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A prototype stilbene beta cell has been designed, manufactured, tested, and compared to a plastic scintillator beta cell used for nuclear explosion monitoring of radioxenon. Measurements of the four isotopes of interest show similar responses for both the plastic scintillator and stilbene cell. However, we show that the stilbene cell exhibits 100-times smaller memory effect than the plastic cell, a capability that is of interest in nuclear explosion monitoring because it improves detection sensitivity, allowing for decreased time between measurements. The resolution of the stilbene cell is improved slightly (decreased by 2.2 keV at 129 keV) when compared to the plastic scintillator cell. The efficiency of the stilbene cell is lower than that of the plastic by an average 15%. These results suggest that the light-collection efficiency of the stilbene cell could be improved in future designs. Pulse shape discrimination analysis decreases the minimum detectable concentration by 1%. The use of this geometry allows for a simple replacement of the plastic cells currently used in the field versus a complete overhaul of the existing radioxenon monitoring stations.

We describe the design and assembly of the LUX-ZEPLIN experiment, a direct detection search for cosmic WIMP dark matter particles. The centerpiece of the experiment is a large liquid xenon time projection chamber sensitive to low energy nuclear recoils. Rejection of backgrounds is enhanced by a Xe skin veto detector and by a liquid scintillator Outer Detector loaded with gadolinium for efficient neutron capture and tagging. LZ is located in the Davis Cavern at the 4850' level of the Sanford Underground Research Facility in Lead, South Dakota, USA. We describe the major subsystems of the experiment and its key design features and requirements. info:eu-repo/semantics/publishedVersion

Quasi-phase-matched GaAs layers up to 600 μm thick have been produced by low-pressure hydride vapor phase epitaxy (HVPE) regrowth on templates fabricated using wafer fusion bonding. We have used this combination of techniques to demonstrate an alternative approach for commercial development of orientation-patterned GaAs (OP-GaAs) for nonlinear optical frequency conversion applications. We report on the characterization of this material and present a comparison with OP-GaAs grown using the conventional all-epitaxial templates. The primary advantages of the wafer fusion method are the avoidance of the molecular beam epitaxy growth of GaAs/Ge/GaAs and the ability to use substrates without misorientation. We have observed that some of the misorientation-related growth defects which affect conventional OP-GaAs growth are not as problematic for regrowth on wafer fusion bonded templates. The overall quality of antiphase domain propagation was similar to conventional OP-GaAs layers. A disadvantage was that growth rates were approximately half the values measured for regrowth on misoriented templates.

Orientation-patterned GaAs (OP-GaAs) has shown promise as an efficient frequency-shifted laser source over the range of 2–12 μm. In order to make OP-GaAs a viable source, efficiency and output power must be significantly increased, which requires minimizing major sources of loss. Low pressure HVPE has been adopted as the most suitable technique for regrowth of thick high quality GaAs layers on OP templates. We have explored process parameters in bulk and OP material to identify and control the sources of point defects, a key contributor to optical losses. Growth on OP templates with periodic [001] and [00−1] domains results in domain specific surface orientation, which should have inhomogeneous defect incorporation. Hall measurements, SIMS depth profiling, and cathodoluminescence (CL) were used to identify point defects in bulk and OP-GaAs. It was found that Si impurities are the primary source of donors, while V Ga were identified as the primary source of acceptors. In order to study the incorporation of impurities in OP-GaAs samples, we intentionally doped samples with Si to increase CL and SIMS detectability. Spatially resolved CL and SIMS revealed regions with significant differences in the defect concentration, which can affect device output.

Evidence of Si complexes was discovered in low temperature photoluminescence (PL) spectra recorded from GaAs grown by hydride vapor phase epitaxy and were measured as a function of secondary HCl flow. In addition, time resolved PL of the samples measured long radiative lifetimes, substantiating the excellent quality of the crystalline growth.

