Showing papers on "Biasing published in 2016"

Near-Infrared Photodetector Based on MoS2/Black Phosphorus Heterojunction

Abstract: Two-dimensional (2D) materials present their excellent properties in electronic and optoelectronic applications, including in ultrafast carrier dynamics, layer-dependent energy bandgap, tunable optical properties, low power dissipation, high mobility, transparency, flexibility, and the ability to confine electromagnetic energy to extremely small volumes. Herein, we demonstrate a photodetector with visible to near-infrared detection range, based on the heterojunction fabricated by van der Waals assembly between few-layer black phosphorus (BP) and few-layer molybdenum disulfide (MoS2). The heterojunction with electrical characteristics which can be electrically tuned by a gate voltage achieves a wide range of current-rectifying behavior with a forward-to-reverse bias current ratio exceeding 103. The photoresponsivity (R) of the photodetector is about 22.3 A W–1 measured at λ = 532 nm and 153.4 mA W–1 at λ = 1.55 μm with a microsecond response speed (15 μs). In addition, its specific detectivity D* is calcul...

Controlled Generation of a p–n Junction in a Waveguide Integrated Graphene Photodetector

Abstract: With its electrically tunable light absorption and ultrafast photoresponse, graphene is a promising candidate for high-speed chip-integrated photonics. The generation mechanisms of photosignals in graphene photodetectors have been studied extensively in the past years. However, the knowledge about efficient light conversion at graphene p–n junctions has not yet been translated into high-performance devices. Here, we present a graphene photodetector integrated on a silicon slot-waveguide, acting as a dual gate to create a p–n junction in the optical absorption region of the device. While at zero bias the photothermoelectric effect is the dominant conversion process, an additional photoconductive contribution is identified in a biased configuration. Extrinsic responsivities of 35 mA/W, or 3.5 V/W, at zero bias and 76 mA/W at 300 mV bias voltage are achieved. The device exhibits a 3 dB bandwidth of 65 GHz, which is the highest value reported for a graphene-based photodetector.

High-Sensitivity Floating-Gate Phototransistors Based on WS2 and MoS2

Abstract: In recent years, 2D layered materials have been considered as promising photon absorption channel media for next-generation phototransistors due to their atomic thickness, easily tailored single-crystal van der Waals heterostructures, ultrafast optoelectronic characteristics, and broadband photon absorption. However, the photosensitivity obtained from such devices, even under a large bias voltage, is still unsatisfactory until now. In this paper, high-sensitivity phototransistors based on WS2 and MoS2 are proposed, designed, and fabricated with gold nanoparticles (AuNPs) embedded in the gate dielectric. These AuNPs, located between the tunneling and blocking dielectric, are found to enable efficient electron trapping in order to strongly suppress dark current. Ultralow dark current (10−11 A), high photoresponsivity (1090 A W−1), and high detectivity (3.5 × 1011 Jones) are obtained for the WS2 devices under a low source/drain and a zero gate voltage at a wavelength of 520 nm. These results demonstrate that the floating-gate memory structure is an effective configuration to achieve high-performance 2D electronic/optoelectronic devices.

Polarization-Insensitive Single- and Broadband Switchable Absorber/Reflector and Its Realization Using a Novel Biasing Technique

Abstract: In this communication, a switchable absorber/reflector based on active frequency-selective surface (AFSS) has been presented for single- as well as broadband applications. The FSS comprises periodic patterns of square loops connected among themselves through p-i-n diodes, which exhibit switchable performances. The novelty of the proposed design lies in its symmetric configuration and biasing network, which makes the structure polarization insensitive unlike the earlier reported AFSSs. A single-band switchable absorber/reflector has initially been realized, which is characterized through an equivalent circuit model to derive the circuit parameters. Later, surface-mount resistors have been implemented in the design to realize a wideband switchable absorber/reflector aimed for C-band applications. Both the structures have used a novel biasing methodology to provide the bias voltage to semiconductor switches without disturbing the original resonance pattern. Furthermore, the fabricated samples, while measuring in an anechoic chamber, show good agreement with the simulated responses under normal incidence as well as for different polarization angles.

