. Thus, there can be biological interaction between the optical, microwave and ELF spectral regions. Frequency modulation of light scattered by bio-fields and its retention in recorded images is discussed. A 'nil-potent' frequency can erase a frequency signature and thence affect a biological system. Homeopathy is interpreted through the biological effects of coherent frequencies derived from the frequency signature of the 'Mother Tincture' and developed through dilution and succussion. A homeopathic potency has a frequency signature therefore it must be able to have a biological effect.

Magnetic-dipole transitions in highly charged ions as a basis of ultraprecise optical clocks.

We evaluate the feasibility of using magnetic-dipole (M1) transitions in highly charged ions as a basis of an optical atomic clockwork of exceptional accuracy. We consider a range of possibilities, including M1 transitions between clock levels of the same fine-structure and hyperfine-structure manifolds. In highly charged ions these transitions lie in the optical part of the spectra and can be probed with lasers. The most direct advantage of our proposal comes from the low degeneracy of clock levels and the simplicity of atomic structure in combination with negligible quadrupolar shift. We demonstrate that such clocks can have projected fractional accuracies below the 10^{-20}-10^{-21} level for all common systematic effects, such as blackbody radiation, Zeeman, ac-Stark, and quadrupolar shifts.

Polarization Conversion for Specular Components of Surface Reflection

Certain characteristics of a material such as the surface reflectivities can be determined even without knowledge of its internal properties. In this letter, a direct relationship (Azzam relationship) and an analytical approximation (Azzam-Sohn-Hong (ASH) approximation) between the vertically and horizontally polarized reflectivities of specular surfaces are derived and validated using the refractive indices of water and metal in a variety of spectral regions. For the purpose of practical remote applications, land, sea water, sea ice, and oil surfaces are considered and compared using the Hong and ASH approximations. Consequently, ASH approximation has an advantage in a variety of spectral bands for materials with a small imaginary part of refractive index, while the Hong approximation does well in the microwave spectral region, or when the imaginary part of the reflective index is not neglected. Thus, a combination of the Hong and ASH approximations is suggested to improve upon previous studies that used only the Hong approximation in a variety of applications.

An Overview of a New Chinese Weather Satellite FY-3A

FengYun-3A (FY-3A), the first satellite in the second generation of the Chinese polar-orbiting meteorological satellites, was launched at Taiyuan, China, launching center on 27 May 2008. Equipped with both sounding and imaging payloads, enabling more powerful observations than the first generation of the FY-1 series, FY-3A carries 11 instruments. Two of them are the same as those on FY-1C/D, while the others, whose spectral bands cover violet, visible, near-infrared, infrared, and microwave spectral regions, are all newly developed. FY-3A instruments can be used to detect and study weather, clouds, radiation, climate, atmosphere, land, ocean, and other environmental features. FY-3A check out took about 5 months following its launch; FY-3A has been operational since January 2009. The plan for the future FY-3 series is to operate two polar-orbiting spacecraft—one in the morning and the other in the afternoon orbit—with different payloads for each spacecraft. This orbit configuration will be further coordina...

THz Polarization Effects on Detection Responsivity of Glow Discharge Detectors (GDDs)

The detection mechanism of glow discharge plasma in neon indicator lamps has been investigated in the terahertz and microwave spectral regions. The influence of the THz radiation polarization on the responsivity of glow discharge detectors (GDD) is considered here. There are two detection mechanisms in the GDD that exist simultaneously, each at the expense of the other. Experiments with GDD neon indicator lamps in the THz regime prove that the dominant detection mechanism of the GDD is enhanced cascade ionization rather than enhanced diffusion current. Our polarization experiments in the THz regime show that when the electric field of the THz radiation is in the same direction as the DC bias electric field of the lamp, the responsivity is about 50% higher than when the THz electric field is orthogonal to the DC field. This supports the concept that the dominant detection mechanism of the GDD in the THz regime is enhanced cascade ionization.

The absolute frequency of the 87Sr optical clock transition

The absolute frequency of the 1S0–3P0 clock transition of 87Sr has been measured to be 429 228 004 229 873.65 (37) Hz using lattice-confined atoms, where the fractional uncertainty of 8.6 × 10−16 represents one of the most accurate measurements of an atomic transition frequency to date. After a detailed study of systematic effects, which reduced the total systematic uncertainty of the Sr lattice clock to 1.5 × 10−16, the clock frequency is measured against a hydrogen maser which is simultaneously calibrated to the US primary frequency standard, the NIST Cs fountain clock, NIST-F1. The comparison is made possible using a femtosecond laser based optical frequency comb to phase coherently connect the optical and microwave spectral regions and by a 3.5 km fibre transfer scheme to compare the remotely located clock signals.

Tuning the polarization state of enhanced transmission in gratings

The polarization characteristics of enhanced transmission of lamellar gratings with structural dimensions on the subwavelength scale were studied and experimental results were compared to numerical models. The ability to tune the polarization state of the transmitted beam by varying the grating’s structural parameters is discussed. Gratings were fabricated and tested in the microwave spectral region, and the results were compared to theoretically modeled results. Enhanced transmission produced by cavity modes was experimentally verified for both s-polarized and p-polarized incident beams of light. Applications of these results to photonic devices in the visible, infrared, and microwave spectral regions are discussed.

A Tunable Photonic Microwave Filter With a Complex Coefficient Using an Optical RF Phase Shifter

A novel tunable photonic microwave filter with complex coefficients is proposed and experimentally demonstrated. The complex coefficient is generated using a wideband tunable optical RF phase shifter that consists of two electrooptic intensity modulators. The phase of the RF signal is shifted by simply adjusting the bias voltages applied to the two electrooptic intensity modulators, and the phase shift remains constant over the microwave spectral region of interest. A two-tap photonic microwave filter with one tunable complex coefficient, with a wide and continuous tuning range, is experimentally demonstrated.

The Successive-Order-of-Interaction Radiative Transfer Model. Part I: Model Development

Abstract This study, the first part of a two-part series, develops the method of “successive orders of interaction” (SOI) for a computationally efficient and accurate solution for radiative transfer in the microwave spectral region. The SOI method is an iterative approximation to the traditional adding and doubling method for radiative transfer. Results indicate that the approximations made in the SOI method are accurate for atmospheric layers with scattering properties typical of those in the infrared and microwave regions. In addition, an acceleration technique is demonstrated that extends the applicability of the SOI approach to atmospheres with greater amounts of scattering. A comparison of the SOI model with a full Monte Carlo model using the atmospheric profiles given by Smith et al. was used to determine the optimal parameters for the simulation of microwave top-of-atmosphere radiances. This analysis indicated that a four-stream model with a maximum initial-layer optical thickness of approximately 0.01 was optimal. In the second part of this series, the accuracies of the SOI model and its adjoint are demonstrated over a wide range of microwave remote sensing scenarios.