Making color: When two red photons make a blue photon

Making color: When two red photons make a blue photon

Can scientists generate any color of light? The answer is not really, but the invention of the laser in 1960 opened new doors for this endeavor. Scientists have now demonstrated a new semiconductor microstructure that performs frequency conversion. This design is a factor of 1000 smaller than previous devices.

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Color is strange, mainly due to perception. Setting aside complex brain processes, what we see is the result of light absorption, emission, and reflection. Trees appear green because atoms inside the leaves are emitting and/or reflecting green photons. Semiconductor LED brake lights emit single color light when electrical current passes through the devices.

Here's a question: Can scientists generate any color of light? The answer is not really, but the invention of the laser in 1960 opened new doors for this endeavor. An early experiment injected high-power laser light through quartz and out popped a different color. This sparked the field of nonlinear optics and with it, a new method of color generation became possible: frequency conversion.

Not all crystals can perform this trick and only through careful fabrication of certain materials is frequency conversion possible. In a result published in Nature Communications, scientists demonstrate a new microstructure that does what's called second harmonic generation (SHG), where the output light has twice the frequency as the input. This new device is a factor of 1000 smaller than previous frequency converters.

You can't really get something from nothing here. Physics demands that both energy and momentum are conserved in the frequency-doubling process. The energy of light is directly related to its frequency through a fundamental constant, thus this conservation law is automatically satisfied. Two photons of fixed energy pass into the conversion crystal and the output photon has a frequency, thus energy, equal to their sum.

The challenging part is momentum conservation and achieving it takes careful engineering. This difficulty arises because light has an associated direction of travel. Materials bend and delay light, and how it occurs is very material dependent. Even more, different frequencies (colors) are bent and delayed differently by a given material. This is called dispersion and is perhaps most familiar as a rainbow, where the constituent colors of sunlight are separated.

Even with dispersion, some materials have naturally occurring refractive properties that allow momentum-matching, and thus frequency conversion. Until about 20 years ago, these materials were the only option for frequency conversion. In the 1990s, scientists began to tackle the momentum conservation issue using a technique called quasi-phase matching (QPM).

When a light wave enters and moves through a crystal its properties such as velocity are altered depending on its color. In the case of second-harmonic generation, the injection light strongly interacts with the medium and a second color, having twice the frequency, is generated. Due to dispersion, the second light wave will be delayed. In QPM, scientists vary the spacing and orientation between the internal crystal layers to compensate for the delay, such that momentum conservation between the injection and output light is conserved. This method of QPM is successful but can be difficult from a fabrication point-of-view. Moreover, miniaturizing their overall size for integration onto chips is limited. This is because the frequency conversion process depends on the physical length of the interaction medium, thus scaling down these types of crystals will lead to an inherent reduction in efficiency.

Now this team has demonstrated a new, arguably simpler way, to achieve QPM and thus frequency conversion. In the new design, gallium arsenide (GaAs) is fabricated into a micrometer-sized disk 'whispering gallery' cavity. Notably, GaAs has one of the largest second-harmonic frequency conversion constants measured. Previously, scientists have harnessed its extremely nonlinear properties through the layer-varying QPM method, leading to device sizes in the centimeter range. This new device is 1000 times smaller.

In the experiment, light from a tapered optical fiber is injected into the cavity. When light travels in a loop with the proper orientation, as opposed to a linear geometry, QPM, and therefore color conversion is achieved. This team skirts around the miniaturization problem because the light can interact many times with the medium by circulating around the disk, yet the overall size can remain small. Using a cavity also means that since the power builds up in the microdisk, less injection power can be used. Think of the architectural example of a whispering gallery -- wherein sound waves add together such that small input signals (whispers) can be heard. This resonant enhancement also happens for light trapped inside microdisk cavities.

NIST scientist and author Glenn Solomon continues, "Through a combination of microcavity engineering and nonlinear optics, we can create a very small frequency conversion device that could be more easily integrated onto optical chips."

Lead author Paulina Kuo, who is currently doing research at NIST in the Information Technology Laboratory,adds, "I am excited because this method for phase-matching is brand new. It is amazing that the crystal itself can provide the phase-matching to ensure momentum conservation, and it's promising to see efficient optical frequency conversion in a really tiny volume."

In terms of future quantum information applications, the simple harmonic generation process can be considered as parametric down conversion (PDC) in reverse. PDC is a method for generating entangled photon pairs and so this device could provide a new technique for accomplishing this.

Gallium arsenide (GaAs) is a common semiconductor and has added benefits such as transmitting and emitting in the infrared (IR) and near IR light, respectively. IR-colored light has applications that include telecommunications and chemical sensing. Kuo adds, "The presence of an absorbing species affects the cavity resonance conditions and, in turn, the amount of frequency conversion in the microdisk. Thus, this device could be used in novel sensing applications."

Bones of a Previously Unknown Species Prove to be One of the Oldest Seabirds

Bones of a Previously Unknown Species Prove to be One of the Oldest Seabirds

Fossils discovered in Canterbury, New Zealand reveal the nature of one of the world's oldest flying seabirds. Thought to have lived between 60.5 and 61.6 million years ago, the fossil is suggested to have formed shortly after the extinction of dinosaurs and many marine organisms.

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Fossils discovered in Canterbury, New Zealand reveal the nature of one of the world's oldest flying seabirds. Thought to have lived between 60.5 and 61.6 million years ago, the fossil is suggested to have formed shortly after the extinction of dinosaurs and many marine organisms.

Bones of the bird were discovered in 2009 by Leigh Love, an amateur fossil collector. The new species, Australornis lovei has been named as such in honour of Love's discovery.

The bird lacks key morphological features of penguins, though it was found near the fossils of the Waimanu manneringi, the oldest penguin, of which it is also estimated to be the same age.

The research is published in Journal of the Royal Society of New Zealand by Dr Gerald Mayr and Dr Paul Scofield. The authors claim the discovery 'represents one of the most significant records of a marine Paleocene bird from the Southern Hemisphere' and supports the 'emerging view that most modern birds were already diversified in the earliest Paleogene'.

Despite the distinctness of this new species, its derived features are not limited to a single bird group. It does resemble an extinct species from Antarctica, however, highlighting the links between Antarctica and New Zealand in the late Cretaceous period.

Timing Is Everything: How the Brain Links Memories of Sequential Events

Timing Is Everything: How the Brain Links Memories of Sequential Events

Suppose you heard the sound of skidding tires, followed by a car crash. The next time you heard such a skid, you might cringe in fear, expecting a crash to follow -- suggesting that somehow, your brain had linked those two memories so that a fairly innocuous sound provokes dread.

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MIT neuroscientists have now discovered how two neural circuits in the brain work together to control the formation of such time-linked memories. This is a critical ability that helps the brain to determine when it needs to take action to defend against a potential threat, says Susumu Tonegawa, the Picower Professor of Biology and Neuroscience and senior author of a paper describing the findings in the Jan. 23 issue of Science.

"It's important for us to be able to associate things that happen with some temporal gap," says Tonegawa, who is a member of MIT's Picower Institute for Learning and Memory. "For animals it is very useful to know what events they should associate, and what not to associate."

The interaction of these two circuits allows the brain to maintain a balance between becoming too easily paralyzed with fear and being too careless, which could result in being caught off guard by a predator or other threat.

The paper's lead authors are Picower Institute postdocs Takashi Kitamura and Michele Pignatelli.

Linking memories

Memories of events, known as episodic memories, always contain three elements -- what, where, and when. Those memories are created in a brain structure called the hippocampus, which must coordinate each of these three elements.

