OT-Terahertz speed -"A read head for quantum computers? Graphene layer reads optical information from nanodiamonds"

Vision of a future quantum computer with chips made of diamond and graphene. Credit: Christoph Hohmann, NIM

 http://phys.org/news/2014-12-quantum-graphene-layer-optical-nanodiamonds.html#jCp

Nitrogen-vacancy centers in diamonds could be used to construct vital components for quantum computers. But hitherto it has been impossible to read optically written information from such systems electronically. Using a graphene layer, a team of scientists headed by Professor Alexander Holleitner of the Technische Universität München has now implemented just such a read unit.

Ideally, diamonds consist of pure carbon. But natural diamonds always contain defects. The most researched defects are nitrogen-vacancy centers comprising a nitrogen atom and a vacancy. These might serve as highly sensitive sensors or as register components for quantum computers. However, until now it has not been possible to extract the optically stored information electronically.

A team headed by Professor Alexander Holleitner, physicist at the TU München and Frank Koppens, physics professor at the Institut de Ciencies Fotoniques near Barcelona, have now devised just such a methodology for reading the stored information. The technique builds on a direct transfer of energy from nitrogen-vacancy centers in nanodiamonds to a directly neighboring graphene layer.

Non-radiative energy transfer

When laser light shines on a nanodiamond, a light photon raises an electron from its ground state to an excited state in the nitrogen-vacancy center. "The system of the excited electron and the vacated ground state can be viewed as a dipole," says Professor Alexander Holleitner. "This dipole, in turn, induces another dipole comprising an electron and a vacancy in the neighboring graphene layer."

In contrast to the approximately 100 nanometer large diamonds, in which individual nitrogen-vacancy centers are insulated from each other, the graphene layer is electrically conducting. Two gold electrodes detect the induced charge, making it electronically measureable.

Laboratory set-up measuring the interaction between graphene and nano-diamonds with implanted nitrogen-vacancy centers. Credit: Astrid Eckert / TUM

Picosecond electronic detection

Essential for this experimental setup is that the measurement is made extremely quickly, because the generated electron-vacancy pairs disappear after only a few billionths of a second. However, the technology developed in Holleitners laboratory allows measurements in the picosecond domain (trillionths of a second). The scientists can thus observe these processes very closely.

"In principle our technology should also work with dye molecules," says doctoral candidate Andreas Brenneis, who carried out the measurements in collaboration with Louis Gaudreau. "A diamond has some 500 point defects, but the methodology is so sensitive that we should be able to even measure individual dye molecules."

As a result of the extremely fast switching speeds of the nanocircuits developed by the researchers, sensors built using this technology could be used not only to measure extremely fast processes. Integrated into future quantum computers they would allow clock speeds ranging into the terahertz domain.

 Explore further: Planting imperfections at specific spots within diamond lattice could advance quantum computing

More information: Ultrafast electronic readout of diamond nitrogen-vacancy centres coupled to graphene, Andreas Brenneis, Louis Gaudreau, Max Seifert, Helmut Karl, Martin S. Brandt, Hans Huebl, Jose A. Garrido, Frank H. L. Koppens and Alexander W. Holleitner, Nature Nanotechnology, Advanced online publication, December 1, 2014 – DOI: 10.1038/nnano.2014.276

Journal reference: Nature Nanotechnology 

Read more at: http://phys.org/news/2014-12-quantum-graphene-layer-optical-nanodiamonds.html#jCp

A Meta-Mirror for Nonlinear Optics

Stewart Wills   http://www.osa-opn.org/home/newsroom/2014/july/a_meta-mirror_for_nonlinear_optics/#.U8fhBJRdV8E

The 400-nanometer-thick nonlinear mirror produces frequency-doubled output through the interaction between the gold plasmonic metasurface and the underlying MQW semiconductor layer. (Source: University of Texas, Austin.)

