Gravitational Waves and How Laser Interferometry enabled their Discovery
The study of the universe has mostly been possible due to the electromagnetic nature of radiation, which falls within X-ray, ultraviolet, visible and radio frequencies. When any celestial object emits radiation within these frequencies, we are able to determine something unique about its properties.
Gravitational waves are an entirely new phenomenon different from anything on the electromagnetic spectrum. In 1915, Albert Einstein proposed a unique way of looking at gravity with his theory of general relativity. Rather than thinking of gravity as a force pushing and pulling objects in different directions, he described gravity as being manifested in a curvature of spacetime. This means, the space (and time) around a massive object is curved, which then dictates how passing objects can move through that space. A consequence of the general relativity framework is that when objects accelerate through this curving of spacetime, they produce ripples known as gravitational waves. These waves propagate through space, compressing it in one direction and stretching it in another1.
These ripples in spacetime are extremely small, reportedly one-thousand of the diameter of a proton, which is the reason why these gravitational waves were so hard to detect. There had to be a celestial event massive enough to generate lots of ripples, and detectors sensitive enough to capture these tiny distortions. The technique that made this possible is called Laser Interferometry.
At the Laser Interferometer Gravitational-Wave Observatory (LIGO) an experiment uses 2 L-shaped detectors, one in Washington State and the other in Louisiana, to search for these ripples. Each detector bounces laser light back and forth between two mirrors located at the end of two perpendicular, 4 kilometer long legs. LIGO splits the laser light into 2 beams that are out of phase with each other. When they recombine, they should cancel each other out. If a gravitational wave passes through them however, a tiny distortion in spacetime will result in distance between the 2 mirrors changing slightly. The combining beams will not cancel each other, and a distortion signal will be detected.
The scientists at LIGO reported that the distance between the 2 mirrors changed by a fraction of the diameter of a proton. They conclude that the initial celestial event was a collision between two huge black holes about 1.3 billion years ago. From fitting the waveform of the gravitational wave detection and comparing it to simulations done with a supercomputer, astronomers can tell that the two black holes were originally 29 and 36 times the mass of our Sun. The discovery is not just proof of gravitational waves, but the strongest confirmation yet for the existence of black holes.
What makes this discovery very exciting for scientists is that the fundamental nature of gravitational waves (that is, how they propagate, behave, distort objects that come in their path etc.) is very different from that of electromagnetic waves. This provides us an entirely new way of observing the universe. While the study of Black Holes, Neutrino stars and Supernova occurrences is predicted to greatly benefit from this new-found method, scientists cannot predict what other phenomena will be discovered by observing gravitational waves. As laser interferometric techniques improve, so will the sensitivity of LIGO-like experiments. We can thus expect many more detections in the years to come.
1: Five Reasons You should Care About The Discovery of Gravitational Waves- Dr. Sabrina Stierwalt, Everyday Einstein, Feb 12, 2016
3. 'Gravitational Waves Discovered From Colliding Black Holes'- Scientific American: Online Link-Sci-Am
SERS Probe Traces Cadmium Levels in Drinking Water
Researchers at Alcorn State U. in collaboration with Jackson State U. have developed a highly sensitive plasmonic probe based on Surface Enhanced Raman Spectroscopy (SERS) to detect trace- level Cadmium in different samples of drinking water. Laserglow's LRS-0671 DPSS laser was used as an excitation source in the experimental setup.
Cadmium is one of the most toxic elements and is reported to be the seventh-highest on the list of most hazardous substances found in the environment by the Center of Disease Control and Prevention (CDC, USA). Surface-enhanced Raman spectroscopy (SERS) is a powerful optical technique for sensitive and selective detection of toxic metals in aqueous solution. In this experiment, researchers chose Alizarin - a highly reactive Raman active dye (which is also environmentally friendly) as a Raman reporter. It was functionalized on a plasmonic gold surface, and then a combination of 3-mercaptopropionic acid and 2, 6-pyridinedicarboxylic acid at pH 8.5 was immobilized on the surface of the gold nanoparticle. At the introduction of Cadmium, the gold nanoparticle provide an excellent hotspot for Alizarin dye and Raman signal enhancement. The team of researchers were able to achieve as low as 10 ppt (parts-per-thousand) sensitivity from various drinking water sources against other Alkali and heavy metal ions.
The team is confident that the developed SERS probe's simplicity has great potential for prototype scale up for field application. The research was first published in October 2015, and available to view online on Nov 24, 2015 on ScienceDirect.com.
For a detailed explanation of the experimental setup and discussion of results, access the full paper at: Science Direct Full Paper
Hollow-Core Fiber Creates a New Laser-Wavelength Long-Sought By Developers
Researchers at the U. Bath, UK have created a new kind of laser capable of pulsed and continuous mid-infrared (IR) emission between 3100 to 3200 nm- a wavelength range which has been very hard to achieve by laser manufacturers. This creates a new opportunity for scientists to utilize this radiation in applications like spectroscopy, geo-sensing and material detection.
The laser, first announced in the Journal Optica, combines aspects of both gas and fiber lasers. Placing a suitable gas inside of a hollow optical fiber allowed the researchers to create a fiber gas laser with mid-IR emission. Conventional lasers lose power beyond 2800 nm, and quantum cascade lasers don't work until 3500 nm and beyond. This range in between however, is now possible to achieve with help of silica hollow-core fibers. Unlike solid-core silica fibers, hollow-core fibers do not absorb wavelengths higher than 2800 nm. Rather, they simply confine the light to the fiber.
In this study, the researchers used acetylene gas as the gain medium as it's known to emit in the mid-IR region. The hollow-core fibers provided a way to trap the light and the gas in the same place so that they can interact for a very long distance. The mid-IR range light produced was then fed into a feedback fiber, which then seeded another cycle of light amplification, thus resulting in a stable mid-IR emission without the need of added power.
The team at Bath is confident that they could use this method of hollow-core fibers with other gases, allowing for emissions up to 5000 nm and beyond. Read more at:
Phys.org full article