Nonlinear optical frequency conversion is an effective technique for generating infrared (IR) and terahertz (THz) wavelengths not readily available from existing laser sources. Birefringent materials such as LiNbO3 are often used to generate wavelengths where gaps exist, but are unsuitable in the mid-IR, far-IR, and THz regions as these materials are often opaque in these regions. As an alternative, GaAs has been employed for frequency conversion in these regions using quasi-phase-matching (QPM) to overcome the material’s lack of birefringence. QPM has been successfully demonstrated in GaAs using fused stacks of thin alternately oriented layers or inverted orientation patterned (OP) grating templates overgrown with thick columnar GaAs layers. Although GaAs has a high nonlinear coefficient d14 = 170 pm/V at 1.064 μm and good thermal conductivity (52 W/m K), it suffers from strong two-photon absorption below 1.7 μm making it inefficient when pumped with a source less than or equal to this wavelength. GaP also has a high nonlinear coefficient d14 = 71 pm/V at 1.064 μm, better thermal conductivity (110 W/m K) and much lower two-photon absorption in the 1 μm region. Therefore, OPGaP is desirable for NLO applications in the mid-IR and THz that use commercially available pump lasers in the 1.06–1.55 μm wavelength range. In this work the fabrication of OPGaP templates suitable for thick columnar hydride vapor phase epitaxial growth of GaP is reported using a commercially viable wafer fusion technique.

Growth of bulk gallium nitride by the ammonothermal method results in thick, faceted specimens presenting polar and non-polar surfaces. In order to investigate defect density, samples need to be cut, polished, and etched. The polishing method must remove surface damage or etch tests will be inaccurate. We examined the efficacy of different polishing and etching methods on the c, a, and m planes. Polishing methods included fixed-grit and chemo-mechanical solutions; results were measured by atomic force microscopy (AFM). Three etchants were tried: 85% hot phosphoric acid, hot KOH-NaOH eutectic, and photo-enhanced chemical (PEC) etching. The eutectic etch resulted in repeatable counts on the gallium face. The non-polar planes did not reveal etch pits (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High quality GaN crystals have been successfully grown by the ammonothermal method in alkaline ammonia solutions using hydride vapor phase epitaxy (HVPE) seeds. The grown crystals, over 1 mm thick, are clear and possess excellent structural and optical properties. The crystalline structure of the as-grown bulk GaN is as good as, or better than the HVPE seeds as measured by high resolution X-ray rocking curves with 100 arcsec of full width at half maximum (FWHM) on (0 0 2) and 90 arcsec on (1 0 2) diffractions. The crystal quality is improved through a process of careful seed selection and controlled heating during nucleation, so that the ammonothermal growth replicates the seed crystals on both the nitrogen and gallium faces. The results are confirmed by low temperature photoluminescence spectra resolving donor-bound and free excitons as well as multiple phonon replicas, and further by room temperature cathodoluminescence indicating reduced yellow-band emission. Successful growth of high quality GaN crystals on HVPE seeds will facilitate the scale-up to large area growth by use of large area GaN HVPE templates as seeds.

