The problem of trying to see through optically scattering media is a one we commonly encounter in everyday scenarios such as driving through the fog. This problem is also very challenging in the field of biomedical optics where the strong scattering nature of biological tissue makes it hard to see beneath superficial layers. The goal of my research is to develop new techniques and tools to enable us to see deep through and inside scattering media, with a particular focus on applications in biomedicine.
Have you ever stopped to think about why our bodies appear opaque? You may suspect that light is strongly absorbed by the tissue of our bodies, preventing light from passing through. However, if you put a flashlight up to your palm and look at the back of your hand, you'll notice a reddish, diffuse glow which suggests that the light is not totally absorbed. This is because the characteristic length that a photon in the visible range (~500-800 nm) will travel before being absorbed by biological tissue is on the order of centimeters. So if absorption is not the problem, what is going on here?
While the absorption of light is not the dominant factor limiting our ability to see through biological tissue, the scattering of light by tissue is much stronger and leads to the opaque appearance of tissue. Elastic optical scattering, while a bit less intuitive than absorption, is due to the heterogenous refractive indices of the components that make up the medium. This means that light, which normally propogates in a straight line in free space or nearly homogeneous media like air and water, is forced to deviate from its straight trajectory. The result is that we can't clearly see through the human body or fog with visible light, since the cells and structure of tissue in our body and water droplets in fog distort the light by scattering it.
While we can still vaguely see through a thin slice of human tissue under a microscope where the light is not scattered too strongly, once the light propagates even 1 millimeter in typical biological tissue it is so strongly scattered that it loses any memory of its input direction the medium looks completely opaque. At this point, it would seem that we are up the proverbial creek without a paddle. If scattering distorts the light trajectories so that the individual photons don't follow the expected trajectories, conventional optical imaging has reached its limit. However, is there anything we can do to retrieve the information from the scattered photons?
In fact, there is a way for us to recover the information encoded in scattered wavefront. Since the scattering of light is due to its interaction with individual particles in the tisssue, this process is deterministic and therefore while complicated, is not random. To recover the information in the scattered wave, we can exploit the physics of the way the light propagates, in particular the time-reversible symmetry of the wave equation.
In essence, we can understand the time-reversal symmetry of scattering in the following way. If we send a collimated plane wave into a scattering media, as it encounters the refractive index inhomogeneity of the medium, the wavefront will scatter and become distorted. This is illustrated by Figure 1 below (light traveling from left to right).
Fig. 1: Forward Wavefront Propagation
However, now imagine we have a way to record the propagation and scattering of the light. Then, if we played the recording backwards, the light would look as if it was traveling back through the scattering media and scattering in a way which re-forms the collimated plane wave on the left hand side of the scattering media. This property of the propagation of waves is called time-symmetry and means that the solution to the wave equation is valid in either the forward propagating direction (forward in time) or the backward propagating direction (backward in time). How can we use this property to reverse the effects of scattering?
One approach to exploit this time-symmetric property of light propagation is called optical phase conjugation. The basic idea behind phase conjugation is that if we can generate a copy of the scattered wavefront and send it in the reverse direction, this will satisfy the time-symmetric property of the light propagation and allow us to send light back through the scattering media. This is illustrated by Figure 2 below.
Fig. 2: Time-Reversed Wavefront Propagation
Using optical phase conjugation, we now have an engine with which to undo optical scattering with potential applications in a wide array of fields, from biomedical optics where we want to see deep into tissue with light to atmospheric imaging through fog or other optically scattering media. Futhermore, by coupling the phase conjugation technique with a guidestar to tag light at a certain voxel inside the scattering medium, we can not only focus light through scattering media, but focus it to points inside it. This further opens the door for novel optical techniques in biomedicine and beyond.
Optical scattering is a challenging problem in optics, especially for the life-sciences where light is one of the primary tools used to interrogate biological samples using various microscopy techniques. However, since optical scattering is not a random process, if we can find a way to characterize the scattering behavior of our sample, we can create appropriately shaped wavefronts that effectively undo the effects of scattering. Optical phase conjugation is one way to do this in a high fidelity manner, exploiting the time-symmetric nature of scattering. Future developments of this technology have the potential to lead to new methods of deep tissue imaging, light delivery for non-invasive laser surgery, and photodynamic therapy.
If you have any comments or questions about this topic or my research, I'd be happy to hear from you. Feel free to drop me a line or connect over on the contact page!
Some selected resources where you can learn more about optical scattering and phase conjugation are listed below: