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We report on the multiple response of microgels triggered by a single optical stimulus. Under irradiation, the volume of the microgels is reversibly switched by more than 20 times. The irradiation initiates two different processes: photo-isomerization of the photo-sensitive surfactant, which forms a complex with the anionic microgel, rendering it photo-responsive; and local heating due to a thermo-plasmonic effect within the structured gold layer on which the microgel is deposited. The photo-responsivity is related to the reversible accommodation/release of the photo-sensitive surfactant depending on its photo-isomerization state, while the thermo-sensitivity is intrinsically built in. We show that under exposure to green light, the thermo-plasmonic effect generates a local hot spot in the gold layer, resulting in the shrinkage of the microgel. This process competes with the simultaneous photo-induced swelling. Depending on the position of the laser spot, the spatiotemporal control of reversible particle shrinking/swelling with a predefined extent on a per-second base can be implemented.

We present and experimentally validate a model describing the sensitivity of the tilt angle, expansion and propulsion velocity of a tin micro-droplet irradiated by a 1 {\mu}m Nd:YAG laser pulse to its relative alignment. This sensitivity is particularly relevant in industrial plasma sources of extreme ultraviolet light for nanolithographic applications. Our model has but a single parameter: the dimensionless ratio of the laser spot size to the effective size of the droplet, which is related to the position of the plasma critical density surface. Our model enables the development of straightforward scaling arguments in turn enabling precise control the alignment sensitivity., Comment: 7 pages, 5 figures

We study the feasibility of generating photon pairs in a resonant Vertical-Cavity Surface-Emitting Laser (VCSEL) as a result of a third-order non-linear, four wave mixing interaction. We focus on degenerate four wave mixing in the spontaneous regime where two pump photons are annihilated to create a pair of signal and idler photons. Using the methods of quantum optics, we calculate the two-photon production rate, the spectrum of the generated photons, and the signal-idler cross-correlations. We highlight how the dispersion of the medium in the VCSEL cavity (a regular GaAs configuration) significantly diminishes the two-photon production rate. Based on our results, we enumerate the characteristics of a VCSEL that would be suitable for photon pair generation. Published by AIP Publishing.

In this work, the unknown set of parameters of the Chua system is recovered under the hypothesys that the voltages of the capacitors are available. To this end, focusing on the differential equations, the Volterra kernel-based approach is used to perform an estimation without the uncertainty of the unmeasurable derivatives and the unknown initial conditions.

We report on the operation of a novel laser system delivering high-energy pulses in the near-infrared region. The pulses are nearly Fourier-transform limited (time-bandwidth product of 0.48), providing narrow-band radiation (∼1.5 GHz), with an energy of 225 mJ and 10 Hz repetition rate. The pulse duration of 320 ps covers the intermediate region between Q-switched and mode-locked lasers, and provides high peak powers. The nearly Gaussian beam profile, with a beam quality factor of M2=2.5, enables tight focusing, reaching intensities exceeding 5×1013 W/cm2. The system operation is demonstrated near 780 nm, however, the tunability extends over the range 700–970 nm. The laser system is suitable for high-order harmonic generation in the extreme-ultraviolet region for high-resolution frequency domain spectroscopy.

To investigate the dielectric response of isolated single-walled carbon nanotubes, (SWCNTs), spatially resolved electron energy-loss spectroscopy measurements have been carried out using a scanning transmission electron microscope in a near-field geometry. Spectra have been compared with those acquired on multiwalled carbon nanotubes (MWCNTs) made of different numbers of layers, and with simulations performed within the framework of the continuum dielectric theory, taking into account the local anisotropic character of these nanostructures and adapted to the cylindrical geometry. Experimental data show a dispersion of mode energies as a function of the ratio of the internal and external diameters, as predicted by the continuum dielectric model. For thin MWCNTs, two polarization modes have been identified at 15 and 19 eV, indexed as tangential and radial surface-plasmon modes, respectively, resulting from the coupling of the two surface modes on the internal and external surfaces of the nanotubes. We finally show that the dielectric response of a SWCNT, displaying a single energy mode at 15 eV, can be understood in the dielectric model as the thin layer limit of surface-plasmon excitation of MWCNTs.