To form episodic memories, the hippocampus also communicates with the region of the cerebral cortex just outside the hippocampus, known as the entorhinal cortex. The entorhinal cortex, which has several layers, receives sensory information, such as sights and sounds, from sensory processing areas of the brain and sends the information on to the hippocampus.

Previous research has revealed a great deal about how the brain links the place and object components of memory. Certain neurons in the hippocampus, known as place cells, are specialized to fire when an animal is in a specific location, and also when the animal is remembering that location. However, when it comes to associating objects and time, "our understanding has fallen behind," Tonegawa says. "Something is known, but relatively little compared to the object-place mechanism."

The new Science paper builds on a 2011 study from Tonegawa's lab in which he identified a brain circuit necessary for mice to link memories of two events -- a tone and a mild electric shock -- that occur up to 20 seconds apart. This circuit connects layer 3 of the entorhinal cortex to the CA1 region of the hippocampus. When that circuit, known as the monosynaptic circuit, was disrupted, the animals did not learn to fear the tone.

In the new paper, the researchers report the discovery of a previously unknown circuit that suppresses the monosynaptic circuit. This signal originates in a type of excitatory neurons discovered in Tonegawa's lab, dubbed "island cells" because they form circular clusters within layer 2. Those cells stimulate inhibitory neurons in CA1 that suppress the set of excitatory CA1 neurons that are activated by the monosynaptic circuit.

This circuit creates a counterbalance that limits the window of opportunity for two events to become linked. "This pathway might provide a mechanism for preventing constant learning of unimportant temporal associations," says Michael Hasselmo, a professor of psychology at Boston University who was not part of the research team.

The findings are "an important demonstration of the functional role of different populations of neurons in entorhinal cortex that provide input to the hippocampus," Hasselmo adds.

Deciphering circuits

The researchers used optogenetics, a technology that allows specific populations of neurons to be turned on or off with light, to demonstrate the interplay of these two circuits.

In normal mice, the maximum time gap between events that can be linked is about 20 seconds, but the researchers could lengthen that period by either boosting activity of layer 3 cells or suppressing layer 2 island cells. Conversely, they could shorten the window of opportunity by inhibiting layer 3 cells or stimulating input from layer 2 island cells, which both result in turning down CA1 activity.

The researchers hypothesize that prolonged CA1 activity keeps the memory of the tone alive long enough so that it is still present when the shock takes place, allowing the two memories to be linked. They are now investigating whether CA1 neurons remain active throughout the entire gap between events.

The research was funded by the RIKEN Brain Science Institute, the Howard Hughes Medical Institute, and the JPB Foundation.

Genome of Longest-Living Cancer: 11,000-Year-Old Living Dog Cancer Reveals Its Origin, Evolution

Genome of Longest-Living Cancer: 11,000-Year-Old Living Dog Cancer Reveals Its Origin, Evolution

A cancer normally lives and dies with a person, however this is not the case with a sexually transmitted cancer in dogs. In a study published in Science, researchers have described the genome and evolution of this cancer that has continued living within the dog population for the past 11,000 years.

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Scientists have sequenced the genome of the world's oldest continuously surviving cancer, a transmissible genital cancer that affects dogs. This cancer, which causes grotesque genital tumors in dogs around the world, first arose in a single dog that lived about 11,000 years ago. The cancer survived after the death of this dog by the transfer of its cancer cells to other dogs during mating.

The genome of this 11,000-year-old cancer carries about two million mutations -- many more mutations than are found in most human cancers, the majority of which have between 1,000 and 5,000 mutations. The team used one type of mutation, known to accumulate steadily over time as a "molecular clock," to estimate that the cancer first arose 11,000 years ago.

"The genome of this remarkable long-lived cancer has demonstrated that, given the right conditions, cancers can continue to survive for more than 10,000 years despite the accumulation of millions of mutations," says Dr Elizabeth Murchison, first author from the Wellcome Trust Sanger Institute and the University of Cambridge.

