Microwave (MW) studies of biochemical liquids require the precise determination of permittivity and a convenient sample placement in a sterile channel, which must be cleaned or replaced after testing. These requirements can be effectively met using the proposed MW characterization technique. This technique is based on a high-quality whispering-gallery mode (WGM) quartz resonator. The liquid under test is filled in a capillary tube (CT), which is placed in the resonator hole. A calibration approach based on simulations for the extraction of complex permittivity values of biochemical liquids of sub-microliter volumes was demonstrated and successfully verified. This was achieved through a special procedure for the analysis of the interaction of the electromagnetic field with the liquid in a CT inside a WGM quartz resonator and measurements of its frequency response. The reliability of the technique was validated by simulation and the measurement of liquids with known characteristics. The approach was successfully applied to obtain the complex permittivity of glucose in water solutions in the Ka-band.

For effective technological implementation of HTS-based microstrip resonators, it is of practical interest to study and to compare the dissipative properties of two types of such resonators: excited on whispering gallery modes (WGM) and resonators excited at the fundamental frequency. The analysis was performed in the frequency range from 1.5 GHz to 40 GHz. By using numerical simulation of partial Q-factors it is shown that the value of Q-factor at a temperature of 77 K and lower for the YBa2Cu3O7-delta - based WGM resonator without housing exceeds the Q-factor for the resonator exited with a fundamental mode with housing in the frequency range from 1.5 to 35 GHz. This is an important practical result. The experimental data and the results of numerical simulations are mutually consistent. In the millimeter wavelength range, when the dimensions of traditional resonators become unacceptably small, it will be more convenient to use microstrip WGM resonators.

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Amino acids play essential role for humans. We studied the dielectric properties of the basic aliphatic amino acids and polar positive amino acids in solutions of different concentrations. The high-Q single microwave whispering gallery-mode (WGM) quartz dielectric resonator based technique, enhanced to a number of measurement frequencies, was applied. The technique allows liquid investigation of micro- to nano- liter volumes filled in microfluidic channel on six discrete frequencies in the 30-40 GHz range. The dependencies of the complex permittivity on the molar mass show almost linear behavior for aliphatic amino acids at different concentrations. The study results are in good agreement with the calculated data obtained by Cole-Cole equation.

The accurate measurement of complex liquid permittivity in a frequency range provides important information on liquid properties in comparison to single frequency permittivity investigations. The multifrequency microwave characterization technique based on a single quartz whispering-gallery-mode (WGM) resonator with a microfluidic chip are proposed, developed, and demonstrated. This technique allows the complex permittivity of small volumes of liquid to be measured at six resonant frequencies in the 30-40 GHz frequency range. The calibration is performed by simulating the measurement cell and by plotting the calibration nomogram charts for all six investigated frequencies. The novel approach is applied in studies of the complex permittivity of the L-lysine in water solutions. The results open up possibilities to investigate the complex permittivity of biological liquids at several frequencies and to further develop the microwave dielectrometry of small liquid volumes in a certain frequency range using the quasi-optical nature of a single high-Q WGM resonator.

The paper presents preliminary results on the analysis of the applicability of microwave measurement techniques for study the properties of superconductors in normal state using dielectric resonators excited with whispering gallery modes (WGM). The expressions for microwave surface impedance for a normal conductor are derived. The experimental technique for measuring the surface impedance of superconductors in a normal state using mm-wave sapphire WGM resonator is proposed and described. The reliability of the proposed approach is demonstrated measuring the response of a resonator with copper samples is presented.

Studies of biological solutions require high measurement accuracy, the ability to detect low changes in substance concentration, and small amounts of the liquid under test. A microwave complex permittivity measurement technique based on a high-quality-factor whispering-gallery-mode resonator with a microfluidic chip allows small amounts of dielectric liquids to be investigated with high accuracy. An existing technique based on a sapphire resonator does not provide for a low-concentration detection limit for substances with low molecular weight. Here, we present an advanced technique based on a quartz dielectric resonator. The detection limit obtained for glucose in water solution was found to be about one order of magnitude lower for the quartz resonator cell than for the sapphire-resonator-based measurement cell. The limit is about or lower than the concentration of glucose in human blood. This fact means that the technique can be used as the sensor for investigations of biological solutions in the microwave range. A study of glucose, lactalbumin, and bovine serum albumin was successfully performed in the Ka-band using the developed sensor.