In recent years, researchers have achieved nonlinear optical responses in semiconductor heterostructures that significantly outstrip the responses in traditional nonlinear materials. But there’s been a catch—the nonlinear response has been limited to incident light polarized perpendicular to the semiconductor layers. Now, a team from the University of Texas, Austin, U.S.A., and the Technische Universität München, Germany, report that they have broken through that restriction. As a result, they've been able to fashion a 400-nm-thick “meta mirror” that can produce nonlinear effects at an intensity roughly a million times better than the best traditional nonlinear materials (Nature, DOI: 10.1038/nature13455).

The researchers achieved the advance by coupling the electromagnetic modes in plasmonic metasurfaces with the control of electronic transitions enabled by multi-quantum-well (MQW) semiconductor heterostructures. The team sandwiched an MQW structure—basically a stack of around 100 nanometer-scale semiconductor layers engineered to optimize the nonlinear response—between a metal ground base and a plasmonic metasurface consisting of asymmetric gold “nanocrosses.” As incident light hits the surface, the MQW semiconductor layers confine electrons into the desired quantum states. The nanocrosses at the surface layer, meanwhile, resonate at input and output frequencies to enable the mirror’s nonlinear optical response. The proof-of-concept system was able to take input light with an intensity as low as that of a laser pointer, and reflect frequency-doubled light at an intensity orders of magnitude greater than in traditional nonlinear materials.

The researchers suggest that these new structures could be tuned to work at frequencies from near-infrared to mid-infrared to terahertz, and can be engineered to produce giant responses for a variety of nonlinear processes beyond frequency doubling. The result, they say, could be a new generation of ultrathin, highly nonlinear optical elements that could help advance laser systems for chemical sensing, explosives detection, biomedical research and other areas with an appetite for compact designs spanning a wide range of wavelengths.
 

A million times better

http://www.sciencecodex.com/a_million_times_better-136849

Lasers have a fixed place in many fields of application. Yet, there are still wavelengths for which either no systems exist, or at best only large and expensive ones. On the other hand remote sensing and medical applications call for compact laser systems, for example with wavelengths from the near infrared to the Terahertz region.

A team of researchers at the Technische Universitaet Muenchen (Germany) and the University of Texas Austin (USA) has now developed a 400 nanometer thick nonlinear mirror that reflects frequency-doubled output using input light intensity as small as that of a laser pointer. For a given input intensity and structure thickness, the new nonlinear metamaterials produce approximately one million times higher intensity of frequency-doubled output, compared to the best traditional nonlinear materials.

Furthermore, because the frequency conversion happens over subwavelength scales, the demonstrated nonlinear mirrors are free from the stringent requirement of matching the phase velocities of the input and output waves, which complicates nonlinear optical experiments with bulk nonlinear crystals.

The new structures can be tailored to work at various frequencies from near-infrared to mid-infrared to terahertz and can be designed to produce giant nonlinear response for different nonlinear optical processes, such as second harmonic, sum- and difference-frequency generation, as well a variety of four-wave mixing processes.

The super sandwich

The magical material the physicists have created comprises a sequence of thin layers made of indium, gallium and arsenic on the one hand and aluminum, indium and arsenic on the other. They stacked about 100 of these layers, each between one and twelve nanometers thick, on top of each other and sandwiched them between a layer of gold at the bottom and a pattern of asymmetrical, crossed gold nanostructures on top

This is the 400-nanometer-thick nonlinear mirror that reflects frequency-doubled output using input light intensity as small as that of a laser pointer.

(Photo Credit: University of Texas, Austin)

Tuning the semiconductor layers thicknesses and the gold surface nanostructures geometry, the researchers have two possibilities to adjust the structure to resonate optimally with the desired wavelengths. For the initial demonstration, the material converts light with a wavelength of 8000 nanometers to 4000 nanometers. "Laser light in this frequency range can be used in gas sensors for environmental technology," says Frederic Demmerle, project member at the Walter Schottky Institute of the TU Muenchen.

Smaller than the wavelength

The ability to double the frequency of a beam of light stems from the engineered electron states in the semiconductor material. When the semiconductor layers are only a few nanometers thick, the electrons can only occupy specific energy states and can be resonantly excited by the electromagnetic radiation.

"This kind of structure is called a coupled quantum well," says Frederic Demmerle. "Now, when we stack a further thin layer at a precisely defined distance from the first layer, we can push these electron states closer together or pull them apart, adjusting them precisely to the desired wavelength."

