Wolfgang Porod

We are developing circular-polarization-sensitive infrared detectors called spiral thermoelectrically coupled nanoantennas (TECNAs) for solar imaging and terrestrial sensing applications. In a spiral TECNA, a nanoscale thermocouple is used to measure the localized joule heating caused by induced currents within a spiral nanoantenna structure. This paper discusses the response of spiral TECNAs to elliptical polarization, which must be considered when performing circular polarization measurements of sources of arbitrary polarization. Measurements and simulations show that spiral TECNAs respond maximally to a specific matching elliptical polarization and minimally to the orthogonal, nonmatching elliptical polarization. Given that any polarization can be decomposed into these two basis polarizations, we find that the spiral-TECNA response to an arbitrary polarization scales with the matching-elliptical-polarization component. The spiral TECNA polarization response is analogous to that of RF spiral antennas, despite the intermediate transduction between high-frequency currents and heat.

Oscillatory neural networks have so far been successfully applied to a number of computing problems, such as associative memories, or to handle computationally hard tasks. In this paper, we show how to use oscillators to process time-dependent waveforms with minimal or no preprocessing. Since preprocessing and first-layer processing are often the most power-hungry steps in neural networks, our findings may open new doors to simple and power-efficient edge-AI devices.

We present thermoelectrically coupled nanoantennas (TECNAs) with log-spiral antennas for mid- and long-wave infrared detection. The nanoantenna resonantly absorbs the incident IR radiation and, due to its shape, provides selectivity for circular polarization. We used HFSS simulations to design TECNAs with both left- and right-handed antennas and show the effect of each design parameter of the spiral on the response of the devices. The fabricated devices were characterized at 10.6 μm wavelength, and the measurement results show that the response is about five times larger when the polarization handedness of the incident IR radiation matches to the antenna

We are studying thermoelectrically coupled nanoantennas, referred to as TECNAs, to determine the angle of incidence of laser beams in the long-wave IR regime. TECNAs consist of a nanoantenna and a nanothermocouple that are fabricated over a quasi-spherical cavity etched into the Si substrate. The antenna nature of the TECNAs makes them polarization- and wavelength-selective, and the cavity provides directional selectivity. In this paper, we experimentally demonstrate that the far-field radiation pattern of TECNAs is altered by positioning them at various locations above the cavity. Both experimental results and finite element method simulations show that the resulting radiation pattern of the antenna is determined by the nanoantenna’s position relative to the center of the cavity. Additionally, we highlight that the substrate significantly affects the far-field pattern of TECNAs; under coherent excitation, interference patterns overlay the radiation pattern due to back reflection from the substrate. We have shown that this effect is attributed to the substrate, as the interference pattern is negligible under incoherent illumination.

This work investigates the speed and responsivity of thermoelectrically coupled nanoantennas (TECNAs) in vacuum. TECNAs are long-wave infrared (IR) detectors that absorb electromagnetic radiation via a resonant dipole antenna. Joule heating within the antenna is converted to a voltage via a nanoscale thermocouple (NTC). Heat loss due to air is reduced under vacuum, allowing for improved device responsivity, but at the expense of speed. A finite difference model is introduced that calculates the steady-state and transient thermal transport within various TECNA designs across a range of ambient pressures. The modeled pressure dependence was derived from nanoscale convection coefficients found in the literature. Models were experimentally verified by studying the pressure-dependent and frequency-dependent voltage response of TECNAs exposed to modulated 10.6 μm IR radiation. Improvements in responsivity ranging from 3 to 4.5 times were demonstrated between atmosphere and vacuum, with 90% of improvement occurring between atmospheric pressure and 1 Torr (rough vacuum) in each design. Devices exhibited bandwidths of at least 10 kHz (roughly 100 times faster than common microbolometers) under vacuum with negligible signal attenuation, though future work is needed to fully characterize device frequency response.

