Input Power Density Research Articles

A kinetic study of electron heating and plasma dynamics in microwave microplasmas

Microwave microplasmas ignited in argon are studied using a one-dimensional particle-in-cell with Monte Carlo collision (PIC-MCC) approach. One-dimensional PIC-MCC simulations are performed at specified input power densities to determine the influence of the applied frequency (ranging from 1 to 320 GHz), pressure, and total deposited power on the plasma dynamics. The frequency response study performed at a fixed input power density shows the presence of off-axis peaks in the electron number density profile at intermediate frequencies. These peaks are attributed to the interplay between the production of hot electrons by the oscillating sheath and their inability to diffuse sufficiently at higher operating pressures, thereby resulting in enhanced ionization at off-axis locations. This is confirmed by the pressure dependence study which shows that the electron number density peaks at the mid-point when the microplasma is ignited at lower pressures. As the excitation frequency is increased further, the sheath oscillation heating decreases and eventually vanishes, thereby requiring the bulk plasma to couple power to the electrons which in turn leads to an increase in electron temperature in the plasma bulk and the electron number density peak appearing at the mid-point. When the power coupled to the microplasma is decreased, the sheath oscillation at a given frequency decreases, thereby leading to higher contribution from heating in the bulk plasma which leads to the disappearance of off-axis peaks even at intermediate frequencies. The microplasma dynamics at all conditions considered in this work demonstrate the interplay between the electron momentum transfer collision frequency, the angular excitation frequency, and the plasma frequency.

Absorption saturation measurement using the tapered optical nanofiber in a hot cesium vapor

We report the observation of ultralow-power absorption saturation in a tapered optical fiber (TOF) mounted in a hot cesium (Cs) vapor in a vacuum chamber. The small optical mode area of TOF produces a great influence on optical properties, allowing optical interactions with nanowatt-level power. The comparison of transmission characteristics for the TOF system and free-space vapor is investigated at different input power and atomic density. The unique performance of the Cs-TOF system makes it a promising candidate in resonant nonlinear optical applications with ultralow power.

Experimental and numerical study on thermal performance of Wood's alloy/expanded graphite composite phase change material for temperature control of electronic devices

The current investigation focuses on the feasibility of effectively managing the thermal conditions of electronic devices using Wood's alloy/expanded graphite composite phase change material (PCM). A thermal management system integrated with the composite PCM-based substrate is constructed and the thermal behaviors of this composite PCM under different testing conditions are studied through both experimental and numerical ways. The results indicate that the thermal performances of the composite PCM are dependent on input power density of the electronic chip, heat storage density and thermal conductivity of the PCM, and thickness of the PCM-based substrate. The temperature holding capability of the PCM can be obtained under high input power density. Under low input power density, the substrate only behaves as a heat spreader. Enhancing the heat storage density of the composite PCM is advantageous to obtain longer critical time (the required time for the chip to reach a critical temperature). Also, varying the thermal conductivity of the PCM from 14.9 to 59.6 W⋅m−1⋅K−1 leads to slight changes to the PCM's temperature control performance during the chip's working period under natural cooling conditions, whereas extending the thickness of the substrate can both lower the chip's equilibrium temperature and prolong the critical time.

Stability enhancement of yttrium substituted nickel zinc ferrite/PZT magnetoelectric gyrators under high power conditions

The influence of yttrium (Y) ion substitution on the magneto-mechanical properties of nickel-zinc ferrite (Ni0.7Zn0.3YxFe2-xO4) or NZFO-Y and the power efficiency (η) of magnetoelectric (ME) gyrators consisting of NZFO-Y/Pb(Zr, Ti)O3 layers were studied. X-ray diffraction (XRD) of polycrystalline samples of NZFO-Y indicated that yttrium incorporated into the crystal lattice resulted in an enlargement of the lattice parameter. Also, the density and ionic hopping length were increased, and the porosity was decreased. Together, these resulted in significant changes in the magneto-mechanical coupling (k33,m) and mechanical quality (Qm) factors. It was found that k33,m decreased with the increasing Y content for 0 ≤ x ≤ 0.08, whereas Qm was increased. For x > 0.05, the values of k33,m and Qm were found to be nearly stable with increasing input power density (PD-Ms) up to 47 W/in3. Under this high power driving condition for 20 min, we found that the power conversion stability of ME gyrators was increased with the increasing Y content in the ferrite and that a maximum power conversion efficiency occurred near 77%. These findings offer the potential for ferrite-based ME gyrator applications in power electronic conversion devices.