The response of sapphire WGM resonator to a single-layer graphene introduced onto the dielectric substrate is studied in Ka-band. The experimental investigations were performed for two types of resonator modes: HE and EH. The modes differ by the prevailing component: electric or magnetic of electromagnetic field along the resonator longitudinal axis. The possibility of using these modes for finding graphene conductivity is demonstrated. Based on the experimental and the resonator model data obtained using COMSOL Multiphysics software the graphene conductivity values were obtained. The conductivity values calculated based on results of measurements performed on two mode types with different substrate position relative to the resonator are compared. The results demonstrate reproducibility. The possible source of some difference between the results obtained on different modes is discussed. The recommendations regarding the design of the most suitable configuration for graphene conductivity measurements are proposed.

The microwave characterization technique for the investigation of liquids of small volumes is presented. The technique is developed on the basis of a high-quality whispering gallery mode quartz resonator with a capillary placed in the hole of the resonator in the axial direction. The optimal inner capillary diameter was determined by experimental data obtained in Ka-band for the aqueous solution of glucose filling. The measurements of the bovine serum albumin and lactalbumin solutions confirm the applicability of the technique for studies of the biological liquids.

Antioxidants play a crucial role in the life sciences, as the regulators of biochemical reactions. We studied the dielectric properties of the low-molecular weight antioxidant specific biomarkers sodium ascorbate and glutathione in solutions of different concentrations. The biomarkers are multifunctional metabolites relevant to the reactive oxygen species (ROS) scavenging system of cells. The newly developed high-Q microwave whispering-gallery-mode (WGM) dielectric resonator based technique was applied. The technique allows investigation of liquids of nanoliter volumes filled in microfluidic channel within several milliseconds. The revealed peculiarities in the dependence of permittivity on concentrations of the sodium ascorbate and glutathione solutions are explained by differences in relaxation times and loses introduced by molecules of different shapes. We suggest that this novel approach offers the potential for the detection and characterization of ROS-relevant biomarkers with millisecond-time resolution.

The response of a sapphire whispering gallery mode (WGM) resonator to a single-layer graphene film was studied in the millimeter wave band (frequency of about 40 GHz) at different distances of graphene from the resonator. In the resonator, the HE141δ WGM was excited, in which the longitudinal component of the electric field is predominant. Based on the fitting results of both the response measurement and the numerical simulation of the resonator, the conductivity value was obtained for a known film thickness. The conductivity of our CVD-grown and transferred graphene was found to be (1.02 ± 0.06) × 106 S/m. This deviates slightly from the values obtained through our DC conductivity measurements, reflecting the real parameters of the graphene material after transfer from copper to a quartz substrate. A significant difference was demonstrated between the conductivity values obtained by the fitting procedure and those calculated using the perturbation method. In explanation for the discrepancy, we propose a possible inapplicability of the perturbation method for the cases of both the resonator and mode polarization used in this work. The results of this work show that a WGM resonator technique allows contactless exploration of graphene parameters, such as conductivity or sheet resistance, in the millimeter wave band.

We present the accurate microwave characterization technique for investigation of liquids of nanoliters volumes. This technique is an improved modification of the previously developed approach based on a sapphire whispering gallery mode (WGM) resonator. It is based on a high-quality quartz WGM resonator with microfluidic chip. Experimental data are obtained in Ka-band for the aqueous solution of glucose studied down to the concentration equal to concentration of glucose in human blood. The detection limit of the technique for low molecular weight substances was found to be less than their concentration in real biological samples.

This work presents the results of an accurate real-time microwave characterization of low-molecular-weight antioxidant biomarkers in low-volume solutions using a high-Q whispering gallery-mode dielectric resonator with a microfluidic channel. Sodium ascorbate and glutathione were characterized as the most significant non-enzymatic biomarkers of oxidative stress with respect to the reactive oxygen species scavenging system of cells. Microwave characterization was performed using a high-Q whispering-gallery-mode dielectric resonator with a microfluidic channel. The experiments demonstrated a fast response with high sensitivity in detecting sodium ascorbate and glutathione of different concentrations in biological solutions of small nanoliter volumes at a resonance frequency of about 35.5 GHz. These results indicate great potential for applying real-time microwave whispering-gallery-mode biosensing techniques in the characterization of low-molecular-weight antioxidant-specific biomarkers, which could be used in the laboratory and clinical biomedical research.

WGM resonators can be used for studies of condensed matter, especially for the study of the dielectric liquids properties. However, the electromagnetic properties of the real resonator structures are not well understood. The microwave response of quasioptical radially-two-layered dielectric resonator in the form of a cylindrical ring sandwiched between two flat conducting endplates is studied. Usually real resonator has an additional layer between the leucosapphire ring and a conducting surface, that eliminates a possibility of the rigorous solution for the corresponding electromagnetic problem. The leucosapphire resonator is studied with numerical modeling methods and experimental measurements in the Ka-band. A good agreement between the experimentally measured and calculated values of frequencies and Q-factors is achieved. It is shown that the resonator allows the measurement of the complex permittivity of solutions with an error of about 3 %.

