Page 204 - Principles and Applications of NanoMEMS Physics

Chapter 5

NANOMEMS APPLICATIONS: PHOTONICS

5.1 Introduction

The ability to fabricate nanometer-scale structures has given new impetus
to the field of miniaturization of optical devices, whose ultimate goal might
be articulated as that of integrating optics and electronics in the context of a
monolithic technology. While there are no fundamental limits to the
miniaturization of electronic functions down to nano- and sub-nanometer
scales, the minimum size of devices manipulating optical signals is limited
by diffraction to about half the wavelength (λ 2 n ) [210], which in practical
terms encompasses dimensions in the several hundreds of nanometer [211].
Two approaches have been devised to overcome these limitations, namely,
the design of optical elements based on very high refractive index materials
[212], which is accompanied by high losses in the sub-30 nm size regime
[213], and the conversion of photons into electromagnetic modes whose size
is determined by the size of the waveguide rather than by the wavelength of
the optical field [214]. The latter approach is based on surface plasmons
(SPs), collective oscillations of free electrons resulting from the interaction
of electromagnetic waves with free electrons at a dielectric-metal interface
[215]. In particular, Dickson and Lyon [212] point out that, by employing
SPs to transport light, the minimum waveguide size becomes only limited by
a combination of the Thomas-Fermi screening length, which is ~0.1 nm in
Au, and size effects affecting the dielectric constant, which have an onset at
dimensions less than 5 nm in Au. While we will focus on SP-based
approaches, a third approach to sub-wavelength photonic circuit elements,