Wimbledon

WIMBLEDON 

23 June- 6 July

Winner of wimbledon 2014

1. 
   

   
   
   Wimbledon
   Men's Singles · Men's Doubles · Women's Singles · Women's Doubles
   1		N. Djokovic	67	6	77	5	6	Finals
   4		R. Federer	79	4	64	7	4	Jul 6, Completed
   4		R. Federer	6	6	6			Semifinals
   8		M. Raonic	4	4	4			Jul 4, Completed
   1		N. Djokovic	6	3	77	79		Semifinals
   11		G. Dimitrov	4	6	62	67		Jul 4, Completed
   8		M. Raonic	64	6	6	77		Quarterfinals
   		N. Kyrgios	77	2	4	64		Jul 2, Completed
   All times are India Time
   
   
   
   
   
   

   All times are India Time

   
   

Optical nano-tweezers control nano-objects

The image on the left is an electron beam microscopy image of the extremity of the plasmon nano-tweezers. The image on the right is a sketch illustrating the trapping of a nanoparticle in the bowtie aperture.

Credit: Institute of Photonic Sciences

In current scenario we are very interested in trapping nano particle and study them for best design. 

As science and technology go nano, scientists search for new tools to manipulate, observe and modify the "building blocks" of matter at the nanometer scale. With this in mind, the recent publication in Nature Nanotechnology in which ICFO researchers demonstrate for the first time the ability to use near-field optical tweezers to trap a nano-size object and manipulate it in the 3 dimensions of space, is an exciting achievement. Romain Quidant, ICREA Professor and leader at ICFO of the Plasmon Nano-Optics research group comments that "this technique could revolutionize the field of nanoscience since, for the first time, we have shown that it is possible to trap, 3D manipulate and release a single nano-object without exerting any mechanical contact or other invasive action."

Imagine an elephant trying to grab an object the size of a needle with its gigantic hoof? Clearly this would be a tremendous if not impossible challenge because of the elephant's enormous size in comparison to that of the needle. Now imagine that our needle is a single molecule or tiny object about the size of a few nanometers and we, with our conventional tools, need to trap it and manipulate it in in order to, for example, understand its implication in the development of a disease. We have the same problem, first because a conventional optical microscope is not capable of visualizing a single molecule and second, because the physical limitations of our conventional tweezers are simply not capable of grasping or manipulating such small objects.

Invented in Bell Labs in the 80's, the original optical trapping demonstrated great capability to trap and manipulate small objects of micrometer size dimensions using laser light. By shining a laser light through a lens, it is possible to focus light in a tiny spot, creating an attractive force due to the gradient of the light intensity of the laser and thus attracting an object/specimen and maintaining it in the spot/focus.

While Optical tweezers have changed forever the fields of both biology and quantum optics, the technique has considerable limitations, one of which being its inability to directly trap objects smaller than a few hundreds of nanometers. This drawback prompted the pursuit of new approaches of nano-tweezers based on plasmonics, capable of trapping nano-scale objects such as proteins or nanoparticles without overheating and damaging the specimen. A few years ago, ICFO researchers demonstrated that, by focusing light on a very small gold nano-structure lying on a glass surface which acts as a nano-lens, one can trap a specimen at the vicinity of the metal where the light is concentrated. This proof of concept was limited to demonstrate the mechanism but did not enable any 3D manipulation needed for practical applications.

Now researchers at ICFO have taken this a crucial step further by implementing the concept of plasmonic nano-tweezers at the extremity of a mobile optical fiber, nano-engineered with a bowtie-like gold aperture. Using this approach, they have demonstrated trapping and 3D displacement of specimens as small as a few tens of nanometers using an extremely small, non-invasive laser intensity. Central to the great potential of this technique is that both trapping and monitoring of the trapped specimen can be done through the optical fiber, performing the manipulation of nano-objects in a simple and manageable way outside of the physics research lab.

This technique opens a plethora of new research directions requiring non-invasive manipulation of objects at the single molecule/virus level. It is potentially attractive in the field of medicine as a tool to further understand the biological mechanisms behind the development of diseases. Likewise, it holds promise in the context of nanotechnologies to assemble future miniature devices, among other exciting potential applications
.

FIFA World Cup 2014

Federation Internationale de Football Association

Now we will meet in Russia

The mystery about this world cup is that in its First tournament England was not the part of this game so this has a french name but most people think this has a English meanings. 

A high speed NASA project

NASA’s Warp Drive Project: “Speeds” That Could Take a Spacecraft to Alpha Centauri in Two Weeks Even Though the System is 4.3 Light-Years Away, Video http://b4in.org/c4Hg

A  few months ago, physicist Harold White stunned the aeronautics world when he announced that he and his team at NASA had begun work on the development of a faster-than-light warp drive.

His proposed design, an ingenious re-imagining of an Alcubierre Drive, may eventually result in an engine that can transport a spacecraft to the nearest star in a matter of weeks — and all without violating

The idea came to White while he was considering a rather remarkable equation formulated by physicist Miguel Alcubierre. In his 1994 paper titled, “The Warp Drive: Hyper-Fast Travel Within General Relativity,”

Alcubierre suggested a mechanism by which space-time could be “warped” both in front of and behind a spacecraft.

Michio Kaku dubbed Alcubierre’s notion a “passport to the universe.” It takes advantage of a quirk in the cosmological code that allows for the expansion and contraction of space-time, and could allow for hyper-fast travel between interstellar destinations.

