Abstract-Terahertz modulator based on insulator–metal transition in photonic crystal waveguide

http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-51-20-4589
Fei Fan, Yu Hou, Zi-Wei Jiang, Xiang-Hui Wang, and Sheng-Jiang Chang
A terahertz modulator based on the insulator–metal transition (IMT) in a photonic crystal waveguide (PCW) coated by vanadium dioxide (VO2) film is proposed. The numerical simulations show that a dielectric state and a metallic state with quite different photonic band structures and transmission properties in the proposed PCW are reciprocally converted because of the IMT of VO2, and the pass-bands of this PCW are greatly shifted from 0.68 to 0.8 and 1.02 to 1.25 THz to 0.8–1.45 THz. This PCW significantly enhances the modulation depth and sensitivity compared with bare VO2 film. Extensive investigation demonstrates that the thickness of VO2 film greatly affects the IMT process in the PCW, and limits the ultimate modulation depth of the device. The proposed modulation scheme will be of great significance for potential THz applications.

Abstract-The importance of scattering, surface potential, and vanguard counter-potential in terahertz emission from gallium arsenide

D. L. Cortie and R. A. Lewis

Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia

It is well established that under excitation by short (<1 ps), above-band-gap optical pulses, semiconductor surfaces may emit terahertz-frequency electromagnetic radiation via photocarrier diffusion (the dominant mechanism in InAs) or photocarrier drift (dominant in GaAs). Our three-dimensional ensemble Monte Carlo simulations allow multiple physical parameters to vary over wide ranges and provide unique direct insight into the factors controlling terahertz emission. We find for GaAs (in contrast to InAs), scattering and the surface potential are key factors. We further delineate in GaAs (as in InAs) the role of a vanguard counter-potential. The effects of varying dielectric constant, band-gap, and effective mass are similar in both emitter types.

© 2012 American Institute of Physics

Magnetoplasmonics moves on

Image via CrunchBase

http://nanotechweb.org/cws/article/tech/50110

Researchers at IBM have come across an unexpected phenomenon while studying how plasmons in graphene behave in the presence of a magnetic field. The finding could help the new and upcoming field of magnetoplasmonics, with graphene finding its way into terahertz magneto-optical devices, such as modulators and Faraday rotators.

Excitation of magnetoplasmons in graphene disks

The team, headed by Phaedon Avouris of IBM's TJ Watson Research Center in New York and Zhiqiang Li of the National High Magnetic Field Laboratory in Florida, studied graphene that had been patterned into a periodic array of microdisks. The structure absorbs light by confining it into regions that are hundreds of times smaller than the wavelength of the light by exploiting plasmons that occur within the individual microdisks. Plasmons are quantized collective oscillations of electrons – and they interact strongly with light.

Graphene appears to be emerging as a very promising plasmonic material thanks to the material's unusual electronic properties, which result in its electrons moving extremely fast and behaving like relativistic "Dirac" particles with virtually no rest mass. Graphene absorbs light particularly well in the terahertz and infrared parts of the electromagnetic spectrum – something that could lead to novel applications in photonics and quantum optics. "What is more, unlike plasmons in metals, the plasmons in graphene should
be strongly affected by an external magnetic field – all because graphene electrons behave like massless fermions," explained project leader Hugen Yan.

Longer-lived edge plasmons

The researchers obtained their results by measuring the light transmission spectrum of the graphene disk arrays, using a Fourier transform IR spectrometer together with a silicon bolometer, while a magnetic field was applied perpendicular to the disks. To their surprise, they found that plasmons at the edges of the nanostructures appear to last longer when a magnetic field is applied. This is counter-intuitive, Yan tells nanotechweb.org, because there are potentially more defects in the vicinity of an edge plasmon that should, conversely, reduce its lifetime.

According to the team, the applied magnetic field may be suppressing electron backscattering at the edges of the microdisks, so allowing the plasmons to last longer in the samples. The result is doubly unexpected given that the phenomenon has never been observed before in conventional 2D electron gas systems in the terahertz range.

And that is not all: the lifetimes of the edge plasmons can also be tuned by varying the magnitude of the applied magnetic field, with larger fields encouraging longer-lived plasmons.

"A long plasmon lifetime is a big advantage for applications in chemical and biological sensing, as well as in electric field enhancement, and could lead to a variety of magneto-optical applications in the future," said Yan.

