This work was partially supported by the European H2020 LABYRINTH project (grant agreement H2020-MG-2019-TwoStages-861696), and by the TRUE5G project (PID2019-108713RB-C52PID2019-108713RB-C52 / AEI / 10.13039/501100011033) funded by the Spanish National Research Agency. In addition, the work of Borja Nogales, Ivan Vidal and Diego R. Lopez has partially been supported by the European H2020 5G-VINNI project (grant agreement number 815279). Finally, the authors thank Alejandro Rodr&#237;guez Garc&#237;a for his support during the realization of this work.

<ol>
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The authors have nothing to disclose.

One of the most important aspects of the previously described protocol is its outstanding flexibility to incorporate new computational infrastructures to an NFV ecosystem, regardless of their distribution in terms of geographical location (as long as bandwidth and latency of the network communications with remote sites supports it). This is possible through a VPN-based overlay network architecture, which enables the establishment of a virtual link to connect remote sites to the central premises of the NFV ecosystem. This approach enables the provision of an effective and secure channel to support the NFV and data communications among sites of an NFV ecosystem, reducing the probability of external parties accessing and/or modifying sensitive information regarding NFV orchestration processes and data from deployed services. In this context, the protocol also describes a specific methodology to share securely the VPN credentials with the external sites that will enable the integration of new infrastructures. The protocol has been exemplified using the NFV ecosystem made available at 5TONIC by Universidad Carlos III de Madrid, Telef&#243;nica, and IMDEA Networks Institute, although it is generic to be utilized in other NFV environments satisfying the prior requisites mentioned in step 1 of this protocol.
In addition, it is worth emphasizing the exclusive utilization of open-source tools and software for the protocol implementation. Notwithstanding the potentially beneficial functionalities that could be offered by different proprietary solutions (e.g., Fortinet35), the use of open-source developments has facilitated the integration of all elements encompassed by the protocol due to their inherent characteristics such as cost effectiveness, an extensive software support provided by the open-source community, and a high level of reliability, just to name a few of them. Moreover, the utilization of open-source technologies can also promote synergies between components of similar nature. For instance, in order to monitor the VPN connection status for the clients using the platform, the VPN service implemented throughout the protocol could rely on the open-vpn monitor tool36 (a python-based monitoring tool capable of interoperating with OpenVPN servers).
On the other hand, the protocol specification considers the instantiation of networking services across different sites for validation purposes. In this regard, it is important to highlight that the deployment of services on a given site is subject to the availability of compute, storage and network resources at the site, as well as of specialized equipment that might be needed to perform the deployment (e.g., NFV-enabled SUAVs). This is not a limitation of the protocol, and should be taken into account by stakeholders interested in reproducing the experiment described in this paper.
Moreover, it should be noted that the time required to carry out the deployment of network services highly depends on several factors such as the network path between the orchestrator and the different VIMs, the performance of data communications between the VIM and its managed computational nodes, and also in the intrinsic nature of these computational nodes (not only because of their available computing resources, but also the technologies incorporated to conduct the virtualization of network functions).
Finally, and given the outstanding performance that this platform and its VPN service had on the European projects and collaborative works where it has been used so far (e.g., 5GINFIRE, 5GRANGE or 5GCity, mentioned in the introduction of this document), it will be regarded as an important element in emerging European projects where Universidad Carlos III de Madrid, Telef&#243;nica, and IMDEA Networks Institute participate, such as the Horizon 2020 LABYRINTH, or national projects, like TRUE-5G.

