Intrinsic Response Time Research Articles

Mixed regime of light-matter interaction revealed by phase sensitive measurements of the dynamical Franz-Keldysh effect

Significant changes of the optical properties of semiconductors can be observed by applying strong electric fields capable to modify the band structure at equilibrium. This is known as the Franz-Keldysh effect (FKE). Here we study the FKE in bulk GaAs by combining single cycle THz pumps and broadband optical probes. The experiments show that the phase content of the selected electromagnetic pulses can be used to measure the timescales characteristic for the different regimes of matter-light interactions. Furthermore, the present phase-resolved measurements allow to identify a novel regime of saturation where memory effects are of relevance.

Thermal emittance and response time of a cesium antimonide photocathode

Measurements of the intrinsic emittance and response time of a Cs3Sb photocathode are presented. The emittance is obtained with a solenoid scan technique using a high voltage dc photoemission gun. Photoemission response time is evaluated using a RF deflecting cavity synchronized to a picosecond laser pulse train. We find that Cs3Sb has both small mean transverse energy, 160 ± 10 meV at 532 nm laser wavelength, and a prompt response time (below the resolution of our measurement) making it a suitable material for high brightness electron photoinjectors.

Intrinsic Response Time of Graphene Photodetectors

Graphene-based photodetectors are promising new devices for high-speed optoelectronic applications. However, despite recent efforts it is not clear what determines the ultimate speed limit of these devices. Here, we present measurements of the intrinsic response time of metal–graphene–metal photodetectors with monolayer graphene using an optical correlation technique with ultrashort laser pulses. We obtain a response time of 2.1 ps that is mainly given by the short lifetime of the photogenerated carriers. This time translates into a bandwidth of ∼262 GHz. Moreover, we investigate the dependence of the response time on gate voltage and illumination laser power.

A note on the periodicities in the logistic analyses of growth processes

In real life growth processes there is always an intrinsic finite response time. This means that these systems do not respond instantaneously so that the logistic modeling of such processes should be formally described by a delayed logistic equation. This poses enormous difficulties to their time evolution modeling as there are no analytical solutions to the delayed logistic equation. In this paper we show that by performing a conventional Verhulst logistic modeling of the corresponding time series data, associated with a fine-coarse analysis of the resulting residuals, we can disclose the underlying periodicities due to the finite time response effects.

Time resolving characteristics of HPK and FBK silicon photomultipliers for TOF and PET applications

In time-of-flight measurements, or positron emission tomography experiments where two gamma rays are detected in coincidence, the time resolution of the photodetector is of primary importance. SiPMs are very promising devices for these applications, since their intrinsic response time can be less than 1 ns. However the actual timing resolution of SiPM is affected by the area (capacitance) of the device, by the type of used to pre-amplify the signal, by the dark count rate which is revealed as pure noise, and other second order effects like cross-talk and after pulsing. In this work we report the characteristics of different samples of Hamamatsu Photonics (HPK) and Fondazione Bruno Kessler (FBK) SiPM, with pixel size ranging from 40 to 100 μm. In particular, we have investigated their time response when stimulated with O(50) ps pulsed laser at wavelengths in the range 400–800 nm.

Microfluidic cell sorter with integrated piezoelectric actuator

We demonstrate a low-power (<0.1 mW), low-voltage (<10 Vp-p) on-chip piezoelectrically actuated micro-sorter that can deflect single particles and cells at high-speed. With rhodamine in the stream, switching of flow between channels can be visualized at high actuation frequency (~1.7 kHz). The magnitude of the cell deflection can be precisely controlled by the magnitude and waveform of input voltage. Both simulation and experimental results indicate that the drag force imposed on the suspended particle/cell by the instantaneous fluid displacement can alter the trajectory of the particle/cell of any size, shape, and density of interest in a controlled manner. The open-loop E. Coli cell deflection experiment demonstrates that the sorting mechanism can produce a throughput of at least 330 cells/s, with a promise of a significantly higher throughput for an optimized design. To achieve close-loop sorting operation, fluorescence detection, real-time signal processing, and field-programmable-gate-array (FPGA) implementation of the control algorithms were developed to perform automated sorting of fluorescent beads. The preliminary results show error-free sorting at a sorting efficiency of ~70%. Since the piezoelectric actuator has an intrinsic response time of 0.1–1 ms and the sorting can be performed under high flowrate (particle speed of ~1–10 cm/s), the system can achieve a throughput of >1,000 particles/s with high purity.

