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We describe the design and fabrication of nanostructured silver split-ring resonators and demonstrate their application as dichroic sensors for multipurpose visible wavelength molecular spectroscopy. By producing arrays of these identical nanostructures, with critical dimensions of 30–150 nm, we demonstrate the controllable generation of plasmonic resonances at two common visible laser wavelengths, produced as a consequence of the polarization of the light with respect to structural geometry. We show the application of these devices in surface enhanced Raman spectroscopy, using their dichroic properties to perform molecular sensing of a self-assembled monolayer of 2-mercaptopyridine at both 532 and 633 nm. [ABSTRACT FROM AUTHOR]

Top-gated, few-layer graphene field-effect transistors (FETs) fabricated on thermally decomposed semi-insulating 4H-SiC substrates are demonstrated. Physical vapor deposited SiO2 is used as the gate dielectric. A two-dimensional hexagonal arrangement of carbon atoms with the correct lattice vectors, observed by high-resolution scanning tunneling microscopy, confirms the formation of multiple graphene layers on top of the SiC substrates. The observation of n-type and p-type transition further verifies Dirac Fermions’ unique transport properties in graphene layers. The measured electron and hole mobilities on these fabricated graphene FETs are as high as 5400 and 4400 cm2/V s, respectively, which are much larger than the corresponding values from conventional SiC or silicon. [ABSTRACT FROM AUTHOR]

Graphene is a hexagonally bonded sheet of carbon atoms that exhibits superior transport properties with a velocity of 108 cm/s and a room-temperature mobility of >15 000 cm2/V s. How to grow gate dielectrics on graphene with low defect states is a challenge for graphene-based nanoelectronics. Here, we present the growth behavior of Al2O3 and HfO2 films on highly ordered pyrolytic graphite (HOPG) by atomic layer deposition (ALD). To our surprise, large numbers of Al2O3 and HfO2 nanoribbons, with dimensions of 5–200 nm in width and >50 μm in length, are observed on HOPG surfaces at growth temperature between 200 and 250 °C. This is due to the large numbers of step edges of graphene on HOPG surfaces, which serve as nucleation sites for the ALD process. These Al2O3 and HfO2 nanoribbons can be used as hard masks to generate graphene nanoribbons or as top-gate dielectrics for graphene devices. This methodology could be extended to synthesize insulating, semiconducting, and metallic nanostructures and their combinations. [ABSTRACT FROM AUTHOR]

Electron beam lithography was used to fabricate gold crescent shaped split-ring resonators with 30 nm minimum feature size. By varying the crescent’s arc length over a range of nanometer-scale dimensions the authors demonstrate the tuneability of visible resonances within such structures. Results, which correlate closely with those predicted using finite-difference time-domain modeling, open the way for these devices to be used in near-field biological sensing. [ABSTRACT FROM AUTHOR]

The authors describe the integration of low-loss optical waveguides with lab-on-a-chip structures to produce an integrated optical-microfluidic platform for time-correlated single-photon counting of fluorescent molecules. Waveguides were fabricated using electron beam densification of planar silica on silicon, eliminating any requirement for depositing upper cladding silica layers. Microfluidic channels were dry etched directly through the waveguides and the device was sealed using a poly(dimethylsiloxane) gasket. Time-resolved fluorescence lifetime measurements of the fluorophore nile blue were used as a model system to demonstrate the operation of the microfluidic device, with dye concentrations as low as 1.5 nM (equivalent to <6000 molecules) being measured. [ABSTRACT FROM AUTHOR]

Understanding and controlling plasmon resonances from metallic nanoscale structures have been the focus of much attention recently, with applications including local surface plasmon resonance sensing, surface enhanced Raman spectroscopy, and negative refractive index materials. In this letter the authors demonstrate the fabrication of uniform arrays of split rings from gold and show that such structures are capable of supporting multiple plasmon resonances. The authors show that up to five plasmon resonances can be identified and use finite difference time domain modeling and absorption spectroscopy to fully characterize and identify each resonance. The implications of higher order surface plasmon resonances for sensing are discussed. [ABSTRACT FROM AUTHOR]