Thermoacoustic sound projector: exceeding the fundamental efficiency of carbon nanotubes

The combination of smooth, continuous sound spectra produced by a sound source having no vibrating parts, a nanoscale thickness of a flexible active layer and the feasibility of creating large, conformal projectors provoke interest in thermoacoustic phenomena. However, at low frequencies, the sound pressure level (SPL) and the sound generation efficiency of an open carbon nanotube sheet (CNTS) is low. In addition, the nanoscale thickness of fragile heating elements, their high sensitivity to the environment and the high surface temperatures practical for thermoacoustic sound generation necessitate protective encapsulation of a freestanding CNTS in inert gases. Encapsulation provides the desired increase of sound pressure towards low frequencies. However, the protective enclosure restricts heat dissipation from the resistively heated CNTS and the interior of the encapsulated device. Here, the heat dissipation issue is addressed by short pulse excitations of the CNTS. An overall increase of energy conversion efficiency by more than four orders (from 10−5 to 0.1) and the SPL of 120 dB re 20 μPa @ 1 m in air and 170 dB re 1 μPa @ 1 m in water were demonstrated. The short pulse excitation provides a stable linear increase of output sound pressure with substantially increased input power density (>2.5 W cm−2). We provide an extensive experimental study of pulse excitations in different thermodynamic regimes for freestanding CNTSs with varying thermal inertias (single-walled and multiwalled with varying diameters and numbers of superimposed sheet layers) in vacuum and in air. The acoustical and geometrical parameters providing further enhancement of energy conversion efficiency are discussed.

Innovative experimental setup for thermal and electrical characterization of silicon solar cells under controlled environmental conditions

The optimization of solar cells properties with thermal criteria gives the possibility to achieve higher conversion efficiency in outdoor conditions. An innovative setup that allows the control of the surroundings of a solar cell is described. Under such specific conditions, the cell temperature can be stabilized and measured. The variation of the cell temperature with the applied bias is experimentally observed and quantified. Between short circuit current density (Jsc) and open circuit voltage (Voc), a slight difference of temperature is observed, revealing a variation of the thermal equilibrium between these two points. The resistivity of the absorber and the input power density are found to influence this temperature shift. From the experimental results, it appears that the emissivity of the solar cell increases with the applied voltage due to an increase in the excess carrier concentration. Consequently, the operating temperature at open-circuit is lower than at short-circuit.

Flexible transparent conducting films with embedded silver networks composed of bimodal-sized nanoparticles for heater application

A facile one-pot synthetic method for preparing the Ag nanoparticle inks with a bimodal size distribution was newly devised and they were successfully employed as a conducting filler to form the metal-mesh type transparent conducting electrodes on the flexible substrate. Bimodal-sized Ag nanoparticles were synthesized through the polyol process, and their size variation was occurred via finely tuned composition ratio between Ag+ ions and polymeric capping agents. The prepared bimodal-sized Ag nanoparticles exhibited the form of well-dispersed Ag nanoparticle inks without adding any dispersants and dispersion process. By filling the patterned micro-channels engraved on the flexible polymer substrate using a bimodal-sized Ag nanoparticle ink, a metal-mesh type transparent electrode (transmittance: 90% at 550 nm, haze: 1.5, area: 8 × 8 cm2) was fabricated. By applying DC voltage to the mesh type electrode, a flexible transparent joule heater was successfully achieved with a performance of 4.5 °C s−1 heat-up rate at a low input power density.

