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Black phosphorus (BP) offers considerable promise for infrared and visible photonics. Efficient tuning of the bandgap and higher subbands in BP by modulation of the Fermi level or application of vertical electric fields has been previously demonstrated, allowing electrical control of its above-bandgap optical properties. Here, we report modulation of the optical conductivity below the bandgap (5 to 15 μm) by tuning the charge density in a two-dimensional electron gas induced in BP, thereby modifying its free carrier-dominated intraband response. With a moderate doping density of 7 × 10 12 cm -2 , we were able to observe a polarization-dependent epsilon-near-zero behavior in the dielectric permittivity of BP. The intraband polarization sensitivity is intimately linked to the difference in effective fermionic masses along the two crystallographic directions, as confirmed by our measurements. Our results suggest the potential of multilayer BP to allow new optical functions for emerging photonics applications., (Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

The incorporation of electrically tunable materials into photonic structures such as waveguides and metasurfaces enables dynamic, electrical control of light propagation at the nanoscale. Few-layer black phosphorus is a promising material for these applications due to its in-plane anisotropic, quantum well band structure, with a direct band gap that can be tuned from 0.3 to 2 eV with a number of layers and subbands that manifest as additional optical transitions across a wide range of energies. In this Letter, we report an experimental investigation of three different, anisotropic electro-optic mechanisms that allow electrical control of the complex refractive index in few-layer black phosphorus from the mid-infrared to the visible: Pauli-blocking of intersubband optical transitions (the Burstein-Moss effect); the quantum-confined Stark effect; and the modification of quantum well selection rules by a symmetry-breaking, applied electric field. These effects generate near-unity tuning of the BP oscillator strength for some material thicknesses and photon energies, along a single in-plane crystal axis, transforming absorption from highly anisotropic to nearly isotropic. Lastly, the anisotropy of these electro-optical phenomena results in dynamic control of linear dichroism and birefringence, a promising concept for active control of the complex polarization state of light, or propagation direction of surface waves.

Graphene's unparalleled strength, stiffness, and low mass per unit area make it an ideal material for nanomechanical resonators, but its relatively low quality factor is an important drawback that has been difficult to overcome. Here, we use a simple procedure to fabricate circular mechanical resonators of various diameters from graphene grown by chemical vapor deposition. In addition to highly reproducible resonance frequencies and mode shapes, we observe a striking improvement of the membrane quality factor with increasing size. At room temperature, we observe quality factors as high as 2400 ± 300 for a resonator 22.5 μm in diameter, about an order of magnitude greater than previously observed quality factors for monolayer graphene. Measurements of quality factor as a function of modal frequency reveal little dependence of Q on frequency. These measurements shed light on the mechanisms behind dissipation in monolayer graphene resonators and demonstrate that the quality factor of graphene resonators relative to their thickness is among the highest of any mechanical resonator demonstrated to date.

The properties of polycrystalline materials are often dominated by the size of their grains and by the atomic structure of their grain boundaries. These effects should be especially pronounced in two-dimensional materials, where even a line defect can divide and disrupt a crystal. These issues take on practical significance in graphene, which is a hexagonal, two-dimensional crystal of carbon atoms. Single-atom-thick graphene sheets can now be produced by chemical vapour deposition on scales of up to metres, making their polycrystallinity almost unavoidable. Theoretically, graphene grain boundaries are predicted to have distinct electronic, magnetic, chemical and mechanical properties that strongly depend on their atomic arrangement. Yet because of the five-order-of-magnitude size difference between grains and the atoms at grain boundaries, few experiments have fully explored the graphene grain structure. Here we use a combination of old and new transmission electron microscopy techniques to bridge these length scales. Using atomic-resolution imaging, we determine the location and identity of every atom at a grain boundary and find that different grains stitch together predominantly through pentagon-heptagon pairs. Rather than individually imaging the several billion atoms in each grain, we use diffraction-filtered imaging to rapidly map the location, orientation and shape of several hundred grains and boundaries, where only a handful have been previously reported. The resulting images reveal an unexpectedly small and intricate patchwork of grains connected by tilt boundaries. By correlating grain imaging with scanning probe and transport measurements, we show that these grain boundaries severely weaken the mechanical strength of graphene membranes but do not as drastically alter their electrical properties. These techniques open a new window for studies on the structure, properties and control of grains and grain boundaries in graphene and other two-dimensional materials.

We fabricated large arrays of suspended, single-layer graphene membrane resonators using chemical vapor deposition (CVD) growth followed by patterning and transfer. We measure the resonators using both optical and electrical actuation and detection techniques. We find that the resonators can be modeled as flat membranes under tension, and that clamping the membranes on all sides improves agreement with our model and reduces the variation in frequency between identical resonators. The resonance frequency is tunable with both electrostatic gate voltage and temperature, and quality factors improve dramatically with cooling, reaching values up to 9000 at 10 K. These measurements show that it is possible to produce large arrays of CVD-grown graphene resonators with reproducible properties and the same excellent electrical and mechanical properties previously reported for exfoliated graphene.

Objective: This paper describes the clinical and imaging characteristics of seminoma complicating intraabdominal testes in three patients who had a history of negative findings from surgical exploration of the inguinal canal and scrotum., Conclusion: Because inguinal surgery for cryptorchidism may fail to disclose intraabdominal testes, surgery that has negative findings should be followed up by a close examination of the abdomen. Seminomas arising from an intraabdominal testis can be large and asymptomatic or may simulate other diseases. Torsion may occur and occasionally appear as acute abdomen. The imaging features can be nonspecific, and the history of cryptorchidism may not be provided. Awareness of the clinical and imaging features of neoplasms involving undescended intraabdominal testes is important for appropriate diagnosis and therapy.

Purpose: To compare superior mesenteric artery (SMA) blood flow in healthy volunteers and patients with stenoses in the fasting state and after food intake by using phase-contrast (PC) cine magnetic resonance (MR) imaging., Materials and Methods: Ten healthy subjects, four asymptomatic patients (three with 50% stenosis, one with 70% stenosis), and one symptomatic patient (with 80% stenosis) were studied. All subjects were studied after fasting at least 8 hours and 15, 30, and 45 minutes after ingesting a standard meal., Results: In healthy volunteers, SMA blood flow at all postprandial intervals increased significantly compared with that obtained after fasting (P < or = .0005). The percentage change in SMA blood flow 30 minutes after food intake provided the best distinction between the healthy subjects, the asymptomatic patients, and the symptomatic patient., Conclusion: Cine PC MR imaging is an effective, noninvasive technique for measuring SMA blood flow.

Purpose: To compare the enhancement patterns of focal liver lesions at dynamic breath-hold gadolinium-enhanced multiplanar spoiled gradient-recalled (SPGR) magnetic resonance (MR) imaging with T2 relaxation times in the differentiation of liver hemangiomas from malignancies., Materials and Methods: Forty-seven patients with focal liver lesions underwent MR imaging with spin-echo and gadolinium-enhanced multiplanar SPGR techniques. T2 relaxation times and enhancement patterns were compared for accuracy in liver lesion characterization., Results: Enhancement patterns allowed better characterization of liver lesions than did T2 relaxation times. Only specific patterns of contrast enhancement indicated a hemangioma. Although 18 of the 33 malignancies and 10 of the 12 hemangiomas showed progressive centripetal hyperintense enhancement, only hemangiomas filled in with hyperintense peripheral nodules. Malignancies often filled in with hyperintense thick rinds., Conclusion: Gadolinium-enhanced multiplanar SPGR imaging allows more accurate characterization of liver lesions than does T2 relaxation time.