Microwave Resonant Cavity Research Articles

Design and characterization of a resonant microwave cavity as a diagnostic for ultracold plasmas.

We present the design and commissioning of a resonant microwave cavity as a novel diagnostic for the study of ultracold plasmas. This diagnostic is based on the measurements of the shift in the resonance frequency of the cavity, induced by an ultracold plasma that is created from a laser-cooled gas inside. This method is simultaneously non-destructive, very fast (nanosecond temporal resolution), highly sensitive, and applicable to all ultracold plasmas. To create an ultracold plasma, we implement a compact magneto-optical trap based on a diffraction grating chip inside a 5GHz resonant microwave cavity. We are able to laser cool and trap (7.25 ± 0.03) × 107 rubidium atoms inside the cavity, which are turned into an ultracold plasma by two-step pulsed (nanosecond or femtosecond) photo-ionization. We present a detailed characterization of the cavity, and we demonstrate how it can be used as a fast and sensitive probe to monitor the evolution of ultracold plasmas non-destructively. The temporal resolution of the diagnostic is determined by measuring the delayed frequency shift following femtosecond photo-ionization. We find a response time of 18 ± 2 ns, which agrees well with the value determined from the cavity quality factor and resonance frequency.

Influence of different Milling Process on the properties of CaTiO3-NdAlO3 Dielectric Ceramics and its Microwave Devices

The high performance CaTiO3-NdAlO3 dielectric ceramics were prepared with different millingprocess. The phase composition, microstructure of the CaTiO3-NdAlO3 dielectric ceramics and dielectric properties of its microwave devices were analyzed. The results indicated that CaTiO3-NdAlO3 dielectric ceramics are single-phase orthogonal perovskite-structure. The temperature drift index of the microwave resonator manufactured by CaTiO3-NdAlO3 dielectric ceramics and the assembled cavity filter are consistent with the technical requirements of microwave resonant components and cavity filters for communication base station.

A New Resonance Phenomenon Observed in UWB 14–50 GHz SAR and Its Application to the Retrieval of Thickness and Dielectric Properties of Scene Features

We describe the observation of a previously unreported resonance phenomenon in UWB SAR imaging and its application in a novel scheme for the retrieval of thickness and dielectric properties of scene features. The resonance was observed in a 14-50 GHz laboratory study of very high resolution SAR imagery. It presented as a periodic fluctuation in the backscatter of a scene feature in response to changes in the radar measurement frequency. The complimentary static reflectivity measurements of a range of dielectric materials showed the phenomenon could be understood as a material behaving as a resonant microwave cavity. Modeling simulations were able to accurately reproduce the observed nulling characteristics for visually homogeneous materials (acrylic, MDF, and plasterboard) and which displayed a regular frequency spacing between nulls. Those with inhomogeneous structures (chipboard and plywood) showed much more irregular nulling patterns. The spacing of the backscatter nulls is set by the product of the permittivity and thickness of the material. By exploiting diversity in viewing geometry, the two terms can be separated and measured. The scheme offers a new opportunity in the SAR surveillance and monitoring of man-made structures.

Numerical optimization of plasma generation in a microwave resonant cavity

The paper presents a numerical simulation of the hydrogen plasma generation process in a cylindrical microwave resonant cavity at moderate pressures (10 Torr). We introduce a modification to the chamber by placing several spherical dielectrics with a high permittivity at the central axis. The radii of the spheres are chosen for the Mie resonance for microwave radiation (at 2.45GHz frequency) to occur. For a dimer configuration where two spheres are placed closely together, a maximum of the electrical field magnitude is located in the gap between the spheres where the intense discharge occurs. Positioning the spheres in different spots of the chamber allows controlling the process of the microwave plasma generation. Such modification is promising for a stable discharge initiation and for preventing the discharge breaking in the microwave cavity.

Characterisation of water in honey using a microwave cylindrical cavity resonator sensor

A low-cost microwave cavity resonator sensor is developed for the assessment of honey. When a liquid sample is placed in the cavity, the resonant responses are significantly changed caused by the material perturbation. From the measurement of standard liquid samples with known permittivity, it is well demonstrated that the electric permittivity can be accurately determined. For the honey-water mixtures with the added water content from 10% to 50% w/w, it is found that the resonance frequency decreases with increasing added water content, and its change conforms to the law of mixture. In addition, there is a linear relationship between the added water content and the real part of the permittivity, and a quadratic relationship is found between the real and imaginary parts of the permittivity. These findings agree well with the results of the mixtures with added water contents below 10%. Hence, both the resonant responses of the cavity and the permittivity obtained can be employed for the indication of adulteration and accurate evaluation of the water content. Further, it is shown that the full penetration of the liquid sample can be achieved. The microwave sensor presented here provides an alternative tool for rapid food evaluation.

