Plasmon Excitations across the Charge-Density-Wave Transition in Single-Layer TiSe2.

1T-TiSe2 is believed to possess a soft electronic mode, i.e., plasmon or exciton, that might be responsible for the exciton condensation and charge-density-wave (CDW) transition. Here, we explore collective electronic excitations in single-layer 1T-TiSe2 by using the ab initio electromagnetic linear response and unveil intricate scattering pathways of the two-dimensional (2D) plasmon mode near the CDW phase. We found the dominant role of plasmon-phonon scattering, which in combination with the CDW gap excitations leads to the anomalous temperature dependence of the plasmon line width across the CDW transition. Below the transition temperature TCDW a strong hybridization between the 2D plasmon and CDW excitations is obtained. These optical features are highly tunable due to temperature-dependent CDW-related modifications of electronic structure and electron-phonon coupling and make CDW-bearing systems potentially interesting for applications in optoelectronics and low-loss plasmonics.

Spin Hall effect of transmitted light through α-Li3N-type topological semimetals.

The spin Hall effect of light occurring in topological semimetals provides unprecedented opportunities to exploit novel photonic properties and applications. In particular, pristine α-Li3N-type crystal has recently been predicted to be a type-I nodal-line semimetal based on density functional theory. Herein, the spin Hall effect of transmitted light through thin films of α-Li3N-type topological semimetals is investigated. We show that the prominent intense peak and dip emerging in the spectra of spin Hall shifts occur at the high-energy side of interband absorption of α-Li3N-type semimetals and show redshifts with increasing the incident angle or permittivity of the exit medium. In addition, type-I nodal-line semimetal under a compressive lattice strain is transformed into a type-II one such that the main intense peak and dip show blueshifts. Inversely, the tensile strain induces the formation of a triply degenerate nodal point in α-Li3N-type semimetals, causing the main intense peak and dip to show redshifts. Moreover, the influences of alloying and hole-doping in α-Li3N-type semimetals on the spin Hall effect of light are also discussed. Our findings may provide clear strategies to accurately engineer and detect the structural or phase change in topological materials.

First-principles insights into interaction of CO, NO, and HCN with Ag8.

We use static as well as time-dependent first-principles computations to study interaction of the CO, NO, and HCN molecules with the Ag8 nanocluster. The many-body based GW correction is applied for accurate description of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbital levels. It is argued that the adsorption of these molecules changes the stable structure of Ag8 from Td to the more chemically active D(2d) symmetry. We discuss that the CO, NO, and HCN molecules prefer to adsorb on the atom of the cluster with significant contribution to both HOMO and LUMO, for the accomplishment of the required charge transfers in the systems. The charge back donation is found to leave an excess energy of about 110 meV on the NO molecular bond, evidencing potential application of silver clusters for NO reduction. It is argued that CO and specially NO exhibit strong physical interaction with the silver cluster and hence significantly modify the electronic and optical properties of the system, while HCN makes very week physical bonds with the cluster. The optical absorption spectra of the Ag8 cluster before and after molecule adsorption are computed and a nontrivial red shift is observed in the NO and HCN adsorbed clusters.

Experimental verification of pulse-probing technique for improving phase coherence grating lobe suppression.

Fabrication of high-frequency phased-array ultrasound transducers is challenging because of the small element- to-element pitch required to avoid large grating lobes appearing in the field-of-view. Phase coherence imaging (PCI) was recently proposed as a highly effective technique to suppress grating lobes in large-pitch arrays for synthetic aperture beamforming. Our previous work proposed and theoretically validated a technique called pulse probing for improving grating lobe suppression when transmit beamforming is used with PCI. The present work reports the experimental verification of the proposed technique, in which the data was collected using a high-frequency ultrasound system and the processing was done offline. The data was collected with a 50-MHz, 256-element, 1.26 λ-pitch linear array, for which only the central 64-elements were used as the full aperture while the beam was steered to various angles. By sending a defocused pulse, the PCI weighting factors could be calculated, and were subsequently applied to the conventional transmit-receive beamforming. The experimental two-way radiation patterns showed that the grating lobe level was suppressed approximately 40 dB using the proposed technique, consistent with the theory. The suppression of overlapping grating lobes in reconstructed phased array images from multiple wire-phantoms in a water bath and tissue phantoms further validated the effectiveness of the proposed technique. The application of pulse probing along with PCI should simplify the fabrication of large-pitch phased arrays at high frequencies.