A Review of Sharp-Switching Devices for Ultra-Low Power Applications

Abstract: The reduction of the supply voltage is standard MOSFETs is impeded by the subthreshold slope, which cannot be lowered below 60 mV/decade, even in ideal fully-depleted devices. We review selected CMOS-compatible devices capable of switching more abruptly than MOSFETs, and discuss their merits and limitations. Tunneling FETs (TFETs) are reverse-biased gated PIN diodes where the gate controls the electric field in the interband tunneling junction. Technological solutions for improved performance will be described, including alternative channel materials and geometries, as well as a proposed paradigm shift of increasing the current drive by internal amplification in the bipolar-enhanced TFET. Other emerging sharp-switching mechanisms are reviewed, including the abrupt change in the polarization of ferroelectric materials, mechanical contact in nano-electro-mechanical systems, energy filtering of injected carriers, etc. Recently proposed band modulation feedback transistors are conceptually different from MOSFETs or TFETs. They have similar gated-diode configuration, but are operated in forward-bias mode. Electrostatic barriers are formed (via gate biasing) to prevent electron/hole injection into the channel until the gate or drain bias reaches a turn-on value. Due to bandgap modulation along the channel, these devices can switch abruptly (<1 mV/decade) to a high current.

Electric-field-induced magnetization switching in CoFeB/MgO magnetic tunnel junctions with high junction resistance

Abstract: We show the electric-field induced magnetization switching for CoFeB/MgO magnetic tunnel junctions with thick MgO barrier layer of 2.8 nm, whose resistance-area product is 176 kΩ μm2, and achieve the small switching energy of 6.3 fJ/bit. The increase of the junction resistance is expected to suppress the energy consumption due to the Joule heating during the switching; however, the energy is still dominated by the Joule energy rather than the charging energy. This is because the junction resistance decreases more rapidly for junctions with thicker MgO as bias voltage increases.

Directional dependence of the electronic and transport properties of 2D borophene and borophane

Abstract: Very recently two dimensional layers of boron atoms, so called borophene, have been successfully synthesized. It presents a metallic band structure, with a strong anisotropic character. Upon further hydrogen adsorption a new material is obtained, borophane; giving rise to a Dirac cone structure like the one in graphene. We have performed a first-principles study of the electronic and transport properties of borophene and borophane through the Landauer–Buttiker formalism. We find that borophene presents an electronic current two orders of magnitude larger than borophane. In addition we verified the direction dependence of the electronic current in two perpendicular directions, namely, Ix and Iy; where for both systems, we found a current ratio, η = Ix/Iy, of around 2. Aiming to control such a current anisotropy, η, we performed a study of its dependence with respect to an external strain. Where, by stretching the borophane sheet, η increases by 11% for a bias voltage of 50 mV.

High performance bias-selectable three-color Short-wave/Mid-wave/Long-wave Infrared Photodetectors based on Type-II InAs/GaSb/AlSb superlattices

Abstract: We propose a new approach in device architecture to realize bias-selectable three-color shortwave-midwave-longwave infrared photodetectors based on InAs/GaSb/AlSb type-II superlattices. The effect of conduction band off-set and different doping levels between two absorption layers are employed to control the turn-on voltage for individual channels. The optimization of these parameters leads to a successful separation of operation regimes; we demonstrate experimentally three-color photodiodes without using additional terminal contacts. As the applied bias voltage varies, the photodiodes exhibit sequentially the behavior of three different colors, corresponding to the bandgap of three absorbers. Well defined cut-offs and high quantum efficiency in each channel are achieved. Such all-in-one devices also provide the versatility of working as single or dual-band photodetectors at high operating temperature. With this design, by retaining the simplicity in device fabrication, this demonstration opens the prospect for three-color infrared imaging.