The introduction of the fifth generation of mobile networks (5G) has implied revolutionizing the telecommunications industry since the beginning of the decade, requiring telecommunication operators to address the much more demanding specifications of the new networking services and applications developed under the 5G umbrella1,2. These new specifications include, but are not limited to, data rate increases, wireless transmission latency improvements, and operational costs reduction. Among the technologies that constitute the foundations of the improvements for this new generation, Network Functions Virtualization3 (NFV) has become one of its key enablers. NFV provides the capacity to softwarize network functions, traditionally relaying on specialized hardware, by using generic-purpose physical equipment instead, such as server computers in a datacenter. With this new paradigm, telecommunication operators and vertical industries can deploy network functions and services as a set of software components, and save costs in both service deployment and maintenance, as well as facilitating a much higher network infrastructure elasticity. This approach alleviates or eliminates the necessity to use dedicated (and usually more complex and less reusable) devices for most network and vertical-specific functions, and supports a much higher and denser degree of operational automation, hence reducing deployment and maintenance costs.
Taking into consideration all the advantages that an NFV environment is able to provide, it is natural that a great number of relevant stakeholders from the telecommunications sector have increasingly been involved in testing new service ideas on NFV environments. In this context, Telef&#243;nica and IMDEA Networks Institute have created 5TONIC4, an open research and innovation laboratory focused on 5G technologies. Based in Madrid (Spain), this laboratory has a wide range of technologies available for researches and partners to boost the development and validation of 5G services. In particular, this laboratory has an experimental NFV platform where developers are able to deploy and test their new NFV-based applications and services over on an ETSI-compliant NFV ecosystem5. Thus, experimental conclusions about design choices and technology proposals can be derived in a realistic much more flexible environment than production networks. This platform has been designed to support experimentation activities across multiple external sites, which may be flexibly interconnected to 5TONIC using a well-defined protocol.
The technical solution adopted for the 5TONIC NFV ecosystem considers the utilization of a single NFV orchestrator, implemented using the ETSI-hosted Open Source MANO (OSM) software6. This is the element in charge of managing and coordinating the lifecycle of Network Services (NS). These services may be built as a composition of Virtualized Network/Vertical Functions (VNF), which can be deployed at any of the sites integrated on the NFV platform. The design of the 5TONIC NFV ecosystem has been done in the context of the H2020 5GINFIRE project7,8, where the platform was used to support the execution of more than 25 experiments, selected through a competitive open-call process, across eight vertical-specific experimental infrastructures located in Europe and one in Brazil, the latter connected through a transoceanic link. In addition, the platform was leveraged to build a distributed NFV testbed at a national scale, in Spain, supporting experimentation activities within the Spanish 5GCity project9,10. More recently, an additional Brazilian site has been integrated into the platform, to support joint demonstration activities in the context of a research and innovation cooperation established between Brazil and Europe (i.e., the 5GRANGE project11,12). Last but not least, the infrastructure has been used to support third-party experiments in the scope of the 5G-VINNI project13,14. The geographic distribution of the NFV platform can be seen in Figure 1.
Interested organizations hosting their own NFV infrastructure can flexibly connect to the 5TONIC NFV ecosystem, subject to approval by the 5TONIC Steering Board, become testbed providers within the distributed ecosystem, and be involved in joint experimentation and demonstration activities. To this end, they must feature a VIM (Virtual Infrastructure Manager) compliant with the OSM software stack. The 5TONIC NFV orchestrator is able to interact with the VIMs at the sites involved in a given service deployment, coordinating the allocation and setup of the computing, storage and network resources needed for the instantiation and interconnection of the VNFs that compose a network service, and controlling its lifecycle, from its on-boarding to its final decommissioning.
In order to manage the exchange of control and data traffic within all the interconnected sites, the 5TONIC NFV ecosystem makes use of an overlay network architecture based on Virtual Private Networks (VPN). This approach provides secure PKI-based access to the external sites that are integrated into the 5TONIC ecosystem, allowing the exchange of NFV control information between the OSM software stack and the different VIMs distributed across the testbeds, as well as the exchange of information that is required to manage and configure all the VNFs. Moreover, this overlay network supports the dissemination of data traffic among VNFs that are deployed at different sites.
In this context, this paper details the protocol designed to incorporate an external site to an NFV ecosystem. The protocol assumes that the ecosystem is governed by a single NFV orchestrator, installed at a central site, and external sites feature a VIM solution compliant with the orchestrator software stack. The proposed protocol allows to increment the portfolio of resources of the experimental ecosystem, with the flexible incorporation of NFV sites and vertical-specific infrastructures. This enables the creation of a distributed MANO platform capable of testing and validating novel network and vertical services across multiple sites, under the control of a single NFV orchestrator. In order to illustrate the inner operation of the protocol, the process will be exemplified by adding an external NFV site to the current 5TONIC NFV ecosystem, describing the needed components at the external site and 5TONIC, as well as all the steps to be taken during the integration process. Figure 2 provides an overview of the objective of the integration, with the new NFV-based testbed attached to the 5TONIC platform from where network services can be deployed, by means of VPN connections between the central site and the rest of the external infrastructures.