Listening to the cochlea with high-frequency ultrasound.

We have developed a high-frequency pulsed-wave Doppler ultrasound probe as a promising minimally-invasive technique for measuring intracochlear mechanics without damaging the cochlea. Using a custom high-frequency ultrasound system, we have measured dynamic motion of intracochlear structures by recording the pulsed-wave Doppler signal resulting from the vibration of the basilar and round window membranes. A 45 MHz needle-mounted Doppler probe was fabricated and placed against the round window membranes of eight different fresh human temporal bones. Pulsed-wave ultrasonic Doppler measurements were performed on the basilar membrane and round window membrane during the application of pure tones to the external ear canal. Doppler vibrational information for acoustic input frequencies ranging from 100-2000 Hz was collected and normalized to the sound pressure in the ear canal. The middle ear resonance, located at approximately 1000 Hz, could be characterized from the membrane velocities, which agreed well with literature values. The maximum normalized mean velocity of the round window and the basilar membrane were 180 µm/s/Pa and 27 µm/s/Pa at 800 Hz. The mean phase difference between the membrane displacements and the applied ear canal sound pressure showed a flat response almost up to 500 Hz where it began to accumulate. This is the first study that reports the application of high frequency pulsed wave Doppler ultrasound for measuring the vibration of basilar membrane through the round window. Since it is not required to open or damage the cochlea, this technique might be applicable for investigating cochlear dynamics, in vivo.

A split-aperture transmit beamforming technique with phase coherence grating lobe suppression.

A small element-to-element pitch (~.5λ) is conventionally required for phased array ultrasound transducers to avoid large grating lobes. This constraint can introduce many fabrication difficulties, particularly in the development of highfrequency phased arrays at operating frequencies greater than 30 MHz. In this paper, a new transmit beamforming technique along with sign coherence factor (SCF) receive beamforming is proposed to suppress grating lobes in large-pitch phased-array transducers. It is based on splitting the transmit aperture (N elements) into N/K transmit elements and receive beamforming on all N elements to reduce the temporal length of the transmit grating lobe signal. Therefore, the use of synthetic aperture beamforming, which can introduce relative phase distortions between the echoes received over many transmit events, can be avoided. After each transmit-receive event, the received signals are weighted by the calculated SCF to suppress the grating lobes. After pulsing all sub-apertures, the RF signals are added to generate one line of the image. Simulated 2-way radiation patterns for different K values show that grating lobes can be suppressed significantly at different steering angles. Grating lobes can be suppressed by approximately 20 dB with K = 2 at steering angles greater than 25° and an element pitch greater than 0.75λ. A technique for determining the optimal transmit sub-apertures has been developed.

High-frequency ex vivo ultrasound imaging of the auditory system.

A 50MHz array-based imaging system was used to obtain high-resolution images of the ear and auditory system. This previously described custom built imaging system (Brown et al. 2004a, 2004b; Brown and Lockwood 2005) is capable of 50 microm axial resolution, and lateral resolution varying from 80 microm to 130 microm over a 5.12 mm scan depth. The imaging system is based on a 2mm diameter, seven-element equal-area annular array, and a digital beamformer that uses high-speed field programmable gate arrays (FPGAs). The images produced by this system have shown far superior depth of field compared with commercially available single-element systems. Ex vivo, three-dimensional (3-D) images were obtained of human cadaveric tissues including the ossicles (stapes, incus, malleus) and the tympanic membrane. In addition, two-dimensional (2-D) images were obtained of an intact cochlea by imaging through the round window membrane. The basilar membrane inside the cochlea could clearly be visualized. These images demonstrate that high-frequency ultrasound imaging of the middle and inner ear can provide valuable diagnostic information using minimally invasive techniques that could potentially be implemented in vivo.