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METHOD:PUBLISH
BEGIN:VEVENT
DTSTAMP:20260826T152634Z
LAST-MODIFIED:20140806T163038Z
DTSTART:20140807T140000Z
DTEND:20140807T150000Z
UID:event1305@bu.edu
URL:http://physics.bu.edu/internal/events/show/1305
SUMMARY:Imaging and Scattering in Porous Media
DESCRIPTION:Featuring William Shain\n\nPart of the Preliminary Oral Exam.\n
	\nExamining Committee:\nBennett Goldberg\, Thomas Bifano\, Jerome Mertz\, S
	teve Ahlen\, Pankaj Mehta\n\nAbstract:\nPorous media occur throughout natur
	e. In biology\, one can measure brain activity through the skull by imaging
	 molecules tagged with fluorescent markers. In ceramics\, one can map the p
	ore structure of a ceramic material by imaging the local reflectance. In th
	e oil industry\, one can characterize oil flow in rock by mixing in fluores
	cent beads and tracking them. Because they are porous\, these materials are
	 all strongly scattering media: imaging more than a few pore lengths into t
	he material is frequently difficult\, if not impossible.\nThe primary means
	 of constructing an image is by scanning a focus across the imaging area an
	d capturing the reflected or re-emitted light. For each of these applicatio
	ns\, the goal is to produce a high-contrast\, aberration-free image; freque
	ntly the interesting features are at a depth far greater than the character
	istic pore size. In any porous medium\, the pores frequently have a differe
	nt index of refraction than the surrounding solid material. The index misma
	tch between the solid medium and the pores scatters and reflects the incide
	nt light\, destroying the focus used for imaging. The thicker the material 
	and the stronger the scattering\, the more the focus spreads\, blurring the
	 image. For every material there is a certain thickness (L*) where the focu
	s disappears entirely\, and the phase beyond is randomized. This distance L
	*\, called the transport mean free path\, is considered the maximum imaging
	 depth for traditional (ballistic) optics. The main challenge of our resear
	ch is imaging beyond L*.\nIn my presentation\, I will discuss the theoretic
	al tools used to describe scattering effects; specifically\, the use of bot
	h particle and wave models of light propagation. By considering the particl
	e aspect of light\, one arrives at a diffusive model of photons undergoing 
	multiple scattering from distinct\, separated pores. The statistics of the 
	pore distribution provide predictive power\, where one can calculate low or
	der moments of the intensity distribution at an imaging plane. By consideri
	ng the wave nature of light\, one considers a transmission matrix with rand
	om coefficients. The transmission matrix relates the incident and transmitt
	ed electric fields\, decomposed into 2D spatial modes. This provides a fram
	ework for understanding various techniques for imaging through highly scatt
	ering media.\nI will also discuss one specific technique to imaging beyond 
	the transport mean free path. This method is an iterative procedure called 
	coherent optimization. By using a spatial light modulator\, one can adjust 
	the input field to direct most of the power into a single output mode. This
	 creates an effective focal spot\, allowing for imaging deep into the scatt
	ering medium.\nLastly\, I will discuss the different ways we characterize t
	he optical properties of the porous media: the maximum imaging depth and th
	e maximum intensity enhancement of a particular spatial mode.\n\n![Shain](/
	resources/event-image/1305/8c9d5e6_small)
LOCATION:SCI 352\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
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