Incident Power Levels Research Articles

Hybrid FSS and Rectenna Design for Wireless Power Harvesting

This communication presents a hybrid frequency selective surface (FSS) and rectenna design for wireless power harvesting. To this extent, the center of the original FSS structure acts as a probe-fed patch antenna and an additional layer was added to the rear of FSS structure to mimic the required antenna ground plane, while minimizing its impact on FSS performance. The proposed hybrid solution yields a bandpass frequency response at GSM 900-MHz frequency band, with band reject behavior at 2.4 GHz and, on the other hand, act as a energy harvesting system through the antenna element at the 2.4-GHz ISM band, e.g., for recycling ambient Wi-Fi energy. The developed prototype, consisting of an array of 12 unit cells, was able to power a 56-mW LED at a 2-Hz rate from an incident power level of $-10\;\text{dBm}$ , resulting in an average output power of $300\;\upmu\text{W}$ and an unit cell RF–DC conversion efficiency of 50%.

Photo-induced reduction of graphene oxide coating on optical waveguide and consequent optical intermodulation.

Increased absorption of transverse-magnetic (TM) - polarised light by a graphene-oxide (GO) coated polymer waveguide has been observed in the presence of transverse-electric (TE) - polarised light. The GO-coated waveguide exhibits very strong photo-absorption of TE-polarised light - and acts as a TM-pass waveguide polariser. The absorbed TE-polarised light causes a significant temperature increase in the GO film and induces thermal reduction of the GO, resulting in an increase in optical-frequency conductivity and consequently increased optical propagation loss. This behaviour in a GO-coated waveguide gives the action of an inverted optical switch/modulator. By varying the incident TE-polarised light power, a maximum modulation efficiency of 72% was measured, with application of an incident optical power level of 57 mW. The GO-coated waveguide was able to respond clearly to modulated TE-polarised light with a pulse duration of as little as 100 μs. In addition, no wavelength dependence was observed in the response of either the modulation (TE-polarised light) or the signal (TM-polarised light).

Transmission Line Resistance Compression Networks and Applications to Wireless Power Transfer

Microwave-to-dc rectification is valuable in many applications, including RF energy recovery, dc–dc conversion, and wireless power transfer. In such applications, it is desired for the microwave rectifier system to provide a constant RF input impedance. Consequently, variation in rectifier input impedance over varying incident power levels can hurt system performance. To address this challenge, we introduce multiway transmission line resistance compression networks (TLRCNs) for maintaining near-constant input impedance in RF-to-dc rectifier systems. A development of TLRCNs is presented, along with their application to RF-to-dc conversion and wireless power transfer. We derive analytical expressions for the behavior of TLRCNs, and describe two design methodologies applicable to both single and multistage implementations. A 2.45-GHz four-way TLRCN network is implemented and applied to create a 4-W resistance compressed rectifier system that has narrow-range resistive input characteristics over a 10-dB power range. It is demonstrated to improve the impedance match to mostly resistive but variable input impedance class-E rectifiers over a 10-dB power range. The resulting TLRCN plus rectifier system has >50% RF-to-dc conversion efficiency over a >10-dB input power range at 2.45 GHz (peak efficiency 70%), and standing wave ratio <1.1 over a 7.7-dB range, despite a nonnegligible reactive component in the rectifier loads.

Enhanced Hot-Carrier Effects in InAlAs/InGaAs Quantum Wells

Hot-carrier solar cells require absorber materials with restricted carrier thermalization pathways, in order to slow the rate of heat energy dissipation from the carrier population to the lattice, relative to the rate of carrier extraction. Absorber suitability can be characterized in terms of carrier thermalization coefficient (Q). Materials with lower Q generate steady-state hot-carrier populations at lower levels of incident solar power and, therefore, are better able to perform as hot-carrier absorbers. In this study, we evaluate Q = 2.5±0.2 W·K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> for a In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.52</sub> AlAs/In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> GaAs single-quantum-well(QW) heterostructure using photoluminescence spectroscopy. This is the lowest experimentally determined Q value for any material system studied to date. Hot-carrier solar cell simulations, using this material as an absorber yield efficiency ~39% at 2000X, which corresponds to a >5% enhancement over an equivalent single-junction thermal equilibrium device.