The activation of pro-inflammatory gene programs by nuclear factor-kappaB (NF-kappaB) is primarily regulated through cytoplasmic sequestration of NF-kappaB by the inhibitor of kappaB (IkappaB) family of proteins. IkappaBbeta, a major isoform of IkappaB, can sequester NF-kappaB in the cytoplasm, although its biological role remains unclear. Although cells lacking IkappaBbeta have been reported, in vivo studies have been limited and suggested redundancy between IkappaBalpha and IkappaBbeta. Like IkappaBalpha, IkappaBbeta is also inducibly degraded; however, upon stimulation by lipopolysaccharide (LPS), it is degraded slowly and re-synthesized as a hypophosphorylated form that can be detected in the nucleus. The crystal structure of IkappaBbeta bound to p65 suggested this complex might bind DNA. In vitro, hypophosphorylated IkappaBbeta can bind DNA with p65 and c-Rel, and the DNA-bound NF-kappaB:IkappaBbeta complexes are resistant to IkappaBalpha, suggesting hypophosphorylated, nuclear IkappaBbeta may prolong the expression of certain genes. Here we report that in vivo IkappaBbeta serves both to inhibit and facilitate the inflammatory response. IkappaBbeta degradation releases NF-kappaB dimers which upregulate pro-inflammatory target genes such as tumour necrosis factor-alpha (TNF-alpha). Surprisingly, absence of IkappaBbeta results in a dramatic reduction of TNF-alpha in response to LPS even though activation of NF-kappaB is normal. The inhibition of TNF-alpha messenger RNA (mRNA) expression correlates with the absence of nuclear, hypophosphorylated-IkappaBbeta bound to p65:c-Rel heterodimers at a specific kappaB site on the TNF-alpha promoter. Therefore IkappaBbeta acts through p65:c-Rel dimers to maintain prolonged expression of TNF-alpha. As a result, IkappaBbeta(-/-) mice are resistant to LPS-induced septic shock and collagen-induced arthritis. Blocking IkappaBbeta might be a promising new strategy for selectively inhibiting the chronic phase of TNF-alpha production during the inflammatory response.

This paper describes advances in the development of quasi-phase-matched (QPM) gallium phosphide (GaP) crystals for agile laser sources in the mid-infrared regions between 3–5 and 8–12 μm. In the quest for a nonlinear optical material with the potential to efficiently convert near infrared energy (wavelength ∼1 μm) to a powerful mid-infrared source, we have investigated the growth of GaP by hydride vapor phase epitaxy (HVPE). The process is shown to produce high quality thick layers at rapid growth rates in a low-pressure horizontal reactor. This process was used to grow thick layers on orientation-patterned (OP) templates. The OP-GaP templates were fabricated by lithographic patterning of inverted wafer-fused GaP. HVPE growth on both OP-GaP and OP-GaAs templates was performed, showing that HVPE can successfully replicate the initial template pattern. However, for longer growth duration at given conditions the patterned structure can be lost, with annihilation of every other domain.

Bulk crystals of gallium nitride have been grown using the ammonothermal method. Seeded growth resulted in thick, faceted GaN specimens. The crystal habit was identified and consisted of a hexagonal prism with pyramidal facets. Fast growth rates were observed on the nitrogen face and in the a-direction. The surface morphology and etching characteristics varied by growth sector. These differences have been correlated with characterization by X-ray diffraction, photoluminescence, and secondary ion mass spectroscopy. The material grown on the nitrogen face of the seed was of higher quality than the gallium face growth. The most promising material was grown laterally off the seed in the a-direction.

Orientation-patterned (OP) GaAs crystals are very promising for their use in nonlinear optical applications. In particular, mid-infrared and terahertz lasers can be generated by frequency conversion from shorter-wavelength sources. However, the quality of the crystals is crucial for high conversion efficiency, as the presence of defects with electrooptical signatures can contribute to optical losses. The study of these defects is a step toward the improvement of OP-GaAs crystals. We present here a spectroscopic cathodoluminescence study of the distribution of the main defects. Tentative relations between defects and the optical propagation losses are discussed.

Orientation-patterned GaAs crystals fabricated by periodically reversing the crystal orientation of the GaAs domains have been studied by cathodoluminescence. The main properties concerning the differences between the two domain orientations, 〈 001 〉 / 〈 00 1 〉 , and the walls between the domains have been studied. The CL study reveals that the domain walls are decorated with defects, and the antibonds, As–As and Ga–Ga, are partially inhibited by other point defects. The local strain around the domain walls was mapped, showing non-uniform distribution, probably related to the distribution of point defects around the domain walls.

