Ultrafast creation and annihilation of space-charge domains in a semiconductor superlattice observed by use of Terahertz fields

We report an experimental study indicating ultrafast creation and annihilation of space-charge domains in a semiconductor superlattice under the action of a THz field. Our experiment was performed for an InGaAs/InAlAs superlattice with the conduction electrons undergoing miniband transport. We applied to a superlattice a dc bias that was slightly smaller than a critical bias necessary for the formation of space-charge domains caused by a static negative differential conductivity. Additionally subjecting the superlattice to a strong THz field, resulted in a dc transport governed by the formation of domains if the frequency of the field was smaller than an upper frequency limit (~3 THz). From this frequency limit for the creation and annihilation of domains we determined the characteristic time of the domain buildup. Our analysis shows that the buildup time of domains in a wide miniband and heavily doped superlattice is limited by the relaxation time due to scattering of the miniband electrons at polar optic phonons. Our results are of importance for both an understanding of ultrafast dynamics of pattern formation in nanostructures and the development of THz electronic devices.Received: 25 March 2004, Published online: 23 July 2004PACS: 72.20.Ht High-field and nonlinear effects - 72.30. + q High-frequency effects; plasma effects - 73.21.Cd SuperlatticesK.N. Alekseev: Permanent address: Department of Physical Sciences, P.O. Box 3000, University of Oulu FIN-90014, Finland.     

Atom-scale ptychographic electron diffractive imaging of boron nitride cones

Ptychographic coherent diffractive imaging (CDI) has been extensively applied using both x rays and electrons. The extension to atomic resolution has been elusive. This Letter demonstrates ptychographic electron diffractive imaging at atomic resolution, permitting identification of structure in a boron nitride helical cone at a resolution of order 1 ?, beyond that of comparative Z-contrast images. A scanning transmission electron microscope is used to create a diverging illumination in a defocused Fresnel CDI geometry, providing a robust strategy leading to a unique solution.     

Cytotoxicity and inflammation in human alveolar epithelial cells following exposure to occupational levels of gold and silver nanoparticles

This study explores the viability of rare earth-doped zirconia nanophosphors as probable candidates for white light emission. Undoped ZrO₂ and single- and double-doped ZrO₂:M (where M?=?Tb³⁺ and Eu³⁺) nanophosphors have been synthesized using a simple sonochemical process. The products were characterized using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDS), and photoluminescence spectrophotometry. The SEM micrographs show that resultant nanoparticles have dendritic shape. TEM and HRTEM studies showed that the size of the majority of the nanoparticles were around 28?±?5?nm. Characteristic blue and green emission from Tb³⁺ ions and red from Eu³⁺ dopant ions were observed. The CIE coordinates of the double-doped ZrO₂:Tb³⁺ (1.2?%):Eu³⁺ (0.8?%) nanophosphor lie in the white light region of the chromaticity diagram and show promise as good phosphor materials for new lighting devices.     

Integrated cavity output spectroscopy measurements of NO levels in breath with a pulsed room-temperature QCL

A cavity-enhanced spectrometer is developed for detection of exhaled nitric oxide in human breath. A thermoelectrically cooled, pulsed, quantum cascade laser, coupled to a high-finesse cavity, is used for trace-gas measurements. The trace-gas analyzer operates at 5.2 microns and utilizes integrated cavity output spectroscopy. Effective optical path lengths of 1.5 km are achieved in a 50-cm-length cell with a sample volume of 60 mL. The instrument is also capable of simultaneously measuringCO2 concentration in exhaled breath. Measurements were performed on human breath samples as well as simulated breath samples. Here we report a detection limit of ≤ 1 ppbv in 4 s for NO in human breath samples.     

Numerical simulation of partially coherent broadband optical imaging using the finite-difference time-domain method

Rigorous numerical modeling of optical systems has attracted interest in diverse research areas ranging from biophotonics to photolithography. We report the full-vector electromagnetic numerical simulation of a broadband optical imaging system with partially coherent and unpolarized illumination. The scattering of light from the sample is calculated using the finite-difference time-domain (FDTD) numerical method. Geometrical optics principles are applied to the scattered light to obtain the intensity distribution at the image plane. Multilayered object spaces are also supported by our algorithm. For the first time, numerical FDTD calculations are directly compared to and shown to agree well with broadband experimental microscopy results.     

Measurement of the spatial backscattering impulse-response at short length scales with polarized enhanced backscattering

In this Letter, we describe an easy to implement technique to measure the spatial backscattering impulse-response at length scales shorter than a transport mean free path with resolution of better than 10 μm using the enhanced backscattering phenomenon. This technique enables spectroscopic measurements throughout the visible range and sensitivity to all polarization channels. Through a combination of Monte Carlo simulations and experimental measurements of latex microspheres, we explore the various sensitivities of our technique to both intrinsic sample properties and extrinsic instrumental properties. We conclude by demonstrating the extraordinary sensitivity of our technique to the shape of the scattering phase function, including higher order shape parameters than the anisotropy factor (or first moment).