Singular charge fluctuations at a magnetic quantum critical point.

Strange metal behavior is ubiquitous in correlated materials, ranging from cuprate superconductors to bilayer graphene, and may arise from physics beyond the quantum fluctuations of a Landau order parameter. In quantum-critical heavy-fermion antiferromagnets, such physics may be realized as critical Kondo entanglement of spin and charge and probed with optical conductivity. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy-grown thin films of YbRh2Si2, a model strange-metal compound. We observed frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery suggests that critical charge fluctuations play a central role in the strange metal behavior, elucidating one of the long-standing mysteries of correlated quantum matter.

4.3 µm quantum cascade detector in pixel configuration.

We present the design simulation and characterization of a quantum cascade detector operating at 4.3µm wavelength. Array integration and packaging processes were investigated. The device operates in the 4.3µm CO2 absorption region and consists of 64 pixels. The detector is designed fully compatible to standard processing and material growth methods for scalability to large pixel counts. The detector design is optimized for a high device resistance at elevated temperatures. A QCD simulation model was enhanced for resistance and responsivity optimization. The substrate illuminated pixels utilize a two dimensional Au diffraction grating to couple the light to the active region. A single pixel responsivity of 16mA/W at room temperature with a specific detectivity D* of 5⋅107 cmHz/W was measured.

Nucleation of Ga droplets on Si and SiOx surfaces.

We report on gallium droplet nucleation on silicon (100) substrates with and without the presence of the native oxide. The gallium deposition is carried out under ultra-high vacuum conditions at temperatures between 580 and 630 °C. The total droplet volume, obtained from a fit to the diameter-density relation, is used for sample analysis on clean silicon surfaces. Through a variation of the 2D equivalent Ga thickness, the droplet diameter was found to be between 250-1000 nm. Longer annealing times resulted in a decrease of the total droplet volume. Substrate temperatures of 630 °C and above led to Ga etching into the Si substrates and caused Si precipitation around the droplets. In contrast, we obtained an almost constant diameter distribution around 75 nm over a density range of more than two orders of magnitude in the presence of a native oxide layer. Furthermore, the droplet nucleation was found to correlate with the density of surface features on the 'epi-ready' wafer.

Quantum cascade detector utilizing the diagonal-transition scheme for high quality cavities.

A diagonal optically active transition in a quantum cascade detector is introduced as optimization parameter to obtain quality factor matching between a photodetector and a cavity. A more diagonal transition yields both higher extraction efficiency and lower noise, while the reduction of the absorption strength is compensated by the resonant cavity. The theoretical limits of such a scheme are obtained, and the impact of losses and cavity processing variations are evaluated. By optimizing the quantum design for a high quality cavity, a specific detectivity of 10(9) Jones can be calculated for λ = 8µm and T = 300K.

Resonant metamaterial detectors based on THz quantum-cascade structures.

We present the design, fabrication and characterisation of an intersubband detector employing a resonant metamaterial coupling structure. The semiconductor heterostructure relies on a conventional THz quantum-cascade laser design and is operated at zero bias for the detector operation. The same active region can be used to generate or detect light depending on the bias conditions and the vertical confinement. The metamaterial is processed directly into the top metal contact and is used to couple normal incidence radiation resonantly to the intersubband transitions. The device is capable of detecting light below and above the reststrahlenband of gallium-arsenide corresponding to the mid-infrared and THz spectral region.

THz quantum cascade lasers with wafer bonded active regions.

We demonstrate terahertz quantum-cascade lasers with a 30 µm thick double-metal waveguide, which are fabricated by stacking two 15 µm thick active regions using a wafer bonding process. By increasing the active region thickness more optical power is generated inside the cavity, the waveguide losses are decreased and the far-field is improved due to a larger facet aperture. In this way the output power is increased by significantly more than a factor of 2 without reducing the maximum operating temperature and without increasing the threshold current.

Detectivity enhancement in quantum well infrared photodetectors utilizing a photonic crystal slab resonator.