The IBM team is now planning to study magnetoplasmons in other graphene microstructures, such as graphene rings, dots, ribbons and elliptical disks.

The current work is detailed in Nano Letters.

About the author

Belle Dumé is contributing editor at nanotechweb.org

Abstract-Active nanoplasmonic metamaterials (bridging gap between conventional optics and the nanoworld)

(a–d, Successive steps in the fabrication of an active negative-refractive-index 'double-fishnet' metamaterial. )

http://www.nature.com/nmat/journal/v11/n7/full/nmat3356.html

O. Hess, J. B. Pendry, S. A. Maier, R. F. Oulton, J. M. Hamm,  K. L. Tsakmakidis   

Optical metamaterials and nanoplasmonics bridge the gap between conventional optics and the nanoworld. Exciting and technologically important capabilities range from subwavelength focusing and stopped light to invisibility cloaking, with applications across science and engineering from biophotonics to nanocircuitry. A problem that has hampered practical implementations have been dissipative metal losses, but the efficient use of optical gain has been shown to compensate these and to allow for loss-free operation, amplification and nanoscopic lasing. Here, we review recent and ongoing progress in the realm of active, gain-enhanced nanoplasmonic metamaterials. On introducing and expounding the underlying theoretical concepts of the complex interaction between plasmons and gain media, we examine the experimental efforts in areas such as nanoplasmonic and metamaterial lasers. We underscore important current trends that may lead to improved active imaging, ultrafast nonlinearities on the nanoscale or cavity-free lasing in the stopped-light regime

Advanced Photonix, Inc. Continues Industrial Market Growth in Terahertz Industrial Process Control

http://finance.yahoo.com/news/advanced-photonix-inc-continues-industrial-143100217.html
ANN ARBOR, Mich.--(BUSINESS WIRE)--
Advanced Photonix, Inc.^® (NYSE Amex: API) today announces that it has received orders from its value-added reseller, Thermo Fisher Scientific, for its T-Gauge^® industrial terahertz (THz) gauging system. These systems are scheduled to be delivered late in second quarter of this fiscal year and deployed on the factory floor shortly thereafter.
The T-Gauge^® measurement systems will be utilized for quality control of multilayer commercial industrial products. The system will measure the layers and provide feedback for adjusting the amount of material to ensure the quality of the product and reduce material costs. The advantage of the T- Gauge^® system is that it can measure the layers directly in real-time and closer to the source than other technologies, making a bigger impact on the manufacturing processes.
The T-Gauge^® system is designed to be deployed on the industrial factory floor for process and quality control applications requiring non-contact measuring of physical properties such as; multiple layer thickness measurements, density, contamination by foreign substances, sub-surface defects within a product and moisture analysis. The T-Gauge^® is non-nuclear and operates at speeds that make it ideal for web processing and converting solutions. The system can be used on-line or off-line to help manufacturers to improve quality and reduce costs by collecting more information and tightening tolerances than previously available.
“This marks the first sale of our new T-Gauge^® measurementsystem for deployment on the factory floor and our continued penetration into the industrial market with our world leading terahertz instrumentation. This is an exciting time for us and our partners as we are beginning to see real traction in the market,” commented Richard Kurtz, President and CEO.
About Advanced Photonix, Inc.
Advanced Photonix, Inc.^® (NYSE Amex: API) is a leading supplier with a broad offering of optoelectronic products to a global customer base. We provide optoelectronic solutions, high-speed optical receivers and terahertz instrumentation for telecom, homeland security, military, medical and industrial markets. With our patented technology and state-of-the-art manufacturing we offer industry leading performance, exceptional quality, and high value-added products to our OEM customer base. For more information visit us on the web at www.advancedphotonix.com.
The information contained herein includes forward looking statements that are based on assumptions that management believes to be reasonable but are subject to inherent uncertainties and risks including, but not limited to, unforeseen technological obstacles which may prevent or slow the development and/or manufacture of new products; potential problems with the integration of the acquired company and its technology and possible inability to achieve expected synergies; obstacles to successfully combining product offerings and lack of customer acceptance of such offerings; limited (or slower than anticipated) customer acceptance of new products which have been and are being developed by the Company; and a decline in the general demand for optoelectronic products. API-G

Contact:

Advanced Photonix, Inc.
Richard Kurtz, (734) 864-5688 begin_of_the_skype_highlighting            (734) 864-5688      end_of_the_skype_highlighting
IR@advancedphotonix.com

