Laser Scanning Microscopy of HTS Films and Devices

The work describes the capabilities of Laser Scanning Microscopy (LSM) as a spatially resolved method of testing high_Tc materials and devices. The earlier results obtained by the authors are briefly reviewed. Some novel applications of the LSM are illustrated, including imaging the HTS responses in rf mode, probing the superconducting properties of HTS single crystals, development of twobeam laser scanning microscopy. The existence of the phase slip lines mechanism of resistivity in HTS materials is proven by LSM imaging.Comment: 17 pages, 21 figures, Submitted to Fizika Nizkikh Temperatur (Low Temperature Physics

Evanescent Microwave Microscopy of Porcine Skin Tissue

This thesis describes the application of a custom designed λ/4 resonant coaxial probe to measurements of the complex permittivity of in vitro freshly excised porcine skin tissue. The dielectric properties of the organic material within the near field of the tip of the probe frustrates the electric field and measurably changes the resonant frequency and quality factor Q of the coaxial sensor, where the change in resonant frequency and quality factor of a tuned resonance is measured as a function of tip-sample separation. The design of the probe enables better spatial resolution than previously developed probes. The following studies were performed: (1) measurement of the complex permittivity of porcine tissue over a range of frequencies revealed values consistent with published literature; (2) the change in quality factor from the probe measurements was compared to a visual microscope analysis of histological slices from the same skin location, indicating that subsurface structures such as capillary beds, ducts of exocrine sweat glands, and vein lumen ducts can be detected; and (3) measurements of the dielectric properties of a puncture wound and several burn lesions of varying severity revealed easily detectable changes, indicating that the probe has potential for evaluating and monitoring skin conditions. In addition, a new quantitative two point relationship between the real and imaginary parts of the materials complex permittivity related to the resonant frequency and quality factor shift using the method of images is developed, presented, and used in the above described skin studies. The two-point model describing the probe tip-sample interaction is capable of producing quantitative complex permittivity values for organic and biological materials

Local Complex Permittivity Measurements of Porcine Skin Tissue in the Frequency Range From 1 GHz to 15 GHz by Evanescent Microscop

The near-field evanescent microwave microscope is based on a coaxial transmission line resonator with a silver plated tungsten tip protruding through an end-wall aperture. The sensor is used to measure the local dielectric properties of porcine skin in the frequency range from 1 GHz to 15 GHz. The dielectric property of the skin within the near field of the tip frustrates the electric field and measurably changes the transmission line\u27s resonant frequency and quality factor (Q). The shift of the resonator\u27s frequency and Q is measured as a function of tip-sample separation, and a quantitative relationship between the real and imaginary parts of the local dielectric constant using the method of images is established. The associated changes in quality factor image scans of subsurface tissue structure and dielectric properties of skin surface lesions are presented

Local Dielectric and Strain Measurements in YBa2Cu3O7−δ Thin Films by Evanescent Microscopy and Raman Spectroscopy

The near-field evanescent microwave microscope is based on a coaxial transmission line resonator with a silver-plated tungsten tip protruding through an end-wall aperture. The sensor is used to measure local dielectric properties of thin-film YBa2Cu3O7−δ deposited on three different SrTiO3 bi-crystal substrates having mismatch grain boundary angles of 3°, 6°, and 12°. The measurements in the superconducting state are below critical temperature at T = 79.4 K. The dielectric property of the superconductor within the near field of the tip frustrates the electric field and measurably changes the transmission line\u27s resonant frequency. The shift of the resonator\u27s frequency is measured as a function of tip–sample separation and associated changes in quality factor (ΔQ) image scans of the thin film are presented. A quantitative relationship between the real and imaginary parts of the local dielectric constant and the frequency shift using the method of images is established. The comparison between experimental data and theory based on this method is given and discussed. Raman measurements of the intergranular strain within the YBa2Cu3O7−δ thin film deposited on each SrTiO3 substrate in the region of the bi-crystal junction showed excellent correlation between grain boundary mismatch and induced grain boundary strain. Compressive strains normal to the aaxis (i.e. tensile strains normal to the b axis) were detected across the grain boundary. The magnitude of induced strain as well as its spread away from the grain boundary increased as the mismatch angle increased

Local Dielectric and Strain Measurements in YBa2Cu3O7−δ Thin Films by Evanescent Microscopy and Raman Spectroscopy

