Optical Rectennas Research Articles

Focused Ion Beam Engineering of Carbon Nanotubes for Optical Rectenna Applications

The optical rectenna is a device that converts the optical range of electromagnetic radiations to a direct current. Optical antennas, as for the radio frequency (RF) antennas, require an antenna’s structure in the range of optical light EM wavelengths, i.e., a few hundred nanometers to a few micrometers. Herein, we demonstrate the optical rectenna effect of single-wall carbon nanotubes (SWCNTs) by cutting them in the resonance lengths of monopole nano-antennas (i.e., λ/4) of 100, 150, and 200 nm to convert incident red, green, and blue lights into a direct current. The physical engineering (cutting) of SWCNTs dispersed on SiO2/Si, comparable to one-quarter of the incident monochromatic light wavelength (i.e., red, green, and blue), is carried out with high-energy gallium (Ga) ion beams, with the help of a focused ion beam (FIB) system. The rectenna characteristics of these engineered SWCNTs are investigated using conductive mode atomic force microscopy (C-AFM). This unprecedented approach to investigating the optical rectennas will open more directions to study the rectenna effect at the nanometer scale.

2D Material-Enabled Optical Rectennas with Ultrastrong Light-Electron Coupling.

Optical rectennas extend the electromagnetic wave rectification process into the visible regime and provide a route toward high-performance photodetection and energy harvesting. Here, the promise of 2D materials toward on-chip optical rectennas is demonstrated. A self-aligned patterning process yields lateral MIM structures where a nanometer-sized air gap separates a 2D material contact from a metal electrode. This device can be scalably produced in large arrays using established microfabrication techniques. Different from previous approaches, the performance of the 2D rectenna can be adjusted through electrostatic gating. Optimization of the band alignment leads to strong rectification at wavelengths around 500nm and clear polarization control. Comparison of wavelength-dependent rectenna performance with a photon-assisted tunneling model reveals a tenfold increase in photon-electron coupling over nanotube-based rectennas. The results highlight the potential of 2D material-based rectennas for future quantum computing applications.

Dark electron tunneling current in metal–insulator–metal structures: modeling, fabrication, and measurement

Metal–insulator–metal (MIM) structures have been proposed for infrared photodetectors, optical rectennas, and alternative choices to conventional solar cells. Generally, if two dissimilar electrodes are separated by a sufficiently thin insulator, electrons can travel through the insulating gap region due to the quantum tunneling effect. This tunneling through the insulating gap can be driven by an electromagnetic field or an external bias voltage. In our work, we incorporate the contributions of the Poole–Frenkel (PF) effect on the energy barrier of the MIM junction and the effect of Coulomb interaction between the electron charge and its virtual image charges. Using this modified PF-Coulombic barrier, we perform quantum calculations for electron tunneling using the transfer matrix method and Wentzel–Kramers–Brillouin approximations. This, along with the density of states, the Fermi occupation probability, and the quantum confinement effect is used to derive the dark current density for the MIM diode. To test the theory, MIM Au-TiO2-Ti diodes are designed and fabricated in-house using photolithography, along with electron beam evaporation and sputter depositions. The dark current–voltage (I − V) characteristics of the fabricated MIM photodiodes are measured and show good agreement with theoretical predictions.

Transistor Laser Antenna: Electromagnetic Model in Transmit and Receive Modes

Transistor laser can drive recent innovative technologies like optical antennas and rectennas. To this end, this semiconductor device requires an accurate electromagnetic model capable of determining the antenna characteristics like radiation pattern, directivity, gain, bandwidth, and polarization. Nonetheless, the current semiconductor models of transistor laser describe the absorption and emission of light mainly by simplified expressions and circuit models. These models usually overlook the actual physical geometry and full-wave light-emitting analysis of the device. In this article, a comprehensive computational electromagnetic modeling and characterization is presented for transistor laser. The existing semiconductor and electromagnetic equations are reorganized in a systematic fashion, coupled, and solved numerically to get the electromagnetic field components emitted or absorbed by the device. These fields determine the radiation pattern, directivity, gain, bandwidth, and polarization of transistor laser in transmit and receive modes. The equations involved in the above electromagnetic model are the Poisson and continuity equations incorporating radiative and non-radiative recombination rates, the vector magnetic potential equation interacting with the Hamiltonian operator of electrons in valance and conduction bands, the equation of the dielectric properties fluctuations of semiconductor layers, and the Poynting vector determining the power flow. The constructed model demonstrates agreement with the general performance of the device in experimental reports.