Investigating the effective range of vacuum ultraviolet-mediated breakdown in high-power microwave metamaterials

Metamaterials and periodic structures operating under high-power excitations are susceptible to breakdown. It was recently demonstrated that a localized breakdown created in a given region of a periodic structure can facilitate breakdown in other regions of the structure where the intensity of the incident electromagnetic fields may not be high enough to cause breakdown under normal circumstances. It was also demonstrated that this phenomenon is due to the generation of vacuum ultraviolet radiation at the location of the initial discharge, which propagates to the neighboring regions (e.g., other unit cells in a periodic structure) and facilitates the generation of a discharge at a lower incident power level. In this paper, we present the results of an experimental study conducted to determine the effective range of this physical phenomenon for periodic structures that operate in air and in pure nitrogen gas at atmospheric pressure levels. It is demonstrated that when breakdown is induced in a periodic structure using a high-power pulse with a frequency of 9.382 GHz, duration of 0.8 μs, and peak power level of 25 kW, this phenomenon is highly likely to happen in radii of approximately 16–17 mm from the location of the initial discharge under these test conditions. The results of this study are significant in designing metamaterials and periodic structures for high-power microwave applications as they suggest that a localized discharge created in such a periodic structure with a periodicity less than 16–17 mm can spread over a large surface and result in a distributed discharge.

Wirelessly Powered Passive Systems With Dynamic Energy Storage Mechanism

Wirelessly powered passive sensors extract the required energy from external radiators. However, their sensitivity is typically worse than -20 dBm due to the limitation of the RF power harvesting. We study the theoretical and practical constraints on the RF power scavenging at low incident power levels. The concept of dynamic energy storage is introduced to enable the operation of the wirelessly powered passive systems at very low input power levels. The extra received power beyond the sensitivity level is stored and later used to enhance the sensitivity of the wireless sensor. As a proof of concept, an integrated wirelessly powered passive transponder with dynamic energy storage mechanism and sensitivity enhancement is designed and fabricated in a 0.13- μm CMOS technology. Based on the measurement results, the sensitivity of the wireless sensor with matching circuitry has been improved from -18.2 to -27.5 dBm at 900 MHz. The total chip area is 0.8 mm × 0.9 mm.

Broadband Switching Nonlinear Metamaterial

In this letter, we present a nonlinear metamaterial capable of transitioning between a broadband reflective and broadband nonreflective state depending on the incident power level. Below the activation voltage, broadband reflectivity is created by forbidding propagation through the metamaterial as a result of the effective permittivity and permeability having different signs. Above the activation voltage, reflection is suppressed by matching the effective permittivity and permeability. Full-wave simulations are performed to analyze the electromagnetic characteristics of the structure, with experimental results presented at S-band frequencies in a WR-284 waveguide. These measurements demonstrate a 10 dB reduction in reflected amplitude over an 18% bandwidth, a 3-dB reduction over a greater than 30% bandwidth, and a maximum difference in reflection of nearly 42 dB.

Light-controlled perfect absorption of light

We study coherent perfect absorption (CPA) of light in a Kerr nonlinear metal-dielectric composite medium, illuminated from the opposite ends. Elementary symmetry considerations reveal that equality of the incident light intensities is a prerequisite to ensure CPA in both linear and nonlinear systems for specific system parameters. We also derive the sufficient conditions for having CPA. We further show that, while CPA in a linear system is insensitive to the incident power level, in a nonlinear system CPA can be achieved only for discrete intensities with interesting hysteretic responses. Our unified formulation of CPA and waveguiding identifies them as opposite scattering phenomena. We further investigate light-induced CPA in on-resonant and off-resonant systems.

Passive inter-modulation scattering analysis of reflector considering contact nonlinearity

Passive inter-modulation (PIM) is a form of nonlinear distortion caused by the inherent nonlinearities of the passive devices and components in RF/microwave system. It will degenerate the performance of communication system with broad-band channel and high-sensitivity receiver. Therefore, it is necessary to construct a model to simulate this process in order to predict the level of PIM. This paper is aimed at constructing some plate models with one-dimensional and two-dimensional contact nonlinearity sections illuminated by two-tone waves, and calculating the scattered field at a fixed-point in space using time-domain physical optics method. By taking fast Fourier transform (FFT), we get the spectrum of the scattered field and then analyze the generated PIM products. At the end of this paper, some numerical examples are presented to show the influence rules of the relative factors on PIM. The results indicate the variation of the level of PIM with the number of the nonlinear regions, the nonlinear spacing, and the incident power levels.

A V-Band Divide-by-Three Injection-Locked Frequency Divider in 28 nm CMOS

In this letter, the 28 nm CMOS divide-by-three injection-locked frequency divider (ILFD) operating at the V band is presented for the first time. Based on a differential direct injection scheme, the proposed circuit can perform a division ratio of three while an enhanced locking range is achieved due to superior injection efficiency. From the measured results, the core circuit consumes a dc power of 8.8 mW from a 1 V supply voltage. At an incident power level of 0 dBm, the divider exhibits a locking range from 52.61 to 55.12 GHz. Within this frequency range, the output power and phase noise are kept around -5 dBm and -125 dBc/Hz at a 1 MHz offset, respectively.