Essentially, the empty space behind a starship would be made to expand rapidly, pushing the craft in a forward direction — passengers would perceive it as movement despite the complete lack of acceleration.

White speculates that such a drive could result in “speeds” that could take a spacecraft to Alpha Centauri in a mere two weeks — even though the system is 4.3 light-years away.

In terms of the engine’s mechanics, a spheroid object would be placed between two regions of space-time (one expanding and one contracting).

A “warp bubble” would then be generated that moves space-time around the object, effectively repositioning it — the end result being faster-than-light travel without the spheroid (or spacecraft) having to move with respect to its local frame of reference.

“Remember, nothing locally exceeds the speed of light, but space can expand and contract at any speed,

Silk: A Different Kind of “Fiber Optics”

Like a moth emerging from a cocoon, silk-based devices—optically versatile and friendly to biological systems—are poised to take flight in a range of new, innovative applications.

Image of a Bombyx mori caterpillar spinning silk on the surface of a gold mirror. The “silkworms” spin nearly 1 km of silk in a “figure 8” pattern (visible on the surface of the mirror) to form their cocoons.

Silk fibers have woven their way through thousands of years of history, in the fabric of kings, the protective drapes of warriors, the clothing of fashion models. As a trade commodity, this material once defined the world economy along a “silk road” extending from ancient China through Samarkand (modern Uzbekistan) to the shores of Europe and beyond. Silk’s distinctive feel and shine, as well as its combination of unusual strength and toughness, have made it a coveted material across generations.

But silk is also finding new uses in some surprising places. Blessed with superb and versatile optical properties, physically robust and rooted in a natural biological system, this material can be fashioned into a range of micro- and nanoscale devices for drug delivery, disease diagnosis and biomedical sensing. As a result, the unlikely combination of silk, optics and biology is emerging as a new and potentially powerful tool in the drive toward personalized medicine.

Reverse-engineering a natural material

Although fit to clothe royalty, silk has the most humble beginnings imaginable. Inside the body of the silkworm caterpillar, Bombyx mori, digested mulberry leaves provide raw material for the synthesis of silk protein. The silk gland of the silkworm, which constitutes a large share of the insect’s volume, contains a mixture of silk fibroin protein, salts and water, which is extruded through a spinneret and drawn into a long, thin fiber as the caterpillar spins its cocoon and prepares to transform into a moth.

The fibers in the silk cocoon are very different from the liquid protein solution found within the silk gland. The fibers are long, thin, opaque, flexible and tough—ideal for a cocoon (or a silk scarf), but unworkable in optical devices. Using silk in optical devices requires reverse engineering of the natural fiber generation process, dissolving the native silk fibers to obtain a purified silk solution (water and fibroin) that closely resembles the material in the caterpillar’s silk gland. This versatile solution forms the starting material for silk films, gels, sponges, blocks and other materials.

From silkworm to silk film: 1. Silkworms use the raw material in mulberry leaves to create silk fiber to spin their cocoons. 2. For use in optical devices, cocoons are cooked (left) to isolate silk fibroin fibers (right). 3. These tough fibers are dissolved to obtain liquid silk. 4. The silk solution can then be formed into multiple material formats, including clear free-standing films (shown), nanoparticles, sponges, adhesives and gels, blocks and fibers.

In addition to mechanical strength and toughness, silk fibroin has remarkable optical properties. Silk films are transparent to visible light of all wavelengths and have extremely low surface roughness (less than 5 nm rms). These properties make silk films a suitable material for a wide range of optical devices, including diffraction gratings, phase masks, white-light holograms and photonic crystal lattices—all fashioned simply by casting the fibroin-water solution onto an appropriate surface.

Processed in this way, silk is able to reproduce surface features as small as 10 nm. And silk’s optical capabilities—coupled with its advantages in biomedical applications and systems (see sidebar, “Molecular bubble wrap,” on right)—open up some surprising opportunities for new devices, sensors and treatments.

Creating silk devices

Silk films are probably the simplest and most versatile materials from which regenerated silk can be transformed. The process is simple: silk fibroin solution is cast onto a surface and allowed to dry. As the water evaporates, the silk proteins self-assemble into a transparent sheet that can be lifted free of the surface. The volume and concentration of solution controls the thickness, which can range from a few nanometers to several hundred microns.

When the liquid silk solution is cast onto a patterned substrate, the liquid will fill any crevices on that surface, so that the resulting film conforms to the shape of the mold, leaving a perfect inverse replica with high fidelity and resolution. Suitably designed masters can allow formation of a wide variety of optical devices. On the downside, the simple cast-and-dry process is relatively slow in ambient conditions; it usually takes 12 to 36 hours for the solution to fully dry. Films can also shrink while drying and can become deformed when the film is removed from the master.

As an alternative, silk films can be cast on a flat substrate and patterns can subsequently be embossed or stamped onto the dry silk film. Careful application of heat and pressure results in parts of the film returning to an almost liquid state. In this phase, the films can conform to extremely small features in the master plate. This faster technique makes it possible to rapidly produce large numbers of devices.

Master molds are commonly made from silicon wafers—but, again, these masters are expensive and slow to produce. Protein-on-protein printing, in which one silk film is used to pattern another, has emerged as an alternative, and master silk molds for this process can be quickly and inexpensively produced using several methods, among which is multiphoton laser ablation.