In addition, to showcase the effectiveness of the protocol, the deployment of a simple vertical service will be shown, using the 5TONIC ecosystem and an external site with NFV-capable small unmanned aerial vehicles (SUAVs). The design of the vertical service has been inspired by an experiment presented in Vidal et al.9, which has been simplified for the illustration purposes of this paper. Figure 3 outlines the service, which aims at aiding smart farming activities on a remote area. The service considers a smart farming service provider who uses SUAVs to collect and disseminate the data produced by meteorological sensors scattered over a crop field. For simplicity, the experiment presented in the paper considers a single SUAV and a sensor, capable of providing temperature, humidity, and pressure measurements. In the experiment, the external NFV site hosts a Wi-Fi access point that is deployed as VNF over the SUAV. This VNF offers network access connectivity to the sensor, forwarding the sensed data towards a gateway function. The latter is deployed as a VNF on a ground equipment (a mini-ITX computer). The dissemination of data from the sensor to the gateway function follows a Publish/Subscribe approach based on the Message Queuing Telemetry Transport (MQTT) protocol15. The gateway function processes and then disseminates the data towards an Internet-of-things (IoT) server, which is made available as a VNF at the central site of the NFV ecosystem, based on the Mainflux16 open-source platform. Finally, the scenario assumes a remote area where Internet connectivity is provided by a cellular non-3GPP access network. Hence, the service includes two additional VNFs: 1) an access router VNF, which implements the user-plane protocol stack of a 3GPP user equipment connected to a non-3GPP access network17; and 2) a baseline implementation of a 5G core network, supporting the forwarding of information between the access router and the IoT server VNFs. To this purpose, the 5G core VNF provides a simplified implementation of the user-plane of a non-3GPP interworking function and a user plane function, as defined by 3GPP17.
Finally, Figure 4 represents the most relevant processes involved during the development of the protocol, highlighting their logical interconnections and the entities in charge of their execution.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Bebop 2</td>
			<td>Parrot</td>
			<td></td>
			<td>UAV used in the experiment to transport the RPis and thus, provide mobility to the compute units of external site.</td>
		</tr>
		<tr>
			<td>BME280 Sensor</td>
			<td>Bosch</td>
			<td></td>
			<td>Sensor capable of providing readings of the environmental conditions regarding temperature, barometric pressure, and humidity.&#160;</td>
		</tr>
		<tr>
			<td>Commercial Intel Core Mini-ITX Computer</td>
			<td>Logic Suppy</td>
			<td></td>
			<td>Computer server which hosts the OpenStack controller node (being executed as a VM) of the experiment&#39;s extternal aite. In addition, another unit of this equipment (along with the RPis) conforms the computational resources of the NFV insfrastrucure included in that site.</td>
		</tr>
		<tr>
			<td>Iptables</td>
			<td>Netfilter - Open source tool</td>
			<td></td>
			<td>(Software) An open source command line utility for configuring Linux kernel firewall rulset. Source-code available online: https://www.netfilter.org/projects/iptables/</td>
		</tr>
		<tr>
			<td>Lithium Battery Pack Expansion Board. Model KY68C-UK</td>
			<td>Kuman</td>
			<td></td>
			<td>Battery-power supply HAT (Hardware Attached on Top) for the UAV computation units composing the NFV infrastructure of the external site.</td>
		</tr>
		<tr>
			<td>MacBook Pro&#160;</td>
			<td>Apple</td>
			<td></td>
			<td>Commodity laptop utilized during the experiment to obtain and gather the results as described in the manuscript.</td>
		</tr>
		<tr>
			<td>Mainflux</td>
			<td>Mainflux Labs - Open source platform</td>
			<td></td>
			<td>(Software) Open source Internet of Things (IoT) platform used in the experiment for implementing the virtual network function called as IoT Server VNF. In addition, this platform includes an open-source software based on Grafana which allows the visualization and formatting of the metric data. Source code available online: https://www.mainflux.com/</td>
		</tr>
		<tr>
			<td>Open Source MANO (OSM) - Release FOUR</td>
			<td>ETSI OSM - Open source community</td>
			<td></td>
			<td>(Software) Management and Orchestration (MANO) software stack of the NFV system configured in the experiment. Source-code available online: https://osm.etsi.org/docs/user-guide/</td>
		</tr>
		<tr>
			<td>OpenStack - Release Ocata</td>
			<td>OpenStack - Open source community</td>
			<td></td>
			<td>(Software) Open source software used for setting up both the NFV infrastrucure of the central site and the NFV infrastructure of external site within the experiment. Source-code available online: https://docs.openstack.org/ocata/install-guide-ubuntu</td>
		</tr>
		<tr>
			<td>OpenVPN - Version 2.3.10</td>
			<td>OpenVPN - Open source community</td>
			<td></td>
			<td>Open source software implementing the VPN service presented in the experiment for the creation of the overlay network that will enable the operations of the NFV ecosystem (providing connectivity among all the sites comprising the ecosystem). Source-code available online: https://openvpn.net/&#160;</td>
		</tr>
		<tr>
			<td>Openvpn-monitor</td>
			<td>Python - Open source software</td>
			<td></td>
			<td>(Software) Open source program based on Python code that allows the visualization of the state of the VPN service, as well as the representation of the sites that are connected at every instant. For this purpose, the program check priodically the information provided by the VPN server implemented with OpenVPN. Source-code available online: https://github.com/furlongm/openvpn-monitor&#160;</td>
		</tr>
		<tr>
			<td>Paho-mqtt 1.5.0</td>
			<td>Python - Open source library</td>
			<td></td>
			<td>(Software) Open source library developed in Python code that enables the trasmission of the data read by the sensor through the use of MQTT standard&#160; Source-code available online: https://pypi.org/project/paho-mqtt/</td>
		</tr>
		<tr>
			<td>Ping&#160;</td>
			<td>Debian - Open source tool</td>
			<td></td>
			<td>(Software) An open source test tool, which verifies the connectivity between two devices connected through a communications network. In addition, this tool allows to assess the network performance since it calculates the Round Trip Time (i.e., the time taken to send and received a data packet from the network).&#160; Source-code available online: https://packages.debian.org/es/sid/iputils-ping</td>
		</tr>
		<tr>
			<td>Power Edge R430</td>
			<td>Dell</td>
			<td></td>
			<td>High-profile computer server which provides the computational capacity within the central site presented in the experiment.</td>
		</tr>
		<tr>
			<td>Power Edge R430</td>
			<td>Dell</td>
			<td></td>
			<td>High-profile computer server in charge of hosting the virtual private network (VPN) service. Note that the computing requirements for provisioning this service are high due to the resource consumption of the encryption operations present in the service.</td>
		</tr>
		<tr>
			<td>Power Edge R630</td>
			<td>Dell</td>
			<td></td>
			<td>Equipment used for hosting the virtual machine (VM) on charge of executing the MANO stack. In addition, the OpenStack controller node of the central site is also executed as a VM in this device. Note that the use of this device is not strictly needed. The operations carried out by this device could be done by a lower performance equipment due to the non-high resource specifications of the before mentioned VMs.</td>
		</tr>
		<tr>
			<td>Raspberry PI. Model 3b</td>
			<td>Raspberry Pi Foundation</td>
			<td></td>
			<td>Selected model of Single Board Computer (SBC ) used for providing the computational capacity to the experiment&#39;s external site. In addition, this SBC model is used during the deployment of the included realistic service for interpreting and sending the data collected by a sensor.</td>
		</tr>
		<tr>
			<td>RPi.bme280 0.2.3</td>
			<td>Python - Open source library</td>
			<td></td>
			<td>(Software) Open source library developed in Python code that allows to interface the sensor Bosch BME280, and interpret the readings offered by that sensor. Source-code available online: https://pypi.org/project/RPi.bme280/</td>
		</tr>
	</tbody>
</table>