Watching the Quantum Race of Electrons

http://www.sciencedaily.com/releases/2012/06/120621112539.htm

ScienceDaily (June 21, 2012) — For the first time ever, a German team of physicists has observed the race of two electrons that are liberated from atoms in the course of photoionization, i.e. under the influence of laser radiation. In order to resolve the electron's movement during only 50 femtoseconds, scientists from Hamburg and Kiel Universities and DESY (Deutsches Elektronen-Synchrotron -- one of the world's leading accelerator centres) used an ultra fast terahertz streak camera in combination with a free-electron-laser. The experimental findings will improve the rapidly evolving free-electron-laser technology used in many fields of science -- from physics to biology.

• lThe results are published on June 20th in Physical Review Letters.

The interaction of electrons is crucial for the properties of atoms and molecules which are the building blocks of living organisms and all human-made systems. Understanding the electronic processes in atoms is therefore essential for numerous future applications -- from medicine to engineering. When a photon of sufficiently high energy hits an atom it may kick out an electron. Albert Einstein termed this process "photo ionization" (photo effect) back in 1905 and thus confirmed a theory of the Kiel scientist Max Planck that would soon revolutionize physics: quantum mechanics. Quantum mechanics predicts that the liberated electron runs away from the atom, behaving like a wave. In the 1920s, Lise Meitner and Pierre Auger independently discovered another effect: When applying specific photon energy during photo ionization, a second electron, termed Auger electron, may be liberated from the atom. In cases when the Auger electron runs in the same direction as the first electron a quantum race of electron waves starts.
This race of the two electrons is extremely fast, taking between 1 and 100 femtoseconds (1fs=0.000 000 000 000 001s). "Until now, no experiment has resolved this race in time," says Professor Markus Drescher of the Institute for Experimental Physics at Hamburg University, leader of the experimental part of the project. The key to observing the electron race in the current study was to combine the free-electron laser FLASH at DESY in Hamburg with an ultrafast streak camera. By accurately controlling the timing of the two electromagnetic fields generated by the laser and the camera, the scientists were able to reconstruct the electrons' movement and energy exchange. In this manner, the physicists detected how the Auger electron passes the first one.
At the Institute of Theoretical Physics and Astrophysics of Kiel University, the research group of Professor Michael Bonitz tried to reproduce the experimental results with computer simulations. "After testing several possible explanations for the measured results, our PhD student Sebastian Bauch verified that the experiment indeed observed the quantum race of two electrons," adds Bonitz. The Kiel physicists collaborate with the experimentalists from Hamburg within the project "FLASH" funded by the German Federal Ministry of Education and Research.
The results give valuable information on key properties of the involved laser pulse. The free electron laser technology has been rapidly evolving in recent years. In this context the present results will be important for improving the quality and precision of future free electron laser experiments

Abstract-Exciton Mott transition in Si revealed by terahertz spectroscopy

http://prl.aps.org/accepted/L/9107fYadD4618036a3439285276298d9fd8b96887
Takeshi Suzuki and Ryo Shimano
We investigate the exciton Mott transition in Si by using optical pump and terahertz probe spectroscopy. The density-dependent exciton ionization ratio a is quantitatively evaluated from the analysis of dielectric function and conductivity spectra. The Mott density is clearly determined by the rapid increase in a as a function of electron-hole (e-h) pair density, which agrees well with the value expected from the random phase approximation theory. However, exciton is sustained in the high-density metallic region above the Mott density as manifested by the 1s-2p excitonic resonance that remains intact across the Mott density. Moreover, the charge carrier scattering rate is strongly enhanced slightly above the Mott density due to nonvanishing excitons, indicating the emergence of highly correlated metallic phase in the photo-excited e-h system. Concomitantly, the loss function spectra exhibit the signature of plasmon-exciton coupling, i.e., the existence of a new collective mode of charge density excitation combined with the excitonic polarization at the proximity of Mott density.

Researchers take wireless to ludicrous speed at 2.5 Tbps

http://gigaom.com/mobile/researchers-take-wireless-to-ludicrous-speed-at-2-5-tbps/
By Stacey Higginbotham
Researchers at the University of Southern California, Tel Aviv University and NASA’s Jet Propulsion lab have used their massive brains to show off a way todeliver speeds of 2.5 terabits per secondwirelessly. That’s 500,000 times faster than the current low-end LTE speeds and 5,000 times faster than the 5.5 gigabits per second, which is the fastest theoretical wireless broadband I’ve encountered in my years covering wireless.

