Etai Rosenkrantz

Enhanced absorption of light by charged nanoparticles

We found that various charged nanoparticles (NPs) can raise the attenuation of electromagnetic (EM) radiation over 30 times more efficiently during resonance in comparison to equivalent noncharged particles for a given set of parameters. A condition that indicates a state of resonance between the incident EM radiation and the NP surface excitations is mathematically derived. Our results shed light on the mechanism responsible for the strong absorption of light by such charged NPs. The outcome of this research could help to design a new generation of communication devices as well as a new technique for biological cell imaging.

An innovative modulating retro-reflector for free-space optical communication

Modulating retro-reflectors (MRR) are beneficial for asymmetric free-space optics communication links. An MRR includes an optical retro-reflector and an electro-optic shutter. The main advantage of an MRR configuration is that it shifts most of the power, weight, and pointing requirements onto one end of the link. In this study an innovative device comprising of nanoparticle-embedded ferroelectric thin film is used as an MRR. The new modulator is mounted in front of a passive retro-reflector. In our study we calculated the link budget for lunar exploration scenario. The scenario includes a base station that communicates with several robots or astronauts. In our simulations, the base station illuminates a robot with a continuous-wave beam, i.e. an interrogating beam. The un-modulated beam strikes the MRR, which is located on the robot, and is passively reflected back to the base station carrying the data that has been modulated onto it by the MRR. In this scenario a robot and a base-station are 4km apart, with a clear line of sight. In addition, the innovative MRR is capable of achieving 12dB contrast ratio. Under these assumptions and using the nanoparticle-embedded ferroelectric MRR we calculated the required transmission power for a given bit-rate and BER.

Tunable electro-optic filter based on metal-ferroelectric nanocomposite for VLC

The emerging technology of visible light communications (VLC) will provide a new modality of communication. This technology uses illumination lighting to carry information. We propose to add a smart capability to mitigate interferences from unwanted light sources. This is achieved by adaptively filtering interference light using a tunable filter to block interferences dynamically. In this Letter, we present an innovative concept for a tunable notch filter based on ferroelectric thin films embedded with noble metal nanoparticles. The adaptivity of the filter is achieved by controlling the external applied voltage. This voltage creates an electric field that changes the refractive index of the host film through the linear electro-optic effect. Moreover, the fundamental characteristics of the filter are determined by the layers parameters, such as film thickness, nanoparticles concentration and geometry, and the material of both the host thin film and nanoparticles. We study the tunability of lead zirconate titanate (PZT) embedded with Ag nanoparticles that reaches approximately 50 nm, between 530 and 590 nm. Moreover, we showed that a PZT notch filter embedded with Ag nanoshells has its stop band shifted to shorter wavelengths. These tunable filters can be used as mode selectors inside a laser resonator, spatial light filters for imaging and communication both for VLC and infrared communication.

Modulating Light by Metal Nanospheres-Embedded PZT Thin-Film

We calculated the optical contrast of silver nanospheres embedded in a lead-zirconate-titanate (PZT) film in relation to various volumetric concentrations (<;5%) of the nanospheres using Mie theory to approximate the extinction spectra, after assuming PZT to be isotropic. Then, we determined the nanosphere radius for large shifts in extinction efficiency that yields high contrast (as high as 12.74 dB), this radius was 7.4 nm. A device based on this nanocomposite film will be able to modulate light in the visible spectrum with sharp contrast between its “on” and “off” state at high speed and with low power consumption.

Resonance Frequencies of Electrically Charged Nanoparticles

We develop a procedure to analyze charged nanoparticle (NP) surface modes. Using the resonance condition derived by Rosenkrantz and Arnon, we obtain frequencies at which the electromagnetic (EM) radiation stimulates resonance over a wide range of modes. Our results confirm that the relation between the resonance frequencies and the excess surface charge can be described by a monotonically increasing function. Taking the derivative of this function, it is evident that the lower surface potentials have a greater influence on the resonance frequency. This effect decreases as the surface potential increases. Surface modes contribute to the surface energy of charged NPs, and for this reason, they can modify charged NP optical properties. We found that there is a strong dependence of the resonance frequencies on the electrically charged NP refractive index and surface potential. This dependence can play an important role in nonmetallic nanotechnological devices, such as attenuators and modulators in optical communication and optical detectors in biomedical sensors.