integration of 5g experimentation infrastructures into a multi-site nfv ecosystem

Network Function Virtualization (NFV) has been regarded as one of the key enablers for the 5th Generation of mobile networks, or 5G. This paradigm allows to reduce the dependence on specialized hardware to deploy telecommunications and vertical services. To this purpose, it relies on virtualization techniques to softwarize network functions, simplifying their development and reducing deployment time and costs. In this context, Universidad Carlos III de Madrid, Telef&#243;nica, and IMDEA Networks Institute have developed an NFV ecosystem inside 5TONIC, an open network innovation center focused on 5G technologies, enabling the creation of complex, close to reality experimentation scenarios across a distributed set of NFV infrastructures, which can be made available by stakeholders at different geographic locations. This article presents the protocol that has been defined to incorporate new remote NFV sites into the multi-site NFV ecosystem based on 5TONIC, describing the requirements for both the existing and the newly incorporated infrastructures, their connectivity through an overlay network architecture, and the steps necessary for the inclusion of new sites. The protocol is exemplified through the incorporation of an external site to the 5TONIC NFV ecosystem. Afterwards, the protocol details the verification steps required to validate a successful site integration. These include the deployment of a multi-site vertical service using a remote NFV infrastructure with Small Unmanned Aerial Vehicles (SUAVs). This serves to showcase the potential of the protocol to enable distributed experimentation scenarios.