By exploring the origin of the saturation current (J0), we show that the polymer/cathode interface determines the open circuit voltage (Voc) in polymer:fullerene bulk heterojunction solar cells. We fabricate and characterize P3HT:PCBM solar cells and compare their electrical characteristics using as reference P3HT/cathode Schottky diodes. We demonstrate with thermionic emission theory that J0 has the same origin in both devices. Thus, a general barrier-dependent expression for Voc, valid for non-ohmic cathode contacts is proposed and discussed.

The quantitative analysis of the vibrational properties of carbon nanotubes is a key issue for the interpretation of Raman experiments. In particular, a reliable characterization of the atomic structure of single-wall carbon nanotubes produced under various conditions is mainly based on the interpretation of low-frequency (100–300 cm-1) Raman spectra. In the present work, we analyze the influence of the packing of the tubes on these low-frequency modes. We find that the low-frequency spectra of crystals of single-wall carbon nanotubes present two intense Raman modes instead of a single fully symmetric A1g mode characteristic of isolated tubes. The second mode has a non-negligible intensity for crystals formed with nanotubes of radii larger than 7 Å. For finite number of tubes in a bundle, two breathinglike intense modes appear as a specific signature. Finally, our simulation for inhomogenous bundles made of a large number of tubes does not reveal any specific signature of the individual tubes in the low-frequency Raman spectra., L.H. was supported by the Belgian National Fund (FNRS). V.N.P. acknowledges the financial support by Grant No. G.0347.97 of the Flemish Science Foundation, by the Concerted Action of the University of Antwerpen GOA-BOF-UA, and by the NATO Research program for Belgium. This work was also partly funded by the Belgian Interuniversity Research Project on Reduced Dimensionality Systems (PAI-UAP P4-10). A.R. acknowledges support from the sabbatical program Salvador de Madariaga of the Spanish MEC (PR2000-0335) and the Ecole Polytechniquits RTN network Contract Nos. HPRN-CT-2000-00128 (COMELCAN), HPRN-CT-2000-00167 (NANOPHASE) and the RTD-FET program (SATUNET).

Using the time-dependent Hartree–Fock method two analytical schemes are elaborated for determining the derivatives of frequency-dependent polarizability with respect to atomic Cartesian coordinates. The first scheme is iterative and consists in determining the mixed derivatives of the density matrix with respect to atomic Cartesian coordinates and dynamic electrical fields. The second takes advantage of the 2n+1 rule to express the polarizability derivatives in terms of first-order derivatives. Both schemes are implemented in the GAMESS program. They enable the fully analytical evaluation of the Raman intensities with inclusion of the frequency dispersion. The potential of these methods is illustrated by determining the polarizability derivatives and Raman intensities of small molecules.

The performance of the density functional approach in the relativistic zero order regular approximation for the evaluation of electron spin resonance (ESR) parameters in small metal compounds has been evaluated critically by comparison with experimental data and available theoretical results for 22 linear molecules, characterized by a 2Σ electronic ground state. For most of the molecules studied the calculated magnetic parameters are in good (A tensors) or reasonable (g tensors) agreement with experiment. Effects of spin-orbit coupling and spin polarization on the calculated hyperfine interaction are investigated. These two effects can only be evaluated separately, since the present method does not allow us to take spin-polarization effects into account in spin-orbit coupled density functional calculations. However, while spin-polarization effects are important for all the molecules investigated, spin-orbit effects are non-negligible only for the molecules containing heavier metal atoms. The ESR parameter...