A Simulation Study of Magnetostrictive Material Terfenol-D in Automotive CNG Fuel Injection Actuation

Magnetostriction is the deformation that spontaneously occurs in ferromagnetic materials when an external magnetic field is applied. In applications broadly defined for actuation, magnetostrictive material Terfenol-D (Tb0.3Dy0.7Fe1.9) possesses intrinsic rapid response times while providing small and accurate displacements and high-energy efficiency. These are some of the essential parameters required for fast control of fuel injector valves for decreased engine emissions and lower fuel consumption compared with the traditional solenoid fuel injection system. A prototype CNG fuel injector assembly was designed which included magnetostrictive material Terfenol-D as the actuator material. A 2D cross-sectional geometry of the injector assembly, which incorporated both linear and non-linear magnetic properties of the corresponding materials, was modeled in ANSYS for 2D axisymmetric magnetic simulation. Subsequently, a 3D replica of the CNG flow conduit was modeled in GAMBIT with the resultant injector lift. The meshed conduit was then simulated in FLUENT using the 3D time independent segregated solver with the Standard k  , the Realizable k   and RSM turbulence models to predict the mass flow rate of CNG to be injected. Eventually, the simulated flow rate was verified against mathematically derived static flow rate required for a standard automotive fuel injector considering standard horsepower, BSFC and injector duty cycle.

Radio frequency pulse response of an in-plane-gate field effect transistor

We report radio frequency responses of an in-plane-gate field effect transistor (IPGFET) using an electrical pulse and probe technique. A series of high frequency pulses with the frequency up to 3 GHz were superimposed on the DC gate bias, and the time-averaged drain current was measured. The frequency and amplitude dependence of the time-averaged current depended on both the intrinsic gate response time of the IPGFET and the charging/discharging time of the traps at the etched surface.

Specific features of the development of differential-phase transformer protection systems on the basis of magnetic reed switches

It shown that the magnetic reed switches installed close to flexible current leads of power transformers with power over 6.3 (40) MV A and high voltage 10 (35) kV may be used as a basis of phase-comparison circuits with specified boundary angles in the devices for the protection of these transformers against short circuit. The response threshold of these circuits, the errors caused by the differences between the intrinsic response times, inaccuracies in settings of magnetic reed switches, and regulation of the transformer voltage are estimated. A technique for suppressing the effects of adjacent phases and electric installations is presented.

Highly nonlinear chalcogenide fibres for all-optical signal processing

Chalcogenide glasses offer many attractive properties for all-optical signal processing including large Kerr nonlinearity (up to 500 × silica glass), an intrinsic ultrafast response time and low to moderate two-photon absorption (TPA). These properties together with the convenience of a fibre format allow us to achieve all-optical signal processing at low peak power and in a very compact form. In this paper, several nonlinear processing functions will be demonstrated including: femto-second pedestal free, pulse compression; all-optic wavelength conversion; and all-optical regenerator. In addition, we show enhanced nonlinearity for more efficient signal processing by tapering the As2Se3 fibre. These applications show chalcogenide glass fibres are very promising candidate materials for nonlinear all-optic signal processing.

Control of the non-Markovian dynamics of a qubit

Solid-state implementations of a qubit based on semiconductor quantum dots or Josephson junctions are studied within a driven spin-boson model with Ohmic spectral function. It is shown that direct or indirect optimization allows selection of simple control fields which drive the dissipative system from a given initial state to a selected target state provided that the system’s intrinsic minimal response times are respected. The control field can be used to establish interference between the system-bath and system-control interactions, leading to control over effective transition rates and the dissipative action of the environment.