Numerical Investigation on the Electrical Characteristics and Electron Energy Transformation of Pulsed Dielectric Barrier Discharge for Ozone Generation

A numerical model consisting of 12 species and 65 reactions is developed and experimentally verified to be valid for the investigation of electrical characteristics and electron energy transformation in oxygen-fed pulsed dielectric barrier discharge (DBD) for ozone generation. The simulation results show that there are two obvious discharges with opposite polarity in one pulse which agrees with many experimental observations. The first discharge is at the rising edge of pulse voltage, and the second discharge is at the falling edge. The former has a much higher current density than that of the latter. Moreover, a higher peak voltage results in an earlier and higher ignition voltage as well as a higher maximum current density. The contrary behavior for rising time is observed, and the ignition and maximum current density are independent of pulsewidth. In addition, the temporal distributions of total input power density, electron power density, and power density of electron consumed by reactions are obtained for the first discharge in DBD for the ozone generation. Only 19.35% of the total input energy within the first discharge is absorbed by electrons when peak voltage, rising time, and pulsewidth are 9 kV, 63.8 ns, and 100 ns, respectively, and 41.09% of electron energy is utilized effectively to form ozone.

Enhanced stability of magnetoelectric gyrators under high power conditions

In this study, three different coil-based magnetoelectric (ME) gyrators of different geometries, including gyrators with high power output, have been designed and characterized. These included two magnetostrictive/piezoelectric/magnetostrictive (M-P-M) and one piezoelectric/magnetostrictive/piezoelectric (P-M-P) type ME gyrators, which consisted of nickel zinc ferrite (NZFO) and lead zirconate titanate (PZT) ceramic plates. Compared with M-P-M ME gyrators, the P-M-P ones exhibited a higher power efficiency (η) of 85% when operated at resonance under an optimal magnetic bias field (HBias) of 40 Oe at low power conditions. It retained a relatively high efficiency of η = 79% under a high input power density of 2.87 W/cm3. A low reduction in the magnetomechanical coupling and mechanical quality (k33,m and Qm) factors of the NZFO ferrite layer in the ME gyrator explains the resilience of the P-M-P type structure with increasing power drive. The findings open the possibility of using ME gyrators in high power applications.

Computer Model for EDFA Dynamics Over 1525–1560 nm Band Using a Novel Multi-Wavelength MATLAB Simulink Test Bed for 8-Channels

ABSTRACTA novel erbium-doped fibre amplifier (EDFA) test bed is designed using the dominant technical MATLAB Simulink platform to characterize gain, noise figure and amplified spontaneous emission (ASE) power variations of a forward pumped EDFA operating as functions of amplifier length, injected pump power, signal input power and doping density in the C-band (1525–1560 nm). The numerical modelling of the rate and propagation equations is done. The Simulink model for two-level-based analysis is designed to investigate the dynamic characteristics of an EDFA. A significant addition to the model is the ability to handle multiple channels using the absorption and emission coefficients that vary as per different specifications and the quantum well laser is being modelled to be applied as the pump source. The results are shown and analyzed graphically for designated simulation environment that accurately represents EDFA performance metrics such as gain, population inversion factor , noise figure, quantum conversion efficiency and power conversion efficiency. The present model can be implemented successfully as a test bed to study the EDFA dynamics over the entire third optical communication bandwidth (1525–1690 nm) with the ability to analyze and optimize all the parameters to achieve the desired operating performance before actual physical designing of device as per the application.

Enhancement of electrothermal performance in single-walled carbon nanotube transparent heaters by room temperature post treatment

High-performance transparent heaters were fabricated on glass substrate using spray coating the solution processed single walled carbon nanotubes (SWCNT). Transparent heaters were treated with acid and alcohol mixture (10:1) to display high heating performance. The post treated SWCNT heater with 81% transmittance at 550nm reached a steady-state temperature of 424K with a heating rate of 0.6K/s when 60V is applied. The enhancement of electrothermal performance was studied in terms of heating rate, input power density and applied voltage. The enhanced heating performance attributed due to the effective wetting of the SWCNT network which intern reduce the sheet resistance of the conducting films. Defrosters realized using the post treated heater able to remove frost within a minute when 30V was applied. The excellent power dependent heating performance with high stability indicates that the SWCNT heaters can be used in the temperature controlled heaters, smart windows and defrosters.