We present a bi-waveguide paradigm composed of joined Positive-Index-Material PIM)/Negative-Index-Material (NIM) slabs, demonstrating ultra-slow light propagation stemming from the competing propagation disposition in the PIM and NIM regions. We report for the first time a mesoscopic extended electromagnetic (EM) enhancement covering regions of the order of the free space wavelength, enabled by the slow-light mode in our system. Our dynamic numerical results are consistent with our developed theoretical model, predicting an EM energy accumulation reminiscent of a charging capacitor. Our analysis reveals that spatial compression is not a requirement to EM enhancement in slow-light systems and stresses on the merits of high coupling efficiency, strong temporal compression, monomodality and modal index bandwidth, -all present in our proposed paradigm. Furthermore, we show that the heterostructure waveguide mode is an extra-ordinary entity with a unique energy velocity, that is opposite to the Poynting vector in one of the participant waveguides. We believe these results will inspire new slow-light platforms relevant to the collective harvesting of strong light-matter interactions., Comment: 28 pages, 10 figures

Citation: J. Appl. Phys. 114, 164304 (2013); doi: 10.1063/1.4826529 (Received 24 July 2013; accepted 6 October 2013; published online 22 October 2013) A comprehensive analysis of electrical, electromagnetic (EM), mechanical, and thermal properties of epoxy resin composites filled with 0.25–2.0 wt. % of carbon additives characterized by high surface area, both nano-sized, like carbon nanotubes (CNTs) and carbon black (CBH), and micro-sized exfoliated graphite (EG), was performed. We found that the physical properties of both CNTs- and CBH-based epoxy resin composites increased all together with filler content and even more clearly for CBH than for CNTs. In the case of EG-based composites, good correlation between properties and filler amount was observed for concentrations below 1.5 wt. %. We conclude that CBH and, to a lower extent, EG could replace expensive CNTs for producing effective EM materials in microwave and low-frequency ranges, which are, in addition, mechanically and thermally stable. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4826529]

The magnetohydrodynamic (MHD) spectra of toroidally rotating, axisymmetric tokamaks are analyzed. The continuous spectrum equations for the coupled localized Alfven and slow modes are derived. The effect of flow on this spectrum is threefold: Doppler shift, centrifugal, and Coriolis effects enter. By exploiting a low-β, large aspect ratio expansion, the newly found low frequency Δm=0 Alfven gap [van der Holst et al., Phys. Rev. Lett. 84, 2865 (2000)] located on the rational mode surfaces is analyzed. This gap is created by the geodesic curvature of the field lines, together with finite pressure, and by centrifugal and Coriolis effects. It is determined by a three mode interaction involving a central Alfven mode and two sideband slow modes. From the same scheme, another new purely flow-induced gap inside the Δm=0 gap is found. Also, toroidal flow-induced Alfven eigenmodes (TFAE) are found in the Δm=0 gap. All of these waves may be useful for MHD spectroscopy. Since the gap structures as well as the global ...

Emerging technology based on artificial materials containing metallic structures has raised the prospect for unprecedented control of terahertz waves through components like filters, absorbers and polarizers. The functionality of these devices is static by the very nature of their metallic or polaritonic composition, although some degree of tunability can be achieved by incorporating electrically biased semiconductors. Here, we demonstrate a photonic structure by projecting the optical image of a metal mask onto a thin GaAs substrate using a femtosecond pulsed laser source. We show that the resulting high-contrast pattern of photo- excited carriers can create diffractive elements operating in transmission. With the metal mask replaced by a digital micromirror device, our photo-imprinted photonic structures provide a route to terahertz components with reconfigurable functionality., 12 pages, 5 figures

One of the proposed mechanisms to explain the heating of the solar corona is resonant absorption of magnetic Alfven waves. Numerical studies of this mechanism often involve large scale computations and produce large amounts of data that need to be visualized. In this article we present a method to visualize numerically calculated density and temperature evolutions of heating calculations by simulating the soft x‐ray observations of the soft x‐ray telescope aboard the Yohkoh satellite. The visualization method is applied to two different model calculations of the heating of coronal magnetic loops by the resonant absorption of Alfven waves. For these two cases, information on the variations of temperature and density can be extracted conveniently from the generated images. The resulting images reveal features that are characteristic of the resonant absorption process. This suggests that signatures of resonant absorption can be extracted from real soft x‐ray observations of coronal loops. By varying the parameters of the models and visualizing the results the effects of the parameters on the simulated soft x‐ray images will become known. In turn, this information can be used to identify the heating mechanism in real observations. © 1996 American Institute of Physics.