“Molecular bubble wrap” Silk’s biomedical advantages As a natural and optically favorable material, silk has some significant advantages for use in biomedical optics: Stability. Within the silk matrix, biologically active dopants such as enzymes, other proteins, drugs or antibodies can be stabilized and preserved for long periods without the need for refrigeration. The reason: The silk protein comprises large hydrophobic domains interspersed with small hydrophilic domains—so that, as the liquid silk solution dries, the protein twists and folds, forming hydrated nanoscale pockets protected by the larger hydrophobic domains. These nano-pockets act as a “molecular bubble wrap” that can protect and preserve unstable compounds within the crystalline silk matrix, without refrigeration—a significant plus particularly in the developing world, where electricity can be scarce and unreliable. Toleration. Silk, unlike many polymers, is remarkably well tolerated inside the body. The hydrophobic domains make it relatively inert to the immune system, and the gentle, all-aqueous procedure by which the silk is formed into materials ensures that no toxic or irritating residue is present in medical devices prepared from the silk protein. Controlled degradation. Over time, the silk protein is harmlessly degraded by proteases in the body. The timeline for degradation can be tuned by controlling how the silk protein folds. Air-dried silk dissolves quickly in water, but exposing the silk to alcohol or water vapor changes it from this water-soluble amorphous form to an insoluble crystalline beta sheet structure. The degree of crystallinity allows for programmed degradation, in which the length of time that the silk lasts in vivo can range from a few seconds to years.

The protein sequence of silk includes small residues of the amino acid tryptophan; this, along with tyrosines, is responsible for silk’s characteristic absorption spectrum, which has peaks in the mid-ultraviolet region (270 nm). When illuminated with focused femtosecond pulses of 810 nm light, an efficient three-photon absorption process takes place at the beam focus, converting the light energy into heat and vaporizing the silk. When applied to silk films, this property allows for the rapid generation of arbitrarily shaped patterns with a resolution of around 1 µm. These patterned films can subsequently be used as masters to emboss flat silk films, reducing from weeks to only hours the time from conception of an idea to dozens of completed samples.

Nanoscale patterning requires a different method. Using electron beam writing and basic lithographic techniques perfected in the microchip industry, silicon master stamps can be produced with feature sizes on the order of 10 nm. Once formed, these masters can be used repeatedly to pattern silk films into a variety of diffractive optical elements.

As an example, aperiodic gratings are structures that diffract light in specific ways depending on the pattern of the grating, the angle of incidence and, most important, the refractive index of the surrounding media. Layers as thin as a single molecule coated on these gratings will dramatically change the far-field color pattern. When combined with silk, these gratings offer an exciting new realm of optical sensors in which the sensing element and the readout are bound together, and the entire sensor design is simplified.

Blood sensors and inverse opals

The prospects for these silk devices become compelling when they are pressed into biomedical service. For example, silk’s ability to stabilize biological systems means that micro- and nanopatterned silk can be doped with biologically relevant compounds to act as sensors. As an example of this merging of optical

form with biological function, a proof-of-concept device consisting of a free-standing silk diffraction grating containing human blood was developed by simply mixing blood with the silk aqueous solution and casting the result onto a diffractive substrate. The hemoglobin contained in the free-standing silk-blood grating maintained its ability to bind and unbind oxygen while being used as an optical element. Thus, the diffraction grating could alter its spectral response as a function of the oxygen concentration surrounding the grating—the silk-blood-grating was able to “sense itself” as a function of the surrounding environment.

While the discussion has focused thus far on 2-D optical elements, silk can also be shaped into 3-D structures and photonic crystals. These highly organized lattices have a periodicity on the order of the wavelength of light (around 400 nm); when light impinges on such a structure, the multiple reflections from each layer boundary result in destructive interference for forward-propagating light of certain wavelengths. This means that light from a certain spectral region is unable to propagate through the crystal and must be reflected.

Examples of silk devices at various scales and with differing optical applications: 1. Printed silk waveguides. 2. A silk card containing diffractive optical elements (microlens arrays, phase masks, diffraction gratings). 3. An electron micrograph of a 2-D photonic lattice. 4. An InGaN thin-film LED on a silk substrate, which conforms to animal skin and is wirelessly powered—an example of biological integration of silk-hybrid photonics.

Photonic-crystal structures, though highly organized and incredibly tiny, are found in nature—such a structure, for example, accounts for the shimmering iridescence of a blue morpho butterfly wing. In addition to being beautiful to look at, these photonic crystals are very sensitive to the refractive index of the surrounding medium. In air, they might reflect a specific color as blue or green, while in water the reflectance can shift to the other end of the spectrum to appear orange or red.

With this example in mind, our group has developed an analog in silk: inverse opals. Regular opals consist of a matrix of tightly packed silica spheres—each between 150 and 300 nm in diameter, which is, not coincidentally, about half the wavelength of visible light. The difference in refractive index between the spheres and the gaps surrounding them gives rise to opals’ characteristic play of colors. And, as with many optical phenomena, the inverse structure—spheres of air, surrounded by a matrix of other material—can yield a similar result.

Silk inverse opals are made by first producing opals from poly(methyl methacrylate), or PMMA, spheres that form spontaneously in water. These direct opals can then be immersed in a silk solution which is allowed to crystallize. The PMMA spheres are then dissolved away using acetone, leaving the silk behind. As with all materials made of silk, these inverse opals can be doped with drugs, dyes or enzymes and safely implanted within the body.

