The random raman laser: A new light source for the microcosmos

In modern microscope imaging techniques, lasers are used as light sources because they can deliver fast pulsed and extremely high-intensity radiation to a target, allowing for rapid image acquisition. However, traditional lasers come with a significant disadvantage in that they produce images with blurred speckle patterns -- a visual artifact that arises because of a property of traditional lasers called "high spatial coherence." These speckles greatly reduce image quality in wide-field microscopy, a common technique for making broad swath images of the whole side of a cell or some other part of the microscopic world in order to understand its intricate inner workings. To solve this problem, scientists have sought a laser-like light source with "low spatial coherence." Low spatial coherence means that the electric fields at different positions in the light beam do not oscillate in lockstep, unlike traditional lasers. Now a team of researchers at Texas A&M University has done just that, demonstrating for the first time that a newly emerging technique known as random Raman lasing emission can produce a bright, speckle-free, strobe light source with potential application in high-speed wide-field microscopy. False color images of the interference pattern produced by a double slit for, (A), random Raman laser emission, (B), elastically scattered light and, (C), Helium-Neon laser emission. Strobe photography images of microcavitation bubbles forming, (D), before and, (E),\ after melanasomes are irradiated with 0.625 J/cm2 at 1064 nm. (Image: (A)-(C) Brett Hokr/ Texas A&M University, College Station, TX (D) and (E) Morgan Schmit/Optical Radiation Branch, Joint Base San Antonio, Fort Sam Houston, TX) "The random Raman laser is unlike any existing laser light source," said Brett Hokr, a physicist at Texas A&M University who led the research. "We found that random Raman lasing emission has a low level of spatial coherence. The emission can be used to produce a wide-field speckle-free quality image with a strobe time on the order of a nanosecond. This new, bright, fast, narrowband, low-coherence light source opens the door to many exciting new applications in bio-imaging such as high-speed, wide-field microscopy." Random Raman Lasing: A Newly Emerging Technique Random Raman lasing causes a diffuse material such as a powder to emit laser light. Different from traditional lasers that work by bouncing photons back and forth in a laser cavity, random Raman lasing happens when the light bounces among the powder particles long enough for amplification to occur. According to Hokr, random Raman laser emission is a pulsed emission with a temporal duration on the scale of single nanoseconds and in a narrow spectrum of about 0.1 nanometer, which can emit a million times more photons per unit time per unit wavelength than any other conventional light source, and should have sufficient intensity to allow scientists to acquire a full two-dimensional fluorescent image in a single pulse of the laser. Hokr's team conducted the first spatial coherence measurement of the random Raman laser in two ways -- initially using a classic set-up known as Young's double slit experiment. Barium sulfate power was pumped with 530 microjoule, 50 picosecond laser pulses to generate random lasing that later passed through a double slit, and the team captured images of the interference patterns. The researchers observed that those interference patterns were barely discernible, indicating a very low degree of spatial coherence. To further quantify the overall spatial coherence, they measured something known as the speckle contrast ratio, which gauges the statistical properties of the emission. These measurements were consistent in confirming the presence of a low level of coherence. To further demonstrate that this low coherence truly leads to a speckle-free image, the researchers produced a full-frame, speckle-free microscopic image showing the formation of a cavitation bubble from melanosomes from a several-nanosecond laser pulse at 1064 nanometer radiation. About the Presentation The presentation, "Accurately Simulating Focusing Beams using Monte Carlo Techniques," by Brett H. Hokr; Joel Bixler; Gabe Elpers; Byron Zollars; Robert Thomas; Vladislav Yakovlev; Marlan O. Scully, will take place from 18:00 - 20:00, Tuesday, 12 May 2015, in the San Jose Convention Center, San Jose, California, USA.