Gas response times of nano-scale SnO 2 gas sensors as determined by the moving gas outlet technique

We report on measurements of the gas response time of pure and platinum (Pt) catalysed SnO 2 films using the moving gas outlet method. In this mode of operation changing gas flows are forced directly onto the sensor surface to avoid the formation of a stagnant layer and to reveal gas response times unaffected by the usual diffusive delays. We find that the intrinsic response and recovery time constants τ follow an exponential temperature dependence τ( T) = τ 0exp( E a/ k B T), with E a standing for the activation energies for adsorption or desorption, respectively. We find that catalytic enhancement lowers the activation energies both for adsorption and desorption, with the more pronounced changes generally taking place in the case of the adsorption time constants. We further show that catalyst-induced changes in E a are accompanied by changes in the prefactor τ 0 which partly compensate the accelerating effect of a lower E a.

Exhaust oxygen sensor dynamic study

We used transfer function approach to investigate the dynamics of oxygen sensors in engine exhaust environment, operated in both Lambda and wide range sensing modes. We measured the sensor transfer functions of the sensor responses and compared with the model. All the dynamic mechanisms involved were identified. The dynamic contributions are from the louver-shield, the protection coating-layer, the sensing electrode of the sensor; the exhaust gas pipe, the engine wall-wetting effect, the sample-and-hold effect of the discrete individual cylinder event, and the engine controller module. The engine wall-wetting effect shaped the measured sensor transfer function. The exhaust pipe caused most of the signal delay but no distortion to the signal strength. Within the exhaust sensor, the coating layer and the electrode determined the intrinsic sensor response time.

Climate forcing by the volcanic eruption of Mount Pinatubo

We determine the volcano climate sensitivity λ and response time τ for the Mount Pinatubo eruption, using observational measurements of the temperature anomalies of the lower troposphere, measurements of the long wave outgoing radiation, and the aerosol optical density. Using standard linear response theory we find λ = 0.15 ± 0.06 K/(W/m2), which implies a negative feedback of −1.4 (+0.7, −1.6). The intrinsic response time is τ = 6.8 ± 1.5 months. Both results are contrary to a paradigm that involves long response times and positive feedback.

Comment on “Climate forcing by the volcanic eruption of Mount Pinatubo” by David H. Douglass and Robert S. Knox

Comment on “Climate forcing by the volcanic eruption of Mount Pinatubo” by David H. Douglass and Robert S. Knox

Design and optimization of GaAs∕AlGaAs heterojunction infrared detectors

Design, modeling, and optimization principles for GaAs∕AlGaAs heterojunction interfacial workfunction internal photoemission (HEIWIP) infrared detectors for a broad spectral region are presented. Both n-type and p-type detectors with a single emitter or multiemitters, grown on doped and undoped substrates are considered. It is shown that the absorption, and therefore responsivity, can be increased by optimizing the device design. Both the position and the strength of the responsivity peaks can be tailored by varying device parameters such as doping and the thickness. By utilizing a resonant cavity architecture, the effect of a buffer layer on the response is discussed. Model results, which are in good agreement with the experimental results, predict an optimized design for a detector with a peak response of 9A∕W at 26μm with a zero response threshold wavelength λ0=100μm. For a λ0=15μm HEIWIP detector, background limited performance temperature (BLIP temperature), for 180° field of view (FOV) is expected around 80K. For a λ0=70μm optimized design, a highly doped n-type substrate could increase the peak detectivity from 1.7×1010to3.4×1010Jones at a FOV=180° operated at temperatures below T&amp;lt;TBLIP=13K. Intrinsic response times on the order of picoseconds are expected for these detectors.