Low-pressure hydride vapor phase epitaxy (HVPE) is being used for the regrowth of thick GaAs on orientation-patterned templates for nonlinear optical frequency conversion. We have achieved epitaxial growth rates of 200 μm/h and produced millimeter-thick films in 10-h-long growth runs. A critical problem in the HVPE regrowth of orientation-patterned GaAs is the preservation of the original patterned structure—during thick growth, the domain walls often bend and annihilate. Measurements indicated that the domain wall bending decreased as the growth temperature was reduced. The substrate miscut and the orientation of unpatterned regions also affect the vertical propagation of domain walls. In this paper we discuss the process improvements which have facilitated the production of millimeter-thick layers with nearly vertical domain walls.

T cells enigmatically require caspase-8, an inducer of apoptosis, for antigen-driven expansion and effective antiviral responses, and yet the pathways responsible for this effect have been elusive. A defect in caspase-8 expression does not affect progression through the cell cycle but causes an abnormally high rate of cell death that is distinct from apoptosis and does not involve a loss of NFκB activation. Instead, antigen or mitogen activated Casp8 -deficient T cells exhibit an alternative type of cell death similar to programmed necrosis that depends on receptor interacting protein (Ripk1). The selective genetic ablation of caspase-8, NFκB, and Ripk1, reveals two forms of cell death that can regulate virus-specific T cell expansion.

A method for growing thick epitaxial layers of GaAs on orientation-patterned wafers by low-pressure hydride vapor phase epitaxy is described. For nonlinear optical applications, it is necessary to grow very thick gratings to accommodate the pump laser as it enters through the side and propagates across the patterned layer. The low-pressure method allows for rapid growth at rates well above 100 μm/h on wafers with patterned stripes having 30–500 μm widths. Parasitic deposition of GaAs on the reactor walls upstream of the sample can reduce the growth rate significantly after a few hours. Various techniques are described for eliminating the parasitic growth. Layers over 500 μm thick have been successfully produced during 8 h of growth.

The relaxation of InGaAs/GaAs thin films grown by molecular beam epitaxy is studied by an in situ optical stress sensor technique. Profiles of normalized film strain versus thickness are obtained for several growth temperatures and two InGaAs compositions. The profiles are modeled using Dodson and Tsao's concept of effective stress, and a new kinetic model of dislocation sources associated with the surface roughness, which could be generated by film growth and have a transient response following a growth interrupt.

The NFκB family of dimeric transcription factors regulate inflammatory and immune responses. While the dynamic control of NFκB dimer activity via the IκB-NFκB signaling module is well understood, there is little information on how specific dimer repertoires are generated from Rel family polypeptides. Here we report the iterative construction – guided by in vitro and in vivo experimentation – of a mathematical model of the Rel-NFκB generation module. Our study reveals that IκBβ has essential functions within the Rel-NFκB generation module, specifically for the RelA:RelA homodimer, which controls a subset of NFκB target genes. Our findings revise the current dogma of the three classical, functionally-related IκB proteins by distinguishing between a positive ‘licensing’ factor (IκBβ) that contributes to determining the available NFκB dimer repertoire in a cell’s steady state, and negative feedback regulators (IκBα and -ε) that determine the duration and dynamics of the cellular response to an inflammatory stimulus.

We present results showing that the strain relaxation rate in heteroepitaxial layers depends strongly on the presence of the growth flux. These results are based on real-time stress measurements made during molecular beam epitaxy of InxGa1−xAs/GaAs (x=0.16±0.01) films grown at 452, 477, and 502 °C. Our measurements indicate that the increased relaxation rate during growth is due to an enhancement in the dislocation glide velocity. We propose a model for the dislocation velocity increase in which the growth-induced supersaturation of adatoms lowers the barrier to single kink nucleation.

Orientation patterned (OP)-GaAs crystals are attractive materiasl for mid-infrared and terahertz lasers sources, using non linear optics frequency conversion from shorter wavelength sources. The optical propagation losses are critical to the fabrication of these sources; among the causes of optical losses the generation of defects and the incorporation of impurities must play a relevant role. The control of the incorporation of impurities and defects is, therefore, crucial to improve the performance of the OP-GaAs crystals as non linear optical materials. We present herein a cathodoluminescence (CL) analysis of OP-GaAS crystals intentionally doped with Si, in order to understand the incorporation paths of Si in the OP-GaAs crystals.