Beam test results of a 15 ps timing system based on ultra-fast silicon detectors

Abstract: In this paper we report on the timing resolution of the first production of 50 micro-meter thick Ultra-Fast Silicon Detectors (UFSD) as obtained in a beam test with pions of 180 GeV/c momentum. UFSD are based on the Low-Gain Avalanche Detectors (LGAD) design, employing n-on-p silicon sensors with internal charge multiplication due to the presence of a thin, low-resistivity diffusion layer below the junction. The UFSD used in this test belongs to the first production of thin (50 {\mu}m) sensors, with an pad area of 1.4 mm2. The gain was measured to vary between 5 and 70 depending on the bias voltage. The experimental setup included three UFSD and a fast trigger consisting of a quartz bar readout by a SiPM. The timing resolution, determined comparing the time of arrival of the particle in one or more UFSD and the trigger counter, for single UFSD was measured to be 35 ps for a bias voltage of 200 V, and 26 ps for a bias voltage of 240 V, and for the combination of 3 UFSD to be 20 ps for a bias voltage of 200 V, and 15 ps for a bias voltage of 240 V.

Fundamental Limits on the Subthreshold Slope in Schottky Source/Drain Black Phosphorus Field-Effect Transistors

Abstract: The effect of thickness, temperature, and source–drain bias voltage, VDS, on the subthreshold slope, SS, and off-state properties of black phosphorus (BP) field-effect transistors is reported. Locally back-gated p-MOSFETs with thin HfO2 gate dielectrics were analyzed using exfoliated BP layers ranging in thickness from ∼4 to 14 nm. SS was found to degrade with increasing VDS and to a greater extent in thicker flakes. In one of the thinnest devices, SS values as low as 126 mV/decade were achieved at VDS = −0.1 V, and the devices displayed record performance at VDS = −1.0 V with SS = 161 mV/decade and on-to-off current ratio of 2.84 × 103 within a 1 V gate bias window. A one-dimensional transport model has been utilized to extract the band gap, interface state density, and the work function of the metal contacts. The model shows that SS degradation in BP MOSFETs occurs due to the ambipolar turn on of the carriers injected at the drain before the onset of purely thermionic-limited transport at the source. Th...

Dispersive Thermometry with a Josephson Junction Coupled to a Resonator

Abstract: We have embedded a small Josephson junction in a microwave resonator that allows simultaneous dc biasing and dispersive readout. Thermal fluctuations drive the junction into phase diffusion and induce a temperature-dependent shift in the resonance frequency. By sensing the thermal noise of a remote resistor in this manner, we demonstrate primary thermometry in the range from 300 mK to below 100 mK, and high-bandwidth (7.5 MHz) operation with a noise-equivalent temperature of better than 10 $\mathrm{\mu K/\sqrt{Hz}}$. At a finite bias voltage close to a Fiske resonance, amplification of the microwave probe signal is observed. We develop an accurate theoretical model of our device based on the theory of dynamical Coulomb blockade.

A Thermal Energy Harvesting Power Supply With an Internal Startup Circuit for Pacemakers

Abstract: A complete thermal energy harvesting power supply for implantable pacemakers is presented in this paper. The designed power supply includes an internal startup and does not need any external reference voltage. The startup circuit includes a prestartup charge pump (CP) and a startup boost converter. The prestartup CP consists of an ultralow-voltage oscillator followed by a high-efficiency modified Dickson. Forward body biasing is used to effectively reduce the MOS threshold voltages as well as the supply voltage in oscillator and CP. The steady-state circuit includes a high-efficiency boost converter that utilizes a modified maximum powerpoint tracking scheme. The system is designed so that no failure occurs under overload conditions. Using this approach, a thermal energy harvesting power supply has been designed using 180-nm CMOS technology. According to HSPICE simulation results, the circuit operates from input voltages as low as 40 mV provided from a thermoelectric generator and generates output voltages up to 3 V. A maximum power of 130 $\mu $ W can be obtained from the output of the boost converter, which means that its efficiency is 60%. A minimum voltage of 60 mV and a maximum time of 400 ms are needed for the circuit to start up.