Electrical transport properties of gate tunable graphene lateral tunnel diodes

A detailed study of the electrical transport properties of gate tunable graphene lateral tunnel diodes is presented. The graphene-Al2O3-graphene lateral tunnel diodes are fabricated on Si/SiO2 substrates, and the fabricated devices show rectifying characteristics at the low voltage below 1 V. The rectifying behavior can be controlled by applying back gate voltages. As a result, the devices show high asymmetry and strong nonlinearity current–voltage (I–V) characteristics, which are desirable properties for applications such as optical rectennas and infrared detectors. The electrical transport mechanism of the graphene lateral diodes is analyzed by extracting parameters from the measured I–V characteristics. We find that trap-assisted tunneling from the defect levels in the Al2O3 layer is the most likely mechanism of the forward current of the fabricated graphene lateral diodes.

High-current density and high-asymmetry MIIM diode based on oxygen-non-stoichiometry controlled homointerface structure for optical rectenna

Optical rectennas are expected to be applied as power sources for energy harvesting because they can convert a wide range of electromagnetic waves, from visible light to infrared. The critical element in these systems is a diode, which can respond to the changes in electrical polarity in the optical frequency. By considering trade-off relationship between current density and asymmetry of IV characteristic, we reveal the efficiency limitations of MIM diodes for the optical rectenna and suggest a novel tunnel diode using a double insulator with an oxygen-non-stoichiometry controlled homointerface structure (MOx/MOx−y). A double-insulator diode composed of Pt/TiO2/TiO1.4/Ti, in which a natural oxide layer of TiO1.4 is formed by annealing under atmosphere. The diode has as high-current-density of 4.6 × 106 A/m2, which is 400 times higher than the theoretical one obtained using Pt/TiO2/Ti MIM diodes. In addition, a high-asymmetry of 7.3 is realized simultaneously. These are expected to increase the optical rectenna efficiency by more than 1,000 times, compared to the state-of-the art system. Further, by optimizing the thickness of the double insulator layer, it is demonstrated that this diode can attain a current density of 108 A/m2 and asymmetry of 9.0, which are expected to increase the optical rectenna efficiency by 10,000.

Evaluation of an optical energy harvester for SHM application

In this paper a preliminary study on an array configuration of rectified optical nanoantennas for energy harvesting application is proposed. Currently, the major impediments for the use of the rectified optical nanoantenna known as rectenna are the relatively low conversion efficiency and low power transfer to the load, both of them caused mainly by the mismatch between the impedance of the rectifier (several kilo ohms) and that of the antenna (hundreds of ohm). For this reason, the design of the array represents a crucial point to obtain the maximum energy transfer from the rectenna to the load, represented as a typical DC/DC boost power converter, and modeled by an equivalent input impedance. The novelty of this study confirms that it is possible to obtain dynamically an impedance matching between the array of optical rectennas and the load. According to this method, in the proposed rectenna array, N elements can be serially connected so to allow to the open circuit voltage to be increased and M elements can be parallel connected to give the degree of freedom needed to achieve the impedance matching with the load. Finally, a case study for Structural Health Monitoring (SHM) application is presented.

Optical Rectenna Arrays Using Carbon Nanotubes: From Fundamental Understanding to Next Generation Devices