In the present work, the measurement cell for investigation of small-volume liquids in sub-THz range is proposed. The proposed measurement cell is based on the whispering-gallery mode quartz resonator covered with the plastic layer comprising the microfluidic channel and allows obtaining the complex permittivity of the liquid filling the microfluidic channel. Investigation of the resonator structure was carried out experimentally and using the numerical model of the structure. It is shown that the resonator structure can be used as a measurement cell with additional calibration procedure.

We present the measurement technique for studying the electromagnetic properties of small-volume water solutions of bioliquids in sub-THz band. The measurement cell is based on the quartz resonator covered by the plastic layer with the microfluidic channel filled with the liquid under test. The experimental and numerical studies of the resonator with the microfluidic channel filled with liquids with known properties were carried out to show the suitability of the resonator structure for the permittivity measurements. The resonator structure with the microfluidic channel filled with glucose and albumine water solutions was studied experimentally. The solution permittivity values can be obtained by solving of the inverse electromagnetic problem.

Using WGM resonators allow achieving high accuracy in determining the substances permittivities due to the high Q-factor. The resonator with a microfluidic channel is promising for the study of small-volume liquids, which is especially important for bioliquids. In a Ka-band a sapphire resonator with microfluidic channel has been proposed as a measurement cell, but in the sub-THz range a quartz resonator has more suitable characteristics due to its high Q-factor. In the paper we propose a measurement cell for the determination of complex permittivity of liquids in sub-THz range on the basis of quasi-optical quartz resonator with a layer of plastic comprising a microfluidic channel. Experimental studies of the resonator structure are carried out and a model for numerical research in the software COMSOL Multiphysics is proposed. By means of c omparing the resonator frequencies and Q-factors values with the corresponding values obtained with the numerical model the correction of the model has been carried out for the water-filled microfluidic channel. It is shown that the model for numerical studies correctly describes the resonator structure. The values of the resonator frequency and Q-factor obtained experimentally and numerically, for the microfluidic channel filled with the substances with well-known complex permittivity (methanol, propanol, ethanol, acetone), are in good agreement, which indicates the possibility of using the resonator as a measurement cell allowing the research on small-liquid permittivity using the special calibration procedure.

Studies of biochemical liquids require precise determination of their complex permittivity. We developed a microwave characterization technique on the basis of a high-quality whispering-gallery mode (WGM) sapphire resonator with a microfluidic channel filled with the liquid under test. A novel approach allows obtaining the complex permittivity of biochemical liquids of sub-microliter/nanoliter volumes with high accuracy. The method is based on special procedure of analysis of the interaction of electromagnetic field with liquid in WGM microfluidic resonator and measurements of both the WGM resonance frequency shift and the change of the inverse quality factor. The approach is successfully applied to obtain the complex permittivity in the Ka band of glucose, albumin bovine serum, lactalbumin and cytochrome C aqueous solutions using the developed microwave technique.

While using whispering gallery modes dielectric resonators as measurement cells for finding the electrophysical parameters of materials it is necessary to choose the resonator structure, which is characterized by the acceptable radiation Q-factor values. Calculation of radiation Q-factor by analytical methods is possible for simple symmetric structures only. The numerical study of different shape resonators made of leucosapphire, which are limited with conducting end plates and without them are carried out in Ka-band. The disk resonators with one conducting end plate are studied experimentally too. The influence of tested material brought in the electromagnetic field on the resonator field distribution, frequency and radiation Q-factor is shown. The application opportunity of measurement cells based on such resonators for determination of (super)conductor microwave impedance and permittivity of dielectric liquids is analyzed. The obtained results make it possible to choose the resonator measurement cell with negligible radiation loss.

Radiation losses of Ka-band sapphire whispering gallery mode (WGM) resonators are studied using COMSOL program. The resonators with two and one conducting endplates (CEP) are analyzed. Such resonators are promising for monitoring of microwave surface impedance of (super)conductors. A hemi-cylindrical resonator as a specific kind of the cut disk resonator with two CEPs has a high radiation Q-factor. The numerical studies performed also for different shape resonators with one CEP, show high values of radiation Q-factor for the resonators, among which the hemispherical resonator has the highest radiation Q-factor. Experimental measurements are performed for the resonators with one CEP, where the shape of dielectric disk differs from cylindrical one. The experiment shows smaller values of Q-factor compared with the numerical results. This can be due to additional radiation losses at the coupling elements of quasi-optical dielectric resonators.