Notes from the Editors

# Highlights from CLEO 2015

Physics 8, 57
At the Conference on Lasers and Electro-Optics in San Jose, researchers presented a smartphone app that analyzes DNA, invisibility cloaks for classroom demos, and new optical techniques for studying the brain.

The annual Conference on Lasers and Electro-Optics (CLEO) is the premier global meeting on laser and optics technology. Here is a selection of the presentations from this year’s conference.

##### Invisibility Cloaks For the Classroom

Researchers in Germany showcased a device that will allow them to easily demonstrate the phenomenon of “cloaking” in classrooms and to the public. Robert Schittny from the Karlsruhe Institute of Technology presented a scheme based on silicone doped with nanoparticles that makes an object’s shadow disappear from sight. Unlike most cloaks, which are small and only work in specialized applications, the silicone cloak provides an easy-to-see effect without any specialized equipment.

In theory, a perfect invisibility cloak would bend light around an object so that it emerges as if the object weren’t there. Yet bending white light in this way is physically impossible because it would require all frequency components to travel faster than $c$, the speed of light in vacuum. The same limits don’t exist in highly scattering media like fog or milk, explained Schittny. In these materials, light has to zigzag over multiple scattering centers, and the effective propagation speed is so slow that even the fastest traveling light required by the cloak doesn’t have to go faster than $c$.

In a live demonstration, Schittny placed a metal cylinder inside a block of silicone that was doped with light-scattering nanoparticles. As expected, the cylinder cast a fuzzy shadow when the block was illuminated from one side. But when the metal was surrounded with an additional layer of more lightly doped silicone, its shadow disappeared (see Fig. 1). This extra layer makes light travel around the cylinder in the same time it would take light to travel through the block.

##### A Smartphone that Analyzes DNA

Scientists can detect malaria, cancer, and other diseases by looking for abnormally long or short genes in DNA. But the process typically involves expensive microscopes only found in dedicated labs. At the meeting, Qingshan Wei from the University of California, Los Angeles, reported a new smartphone attachment that, for an estimated price of only 400 US dollars, could turn every phone into a portable DNA microscope.

The device fits into a case half the size of a typical phone. A laser diode in the device illuminates a sample of DNA molecules that have been tagged with fluorescent “marker molecules.” A lens collects the sample’s fluorescence and sends it to the phone’s camera. An app then sends these images to a remote server that, within seconds, analyzes the data to determine the number of DNA molecules and their lengths.

The scheme acquires images with a spatial resolution of less than 2 micrometers, sufficient to image individual DNA strands. For now, the DNA-tagging step still needs to be performed in a lab. But Wei foresees a portable way of doing this using a microfluidic chip attachment that labels the DNA and prepares it for analysis. The researchers next plan to test their device in the field, where they will use DNA analysis to detect the emergence of resistance to malaria drugs.

##### Illuminating the Brain

The emerging field of optogenetics combines genetics and optics to control and observe neurons in the brains of animals. The idea is to insert photosensitive proteins into specific neurons, which can then be turned on and off or monitored using light. At the meeting, two presentations highlighted recent progress in the imaging and stimulation of networks of brain neurons.

Mark Schnitzer from Stanford University, Calfornia, described a compact microscope that can be attached to a mouse’s brain to “read out” the neural activity as the animal carries out certain tasks or responds to its surroundings. Built from micro-optical components, the device weighs only 2 grams, allowing the mouse to move freely during a measurement. His group also developed a microscope with two arms, which can simultaneously image pairs of brain areas with single-cell resolution. This feature, Schnitzer explained, will help study the functional relationship between different parts of the brain.

Valentina Emiliani, from Descartes University in Paris, reported a technique that uses light to precisely “orchestrate” a pattern of neural activity. With a fiber bundle, Emiliani and her co-workers deliver short pulses of blue light to the brain of mice. Using optical techniques such as computer-generated holography, the researchers are able to excite a precisely determined set of cells. The method will permit the control of specific subsets of neurons in the brain, helping elucidate the role they play in the brain’s circuitry.

–Matteo Rini

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