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EM scattering from semiconducting nanowires and nanocones

EM scattering from semiconducting nanowires and nanocones. Vadim Karagodsky. Enhanced Raman scattering from individual semiconductor nanocones and nanowires, L. Cao et al. and J. E. Spanier, Physical Review Letters, 96, 157402 (2006)

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EM scattering from semiconducting nanowires and nanocones

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  1. EM scattering from semiconducting nanowires and nanocones Vadim Karagodsky • Enhanced Raman scattering from individual semiconductor nanocones • and nanowires, L. Cao et al. and J. E. Spanier, Physical Review Letters, 96, • 157402 (2006) • On the Raman scattering from semiconducting nanowires, L. Cao, et al. and • J. E. Spanier, Journal of Raman Spectroscopy, 38, 697-703 (2007) • Electromagnetic scattering from long nanowires, M. E. Pellen et al. and P. C. • Eklund, Antennas and Propagation International Symposium, 2007 IEEE.

  2. Motivation • Similarly to surface plasmon resonance in metallic particles and films, semiconducting nanowires are also demonstrated to provide intense resonant enhancement of visible EM light, and to be excellent scatterers. • The key factor is: subwavelength dimensions. Applications • Sensors and detectors • Couplers • Nano-antenna arrays

  3. Backscattering experiment Si nanocones / Si nanowires / c-Si(100) wafer (bulk) Laser polarization: TM and TE “Nano”-wires (too large): 130nm < diameter < ~1m “Nano”-cones: <5nm  = 0.12rad ~25m

  4. Backscattered intensity 632.8 nm (diameter~250nm) 5 times larger than bulk (near the base) twice as large as bulk

  5. Backscattering enhancement – 632.8 nm Raman Enhancement (RE) = [Inw/Vnw]/[Ibulk/Vbulk] I = scattered intensity; V = probed volume • RE=250~300 at the • nanocone tip. • RE~800 for the 130nm • nanowire. • Good agreement between • nanowires and nanocones. • Small but reproducible • differences between TM • and TE

  6. Backscattering experiment - wavelength dependence • The RE increases • with wavelength. • Qualitative reason: • The enhancement • is controlled by the • ratio: • diameter/wavelength

  7. Theoretical Model Plane wave / infinite cylinder E-field inside the cylinder Definition of average intensity Avg. intensity inside the cylinder

  8. RE as a Quality factor - comparison with experiment Qint=Iint,nw/Ibulk Qscat~Qint RE=QRaman~QintQscat~Qint2 • Reasonable agreement between • theory and experiment • The calculated values are • consistently lower. • The undulations are not • observed. Suggested reason: • Period of undulations: ~70nm • Diameter variation across the • laser spot: ~170nm.

  9. Theoretical Model - calculation results • Normalized units reveal • wavelength insensitivity • for small diameters • The nanowire can be designed • for TM/TE mode selectivity

  10. FDTD simulation - GaP nanowire (polarization dependence) E-field – TM: E-Field – TE:

  11. Conclusions • The Raman enhancement depends on the • diameter, wavelength and polarization. • For small diameters the enhancement over bulk is • up to 3 orders of magnitude, due to resonant • scattering. • Reasonable agreement between theory and • experiment. • The efficient radiation coupling to Si is good for • photonic and sensing properties of Si and Si-based • nanostructures.

  12. Suggestions for improvement: • Measure the entire scattered spectrum – the • enhancement is not necessarily Raman related. • Normalize by scattering cross-section instead of • probed volume. • Revise the Q-factor model for the intensity • enhancement. Thank you

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