The genome of the transmissible dog cancer still harbors the genetic variants of the individual dog that first gave rise to the cancer 11,000 years ago. Analysis of these genetic variants revealed that this dog may have resembled an Alaskan Malamute or Husky. It probably had a short, straight coat that was colored either grey/brown or black. Its genetic sequence could not determine if this dog was a male or a female, but did indicate that it was a relatively inbred individual.

"We do not know why this particular individual gave rise to a transmissible cancer," says Dr Murchison, "But it is fascinating to look back in time and reconstruct the identity of this ancient dog whose genome is still alive today in the cells of the cancer that it spawned."

Transmissible dog cancer is a common disease found in dogs around the world today. The genome sequence has helped scientists to further understand how this disease has spread.

"The patterns of genetic variants in tumors from different continents suggested that the cancer existed in one isolated population of dogs for most of its history," says Dr Murchison. "It spread around the world within the last 500 years, possibly carried by dogs accompanying seafarers on their global explorations during the dawn of the age of exploration."

Transmissible cancers are extremely rare in nature. Cancers, in humans and animals, arise when a single cell in the body acquires mutations that cause it to produce more copies of itself. Cancer cells often spread to different parts of the body in a process known as metastasis. However, it is very rare for cancer cells to leave the bodies of their original hosts and to spread to other individuals. Apart from the dog transmissible cancer, the only other known naturally occurring transmissible cancer is an aggressive transmissible facial cancer in Tasmanian devils that is spread by biting.

"The genome of the transmissible dog cancer will help us to understand the processes that allow cancers to become transmissible," says Professor Sir Mike Stratton, senior author and Director of the Sanger Institute. "Although transmissible cancers are very rare, we should be prepared in case such a disease emerged in humans or other animals. Furthermore, studying the evolution of this ancient cancer can help us to understand factors driving cancer evolution more generally."

One Quarter of the World's Cartilaginous Fish, Namely Sharks and Rays, Face Imminent Extinction

One Quarter of the World's Cartilaginous Fish, Namely Sharks and Rays, Face Imminent Extinction

One quarter of the world's cartilaginous fish, namely sharks and rays, face extinction within the next few decades, according to the first study to systematically and globally assess their fate.

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The International Union for Conservation of Nature's (IUCN's) Shark Specialist Group (SSG), co-chaired by Nick Dulvy, a Simon Fraser University (SFU) Canada Research Chair in Marine Biodiversity and Conservation in British Columbia, conducted the study.

It was published ineLife journal today.Previous studies have documented local overfishing of some populations of sharks and rays. But this is the first one to survey their status through out coastal seas and oceans. It reveals that one-quarter (249) of 1,041 known shark, ray and chimaera species globally fall under three threatened categories on the IUCN Red List.

"We now know that many species of sharks and rays, not just the charismatic white sharks, face extinction across the ice-free seas of the world," says Dulvy. "There are no real sanctuaries for sharks where they are safe from overfishing."

Over two decades, the authors applied the IUCN's Red List categories and criteria to the 1,041 species at 17 workshops involving more than 300 experts. They incorporated all available information on distribution, catch, abundance, population trends, habitat use, life histories, threats and conservation measures.

Sharks and rays are at substantially higher risk of extinction than many other animals and have the lowest percentage of species considered safe. Using the IUCN Red List, the authors classified 107 species of rays (including skates) and 74 species of sharks as threatened. Just 23 percent of species were labeled as being Least Concern.

The authors identified two main hotspots for shark and ray depletion -- the Indo-Pacific (particularly the Gulf of Thailand), the Red Sea and the Mediterranean Sea.

"In the most peril are the largest species of rays and sharks, especially those living in relatively shallow water that is accessible to fisheries. The combined effects of overexploitation -- especially for the lucrative shark fin soup market -- and habit degradation are most severe for the 90 species found in freshwater.