Patterning the front side of an ultra-thin crystalline silicon (c Si) solar cell helps keeping the energy conversion efficiency high by compensating for the light absorption losses. A super-Gaussian mathematical expression was used in order to encompass a large variety of nanopattern shapes and to study their influence on the photonic performance. We prove that the enhancement in the maximum achievable photo-current is due to both impedance matching condition at short wavelengths and to the wave nature of light at longer wavelengths. We show that the optimal mathematical shape and parameters of the pattern depend on the c Si thickness. An optimal shape comes with a broad optimal parameter zone where fabricating errors would have much less influence on the efficiency. We prove that cylinders are not the best suited shape. To compare our model with a real slab, we fabricated a nanopatterned c Si slab via Nano Imprint Lithography., 21 pages, 7 figures

We experimentally and theoretically study the characteristics of semiconductor ring lasers bidirectionally coupled by a single bus waveguide. This configuration has, e.g., been suggested for use as an optical memory and as an optical neural network motif. The main results are that the coupling can destabilize the state in which both rings lase in the same direction, and it brings to life a state with equal powers at both outputs. These are both undesirable for optical memory operation. Although the coupling between the rings is bidirectional, the destabilization occurs due to behavior similar to an optically injected laser system., Comment: 5 pages, 7 figures

The photoluminescence (PL) of Si nanocrystals (Si-nc) embedded in fused silica has been investigated under simultaneous excitations by laser and low energy proton beam. Ionodegradation of the sample, characterized by a rapid decrease and a spectral blueshift of the PL emission has been observed. These effects are associated with the creation of non-radiative centers in the Si-nc. Micro-Raman spectroscopy analysis shows that the proton beam has not changed the average size of Si-nc, but has disturbed a fraction of Si-Si bonds inside the Si-nc, which is consistent with both simulations and direct measurements. A post-annealing performed at 400 C for 1 h can restore the structural properties of the Si-nc, but only a part of their nominal PL emission intensity is recovered. Characterization of the damage induced by low energy proton irradiation reported in this paper makes the use of light ion beams relevant for the experimental investigation of nanostructured systems, such as ionoluminescence measurements. © 2011 American Institute of Physics.

The phonon dispersion of graphene is calculated using a perturbative approach within a density-functional-based nonorthogonal tight-binding model. In the adiabatic approximation, the LO and the TO phonon branches are found to have a finite slope at the Γ and K points of the Brillouin zone, respectively. This linear behavior is due to strong electron-phonon coupling for electron wave vector close to the K point and is a signature of the Kohn anomaly. The explicit account of the dynamic effects results in a strong modification of these phonon branches as well as in a significant broadening of their linewidth in the vicinity of the Γ and K points. In particular, the finite slope of the phonon branches turns to zero. The charge doping of graphene changes the LO and TO branches in the vicinity of the two points and essentially removes the Kohn anomaly with the increase in the doping level. The obtained results are in a good agreement with available experimental data.

We present here an analysis of the electronic-quantum properties of B and N codoping of carbon nanotubes. We show that BN pairs are transparent for conduction electrons. Localized transport phenomena, observed for isolated B or N doping, are recovered when B and N atoms are far apart. We also analyze the effect of BN nanodomains on the quantum conductance and demonstrate the drastic effect of the B-N "parity" and of the shape of the domain. Our simulations have been performed at the density-functional-theory level and within Green's function Landauer approach. However, we show that a one-parameter (on-site energy) model could catch the main features of the transport properties of BN codoping effect on electronic transport

Transitions from the ground state X 1Σ+ v=0 to eleven highly‐excited states of CO were investigated using extreme ultraviolet laser spectroscopy. Excited states were detected using a two‐photon ionization scheme. Improved values of band origins could be deduced from an absolute calibration of the extreme ultraviolet laser radiation against the I2 standard in the visible wavelength region. Accurate predissociation rates followed from linewidth analyses of individual rotational lines.