Orientation patterned (OP)-GaAs crystals have high potential as non linear optical systems. Mid-infrared and terahertz lasers sources can be fabricated with these crystals by frequency conversion from shorter wavelength sources. The optical propagation losses are critical; therefore, the OP-GaAs crystals must have high quality with low incorporation of defects and high homogeneity to reduce the refractive index fluctuations. Defects with electro-optic signature must be characterized in order to reduce their presence. Cathodoluminescence studies of these crystals permit the distribution of the main defects to be established, both extended and point defects. Special attention is paid to the role of the walls between the two domain orientations, and to the incorporation of impurities in Si-doped samples.

The U.S. Air Force is developing orientation-patterned GaAs (OPGaAs) for nonlinear frequency conversion in the 2-5 μm and 8-12 μm regions. We report recent progress in OPO device performance which reflects continued improvement in material quality. Seven new OPGaAs samples, representing four distinct growth regimes, were evaluated in terms of threshold, slope efficiency, and output spectral content using a Q-switched Tm,Ho:YLF laser operating at 2 μm as the pump source. The samples were identical in base template, grating period, length, and AR coating, permitting direct comparison of results to identify favorable growth conditions. As anticipated, performance varied significantly among the sample set, with slope efficiencies from 12% to 35% and thresholds from 9 μJ to 40 μJ. Less anticipated was the low level of uniformity across each sample and between samples grown in the same growth run. Significant variations in slope efficiency and threshold were not uncommon. Still, most of the samples performed manifestly better than previously grown material, indicating an overall improvement in OPGaAs quality. Research continues on understanding growth processes, optical loss mechanisms, and how these translate into device performance.

Millimeter-thick crystals of orientation-patterned GaAs have been grown using low pressure Hydride Vapor Phase Epitaxy for use in the generation of mid-IR and THz radiation.

We report mW-average-power widely tunable (0.5-3.5 THz) monochromatic THz source based on frequency mixing in periodically-inverted GaAs, between the two closely spaced dasiasignalpsila and dasiaidlerpsila waves, inside the resonant cavity of an optical parametric oscillator.

TNF-induced NF-κB activity shows complex temporal regulation whose different phases lead to distinct gene expression programs. Combining experimental studies and mathematical modeling, we identify two temporal amplification steps—one determined by the obligate negative feedback regulator IκBα—that define the duration of the first phase of NF-κB activity. The second phase is defined by A20, whose inducible expression provides for a rheostat function by which other inflammatory stimuli can regulate TNF responses. Our results delineate the nonredundant functions implied by the knockout phenotypes ofiκbαanda20, and identify the latter as a signaling cross-talk mediator controlling inflammatory and developmental responses.

We developed an efficient frequency-tunable THz source using intracavity mixing between the two resonant waves of a synchronously pumped doubly resonant OPO. Three types of quasi-phase-matched GaAs were utilized: optically contacted, orientation-patterned, and diffusion-bonded GaAs.

We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium arsenide (GaAs). We used three types of micro-structured GaAs to quasi-phase-match the interaction: optically contacted, orientation-patterned, and diffusion-bonded GaAs. The room-temperature GaAs was placed in an optical parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO signal and idler waves. 250-GHz-bandwidth radiation was generated with frequencies spanning 0.4-3.5 THz. We measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant oscillator (DRO) configuration, the efficiency increased by 21 times over the singly resonant oscillator (SRO) performance with an optical-to-THz efficiency of 10 -4 and average THz power of 1 mW.

