Coherent Backscattering of Light

by Ming Lu


Department of Physics and Astronomy, SUNY at Stony Brook

Advisors: Dr.John Noe/Prof. Hal Metcalf



OUTLINE




Introduction

  1. What is Coherent Backscattering?
    When coherent light (Laser) is shone at a suspension of small (micrometre sized) spheres in the solution, light is ENHANCED SCATTERED directly anti-parallel to the incident light, instead of being scattered EQUALLY in all directions as would be expected.
  2. Theory
    • The Width of CBS Cone is inversed proportional to the mean free path theta=wavelength/(1.414xL)
    • Peak height of CBS signal is twice of the scattering background.
Back

Experiments

Estimate of the power of signal:

  • Light comes from laser @ 7mW
  • After passing the Beam splitter twice: left 25%
  • Scattering: 3.14x(0.25x0.25mm2)/(43.2x43.2inch2)/(4*3.14)=1.4e-8
  • Reflection from the wall of cell, the plate and the polarizer: left~80%
TOTALLY LEFT:2.8e-9X7mW=20pW
Only PMT can detect it!

Setup

Setup

  • Laser is a He-Ne linear polarized laser with 0.79 mrad divergence.
  • Quarter wave plate converts the incident linear polarized light to circularly polarized light, converts the scattered circularly polarized light back to linear polarized light, which makes the single scattered light (which has a pi phase shift) unable to pass the polarizer.
  • Interference filter (FWHM of pass band = 1nm) blocks the noise light not coming from laser.
Back

Results and Discussion

Horizontal Direction

Vertical Direction

Automatically Scanned CBS Curve(Horizonal)

The Blue one is the "real" CBS signal, while the Green one is the background and the Red one is the originally measured CBS curve. Back

Conclusions

  1. We observed CBS phenomena although the fluctuation are considerable!
  2. The pseudo peaks appeared in the curves above came from the all kinds of reflection, Which can be removed by subtracting the background.
  3. Suggested work to improve the measurement include:
    • Use anti-reflection Beam Splitter, Quarter-wave Plate to reduce the reflection.
    • Use Lock-in Amplifier and Chopper to get rid of the noise.
  4. Suggested work to clear some question include:
    • Turn the cell to measure intensity cell-position-dependece of CBS, to check whether some of the peaks come from back wall reflection.
    • ...

Back



Acknowledgements

I am very appreciative of Dr. John Noe for his comprhensive instruction and help, both in physics and in technique. In his lab, I enjoy a lot from physics.