After carefully following the protocol to incorporate a new site to the central platform and run one network service to validate its proper functionality, Figure 6 depicts a screenshot of the open-vpn-monitor tool. It can be observed how the new site is using the VPN for all its communications, showing how its communications follow the VPN to allow this data exchange and, in consequence, the correct addition of the new site to the VPN service.
As depicted in Figure 3, the network service is delivering information from a sensor located in a remote infrastructure to the server located in the central site. In addition, Figure 7 displays the successful deployment of the network service from the OSM web GUI, showing how the experiment can be properly instantiated in the new remote infrastructure from the MANO stack located within the central site. Moreover, the time required in the experiment to complete the deployment of the service is around eight minutes. This value, along with the time needed to on-board the service descriptors into the orchestration platform (about 9 seconds, with 1.3 seconds per descriptor, considering both the NS and each VNF descriptors), enable to satisfy the Key Performance Indicator (KPI) of 90 minutes for the service creation time, as indicated by the 5G Infrastructure Public Private Partnership34. In this context, the work presented in Vidal et al.9 includes an in-depth analysis of the service creation time with multiple sites using the presented protocol.
Figure 8&#160;displays the data collected from the sensor, including the values of humidity, temperature and pressure respectively. These samples&#160;correspond to all data sent from the sensor to a remote server located in 5TONIC, where these values are stored in a database. All these data demonstrate that the platform is able to deploy practical network services after the inclusion of a new infrastructure, as well as to correctly enable communications between sites.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61946/61946fig01v2.jpg" /> 
Figure 1: VPN service site distribution. Distribution of the VPN service through the platform and their link connectivity (all passing through 5TONIC). <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/61946/61946fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/61946/61946fig02v2.jpg" /> 
Figure 2. Overview of the platform and VPN service. This figure shows all elements of the platform: the central location, along with its NFV Infrastructure, the VPN service and a new infrastructure aggregated to the system. It also includes the connections between its elements. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/61946/61946fig02largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/61946/61946fig03v2.jpg" /> 
Figure 3: Overview of the network service. It depicts the elements involved in the network service, its distribution and its logical, and networking, connectivity. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/61946/61946fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/61946/61946fig4v3.jpg" /> 
Figure 4: Protocol Workflows. Each column represents one section of the protocol, where every action performed is described, its logical connection between them and the component in charge of its execution. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/61946/61946fig4v3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/61946/61946fig05v2.jpg" /> 
Figure 5: Pin configuration scheme. Diagram representing how to make the physical connections between the board pins of the sensors and the GPIO pins of the SBC that incorporates that sensor. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/61946/61946fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/61946/61946fig06v2.jpg" /> 
Figure 6: OpenVPN-monitor snapshot. The picture shows that the aggregated infrastructure is connected to the VPN service, including some of its details regarding its connection. Moreover, the figure also depicts additional connections belonging to other remote infrastructures. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/61946/61946fig06largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/61946/61946fig07v2.jpg" /> 
Figure 7: OSM NS deployment status. OSM graphical interface, showing the successful deployment of the test network service in the remote infrastructure. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/61946/61946fig07largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/61946/61946fig08v3.jpg" /> 
Figure 8: Representative analysis of the data collected by the sensor. (A) Illustration of the temperature data periodically collected by the sensor every 5 seconds. (B) Graphical representation of the humidity data collected by the sensor every 5 seconds. (C) Visual depict of the pressure data collected by the sensor every 5 seconds.&#160;<a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/61946/61946fig08largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Integration of 5G Experimentation Infrastructures into a Multi-Site NFV Ecosystem at JoVE.com

Integration of 5G Experimentation Infrastructures into a Multi-Site NFV Ecosystem

The objective of the described protocol is to support the flexible incorporation of 5G experimentation infrastructures into a multi-site NFV ecosystem, through a VPN-based overlay network architecture. Moreover, the protocol defines how to validate the effectiveness of the integration, including a multi-site vertical service deployment with NFV-capable small aerial vehicles.