Correlation of p-doping in CVD Graphene with Substrate Surface Charges

Abstract: Correlations between the level of p-doping exhibited in large area chemical vapour deposition (CVD) graphene field effect transistor structures (gFETs) and residual charges created by a variety of surface treatments to the silicon dioxide (SiO2) substrates prior to CVD graphene transfer are measured. Beginning with graphene on untreated thermal oxidised silicon, a minimum conductivity (σ(min)) occurring at gate voltage V(g) = 15 V (Dirac Point) is measured. It was found that more aggressive treatments (O2 plasma and UV Ozone treatments) further increase the gate voltage of the Dirac point up to 65 V, corresponding to a significant increase of the level of p-doping displayed in the graphene. An electrowetting model describing the measured relationship between the contact angle (θ) of a water droplet applied to the treated substrate/graphene surface and an effective gate voltage from a surface charge density is proposed to describe biasing of V(g) at σ(min) and was found to fit the measurements with multiplication of a correction factor, allowing effective non-destructive approximation of substrate added charge carrier density using contact angle measurements.

A 60 GOPS/W, −1.8 V to 0.9 V body bias ULP cluster in 28 nm UTBB FD-SOI technology

Abstract: Ultra-low power operation and extreme energy efficiency are strong requirements for a number of high-growth application areas, such as E-health, Internet of Things, and wearable Human–Computer Interfaces. A promising approach to achieve up to one order of magnitude of improvement in energy efficiency over current generation of integrated circuits is near-threshold computing. However, frequency degradation due to aggressive voltage scaling may not be acceptable across all performance-constrained applications. Thread-level parallelism over multiple cores can be used to overcome the performance degradation at low voltage. Moreover, enabling the processors to operate on-demand and over a wide supply voltage and body bias ranges allows to achieve the best possible energy efficiency while satisfying a large spectrum of computational demands. In this work we present the first ever implementation of a 4-core cluster fabricated using conventional-well 28 nm UTBB FD-SOI technology. The multi-core architecture we present in this work is able to operate on a wide range of supply voltages starting from 0.44 V to 1.2 V. In addition, the architecture allows a wide range of body bias to be applied from −1.8 V to 0.9 V. The peak energy efficiency 60 GOPS/W is achieved at 0.5 V supply voltage and 0.5 V forward body bias. Thanks to the extended body bias range of conventional-well FD-SOI technology, high energy efficiency can be guaranteed for a wide range of process and environmental conditions. We demonstrate the ability to compensate for up to 99.7% of chips for process variation with only ±0.2 V of body biasing, and compensate temperature variation in the range −40 °C to 120 °C exploiting −1.1 V to 0.8 V body biasing. When compared to leading-edge near-threshold RISC processors optimized for extremely low power applications, the multi-core architecture we propose has 144× more performance at comparable energy efficiency levels. Even when compared to other low-power processors with comparable performance, including those implemented in 28 nm technology, our platform provides 1.4× to 3.7× better energy efficiency.

Two-Volt Josephson Arbitrary Waveform Synthesizer Using Wilkinson Dividers.

Abstract: The root-mean-square (rms) output voltage of the National Institute of Standards and Technology (NIST) Josephson arbitrary waveform synthesizer (JAWS) has been doubled from 1 V to a record 2 V by combining two new 1 V chips on a cryocooler. This higher voltage will improve calibrations of ac thermal voltage converters and precision voltage measurements that require state-of-the-art quantum accuracy, stability, and signal-to-noise ratio. We achieved this increase in output voltage by using four on-chip Wilkinson dividers and eight inner/outer dc blocks, which enable biasing of eight Josephson junction (JJ) arrays with high-speed inputs from only four high-speed pulse generator channels. This approach halves the number of pulse generator channels required in future JAWS systems. We also implemented on-chip superconducting interconnects between JJ arrays, which reduces systematic errors and enables a new modular chip package. Finally, we demonstrate a new technique for measuring and visualizing the operating current range that reduces the measurement time by almost two orders of magnitude and reveals the relationship between distortion in the output spectrum and output pulse sequence errors.