Using the semiconductor material grown at TU Muenchen, a team of researchers at the University of Texas, led by Prof. Mikhail Belkin and Prof. Andrea Alu, designed a pattern of crossed gold structures tailored to have resonances at particular input and output frequencies and fabricated then on top of the semiconductor layer. It is this specific combination of semiconductor material and gold nanostructures engineering that produces giant nonlinear response.

Although the patterns are considerably smaller than the wavelength of the incoming light, the metallic structures ensure that the light is optimally coupled to the material. Their special design also causes a strong increase in field strength at specific locations, which further amplifies the nonlinear response.

Towards the terahertz region

In the future, the team envisions using new materials realized along these lines for other nonlinear effects. "Alongside frequency doubling, our structures may be designed for sum- or difference-frequency generation," says graduate student Jongwon Lee, at the University of Texas, the lead author on the paper. "These kinds of elements could be used to produce and detect terahertz radiation – which is of interest for sensing and imaging applications, e.g., in medicine, because it does not harm biological tissue."

"This work opens a new paradigm in nonlinear optics by exploiting the unique combination of exotic wave interaction in metamaterials and of quantum engineering in semicondcutors." says Professor Andrea Alu.

"On the applications side, our work unveils a pathway towards the development of ultrathin nonlinear optical elements for efficient frequency conversion that will operate without stringent phase-matching constrains of currently-used bulk nonlinear crystals," says Professor Mikhail Belkin.

Graphene snapped: Laser spectroscopy freezes picosecond action

http://www.spectroscopynow.com/coi/cda/detail.cda?id=26772&type=Feature&chId=1&page=1

My Note: Another viewpoint on a story posted previously

Photocurrent pumped and probed

When graphene is stimulated optically it produces a photocurrent on a time scale of mere picoseconds. A German research team has now used the pump-probe method of time-resolved laser spectroscopy to take a snapshot of this process as it happens.

Graphene, the single layer graphite-like material that earned the 2011 Nobel Prize for Physics for its developers in Manchester, England, continues to offer intriguing glimpses into a world of future optoelectronic devices. Indeed, photodetectors composed of this carbon allotrope can both conduct and process light signals and electric signals extremely quickly. Graphene absorbs about 2 percent of incident light across a range of wavelengths. Until now, observing such a very rapid process in terms of photocurrent was not possible.

Alexander Holleitner and Leonhard Prechtel of the Technische Universitaet Muenchen (TUM), Germany have now devised an approach to measuring how this photocurrent changes over such very short time scales. The work could allow researchers to investigate graphene's high conductivity and other properties in much finer detail than was previously possible. The work could open up a whole range of applications and lead to the development of viable optoelectronic devices based on graphene.

Holleitner and Prechtel, who both work at the Walter Schottky Institut at TUM, have collaborated with members of the Cluster of Excellence Nanosystems Initiative Munich (NIM), physicists from the Universität Regensburg, Germany (Dieter Schuh), Eidgenössische Technische Hochschule Zürich (ETH), Switzerland (Werner Wegscheider), Rice University, Houston, Texas, USA (Pulickel Ajayan) and Shinshu University in Wakasato, Japan (Li song). They explain that the central element of the photodetector that they are studying contains a freely suspended graphene integrated into an electrical circuit via metallic coplanar stripline contacts. Time-resolved laser spectroscopy involved exciting electrons in the co-planar stripline circuit and monitoring the result of the stimulation with a second laser.

Terahertz observations

The technique has the added advantage of allowing the researchers to make other observations simultaneously. They have now found evidence that graphene, when optically stimulated, emits radiation in the terahertz (THz) range. This lies between infrared light and microwave radiation in the electromagnetic spectrum (0.3 to 3.0 THz; 0.1 to 1.0 mm). " Various graphene-based terahertz sources and detectors have been proposed, as the frequency of plasma waves, the gap of graphene nanoribbons, and the tunable bandgap in bilayer graphene lies in the terahertz range," the team explains.

