One of the differences between a laser and a desk lamp is that the propagation of the laser is spatially congruent, meaning that the crests and troughs of the light wave are interrelated. Waves launched from a lamp are irrelevant waves, manifesting a messy state and, on the other hand, are often considered irrelevant.
This is a bit of a wording, however. In theory, almost all light, even "irrelevant" light, can have a high degree of spatial coherence. However, the detection of its coherence requires the detection of light on very small length scales, which can not be achieved using conventional techniques.
Today, Domenico Pacifici lab researchers and professors at Brown University's School of Engineering have found a way to detect spatially coherent beams for detection on hundreds of nanometers, which is smaller than ever before. This study provides the first experimental verification of nanoscale optical coherence theory.
"There is a very small length-scale light, which is generally irrelevant, but we lack quantitative experimentation," said Drew Morrill, who is the lead author of the experimental paper. "This level of coherence contains very meaningful information that we now have access to, which is very useful for characterizing light sources and potentially valuable for new imaging and microscopy techniques."
Morrill, now a graduate student at the University of Colorado, works at Brown University. This research was co-authored by Pacifici and Lee Dong-fang, a postdoctoral scholar at Brown University, in a paper published in Nature Photonics.
The traditional way to test the extent to which light is spatially coherent involves the equipment that can split the wavefront of the light beam. The most famous is Young's interferometer, also known as double-slit experiment. The experiment consists of a light source pointing to the detector screen, with an opaque barrier in between. There are two small slots in the middle of the barrier, allowing two beams to pass through. When light passes through the gap, some light waves bend toward each other, causing them to reorganize. The coherence of this complex wave will create an interference pattern, a series of light and dark stripes on the detector's screen. By measuring the contrast between these light and dark spots, researchers can quantify the coherence of light.
The problem is that for a light source with a very low spatial coherence, a double-slit experiment does not work because the length scale in which the interference pattern appears is very small. Interference on a small scale requires two slits to be placed in close proximity. But when the distance between the two slits is close to the wavelength of the light they show, the experiment collapses. The interferometer was unable to divide and reorganize the beam to find interference.
"The interference fringes are blurred, to the point where it is difficult to quantify the extent of interference," Morrill said. "But if you can get the basic limitations of a two-slit experiment, you should theoretically see these streaks."
To overcome these limitations, researchers employed a different interferometer that exploited the interaction of surface plasmons with metal light and electrons. Instead of two slits, the slit plasma interferometer is already in the silver surface groove. Light is emitted into the groove to generate surface plasmon (SPP), an electron density wave across the silver surface. The SPP propagates toward the slit and is recombined with the beam passing through the slit. Since SPP is associated with the original beam, it has a smaller wavelength because it will have a 90-degree angle of diffraction toward the nip, and the slot in the plasma interferometer may be more flexible than a Young's two-slit interferometer Put together close together.
Researchers have collected hundreds of these tiny interferometers, which are designed and prepared on a nanoscale chip. They used the chip to measure the coherence length of the wavelength bandwidth in the hundreds of visible light spectrum of xenon lamps. For blue-to-green light, the measured coherence length drops down to 330 nm, which is less than the 500 nm wavelength of the incident light source.
The first experimental result confirmed the coherence theory smaller than the wavelength of light wave.
"That was an exciting result," Morrill said. "Without experimental verification, we really do not know if these equations will help with these small-scale experiments, but it turns out to be really helpful."
On the application side, plasma chips will help manufacturers of light sources for microscope light, holography and other applications, which will better characterize light sources. The system is integrated on a single chip, which will help make the description of the light source faster and easier.
"With a certain intensity of light through a dense plasma interferometer, you can document its spatial coherence in snapshots, which takes only a few seconds," said Lee, who led the production of meters.
"We are providing scientists with a new tool for quantifying the degree of coherence on a scale that was previously impossible to scale," Pacifici said.
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