Microfluidic devices

The combination of properties exhibited by silk makes it a useful and versatile platform for microfluidic sensors. These sensors work by flowing tiny volumes of liquid through very small channels. Most medical testing equipment requires several milliliters of fluid, while microfluidic sensors typically require about a thousand times less volume. This allows for more tests to be done on the same volume of sample in a smaller amount of space. Lab-on-a-chip devices are currently being produced that can replace roomfuls of equipment with a device that can be held in one hand.

Inverse opals: (Left) Free-standing silk film with nanopatterned 3-D inverse opals, displaying iridescence according to the lattice constant (Λ) of the devices. (Top right) The devices change color when infiltrated with a fluid, thus changing the index contrast of the photonic crystal lattice. (Bottom right) Electron microscope image of the different silk inverse opals.

Silk is well suited for these sensors. Sensitive molecules can be doped into a silk film and stabilized in the device. Depending on the sample and the target to be analyzed, the response can be read colorometrically (taking advantage of the optical transparency of silk) or using an optical element such as a diffraction grating to measure spectral changes. When packaged into a microfluidic device, these sensors can be more robust and longer lasting than if they were made from more conventional materials.

Toward implantable optics

When light interacts with tissue, absorption, single-scattering and multiple-scattering processes all play a role in directing where the photons travel. Implantable optical devices have tremendous potential to guide light within the body, allowing for enhanced imaging, optogenetic stimulation or more targeted light delivery for photodynamic therapies. Silk has the optical, mechanical and biological properties necessary to excel in this role.

One example is the use of implantable silk mirrors to increase the signal of optical imaging methods. Microprism arrays (MPAs) have the ability to collect incident light from a wide angle and reflect it at a much narrower angle. These structures are found in road signs and running shoes that seem to glow when illuminated by a car’s headlights. Simply casting silk solution over an MPA master will result in a silk MPA. When implanted, these MPAs collect incident light and reflect it back. By scanning an illumination source and a detector over the skin, the precise location of the MPA can be seen by an increase in the amount of reflected light. This increase in signal could enhance optical imaging deep within the body.

Implantable silk mirrors: (Top left) Electron microscope image of silk microprism arrays degrading in the presence of enzymes. (Top right) Silk microprism array loaded with the chemotherapeutic doxorubicin. When these silk devices are loaded with drugs in this manner, the drug is released and correspondingly the microprisms change their geometry, allowing for correlation of the reflectivity from the silk device and the amount of drug delivered (bottom).

The silk mirrors can also be doped with drugs or other compounds. As the mirrors degrade in the body, the drug will be slowly released. As the drug is released, the amount of light reflected from the mirrors will also degrade—which means that the amount of drug that has been delivered can be determined using the change in reflectance. This simple technique offers a way to monitor an injury site optically, deliver a drug where and when it’s needed and assess how well the drug is being delivered.

A new silk road for biomedicine

The integration between photonics and biology could revolutionize the way healing, surgical integration, vascular ingrowth, brain function, cancer progression or regression and many other common health issues are tracked. Even everyday health monitoring could benefit from using the physiology of the body to modulate photons and relay information to an external optical receiver. The impact of an implantable device that matches the performance of current technology and simultaneously can be an integral part of the physiological fabric is potentially immense. The option to move from outside-in monitoring to inside-out monitoring would redefine portability, bring long-term sensing directly to the site of interest and contribute to the development of personalized medicine.

Silk could help to enable just such a revolution. It is a synergistic material, and each of its characteristics—optical properties comparable to glass; strength comparable to Kevlar; biocompatibility; the abililty to design, tune and control its degradation lifetime; and the ability to stabilize drugs—is crucial to the next generation of advanced medical devices. The combination of all of these qualities in one material truly makes silk more than the sum of its parts.

First Broadband Wireless Connection…to the Moon

WASHINGTON, May 22, 2014—If future generations were to live and work on the moon or on a distant asteroid, they would probably want a broadband connection to communicate with home bases back on Earth. They may even want to watch their favorite Earth-based TV show. That may now be possible thanks to a team of researchers from the Massachusetts Institute of Technology’s (MIT) Lincoln Laboratory who, working with NASA last fall, demonstrated for the first time that a data communication technology exists that can provide space dwellers with the connectivity we all enjoy here on Earth, enabling large data transfers and even high-definition video streaming.

At CLEO: 2014, being held June 8-13 in San Jose, California, USA, the team will present new details and the first comprehensive overview of the on-orbit performance of their record-shattering laser-based communication uplink between the moon and Earth, which beat the previous record transmission speed last fall by a factor of 4,800. Earlier reports have stated what the team accomplished, but have not provided the details of the implementation.

“This will be the first time that we present both the implementation overview and how well it actually worked,” says Mark Stevens of MIT Lincoln Laboratory. “The on-orbit performance was excellent and close to what we’d predicted, giving us confidence that we have a good understanding of the underlying physics,” Stevens says.

The team made history last year when their Lunar Laser Communication Demonstration (LLCD) transmitted data over the 384,633 kilometers between the moon and Earth at a download rate of 622 megabits per second, faster than any radio frequency (RF) system. They also transmitted data from the Earth to the moon at 19.44 megabits per second, a factor of 4,800 times faster than the best RF uplink ever used.

“Communicating at high data rates from Earth to the moon with laser beams is challenging because of the 400,000-kilometer distance spreading out the light beam,” Stevens says. “It’s doubly difficult going through the atmosphere, because turbulence can bend light—causing rapid fading or dropouts of the signal at the receiver.”