These researchers have managed to achieve this epic speed at a distance of less than one meter using what researchers call “twisted signals.” According to Extreme Tech, which explains it so well:

These twisted signals use orbital angular momentum (OAM) to cram much more data into a single stream. In current state-of-the-art transmission protocols (WiFi, LTE, COFDM), we only modulate the spin angular momentum (SAM) of radio waves, not the OAM. If you picture the Earth, SAM is our planet spinning on its axis, while OAM is our movement around the Sun. Basically, the breakthrough here is that researchers have created a wireless network protocol that uses both OAM and SAM.

The resulting technology offers the spectral efficiency of 95.7 bits per hertz. To put that into perspective, today on Verizon’s LTE network, the equipment delivers 1.5 bits per hertz of spectrum. By delivering so much data per hertz of spectrum, the barriers toward building ever-faster networks as defined by Shannon’s Law would become fundamentally reset, allowing the next generation of engineers to build networks unimaginable to today’s generation.

Obviously, the usual caveats around new technologies apply. So far this is in the lab only. The speeds aren’t maintained for long distances and are based on lightwaves as opposed to radio waves (this means line of sight is essential). There’s no indication of how much power chips to deliver this type of speed would consume, and there’s no actual ecosystem in place to support or even twist those wavelengths.

That being said, there’s no Moore’s Law governing wireless networks, which is a real problem given how much data we are demanding via mobile networks. Granted, Wi-Fi andmore efficient technologies will help, but we need a fundamental breakthrough on the physics side to keep up with wireless consumption. Terahertz spectrum and chips are one way, and these twisted signals might be another. As a plus, they could work on fiber optic networks too, which means we might see another boost in broadband capacity along our long haul and core networks.

Broadband Array of Invisibility Cloaks in the Visible Frequency Range

http://www.2physics.com/2012/06/broadband-array-of-invisibility-cloaks.html

Author: Vera Smolyaninova

Affiliation: Dept. of Physics Astronomy and Geosciences, Towson University, MD, USA

Ever since the first experimental demonstrations in the microwave and visible ranges [1,2], invisibility cloaks have stimulated progress in the fields of metamaterials and transformation optics. Very recently, Farhat and co-workers [3] suggested that arrays of invisibility cloaks may have interesting electromagnetic properties, and suggested some potential applications in noninvasive probing, sensing and communication. Our team, Vera Smolyaninova and Kurt Ermer from Towson University, and Igor Smolyaninov from the University of Maryland demonstrated the first experimental realization of an invisibility cloak array.

Our experiment is based on the recent demonstration of broadband invisibility cloak, which relies on a curved waveguide mimicking the metamaterial properties necessary for cloaking [4]. Since a gap between a gold-coated spherical lens touching a gold-coated planar glass slide provides a good approximation of the required waveguide shape, such geometry can be easily transformed into a large array of broadband invisibility cloaks using commercially available microlens arrays. This work is reported in the May issue of the New Journal of Physics [5]. In the experiments, conducted at Towson University, very large arrays of roughly 25000 cloaks were used to “hide” approximately 20% of the surface area. This is the first experimental arrangement which lets you study mutual interactions of a very large number of invisibility cloaks.

Past 2Physics article by Vera Smolyaninova:
June 06, 2009: "Large Broadband Invisibility Cloak for Visible Light"
by Vera Smolyaninova and Vlad Shalaev

Igor Smolyaninov of University of Maryland

Unlike the so-called “carpet cloaks” which hide objects on the metallic mirror background, every cloak in our array guides light around the cloaked area. While the former approach is akin to rendering bumps on a carpet invisible by allowing them to blend in with the carpet, classical cloaking concentrates on enabling light to flow around an object. Typically, such classical cloaks [1,2] require sophisticated metamaterial nanofabrication. Each material has its own refractive index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. Natural materials typically have refractive indices greater than one. Refraction occurs as electromagnetic waves bend when passing from one material into another. It causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside.

Kurt Ermer of Towson University

Unlike natural materials, metamaterials are able to produce the index of refraction ranging from very large positive values of the order of 100 to less than zero. In particular, artificial metamaterials needed for cloaking must have the index of refraction, which varies from zero to one. Unfortunately, such artificial metamaterials have very large losses. In our cloak array the precisely tapered shape of the waveguide around each cloak alters the refractive index in the same way as in metamaterials, gradually increasing the index from zero to 1 along the curved surface of each microlens. Since light propagates mainly through the air gap, losses in this design are very low, and the resulting structure is broadband. It works across the whole visible light spectrum.