(Invited) Non-Thermal Plasmas for the Production of Sustainable Functional Materials

This talk will highlight the potential of non-thermal plasmas as a viable synthetic approach for materials that are difficult to produce using other techniques. Materials such as titanium and zirconium nitride are impossible to produce in nanoparticle form using liquid phase chemistry techniques, and are not compatible with approaches such as flame and spray pyrolysis because they are not oxide-based ceramics. At the same time, they are quickly attracting interest because they are predicted to show plasmonic response in the visible, making them great candidates for the replacement of silver and gold-based plasmonics. Our recent investigation in this material system confirms that low-temperature, non-thermal plasma processes are compatible with the synthesis of titanium and zirconium nitride nanocrystals. Particles are produced using metal chlorides and ammonia as precursors. The plasma reactor is designed in a continuous flow configuration, where precursors are continuously fed into the plasma volume and nanoparticles are continuously collected downstream of the plasma by filtering. This technique allows producing TiN and ZrN nanocrystals with average size below 10 nm, with narrow size distribution (all particles are within 5 nm and 15 nm) and with perfect stoichiometry. In partial agreement with the theoretical predictions, such particles show plasmon resonance in the visible and in the near-infrared part of the spectrum. Yet the plasmon energy is lower than theoretically predicted, a behavior that has not been observed before and that we attribute to the presence of a native oxide shell. This observation is justified by simple experiments in which the particles are annealed in air at moderate temperature (<300°C), leading to the progressive growth of an oxide shell, as confirmed by elemental analysis and by XPS. The plasmon peak energy continuously red shifts during the annealing process, suggesting that a strategy for limiting oxide growth needs to be developed to enable the use of these nanomaterials for refractory plasmonic applications. In addition to discussing the properties of these novel plasma-produce nanomaterials, this talk will summarize our most recent findings with respect of their nucleation and growth. A combination of in-situ FTIR and optical emission spectroscopy measurements suggests that the nucleation process is initiated via atomic hydrogen-induced dissociation of the chlorine-based metal precursor, in this case titanium tetrachloride. This talk will also cover our most recent studies on the interaction between nanoparticles and plasmas. Despite operating near room temperature, and despite the short residence time (tens to hundreds of milliseconds) these systems are capable of producing nanocrystals of high melting point materials. Moreover, there is mounting evidence that such systems are compatible with the production of a broad range of high melting point materials. Therefore it is necessary to achieve a detailed understanding of the phenomena occurring in such reactors to fully take advantage of their capabilities as a nanomaterial synthetic approach. Using the silane-based chemistry as a testbed, we have performed careful in-situ FTIR characterization of the surface of silicon nanoparticles immersed in a non-thermal plasma. The hydrogen surface coverage decreases as the input power density increases. We attribute this to plasma-induced heating of nanoparticles to temperatures sufficiently high to lead to hydrogen desorption. This observation confirms the prediction of several theoretical studies that there is a non-equilibrium between nanoparticle temperature in a dusty plasma and the background gas temperature.