There has been much discussion for decades over the proper concepts of minimum dissipation per logic gate and what is required for computation, as well as early arguments over logically reversible machines. Here these arguments, and the thermodynamics related to them, are discussed in terms of what is required for a computing machine. This gives a set of requirements for a valid computer that arise already from Turing and a set of limitations on energy dissipation and entropy. Here, the requirements set by Turing on what can and cannot be a computing machine are introduced, and how these limits affect not only the machine, but individual bits and a general need for irreversibility, are discussed. Then, it is shown that there is a minimum dissipation for a bit operation, and this is imposed both by Turing’s requirements as well as by noise in the system. Finally, it is shown that information entropy differs from physical entropy and care must be taken in trying to connect the two quantities. How these requirements, and new ones, affect quantum computation is also discussed.

The field of design and design automation of micro/nano circuits and systems promotes interdisciplinary research spanning computer science, computer engineering, and electrical engineering. This field has created key technologies without which it would be impossible to achieve advances in information processing, which is an inseparable part of our everyday lives. For example, fundamental principles and tools created by this field have empowered Moore char63s law scaling for over 50 years. Without design and design automation of circuits and systems, it would be impossible to create multibillion transistor integrated circuits (ICs) that form the foundations of today char63s information age.

There has been growing interest in using spin waves as an alternative information carrier or state variable. One proposed use of spin waves is to utilize the diffraction and interference of spin waves to perform highly parallelized computing of specific functions. This new class of device would require isotropic dispersion relations. The subject of this work is a transducer design to convert electrical signals into short-wavelength forward-volume spin-waves. Short-wavelength spin waves would enable spin-wave devices to scale down to smaller sizes and potentially allow the use of films that are CMOS compatible but suffer from higher damping. This work presents a new design of electrical-to-spin-wave transducers that can excite short-wavelength spin waves by first exciting large-wavelength (1.43 μm for example) spin-waves using relatively large coplanar waveguides. The spin-waves are then transformed into short-wavelength (132 nm) spin-waves by traveling into an adjacent film where their wavelengths decrease significantly. Simulations show that this design is able to excite spin waves with significantly larger amplitudes (> 10×) than an appropriately sized CPW on top of the magnetic film with the same input power.

We present the design and experimental realization of a device that acts like a spin-wave lens i.e., it focuses spin waves to a specified location. The structure of the lens does not resemble any conventional lens design, it is a nonintuitive pattern produced by a machine learning algorithm. As a spin-wave design tool, we used our custom micromagnetic solver 'SpinTorch' that has built-in automatic gradient calculation and can perform backpropagation through time for spin-wave propagation. The training itself is performed with the saturation magnetization of a YIG film as a variable parameter, with the goal to guide spin waves to a predefined location. We verified the operation of the device in the widely used mumax $^{3}$ micromagnetic solver, and by experimental realization. For the experimental implementation, we developed a technique to create effective saturation-magnetization landscapes in YIG by direct focused-ion-beam irradiation. This allows us to rapidly transfer the nanoscale design patterns to the YIG medium, without patterning the material by etching. We measured the effective saturation magnetization corresponding to the FIB dose levels in advance and used this mapping to translate the designed scatterer to the required dose levels. Our demonstration serves as a proof of concept for a workflow that can be used to realize more sophisticated spin-wave devices with complex functionality, e.g., spin-wave signal processors, or neuromorphic devices.

Magnonics addresses the physical properties of spin waves and utilizes them for data processing. Scalability down to atomic dimensions, operation in the GHz-to-THz frequency range, utilization of nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a few of many advantages offered by magnons. Although magnonics is still primarily positioned in the academic domain, the scientific and technological challenges of the field are being extensively investigated, and many proof-of-concept prototypes have already been realized in laboratories. This roadmap is a product of the collective work of many authors, which covers versatile spin-wave computing approaches, conceptual building blocks, and underlying physical phenomena. In particular, the roadmap discusses the computation operations with the Boolean digital data, unconventional approaches, such as neuromorphic computing, and the progress toward magnon-based quantum computing. This article is organized as a collection of sub-sections grouped into seven large thematic sections. Each sub-section is prepared by one or a group of authors and concludes with a brief description of current challenges and the outlook of further development for each research direction.