To outmaneuver problems with fading of the signal over such a distance, the demonstration uses several techniques to achieve error-free performance over a wide range of optically challenging atmospheric conditions in both darkness and bright sunlight. A ground terminal at White Sands, New Mexico, uses four separate telescopes to send the uplink signal to the moon. Each telescope is about 6 inches in diameter and fed by a laser transmitter that sends information coded as pulses of invisible infrared light. The total transmitter power is the sum of the four separate transmitters, which results in 40 watts of power.

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The reason for the four telescopes is that each one transmits light through a different column of air that experiences different bending effects from the atmosphere, Stevens says. This increases the chance that at least one of the laser beams will interact with the receiver, which is mounted on a satellite orbiting the moon. This receiver uses a slightly narrower telescope to collect the light, which is then focused into an optical fiber similar to fibers used in terrestrial fiber optic networks.

From there, the signal in the fiber is amplified about 30,000 times. A photodetector converts the pulses of light into electrical pulses that are in turn converted into data bit patterns that carry the transmitted message. Of the 40-watt signals sent by the transmitter, less than a billionth of a watt is received at the satellite—but that’s still about 10 times the signal necessary to achieve error-free communication, Stevens says.

Their CLEO: 2014 presentation will also describe how the large margins in received signal level can allow the system to operate through partly transparent thin clouds in the Earth’s atmosphere, which the team views as a big bonus.

“We demonstrated tolerance to medium-size cloud attenuations, as well as large atmospheric-turbulence-induced signal power variations, or fading, allowing error-free performance even with very small signal margins,” Stevens says.

While the LLCD design is directly relevant for near-Earth missions and those out to Lagrange points – areas where the forces between rotating celestial bodies are balanced, making them a popular destination for satellites -- the team predicts that it’s also extendable to deep-space missions to Mars and the outer planets.

'Landmark achievement' for LOC

A company established by Croydon LOC has launched a community ophthalmology service following a successful competitive tendering process put together with the help of the LOC Support Unit (LOCSU). It is believed that this is the first service of its kind in the country to make extensive use of optical practices which will act as the core of the service.

The aim of the service, provided by the new London-based company, Complete Ophthalmic Services CIC, is to provide patients with recently occurring eye conditions with treatment closer to home, refine cataract and glaucoma referrals, and generally reduce the number of patients needing to attend secondary care.

Loksabha Election

The Indian general election of 2014 was held to constitute the 16th Lok Sabha, electing members of parliament to all 543 parliamentary constituencies of India. Running in nine phases from 7 April to 12 May 2014, this was the longest election in the country's history.^[1][2] According to the Election Commission of India, 814.5 million people were eligible to vote, with an increase of 100 million new voters since the last general election in 2009^[3], making this the largest-ever election in the world.^[4]A total of 8,251 candidates contested for the 543 Lok Sabha seats.^[5] The average election turnout over all nine phases was around 66.38%, the highest ever in the history of Indian general elections.^[5]

The result of election was declared on 16 May, fifteen days before the 15th Lok Sabha completes its constitutional mandate on 31 May 2014.^[6] The counting exercise was held at 989 counting centers.^[5]The National Democratic Alliance emerged as the majority coalition, winning 336 seats and the Bharatiya Janata Party as the single largest party winning 282 seats. The United Progressive Alliance won 59 seats, and the Congress, 44

Ultrafast and Ultrashort

Ultrashort-pulse lasers have been used for decades in industrial settings requiring powerful, precise processing of all kinds of materials. But innovation is hardly dead in this marketplace. In academic labs, advances in materials and laser configurations promise to open up a variety of new applications outside of the technology’s core area of industrial micromachining. And even within that core market, suppliers are pushing the envelope toward ever-shorter and more stable pulses, extending micromachining into previously untapped segments. Here’s an update on some recent developments on the ultrafast-laser scene.

The graphene difference

On the R&D front, particular interest currently centers on developing ultrafast lasers in the infrared, especially at around 2 µm. That’s a wavelength with broad potential applications—in telecom; in medicine (because the wavelength is “eye safe” and because water’s high absorption at near 2 µm opens up possibilities for targeted microsurgery); and in lidar and free-space optical communications (due to the transparency of the atmosphere at that wavelength). Unfortunately, ultrafast lasers in the infrared tend to be expensive and space-consuming, requiring an optical table and many components, which has largely limited their use to lab settings.

But researchers are starting to find ways around that conundrum. And one approach, as with so much of materials science and engineering today, rests on the remarkable properties of a new material, graphene—the one-atom-thick layers of carbon whose discovery and characterization snagged the 2010 Nobel Prize in Physics. Among many other uses, graphene turns out to have strong potential as a versatile, cheap material for saturable absorbers in pulsed lasers.

Saturable absorbers reside inside the laser cavity, and can be used to absorb just the right amount of light and reflect it to generate short pulse durations at high repetition rates. Graphene can absorb a wide range of wavelengths, including visible and infrared, due to its lack of a band gap. Graphene is also resistant to chemical and mechanical stress, making it a great candidate for use as a mode-locker inside laser cavities. And it’s inexpensive and easy to fabricate.

An ultrafast tunable VECSEL in the lab at ETH Zurich uses a single-layer of graphene as a saturable absorber in a 6-cm-long cavity.

