Better Sense of Incoherent Light

A new technique detects spatial coherence in light at smaller scales than had been possible. The image shows visibility curves appearing at the nanoscale, the telltale sign of spatial coherence. (Source: Pacifici Lab / Brown U)

A new technique detects spatial coherence in light at smaller scales than had been possible. The image shows visibility curves appearing at the nanoscale, the telltale sign of spatial coherence. (Source: Pacifici Lab / Brown U)

One of the dif­ferences between lasers and desk lamps is that laser light is spatially coherent, meaning the peaks and valleys of the light waves are correlated with each other. The jumbled, uncorre­lated waves coming from a desk lamp, on the other hand, are often said to be incoherent. That’s a bit of a misnomer, however. In theory, virtually all light — even inco­herent light — can have a high degree of spatial coherence. But detecting that coherence requires probing light at extremely small length scales that cannot be accessed using traditional techniques.

Now, researchers in the lab of Domenico Pacifici, professor in Brown University’s School of Engineering, have found a way to detect spatial coherence in light beams at the scale of a few hundred nanometers — a much smaller scale than has ever been possible. The research provides the first experi­mental veri­fication of optical coherence theory at the nanoscale. “There’s a very small length scale at which light that’s often said to be inco­herent behaves coherently, but we’ve lacked experimental techniques to quantify it,” said Drew Morrill. “That degree of coherence contains meaningful information we can now access, which could be useful in charac­terizing light sources and poten­tially for new imaging and micro­scopy techniques.” Morrill, now a graduate student at the University of Colorado, performed the work as an undergraduate at Brown.

Tradi­tional methods for testing the extent to which light is spatially coherent involve devices that can split the wavefront of a light beam. The most famous of these is the Young inter­ferometer, also known as the double slit experiment. The expe­riment consists of a light source aimed at a detector screen, with an opaque barrier between the two. The barrier has two small slits in it, allowing two rays of light to pass through. As the two rays emerge from the slits, some of the light waves are bent toward each other, causing them to recombine. Recom­bining waves that are coherent will create an inter­ference pattern on the detector screen. By measuring the contrast of those light and dark patches, researchers can quantify the light’s coherence.

The problem is that for light sources with very low spatial coherence, the double slit experiment doesn’t work as well because the length scales at which the inter­ference patterns appear is very small. Producing inter­ference over small length scales requires the two slits to be placed very close together. But when the distance between the two slits gets close to of the wave­length of the light shown upon them, the experiment breaks down. The inter­ferometer can no longer split and recombine the beam properly to look for interference. “The inter­ference fringes are smeared out, making it difficult to quantify the degree of coherence,” Morrill said. “But if you could get around the fun­damental limi­tations of the double slit experiment, theore­tically you should be able to see those fringes.”

To get around those limi­tations, the researchers employed a different kind of inter­ferometer that makes use of plasmonics, the inter­action between light and electrons in a metal. Instead of two slits, the plasmonic inter­ferometer has a slit and a groove in a surface made of silver. Light hitting the groove creates a surface plasmon polariton (SPP), a density wave of electrons moving across the silver surface. The SPP propagates toward the slit, where it recombines with the light going through the slit. Because the SPP is related to the original beam of light but has a smaller wavelength, and because it diffracts at a 90-degree angle toward the slit, the groove and slit in the plasmonic inter­ferometer can be placed closer together than the two slits in the Young inter­ferometer.

The researchers amassed hundreds of these tiny inter­ferometers, designed and fabricated with nanometric precision, on a microchip. They used that chip to the measure coherence lengths of a broadband xenon lamp for hundreds of wave­lengths across the visible spectrum. For blue-green light, measured coherence lengths dropped as low as 330 nanometers — smaller than the 500 nanometer incident wavelength of the light source. The results are the first experi­mental confir­mation of coherence theory at or below the wavelength of light.

“That was a really exciting result,” Morrill said. “Without experi­mental verifi­cation, we really didn’t know if these equations held up for these small scales, but it turns out that they do.” In terms of potential appli­cations, the plasmonic chip could help manu­facturers of light sources for micro­scopy, holography and other appli­cations to better characterize their light sources. The inte­gration of the inter­ferometers on a single chip makes the processes of charac­terizing a light source quick and easy.

“You can just record the degree of spatial coherence in a single snapshot by taking a picture of the light intensity through the densely spaced plasmonic inter­ferometers, which only takes a few seconds,” said Li, who led the fabri­cation of the meter. “We’re providing scientists with a new tool to quantify the degree of coherence of light at a length scale that hadn’t been possible before,” Pacifici said. (Source: Brown Univ.)

Reference: D. Morrill et al.: Measuring subwavelength spatial coherence with plasmonic interferometry, Nat. Phot., online 

Link: Pacifici Group, School of Engineering, Brown University, Providence, Rhode Island, USA

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