Nanolithography based on two-surface-plasmon-polariton-absorption

Lithography is one of most important technologies for fabricating micro- and nano-structures. Limited by the light diffraction limit, it becomes more and more difficult to reduce the feature size of lithography. Surface plasmon polariton (SPP) is due to the interaction between electromagnetic wave and oscillation of free-electron on metal surface. For the shorter wavelength, higher field intensity and abnormal dispersion relation, the SPP would play an important role in breaking through the diffraction limit and realizing nanolithography. In this paper, we theoretically and experimentally study the optical nonlinear effect of SPP (two-SPP-absorption) in the photoresist and its application of nanolithography with large field. First, the concept and features of two-SPP-absorption are introduced. Like two-photo-absorption, the two-SPP-absorption based lithography is able to realize nanopatterns beyond the diffraction limit: 1) the absorption rate quadratically depends on the light intensity, which can further squeeze the exposure spot; 2) the pronounced power threshold provides a possibility for precisely controlling the linewidth by manipulating the illumination power. Nevertheless, unlike the two-photo-absorption lithography which focuses light onto a single spot and scans point by point, the two-SPP-absorption method could obtain the subwavelength field pattern by simply illuminating the plasmonic mask. The subwavelength field pattern due to the short wavelength of SPP would further result in the overcoming-diffraction-limit resist pattern. Besides, the highly concentrated SPP field leads to the strong electromagnetic field enhancement at the metal-dielectric interface, which could reduce the input power density of exposure source or enlarge the exposure area. Then the two-SPP absorption is realized under the illuminations of femtosecond lasers with vacuum wavelengths of 800 nm and 400 nm. Meanwhile, the interference periodic patternis realized and it is observed that the linewidth could be adjusted by controlling the exposure dose. The minimum linewidth of resist pattern is only one tenth of the vacuum wavelength. By utilizing the features of two-SPP-absorption, namely shorter wavelength, enhanced field and threshold effect, the lithography field could be of millimeter size, which is about four to five orders of magnitude larger than the characteristic size of nanostructure. Therefore, this two-SPP-absorption scheme could be used for large-area plasmonic lithography beyond the diffraction limit with the help of various plasmonic structures and modes.

The OES Diagnosis in Removal of HCHO by the Uniform Bipolar Nanosecond-Pulsed DBD Using Wire-Cylinder Electrode Configuration in Atmospheric N2

In this paper, the uniform bipolar nanosecondpulsed dielectric barrier discharge (NPDBD) with 15-ns rising time generating in a wire-cylinder reactor is employed to remove the gaseous formaldehyde in atmospheric pressure N 2 . The image, the waveforms of pulse voltage and discharge current, the optical emission spectra of the discharges, and the removal rate of formaldehyde are recorded and calculated, respectively. It is found that the emission intensity of OH (A 2 Σ → X 2 Π, 0-0) rises when increasing the input power density (PD). Furthermore, with the increase of input PD and the emission intensity of OH (A 2 Σ → X 2 Π, 0-0), the removal rate of formaldehyde rises correspondingly. The results also indicate that when the O 2 concentration rises within 0%-5%, the removal rate of formaldehyde increases, however, when the O 2 concentration rises within 5%-20%, the formaldehyde removal rate decreases correspondingly. In addition, at the same conditions, the DBD synergistically removing formaldehyde with Al 2 O 3 and TiO 2 could increase the removal rate by 10%-30% compared with the discharge only.

Thermal gradient induced tweezers for the manipulation of particles and cells.

Optical tweezers are a well-established tool for manipulating small objects. However, their integration with microfluidic devices often requires an objective lens. More importantly, trapping of non-transparent or optically sensitive targets is particularly challenging for optical tweezers. Here, for the first time, we present a photon-free trapping technique based on electro-thermally induced forces. We demonstrate that thermal-gradient-induced thermophoresis and thermal convection can lead to trapping of polystyrene spheres and live cells. While the subject of thermophoresis, particularly in the micro- and nano-scale, still remains to be fully explored, our experimental results have provided a reasonable explanation for the trapping effect. The so-called thermal tweezers, which can be readily fabricated by femtosecond laser writing, operate with low input power density and are highly versatile in terms of device configuration, thus rendering high potential for integration with microfluidic devices as well as lab-on-a-chip systems.