High quality factor photonic cavity for dark matter axion searches.

Searches for dark matter axions involve the use of microwave resonant cavities operating in a strong magnetic field. Detector sensitivity is directly related to the cavity quality factor, which is limited, until recently, to the use of non-superconducting metals by the presence of the external magnetic field. In this paper, we present a cavity of novel design whose quality factor is not affected by a magnetic field. It is based on a photonic structure by the use of sapphire rods. The quality factor at cryogenic temperature is in excess of 5 × 105 for a selected mode.

Pure electric and magnetic fields applied to reduced graphene oxide for defect repair and oxygen removal

The rapid repair of graphene oxide (GO) was demonstrated using a microwave cavity resonator that separates pure electric and magnetic fields from microwaves. Reduced graphene oxide (rGO), a high-quality graphene-like material, was obtained using our method. Raman spectroscopy of rGO after microwave treatment revealed that the D band intensity decreased as the temperature increased, while the G and G′ band intensities increased with increasing temperature. Furthermore, the cavity resonator provided better defect repair and oxygen removal from GO than conventional heating, and the microwave magnetic field yielded better quality rGO than the microwave electric field. Microwave heating removed surface impurities and repaired defects in GO more effectively than conventional heating, while suppressing graphitisation. These findings provide guidelines for the microwave-assisted reduction of GO for the fast and large-scale production of graphene.

Highly sensitive open-ended coaxial cable-based microwave resonator for humidity sensing

A novel portable open-ended hollow coaxial cable resonator (OE-HCCR) is proposed and demonstrated for direct relative humidity (RH) measurements without the assistance of any humidity-sensitive material. The sensor is fabricated from a homemade hollow coaxial cable platform that provides ease of modification and improved robustness. The microwave cavity resonator is a metal post-coupled open-ended coaxial resonator, which includes two highly-reflective reflectors, i.e., the metal post and the metal plate-backed open end of the coaxial line. The gap distance between the metal plate and the open end is defined by a circular spacer. By simple one-step adjustments of the thickness of the spacer, the RH measurement sensitivity of the OE-HCCR can be greatly enhanced. The device physics is discussed; numerical calculations and experimental investigations are conducted to verify the sensitivity enhancement of the OE-HCCR for RH sensing. Due to the increased sensitivity, this is the first report of an open-ended coaxial line that is able to directly sense gaseous analyte, to the best of our knowledge. The proposed OE-HCCR with the sensitivity enhancement has the potential for highly sensitive chemical sensing combined with functional materials.

Study of the dielectric response and thermo-dynamical properties of PentylCyano Biphenyl (PCB) Liquid Crystal using microwave cavity spectrometer as a probe

A comparable study of the dielectric response of PentylCyano Biphenyl (PCB) nematic liquid crystals over the temperature range 297–316°K is reported for two frequencies i.e. 9.0 GHz and 20.9 GHz. The loaded resonant microwave cavity operated in TM010 mode at two different frequencies mentioned above. The theory of perturbation is used to analyze the data. The computation of Activation energy and Relaxation time was done. Calculation for the thermo-dynamical parameters such as ΔG, ΔH and ΔS was done for a suitable temperature range. It has been observed that the change in temperature and the change in frequency both effects the properties such as relaxation time, activation energy, entropy and enthalpy. This method is well known in scientific community for its requirement of small quantity of sample and inherent accuracy. The possible applications are reversible optical memories, beam steering antenna, and fabricating frequency agile components.

Probing Collisional Plasmas with MCRS: Opportunities and Challenges

Since the 1940s, Microwave Cavity Resonance Spectroscopy (MCRS) has been used to investigate a variety of solids, gases, and low-pressure plasmas. Recently, the working terrain of the diagnostic method has been expanded with atmospheric-pressure plasmas. This review discusses the advancements that were required for this transition and implications of studying highly collisional, with respect to the probing frequencies, plasmas. These developments and implications call for a redefinition of the limitations of MCRS, which also impact studies of low-pressure plasmas using the diagnostic method. Moreover, a large collection of recommendations concerning the approach and its potential for future studies is presented.