The near-field evanescent microwave microscope is based on a coaxial transmission line resonator with a silver-plated tungsten tip protruding through an end-wall aperture. The sensor is used to measure local dielectric properties of thin-film YBa2Cu3O7−δ deposited on three different SrTiO3 bi-crystal substrates having mismatch grain boundary angles of 3°, 6°, and 12°. The measurements in the superconducting state are below critical temperature at T = 79.4 K. The dielectric property of the superconductor within the near field of the tip frustrates the electric field and measurably changes the transmission line\u27s resonant frequency. The shift of the resonator\u27s frequency is measured as a function of tip–sample separation and associated changes in quality factor (ΔQ) image scans of the thin film are presented. A quantitative relationship between the real and imaginary parts of the local dielectric constant and the frequency shift using the method of images is established. The comparison between experimental data and theory based on this method is given and discussed. Raman measurements of the intergranular strain within the YBa2Cu3O7−δ thin film deposited on each SrTiO3 substrate in the region of the bi-crystal junction showed excellent correlation between grain boundary mismatch and induced grain boundary strain. Compressive strains normal to the aaxis (i.e. tensile strains normal to the b axis) were detected across the grain boundary. The magnitude of induced strain as well as its spread away from the grain boundary increased as the mismatch angle increased

Structural investigations and magnetic properties of sol-gel Ni0.5Zn0.5Fe2O4 thin films for microwave heating

Nanocrystalline Ni0.5Zn0.5Fe2O4 thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology, magnetic, and microwave absorption properties of the films calcined in the 673–1073 K range were studied with x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, atomic force microscopy, vibrating sample magnetometry, and evanescent microwave microscopy. All films were uniform without microcracks. Increasing the calcination temperature from 873 to 1073 K and time from 1 to 3 h resulted in an increase of the grain size from 12 to 27 nm. The saturation and remnant magnetization increased with increasing the grain size, while the coercivity demonstrated a maximum near a critical grain size of 21 nm due to the transition from monodomain to multidomain behavior. The complex permittivity of the Ni–Zn ferrite films was measured in the frequency range of 2–15 GHz. The heating behavior was studied in a multimode microwave cavity at 2.4 GHz. The highest microwave heating rate in the temperature range of 315–355 K was observed in the film close to the critical grain size

Microwave characterization of nanostructured ferroelectric Ba0.6Sr0.4TiO3 thin films fabricated by pulsed laser deposition

A series of nanostructured ferroelectric thin films of barium strontium titanate were fabricated using a pulsed laser deposition system with real-time in situ process control. Pulsed laser deposition parameters were controlled during the growth of nanostructured thin films for use in the development of high frequency tunable microwave devices. The thin films were all grown at the same substrate temperature and laser beam energy density, but the chamber oxygen partial pressure (COPP) was varied systematically from 19 mTorr through 1000 Torr. Structural and electromagnetic characterization was performed using atomic force microscopy and evanescent microwave microscopy, respectively. Atomic force microscopy showed a linear increase in grain size with increases in the ambient oxygen pressure from 38 to 150 mTorr and from 300 mTorr to 1000 Torr. The correlation of the microwave properties with the epitaxial film microstructure can be attributed to stresses and polarizability in the film. Microwave characterization showed that a COPP of 75 mTorr yielded the most desirable film in terms of tunability and loss tangent over a wide frequency range

Microwave characterization of nanostructured ferroelectric Ba0.6Sr0.4TiO3 thin films fabricated by pulsed laser deposition

A series of nanostructured ferroelectric thin films of barium strontium titanate were fabricated using a pulsed laser deposition system with real-time in situ process control. Pulsed laser deposition parameters were controlled during the growth of nanostructured thin films for use in the development of high frequency tunable microwave devices. The thin films were all grown at the same substrate temperature and laser beam energy density, but the chamber oxygen partial pressure (COPP) was varied systematically from 19 mTorr through 1000 Torr. Structural and electromagnetic characterization was performed using atomic force microscopy and evanescent microwave microscopy, respectively. Atomic force microscopy showed a linear increase in grain size with increases in the ambient oxygen pressure from 38 to 150 mTorr and from 300 mTorr to 1000 Torr. The correlation of the microwave properties with the epitaxial film microstructure can be attributed to stresses and polarizability in the film. Microwave characterization showed that a COPP of 75 mTorr yielded the most desirable film in terms of tunability and loss tangent over a wide frequency range