Low Threshold Light Emission from Reverse-Rib n+ Ge Cavity Made by P Diffusion

Strain-tensile n+ Ge is an enabler of monolithically integrated light sources on a Si chip. The challenges are (1) high lasing thresholds and (2) n+ doping methods. (1) The threshold currents reported for electrically pumped Ge lasers are 280 and 510 kA/cm2 [1, 2]; much higher than III-V lasers (less than 1 kA/cm2) and even theoretical prediction of the Ge laser (~6 kA/cm2) [3]. (2) Ge can be used as photodetectors (PDs) and modulators (MODs), which need different in-depth dopant profiles. Ge epitaxy cannot control in-plane P concentration locally unless otherwise one performs multiple epitaxy. In the present study we tried to reduce the threshold using specific epi-structures and to diffuse Phosphorus (P) to make n+ Ge, and successfully observed a threshold in light emission using P diffusion for the first time. We have reported that a reverse-rib structure can be made using the epitaxial lateral overgrowth (ELO) technique [4], and can effectively isolate light modes from Si substrate. It further reduces light scattering at the Ge/SiO2 sidewall formed by dry etching as shown in Fig. 1 [5], some of which should decrease the optical net gain. In addition, our group reported that the Ge/Si interfaces if located in the pn junction enhance significantly non-radiative recombination [6]. Thus, n+ Ge was grown on n+ Si. P diffusion was employed to achieve 1 × 1019 P/cm3 both in the reverse-rib and the blanket Ge epilayers using a well-controlled diffusion method [7]. Optical pumping was used to excite the structure. 40-nm thick SiO2 was formed by thermal oxidation on an n+ Si wafer and patterned in line-and-space shape along [110] orientation by electron beam lithography followed by a wet etching. Epitaxial Ge layers were grown on the wafer using ultra high vacuum chemical vapor deposition (UHVCVD) at 700 ˚C. The epilayers were 1 µm thick at reverse-rib areas, while 1.8 µm thick at blanket areas. P diffusion was performed at 850 ˚C for 5 minutes, which achieves uniform P concentration of 1 × 1019 /cm3 through these Ge epilayers. Schematic of the measurement system is shown in Fig. 2. A continuous wave YAG:Nd laser (wavelength: 1.07 µm, power: ~kW/cm2) was used for optical pumping and a Ge PD was used to measure the relation of light emission and optical pumping. The effective input power density should be actually smaller than the set value since the reverse-rib Ge epilayer (300 µm wide and 3 mm long) is wider than the pumping laser (30 µm wide and 4 mm long), causing out-diffusion of photo-excited minority carriers (holes) to non-pumped areas. The measurements were carried out at room temperature. Figure 3 shows a cross sectional TEM image of the as-grown reverse-rib Ge. The air semi-cylinders are observed over the SiO2 masks (invisible in this image) since Ge will not grow on SiO2 in this condition. The air semi-cylinders are 150 nm high and 450 nm wide, which is big enough to isolate light modes from the Si substrate according to a mode solver simulation. Raman scattering spectroscopy reveals 0.17 % tensile strain at the top surface of the reverse-rib Ge while there should be strain relaxation around the air semi-cylinders. The results of the optical pumping measurements are summarized in Fig. 4. Light emission from the reverse-rib Ge shows a sharp threshold at the input power density of 8 kW/cm2 (set value). The results can be understood as either amplified spontaneous emission (ASE) or lasing. We will soon report the spectra to identify the results. Assuming lasing, the threshold of 8 kW/cm2 is very small compared to the value reported (30 kW/cm2) [8].