Terahertz radiation is useful for penetrating matter for materials testing, scanning suspect packages at borders and in medical imaging. THz radiation is also at the heart of modern body scanners used at airports to reveal objects hidden beneath a passenger's clothes, for instance.

The researchers explain that the action occurs at the interface between the graphene and the metal stripline connectors. "We demonstrate that built-in electric fields give rise to a photocurrent with a full-width-half- maximum of about 4 picoseconds and that a photothermoelectric effect generates a current with a decay time of about 130 ps," they say. Additionally, given that the optically pumped graphene generates electromagnetic radiation up to 1 THz there might be wide applications for this discovery. "Our results may prove essential to build graphene-based ultrafast photodetectors, ultrafast photoswitches, photovoltaic cells and terahertz sources," the team concludes.

Related links

• Nature Commun, 2012, online: "Time-resolved ultrafast photocurrents and terahertz generation in freely suspended graphene"

Ultra-fast photodetector and terahertz generator

http://www.physorg.com/news/2012-01-ultra-fast-photodetector-terahertz.html
January 31, 2012

Enlarge

Photodetectors made from graphene can process and conduct light signals as well as electric signals extremely fast. Within picoseconds the optical stimulation of graphene generates a photocurrent. Until now, none of the available methods were fast enough to measure these processes in graphene. Scientists at the Technische Universitaet Muenchen now developed a method to measure the temporal dynamics of this photo current. Furthermore they discovered that graphene can emit terahertz radiation. Credit: Image: TUM

Photodetectors made from graphene can process and conduct light signals as well as electric signals extremely fast. Within picoseconds the optical stimulation of graphene generates a photocurrent. Until now, none of the available methods were fast enough to measure these processes in graphene. Scientists at the Technische Universitaet Muenchen, Germany, now developed a method to measure the temporal dynamics of this photo current. Furthermore they discovered that graphene can emit terahertz radiation.

Graphene leaves a rather modest impression at a first sight. The material comprises nothing but carbon atoms ordered in a mono-layered "carpet". Yet, what makes graphene so fascinating for scientists is its extremely high conductivity. This property is particularly useful in the development of photodetectors. These are electronic components that can detect radiation and transform it into electrical signals.

Graphene's extremely high conductivity inspires scientists to utilize it in the design of ultra-fast photodetectors. However, until now, it was not possible to measure the optical and electronic behavior of graphene with respect to time, i.e. how long it takes between the electric stimulation of graphene and the generation of the respective photocurrent.

Alexander Holleitner and Leonhard Prechtel, scientists at the Walter Schottky Institut of the TU Muenchen and members of the Cluster of Excellence Nanosystems Initiative Munich (NIM), decided to pursue this question. The physicists first developed a method to increase the time resolution of photocurrent measurements in graphene into the picosecond range. This allowed them to detect pulses as short as a few picoseconds. (For comparison: A light beam traveling at light speed needs three picoseconds to propagate one millimeter.)

The central element of the inspected photodetectors is freely suspended graphene integrated into electrical circuits via metallic contacts. The temporal dynamics of the photocurrent were measured by means of so-called co-planar strip lines that were evaluated using a special time-resolved laser spectroscopy procedure – the pump-probe technique. A laser pulse excites the electrons in the graphene and the dynamics of the process are monitored using a second laser. With this technique the physicists were able to monitor precisely how the photocurrent in the graphene is generated.

At the same time, the scientists could take advantage of the new method to make a further observation: They found evidence that graphene, when optically stimulated, emits radiation in the terahertz (THz) range. This lies between infrared light and microwave radiation in the electromagnetic spectrum. The special thing about THz radiation is that it displays properties shared by both adjacent frequency ranges: It can be bundled like particle radiation, yet still penetrates matter like electromagnetic waves. This makes it ideal for material tests, for screening packages or for certain medical applications.

More information: Time-resolved ultrafast photocurrents and terahertz generation in freely suspended grapheme. Leonhard Prechtel, Li Song, Dieter Schuh, Pulickel Ajayan, Werner Wegscheider, Alexander W. Holleitner,Nature Communications, http://dx.doi.org/ … 8/ncomms1656

Provided by Technische Universitaet Muenchen