Effect of a current blocking barrier on a 2–6 μm p-GaAs/AlGaAs heterojunction infrared detector

We report the performance of a 30 period p-GaAs/AlxGa1 − xAs heterojunction photovoltaic infrared detector, with graded barriers, operating in the 2–6 μm wavelength range. Implementation of a current blocking barrier increases the specific detectivity (D*) under dark conditions by two orders of magnitude to ∼1.9 × 1011 Jones at 2.7 μm, at 77 K. Furthermore, at zero bias, the resistance-area product (R 0 A) attains a value of ∼7.2 × 108 Ω cm2, a five orders enhancement due to the current blocking barrier, with the responsivity reduced by only a factor of ∼1.5

Mid-infrared detection in p-GaAs/AlGaAs heterostructures with a current blocking barrier

For the infrared detection in the 3-5 μm range, p-GaAs/AlxGa1-xAs heterojunction is an attractive material system due to light hole/heavy hole and spin-orbit split-off intra-valance band transitions in this wavelength range. Varying the Al mole fraction (x) provides the tuning for the wavelength threshold, while graded AlxGa1-xAs potential barriers create an asymmetry to allow a photovoltaic operation. The photovoltaic mode of operation offers the advantage of thermal noise limited performance. In our preliminary work, a 2 - 6 μm photovoltaic detector was studied. Implementation of an additional current blocking barrier improved the specific detectivity (D∗) by two orders of magnitude, to 1.9×1011 Jones at 2.7 μm, at 77K. At zero bias, the resistance-area product (R0A) had a value of ∼ 7.2×108 Ω cm2, which is five orders higher in magnitude (with a corresponding reduction of the responsivity by only a factor of ∼ 1.5), compared to the R0A value without the blocking barrier. A photoresponse was observed up to 130K

Charge Induced Vortex Lattice Instability

It has been predicted that superconducting vortices should be electrically charged and that this effect is particularly enhanced for, high temperature superconductors.\cite{kho95,bla96} Hall effect\cite{hag91} and nuclear magnetic resonance (NMR) experiments\cite{kum01} suggest the existence of vortex charging, but the effects are small and the interpretation controversial. Here we show that the Abrikosov vortex lattice, characteristic of the mixed state of superconductors, will become unstable at sufficiently high magnetic field if there is charge trapped on the vortex core. Our NMR measurements of the magnetic fields generated by vortices in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+y}$ single crystals\cite{che07} provide evidence for an electrostatically driven vortex lattice reconstruction with the magnitude of charge on each vortex pancake of $\mathbf{\sim 2}$x$\mathbf{10^{-3} e}$, depending on doping, in line with theoretical estimates.\cite{kho95,kna05}Comment: to appear in Nature Physics; 6 pages, 7 figure

Room-Temperature operation of a quantum well mid-infrared detector embedded in nano-antennae array at critical optical coupling

We present the first room temperature photodection of hundreds on nanowatts using a quantum well mid-infrared detector at 9μm, with a background-limited temperature of 82K and a corresponding background-limited specific detectivity of 1.4×1010 cmHz1/2/W. The photonic architecture consists of an array of double metal nano-antennae and allows to reduce the dark current and increase the absorbed electromagnetic field inside the active region, so to prove a high temperature photoresponse

Extended wavelength infrared photodetectors

Extension of the wavelength threshold of an infrared detector beyond λt=hc/Δ is demonstrated, without reducing the minimum energy gap (Δ) of the material. Specifically, a photodetector designed with Δ=0.40  eV, and a corresponding λt=3.1  μm, was shown to have an extended threshold of ∼45  μm at 5.3 K, at zero bias. Under negative and positive applied bias, this range was further extended to ∼60 and ∼68  μm, respectively, with the photoresponse becoming stronger at increased biases, but the spectral threshold remained relatively constant. The observed wavelength extension arises from an offset between the two potential barriers in the device. Without the offset, another detector with Δ=0.30  eV showed a photoresponse with the expected wavelength threshold of ∼4  μm

Indolin-2-one compounds targeting thioredoxin reductase as potential anticancer drug leads

Several compounds bearing the indolinone chemical scaffold are known to possess anticancer properties. For example, the tyrosine kinase inhibitor sunitinib is an arylideneindolin-2-one compound. The chemical versatility associated with structural modifications of indolinone compounds underlies the potential to discover additional derivatives possessing anticancer properties. Previously synthesized 3-(2-oxoethylidene)indolin-2-one compounds, also known as supercinnamaldehyde (SCA) compounds in reference to the parent compound 1 [1-methyl-3(2-oxopropylidene)indolin-2-one], bear a nitrogen-linked α,β-unsaturated carbonyl (Michael acceptor) moiety. Here we found that analogs bearing N-substituents, in particular compound 4 and 5 carrying an N-butyl and N-benzyl substituent, respectively, were strongly cytotoxic towards human HCT 116 colorectal and MCF-7 breast carcinoma cells. These compounds also displayed strong thioredoxin reductase (TrxR) inhibitory activity that was likely attributed to the electrophilicity of the Michael acceptor moiety. Their selectivity towards cellular TrxR inhibition over related antioxidant enzymes glutathione reductase (GR), thioredoxin (Trx) and glutathione peroxidase (GPx) was mediated through targeting of the selenocysteine (Sec) residue in the highly accessible C-terminal active site of TrxR. TrxR inhibition mediated by indolin-2-one compounds led to cellular Trx oxidation, increased oxidative stress and activation of apoptosis signal-regulating kinase 1 (ASK1). These events also led to activation of p38 and JNK mitogen-activated protein kinase (MAPK) signaling pathways, and cell death with apoptotic features of PARP cleavage and caspase 3 activation. In conclusion, these results suggest that indolin-2-one-based compounds specifically targeting TrxR may serve as novel drug leads for anticancer therapy

Discrete Vernier tuning in terahertz quantum cascade lasers using coupled cavities

Terahertz-frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of coherent radiation in the 1–5 THz region of the electromagnetic spectrum . The emission spectra of THz QCLs typically exhibit multiple longitudinal modes characteristic of Fabry–Pérot (FP) cavities. However, widely-tunable (single-mode) THz QCLs would be ideally suited to many THz-sensing applications, such as trace gas detection, atmospheric observations , and security screening . Here we demonstrate discrete Vernier tuning using a simple two-section coupled-cavity geometry. A monolithic THz QCL ridge cavity was etched using focused ion beam milling to create two coupled FP cavities separated by an air gap. In this scheme, one of the two sections (the ‘lasing section’) is electrically driven above the lasing threshold, while the other is driven below threshold and acts as a ‘tuning section’. The lengths of the two sections and the air gap were designed such that the longitudinal FP modes of the respective sections coincide at a selected (‘resonant’) frequency. The dominant lasing mode of the coupled cavity occurs at this frequency owing to the reduction in threshold . A small perturbation to the frequency of the modes in either section of the device will detune the resonance, causing the dominant mode of the coupled-cavity to ‘hop’ to a different frequency, in a manner analogous to the Vernier effect. The longitudinal modes of the tuning section are controlled by perturbing its refractive index through current-induced heating

Antenna-Coupled Microcavity Enhanced THz Photodetectors

Plasmonic THz photodetectors have been realized in this work, by implementing the active region of a 5 THz quantum well detector with an antenna-coupled microcavity array. Our results demonstrate a clear improvement in responsivity, polarization insensitivity and background limited performance