Multi-machine scaling of the main SOL parallel heat flux width in tokamak limiter plasmas

As in many of today’s tokamaks, plasma start-up in ITER will be performed in limiter configuration on either the inner or outer midplane first wall (FW). The massive, beryllium armored ITER FW panels are toroidally shaped to protect panel-to-panel misalignments, increasing the deposited power flux density compared with a purely cylindrical surface. The chosen shaping should thus be optimized for a given radial profile of parallel heat flux, in the scrape-off layer (SOL) to ensure optimal power spreading. For plasmas limited on the outer wall in tokamaks, this profile is commonly observed to decay exponentially as , or, for inner wall limiter plasmas with the double exponential decay comprising a sharp near-SOL feature and a broader main SOL width, . The initial choice of , which is critical in ensuring that current ramp-up or down will be possible as planned in the ITER scenario design, was made on the basis of an extremely restricted L-mode divertor dataset, using infra-red thermography measurements on the outer divertor target to extrapolate to a heat flux width at the main plasma midplane. This unsatisfactory situation has now been significantly improved by a dedicated multi-machine ohmic and L-mode limiter plasma study, conducted under the auspices of the International Tokamak Physics Activity, involving 11 tokamaks covering a wide parameter range with Measurements of in the database are made exclusively on all devices using a variety of fast reciprocating Langmuir probes entering the plasma at a variety of poloidal locations, but with the majority being on the low field side. Statistical analysis of the database reveals nine reasonable engineering and dimensionless scalings. All yield, however, similar predicted values of mapped to the outside midplane. The engineering scaling with the highest statistical significance, , dependent on input power density, aspect ratio and elongation, yields = [7, 4, 5] cm for = [2.5, 5.0, 7.5] MA, the three reference limiter plasma currents specified in the ITER heat and nuclear load specifications. Mapped to the inboard midplane, the worst case (7.5 MA) corresponds to mm, thus consolidating the 50 mm width used to optimize the FW panel toroidal shape.

A Methodology to Measure Input Power and Effective Area for Characterization of Direct THz Detectors

This paper presents a test setup and measurement methodology for the characterization of direct terahertz (THz) detectors. The detector under test is illuminated by a continuous wave THz source and its voltage response is measured using a lock-in amplifier. A reference detector is used to measure the input power density of the receiver antenna. The calculation of the effective area is done by measuring $E$ and $H$ plane radiation patterns of the receiver antenna. Several measurement techniques are shown in order to achieve a reliable characterization of the detector with different advantages and disadvantages. In addition, a setup modification is done for the removal of the electronically induced background noise due to the coupling of the signals through cables. Based on the measured input power, the performance parameters of an antenna-coupled field-effect transistor (FET)-based THz detector are measured and presented. This measurement methodology can also be used to characterize other direct THz detectors.

Sensitive and Efficient RF Harvesting Supply for Batteryless Backscatter Sensor Networks

This work presents an efficient and high-sensitivity radio frequency (RF) energy harvesting supply. The harvester consists of a single-series circuit with one double diode on a low-cost, lossy FR-4 substrate, despite the fact that losses decrease RF harvesting efficiency. The design targeted minimum reflection coefficient and maximum rectification efficiency, taking into account not only the impedance matching network, but also the rectifier microstrip trace dimensions and the load. The simulated and measured rectenna efficiency was 28.4% for $-\hbox{20-dBm}$ power input. In order to increase sensitivity, i.e., ability to harvest energy and operate at low power density, rectennas were connected in series configuration (voltage summing), forming rectenna arrays. The proposed RF harvesting system ability was tested at various input power levels, various sizes of rectenna arrays, with or without a commercial boost converter, allowing operation at RF power density as low as 0.0139 $\mu \hbox{W/cm}^{2}$ . It is emphasized that the boost converter, whenever used, was self-started, without any additional external energy. The system was tested in supplying a scatter radio sensor, showing experimentally the effect of input power density on the operational cold start duration and duty cycle of the sensor.

Physical origin of twice threshold phenomena in the transmission of the nonlinear photonic crystal molecules

We investigate the dynamics response of photonic crystal (PC) molecules with Kerr nonlinearity to the input continuous wave (CW) and focus on the jump action of the transmission on the input power density with the theoretical analyses and the simulation which is based on the finite-difference time-domain technique. It is found that the twice transmission jump behaviors exist when the frequency of input CW source be reasonable chosen and the photonic crystal (PC) atoms have the nearly same resonant frequency and different size. Therefore, these kinds of PC molecules can be employed to build new type optical diodes.