I'd like to discuss photonic crystals,
they can be the ultimate approach to
obtaining a small cavity with high queue.
So what is a photonic crystal?
It's nothing more than a periodic array of
dielectric material that allows for "band structure"
of photons in an analogous way to
the away an electronic band structure is
created by the periodic arrangement
of atoms in the crystal.
You can see here a 1D photonic crystal
essentially is nothing more than a multi-layer film,
that's been used for a long time.
You can also see examples on this slide of
2D and 3D photonic crystals that
are cases of generalized multilayer films.
So what happens is that
this periodic dielectric constant modulate
and scatters the light in
a similar way to the periodic potential
and the way that affects electrons and holes.
So we can think about some direct analogies
between the two cases.
In the photonic crystal,
the photonic bands act just as
electronic bands created in crystals.
They create photonic band gaps where
light propagation and generation of light is prohibited,
we can define a bullion zone and we can also
dope photonic crystals by adding an impurity or a defect.
If we're in the band gap,
we create an allowed mode in
the band gap and the mode can be
localized at the defect due to
the strong attenuation in the photonic crystal.
So the linear defect can act
essentially as a very tightly confined structure,
can find not only by
the photonic crystal but also by the waveguide itself.
So before continuing, I think it's
important to review the definition of a photonic crystal.
It's got the periodic refractive index,
we get bloch waves and we get
a strongly modified dispersion surface.
We can use this to generate
novel optical properties including a photonic band gap,
waveguides and cavities, a super-prism,
self-collimation, and negative refraction.
So I'd now like to spend some time
discussing the photonic crystal micro cavity.
We can create a point defect that acts like
a micro cavity with high Q and small mode volume.
So essentially, what we can do is we can make
a waveguide where we essentially remove one of the holes,
so we have a defect in the middle,
and you can see some work from MIT showing this.
So this is a silicon waveguide
and you can see the defect in
the middle and you can also
see your cross-section of the structure.
So essentially, get confinement
by the photonic crystal
and then also through the waveguide.
So here's a scan showing you
the transmission versus wavelength and what
you can see is that these structures are designed to have
a band gap from 1350-1750,
but here you see this resonance,
and that's actually from
the defect, what I was showing you.
So we can extend this concept to two-dimensions,
and so essentially we can have a two-dimensional array of
holes where one is missing and create a cavity.
We can create 2D confinement by
the band gap and vertical confinement
by the index profile,
and then use this to enhance the Purcell effect.
So here's a figure
from a science paper showing you a 2D photonic crystal,
and here's the zoomed in view showing you the defect.
Here are some results showing you
essentially the power versus wavelength and
this sharp tip on the right is due to wavelength.
So the Purcell effect can create
strong enhancement of spontaneous emission
in a high Q to V cavity.
It can be used to generate the thresholdless laser,
as well as an ultrafast laser.
Here's another example of
a 2D photonic crystal cavity used for lasing
and here you can see the array of
the 2D holes and here you can see again,
this sharp peak on the right
is essentially the lasing transition.
Here, what you're looking at is you're looking
at the intensity versus
wavelength and here you can see that
this laser has an ultralow threshold.
We can also extend the technique to
three dimensions and essentially
here we'll have full 3D confinement,
but the fabrication becomes very challenging.