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Research: Wavefront Shaping

Time-reversal Wavefront Engineering

Light scattering in biological tissues poses a significant challenge to our ability to study intact specimens beyond superficial depths (Fig. 1). Conventional optical focusing methods treat scattered light as noise and rely on unscattered light, which exponentially decreases with depth. However, scattering is a deterministic process and is reversible through optical phase conjugation (OPC). Using this technique, our group previously demonstrated focusing of light through biological tissues (Fig. 2)1, 2.

Scattering in biological tissues limits focusing to superficial layers or thin samples.jpg
Fig. 1: Scattering in biological tissues limits focusing to superficial layers or thin samples.

Wavefront distortions due to scattering can be reserved through optical phase conjugation.jpg

OPC foci obtained through 3 mm (a), 6 mm (b), and 10 mm (c) thick chicken breast tissues..jpg

Fig. 2: (Top) Wavefront distortions due to scattering can be reserved through optical phase conjugation. (Bottom) OPC foci obtained through 3 mm (a), 6 mm (b), and 10 mm (c) thick chicken breast tissues. (d) Photo of the 10 mm thick tissue sample.

Time Reversal of Ultrasound-Encoded Light (TRUE)

An important goal of biomedical optics, however, is to focus inside. To this end, Xu et. al. proposed combining optical phase conjugation with ultrasound encoding in a method named time-reversal of ultrasound encoded light (TRUE)3. To realize high resolution, high intensity TRUE focusing in thick tissues, our group utilized a digital optical phase conjugate mirror (DOPC) that enables high gain4 (Fig. 3). We illustrate the potential of our method for fluorescence bioimaging in the diffusive regime by imaging complex fluorescent objects and tumor microtissues ~ 2.5 mm deep in biological tissues, at a lateral resolution of ~ 40 µm (Fig. 4).

A portion of the light passing through the ultrasound focus is frequency-shifted via the acousto-optic effect.jpg
Fig. 3: A portion of the light passing through the ultrasound focus is frequency-shifted via the acousto-optic effect. The frequency-shifted light is selectively phase conjugated by the digital optical phase conjugation mirror (DOPC)4.

Images of quantum dot "CIT" feature an fluorescently.jpg
Fig. 4: Images of quantum dot "CIT" feature an fluorescently dyed cancer microtissues (top) Epifluorescence before embedding (middle) Epifluorescence after embedding (bottom) Digital TRUE image.

Time Reversal of Variance-Encoded Light (TROVE)

The resolution of the TRUE method is fundamentally limited by the size of the ultrasound focus. In TROVE, we overcome this limitation and achieve single optical speckle resolution by using speckle statistics to encode the position of each optical speckle within the ultrasound focus. To do so, we illuminate the sample with a set of input wavefronts and measure the scattered ultrasound tagged wavefront that exits the sample. With the ability of the DOPC to digitally analyze and manipulate optical wavefronts, these wavefronts can be processed and decomposed into an eigenset of optimal wavefront solutions. When time-reversed, each of these wavefront solutions would focus to a single speckle-size limited spot. For example, the solution that exhibits the highest eigenvalue would time-reverse back to the exact center of the ultrasound focus. This essentially allows us to achieve optical speckle size limited, high resolution focusing.5

TROVE makes use of speckle statistics to enable optical.jpg
Fig. 5: TROVE makes use of speckle statistics to enable optical speckle sized focusing.

Results obtained with TROVE and comparison with TRUE.jpg
Fig. 6: Results obtained with TROVE and comparison with TRUE.

Optical-channel-based intensity streaming (OCIS)

Conventional wavefront shaping controls the wavefront of incident light field to focus inside the scattering medium. Here, we tried to use the intensity to generate a focus inside scattering medium. When a point source (guidestar) emits light, the output will be speckles due to random scattering inside the scattering medium. What we know is that light interferes constructively for those bright speckles (each speckle is treated as an optical channel). By optical reciprocity, if the light enters the medium at those bright speckles, light will constructively interfere at the guidestar's location. Speckle at anywhere else does not exhibit such behavior. If we manipulate the light such that it enters only at those location where bright speckles (bright optical channels) are recorded, we can effectively focus inside the scattering medium.6

ocis_concept

Fig. 7: Concept of OCIS. By choosing areas that gives us a bright speckle, we can effectively focus through the scattering medium.

Further information on the effects of Time-reversal Wavefront Engineering in Biological Media can be found here.

References

  1. Yaqoob, Z., et al., Optical phase conjugation for turbidity suppression in biological samples. Nature Photonics 2(2): 110-115, (2008).
  2. McDowell, E.J., et al., Turbidity suppression from the ballistic to the diffusive regime in biological tissues using optical phase conjugation. Journal of Biomedical Optics 15(2): 025004-025004, (2010).
  3. Xu, X., Liu, H., and Wang, L.V., Time-reversed ultrasonically encoded optical focusing into scattering media. Nature Photonics 5, 154-157, (2011).
  4. Wang, Y.M., Judkewitz, B., DiMarzio, C.A. and Yang, C., Deep-tissue focal fluorescence imaging with digitally time-reversed ultrasound-encoded light. Nature Communications 3: 928, (2012).
  5. Judkewitz, B., Wang, Y.M., Horstmeyer, R., Mathy, A. and Yang, C., Speckle-scale focusing in the diffusive regime with time reversal of variance-encoded light (TROVE). Nature Photonics 7: 300-305, (2013).
  6. Ruan, H., Xu, J. & Yang, C. Optical information transmission through complex scattering media with optical-channel-based intensity streaming. Nature Communications 12: 2411, (2021).