The stability of the polar Zn-terminated ZnO surface is probed by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy (STS). Surface states in the bandgap of ZnO are evidenced by STS and their presence is correlated with the local surface corrugation. Very defective surface regions are characterized by a bulk electronic structure showing a wide bandgap while nanometer-scale defect free regions exhibit a narrower bandgap and surface states. We also image atomically resolved (root 3 x root 3)R30 degrees reconstructions on the defect-free areas. (C) 2009 American Institute of Physics. [doi:10.1063/1.3238288]

The amount of solar energy reaching the active (photovoltaic) layer in a thin-film solar cell can be increased by reducing the Fresnel reflection losses at the interfaces. By using corrugated interfaces (at the wavelength scale), adiabatic propagation of the electromagnetic radiation is achieved over a broad wavelength range throughout the structure, which leads to an increase in the light that is absorbed in the active layer and, ultimately, to the improvement of the photovoltaic conversion efficiency. In this article, we have considered the case of corrugated thin-film solar cell structures and we have studied theoretically the optimization of such structures from the point of view of photonics. The focus was put on periodic pyramidal interface corrugations because they were similar to those existing at the surface of corrugated transparent electrodes on which active layers can be deposited. Because of their technological importance, we chose to work with fluorine-doped tin oxide as front electrode material and with amorphous silicon as active material. Using an original three dimensional transfer matrix method, we solved the electromagnetic wave propagation problem in the general case of laterally periodic stratified media and we compared this solution with effective medium approximated solution. On the basis of typical pyramid sizes, we demonstrated, through numerical simulations, the optimization of the global light energy intake by means of corrugations of increasing complexity. The best structures were found to be based on pyramid arrays having subwavelength periods and aspect ratio values close to one. Typically, a pyramidal structure with base and height both equal to 300 nm led to a global energy intake equal to I=0.98 (integrated over the spectral range 400-710 nm), which represented a 24% improvement in comparison with the global energy intake of a planar structure (I=0.79).

A newly developed hyphenated technique is presented combining an existing rheometer and differential scanning calorimeter into a single experimental setup. Through the development of a fixation accessory for differential scanning calorimeter (DSC) crucibles and a novel rotor, the simultaneous measurement is performed inside the well-controlled thermal environment of a Tzero DSC cell. Hence, the evolution of thermal and flow properties of a material can be simultaneously measured using steady or oscillatory shear measurements and regular or modulated temperature DSC measurements. Along with the construction of a prototype, a validation of the design was performed. The technique offers interesting opportunities for the investigation of flow-induced transitions, for instance, crystallization or phase separation, and provides an asset for high-throughput screening of materials. The potential of the novel technique is demonstrated by two case studies: the chemorheology during the cure of a thermosetting epoxy-amine system and the flow-induced crystallization of syndiotactic polypropylene.

In order to develop optical fiber hydrogen sensors, thin film materials with a high optical contrast between the metallic and hydrided states are needed. Magnesium exhibits such a contrast but cannot be easily hydrogenated at room temperature. However, thin films of Pd-doped Mg (MgPdy with 0.023⩽y⩽0.125) prepared by magnetron dc sputtering can easily be hydrided at room temperature and 0.5bar H2 within a few minutes. The rate of first hydrogenation increases linearly with increasing Pd concentration. Hydrogenation induces high variations of transmission (ΔT up to 20%) and reflection (ΔR up to 70%) of light (0.5eV⩽ℏω⩽6.0eV corresponding to 2500nm⩾λ⩾210nm). The optical properties can be understood by considering Pd as a deep donor in semiconducting MgH2.