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Many uses in researching quantum dots

It's easier to dissolve a sugar cube in a glass of water by crushing the cube first, because the numerous tiny particles cover more surface area in the water than the cube itself. In a way, the same principle applies to the potential value of materials composed of nanoparticles.
Ari Chakraborty is an assistant professor of chemistry at Syracuse University. Because nanoparticles are so small, millions of times smaller than the width of a human hair, they have "tremendous surface area," raising the possibility of using them to design materials with more efficient solar-to-electricity and solar-to-chemical energy pathways, says Ari Chakraborty, an assistant professor of chemistry at Syracuse University.

"They are very promising materials," he says. "You can optimize the amount of energy you produce from a nanoparticle-based solar cell."

Chakraborty, an expert in physical and theoretical chemistry, quantum mechanics and nanomaterials, is seeking to understand how these nanoparticles interact with light after changing their shape and size, which means, for example, they ultimately could provide enhanced photovoltaic and light-harvesting properties. Changing their shape and size is possible "without changing their chemical composition," he says. "The same chemical compound in different sizes and shapes will interact differently with light."

Specifically, the National Science Foundation (NSF)-funded (Award: Theoretical investigation of optical properties of quantum dots using explicitly correlated methods) scientist is focusing on quantum dots, which are semiconductor crystals on a nanometer scale. Quantum dots are so tiny that the electrons within them exist only in states with specific energies. As such, quantum dots behave similarly to atoms, and, like atoms, can achieve higher levels of energy when light stimulates them.

Chakraborty works in theoretical and computational chemistry, meaning "we work with computers and computers only," he says. "The goal of computational chemistry is to use fundamental laws of physics to understand how matter interacts with each other, and, in my research, with light. We want to predict chemical processes before they actually happen in the lab, which tells us which direction to pursue." These atoms and molecules follow natural laws of motion, "and we know what they are," he says. "Unfortunately, they are too complicated to be solved by hand or calculator when applied to chemical systems, which is why we use a computer." The "electronically excited" states of the nanoparticles influence their optical properties, he says. "We investigate these excited states by solving the Schrödinger equation for the nanoparticles," he says, referring to a partial differential equation that describes how the quantum state of some physical system changes with time. "The Schrödinger equation provides the quantum mechanical description of all the electrons in the nanoparticle. "However, accurate solution of the Schrödinger equation is challenging because of large number of electrons in system," he adds. "For example, a 20 nanometer CdSe quantum dot contains over 6 million electrons. Currently, the primary focus of my research group is to develop new quantum chemical methods to address these challenges. The newly developed methods are implemented in open-source computational software, which will be distributed to the general public free of charge." Solar voltaics, "requires a substance that captures light, uses it, and transfers that energy into electrical energy," he says. With solar cell materials made of nanoparticles, "you can use different shapes and sizes, and capture more energy," he adds. "Also, you can have a large surface area for a small amount of materials, so you don't need a lot of them." Nanoparticles also could be useful in converting solar energy to chemical energy, he says. "How do you store the energy when the sun is not out?" he says. "For example, leaves on a tree take energy and store it as glucose, then later use the glucose for food. One potential application is to develop artificial leaves for artificial photosynthesis. There is a huge area of ongoing research to make compounds that can store energy." Medical imaging presents another useful potential application, he says. "For example, nanoparticles have been coated with binding agents that bind to cancerous cells," he says. "Under certain chemical and physical conditions, the nanoparticles can be tuned to emit light, which allows us to take pictures of the nanoparticles. You could pinpoint the areas where there are cancerous cells in the body. The regions where the cancerous cells are located show up as bright spots in the photograph." Chakraborty is conducting his research under an NSF Faculty Early Career Development (CAREER) award. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research within the context of the mission of their organization. NSF is funding his work with $622,123 over five years. As part of the grant's educational component, Chakraborty is hosting several students from a local high school--East Syracuse Mineoa High School--in his lab. He also has organized two workshops for high school teachers on how to use computational tools in their classrooms "to make chemistry more interesting and intuitive to high school students," he says. "The really good part about it is that the kids can really work with the molecules because they can see them on the screen and manipulate them in 3-D space," he adds. "They can explore their structure using computers. They can measure distances, angles, and energies associated with the molecules, which is not possible to do with a physical model. They can stretch it, and see it come back to its original structure. It's a real hands-on experience that the kids can have while learning chemistry."