A λ/8 graphene saturable absorber mirror (G-SAM) features a square of single-layer graphene 5 mm across (red dotted line) on a layer of SiO₂. The graphene is fabricated on copper and transferred to a quartz mirror in the laser cavity.

In 2009, Andrea Ferrari, professor of nanotechnology at the University of Cambridge, U.K., and colleagues began to demonstrate the practical use of graphene as a mode-locker at several infrared wavelengths. In late 2013, Ferrari’s team demonstrated graphene mode locking at 2 µm in a solid-state laser that generated 410 fs pulses, 270 mW average output power, and 110 MHz repetition rate. The graphene mode-locked laser requires a single layer of graphene inside the laser cavity, enabling a laser that is very compact and tunable. Experiments have demonstrated that the same graphene sample used at 1 µm, 1.3 µm and 1.5 µm is usable at 2 µm.

“This shows how simple it is to introduce a graphene sample inside the laser cavity,” says Ferrari.

Moving toward VECSELs

More recently, in collaboration with Professor Ursula Keller’s group at ETH Zurich, Switzerland, Ferrari’s team has focused on a new generation of mode-locked vertical-external-cavity surface-emitting lasers (VECSELs) using graphene-based saturable absorbers (GSLs). In contrast to the tiny enclosed cavity of a conventional vertical-cavity surface-emitting laser (VCSEL), a VECSEL consists of an external cavity formed by high-reflection mirrors and an output coupler, with a typical cavity measuring a few millimeters to tens of centimeters. The gain chip consists of a highly reflective bottom (to reflect the laser and pump light), an active semiconductor gain section and an anti-reflective top layer.

VECSELs thus combine the advantages of compact semiconductor lasers and diode-pumped solid-state lasers, achieving low timing jitter, excellent beam quality, and high average and peak power. Ultrafast VECSELs (also called semiconductor disk lasers or optically pumped semiconductor lasers) have potential as pulsed sources for multiphoton microscopy, fiber optic communications, supercontinuum generation and ultracompact stabilized frequency combs.

The Ferrari-Keller collaboration has used a GSL to mode-lock a VECSEL from 935 to 981 nm. The VECSEL is widely tunable over that 46 nm band, which makes it the widest-tunable VECSEL reported to date.

The group is considering further applications, such as graphene-based waveguides and nonlinear devices, as well as fiber-based ultrafast lasers. Graphene has other applications in photonics as well, including solar cells, photodetectors and for integration into optical components such as output coupler mirrors, dispersive mirrors and dielectric coatings on gain materials.

Passively mode-locked VECSELs are only a few years old and, according to Dr. Rüdiger Paschotta, the founder of RP Photonics Consulting GmbH, could be poised for significant further advances in performance, especially combining multi-gigahertz repetition rates, multi-watt output powers and sub-picosecond pulse durations. What’s more, the application of wafer-scale technologies may allow very-low-cost mode-locked VECSELs, opening up a range of new applications where cost is a factor.

An Ultrashort Menu Ultrashort-pulse lasers, also called ultrafast lasers, generate pulses lasting only femtoseconds (10–15 s) or picoseconds (10–12 s). The short pulse widths are achieved via gain switching or, more commonly, passive mode locking, in which the pulse is circulated within a laser cavity, typically using a saturable absorber. Here are some common varieties: Mode-locked diode lasers can be either external-cavity diode lasers or monolithic devices. They may use any type of mode locking (active, passive or a hybrid) and typically generate high pulse repetition rates in the multi-gigahertz range with moderate pulse energies. Ultrafast lasers can also be diode-pumped with ytterbium- or chromium-based crystals and passively mode-locked with a semiconductor saturable absorber mirror. Ti:sapphire lasers, among the more common ultrafast varieties in the femtosecond realm, enable very short pulses of approximately 5 fs and typical average peak powers of hundreds of milliwatts for 80 MHz pulse frequencies. They are tunable across a 700–1,000 nm spectral range, but require pumping from a green laser to create Kerr-lens-effect passive mode locking, which can make them complex, bulky and more expensive than other types. Ti:sapphire lasers are used in all kinds of applications that require tuning with the shortest pulses, high peak power and lower pulse energies. Fiber lasers, in which an optical fiber constitutes the gain medium, can also be pulsed via passive mode locking, but the resulting pulses are not as short and have less peak power than bulk lasers.

Micromachining: The ultrafast difference

Meanwhile, the commercial use of ultrafast lasers for precise, powerful industrial applications like micromachining and processing is well established on all types of materials, ranging from textiles and polymers to metals and semiconductor wafers. And the advent of ever-shorter, ever-more-stable pulses is enabling more efficient, cost-effective micromachining and is increasing the role of these lasers in other industries. Micromachining with femtosecond lasers can produce repeatable, precise features down to 10 µm in size, like holes and grooves on surfaces or structures. Such tiny structured surfaces might be used in microfluidic devices or a lab-on-a-chip.

Medical stents—the tiny mesh tubes inserted into a natural passage or conduit in the body, such as a blood vessel, to counteract a localized flow constriction, such as arterial blockage—form an interesting case study in comparing nanosecond and picosecond/femtosecond technology.

Stents are conventionally manufactured in metals using Nd:YAG lasers, the pulse duration of which varies from milliseconds to nanoseconds. The longer pulse lengths produce heat-affected zones and melting along the cut edges, and a deposition of solidified droplets stuck to the tube surface. The result: burrs along the cut that can’t easily be cleaned on the smallest of stents, preventing their use in smaller veins. In addition, Nd:YAG lasers aren’t effective at processing all of the potential materials that would be useful for making stents.

