104
P. Vasa
imations made in deriving the probability dynamics discussed above breakdown
[8, 9, 24].
As we have seen, a resonator is essential to observe coherent light-matter interactions involving vacuum field [5, 6]. All of the light-matter interactions discussed
in this Chapter are observed in optical resonators having characteristic length scales
comparable to or shorter than the light wavelength. Reflection and interference of
light play an important role in designing these resonators. As such a pair of reflecting
mirrors is sufficient to construct an optical resonator or a Fabry-Perot cavity, which
is the simplest resonator. However, the performance or the Q-factor is significantly
improved when instead of a single pair of mirrors, multi-layers of several pairs of
alternating transparent (dielectric) materials with different refractive indices are used
as in Bragg reflector. Using modern fabrication technology Q-factors as high as 10
6
have been demonstrated for Bragg reflectors [6–9, 14]. Designing and fabricating
optical resonators involve ability to manipulate optical fields and light propagation,
which is central to optics and photonics. Traditionally, the flow of electromagnetic
energy is controlled using elements such as mirrors, lenses, fibres and diffractive
elements. These components are diffraction limited posing a significant challenge to
highest attainable spatial resolution, miniaturization and high-density integration of
optical devices. Recently, more complex structures like photonic crystals and metamaterials have been developed to achieve optimal control with sub-wavelength precision. Photonic crystal is a type of Bragg resonator, in which multiple interferences
from periodic interfaces confine light in one-, two as well as three-dimensions. Nevertheless, these resonators at best can be on the scale of the wavelength. It is highly
desirable to achieve manipulation of optical fields and control of light-mater interactions with ∼ 10 nm or even higher precision, as it holds promise for enhancing the
performance and efficiency of several photonic functionalities like photo-detection,
-emission and chemical sensing [6, 7]. The main approach to circumvent diffraction
limit over a broad range of wavelengths is to exploit surface plasmon polaritons
(SPPs) in sub-wavelength metal structures. SPPs are hybrid modes of light waves
coupled to free electron oscillations in a metal that can be laterally confined below
the diffraction limit [29–32]. The spatial extent of the SPP fields is dictated by the
geometry of the metallic nanostructures rather than by the wavelength of the light.
The unique optical properties of metals allow field localization on the scale of the skin
depth of the metal, which remains sub-micron throughout the broad spectral range.
By choosing appropriate geometries, e.g. a gap waveguide, metallic nanowires or
chains of metallic nanoparticles as novel SPP waveguides that can transport light on
the nanoscale, the ultimate achievable localization is typically below ∼ 1 nm, limited
by the charge-screening length or the Thomas-Fermi screening length of metals [8,
9, 14, 29–32]. Apart from possible manipulation of light, focusing with ∼ 10 nm or
even higher resolution could result in a strong field enhancement, enabling efficient
manipulation of light-matter interaction and boost optical nonlinearities. For example, plasmonic substrates are vital for the enormous signal enhancement achieved
in surface enhanced Raman spectroscopy (SERS)-a technique that can detect a single molecule [33, 34], for bio-sensing and in magneto-optical effects like inverse
Faraday effect [8, 14, 30].
P. Vasa
imations made in deriving the probability dynamics discussed above breakdown
[8, 9, 24].
As we have seen, a resonator is essential to observe coherent light-matter interactions involving vacuum field [5, 6]. All of the light-matter interactions discussed
in this Chapter are observed in optical resonators having characteristic length scales
comparable to or shorter than the light wavelength. Reflection and interference of
light play an important role in designing these resonators. As such a pair of reflecting
mirrors is sufficient to construct an optical resonator or a Fabry-Perot cavity, which
is the simplest resonator. However, the performance or the Q-factor is significantly
improved when instead of a single pair of mirrors, multi-layers of several pairs of
alternating transparent (dielectric) materials with different refractive indices are used
as in Bragg reflector. Using modern fabrication technology Q-factors as high as 10
6
have been demonstrated for Bragg reflectors [6–9, 14]. Designing and fabricating
optical resonators involve ability to manipulate optical fields and light propagation,
which is central to optics and photonics. Traditionally, the flow of electromagnetic
energy is controlled using elements such as mirrors, lenses, fibres and diffractive
elements. These components are diffraction limited posing a significant challenge to
highest attainable spatial resolution, miniaturization and high-density integration of
optical devices. Recently, more complex structures like photonic crystals and metamaterials have been developed to achieve optimal control with sub-wavelength precision. Photonic crystal is a type of Bragg resonator, in which multiple interferences
from periodic interfaces confine light in one-, two as well as three-dimensions. Nevertheless, these resonators at best can be on the scale of the wavelength. It is highly
desirable to achieve manipulation of optical fields and control of light-mater interactions with ∼ 10 nm or even higher precision, as it holds promise for enhancing the
performance and efficiency of several photonic functionalities like photo-detection,
-emission and chemical sensing [6, 7]. The main approach to circumvent diffraction
limit over a broad range of wavelengths is to exploit surface plasmon polaritons
(SPPs) in sub-wavelength metal structures. SPPs are hybrid modes of light waves
coupled to free electron oscillations in a metal that can be laterally confined below
the diffraction limit [29–32]. The spatial extent of the SPP fields is dictated by the
geometry of the metallic nanostructures rather than by the wavelength of the light.
The unique optical properties of metals allow field localization on the scale of the skin
depth of the metal, which remains sub-micron throughout the broad spectral range.
By choosing appropriate geometries, e.g. a gap waveguide, metallic nanowires or
chains of metallic nanoparticles as novel SPP waveguides that can transport light on
the nanoscale, the ultimate achievable localization is typically below ∼ 1 nm, limited
by the charge-screening length or the Thomas-Fermi screening length of metals [8,
9, 14, 29–32]. Apart from possible manipulation of light, focusing with ∼ 10 nm or
even higher resolution could result in a strong field enhancement, enabling efficient
manipulation of light-matter interaction and boost optical nonlinearities. For example, plasmonic substrates are vital for the enormous signal enhancement achieved
in surface enhanced Raman spectroscopy (SERS)-a technique that can detect a single molecule [33, 34], for bio-sensing and in magneto-optical effects like inverse
Faraday effect [8, 14, 30].