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Random Raman Laser Could Reveal Secrets Of Microscopic Life

Scientists created a “random Raman laser” that could have promising applications in high-speed wide-field microscopy, potentially revealing secrets of the unseen world.

"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." (Photo : Brett Hokr/ Texas A&M University, College Station, TX (D) and (E) Morgan Schmit/Optical Radiation Branch, Joint Base San Antonio)

“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.” (Photo : Brett Hokr/ Texas A&M University, College Station, TX (D) and (E) Morgan Schmit/Optical Radiation Branch, Joint Base San Antonio)

In today’s microscopic imaging techniques, lasers are used as light sources because of their ability to deliver pulses of intense radiation to a target, allowing for ultra-fast image acquisition, the Optical Society reported. However, this technique comes with disadvantages because it produces images blurred by speckled patterns.

The speckles occur because of a property of conventional lasers called “high spatial coherence,” so in order to remedy thisa team of researcher sought out a laser-like light source with “low spatial coherence.” This means the new light source would have electric fields at different positions in the light beam that do not oscillate in the lockstep as they do in traditional lasers. The new random Raman lasing emissions offer a bright, speckle-free, strobe light source.

“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.”

The new technique causes a diffuse material, such as powder, to emit laser light. This is different from conventional lasers, which bounce photons back and forth in a laser cavity.  Raman lasing occurs when light bounces around the powder particles long enough for amplification to occur. The emission is pulsed with a “temporal duration on the scale of single nanoseconds and in a narrow spectrum of about 0.1 nanometer,” which allows for a million times more photons per unit time per unit wavelength of any known technique.

The research team conducted the first spatial coherence measurement of the random Raman laser by setting up Young’s double slit experiment, in which Barium sulfate power was pumped with 530 microjoule, 50 picosecond laser pulses to create random lasing that passed through a double slit. They observed the interference patterns were barely discernible, meaning low spatial coherence had been achieved. The team also measured “speckle contrast ratio,” which shows the statistical properties of emissions. These measurements also confirmed a low level of coherence.

In a final demonstration of the technique’s effectiveness, the researchers produced a “full-frame, speckle-free microscopic image,” showing the formation of a cavitation bubble from melanosomes.

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