Optimum LED wavelength for underwater optical wireless communication at turbid water

Underwater optical wireless communication is an emerging technology, which can provide high data rate. High data rate communication is required for applications such as underwater imaging, networks of sensors and swarms of underwater vehicles. These applications pursue an affordable light source, which can be obtained by light emitting diodes (LED). LEDs offer solutions characterized by low cost, high efficiency, reliability and compactness based on off-the-shelf components such as blue and green light emitting diodes. In this paper we present our recent theoretical and experimental results in this field.

Reducing energy consumption of data centers using optical wireless links

Data Center (DC) servers consume a huge amount of energy. However, a part of this energy (cooling and information technology devices) can be saved by shutting down racks containing idle servers. This can be achieved by rerouting workloads from partially loaded racks, and thus allowing additional racks to switch into idle mode. Workload rerouting requires additional wired links or by optical wireless communication (OWC) links, which can be deployed on top the existing network. In comparison to classic wired infrastructure, OWC deployment and maintenance is faster and simpler, and it offers dynamic configuration of the network. Moreover, OWC can be used to augment the performance of virtualization techniques, which allow load balancing. In this paper we developed a mathematical model and an optimization algorithm along with numerical simulations, which evaluate the energy consumption in DCs. Results show that for a uniform distributed load between 24–100% and a 100% deployment of OWC links, 33% of the IT and cooling energy consumed can be saved.

1550 nm modulating retroreflector based on coated nanoparticles for free-space optical communication

Nowadays, there is a renaissance in the field of space exploration. Current and future missions depend on astronauts and a swarm of robots for reconnaissance. In order to reduce the power consumption, weight, and size of the robots, an asymmetric communication system may be used. This is achieved by installing modulating retroreflectors (MRRs) on one side of the link and an interrogating laser on the other side. In this paper, we theoretically study an innovative device that can serve as an MRR in the infrared range of the spectrum. The device is based on a ferroelectric PZT thin film containing TiO2 coated Ag nanoparticles, which exhibit strong plasmonic resonance in the infrared range. After intensive analyses, which included calculations and simulations, we were able to design the device to operate at the 1550 nm wavelength. This is of great importance since the design of devices operating at 1550 nm as this wavelength is a mature technology widely used in free-space optics. Hence, this MRR can serve in asymmetric communication links relying on 1550 nm transmissions, which are also eye-safe. To the best of our knowledge, this is the first time coated metal nanoparticles have been proposed to modulate light in the infrared region. The performance of this device is unique, reaching a 17.5 dB modulation contrast with only a ±2 V operating voltage. This modulator may also be used for terrestrial communication such as fiber optics and optical interconnects in future data centers.

Modulated retro reflector for VLC applications

Visible light communication (VLC) is a new emerging technology that uses standard visible light to transmit broadband data streams in addition to illumination. In our research we have theoretically studied an innovative device that can serve as a modulating retro-reflector (MRR) for VLC applications. The device comprises of a nanocoposite of ferroelectric thin-film embedded with noble metal nano-shells. In comparison to the nano-spheres, the nano-shells provide more flexibility in the design of the device. This MRR can be used in asymmetric communication links as an optical transceiver for mobile devices. The main conclusion from our study is that a nanocomposite based MRR can save power, complexity, dimensions and weight in comparison to standard communication links. this fact is very important for mobile platforms.

Local Surface Plasmon Tuning for Optical Devices

Confining light to nanoscale dimensions has become possible with surface plasmons. However, active control of plasmonic responses remains a hurdle for building plasmonic optical devices. In this letter, we analytically derive a model describing the gap between the local surface plasmon (LSP) modes of a spherical nanoparticle in anisotropic media as a function of applied external electric field. Anisotropic ferroelectric materials exhibit birefringence that can be controlled through the electro-optic effect. Hence, the splitting of LSP frequencies in ferroelectrics embedded with nanoparticles can be controlled. In other words, by applying an external voltage a transition between the anisotropic and the isotropic phases can be achieved. As a result, the gap created by anisotropy can be tuned by the applied voltage. We derive and provide an explanation to the required field intensity according to the coercive field of ferroelectric materials. This simple approach is the basis for a method to actively tune the amplitude and the polarizability of light. The method can be considered for photonic applications, such as optical switches and biomedical sensors.