Network Function Virtualization (NFV) has been regarded as one of the key enablers for the 5th Generation of mobile networks, or 5G. This paradigm allows ...

integration-5g-experimentation-infrastructures-into-multi-site-nfv

Universidad de Cartagena UDEC

Research

JoVE Journal

Engineering

3.7K Views.  Universidad Carlos III de Madrid. The objective of the described protocol is to support the flexible incorporation of 5G experimentation infrastructures into a multi-site NFV ecosystem, through a VPN-based overlay network architecture. Moreover, the protocol defines how to validate the effectiveness of the integration, including a multi-site vertical service deployment with NFV-capable small aerial vehicles.

Video: Integration of 5G Experimentation Infrastructures into a Multi-Site NFV Ecosystem

Bringing the Visible Universe into Focus with Robo-AO

The angular resolution of ground-based optical telescopes is limited by the degrading effects of the turbulent atmosphere. In the absence of an atmosphere, the angular resolution of a typical telescope is limited only by diffraction, i.e., the wavelength of interest, &lambda;, divided by the size of its primary mirror's aperture, D. For example, the Hubble Space Telescope (HST), with a 2.4-m primary mirror, has an angular resolution at visible wavelengths of ~0.04 arc seconds. The atmosphere is composed of air at slightly different temperatures, and therefore different indices of refraction, constantly mixing. Light waves are bent as they pass through the inhomogeneous atmosphere. When a telescope on the ground focuses these light waves, instantaneous images appear fragmented, changing as a function of time. As a result, long-exposure images acquired using ground-based telescopes - even telescopes with four times the diameter of HST - appear blurry and have an angular resolution of roughly 0.5 to 1.5 arc seconds at best.
Astronomical adaptive-optics systems compensate for the effects of atmospheric turbulence. First, the shape of the incoming non-planar wave is determined using measurements of a nearby bright star by a wavefront sensor. Next, an element in the optical system, such as a deformable mirror, is commanded to correct the shape of the incoming light wave. Additional corrections are made at a rate sufficient to keep up with the dynamically changing atmosphere through which the telescope looks, ultimately producing diffraction-limited images.
The fidelity of the wavefront sensor measurement is based upon how well the incoming light is spatially and temporally sampled1. Finer sampling requires brighter reference objects. While the brightest stars can serve as reference objects for imaging targets from several to tens of arc seconds away in the best conditions, most interesting astronomical targets do not have sufficiently bright stars nearby. One solution is to focus a high-power laser beam in the direction of the astronomical target to create an artificial reference of known shape, also known as a 'laser guide star'. The Robo-AO laser adaptive optics system2,3 employs a 10-W ultraviolet laser focused at a distance of 10 km to generate a laser guide star. Wavefront sensor measurements of the laser guide star drive the adaptive optics correction resulting in diffraction-limited images that have an angular resolution of ~0.1 arc seconds on a 1.5-m telescope.

Light from astronomical objects must travel through the earth's turbulent atmosphere before it can be imaged by ground-based telescopes. To enable direct imaging at maximum theoretical angular resolution, advanced techniques such as those employed by the Robo-AO adaptive-optics system must be used.

The angular  resolution of ground-based optical telescopes is limited by the degrading  effects of the turbulent atmosphere. In the absence of an ...

Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics Beamline of the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory1-4. This video demonstrates how the complex chemical structures of laboratory-based model flames are analyzed using flame-sampling mass spectrometry with tunable synchrotron-generated vacuum-ultraviolet (VUV) radiation. This experimental approach combines isomer-resolving capabilities with high sensitivity and a large dynamic range5,6. The first part of the video describes experiments involving burner-stabilized, reduced-pressure (20-80 mbar) laminar premixed flames. A small hydrocarbon fuel was used for the selected flame to demonstrate the general experimental approach. It is shown how species&#8217; profiles are acquired as a function of distance from the burner surface and how the tunability of the VUV photon energy is used advantageously to identify many combustion intermediates based on their ionization energies. For example, this technique has been used to study gas-phase aspects of the soot-formation processes, and the video shows how the resonance-stabilized radicals, such as C3H3, C3H5, and i-C4H5, are identified as important intermediates7. The work has been focused on soot formation processes, and, from the chemical point of view, this process is very intriguing because chemical structures containing millions of carbon atoms are assembled from a fuel molecule possessing only a few carbon atoms in just milliseconds. The second part of the video highlights a new experiment, in which an opposed-flow diffusion flame and synchrotron-based aerosol mass spectrometry are used to study the chemical composition of the combustion-generated soot particles4. The experimental results indicate that the widely accepted H-abstraction-C2H2-addition (HACA) mechanism is not the sole molecular growth process responsible for the formation of the observed large polycyclic aromatic hydrocarbons (PAHs).

Gas sampling from laboratory-scale flames with online analysis of all species by mass spectrometry is a powerful method to investigate the complex mixture of chemical compounds occurring during combustion processes. Coupled with tunable soft ionization via synchrotron-generated vacuum-ultraviolet radiation, this technique provides isomer-resolved information and potentially fragment-free mass spectra.

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics ...

Atomic Force Microscopy of Red-Light Photoreceptors Using PeakForce Quantitative Nanomechanical Property Mapping

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be measured to ~10 pN by a laser and sectored detector, which can be converted to image topography. Amplitude modulation or &#8220;tapping mode&#8221; AFM involves the probe making intermittent contact with the surface while oscillating at its resonant frequency to produce an image. Used in conjunction with a fluid cell, tapping-mode AFM enables the imaging of biological macromolecules such as proteins in physiologically relevant conditions. Tapping-mode AFM requires manual tuning of the probe and frequent adjustments of a multitude of scanning parameters which can be challenging for inexperienced users. To obtain high-quality images, these adjustments are the most time consuming.
PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) produces an image by measuring a force response curve for every point of contact with the sample. With ScanAsyst software, PF-QNM can be automated. This software adjusts the set-point, drive frequency, scan rate, gains, and other important scanning parameters automatically for a given sample. Not only does this process protect both fragile probes and samples, it significantly reduces the time required to obtain high resolution images. PF-QNM is compatible for AFM imaging in fluid; therefore, it has extensive application for imaging biologically relevant materials.
The method presented in this paper describes the application of PF-QNM to obtain images of a bacterial red-light photoreceptor, RpBphP3 (P3), from photosynthetic R. palustris in its light-adapted state. Using this method, individual protein dimers of P3 and aggregates of dimers have been observed on a mica surface in the presence of an imaging buffer. With appropriate adjustments to surface and/or solution concentration, this method may be generally applied to other biologically relevant macromolecules and soft materials.

A method for investigating the structure of a protein photoreceptor using atomic force microscopy (AFM) is described in this paper. PeakForce Quantitative Nanomechanical Property Mapping (PF-QNM) reveals intact protein dimers on a mica surface.

Atomic force microscopy (AFM) uses a pyramidal tip attached to a cantilever to probe the force response of a surface. The deflections of the tip can be ...

Energy Dispersive X-ray Tomography for 3D Elemental Mapping of Individual Nanoparticles

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial resolution, and is even capable of providing atomically resolved elemental maps. In this technique, a highly focused electron beam is incident upon a thin sample and the energy of emitted X-rays is measured in order to determine the atomic species of material within the beam path. This elementally sensitive spectroscopy technique can be extended to three dimensional tomographic imaging by acquiring multiple spectrum images with the sample tilted along an axis perpendicular to the electron beam direction.
Elemental distributions within single nanoparticles are often important for determining their optical, catalytic and magnetic properties. Techniques such as X-ray tomography and slice and view energy dispersive X-ray mapping in the scanning electron microscope provide elementally sensitive three dimensional imaging but are typically limited to spatial resolutions of &gt; 20 nm. Atom probe tomography provides near atomic resolution but preparing nanoparticle samples for atom probe analysis is often challenging. Thus, elementally sensitive techniques applied within the scanning transmission electron microscope are uniquely placed to study elemental distributions within nanoparticles of dimensions 10-100 nm.
Here, energy dispersive X-ray (EDX) spectroscopy within the STEM is applied to investigate the distribution of elements in single AgAu nanoparticles. The surface segregation of both Ag and Au, at different nanoparticle compositions, has been observed.

