As modern computing gets continuously pushed up against the von Neumann Bottleneck -limiting the ultimate speeds for data transfer and computation-new computing methods are needed in order to bypass this issue and keep our computer's evolution moving forward, such as hybrid computing with an optical co-processor, all-optical computing, or photonic neuromorphic computing. In any of these protocols, we require 1 arXiv:1911.03536v1 [physics.app-ph] 7 Oct 2019 an optical memory: either a multilevel/accumulator memory, or a computational memory. Here, we propose and demonstrate a 2-dimensional 4-bit fully optical non-volatile memory using Ge 2 Sb 2 Te 5 (GST) phase change materials, with encoding via a 1550 nm laser. Using the telecom-band laser, we are able to reach deeper into the material due to the low-loss nature of GST at this wavelength range, hence increasing the number of optical write/read levels compared to previous demonstrations, while simultaneously staying within acceptable read/write energies. We verify our design and experimental results via rigorous numerical simulations based on finite element and nucleation theory, and we successfully write and read a string of characters using direct hexadecimal encoding.
Plasmonic structures have long proved their capabilities to concentrate and manipulate light in micro-and nano-scales that facilitate strong light-matter interactions. Besides electromagnetic properties, ultra-small plasmonic structures may lead to novel applications based on their mechanical properties. Here we report efficient coupling between optical absorption and mechanical deformation in nanoscales through plasmonically enhanced fishbone nanowires. Using tailorable absorbers, free-space radiation energy is converted into heat to thermally actuate the suspended nanowires whose deformation is sensed by the evanescent fields from a waveguide. The demonstration at 660 nm wavelength with above 30% absorption shows the potential of the device to detect nW/√Hz power in an uncooled environment. IntroductionThe fundamental mechanism of infrared detection is energy transduction from the electromagnetic domain to others. Depending on the energy transduction mechanism, most of the infrared detectors can be classified as either photon detection or thermal sensing [1]. The semiconductor-based photonic detectors [2][3][4] have the advantages of high signal-to-noise ratio and fast response time. However, these advantages come at the expense of bulkiness, high cost, and power-inefficiency due to the use of cryogenic cooling. On the other hand, thermal detectors that utilize the temperature-induced changes in material properties are less expensive, more power efficient, and compatible with room temperature operations. Up to date, several uncooled thermal detectors have been demonstrated based on pyroelectricity [5][6][7], thermoelectricity [8][9][10], conductivity [11][12][13][14], piezoelectricity [15], optical resonance [16], mechanical deflection [17,18], etc. Thermo-mechanical detectors rely on the structural deformation upon exposure to radiation. As structure sizes go into nanoscales, finding an efficient light concentrator that can generate enough temperature gradient in subwavelength dimensions becomes one of the fundamental challenges. Fortunately, the plasmonic structures address this challenge by enhancing the light-matter interaction and boosting the absorption [19,20] in nanoscales. Another challenge lies in converting the tiny mechanical deflection into a measurable quantity. In doing so, optical approaches often utilize trigonometry [21,22] and interferometry [23,24] to amplify the displacement. However, these optical systems require discrete bulky lenses and detectors, hence they are difficult to miniaturize. Therefore, it is highly desirable to have efficient actuation in plasmonic nanomechanical structures with compact and sensitive on-chip transduction [25-27] and on-chip optical readout.
While over the past decade architected cellular materials have been shown to possess unique mechanical properties, their thermal properties have received relatively little attention. Here, we investigate thermal transport in hollow nickel microlattices as a function of temperature and mechanical loading using infrared thermography. The effective thermal conductivity of hollow nickel microlattices with 99.9% porosity and 1 µm layer thickness is as low as 0.049 W m−1 K−1 at 320 K and increases to 0.075 W m−1 K−1 at 480 K, an increase we attribute to internal thermal radiation. By measuring the emissivity and using the Stephan-Boltzmann law, we estimate the contribution of thermal radiation in the effective thermal conductivity to range from 20% at 320 K to 49% at 480 K. The high porosity of microlattices strongly limits solid conduction and makes surface radiation very important in thermal transport. We further explore the impact of the strut surface condition by comparing hollow doped nickel microlattices with a smooth surface to those with a rough surface: the emissivity increases from 0.24 to 0.43, leading to increased thermal radiation contributions of 41% at 320 K to 58% at 480 K. Under mechanical loading, as the strain increases from 0% to 50%, decreasing the angle between the struts and the horizontal plane from 60° to 38°, the effective thermal conductivity decreases from 0.049 W m−1 K−1 to 0.016 W m−1 K−1. These findings indicate that architected cellular materials provide an excellent platform to control thermal properties independently on mechanical properties and to potentially develop thermal and thermomechanical metamaterials.
Shape memory polymers are gaining significant interest as one of the major constituent materials for the emerging field of 4D printing. While 3D-printed metamaterials with shape memory polymers show unique thermomechanical behaviors, their thermal transport properties have received relatively little attention. Here, we show that thermal transport in 3D-printed shape memory polymers strongly depends on the shape, solid volume fraction, and temperature and that thermal radiation plays a critical role. Our infrared thermography measurements reveal thermal transport mechanisms of shape memory polymers in varying shapes from bulk to octet-truss and Kelvin-foam microlattices with volume fractions of 4%–7% and over a temperature range of 30–130 °C. The thermal conductivity of bulk shape memory polymers increases from 0.24 to 0.31 W m−1 K−1 around the glass transition temperature, in which the primary mechanism is the phase-dependent change in thermal conduction. On the contrary, thermal radiation dominates heat transfer in microlattices and its contribution to the Kelvin-foam structure ranges from 68% to 83% and to the octet-truss structure ranges from 59% to 76% over the same temperature range. We attribute this significant role of thermal radiation to the unique combination of a high infrared emissivity and a high surface-to-volume ratio in the shape memory polymer microlattices. Our work also presents an effective medium approach to explain the experimental results and model thermal transport properties with varying shapes, volume fractions, and temperatures. These findings provide new insights into understanding thermal transport mechanisms in 4D-printed shape memory polymers and exploring the design space of thermomechanical metamaterials.
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