EM scattering from semiconducting

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 lt diameter lt 1?m
Nano-cones
lt5nm
? 0.12rad
25?m
Laser Ar HeNe Diode
Wavelength (nm) 514.5 632.8 785
Power (mW) 0.3 0.8 0.08
Gaussian width (?m) 1.0 1.2 1.5
4
Backscattered intensity 632.8 nm
(near the base) twice as large as bulk
(diameter250nm) 5 times larger than bulk
5
Backscattering enhancement 632.8 nm
Raman Enhancement (RE) Inw/Vnw/Ibulk/Vbulk
I scattered intensity V probed volume

• RE250300 at the
• nanocone tip.
• RE800 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
QintIint,nw/Ibulk
QscatQint
REQRamanQintQscatQint2

• 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

• The nanowire can be designed
• for TM/TE mode selectivity

• Normalized units reveal
• wavelength insensitivity
• for small diameters

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