"A whole bunch of wildly charismatic species is at risk. Rays, including the majestic manta and devil rays, are generally worse off than sharks. Unless binding commitments to protect these fish are made now, there is a real risk that our grandchildren won't see sharks and rays in the wild."

Losing these fish will be like losing whole chapters of our evolutionary history says Dulvy. "They are the only living representatives of the first lineage to have jaws, brains, placentas and the modern immune system of vertebrates."

The potential loss of the largest species is frightening for many reasons, says Dulvy. "The biggest species tend to have the greatest predatory role. The loss of top or apex predators cascades throughout marine ecosystems."

The IUCN SSG is calling on governments to safeguard sharks, rays and chimaeras through a variety of measures, including the following: prohibition on catching the most threatened species, science-based fisheries quotas, protection of key habitats and improved enforcement.

Gold Nanoparticles Help to Develop a New Method for Tracking Viruses

Gold Nanoparticles Help to Develop a New Method for Tracking Viruses

Researchers at the Nanoscience Center (NSC) of University of Jyväskylä in Finland have developed a novel method to study enterovirus structures and their functions. The method will help to obtain new information on trafficking of viruses in cells and tissues as well as on the mechanisms of virus opening inside cells. This new information is important for example for developing new antiviral drugs and vaccines. The study was published in the journalProceedings of the National Academy of Science.

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Enteroviruses are pathogenic viruses infecting humans. This group consists of polioviruses, coxsackieviruses, echoviruses and rhinoviruses. Enteroviruses are the most common causes of flu, but they also cause serious symptoms such as heart muscle infections and paralysis. Recently, enteroviruses have been linked with chronic diseases such as diabetes.

The infection mechanisms and infectious pathways of enteroviruses are still rather poorly known. Previous studies in the group of Dr. Varpu Marjomäki at the NSC have focused on the cellular factors that are important for the infection caused by selected enteroviruses. The mechanistic understanding of virus opening and the release of the viral genome in cellular structures for starting new virus production is still largely lacking. Furthermore, the knowledge of infectious processes in tissues is hampered by the lack of reliable tools for detecting virus infection.

The newly developed method involves a chemical modification of a known thiol-stabilized gold nanoparticle, the so-called Au102 cluster that was first synthesized and structurally solved by the group of Roger D Kornberg in 2007 and later characterized at NSC by the groups of prof. Hannu Häkkinen and prof. Mika Pettersson in collaboration with Kornberg. The organic thiol surface of the Au102 particles is modified by attaching linker molecules that make a chemical bond to sulfur-containing cysteine residues that are part of the surface structure of the virus. Several tens of gold particles can bind to a single virus, and the binding pattern shows up as dark tags reflecting the overall shape and structure of the virus. The gold particles allow for studies on the structural changes of the viruses during their lifespan.

The study showed also that the infectivity of the viruses is not compromised by the attached gold particles which indicates that the labeling method does not interfere with the normal biological functions of viruses inside cells. This facilitates new investigations on the virus structures from samples taken from inside cells during the various phases of the virus infection, and gives possibilities to obtain new information on the mechanisms of virus uncoating (opening and release of the genome). The new method allows also for tracking studies of virus pathways in tissues. This is important for further understanding of acute and chronic symptoms caused by viruses. Finally, the method is expected to be useful for developing of new antiviral vaccines that are based on virus-like particles.

The method was developed at the NSC as a wide cross-disciplinary collaboration between chemists, physicists and biologists. Researchers involved in the work are Tanja Lahtinen, Kirsi Salorinne, Jaakko Koivisto and Mika Pettersson from the Department of Chemistry, Sami Malola from the Department of Physics and Mari Martikainen and Varpu Marjomäki from the Department of Biology and Environmental Science. The research was coordinated by Docent Varpu Marjomäki and the Scientific Director of NSC, professor Hannu Häkkinen.