The use of energy dispersive X-ray tomography in the scanning transmission electron microscope to characterize elemental distributions within single nanoparticles in three dimensions is described.

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired. In order to enable rapid characterization of materials that may be carried out in parallel with improvements to their growth methodology, a method of device fabrication for very small samples has been devised to permit the characterization of novel materials as soon as a preliminary batch has been produced. A slight variation of this methodology is applicable to producing devices using exfoliated samples of two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs), as well as multilayer heterostructures of such materials. Here we present detailed protocols for the experimental device fabrication of fragments and flakes of novel materials with micron-sized dimensions onto substrate and subsequent measurement in a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system at temperatures down to 0.300 K and magnetic fields up to 12 T.

We describe the methodology of mechanical exfoliation and deposition of flakes of novel materials with micron-sized dimensions onto substrate, fabrication of experimental device structures for transport experimentation, and the magnetotransport measurement in a dry helium close-cycle cryostat at temperatures down to 0.300 K and magnetic fields up to 12 T.

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film ...

Magnetically Induced Rotating Rayleigh-Taylor Instability

Classical techniques for investigating the Rayleigh-Taylor instability include using compressed gasses1, rocketry2 or linear electric motors3 to reverse the effective direction of gravity, and accelerate the lighter fluid toward the denser fluid. Other authorse.g. 4,5,6 have separated a gravitationally unstable stratification with a barrier that is removed to initiate the flow. However, the parabolic initial interface in the case of a rotating stratification imposes significant technical difficulties experimentally. We wish to be able to spin-up the stratification into solid-body rotation and only then initiate the flow in order to investigate the effects of rotation upon the Rayleigh-Taylor instability. The approach we have adopted here is to use the magnetic field of a superconducting magnet to manipulate the effective weight of the two liquids to initiate the flow. We create a gravitationally stable two-layer stratification using standard flotation techniques. The upper layer is less dense than the lower layer and so the system is Rayleigh-Taylor stable. This stratification is then spun-up until both layers are in solid-body rotation and a parabolic interface is observed. These experiments use fluids with low magnetic susceptibility, |&#967;| ~ 10-6 - 10-5, compared to a ferrofluids. The dominant effect of the magnetic field applies a body-force to each layer changing the effective weight. The upper layer is weakly paramagnetic while the lower layer is weakly diamagnetic. When the magnetic field is applied, the lower layer is repelled from the magnet while the upper layer is attracted towards the magnet. A Rayleigh-Taylor instability is achieved with application of a high gradient magnetic field. We further observed that increasing the dynamic viscosity of the fluid in each layer, increases the length-scale of the instability.

We present a protocol for preparing a two-layer density-stratified liquid that can be spun-up into solid body rotation and subsequently induced into Rayleigh-Taylor instability by applying a gradient magnetic field.

Classical techniques for investigating the Rayleigh-Taylor instability include using compressed gasses1, rocketry2 or linear electric motors3 to reverse ...

Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with one twist grain boundary. This apparatus utilizes high uniaxial pressure and a pulsed direct current for rapid consolidation of material. Diffusion bonding of strontium titanate bicrystals without fracture, in a spark plasma sintering apparatus, is possible at high pressures due to the unusual temperature dependent plasticity behavior of strontium titanate. We demonstrate a method for the successful formation of bicrystals at accelerated time scales and lower temperatures in a spark plasma sintering apparatus compared to bicrystals formed by conventional diffusion bonding parameters. Bond quality was verified by scanning electron microscopy. A clean and atomically abrupt interface containing no secondary phases was observed using transmission electron microscopy techniques. Local changes in bonding across the boundary was characterized by simultaneous scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy.

A viable technique for the formation of strontium titanate bicrystals at high pressure and fast heating rate via the spark plasma sintering apparatus is developed.

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (&#8804;150 &#176;C) growth of ultrathin films (100-1000 &#197;) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films&#39; optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

Thin films (100-1000 &#197;) of vanadium dioxide (VO2) were created by atomic-layer deposition (ALD) on sapphire substrates. Following this, the optical properties were characterized through the metal-insulator transition of VO2. From the measured optical properties, a model was created to describe the tunable refractive index of VO2.

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with ...