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.

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

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

~25m

<5nm = 0.12rad

Laser polarization: TM and TE

“Nano”-wires (too large):130nm < diameter <

~1m“Nano”-cones:

Si nanocones / Si nanowires / c-Si(100) wafer (bulk)

Backscattered intensity 632.8 nm

(near the base)twice as large

as bulk

(diameter~250nm)5 times larger

than bulk

Backscattering enhancement – 632.8 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) = [Inw/Vnw]/[Ibulk/Vbulk]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

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.

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