The research was funded by the Academy of Finland and the TEKES FiDiPro -project NOVAC (Novel methods for vaccination and virus detection).

New Evidence of 'Nordic Grog' Discovered in Scandinavia

New Evidence of 'Nordic Grog' Discovered in Scandinavia

A blazing fire was not the only thing to keep Bronze and Iron Age Scandinavians warm through long cold winters. From northwest Denmark, circa 1500-1300 BC, to the Swedish island of Gotland as late as the first century AD, Nordic peoples were imbibing an alcoholic "grog" or extreme hybrid beverage rich in local ingredients, including honey, bog cranberry, lingonberry, bog myrtle, yarrow, juniper, birch tree resin, and cereals including wheat, barley and/or rye -- and sometimes, grape wine imported from southern or central Europe.

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New research published in the Danish Journal of Archaeology examines evidence derived from samples inside pottery and bronze drinking vessels and strainers from four sites in Demark and Sweden. The research proves the existence of an early, widespread, and long-lived Nordic grog tradition, one with distinctive flavors and probable medicinal purposes  and the first chemically attested evidence for the importation of grape wine from southern or central Europe as early as 1100 BC, demonstrating both the social and cultural prestige attached to wine, and the presence of an active trading network across Europe  more than 3,000 years ago.

"Far from being the barbarians so vividly described by ancient Greeks and Romans, the early Scandinavians, northern inhabitants of so-called Proxima Thule, emerge with this new evidence as a people with an innovative flair for using available natural products in the making of distinctive fermented beverages," notes Dr. Patrick E. McGovern, lead author of the paper. "They were not averse to adopting the accoutrements of southern or central Europeans, drinking their preferred beverages out of imported and often ostentatiously grand vessels. They were also not averse to importing and drinking the southern beverage of preference, grape wine, though sometimes mixed with local ingredients."

To reach their conclusions the researchers, based at the University of Pennsylvania Museum of Archaeology and Anthropology, obtained ancient residue samples from four sites in a 150-mile radius of southern Sweden and encompassing Denmark. The oldest, dated 1500 -- 1300 BC, was from Nandrup in northwestern Denmark, where a warrior prince had been buried in an oak coffin with a massively hafted bronze sword, battle-ax, and pottery jar whose interior was covered with a dark residue that was sampled. A second Danish sample, dated to a later phase of the Nordic Bronze Age from about 1100 -- 500 BC, came from a pit hoard at Kostræde, southwest of Copenhagen. A brownish residue filling a perforation of a bronze strainer, the earliest strainer yet recovered in the region, was sampled. A third Danish sample was a dark residue on the interior base of a large bronze bucket from inside a wooden coffin of a 30-year-old woman, dating to the Early Roman Iron Age, about 200 BC, at Juellinge on the island of Lolland, southwest of Kostræde. The bucket was part of a standard, imported Roman wine-set, and the woman held the strainer-cup in her right hand. A reddish-brown residue filling the holes and interior of a strainer-cup, again part of imported Roman wine-set, provided the fourth sample. Dating to the first century AD, the strainer-cup was excavated from a hoard, which also included a large gold torque or neck ring and a pair of bronze bells, at Havor on the Swedish island of Gotland in the Baltic Sea.

According to Dr. McGovern, the importation of southern wine grew apace in the Bronze and Iron Ages, and eventually eclipsed the grog tradition -- but never completely. Many of the ingredients in Nordic grog went on to be consumed in birch beer and as the principal bittering agents (so-called gruit) of medieval beers, before hops gained popularity, and the German purity law (Reinheitsgebot) which limited ingredients of beer to barley, hops and water was enacted in Bavaria in 1516 and eventually became the norm in northern Europe.

"About the closest thing to the grog today is produced on the island of Gotland in the Baltic Sea," Dr. McGovern noted. "You can taste Gotlandsdryka in farmhouses. It's made from barley, honey, juniper, and other herbs like those in the ancient version."