Novel wave phenomena in two- and three-dimensional (2D and 3D) phononic crystals were investigated experimentally using ultrasonic techniques. Resonant tunneling of ultrasonic waves was successfully observed for the first time by measuring the transmission of ultrasound pulses through a double barrier consisting of two 3D phononic crystals separated by a cavity. This effect is the classical analogue of resonant tunneling of a quantum mechanical particle through a double potential barrier, in which transmission reaches unity at resonant frequencies. For phononic crystals, the tunneling peak was found to be less than unity, an effect that was explained by absorption. The dynamics of resonant tunneling was explored by measuring the group velocities of the ultrasonic pulses. Very slow and very fast velocities were found at frequencies close to and at the resonance, respectively. These extreme values are less than the speed of sound in air and greater than the speed of sound in any of the crystal’s constituent materials. Negative refraction and focusing effects in 2D phononic crystals were also observed. Negative refraction of ultrasound was demonstrated unambiguously in a prism-shaped 2D crystal at frequencies in the 2nd pass band where the wave vector and group velocity are opposite. The Multiple Scattering Theory and Snell’s law allowed theoretical predictions of the refraction angles. Excellent agreement was found between theory and experiment. The negative refraction experiments revealed a mechanism that can be used to focus ultrasound using a flat phononic crystal, and experiments to demonstrate the focusing of ultrasound emitted by several point sources were successfully carried out. The importance of using phononic crystals with circular equifrequency contours, as well as matching the size of the contours inside and outside the crystal, was established. Both conditions were satisfied by a flat phononic crystal of steel rods, in which the liquid inside the crystal (methanol) was different from the outside medium (water). The possibility of achieving subwavelength resolution using this phononic crystal was investigated with a subwavelength line source (a miniature strip-shaped transducer, approximately lambda/5 wide, where lambda is sound wavelength in water). A resolution of 0.55lambda was found, which is just above the diffraction limit lambda/2. / October 2007

Novel wave phenomena in two- and three-dimensional (2D and 3D) phononic crystals were investigated experimentally using ultrasonic techniques. Resonant tunneling of ultrasonic waves was successfully observed for the first time by measuring the transmission of ultrasound pulses through a double barrier consisting of two 3D phononic crystals separated by a cavity. This effect is the classical analogue of resonant tunneling of a quantum mechanical particle through a double potential barrier, in which transmission reaches unity at resonant frequencies. For phononic crystals, the tunneling peak was found to be less than unity, an effect that was explained by absorption. The dynamics of resonant tunneling was explored by measuring the group velocities of the ultrasonic pulses. Very slow and very fast velocities were found at frequencies close to and at the resonance, respectively. These extreme values are less than the speed of sound in air and greater than the speed of sound in any of the crystal’s constituent materials. Negative refraction and focusing effects in 2D phononic crystals were also observed. Negative refraction of ultrasound was demonstrated unambiguously in a prism-shaped 2D crystal at frequencies in the 2nd pass band where the wave vector and group velocity are opposite. The Multiple Scattering Theory and Snell’s law allowed theoretical predictions of the refraction angles. Excellent agreement was found between theory and experiment. The negative refraction experiments revealed a mechanism that can be used to focus ultrasound using a flat phononic crystal, and experiments to demonstrate the focusing of ultrasound emitted by several point sources were successfully carried out. The importance of using phononic crystals with circular equifrequency contours, as well as matching the size of the contours inside and outside the crystal, was established. Both conditions were satisfied by a flat phononic crystal of steel rods, in which the liquid inside the crystal (methanol) was different from the outside medium (water). The possibility of achieving subwavelength resolution using this phononic crystal was investigated with a subwavelength line source (a miniature strip-shaped transducer, approximately lambda/5 wide, where lambda is sound wavelength in water). A resolution of 0.55lambda was found, which is just above the diffraction limit lambda/2.

Novel wave phenomena in two- and three-dimensional (2D and 3D) phononic crystals were investigated experimentally using ultrasonic techniques. Resonant tunneling of ultrasonic waves was successfully observed for the first time by measuring the transmission of ultrasound pulses through a double barrier consisting of two 3D phononic crystals separated by a cavity. This effect is the classical analogue of resonant tunneling of a quantum mechanical particle through a double potential barrier, in which transmission reaches unity at resonant frequencies. For phononic crystals, the tunneling peak was found to be less than unity, an effect that was explained by absorption. The dynamics of resonant tunneling was explored by measuring the group velocities of the ultrasonic pulses. Very slow and very fast velocities were found at frequencies close to and at the resonance, respectively. These extreme values are less than the speed of sound in air and greater than the speed of sound in any of the crystal’s constituent materials. Negative refraction and focusing effects in 2D phononic crystals were also observed. Negative refraction of ultrasound was demonstrated unambiguously in a prism-shaped 2D crystal at frequencies in the 2nd pass band where the wave vector and group velocity are opposite. The Multiple Scattering Theory and Snell’s law allowed theoretical predictions of the refraction angles. Excellent agreement was found between theory and experiment. The negative refraction experiments revealed a mechanism that can be used to focus ultrasound using a flat phononic crystal, and experiments to demonstrate the focusing of ultrasound emitted by several point sources were successfully carried out. The importance of using phononic crystals with circular equifrequency contours, as well as matching the size of the contours inside and outside the crystal, was established. Both conditions were satisfied by a flat phononic crystal of steel rods, in which the liquid inside the crystal (methanol) was different from the outside medium (water). The possibility of achieving subwavelength resolution using this phononic crystal was investigated with a subwavelength line source (a miniature strip-shaped transducer, approximately lambda/5 wide, where lambda is sound wavelength in water). A resolution of 0.55lambda was found, which is just above the diffraction limit lambda/2.

We are reporting on a study of the near-field sensitivity and resolution of a metal-dielectric probe (MDP). The propagation of the electromagnetic field across the probe was studied experimentally by means of time-domain terahertz spectroscopy and numerically simulated by CST MicroWave Studio 2008. Several localised areas at the probe end facet were distinguished and showed to be sensitive to the local dielectric properties and local anisotropy of the sample. Contrast and sensitivity measurements were conducted in several configurations of a MDP; the results were confirmed by simulations. The acquired data were analysed by using singular value decomposition that enabled separating independent physical phenomena in the measured datasets and filtering external disturbances out of the signal. Independent components corresponding to the changes in the output terahertz pulse upon varying the probe-sample distance and reflecting the local anisotropy in a ferroelectric barium titanate (BaTiO3) crystal were extracted and identified. The domain structure with characteristic dimensions of about 5 um was resolved during imaging experiments on the ferroelectric BaTiO3 sample, i.e. the resolved structures were ten times smaller than the characteristic dimensions of the end facet of the probe and forty times smaller than...