Picosecond and femtosecond lasers, in contrast, can overcome this problem through a process called cold ablation. This occurs when intensity of a high-energy laser pulse exceeds the ablation threshold and penetrates more deeply into the material, causing it to sublimate into a plasma and eliminating thermal debris.

Femtosecond lasers that enable cold ablation can produce high-quality stents with no post-processing, not only in metals but also in polymers that degrade naturally over time. Use of lasers with pulse lengths shorter than 12 ps can help to avoid melt debris that could alter the chemistry of the human body when the device is inserted. Such an approach requires more stable pointing, which, if mastered, can drill thicker materials with fewer shots without losing accuracy—useful not just in biomedicine but in other industries requiring high-throughput, precision processing, such as automotive, consumer electronics, mobile displays and 3-D semiconductors.

(Top) Femtosecond laser technology enables precision engraving of a lab-on-a-chip, which incorporates the functionalities of an entire laboratory onto a single glass substrate. (Bottom left) Femtosecond lasers can engrave precision patterns on tiny objects, such as this 2-mm-diameter stent made of a nickel titanium alloy. (Bottom right) Femtosecond lasers like TRUMPF’s TruMicro 5070 perform micromachining with a negligible heat-affected zone, like this microhole drilled into a printed circuit board substrate.

As always, the choice of pulse length is partly a cost judgment. Femtosecond lasers typically cause less melt, but are more expensive. Laser developer and manufacturer Coherent, based in Santa Clara, Calif., U.S.A., has recently expanded its family of industrial picosecond lasers (based on a modular fiber/regenerative-amplifier technology) with the Talisker HE and RAPID series, whose high-energy pulses, according to the company, enable drilling, cutting and scribing of thicker materials than other ultrafast lasers.

Notes Mark Thompson, senior product manager at Coherent, the Talisker HE and HYPER RAPID-HE lasers are targeted at three different wavelengths with different output options: 200 µJ pulse energy in the near-infrared (1,064 nm) for materials that better dissipate heat, like metals, polymers, glass and ceramics; 120 µJ per pulse in the green (532 nm) for glass; or 40 µJ pulse energy in the ultraviolet (355 nm) for low-melting-point materials like transparent polymers.

“The key spec to achieve is productivity—you want to make quality parts as quickly as possible,” says Thompson. To do that, manufacturers need to concentrate the pulse energy and reduce the spot size, or fluence, to reduce the thermal heating that causes melt debris.

Femtosecond stability

Beyond cost and productivity, says Thompson, another key issue is stability. “Can it withstand 24-7 production in New York during the winter or China in the summer? Can the laser survive international shipping by sea freight? Stability is what distinguishes scientific ultrafast from robust, industrial ultrafast.”

In January 2014, industrial laser manufacturer TRUMPF, based in Farmington, Conn., U.S.A., introduced the TruMicro 5070 femtosecond laser, which is based on its high-power picosecond laser series. The TruMicro 5070 offers pulse durations of 800 ± 200 fs and pulse energy as high as 200 µJ. Featuring repetition rates of 400 and 600 kHz, the new infrared laser emits at 1030 nm with a remarkable 80 W of average output power.

The TruMicro 5070, according to TRUMPF, is designed to provide consistent results in demanding industrial production environments. The system separates pulse generation and pulse emission and uses feedback systems to monitor individual pulses to maintain constant power, pulse energy and duration, and beam quality. Initial applications include drilling holes in circuit boards and processing polymers used in medical implants. According to Sascha Weiler, program manager, Micro Processing, at TRUMPF, these advantages, along with the higher power and shorter pulse duration, “directly translate into throughput, reducing the ‘cost per watt’ and therefore the ‘cost per part.’”

The challenge for commercial femtosecond lasers, according to Weiler, is to translate power into speed while reducing price—an 80W CW laser will be always cheaper than an 80W fs laser. But, says Weiler, the price of ps/fs lasers hinges on economies of scale, so prices should come down as these lasers find more and more applications.

Improving throughput with parallel beams

In addition to pulse speed, maximizing the speed of the production process itself is another challenge in femtosecond micromachining. Conventional material processes scan sequentially over the target, dot by dot. Rotary scan heads can help by moving the beam very quickly from point to point. Another option, however, is to split the original beam into several beams for parallel processing.

Dispersion-compensated module: (Top) The diffractive-refractive dispersion-compensated module uses a femtosecond Ti:sapphire laser and optics to create multiple beams for improved multifocal micromachining. Optical microscopy reveals the details of the ablated surface of a stainless steel sheet with (bottom left) the dispersion-compensated module and (bottom right) a conventional setup. In the bottom part of the figure, the SEM images of the holes correspond to different diffraction orders.

Researchers with the Group of Research in Optics at Universitat Jaume I (GROC–UJI) in Castellón de la Plana, Spain, recently investigated a promising parallel-processing technique. According to Gladys Mínguez-Vega, associate professor of physics and a GROC–UJI member, a split-beam solution makes sense because it eliminates the extensive attenuation required for current regenerative or multipass amplifier systems, which produce pulse energies in the millijoule range. Furthermore, splitting the beam into an array of multiple beams improves production and throughput, reduces fabrication time and maximizes full use of the laser’s energy.