We characterize the performance of a quantum well infrared photodetector (QWIP), which is fabricated as a photonic crystal slab (PCS) resonator. The strongest resonance of the PCS is designed to coincide with the absorption peak frequency at 7.6 µm of the QWIP. To accurately characterize the detector performance, it is illuminated by using single mode mid-infrared lasers. The strong resonant absorption enhancement yields a detectivity increase of up to 20 times. This enhancement is a combined effect of increased responsivity and noise current reduction. With increasing temperature, we observe a red shift of the PCS-QWIP resonance peak of -0.055 cm(-1)/K. We attribute this effect to a refractive index change and present a model based on the revised plane wave method.

Electrically controllable photonic molecule laser.

We have studied the coherent intercavity coupling of the evanescent fields of two microdisk terahertz quantum-cascade lasers. The electrically controllable optical coupling of the single-mode operating lasers has been observed for cavity spacings up to 30 mum. The strongest coupled photonic molecule with 2 mum intercavity spacing allows to conditionally switch the optical emission by the electrical modulation of only one microdisk. The lasing threshold characteristics demonstrate the linear dependence of the gain of a quantum-cascade laser on the applied electric field.

Active photonic crystal terahertz laser.

We present the design and the realization of active photonic crystal (PhC) semiconductor lasers. The PhC consists of semiconductor nanostructure pillars which provide gain at a quantized transition energy. The vertical layer sequence is that of a terahertz quantum cascade laser. Thereby, the artificial crystal itself provides the optical gain and the lateral confinement. The cavities do not rely on a central defect, the lasing is observed in flat-band regions at high symmetry points. The experimental results are in excellent agreement with the finite-difference time-domain simulations. For the vertical confinement a double-metal waveguide is used. The lasers are showing a stable single-mode emission under all driving conditions. Varying the period of the PhC allows to tune the frequency by 400 GHz.

Terahertz photonic crystal resonators in double-metal waveguides.

We present the design and the fabrication of photonic crystals with a complete bandgap for TM-modes used as a resonator for terahertz quantum-cascade lasers (QCL), which are lasing around 2.7 THz. The emission of the devices with and without a photonic crystal shows a shift in the emission from the gain maximum to the bandgap of the crystal. The devices are built up by a core, which provides the optical gain, and by a surrounding photonic crystal, which acts as a frequency selective mirror. The whole device is processed into a double-metal waveguide.

Towards functional group-specific detection in high-performance liquid chromatography using mid-infrared quantum cascade lasers.

A distributed feedback quantum cascade laser was applied for the first time as a powerful light source for mid-infrared (MIR) detection in liquid chromatography. Fructose and glucose in red wine were separated with an isocratic HPLC system, which was connected to a custom-made flow cell. This flow cell was constructed of two diamond windows with adjustable spacing and two hollow wave-guides for guiding the incoming and outgoing light. The HPLC column based on an ion-exchange resin with calcium(II) counter ion was run at 80 degrees C with 0.04% formic acid as the mobile phase. Under these conditions the carbohydrates could not be completely separated from the organic acids also present in wine. However, the emission of the laser at 1067 cm(-1) matches the absorption maximum of fructose and glucose, whereas the organic acids do not absorb appreciably at this wavenumber. Thus group-specific detection could be achieved. Additionally, the optical path length could be increased from 25 to 125 microm, which is very promising in gaining enhanced sensitivity compared to Fourier transform IR detection.

Spectroscopy in the gas phase with GaAs/AlGaAs quantum-cascade lasers.

We demonstrate what we believe is the first application of the recently developed electrically pumped GaAs/AlGaAs quantum-cascade lasers in a spectroscopic gas-sensing system by use of hollow waveguides. Laser light with an emission maximum at 10.009 microm is used to investigate the mid-infrared absorption of ethene at atmospheric pressure. We used a 434-mm-long silver-coated silica hollow waveguide as a sensing element, which served as a gas absorption cell. Different mixtures of helium and ethene with known concentrations are flushed through the waveguide while the laser radiation that passes through the waveguide is analyzed with a Fourier-transform infrared spectrometer. The experimentally obtained discrete ethene spectrum agrees well with the calculated spectrum. A detection threshold of 250 parts per million is achieved with the current setup.

Removal of endoproteinases from biological fluids by "sandwich affinity chromatography" with alpha 2-macroglobulin bound to zinc chelate-Sepharose.