Orientation-patterned GaP is a promising nonlinear material for frequency conversion in the mid and longwave IR (2-5 µm and 8-12 µm) by quasi-phase matching. As an alternative to OPGaAs, OPGaP has the advantage of having lower two-photon absorption in the convenient pumping range 1 – 1.7 µm. We report recent results on development of thick QPM GaP for high power tunable laser sources radiating in the mid IR. Two are the major challenges to producing OPGaP: development of simple techniques for preparation of patterned templates and a technology for fast epitaxial growth of thick, high quality GaP on these templates. The focus was to adapt/simplify the wafer fusion process for OPGaP template preparation. Then increase the growth rate and layer thickness of regrown material, while maintaining vertical domain propagation. The growth experiments were conducted in a horizontal quartz reactor, using a standard hydride vapor phase epitaxial process. The growth was performed on: (i) plain (100) GaP; (ii) half-patterned (HP) and (iii) orientation-patterned (OP) templates, fabricated on (100) GaP. Up to 370 µm thick layers with high crystal and optical quality were reproducibly grown on plain material. Growth on HP templates resulted in up to 470 µm thick layers with rectangular mesa‟s shape. These results were used to determine the optimal substrate and pattern orientations. HVPE growth on OP templates achieved stable growth rates of 50-70 µm/h with domain walls propagating vertically, following the periodicity of the initial pattern, and resulted in the first 350 µm thick device quality OPGaP.

The fabrication of thick orientation-patterned GaAs (OP-GaAs) films is reported using a two-step process where an OP-GaAs template with the desired crystal domain pattern was prepared by wafer fusion bonding and then a thick film was grown over the template by low pressure hydride vapor phase epitaxy (HVPE). The OP template was fabricated using molecular beam epitaxy (MBE) followed by thermocompression wafer fusion, substrate removal, and lithographic patterning. On-axis (100) GaAs substrates were utilized for fabricating the template. An approximately 350 μm thick OP-GaAs film was grown on the template at an average rate of ~70 μm/hr by HVPE. The antiphase domain boundaries were observed to propagate vertically and with no defects visible by Nomarski microscopy in stain-etched cross sections. The optical loss at ~2 μm wavelength over an 8 mm long OP-GaAs grating was measured to be no more than that of the semi-insulating GaAs substrate. This template fabrication process can provide more flexibility in arranging the orientation of the crystal domains compared to the Ge growth process and is scalable to quasi-phase-matching (QPM) devices operating from the IR to terahertz frequencies utilizing existing industrial foundries.

Orientation-patterned GaAs (OPGaAs) shows great promise as a nonlinear optical material for frequency conversion in the 2-5 micron and 8-12 micron regions. We report recent progress in each of the three main areas of OPGaAs development: fabrication of patterned templates using a combination of wafer bonding and MBE techniques; thick-layer HVPE growth; and material and OPO device characterization. This work has led to significant improvements in material quality, specifically reduced optical loss, increased sample thickness, improved patterned domain fidelity, and greater material uniformity. Advances in material quality have in turn enabled demonstration of OPO devices operating in the 3-5 micron spectral region. Optical loss and OPO performance measurements on a series of OPGaAs samples are presented, with the goal of understanding how these properties are influenced by growth conditions, and how OPO performance may be improved. Research continues on understanding loss mechanisms, correlating performance with material properties, transitioning the technology into an industrial process, and extending it to additional materials.

We report mW-average-power widely tunable (0.5-3.5 THz) monochromatic THz source based on frequency mixing in periodically-inverted GaAs, between the two closely spaced 'signal' and 'idler' waves, inside the resonant cavity of a synchronously-pumped picosecond OPO.

We review nonlinear-optical applications of micro-structured quasi-phase matched GaAs. These include new sources of mid-IR coherent radiation, pump-polarization-insensitive devices, as well as sources of tunable terahertz radiation, based on optical rectification or intracavity frequency mixing.

We demonstrated an optical parametric oscillator based on GaAs with a circularly polarized pump. The threshold was lower and conversion efficiency was higher than with a [001]-linearly polarized pump, which is consistent with GaAs symmetry.