Addendum: Mapping electron dynamics in highly transient EUV photon-induced plasmas: a novel diagnostic approach using multi-mode microwave cavity resonance spectroscopy (2018 J. Phys: D. Appl. Phys. 52 034004)

A new approach for an in-line beam monitor for ionizing radiation was introduced in a recent publication (Beckers J et al 2018 J. Phys. D: Appl. Phys. 52 034004). Due to the recent detection and investigation of an additional third decay regime of the afterglow of an extreme ultraviolet photon-induced plasma described in a later article (Platier B et al 2020 Appl. Phys. Lett. 116 103703) there is an additional reason for a minimum number of photons for this approach to work. Near or below this threshold, we explain that the response time of the diagnostic method is a limiting factor. Further, a second limit for the number of photons within a pulse is formalized related to the trapping of highly energetic free electrons.

Magnetic field-enhanced beam monitor for ionizing radiation.

For the microwave cavity resonance spectroscopy based non-destructive beam monitor for ionizing radiation, an addition-which adapts the approach to conditions where only little ionization takes place due to, e.g., small ionization cross sections, low gas pressures, and low photon fluxes-is presented and demonstrated. In this experiment, a magnetic field with a strength of 57 ± 1 mT was used to extend the lifetime of the afterglow of an extreme ultraviolet-induced plasma by a factor of ∼5. Magnetic trapping is expected to be most successful in preventing the decay of ephemeral free electrons created by low-energy photons. Good agreement has been found between the experimental results and the decay rates calculated based on the ambipolar and classical collision diffusion models.

Characterization of Cryogenic Material Properties of 3-D-Printed Superconducting Niobium Using a 3-D Lumped Element Microwave Cavity

We present an experimental characterization of the electrical properties of 3-D-printed niobium. The study was performed by inserting a 3-D-printed Nb post inside an aluminum cylindrical cavity, forming a 3-D lumped element reentrant microwave cavity resonator. The resonator was cooled to temperatures below the critical temperature of niobium (9.25 K) and then aluminum (1.2 K) while measuring the quality factors of the electromagnetic resonances. This was then compared with finite element analysis of the cavity and a measurement of the same cavity with an aluminum post of similar dimensions and frequency, to extract the surface resistance of the niobium post. The 3-D-printed niobium exhibited a transition to the superconducting state below 9.25 K, as well as a surface resistance of 3.1 ± 0.54 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Ω. This value was comparable to many samples of traditionally machined niobium previously studied without specialized surface treatment. Furthermore, this study demonstrates a simple new method for characterizing the material properties of a relatively small and geometrically simple sample of the superconductor, which could be easily applied to other materials, particularly 3-D-printed materials. Further research and development in additive manufacturing may see the application of 3-D-printed niobium in not only superconducting cavity designs but also in the innovative technology of the future.

Radio Frequency Studies of Soot Loading and Ammonia Storage on a Diesel Particulate Filter with a SCR Catalyst Coating

In the exhaust system of some diesel vehicles, the diesel particulate filter (DPF) is loaded with a selective catalyst reduction (SCR) wash coat. This combined catalyst, referred to as a SDPF, serves two functions by trapping soot and by reducing NOx emissions. The latter is accomplished via reactions of NOx with NH3 stored on the SCR wash coat. The NH3 is dosed, as urea, upstream of the SDPF.

Optical and Microcavity Modes Entanglement by Means of Plasmonic Opto-Mechanical System

In this study, plasmonic opto-mechanical tripartite system is proposed to improve the performance of the traditional tripartite opto-mechanical system. In the new design, significantly, optical cavity and microwave cavity modes are directly coupled to each other. The originality of this work consists in embedding a microsphere in the optical cavity where the plasmon-plasmon interaction between the metal plates generates a plasmon mode inside the optical cavity and changes the electric field distribution. The plasmonic property influences the microsphere electrical properties and interacts with the photonic mode inside the optical cavity by which the microwave cavity properties are also affected through coupling to the optical cavity. Microsphere introduces a capacitor as a function of plasmonic properties that can strongly influence the microwave cavity resonance frequency. That is the feature that we want to utilize to enhance the performance of the system at high temperature. The results show that the optical cavity and microwave cavity modes remain entangled at high temperature. It is contributed to the plasmonic-based capacitor induced by the microsphere which is not affected by the thermally induced photons (noise). It is worth mentioning that the induced noise strongly restricts the traditional tripartite system operated with a wide bandwidth.