Optical rectennas have emerged as promising devices for high frequency optoelectronic applications such as infrared and optical sensing and energy conversion. We achieve optical rectification using vertical multiwall carbon nanotube (CNT) arrays. The CNT nanoantennas absorb electromagnetic waves and ultrafast metal-insulator-metal tunnel diodes fabricated at the CNT tips convert the resulting terahertz AC current into DC electricity. Since the first demonstration in 2015, considerable progress to CNT optical rectennas has been made. The latest generation of CNT rectenna uses an innovative multi-insulator structure to provide exceptional diode properties and enable air stable materials. We applied photon-assisted tunneling theory to model our device’s optical-frequency behavior. Comparing the rectenna model to our optical experiments through a range of wavelengths (404–1064 nm) has yielded a better fundamental understanding that will guide future device design. The model provides strong evidence that the CNT antennas behave as dipole antennas and implicates a 780 THz diode cutoff frequency that reveals room for performance improvement. We report ongoing research developing a novel flexible CNT rectenna device structure. We use polydimethylsiloxane (PDMS) to infiltrate the CNT array and create a mechanically flexible substrate and planarized diode surface. This structure enables transparent, conductive top electrodes with PEDOT:PSS. Our recent efforts greatly enhance the commercial development of efficient, low cost, and application tunable CNT rectennas.

Photon-Assisted Tunneling in Carbon Nanotube Optical Rectennas: Characterization and Modeling

We present optical characterization and modeling of carbon nanotube (CNT) rectennas featuring multi-insulator, metal–insulator–metal tunneling diodes. The diodes use four layers of Al2O3 and ZrO2 dielectrics to obtain strong nonlinearity and highly asymmetric current density at low turn-on voltage. The CNT rectenna devices show energy conversion in the full optical spectrum (404–980 nm). We introduce the theory of photon-assisted tunneling (PAT) to model the optical behavior based on unilluminated diode characteristics. Our model shows agreement between PAT and our experimental results, and fitting suggests a wavelength-dependent optical voltage. We discuss the impact of rectenna parameters and elucidate performance limits to our CNT rectenna device.

Avoiding erroneous analysis of MIM diode current-voltage characteristics through exponential fitting

Accurate fitting of measured current-voltage [I(V)] data is crucial to the correct analysis and understanding of metal-insulator–metal (MIM) diodes, especially for optical rectennas. With the commonly used polynomial fitting of the I(V) data, the order of the fit can drastically affect the diode performance metrics such as resistance, responsivity, and asymmetry. Additionally, the resulting fitting coefficients provide no useful parameters. An exponential-based equation can fit the I(V) data well, can avoid artifacts from the choice of order of the polynomial, and allows for the accurate calculation of diode performance metrics directly from the fitting coefficients. Connecting the performance metrics to fitting coefficients shows a correspondence between zero-bias responsivity and asymmetry at any given voltage.

High performance MIIM diode based on cobalt oxide/titanium oxide

Optical rectennas for infrared energy harvesting commonly incorporate metal/double-insulator/metal diodes. Required diode characteristics include high responsivity and low resistance near zero bias with a sub-micron area, which have not been obtainable simultaneously. Diodes based on a new material set, Co/Co3O4/TiO2/Ti and an area of 0.071 μm2, provide a median maximum responsivity of 4.1 A/W, a median zero-bias responsivity of 1.2 A/W, and a median resistance of 14 kΩ. The highest performing diode has a maximum responsivity of 4.4 A/W, a zero-bias responsivity of 2.2 A/W, and a resistance of 18 kΩ.

Optical rectenna operation: where Maxwell meets Einstein

Optical rectennas are antenna-coupled diode rectifiers that receive and convert optical-frequency electromagnetic radiation into DC output. The analysis of rectennas is carried out either classically using Maxwell’s wave-like approach, or quantum-mechanically using Einstein’s particle-like approach for electromagnetic radiation. One of the characteristics of classical operation is that multiple photons transfer their energy to individual electrons, whereas in quantum operation each photon transfers its energy to each electron. We analyze the correspondence between the two approaches by comparing rectenna response first to monochromatic illumination obtained using photon-assisted tunnelling theory and classical theory. Applied to broadband rectenna operation, this correspondence provides clues to designing a rectenna solar cell that has the potential to exceed the 44% quantum-limited conversion efficiency. The comparison of operating regimes shows how optical rectenna operation differs from microwave rectenna operation.

Simple Figure of Merit for Diodes in Optical Rectennas

Harvesting infrared and visible light energy using optical rectennas—diode-coupled optical antennas—is strongly dependent on the diode current–voltage characteristics. Predicting the conversion efficiency is an arduous procedure that involves calculating the efficiency by solving the nonlinear diode circuit using the theory of photon-assisted tunneling (PAT). Because of the complexity of the calculations, many rectenna diodes have been reported without indications of how well these diodes will actually function in a rectenna. To provide a quick assessment of the diodes’ usefulness in optical rectennas, we provide a figure of merit ( FOM ) that incorporates the diodes’ current–voltage and capacitance, the antenna radiation resistance, and the illumination parameters.