Theoretical work for the design was led by the University of Maryland, with Towson University leading work to fabricate the device and demonstrate its cloaking properties. The cloaking array device is formed by two gold-coated surfaces, one surface being a commercially available microlens array, and the other a flat glass slide. Individual cloaks in the array were separated by about 30 microns, or roughly the width of a human hair, so that a 5 by 5 millimeter squared microlens array would make approximately 25000 individual invisibility cloaks. Instead of being reflected as normally would happen, the light flows around each cloak and shows up on the other side, like water flowing around an array of stones.

[Click on image to see a higher resolution version]

Building and studying the arrays of invisibility cloaks offers more refined experimental tools to test individual cloak performance. Compared to the characterization of individual cloaks, the angular performance of cloak arrays appears to be more sensitive to cloak imperfections. For example, cloak arrays perform better when light is sent in along the row directions. These findings may be useful in such related areas as acoustic and surface-wave cloaking, as well as in the potential practical applications listed above.

On the other hand, since light is “stopped” near each cloak, and the cloak radius depends on the light wavelength, the cloak array produced in our study may be used in the spectrometer on the chip applications. The “trapped rainbow” effect observed near each cloak [6] may find applications in such fields as biosensing and testing for genetic decease. Stopping light at the cloak boundary leads to considerable enhancement of fluorescence near each cloak in the array [6].

The work was funded by the National Science Foundation.

References
[1] “Metamaterial electromagnetic cloak at microwave frequencies”, D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies”, Science 314, 977-980 (2006). Abstract.
[2] “Two-dimensional metamaterial structure exhibiting reduced visibility at 500 nm”,I.I. Smolyaninov, Y.J. Hung, and C.C. Davis, Optics Letters 33, 1342-1344 (2008). Abstract.
[3] “Understanding the functionality of an array of invisibility cloaks”, M. Farhat, P.-Y. Chen, S. Guenneau, S. Enoch, R. McPhedran,C. Rockstuhl, and F. Lederer, Phys. Rev. B 84 235105 (2011). Abstract.
[4] “Anisotropic metamaterials emulated by tapered waveguides: application to electromagnetic cloaking”, I.I. Smolyaninov, V.N. Smolyaninova, A.V. Kildishev, and V.M. Shalaev, Phys. Rev. Letters 103, 213901 (2009). Abstract. 2Physics Article.
[5] “Experimental demonstration of a broadband array of invisibility cloaks in the visible frequency range”, V.N. Smolyaninova, I.I. Smolyaninov, and H.K. Ermer, New J. Phys. 14, 053029 (2012). Full Article.
[6] “Trapped rainbow techniques for spectroscopy on a chip and fluorescence enhancement” V.N. Smolyaninova, I.I. Smolyaninov, A.V. Kildishev, and V.M. Shalaev, Applied Physics B 106, 577-581 (2012).Abstract. arXiv:1101.336.

Labels: Invisibility Cloak 2, Metamaterial 2

Abstract-Nonlinear effects in the Josephson-vortex terahertz photonic crystal: Second harmonic generation

A. Wall-Clarke and S. Savel'ev 
Department of Physics, Loughborough University, Loughborough LE11 3TU, United Kingdom

Received 20 April 2012; published 20 June 2012

Received 20 April 2012; published 20 June 2012

This paper considers nonlinear effects on the propagation of Josephson plasma waves in the Josephson-vortex photonic crystal in layered superconductors with the possibility of exciting the second harmonic, tunable by magnetic field. This would enable an increase in the operable frequency of THz superconducting devices such as generators, detectors, and filters. We demonstrate numerically that the second harmonic can be strongly amplified for certain resonance frequencies. The linear spectrum for the second harmonic has also been analyzed, finding the possible resonance points with the first harmonic spectrum. An analytical approach describing a spatial distribution of the THz Josephson plasma waves at and near the resonances is also developed.

©2012 American Physical Society

URL:

http://link.aps.org/doi/10.1103/PhysRevB.85.214521

DOI:

10.1103/PhysRevB.85.214521

PACS:

74.25.Uv, 42.70.Qs