Sijia Gu

Broadband dielectric characterization of aqueous saline solutions by an interferometer-based microwave microscope

The complex dielectric permittivity of aqueous saline solutions has been determined in the frequency range [2–18 GHz] with a home-made near-field microwave microscope. The instrument is built on a vector network analyzer, a matching network, and an evanescent microwave probe. The interferometer-based matching network enables highly reproducible, sensitive, and accurate measurements on the entire frequency band of operation. NaCl solutions concentrations ranging from 0 to 160 mg/ml are investigated at 25 °C. A maximum measurement sensitivity for NaCl concentrations is found to be equal to 2.3 dB/(mg/ml) and 7.7°/(mg/ml) for magnitude and phase-shift, respectively. To translate the measurement data (S parameters) to the corresponding complex permittivities, an inversion procedure based on a simple calibration model is applied. The resulting complex permittivities are found to be in a very good agreement with those calculated by Cole-Cole model.

Measurement accuracy and repeatability in near-field scanning microwave microscopy

We report on the accuracy and repeatability tests for near-field scanning microwave microscopy applications by associating a network analyzer and an evanescent microwave probe (EMP). A broadband matching network based on an interferometric technique is used to achieve a strong electromagnetic coupling between the probe tip and the material in the frequency range 1-20 GHz. The electromagnetic coupling between the probe and a planar metallic sample is investigated using numerical simulations based on finite element method (FEM). Experimental validations show that the measurement sensitivity is enhanced in the vicinity of the probe tip. Measurement accuracy and repeatability of the system are provided that are instructive and beneficial to further experiments.

Dielectric properties characterization of saline solutions by near-field microwave microscopy

Saline solutions are of a great interest when characterizations of biological fluids are targeted. In this work a near-field microwave microscope is proposed for the characterization of liquids. An interferometric technique is suggested to enhance measurement sensitivity and accuracy. The validation of the setup and the measurement technique is conducted through the characterization of a large range of saline concentrations (0–160 mg ml−1). Based on the measured resonance frequency shift and quality factor, the complex permittivity is successfully extracted as exhibited by the good agreement found when comparing the results to data obtained from Cole–Cole model. We demonstrate that the near field microwave microscope (NFMM) brings a great advantage by offering the possibility to select a resonance frequency and a quality factor for a given concentration level. This method provides a very effective way to largely enhance the measurement sensitivity in high loss materials.

A novel inkjet-printed chipless RFID-based passive fluid sensor platform

In this paper, we present a novel chipless RFID-based passive fluid sensor platform which enables a reliable remote fluid identification. The proposed sensor platform consists of a capacitive gap-loaded tunable filter for fluid sensing and two broadband perpendicularly polarized microstrip monopole antennas for a dual-polarization wireless interrogation approach, that allows for drastically reduced ambient clutter interference. Two different fluids (acetone and water) are tested to verify the exceptional sensing capability of the proposed platform, while inkjet-printing technology has been used for its low-cost conformal implementation.

Broadband non-contact characterization of epitaxial graphene by near-field microwave microscopy

In this paper, a broadband non-destructive and non-contact local characterization of graphene fabricated by epitaxial method on silicon carbide is demonstrated by using an interferometer-based near-field microwave microscope. First, a method has been proposed to extract the dielectric properties of silicon carbide, and finally, the graphene flake has been characterized as a resistance (∼20 kΩ) and a small inductance (360 pH) in the frequency band (2-18 GHz). The advantage of the proposed method is that there is no need to fabricate electrodes on the sample surface for the characterization. The instrument proposed is a good candidate for the local characterization of 2D materials.

Materials characterization by near-field scanning microwave microscopy

Near-field scanning microwave microscope (NFSMM) has potentials to achieve characterizations of materials with high spatial resolution and sensitivity. A wide range of applications have been addressed such as dielectric constant measurement, impedance analysis of metals and semiconductor materials/devices, gas and chemical sensing, etc. [1, 2]. In this work, we propose a home-made NFSMM to perform materials nondestructive characterization. This system includes a vector network analyzer (VNA), a matching network and an evanescent microwave probe (EMP) [3, 4]. The configuration of the system is given in Fig. 1. The broadband matching network is composed of a hybrid coupler and an impedance tuner. This matching circuit permits the measurements with high sensitivity and accuracy in the frequency range from 2 GHz to 18 GHz. It is well known that the evanescent wave is concentrated around the probe apex and decays rapidly with the distance away from the probe tip. In this study, an EMP with apex of 66 μm is used to achieve a very local characterization of materials with a sub-wavelength resolution when the tip-material separation is in the order of the apex size. The electromagnetic coupling between the probe and the material under test is also investigated using numerical simulations (ANSYS/HFSS™) based on finite element method (FEM). Thanks to the fact that microwave penetrates into the material, the electric properties (impedance and dielectric constant) of materials can be quantitatively determined based on the wave reflected by the material. The proposed microscopy platform thus allows a straightforward estimation of material properties rather than just topographic information. Furthermore, another advantage of the system is the nondestructive characterization possibility, which avoids injuring the sample surface.

2-20 GHz non-uniform coupler for six-port reflectometer

In this paper a six-port reflectometer to be included in a near-field microwave microscope (NFMM) is presented. The reflectometer, made of five couplers based on non-uniform and offset parallel-coupled transmission lines, is proposed to achieve a broad frequency band [2-20 GHz], corresponding to the frequency range of the interferometer based NFMM. It becomes then possible to replace the vector network analyzer classically used in NFMMS.