MONDAY MORNING
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and Related Phenomena, Vol. 9 of Topics in Applied Physics, 2nd ed., J.C. Dainty, ed., (Springer Verlag, Berlin, 1984). 2. C.A. Thompson, K.J. Webb, and A.M. Weiner, “Diffusive media characterization using laser speckle,” Appl. Opt. 36, 3726-3734 (1997). 3. B. Chance, K. Kang, and E. Sevick, “Photon diffusion in breast and brain: spectroscopy and imaging,” Optics and Photonics News 4,9-13 (1993).
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CMJ6 Fig. 1. Experimental Setup.
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CMJ6 Fig. 2. Experimental contrast data for 9 mm and 18 mm thick samples and associated theoretical Contrast values calculated with ps = 7.5 cm-’.
spectrum over a range of many GHz. As scattering media we used commercially available white acrylics with scattering due to -50 nm TiO, particles suspended in the acrylic background. The laser output was focused onto the front face of our plastic samples, and the resulting speckle pattern from the back face was collected on a cooled CCD camera. The laser frequencywas modulated using a 200 Hz voltage ramp, which was much faster than the integration time of the CCD array. Therefore, the collected speckle data resulted from the time integration over multiple frequency scan periods, and the effective frequency spectrum was the time-integrated spectrum over the modulation period. This allowed us to synthesize various spectral linewidths by varying the amplitude of the voltage drive to the PZT. These effective spectra, measured with a Fabry-Perot etalon, had an approximately
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CMJ6 Fig. 3. Experimental contrast ratio data for 9 mm thick samples of two different white acrylics.
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rectangular lineshape determined by the use of a ramp modulation waveform. We collected speckle data for 16 different modulation amplitudes, resulting in 16 different effective linewidths, for several plastic types and thicknesses. From each speckle pattern, the contrast ratio, defined as the ratio of the intensity standard deviation to the mean intensity, was calculated. Theoretical values for the contrast were also calculated by evaluation of a theoretical expression from,* which depends on the laser power spectrum S(X) and the photon path-length probability distribution p(l) for the output point. We obtainedp(l) from the solution of the photon diffusion equation for a homogeneous medium with scattering coefficient ps, where ps is adjusted to fit the data. Figure 2 shows experimental contrast ratio data for 9 mm and 18 mm thicknesses of a white acrylic along with the associated theoretical contrast values. The shape ofthe experimental data follows that of the theory quite well. The contrast falls off more quickly for the thicker sample due to increased scatter, which leads to a larger variance in photon travel times. To fit our data, we used p = 7.5 cm-’ instead of p,s = 40 cm-’ in earlier work* performed at the HeNe wavelength. This difference may be explained in part by the wavelength difference (632.8 nm vs. 850 nm) and the h4 dependence expected for Rayleigh scattering. Note also that we easily perform measurements up to linewidths of 12 GHz - a frequency range that is an order of magnitude higher than has been demonstrated with photon diffusion instruments. Figure 3 shows experimental data for two plastics, with different ps, both of 9 mm thickness. Similar to Fig. 2, the contrast falls off more quicklywith increasing linewidth in the case of heavier scatter (ps for the less scattering case is approximately 1/4 (LS for the more scattering plastic). Again, this data agrees with the theory quite well. In conclusion, we have demonstrated characterization of scattering parameters in thick, optically scattering media via speckle contrast measurements performed using variable coherence length light. This technique to synthesize laser linewidths by using a frequency modulated laser gives the flexibility to tailor the source coherence properties to best match different materials possessing different scattering properties. 1. J.W. Goodman, “Statistical properties of laser speckle patterns,” in Laser Speckle
JMB
10:30 am-12:30 pm Rooms 327/329
Quantum Dots and Wires Yasuhiko Arakawa, University of Tokyo, Japan, Presider
JMBl (Invited)
10:30 am
Optlcal spectroscopy of single semlconductor quantum dots Erez Dekel, David Gershoni, Eitan Ehrenfreund, Pierre M. Petroff,* Physics Department, Technion-Israel Institute of Technology, Physics Department, Technion, Haifa, 32000 Israel; E-mail:
[email protected] Low temperature confocal optical microscopy is used to spectroscopically study emission from a single semiconductor quantum dot. The spectrally sharp transitions between discrete confined multiexcitonic states are quantitatively explained using a few interacting carrier Hamiltonian *Materials Department, University of California, Santa Barbara, California 93106 USA; Email:
[email protected]
JMB2
11:OO am
Spectroscopy of self-assembled quantum dots in ZnSe Hailong Zhou, A.V. Nurmikko, M. Kobayashi? A. Yoshikawa,* Division of Engineering and Department of Physics, Brown University, Providence, Rhode Island 02912 USA The growth and study of self-assembled quantum dots in semiconductors commonly occurs in circumstances where a large lattice mismatch between the constituent materials drives the layered system to form a high density of quantum dots (QD) along a well defined plane (e.g. in the InAs/GaAs system). The case of smaller lattice mismatch is also of contemporary interest since the QD formation in this instance proceeds via a different energetic pathway, leading to a potentially wide range in the control of the QD size, shape, and density. Such “tuning” of quantum dots in wide bandgap semiconductors, in turn, is of potential interest in designing submicron scale high efficiency green and blue light emitters. We report here results on spectroscopic inves-