Atomic microwave-to-optical signal transduction via magnetic-field coupling in a resonant microwave cavity

Atomic vapors offer many opportunities for manipulating electromagnetic signals across a broad range of electromagnetic spectra. Here, a microwave signal with an audio frequency modulation encodes information in an optical signal by exploiting an atomic microwave-to-optical double resonance and magnetic-field coupling that is amplified by a resonant high-Q microwave cavity. Using this approach, audio signals are encoded as amplitude or frequency modulations in a GHz carrier, transmitted through a cable or over free space, demodulated through cavity-enhanced atom-microwave interactions, and, finally, optically detected to extract the original information. This atom-cavity signal transduction technique provides a powerful means by which to transfer information between microwave and optical fields, all using a relatively simple experimental setup without active electronics.

Resonant microwaves probing acoustic waves from an RF plasma jet

Microwave cavity resonance spectroscopy is introduced and demonstrated as a new approach to investigate the generation of acoustic waves by a pulsed radio-frequency driven atmospheric-pressure plasma jet. Thanks to recent advancements in the diagnostic method, the lower detection limit for pressure changes in air is ∼0.3 Pa. Good agreement with conventional pressure transducer measurements with respect to the temporal evolution, the pressure amplitude and the spectral response is found. Fourier analysis revealed that the acoustic waves induced by the plasma can most likely be attributed to standing waves in the discharge geometry. Additionally, the plasma-induced acoustic waves of a few (tens of) Pa are proposed as an active mechanism in plasma medicine.

Transition from ambipolar to free diffusion in an EUV-induced argon plasma

Extreme Ultraviolet (EUV) optical components used in EUV lithography tools are continuously impacted by an exotic and highly transient type of plasma: EUV-induced plasma. Such an EUV-induced plasma is generated in a repetitive fashion upon sending a pulsed beam of high energy (92 eV) photons through a low-pressure background gas. Although its formation occurs on a time scale of ∼100 ns, it is the plasma's decay dynamics on longer time scales that dictates the fluxes and energy distribution of the produced ions. Therefore, the plasma decay also determines the overall impact on plasma-facing EUV optical components. Enabled by electron density measurements using Microwave Cavity Resonance Spectroscopy at a much higher sensitivity, we clearly show the breakdown of the ambipolar field in an EUV photon-induced plasma below electron densities of ∼2 × 1012 m−3 and the—until now—unidentified transition from ambipolar diffusion-driven decay into a decay regime driven by free diffusion. These results not only further improve the understanding of elementary processes in this type of plasma but also have a significant value for modeling and predicting the stability and lifetime of optical components in EUV lithography.

Resonant microwaves probing the spatial afterglow of an RF plasma jet

The electron density and effective electron collision frequency in the spatial afterglow of a pulsed radio frequency driven atmospheric-pressure plasma jet are obtained by using microwave cavity resonance spectroscopy in a temporal manner with an ∼1 μs resolution. During the “plasma on” phase, values of 1.7 ± 0.3×1018 m−3 for the electron density and 0.12 ± 0.01 THz for the electron collision frequency were found. These values and standard deviations represent the collective measurement set with repetition rates ranging from 125 to 8000 Hz. The spread in the plasma parameters during this phase within one repetition frequency is smaller than 3%. It is observed that remnant species, e.g., metastables, of previous discharges influence the decay of the plasma. The work reported is enabled by recent developments in the applied diagnostic with respect to the resolution in the plasmas' permittivity. Moreover, a multiplying probe is used for the electrical characterization of the plasma and the presence of the cavity did not influence the plasma impedance. This strongly suggests that the cavity did not affect the discharge.

Microwave cavity resonance spectroscopy of ultracold plasmas

We present an in situ study of the electron dynamics of an ultracold $^{85}\mathrm{Rb}$ plasma using microwave cavity resonance spectroscopy with submicrosecond temporal resolution and vastly improved sensitivity. We observe a three times faster plasma decay than predicted by a self-similar Gaussian expansion model, which might be attributed to collisional microwave heating and plasma-wall interactions. The demonstrated sensitivity enables nondestructive probing of other (nearly) ionized media of fundamental importance as well, such as cold Rydberg gases and antihydrogen plasmas.