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, (2006)  On the Raman scattering from semiconducting nanowires, L. Cao, et al. and J. E. Spanier, Journal of Raman Spectroscopy, 38, (2007)  Electromagnetic scattering from long nanowires, M. E. Pellen et al. and P. C. Eklund, Antennas and Propagation International Symposium, 2007 IEEE.

 Sensors and detectors  Couplers  Nano-antenna arrays  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 Motivation

Backscattering experiment LaserAr + HeNeDiode Wavelength (nm) Power (mW) Gaussian width (  m) ~1.0~1.2~1.5 ~25  m <5nm  = 0.12rad Laser polarization: TM and TE “Nano”-wires (too large): 130nm < diameter < ~1  m “Nano”-cones: Si nanocones / Si nanowires / c-Si(100) wafer (bulk)

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

Backscattering enhancement – nm  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 Raman Enhancement (RE) = [I nw /V nw ]/[I bulk /V bulk ] I = scattered intensity; V = probed volume

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

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

RE as a Quality factor - comparison with experiment Q int =I int,nw /I bulk Q scat ~Q int RE=Q Raman ~Q int Q scat ~Q int 2  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.

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

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

 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. Conclusions

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