Giant Photoresponsivity of Midinfrared Hyperbolic Metamaterials in the Photon-Assisted-Tunneling Regime

Conventional solar cells cannot collect energy from abundant midinfrared light, because there are no suitable semiconductors of the right band gap. Hyperbolic metamaterials (HMMs) are proposed to realize broadband, omnidirectional, sensitive diodes operating in this spectral region. The authors show how the slow-light modes supported by HMMs can trap incident radiation in metal-insulator-metal tunnel junctions, for ultrafast optical rectification and photon-to-electron energy conversion that is orders of magnitude more efficient than in conventional optical rectennas.

Design of a sector bowtie nano-rectenna for optical power and infrared detection

We designed a sector bowtie nanoantenna integrated with a rectifier (Au–TiO x –Ti diode) for collecting infrared energy. The optical performance of the metallic bowtie nanoantenna was numerically investigated at infrared frequencies (5–30 μm) using three-dimensional frequency-domain electromagnetic field calculation software based on the finite element method. The simulation results indicate that the resonance wavelength and local field enhancement are greatly affected by the shape and size of the bowtie nanoantenna, as well as the relative permittivity and conductivity of the dielectric layer. The output current of the rectified nano-rectenna is substantially at nanoampere magnitude with an electric field intensity of 1 V/m. Moreover, the power conversion efficiency for devices with three different substrates illustrates that a substrate with a larger refractive index yields a higher efficiency and longer infrared response wavelength. Consequently, the optimized structure can provide theoretical support for the design of novel optical rectennas and fabrication of optoelectronic devices.

Rectennas at optical frequencies: How to analyze the response

Optical rectennas, antenna-coupled diode rectifiers that receive optical-frequency electromagnetic radiation and convert it to DC output, have been proposed for use in harvesting electromagnetic radiation from a blackbody source. The operation of these devices is qualitatively different from that of lower-frequency rectennas, and their design requires a new approach. To that end, we present a method to determine the rectenna response to high frequency illumination. It combines classical circuit analysis with classical and quantum-based photon-assisted tunneling response of a high-speed diode. We demonstrate the method by calculating the rectenna response for low and high frequency monochromatic illumination, and for radiation from a blackbody source. Such a blackbody source can be a hot body generating waste heat, or radiation from the sun.

Nonlinear Photon-Assisted Tunneling Transport in Optical Gap Antennas

We introduce strongly coupled optical gap antennas to interface optical radiation with current-carrying electrons at the nanoscale. The transducer relies on the nonlinear optical and electrical properties of an optical gap antenna operating in the tunneling regime. We discuss the underlying physical mechanisms controlling the conversion involving d-band electrons and demonstrate that a simple two-wire optical antenna can provide advanced optoelectronic functionalities beyond tailoring the electromagnetic response of a single emitter. Interfacing an electronic command layer with a nanoscale optical device may thus be facilitated by the optical rectennas discussed here.

Quantum theory of operation for rectenna solar cells

Optical rectennas, sub-micrometre antenna-coupled diodes, can directly rectify solar and thermal electromagnetic radiation, and have been proposed as an alternative to conventional semiconductor photovoltaics. We develop a comprehensive description of the operating principle of rectenna solar cells. In prior work classical concepts from microwave rectenna theory have been applied to the analysis of photovoltaic power generation using these ultra-high frequency rectifiers. Because of their high photon energy the interaction of petahertz frequency waves with fast-responding diodes requires a semiclassical analysis. We use the theory of photon-assisted transport to derive the current–voltage [I(V)] characteristics of metal/insulator/metal tunnel diodes under illumination. We show how power is generated in the second quadrant of the I(V) characteristic, derive solar cell parameters, and analyse the key variables that influence the performance under monochromatic radiation and to a first order approximation. The efficiency improves with reduced dark current under reverse bias and increasing incident electromagnetic power.