A new method for removing nearly all active endoproteinases from fluids called "sandwich affinity chromatography" is described. It is based on strong chelate binding of alpha 2-macroglobulin (alpha 2M) and its proteinase complexes to Zn2+-bis-carboxymethylamino-Sepharose (Zn chelate-Sepharose) and its ability to complex most active endoproteinases. The preferred performance minimizing unspecific protein adsorption is binding first alpha 2M to Zn chelate-Sepharose and then adsorbing the proteinase to the alpha 2M-Zn chelate-Sepharose using elevated salt concentrations. A suitable standard buffer, in which most proteases and alpha 2M are active and the protease-alpha 2M complex remains bound to Zn chelate-Sepharose, is 0.02 mol/liter sodium phosphate, pH 6.5, containing 0.15 mol/liter NaCl. As an example, the reaction of trypsin with alpha 2M-Zn chelate-Sepharose was studied. After saturating Zn chelate-Sepharose first with alpha 2M and then with trypsin under standard conditions, the bound alpha 2M equals the bound trypsin activity (measured with Chromozym TRY). The specific binding capacity of alpha 2M-Zn chelate-Sepharose for proteases was determined in this way to be 30-40 U trypsin, i.e., 0.40-0.54 mg/ml of gel. The balance and the fact that the bound trypsin is inaccessible to soybean trypsin inhibitor indicate that at these conditions no unspecific trypsin binding occurs. Chymotrypsin, thermolysin, elastase, bromelain, ficin, and papain are also bound at standard conditions but not exoproteases like carboxypeptidases A and Y. Advantages of the sandwich affinity chromatography are the simple loading procedure by adsorption, the high capacity of the gel material, and the possibility to reuse the Zn chelate-Sepharose after eluting reacted alpha 2M and reloading with new alpha 2M.

Antithrombin BM from human plasma: an antithrombin binding moderately to heparin.

A human antithrombin was purified app. 60 fold from Cohn fraction IV, to give a single band of about 70.000 molecular weight in polyacrylamide gel electrophoresis. Compared to the similar antithrombin III, this glycoprotein binds only moderately to porcine heparin (hence its name Antithrombin BM), thus requiring higher heparin concentration for full thrombin inhibitor function, and lower ionic strength for elution from a heparin sepharose column. In these respects it resembles "heparin cofactor A", which is, however, characterized by a substantially larger molecular weight. From AT III, AT BM further differs in its absolute dependency on the presence of heparin(oids) for antithrombin activity, in its more pronounced inhibitory specificity largely restricted to thrombin, and in the absence of substantial immunological crossreaction with antibody to AT III. Based on comparative measurements of antithrombin activity in the presence of different amounts of heparin, up to 40% of the antithrombin activity present in human blood may be attributed to AT BM. The in vivo role of this new inhibitor remains to be elucidated.

Expression in Escherichia coli of biologically active enzyme by a DNA sequence coding for the human plasminogen activator urokinase.

We have isolated clones of Escherichia coli strain K-12 that contain a hybrid pBR322 plasmid having a 4.2-kilobase insert of a DNA transcript of the mRNA of human plasminogen activator, urokinase. The bacterially produced enzyme has properties similar to those of urokinase from human fetal kidney cells. Both enzymes occur in discrete forms ranging from 32,000 to 150,000 daltons in size. They react with antibody to purified urokinase from human kidney cells, bind to a benzamidine-Sepharose column, and induce plasminogen-dependent lysis of a fibrin clot.

Multipolymer systems.

Different polymers can be combined to yield a wide variety of composite materials: layered sheets and films, homogeneous and heterogeneous blends, interpenetrating polymer networks, bicomponent fibers, and others. Some properties of a multipolymer material are roughly additive, but synergistic interactions can yield properties and performances superior to those of the individual constituents. Consequently, the use of polymers in combination is a rapidly growing component of polymer materials technology.

A simple method for making new transducing lines of coliphage lambda.

A variety of novel transducing lines of phage lambda can be obtained by induction of a mixed culture of abnormal lysogens. Such a culture is simply made by mass lysogenization of a host lacking the normal prophage attachment site.