Liquid sensing in aquatic environment using high quality planar microwave resonator

Abstract: This paper describes a non-contact liquid sensor operating in an aquatic environment using an active, feedback loop assisted, planar, micro strip microwave resonator. The proposed sensor has the ability to operate in noncontact fashion within a distance of 0 to 8 cm. The active loop technique is shown to increase the primary quality factor from 210 to 500,000 in air when measured at a resonant frequency of 1.52 GHz. The quality factor of the proposed resonator is adjustable with the direct current (DC) bias voltage and is set to 450,000 in the presence of an aquatic medium. The ability to adjust the quality factor of the reported microwave sensor allows for larger dynamic range and permittivity variation monitoring. Different shifts in resonant frequency are reported in the presence of various liquid samples in tubes which are submerged in deionized (DI) water. The proposed device is used to distinguish between Water, Ethanol, Methanol, Isopropanol, and Acetone in a submerged tube inside a water filled container.

Hotspot relaxation dynamics in a current-carrying superconductor

Abstract: We experimentally studied the dynamics of optically excited hotspots in current-carrying WSi superconducting nanowires as a function of bias current, bath temperature, and excitation wavelength. We observed that the hotspot relaxation time depends on bias current, temperature, and wavelength. We explained this effect with a model based on quasiparticle recombination, which provides insight into the quasiparticle dynamics of superconductors.

Tunable two dimensional optical beam steering with reconfigurable indium tin oxide plasmonic reflectarray metasurface

Abstract: In this paper, an electrically tunable reflectarray metasurface for two dimensional (2D) beam steering is designed by integration of a thin layer of indium tin oxide (ITO) material into a metal-insulator-metal (MIM) plasmonic unit-cell. The reflectarray is composed of square-shaped patch nanoantennas placed on a stack of insulator-ITO-metallic ground plane. The resonant characteristic of unit-cell and the accumulation of carrier density at the interface of ITO-insulator play key roles in obtaining over 250° of phase agility at around 218 terahertz (THz), by electrically varying the bias voltage. An array of unit-cells with integrated ITO and 2D voltage biasing distribution (from the side) offer the possibility of designing a reconfigurable antenna in which the main beam can be steered to relatively large angles in both θ- and - planes at carefully selected operating frequency. The significant advantage of this design is the dynamically adjustable radiation pattern in both azimuth and elevation planes even after fabrication.

Gate-tunable rectification inversion and photovoltaic detection in graphene/WSe2 heterostructures

Abstract: We studied electrical transport properties including gate-tunable rectification inversion and polarity inversion, in atomically thin graphene/WSe2 heterojunctions. Such engrossing characteristics are attributed to the gate tunable mismatch of Fermi levels of graphene and WSe2. Also, such atomically thin heterostructure shows excellent performances on photodetection. The responsivity of 66.2 mA W−1 (without bias voltage) and 350 A W−1 (with 1 V bias voltage) can be reached. What is more, the devices show great external quantum efficiency of 800%, high detectivity of 1013 cm Hz1/2/W, and fast response time of 30 μs. Our study reveals that vertical stacking of 2D materials has great potential for multifunctional electronic and optoelectronic device applications in the future.

4H-SiC p-i-n diode as Highly Linear Temperature Sensor

Abstract: The linear dependence on temperature of the voltage drop $V_{D}$ across a forward-biased 4H-SiC p-i-n diode is investigated experimentally. The results show that the fabricated temperature sensor has a high degree of linearity in the range from room temperature up to 573 K corresponding to a root-mean-square error lower than 0.5%. A maximum sensitivity of 2.66 mV/K was calculated. The low saturation current of the p-i-n diode, well below the forward biasing current also at high temperatures, reduces the nonlinear effects in the $V_{D}$ – $T$ characteristic allowing the design and fabrication of highly linear sensors operating in a wider temperature range.

