Levente Klein

Silicon nanowire piezoresistance: Impact of surface crystallographic orientation

We investigate piezoresistance in lithographically defined silicon nanowires of various cross-sectional aspect ratios. Both ⟨110⟩- and ⟨100⟩-oriented nanowires are investigated under ⟨110⟩-oriented strain. The nanowire thickness is varied from 23 to 45 nm and the nanowire width is varied from 5 to 113 nm. Our data shows piezoresistance in silicon nanowires being a surface induced effect with {110} surfaces inducing a much larger piezoresistance than {100} surfaces. This is consistent with a higher density of surface states on {110} surfaces than on {100} surfaces. Our experimental findings support recent computational work pointing toward surface states being the source of giant piezoresistance in silicon nanowires.

Enhanced thermal emission from individual antenna-like nanoheaters

Here we report polarization-sensitive, thermal radiation measurements of individual, antenna-like, thin film Platinum nanoheaters. These heaters confine the lateral extent of the heated area to dimensions smaller (or comparable) to the thermal emission wavelengths. For very narrow heater structures the polarization of the thermal radiation shows a very high extinction ratio as well as a dipolar-like angular radiation pattern. A simple analysis of the radiation intensities suggests a significant enhancement of the thermal radiation for these very narrow heater structures.

Sub-wavelength bolometers: Uncooled platinum wires as infrared sensors

We present characterization results of microscopic platinum wires as bolometers. The wire lengths range from 16 μm down to 300 nm. Thus they are in many cases significantly smaller in size than the wavelength of the radiation from the 1200 K blackbody source they were exposed to. We observe a steep rise in both responsivity ℜ and detectivity D* with decreasing wire size, reaching ℜ = 3.1×10(4) V/W and D* = 2.7×10(9) cm Hz(1/2)/W at room temperature for a 300×300 nm(2) device. Two significant advantages of such small wires as bolometers are their low power requirement and fast response time. Our numerical estimations suggest response times in the order of nanoseconds for the smallest samples. They could help improve resolution and response of thermal imaging devices, for example. We believe the performance may be further improved by optimizing the design and operating parameters.

Coherence properties of infrared thermal emission from heated metallic nanowires

Coherence properties of the infrared thermal radiation from individual heated nanowires are investigated as a function of nanowire dimensions. Interfering the thermally induced radiation from a heated nanowire with its image in a nearby moveable mirror, well-defined fringes are observed. From the fringe visibility, the coherence length of the thermal emission radiation from the narrowest nanowires was estimated to be at least 20μm, which is much larger than expected from a classical blackbody radiator. Significant increases in coherence and emission efficiency are observed for smaller nanowires.

PAIRS: A scalable geo-spatial data analytics platform

Geospatial data volume exceeds hundreds of Petabytes and is increasing exponentially mainly driven by images/videos/data generated by mobile devices and high resolution imaging systems. Fast data discovery on historical archives and/or real time datasets is currently limited by various data formats that have different projections and spatial resolution, requiring extensive data processing before analytics can be carried out. A new platform called Physical Analytics Integrated Repository and Services (PAIRS) is presented that enables rapid data discovery by automatically updating, joining, and homogenizing data layers in space and time. Built on top of open source big data software, PAIRS manages automatic data download, data curation, and scalable storage while being simultaneously a computational platform for running physical and statistical models on the curated datasets. By addressing data curation before data being uploaded to the platform, multi-layer queries and filtering can be performed in real time. In addition, PAIRS offers a foundation for developing custom analytics. Towards that end we present two examples with models which are running operationally: (1) high resolution evapo-transpiration and vegetation monitoring for agriculture and (2) hyperlocal weather forecasting driven by machine learning for renewable energy forecasting.

Corrosion management for data centers

The recent interest in air-side cooling and the spread of data centers into geographies with higher levels of atmospheric contamination is requiring more attention towards air quality management in data centers. One concern of air side economization is an increase in contamination levels potentially leading to more failures and outages of the IT equipment. In this paper we describe a corrosion measurement and management technology that enables high accuracy and real time monitoring of the gaseous contamination. The synergistic effects of indoor air temperature and relative humidity on corrosion rates are investigated and the spatial and the temporal variations of the corrosivity are established. Filtering of the outside air, both for particulate and gaseous contamination can mitigate air contamination in data centers. Implementing a facility wide air quality monitoring system promises the safe use of air-side economizers and would establish appropriate filtering, which enable early prevention of critical situations for information technology (IT) equipment operations.

Changing the emission of polarized thermal radiation from metallic nanoheaters.

The polarization of the thermal radiation emitted from individual nanoheaters is investigated for nanoheaters with widths ranging from 500 nm to 2000 nm. The polarization is oriented along the long axis of the nanoheater for widths below 600 nm and rotates by 90 degrees and becomes perpendicular for widths above 900 nm. For certain width nanoheaters the orientation of the polarization of the thermal emission can be rotated from parallel to perpendicular by changing the temperature of the nanoheater. The change in the direction of the emitted thermal radiation is explained by thermally excited transverse plasmon modes.

Corrosion risk management in IT facilities

Environmental conditions in data centers determine the reliability of the IT equipment and operational uptimes. Due to continuous increase in energy usage and a trend towards sustainability, leads many data center operators to lean towards implementing energy efficiency measures. Many of todays data center are operated in a narrow temperature and relative humidity band, however these may change as ASHRAE recently expanded the limits of the temperature and relative humidity. These expansions are triggered by recent interest to implement air side economizer based cooling in data centers. For air side economized data centers besides pollutants that can be introduced unintentionally, the daily variation of temperature and relative humidity could possibly lead to condensation or corrosion. The variation of the corrosion rate due to daily changes of the indoor pollutant concentrations combined with temperature and humidity cycling are presented for a data center.

Sustainable data centers powered by renewable energy

The energy consumption of data centers (DCs) has dramatically increased in recent years, primarily due to the massive computing demands driven by communications, banking, online retail, and entertainment services. In todays data centers, the cooling and infrastructure operations require almost the same energy as the IT operations. The large energy consumption in data centers prompted government agencies, industries, professional organizations, and academic institutions to investigate sustainable growth paths. We discuss such scenarios based on current trends and projections and propose the required innovations to achieve a 10 fold increase in “performance per Watt” of IT operations. We discuss three possible technology components that would improve the operational performance of data centers: (1) integration of renewable energy sources (2) increasing energy efficiency through IT consolidation and workload optimization, and (3) multifunctional sensor networks for better cooling and infrastructure management. We discuss the key requirements and how these technologies can be combined to achieve a sustainable path.

A tele-operative RMMT system facilitating the management of cooling and energy in data centers

This paper presents a tele-operative robotic mobile measurement technique for collecting temperature data in data centers. It provides highly effective data support to the analysis of data center thermal profiles, and plays an active role in improving data center cooling and energy efficiencies. With this technique, a human operator controls the navigation of the teleoperative mobile measurement platform and triggers the data acquisition operation through a wirelessly connected console. Comparing with traditional manual operation, it largely reduces the intensity of human labor and improves the operational efficiency. The effectiveness of the proposed technique has been verified through a demonstrative data scanning in a real data center environment.