An ultrasensitive nanocalorimeter for use with pl-scale biological samples using silicon microfabrication technology has been developed in which a 720 pl reaction vessel, a calibration heater, and a thermoelectric transducer of 125 µK sensitivity were integrated into a single multilayer thin-film configuration. The resolution of the system ranged from 10 to 25 nW depending on the heat capacity, conductance and power density of the samples studied. The device has been used in heat conduction measurements of the energy released from the enzyme catalyzed hydrolysis of hydrogen peroxide using purified catalase, and for the determination of the catalase activity within a single mouse hepatocyte. The nanocalorimeter has the potential for integration in a high-density array format, where the change in temperature from ultralow volume cellular assays could be used as a generic analytical tool for high throughput screening of bioactive compounds. [ABSTRACT FROM AUTHOR]

Experimentally we compare the densification induced by electron beam irradiation of Ge-doped silica, produced by flame hydrolysis deposition with the densification of thermally produced SiO[sub 2]. By comparing these results to the predictions made by elasticity theory, we find good agreement for the thermal SiO[sub 2] by considering a single region of electron beam damage. For the Ge-doped flame-hydrolysis-deposited silica, we need to include in the model a second, shallow region, which densifies to a greater extent. X-ray photoelectron spectroscopy measurements suggest that the thickness of this additional region is comparable to a layer that was found to be depleted of Ge. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Negative transconductance is reported in a resistive gate metal-semiconductor field-effect transistor (MESFET) for the first time. The negative transconductance arises as a result of the negative differential mobility of electrons in the GaAs channel, and is adequately described by a simple equivalent circuit model. The experimental device is fabricated with a planar ion-implanted MESFET process, and shows promise as a microwave signal source for use in monolithic microwave integrated circuits. [ABSTRACT FROM AUTHOR]

Potassium dihydrogen phosphate (KDP) is widely used for high-power laser frequency conversion. Its applications, however, are limited by a characteristically low laser damage threshold. This damage is associated with extrinsic impurity inclusions, whose exact nature has not been previously identified. We have now demonstrated that KDP grown in solutions containing organic impurities incorporates select organic compounds into the crystal matrix. Furthermore, we have observed a strong correlation between the concentration of these compounds and the crystal laser damage threshold and density. [ABSTRACT FROM AUTHOR]

We report photocollection efficiency measurements of p+in GaAs/AlAs/GaAs diodes fabricated on films grown by molecular beam epitaxy. Both the zero-bias and bias-dependent photocollection characteristics can be explained by assuming that the band discontinuity between AlAs and GaAs is mostly accommodated in the valence band. [ABSTRACT FROM AUTHOR]

We report photosensitive capacitance-voltage characteristics of n+-GaAs/Al0.3Ga0.7As/p-GaAs heterostructures grown by molecular beam epitaxy. Inversion layers are observed due to minority charge retention in the triangular potential well at the AlGaAs/p-GaAs interface. Details of the device structure and experimental evaluation utilizing capacitance-voltage and current-voltage techniques are presented. [ABSTRACT FROM AUTHOR]

We report the first direct measurement of electron storage time at the AlxGa1-xAs/GaAs interface as a function of Al mole fraction x. Storage capacitors with x=0.4, 0.6, 0.8, and 1.0 were fabricated and their inversion layer electron retention times measured. The optimal mole fraction for electron retention is in the range 0.4≤x≤0.6. [ABSTRACT FROM AUTHOR]

The capacitance-voltage (C-V) and the current-voltage characteristics of metal/ZnSe/p-GaAs capacitors have been investigated; both epitaxial layers were grown by molecular beam epitaxy. In the capacitor structures highly resistive, stoichiometric ZnSe functioned as the pseudoinsulator on the doped GaAs layers. The capacitance-voltage measurements demonstrated that the capacitors could be biased from accumulation through depletion, and into deep depletion, with current in the range of 10-8 A cm-2. Very little frequency dispersion was observed in the C-V data when measured from 1 kHz to 1 MHz. From the high-frequency C-V curve, the surface state density as a function of position in the GaAs band gap was determined. Surface state densities were comparable to densities reported for (Al,Ga)As/GaAs heterojunctions. [ABSTRACT FROM AUTHOR]