A High-Power Widely Tunable Limiter Utilizing an Evanescent-Mode Cavity Resonator Loaded With a Gas Discharge Tube

Abstract: A new plasma-based widely tunable power limiter is introduced and experimentally investigated in this paper. The proposed limiter is composed of a high- $Q$ evanescent-mode cavity resonator loaded with a gas discharge tube in its gap area over the post. Increasing input power results in enhancing electric field over the gap, which eventually leads to microwave gas breakdown and plasma formation. The limiter loss after the breakdown depends on the electron density in the tube, which itself is a function of input power. Hence, the limiting mechanism is self-sustained. The threshold power is tunable either coarsely by selecting different tubes with various breakdown voltages or finely by dc biasing the tube. In a fabricated proof-of-concept limiter at 2.16 GHz with 22% bandwidth, preionization by an external dc bias provided 15.5 dB of limiting power tunability in the range of 370 mW–13 W, while the limiter could easily handle up to 100-W input power. The response time of this limiter is $ \alpha $ -discharge regime, the limiter performance is quite stable and no degradation was observed after a long operation under high input power.

85–440 K Temperature Sensor Based on a 4H-SiC Schottky Diode

Abstract: The performance of a 4H-SiC Schottky diode for thermal sensing in the wide temperature range from $T=85$ up to 443 K is presented. The linear dependence on temperature of the forward voltage drop, for different bias currents, is investigated through an analytical study of the temperature-dependent physical Schottky diode parameters. A high sensitivity of 1.18 mV/K was observed for a constant bias current of $I_{D}=80 \,\,\mu \text{A}$ . The device exhibits a good degree of linearity with a calculated root mean square error, with respect to the best-linear fitting model, lower than 2.7 mV. Moreover, the proposed sensor shows a good repeatability maintaining a stable output over more cycles of measurements, from (down to) 85 up to (from) 443 K, in a long period of time.

Nonequilibrium spin injection in monolayer black phosphorus

Abstract: Monolayer black phosphorus (MBP) is an interesting emerging electronic material with a direct band gap and relatively high carrier mobility. In this work we report a theoretical investigation of nonequilibrium spin injection and spin-polarized quantum transport in MBP from ferromagnetic Ni contacts, in two-dimensional magnetic tunneling structures. We investigate physical properties such as the spin injection efficiency, the tunnel magnetoresistance ratio, spin-polarized currents, charge currents and transmission coefficients as a function of external bias voltage, for two different device contact structures where MBP is contacted by Ni(111) and by Ni(100). While both structures are predicted to give respectable spin-polarized quantum transport, the Ni(100)/MBP/Ni(100) trilayer has the superior properties where the spin injection and magnetoresistance ratio maintains almost a constant value against the bias voltage. The nonequilibrium quantum transport phenomenon is understood by analyzing the transmission spectrum at nonequilibrium.

Intense THz Pulses with large ponderomotive potential generated from large aperture photoconductive antennas.

Abstract: We report the generation of free space terahertz (THz) pulses with energy up to 8.3 ± 0.2 µJ from an encapsulated interdigitated ZnSe Large Aperture Photo-Conductive Antenna (LAPCA). An aperture of 12.2 cm2 is illuminated using a 400 nm pump laser with multi-mJ energies at 10 Hz repetition rate. The calculated THz peak electric field is 331 ± 4 kV/cm with a spectrum characterized by a median frequency of 0.28 THz. Given its relatively low frequency, this THz field will accelerate charged particles efficiently having very large ponderomotive energy of 15 ± 1 eV for electrons in vacuum. The scaling of the emission is studied with respect to the dimensions of the antenna, and it is observed that the capacitance of the LAPCA leads to a severe decrease in and distortion of the biasing voltage pulse, fundamentally limiting the maximum applied bias field and consequently the maximum energy of the radiated THz pulses. In order to demonstrate the advantages of this source in the strong field regime, an open-aperture Z-scan experiment was performed on n-doped InGaAs, which showed significant absorption bleaching.