Investigation Into the Temperature Dependence of Ferrimagnetic Nonlinear Transmission Lines

In pulsed power systems, coaxial based nonlinear transmission lines (NLTLs) loaded with ferrimagnetic materials act as pulse sharpeners or high power microwave sources. Microwave generation comes by way of nonlinearities present in the ferrimagnetic material as well as excitation of damped gyromagnetic precession at large incident power levels. Ferrimagnetic properties highly depend on operating temperature; therefore, there exists a need to understand operational performance of ferrite loaded NLTLs under different temperature environments. Ferrite samples are chilled or heated to temperatures between -20°C to 150 °C, providing a wide range of possible operating temperatures. The Curie temperature of the tested samples is approximately 120 °C; therefore, this study allows observation of precession performance in possible operating temperatures as well as a brief look at the consequences of exceeding the Curie temperature. The design, testing, and results for an NLTL measuring 0.3 m in length with ferrite inner and outer diameters of 3 mm and 6 mm, respectively, are detailed. Results reveal precessional performance, both peak power and frequency of oscillations, versus temperature.

Wave shadowing and modulation of microwave backscatter from the ocean

Shadowing and modulation of microwave backscatter by ocean waves are studied using coherent X‐band radars. Two types of shadowing are investigated: geometric shadowing (complete blockage of incident rays) and partial shadowing (polarization‐dependent diffraction combined with weak scatterers). We point out that the frequency of occurrence of zero signal‐to‐noise ratio samples cannot depend on the incident power level or the polarization if geometric shadowing occurs but can if partial shadowing exists. We then compare this behavior with observations, and show that the data do not support the hypothesis that geometric shadowing plays a significant role in low‐grazing‐angle microwave scattering from the ocean surface. Furthermore, our data indicate that partial shadowing only depends significantly on polarization for the steep waves found near shorelines. We also study the modulation of microwave backscatter by ocean waves using these data by looking at the phase differences between received power and scatterer velocity. These phase differences appear to be rather well explained by standard composite surface theory at VV polarization, having values that are positive looking up wave and negative looking down wave. For HH polarization, however, breaking effects come into play and overshadow composite surface effects of free waves. They cause the phase difference to be near zero for up wave looks and near 180° for down‐wave looks. A simple model that involves both breaking and freely propagating waves but does not include any shadowing effects is shown to account for observed phase differences at both polarizations to within about 10°.

Characterization and Modeling of UHF RFID Channels for Ranging and Localization

A comprehensive characterization and model of the UHF RFID channel is presented for narrowband through ultra-wideband tag localization systems. The analyses are based on ultra-wideband channel measurements in a warehouse portal, centered around 900 MHz. Measured scenarios include an electromagnetically transparent pallet and a pallet containing liquids, each for a portal shielded by metal backplanes and for a portal shielded by absorbing material. The presented analyses cover the individual channels to and from the tag, the feedback channel, and the backscatter channel, for bi- and monostatic reader setups. We find that the direct path is rarely dominant on the backscatter link despite clear line-of-sight conditions and directive reader antennas. The power ratio between the direct and all indirect paths ranges from -20 through 5 dB for the more common metal portal, and RMS delay spreads are in the range of 10-80 ns. Since only the direct (line-of-sight) path carries the correct distance/direction information, tag localization in such portals requires high robustness with respect to weak line-of-sight components. We also show that classical channel models in UHF RFID, despite predicting the incident power level at the tag accurately, produce far too optimistic estimates of channel parameters relevant to ranging and localization.

Power Linearity Measurement in Terahertz Time-Domain Spectroscopy Using Metalized Film Attenuators

We describe a method for evaluating the power linearity over a wide dynamic range of a terahertz time-domain spectroscopy (THz-TDS) system. The dynamic range is achieved by means of metalized film attenuators (MFAs). The evaluation was based on repeated measurements of transmittance in a particular sample at different incident power levels. We apply the method to both focused- and collimated-beam systems and show that the method can be applied to reflectance measurements as well.

Complementary Control of Oscillating Water Column-Based Wave Energy Conversion Plants to Improve the Instantaneous Power Output

In this paper, an oscillating water column-based wave energy conversion plant is modeled and controlled by means of two complementary control strategies in order to improve the conversion of wave energy into electric power. This wave power generation system consists of a capture chamber, a Wells turbine, and an induction generator. The improvement relays on the implementation of a control scheme that combines two different control methods: a rotational speed control and an airflow control implemented by means of a throttle valve in series with the turbine. The use of rotational speed control provides a fast way to react to the abrupt and short changes in the turbine speed, ensuring that the average power of the generator is adequately adjusted according to the incident wave power level. Besides, a throttle-valve is used to control the flow through the turbine so as to increase the amount of energy produced, particularly at the higher incident wave power levels. These two control strategies complement each other, maximizing and improving the quality of supply by controlling and smoothing the generated power for different scenarios of wave oscillations, variations of wave grouping, and changes in the sea state.