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From brittle to plastic in one breath

What if peanut brittle, under certain conditions, behaved like taffy? Something like that happens to a two-dimensional dichalcogenide analyzed by scientists at Rice University. Rice researchers calculated that atomically thin layers of molybdenum disulfide can take on the qualities of plastic through exposure to a sulfur-infused gas at the right temperature and pressure. That means one can deform it without breaking it — a property many materials scientists who study two-dimensional materials should find interesting, according to Rice theoretical physicist Boris Yakobson and postdoctoral researcher Xiaolong Zou; they led the study that appeared in the American Chemical Society journal ("Environment-Controlled Dislocation Migration and Superplasticity in Monolayer MoS2"). Calculations by Rice University scientists show that a two-dimensional layer of molybdenum disulfide can become superplastic by changing its environmental conditions. In an atmosphere with sulfur and under the right temperature and pressure, the energy barrier is lowered, allowing dislocations along the grain boundaries to shift and changing the material’s properties. S2 refers to a disulfur molecule; VS2 is a two-sulfur-atom vacancy. (Image: Xiaolong Zou/Rice University) Molybdenum disulfide, the object of study in many labs for its semiconducting properties, interested the Rice lab because of the characteristics of its grain boundaries. Two-dimensional materials like graphene are actually flat, atom-thick sheets. But 2-D molybdenum disulfide is a sandwich, with layers of sulfur above and below the molybdenum atoms. When two sheets join at different angles during growth in a furnace, atoms at the boundaries have to compensate by improvising “defective” arrangements, called dislocations, where they come together. The researchers determined it may be possible to promote the movement of those dislocations through environmental control of the gas medium. This would change the material’s properties to give it superplasticity, which allows it to be deformed beyond its usual breaking point. Plastic materials can be rearranged and will hold their new shape. For example, a plumber can bend a metal pipe; that bendable quality is plasticity. Yakobson noted such materials can become brittle again with further changes in the environment. “Generally, the coupling of chemistry and mechanics is quite rare and scientifically difficult to understand,” said Yakobson, whose group at Rice analyzes materials by calculating the energies that bind their atoms. “Corrosion is the best example of how chemistry affects mechanical behavior, and the science of corrosion is still in development.” Calculations by Rice University scientists show that a two-dimensional layer of molybdenum disulfide can become superplastic by changing its environmental conditions. In an atmosphere with sulfur and under the right temperature and pressure, the energy barrier is lowered, allowing dislocations along the grain boundaries to shift and changing the material’s properties. S2 refers to a disulfur molecule; VS2 is a two-sulfur-atom vacancy. (Image: Xiaolong Zou/Rice University) For molybdenum disulfide, they found two mechanisms by which boundaries could overcome activation energy barriers and lead to superplasticity. In the first, called direct rebonding, only one molybdenum atom in a dislocation would shift in response to external forces. In the second, bond rotation, several atoms would shift in opposite directions. They calculated that the barrier for direct rebonding, while less dramatic, is much lower than for bond rotation. “Through the rebonding path, the mobility of this defect changes by several orders of magnitude,” Yakobson said. “We know from the mechanics of materials that brittle or ductile qualities are defined by the mobility of these dislocations. What we show is that we can affect the tangible property, the stretchability, of the material.” Yakobson suggested it may be possible to tune the plasticity of dichalcogenides in general and that it may also be possible to eliminate the defects from a 2-D dichalcogenide sheet by treating the dislocations “to allow them to rapidly diffuse away and vanish or to form interesting aggregated states.” That would likely open the way to the easier manufacture of dichalcogenides that need particular electrical or mechanical properties for applications, he said. “We think of these two-dimensional materials as an open canvas, theoretically speaking,” he said. “You can very quickly read and write changes to them. Bulk materials don’t have this openness, but here, every atom is in immediate proximity to the environment.”

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