Surface state controlled ultrahigh selectivity and sensitivity for UV photodetectors based on individual SnO2 nanowires

Abstract: For nanostructures with a large surface-to-volume ratio, quantities of dangling bonds can create affluent surface states due to the breaking of lattice periodicity on their surfaces. Herein, surface states are utilized for the development of high-performance photodetectors based on individual SnO2 nanowires. The photodetectors not only show ultrahigh selectivity and sensitivity to ultraviolet (UV) light of about 340 nm with and without bias, but also show weak response to sub-bandgap visible (VIS) and near infrared (NIR) light. Moreover, their detectivity is strongly dependent on externally applied bias voltage and illumination light intensity. The abundant surface states result in the formation of back-to-back diodes for two-terminal nanowire-based devices. The photoconductive mechanism is controlled by a photovoltaic effect of a surface state related reverse biased diode in contact with a negative electrode. Under light illumination, the efficient separation of photoexcited carriers at the Ag and SnO2 interface and the fast drift of electrons separated into the core of n-type SnO2 nanowires toward another end lead to ultrahigh gain, a large photo-to-dark current ratio, and high-speed response for the nanostructure photodetectors at a relatively high bias voltage. These results demonstrate that the surface states of nanostructures are promising for the development of high performance photodetectors, and meanwhile render an interpretation particularly valuable for the photoconductive mechanism of nanostructures.

Impedance spectra of Fe-doped SrTiO3 thin films upon bias voltage: inductive loops as a trace of ion motion

Abstract: Mass and charge transport properties of slightly Fe-doped SrTiO3 (Fe:STO) thin films on a conducting substrate were investigated by means of impedance spectroscopy under different bias voltages and I–V measurements with varying scan rates. At measurement temperatures between 325 °C and 700 °C the applied bias voltage caused an unusual “inductive loop” in the low frequency range of impedance spectra. DC measurements showed that current–voltage curves strongly depend on the scan rate, indicating that different states of the sample became accessible to probe. Both findings can be understood in terms of bias induced ion motion, i.e. by stoichiometry polarization within the Fe:STO thin films upon voltage. Hence, the appearance of an “inductive loop” in the impedance spectra is considered a very general feature that might exist for many materials, particularly in oxide thin films. It may indicate ion motion and stoichiometry variations taking place in the corresponding frequency range.

Membrane distributed-reflector laser integrated with SiOx-based spot-size converter on Si substrate.

Abstract: We demonstrate monolithic integration of a 50-μm-long-cavity membrane distributed-reflector laser with a spot-size converter, consisting of a tapered InP wire waveguide and an SiOx waveguide, on SiO2/Si substrate. The device exhibits 9.4-GHz/mA0.5 modulation efficiency with a 2.2-dB fiber coupling loss. We demonstrate 25.8-Gbit/s direct modulation with a bias current of 2.5 mA, resulting in a low energy cost of 132 fJ/bit.

Composite biasing in Monte Carlo radiative transfer

Abstract: Biasing or importance sampling is a powerful technique in Monte Carlo radiative transfer, and can be applied in different forms to increase the accuracy and efficiency of simulations. One of the drawbacks of the use of biasing is the potential introduction of large weight factors. We discuss a general strategy, composite biasing, to suppress the appearance of large weight factors. We use this composite biasing approach for two different problems faced by current state-of-the-art Monte Carlo radiative transfer codes: the generation of photon packages from multiple components, and the penetration of radiation through high optical depth barriers. In both cases, the implementation of the relevant algorithms is trivial and does not interfere with any other optimisation techniques. Through simple test models, we demonstrate the general applicability, accuracy and efficiency of the composite biasing approach. In particular, for the penetration of high optical depths, the gain in efficiency is spectacular for the specific problems that we consider: in simulations with composite path length stretching, high accuracy results are obtained even for simulations with modest numbers of photon packages, while simulations without biasing cannot reach convergence, even with a huge number of photon packages.