Delivery light energy ultra-deep into turbid medium by controlling photons to travel in open channels

Due to the inhomogeneity of biological tissue and the impact between photons and microscope particles, photons are inevitably scattered and diverged from the incident direction when traveling in the scattering medium. Besides, the number of scattering photons decreases exponentially with the propagation distance. Therefore, light scattering is considered a fundamental limitation for deep imaging and focusing.

 

In a new paper published in Light Science & Application, a team of scientists, led by Dr. Jing Cao from Beckman Laser Institute, University of California, Irvine, USA, and co-workers have proposed an exploration in enhancing the delivery of light energy ultra-deep into the scattering medium, as well as focusing light within the sample without any guide-star assistant. This method is based on a flying spot Reflection Matrix Optical Coherence Tomography (RMOCT). After obtaining the sample’s reflection matrix, a time reversal operator is calibrated with Tikhonov regularization and deviation criterion to find out multiple scattering photons, which have arrived at the target layer once, from the whole multiple scattering photons. Then with the help of reflection matrix inversion to get the matched wavefronts and a phase-only spatial light modulate to renew these matched wavefronts, these photons would travel in the so-called open channels. The propagation of photons in these channels will experience a high transmission coefficient directly to the desired focusing point.

 

These scientists claimed:

“Although there are a lot of limitations and urgent issues in beam focusing within the scattering medium, our method still shows the positive in overcoming these barriers. With the combination of reflection matrix measurement, decomposition of time reversal, and wavefront shaping technology, we have presented a systematic method in the investigation of (1) photons propagation process within the sample, (2) filter out single from multiple scattering photons, and (3) control photons in saving most of their energy when traveling to the target.”

 

 “Our method has two advantages. (1) We present a method to get the model energy matrix to help us “visualize” the degree of focus within the sample. It acts as a camera inserted into the sample to investigate the degree of focus. It is the basis of our proposed method, which provides a powerful tool for research about light focusing inside the sample. (2) With the help of the energy matrix and wavefront shaping, we realize the light focusing deep within the scattering medium without guide-stars. It provides applications within a broad realm of biomedicine.” they added.

 

“In addition, the ability to separate multiple scattering (ξ_MS1) photons that have reached the target once from the other multiple scattering (ξ_MS2) photons may also be beneficial for optical imaging methods. Modern advanced optical imaging methods adopt spatial pinhole filter, coherence/time gating, and other strategies to capture the ballistic and single scattering photons to recover the images. But these ξ_MS1 photons also carrier the useful information of the imaging plane. We believe the possibility of finding out these photons may pave the way for the emergence of new technologies in light manipulation, control light propagation, and deep imaging.” the scientists forecasted.

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