Ultrafast superconducting single-photon detectors for near-infrared-wavelength quantum communications

The paper reports progress on the design and development of niobium-nitride, superconducting single-photon detectors (SSPDs) for ultrafast counting of near-infrared photons for secure quantum communications. The SSPDs operate in the quantum detection mode, based on photon-induced hotspot formation and subsequent appearance of a transient resistive barrier across an ultrathin and submicron-width superconducting stripe. The devices are fabricated from 3.5 nm thick NbN films and kept at cryogenic (liquid helium) temperatures inside a cryostat. The detector experimental quantum efficiency in the photon-counting mode reaches above 20% in the visible radiation range and up to 10% at the 1.3–1.55 μn infrared range. The dark counts are below 0.01 per second. The measured real-time counting rate is above 2 GHz and is limited by readout electronics (the intrinsic response time is below 30 ps). The SSPD jitter is below 18 ps, and the best-measured value of the noise-equivalent power (NEP) is 2 × 10−18 W/Hz1/2. at 1.3 μm. In terms of photon-counting efficiency and speed, these NbN SSPDs significantly outperform semiconductor avalanche photodiodes and photomultipliers.

Chemical Sensing Systems Using Xerogel-Based Sensor Elements and CMOS Photodetectors

We present the first example of an integrated complementary metal-oxide-semiconductor (CMOS) photodetector coupled with a solid-state xerogel-based thin-film sensor to produce a compact chemical sensor system. We compare results using two different CMOS-based detector systems to results obtained by using a standard photomultiplier tube (PMT) or charge-coupled device (CCD) detector. Because the chemical sensor elements are governed by a Stern-Volmer relationship, the Stern-Volmer quenching constant is used as the primary comparator between the different detectors. All of the systems yielded Stern-Volmer constants from 0.042 to 0.049 O/sub 2/%/sup -1/. The results show that the CMOS detector system yields analytical data that are comparable to the CCD- and PMT-based systems. The disparity between the data obtained from each detector is primarily associated with the difference in how the signals are obtained by each detector as they presently exist. We have also observed satisfactory reversibility in the operation of the sensor system. The CMOS-based system exhibits a response time that is faster than the chemical sensor element's intrinsic response time, making the CMOS suitable for time-dependent measurements. The CMOS array detector also uses less than 0.1% the power in comparison to a standard PMT or CCD. The combined xerogel/CMOS system represents an important step toward the development of a portable, efficient sensor system.

Nano-structured superconducting single-photon detectors

NbN detectors, formed into meander-type, 10×10-μm2 area structures, based on ultrathin (down to 3.5-nm thickness) and nanometer-width (down to below 100nm) NbN films are capable of efficiently detecting and counting single photons from the ultraviolet to near-infrared optical wavelength range. Our best devices exhibit QE >15% in the visible range and ∼10% in the 1.3–1.5-μm infrared telecommunication window. The noise equivalent power (NEP) ranges from ∼10−17W/Hz1/2 at 1.5μm radiation to ∼10−19W/Hz1/2 at 0.56μm, and the dark counts are over two orders of magnitude lower than in any semiconducting competitors. The intrinsic response time is estimated to be <30ps. Such ultrafast detector response enables a very high, GHz-rate real-time counting of single photons. Already established applications of NbN photon counters are non-invasive testing and debugging of VLSI Si CMOS circuits and quantum communications.

Time-resolved x-ray diffraction study of the relaxation process of electric-field-induced strain in KD2PO4

In this article, we report on measurements of the strain relaxation process in KD2PO4. The application of a static electric field to a piezoelectric crystal generates stresses, which are released by the introduction of strain. Time-resolved x-ray diffraction experiments on square platelike samples indicated that the strain relaxation process strongly depends on the rise time of the electric field. In the case of an instantaneously applied electric field (rise time 30 ns, i.e., much shorter than the response time of the crystal), the induced strain waves were only weakly damped and still present 400 μs after activating the external electric field (which corresponds to roughly 50 round trips of the strain wave). The strain waves can be suppressed completely by employing a rise time equivalent to the intrinsic response time of the crystal. Measurements on a bar-shaped crystal indicated that the strain waves are generated at the edges of the crystal and propagate along the length of the crystal at the speed of sound. This was concluded from the time of arrival of the strain waves, as well as from the measured vibration frequency.