We present an approach to reduce the convective flow in an electrostatically levitated liquid droplet to such an extent that diffusion is the dominant mechanism for mass transport, thus enabling direct measurements of atomic diffusion in reactive liquids at elevated temperatures. Convection is minimized by containerless processing, and reducing temperature variations in the sample. The diffusion tracer is deposited in situ in the electrostatic levitation device used for containerless processing. Uniform noncontact heating of the sample is achieved by laser heating with multiple beams arranged symmetrically, e.g., in a tetrahedral geometry. The expected temperature variations and the resulting convection flows are estimated for a Zr-based glass-forming alloy. The analysis suggests that diffusion experiments are possible throughout the entire undercooled liquid temperature range of this alloy and, in microgravity, up to 50 K above the liquidus temperature.

A high-power, frequency-stabilized laser for cooling of metastable helium atoms using the 2 S13 →3 P23 transition at 389 nm has been developed. The 389 nm light is generated by frequency doubling of a titanium:sapphire laser in an external enhancement cavity containing a lithium-triborate nonlinear crystal. With a maximum conversion efficiency of 75%, 1 W of useful 389 nm power is produced out of 2 W at 778 nm. While being stabilized to the 2 S13 →3 P23 transition, the 389 nm frequency is tunable over ±150 MHz with respect to the field-free atomic resonance frequency. This is accomplished by Zeeman tuning of the absorption lines used in the frequency-stabilization scheme. The setup for saturated absorption spectroscopy in an rf discharge cell, used to stabilize the 389 nm laser to the atomic transition, is described in detail. © 2005 American Institute of Physics.

The dielectric response function of single-walled carbon nanotubes is calculated within tight-binding models with different levels of complexity. First, the effects of the orbital basis set of the model, the orbitals overlap, and the structural optimization on the electronic band structure and the dielectric function of three small-radius nanotubes are investigated in detail. Second, the optical transition energies for a large number of nanotubes are derived from the peak positions of the imaginary part of the dielectric function for parallel and perpendicular light polarization. These results can be useful for the assignment of absorption spectra of nanotube samples and for the determination of the conditions for resonant Raman scattering from nanotubes. The obtained results are compared to recent spectrofluorimetric data on isolated single-walled carbon nanotubes.

During prolonged application of a gate bias, organic field-effect transistors show a gradual shift of the threshold voltage towards the applied gate bias voltage. The shift follows a stretched-exponential time dependence governed by a relaxation time. Here, we show that a thermodynamic analysis reproduces the observed exponential dependence of the relaxation time on the oxidation potential of the semiconductor. The good fit with the experimental data validates the underlying assumptions. It demonstrates that this operational instability is a straightforward thermodynamically driven process that can only be eliminated by eliminating water from the transistor. © 2011 American Institute of Physics.

We synthesized Sm-Co/α-Fe nanocomposite film-magnets, approximately 10μm in thickness, composed of 780 layers by the pulse laser deposition method. Transmission electron microscopic observations revealed that the synthesized film is composed of Sm-Co and α-Fe layers with the well-controlled α-Fe thickness of approximately 10-20 nm, which is suitable one predicted by the micromagnetic simulation. In spite of the enhanced interlayer diffusion of Fe and Co by annealing for crystallization, the (BH)max value of 100 kJ/m3 was obtained at the averaged compositions of Sm/(Sm + Co) = 0.16 and Fe/(Sm + Co + Fe) = 0.47. The α-Fe fraction for obtaining the highest (BH)max value was smaller than that expected from the micromagnetic simulation. Although the annealing for crystallization lay the easy direction of magnetization in the plane, the film is not expected to have strong crystallographic texture., Journal of Applied Physics, 115(17), 17A748; 2014

The cooling effect of field emission from an n-type semiconductor was theoretically investigated in quest for a solid state cooler. The vacuum potential was exactly expressed in terms of the semiconductor cathode geometry. This leaded to the more accurate configuration-dependent calculations of the energy exchange and the cooling power. It has been shown that a sharper tip of semiconductor can yield either a larger field emission current density or a larger energy exchange, according to the applied bias. For an atomic size tip, the n-Si cathode yielded the cooling power density Γ = 2.0, 75, and 713 W/cm2 at temperature T = 300, 600, and 900 K, respectively. This implies that an optimized configuration of an n-Si cathode produces a significant electron emission cooling, especially at high temperatures.