Zincblende semiconductors (GaAs, GaP) show great potential for quasi-phase-matched (QPM) THz generation because of their small (20 times less than in lithium niobate) absorption coefficient at terahertz frequencies, small mismatch between the optical group and THz phase velocities, high thermal conductivity, and decent electro-optical coefficient. Terahertz-wave generation was demonstrated recently in QPM GaAs, using optical rectification of femtosecond pulses. Here we report on a new efficient widely tunable (0.5-3.5 THz) source of THz radiation based on quasi-phase-matched GaAs crystal. The source is based on difference frequency generation inside the cavity of a synchronously pumped near-degenerate picosecond OPO and takes advantage of resonantly enhanced both the signal and the idler waves. THz average power as high as 1 mW was achieved in a compact setup.

Microstructured semiconductors, like orientation-patterned GaAs, achieve quasi-phasematching through controlled, periodic inversions of the crystallographic orientation. These nonlinear optical materials are becoming more mature and are being used to generate mid-infrared and THz radiation.

We demonstrate an optical parametric oscillator based on GaAs pumped with both a circularly polarized and a pseudo-depolarized pump. High symmetry in the nonlinear susceptibility tensor of GaAs enables pumping with “exotic” polarizations.

We generated 1 of mW average THz power using quasi-phasematched GaAs as a frequency mixer between an optical parametric oscillator’s (OPO) signal and idler waves. The output frequency was tunable from 0.65-3.4 THz.

We report a new highly efficient source of frequency-tunable (0.5-3.5 THz) narrow-bandwidth terahertz wave packets with up to 1 mW average power, based on parametric down-conversion in quasi-phase-matched GaAs. Different lasers were employed as a pump source, including femtosecond OPA/DFG system (wavelength range 2-4μm), Tm-fiber femtosecond laser (wavelength ~2μm), and near-degenerate synchronously-pumped picosecond OPO system with extra- and intracavity THz generation. We prove experimentally that the optical-to-terahertz conversion efficiency is fluence-dependent, with the scaling factor being the same for femtosecond (optical rectification) and picosecond (difference frequency generation) pump pulses, with optical-to-terahertz conversion efficiency on the order of 0.1% per μJ.

We demonstrate a new source of frequency-tunable THz wave packets based on parametric down-conversion process in orientation-patterned GaAs (OP-GaAs) that produces muW-level THz average powers at the repetition rate of 100 MHz. The OP-GaAs crystal is pumped by a compact all-fiber femtosecond laser operating at the wavelength of 2 mum. Such combination of fiber laser and OP-GaAs technologies promises a practical source of THz radiation which should be suitable for many applications including imaging and spectroscopy.

A recently proposed THz laser concept in homoepitaxially grown p-Ge with layered doping is reviewed. Prospects for realizing a similar design in Si or GaAs are considered.

Recent work has confirmed the accuracy of real-time measurements of the stress-thickness product collected using optical monitoring of the wafer curvature during MBE growth. These experiments yield plots of the strain relieved vs. film thickness, thus providing direct comparison with kinetic models of dislocation-mediated relaxation. The present contribution documents the experimental relaxation profile and its change with growth temperature in the range of approximately 400-500/spl deg/C for two values of In composition, x /spl sim/ 0.15 and x /spl sim/ 0.30.

The authors demonstrate an efficient room temperature source of terahertz radiation using femtosecond laser pulses as a pump and GaAs structures with periodically inverted crystalline orientation, such as diffusion-bonded stacked GaAs and epitaxially grown orientation-patterned GaAs, as a nonlinear optical medium. By changing the GaAs orientation-reversal period (504–1277μm), or the pump wavelength (2–4.4μm), we were able to generate narrow-bandwidth (∼100GHz) terahertz wave packets, tunable between 0.9 and 3THz, with the optical-to-terahertz photon conversion efficiency of 3.3%.