Power Linearity Measurement in Terahertz Time-Domain Spectroscopy Using Metalized Film Attenuators

We describe a method for evaluating the power linearity over a wide dynamic range of a terahertz time-domain spectroscopy (THz-TDS) system. The dynamic range is achieved by means of metalized film attenuators (MFAs). The evaluation was based on repeated measurements of transmittance in a particular sample at different incident power levels. We apply the method to both focused- and collimated-beam systems and show that the method can be applied to reflectance measurements as well.

Measurement of the two-photon absorption cross section by means of femtosecond thermal lensing

We present a variation of the single-beam thermal lensing experiment to determine the two-photon absorption cross sections of classical fluorophores. The approach is based on comparison of two thermal lensing signals simultaneously induced by a one- and two-photon absorption process from a high-repetition-rate femtosecond laser system. As a consequence of this comparison, a simplified expression independent of the several experimental parameters is obtained. Additionally, because of the low incident power levels required, undesirable optical effects such as Kerr or Raman scattering are avoided. Our experimental results agree well with those recently published for luminescent methods, validating the approach.

Dual-Loaded Modulated Dipole Scatterer as an Embedded Sensor

The modulated scatterer technique (MST) has shown great promise as an embedded sensor for material characterization and flaw detection in nondestructive testing and evaluation (NDT&E) applications. In particular, embedded MST probes facilitate real-time measurements of the dielectric properties of a host structure in the vicinity of the probe, i.e., localized measurements. MST is based on illuminating a loaded scatterer/probe, which is usually a dipole antenna loaded with a p-i-n diode, with an electromagnetic wave, and measuring the backscattered signal while the diode is modulated. Modulating the p-i-n diode by turning it “off” and “on” changes the dipole input impedance, which, in turn, results in two different backscattered signals. These signals are functions of the dielectric properties of interest, i.e., dielectric constant, and other measurement parameters, e.g., scatterer-to-antenna distance, incident power level, and incidence angle. Using the complex ratio of these signals renders measurement independence with respect to many measurement parameters and results in a measurement that is sensitive primarily to the parameter of interest (i.e., the dielectric constant of the material surrounding the probe). However, the modulated backscattered signal must be separated from static or unmodulated signals present at the receiving antenna before the ratio can be computed. This can be accomplished in a number of ways; each has its own advantages and drawbacks with respect to system complexity, signal processing requirements, and accuracy. To improve on the previous designs, in this paper, we introduce a new probe that simultaneously utilizes two p-i-n diodes. Using the four possible combinations of the measured backscattered signals from such a probe, in conjunction with a differential measurement scheme and subsequent ratio calculation, simultaneously results in the desired independence from several measurement parameters and the removal of the static terms. The operation of the proposed dual-loaded probe is described and analyzed in this paper. Experimental verification of the probe performance is also reported herein.

1.5-GHz voltage controlled oscillator with 3% tuning bandwidth using a two-pole DSBAR filter

First results on a novel voltage controlled oscillator (VCO) in the lower gigahertz range, featuring excellent phase noise and high power efficiency are presented. The heart of the VCO is a recently reported novel miniature two-pole decoupled stacked bulk acoustic resonator (DSBAR) filter. With its single 180° phase transition over the 1 dB bandwidth, linear phase, and maximum 1 dB insertion loss, it provides stable single-mode operation over 45 MHz (≈3%) of tuning bandwidth and has negligible heat dissipation when operated at incident power levels of 100 mW or greater. The 1.55-GHz laboratory VCO prototypes operate at 5 V supply voltage, 50 mA supply current, 15 dBm of output power, and >13% efficiency, demonstrating -84 and < -180 dBc/Hz phase noise suppression at 1 kHz carrier offset and in the thermal noise region, respectively. VCOs with cascaded DSBAR filters for further phase noise reduction are also demonstrated.

Raman Tweezers Spectroscopy of Live, Single Red and White Blood Cells

An optical trap has been combined with a Raman spectrometer to make high-resolution measurements of Raman spectra of optically-immobilized, single, live red (RBC) and white blood cells (WBC) under physiological conditions. Tightly-focused, near infrared wavelength light (1064 nm) is utilized for trapping of single cells and 785 nm light is used for Raman excitation at low levels of incident power (few mW). Raman spectra of RBC recorded using this high-sensitivity, dual-wavelength apparatus has enabled identification of several additional lines; the hitherto-unreported lines originate purely from hemoglobin molecules. Raman spectra of single granulocytes and lymphocytes are interpreted on the basis of standard protein and nucleic acid vibrational spectroscopy data. The richness of the measured spectrum illustrates that Raman studies of live cells in suspension are more informative than conventional micro-Raman studies where the cells are chemically bound to a glass cover slip.