The fabrication and current-voltage characteristics of the first depletion-mode field-effect transistors based on a pseudomorphic ZnSe/n-GaAs heterointerface are described. The devices are doped-channel field-effect transistors produced by means of interrupted growth with the use of two separate molecular beam epitaxy systems. Very strong (visible to the naked eye) reflection high-energy electron diffraction intensity oscillations persist for 120 periods when ZnSe is nucleated on the GaAs epilayer. The current-voltage characteristics of the transistors are close to ideal; channel modulation indicates that the Fermi level is not pinned at the ZnSe/GaAs interface. [ABSTRACT FROM AUTHOR]

A buried five-period 94 Å Al0.3Ga0.7As/ 56 Å GaAs superlattice is employed to improve the dynamic retention of electrons at an AlGaAs/GaAs heterojunction. The improvement is attributed to the impurity trapping and gettering properties of this superlattice and the amount of time our molecular beam epitaxy system had been closed. Electron retention time constants of 29 s were obtained at a temperature of 77 K. This is more than 200 times longer than previously reported time constants and more than adequate for dynamic logic or memory applications compatible with AlGaAs/GaAs heterojunction field-effect transistors. [ABSTRACT FROM AUTHOR]

We discuss the nature of errors which may occur when the spectra of random signals not obeying Gaussian statistics are measured with typical rf spectrum analyzers. These errors depend on both the noise statistics and the process used to detect the random signal after it has been passed through a narrow bandpass filter within the spectrum analyzer. In general, for random signals not obeying Gaussian statistics, the output of the bandpass filter must be measured with a square law detector if the resulting measurement is to be strictly proportional to the power spectrum of the input signal. We compare measurements of the power spectra of non-Gaussian noise using a commerical spectrum analyzer with its resident envelope detector, with measurements by the same analyzer fitted with a square law detector. Differences of about 5% were observed. [ABSTRACT FROM AUTHOR]

We have studied the capacitance-voltage (CV) and current-voltage (IV) characteristics of molecular beam epitaxially (MBE) grown p+-GaAs/AlAs/n-GaAs heterostructures as a function of illumination level and temperature. Under illumination we find clear evidence of a hole inversion layer at the AlAs/n-GaAs interface at 77 K, with a maximum inversion layer carrier concentration of about 4.1×1011 cm-2. The samples are essentially free of trapping in the AlAs, as indicated by the almost complete lack of hysteresis in the CV curves. At room temperature under steady-state illumination we see both a voltage shift and a peak in the CV curves. The shift is attributed to the formation of an inversion layer, while the peak is believed due to the ac response of space charge flowing through the AlAs. In contrast to recent reports on similar samples grown by metalorganic chemical vapor deposition, the MBE grown samples appear to be suitable for fabricating field-effect devices. [ABSTRACT FROM AUTHOR]

We report an investigation of the long term retention characteristic of a hole inversion layer at the interface between an AlAs/GaAs superlattice and GaAs in the dark. The retention time constant at 77 K is measured to be 34 h. From the temperature dependence of the retention time constant, an effective valence-band discontinuity of 0.17 eV is obtained. [ABSTRACT FROM AUTHOR]

A multitasking extension has been developed for computers based on the Z80A CPU; it enables the Sinclair ZX Spectrum 48 K to be used for data acquisition and analysis simultaneously. [ABSTRACT FROM AUTHOR]

For the purpose of calculating (ro-)vibrational spectra, rate constants, scattering cross sections, etc. product basis sets are very popular. They, however, have the important disadvantage that they are unusably large for systems with more than four atoms. In this paper we demonstrate that it is possible to efficiently use a basis set obtained by removing, from a product basis set, functions associated with the largest diagonal Hamiltonian matrix elements. This is done by exploiting the fact that for every factor of every term in the Hamiltonian, there is a basis-set order in which the matrix representation of the factor is block diagonal. Due to this block diagonality the Lanczos algorithm can be implemented efficiently. Tests with model Hamiltonians with as many as 32 coordinates illustrate the merit of the ideas.