The kinetics of dislocation-mediated strain relaxation have been investigated during InxGa1−xAs∕GaAs molecular beam epitaxy growth. We use in situ curvature measurement to determine the film stress as a function of time during deposition. The relaxation behavior includes a temperature dependent initial relaxation stage which partially relieves the misfit strain, followed by a rapid strain relaxation which commences at a temperature independent thickness and proceeds at a comparable rate at all temperatures studied. We discuss these observations in terms of the mobile dislocation density evolution and the dependence of the strain relaxation rate on the driving force and temperature.

In situ stress measurements during molecular-beam epitaxy growth of InxGa1−xAs∕GaAs provide insight into the relaxation behavior of thin films grown on mismatched substrates. Strain relaxation in the materials studied occurs due to the formation and glide of dislocations. Measurements of additional relaxation during growth interruptions show that relaxation is kinetically limited during the early stages of growth. In thicker films, the residual strain is not significantly affected by growth conditions. We find that relaxation in thick layers does not proceed as far as predicted by models which consider the formation of dislocation arrays or dislocation blocking, and we discuss this discrepancy between theory and experiment. We discuss the effect of a nonuniform dislocation array configuration on the overall relaxation behavior. Results from dislocation dynamics simulations are presented which provide evidence that nonuniform dislocation array distributions limit the extent of plastic relaxation.

In situ stress monitoring has revealed that the strain relaxation rate of InxGa1−xAs∕GaAs thin films is significantly enhanced (over 20 times faster at 500 °C) by the presence of a growth flux. We have explained this effect with a model in which the energy for dislocation single kink nucleation is lowered in the presence of a supersaturation of adatoms. In this paper, we present additional measurements showing that the growth rate dependence of this enhancement agrees well with our model. We also address alternate explanations for these observations.

In situ real-time stress monitoring is used to study the kinetics of stress relaxation during molecular beam epitaxy growth of strained thin films of InxGa1−xAs/GaAs. We present measurements of the temperature-dependent relaxation behavior obtained at 443 and 469 °C. To study the relationship between the dislocation structure and the stress relaxation, samples were grown to different thicknesses for transmission electron microscopy (TEM) analysis. The combination of the information from the real-time stress monitoring and observations from TEM provides insight into the mechanisms of stress relaxation.

SummaryThe N-terminal nuclear export sequence (NES) of inhibitor of nuclear factor kappa B (NF-κB) alpha (IκBα) promotes NF-κB export from the cell nucleus to the cytoplasm, but the physiological role of this export regulation remains unknown. Here we report the derivation and analysis of genetically targeted mice harboring a germline mutation in IκBα NES. Mature B cells in the mutant mice displayed nuclear accumulation of inactive IκBα complexes containing a NF-κB family member, cRel, causing their spatial separation from the cytoplasmic IκB kinase. This resulted in severe reductions in constitutive and canonical NF-κB activities, synthesis of p100 and RelB NF-κB members, noncanonical NF-κB activity, NF-κB target gene induction, and proliferation and survival responses in B cells. Consequently, mice displayed defective B cell maturation, antibody production, and formation of secondary lymphoid organs and tissues. Thus, IκBα nuclear export is essential to maintain constitutive, canonical, and noncanonical NF-κB activation potentials in mature B cells in vivo.

Orientation patterned (OP)-GaAs crystals show promise for use in tunable coherent light sources in the infrared (IR) and terahertz (THz). These structures consist of an alternating array of [001]/[00-1] oriented domains grown by hydride vapor phase epitaxy (HVPE). Material characteristics concerning the propagation losses, the crystal dimensions, and the grating size must be taken into account to implement optical devices for specific wavelength ranges and operating modes (CW or pulsed). The analysis of the main factors contributing to optical loss in the OP crystals is a crucial step toward their use in many promising applications. We present a cathodoluminescence study, where the main defects and their distribution over the OP-GaAs crystals are revealed, with special emphasis paid to the properties of the domain walls.