In the parallel-beam setup, the group used a suited dispersion-compensated module (DCM) to focus the light. Consisting of a diffractive optical element (DOE), one refractive lens and two diffractive lenses, the DCM increased the ablation area by nearly a factor of three. The DCM also mitigated first-order chromatic aberration and pulse stretching on the target material caused by use of DOEs. The setup projected a 30 fs laser pulse in a parallel-processing configuration onto a stainless steel sample to simultaneously create 52 blind holes measuring less than 5 µm across.

“We can increase the speed of manufacturing by a factor of 52—or even more by changing the system parameters—without a loss of quality,” says Mínguez-Vega. “We believe this may be of interest to manufacturers that want a higher throughput of their millijoule laser for micromachining applications.” While the experimental system used expensive diffractive optical elements fabricated on a glass substrate (costing about €4000), the team suggests using polymer materials for cost-effective, high-volume production DOEs.

Banishing microcracks

Another potential application area for such parallel processing is cutting the scratch-resistant cover glass for mobile phones. If the glass is cut by a conventional long pulse, the pulse duration is sufficient to thermally generate microcracks in the material, to the detriment of quality. Femtosecond lasers concentrate the energy in such a short pulse duration that heat cannot be transferred to the surrounding material. The cutting process then becomes truly athermal, and microcracks don’t develop. Many other applications may benefit from industrial processing with precision femtosecond lasers, including patterning of thin-film solar cells, chip manipulation and fabrication of nanoparticles.

The GROC–UJI team next plans to study how to alleviate the mechanical tolerances in femtosecond laser microprocessing. To generate feature size with high quality and small dimensions, the most widely adopted method is to focus the light with a refractive lens. This allows concentration of the beam energy onto a small area, the minimum size of which is restricted by the diffraction limit and the beam aberrations.

In addition, precision micromachining is highly susceptible to minor displacements caused by environmental disturbances such as temperature variation or system vibration. This can be a bottleneck for the fabrication of nanostructures, and micro-processing of rough samples or of large areas where active compensation of sample tilt would be required. Sophisticated monitoring and control of the focal position not only slows the speed of the material processing due to the continuous realignment, but also increases the cost of the beam delivery system. Researchers are currently working hard to obtain stable laser modules with high mechanical tolerances.

“We want to face the problem of alleviating mechanical tolerances in femtosecond micromachining by using diffractive lenses for light focusing instead of the refractive ones,” says Mínguez-Vega. “Due to the strong dependence of the focal length of a diffractive lens on the wavelength of light, a diffractive lens generates a longer focus and the position of the target becomes less critical". 

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At Photonics West SCHOTT introduces coatings with high laser damage threshold

The international technology group and specialty glass expert SCHOTT is now offering a wide range of coated materials for laser applications. Thanks to improvements of the coating capabilities at its site in Yverdon-les-Bains, Switzerland, the company is able to offer improved coated active glass devices as well as passive components made from materials such as SCHOTT N-BK7^® and FK5 optical glasses, as well as fused silica, filter glass, and ZERODUR^® glass-ceramic. Now, a coating is available with a higher laser damage threshold, which is relevant for the development of next-generation laser technology in various fields. SCHOTT is displaying its products at Photonics West (Booth #1601).

SCHOTT delivers laser glass coated with a high laser damage threshold, which is relevant for the development of next-generation laser technology in various fields. Photo: SCHOTT.

“We have been working diligently to make our laser components fit for use in large, demanding projects and a wide range of applications,” says Todd Jaeger, Sales Manager for Advanced Optics at SCHOTT North America, Inc. New SPIE test results confirm the improved performance of these components. One of the decisive factors is the laser damage threshold which must be high enough to allow for successful high-power applications that rely on a very high repetition rate of high-energy beams in a very short period of time. SCHOTT’s excellent material processing and polishing skills as well as its improved high quality dielectric coating technology make the materials the most resistant against laser induced damage. Furthermore new optical coating designs allow for broadband high reflection and dispersion control needed for ultrashort pulse, high-power applications. “While laser glass is produced and post-processed at our site in Duryea, Pennsylvania, the coating steps are carried out at our facility in Switzerland”, said Jaeger.

Improved material for use in new rangefinder generation
Also highlighted will be SCHOTT’s LG940 glass, ideal for "eye-safe" laser transmitters used in defense and industrial applications such as laser rangefinders and laser target designation systems. LG940, operating near the 1.5um wavelength, has increased strength, and excellent laser and optical properties for use at higher repetition rates than previous products. For the warfighter, SCHOTT LG940 enables the widespread production of novel laser amplifier designs for compact, low power, reliable rangefinders for soldier systems, ground and air vehicles with reduced weight and system costs.

In addition, SCHOTT expanded its portfolio with the ‘eye-safe’ phosphate laser glass LG940, which works in a short pulse regime for use in medical applications such as cosmetic laser treatments in dermatology. New applications in ophthalmic optics are also now possible.

Wide range of passive materials for laser applications
SCHOTT produces a range of components for improving lasing efficiencies and powers. These components such as mirrors, polarizers, and beam splitters are made of passive materials such as fused silica, optical glass (SCHOTT N-BK7^®, FK5), filter glass or ZERODUR^® glass-ceramic. All materials offer extremely high accuracy and cater to demands for the highest quality. Passive laser glass can, for example, be used as laser pumping cavity filters, which absorb undesired pumping light in the UV range, preventing solarization of the laser glass.

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