We study the low-frequency region of the Raman spectra of infinite bundles of identical single-walled carbon nanotubes within a Valence force field model of the lattice dynamics and a bond-polarizability model for the Raman intensity. It is obtained that two breathinglike phonon modes are present in such bundles for each tube type. Based on this prediction, an alternative interpretation of the experimental Raman spectra is proposed. In particular, an explanation is given for the appearence of resonant Raman lines that are predicted as nonresonant within the noninteracting tube picture.

The first comprehensive high-resolution photoabsorption spectrum of (14)N(15)N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet and span 100 000-109 000 cm(-1) (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N(2) that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schrödinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.

We study the rectification properties of geometrically asymmetric metal-vacuum-metal junctions. In particular, we focus on systems in which the cathode metal supports a hemispherical protrusion. By using a transfer-matrix methodology to take account of three-dimensional aspects of the problem, we compute the forward and backward currents that flow in this device when it is subject to positive or negative external biases. These currents enable the calculation of the rectification properties of the device in the limit of quasistatic fields. We also determine the power this device could provide to an external load. We study in detail how these properties depend (i) on the magnitude of the bias established between the two metallic electrodes, (ii) on the separation between the two electrodes, (iii) on differences in the work function of the two metals, (iv) on differences in the temperature of the two metals, and (v) on the height of the protrusion. These calculations provide quantitative results for the use of these junctions as an energy converter and the efficiency with which the energy of incident radiation is being converted into a dc current and delivered to an external load.

The authors present a charge-dipole model for the calculation of the polarization of fullerenes and carbon nanotubes. By expressing the electrostatic interactions in terms of normalized propagators, the technique achieves a better consistency and an improved stability. In its most elementary form, the model depends on a single parameter and provides an excellent agreement with reference data. The technique is illustrated with a C720 fullerene and enables one to quantify the role of free charges in the polarization of these structures. The positioning of these charges suggests that field-induced adsorption of small molecules should be enhanced at pentagonal rings and defects.

We present a monopole-dipole model to compute the polarization of metallic carbon nanotubes. The parameters of this model are adjusted on experimental data. In particular, we aimed at reproducing the mean polarizability of C60 and C70, the transverse polarizability of some carbon nanotubes, as well as the internal over external value ratio of transverse electric fields. As an application, we compute the axial polarizability of finite (5,5) carbon nanotubes. The comparison of these results with those obtained using other techniques shows that the consideration of net charges gives a better account of the metallicity of this kind of nanotubes.

Two analytical procedures based on the time-dependent Hartree–Fock (TDHF) scheme are elaborated to evaluate the first derivatives of the dynamic first hyperpolarizability with respect to atomic Cartesian coordinates. In the first scheme, the mixed third-order TDHF equations are solved iteratively to obtain the third derivatives of the linear combination of atomic orbitals coefficients twice with respect to external dynamic electric fields and once with respect to atomic Cartesian coordinates. The second takes advantage of the 2n+1 rule and interchange relations to express the first derivatives of the dynamic first hyperpolarizability with respect to atomic Cartesian coordinates in terms of lower-order derivatives. Both procedures have been implemented in the GAMESS quantum chemistry package. Applications focus on the characterization of the frequency dispersion of the first derivatives of the first hyperpolarizability with respect to vibrational normal coordinates of H2O, NH3, and CH4 as well as on the de...