# Presentation: 180487

## Speckle Evolution of Diffusive and Localized Waves  Azriel Z. Genack, Queens College of CUNY, DMR 0538350 

**Speckle Evolution of Diffusive and Localized Waves**** ** **Azriel Z. Genack, Queens College of CUNY, DMR 0538350**** **

- The addition of randomly scattered waves gives rise to a speckled intensity pattern for any type of wave, including light, sound and electrons. The evolution of such patterns as the incident frequency is scanned is observed for microwave radiation. Though random, the patterns observed have an intricate structure. A network of phase singularities appears about points of vanishing intensity. We show that the motion of these points of darkness provides a diagnostic of the nature of wave transport within the scattering object.
- For waves strongly localized within the sample, the speckle pattern develops in fits and starts as opposed to the smooth development for spatially extended waves. This work provides measures of speckle evolution which can be exploited to monitor motion of hidden objects and the development of structural defects.

- Physical Review Letters **99**, 063902 (2007).

- Patterns of intensity and phase produced by microwave radiation transmitted through a strongly scattering sample filling a metal tube. The resulting speckle pattern of intensity (left) and phase (right) are magnified in the lower panel. The motion of points of undetermined phase, seen as the center of a pinwheel, give a clear diagnostic of the nature of waves inside the sample.

**Notes:**
The nature of wave propagation in disordered samples reflects the spatial extent of the wave within the medium. We have therefore measured the variation of the microwave field with space and frequency inside an open absorbing waveguide filled with randomly juxtaposed dielectric slabs. Studies were made in the range of frequencies in which the average frequency spacing between modes of electromagnetic oscillation exceeds the typical frequency width over which these modes can be excited. Whenever lines overlap in frequency, we find multiple peaks in the spatial variation of the field. Only then is substantial energy transferred beyond the first half of the sample. When the frequency variation of the field throughout the sample is decomposed into a sum of electromagnetic modes their central frequencies and widths are found to be essentially independent of position. This confirms the mode decomposition which allows us to find the actual shape of the modes underlying the field distribution. When several modes overlap we find the fundamental modes have a number of peaks in space essentially equal to the number of overlapping modes. 

## Speckle Evolution of Diffusive and Localized Waves  Azriel Z. Genack, Queens College of CUNY, DMR 0538350 

- Speckle techniques being developed via microwave and optical measurements may find applications to surveying geological formations and ice caps, as well as inspecting aerospace components. Transmitted optical speckle patterns may also find application to locating tumors. Measurements of speckle though glass stacks may point the way to probing naturally occurring layered media such as the earth’s crust.

**Education and outreach:**
- Three high school students who participated in research on wave propagation in the summer of 2006 plan to continue in physics or engineering. Elizabeth Shechner was a semi-finalist in the Intel Science program and will go on to Harvard, Gennady Rozenberg will go on to the Columbia School of Engineering, and Yitzchak Locerman, published his first scinetific paper as a Freshman at Queens College. Two graduate students, Jing Wang and Jongchul Park are about to defend their Ph.D. proposal this September. One postdoctoral fellow, Sheng Zhang has made a transition from quantum computimg to experimetnal physics.

- Speckle Evolution of Diffusive and Localized Waves  **Azriel Z. Genack, Queens College of CUNY, DMR 0538350**** **