Artificial atoms is the name we give to certain manmade nanostructures that emulate the behaviour
of real atoms but give us the flexibility to design them for specific purposes. Real
atoms are the fundamental building block of all the objects around us and in the most
simple version that many people may have in their mind, they're constituted of a positive
charge in the nucleus, surrounded by negative charges which are the electrons that orbit
around the nucleus. A bit like the moon around the Earth. Now this is in fact a wrong picture,
that's not the way atoms work because of quantum mechanics, because particles are also waves
and so the electrons that orbit around the nucleus in fact, do not orbit like the moon
around the Earth, but they have to orbit in some very well defined trajectories to satisfy
the requirement that the wave associated with the electron fits into the certain space that's
allowed to it. So to these electrons orbiting around the nucleus is associated a wave and
you need to ensure that there is an integer number of wavelengths of half wavelengths
in the trajectory of the electron.
A perhaps better way to visualise that is to think of the nucleus of the atom, the positive
charge of the atom, as creating an attractive potential, a binding potential for the negative
charge of the electrons. And then inside of this potential energy, the electron can live
but again, because it's also a wave, it also has to satisfy the condition that it needs
to fit in the space that's available to it. So what we find is that the electron can only
have a very well defined set of energy levels that is determined by the wavelength associated
with the mass of the electron. This is a little bit like what happens in musical instruments.
If you take the strings of a guitar, the string is attached at two ends and when you pick
the string, you don't get just any random noise, you get a note and that note contains
only a certain frequency because there is a fundamental mode of vibration of the guitar
string or it can contain some higher harmonics. That's when the string does an entire wavelength
within the anchoring points and so on.
So the same thing happens here with the electrons bound to the atom. Now, the interesting thing
is to wonder what is it that happens when an electron changes from one stable state
to another? So when it changes from having half a wavelength to having a full wavelength
between the atom. What has to happen is that energy is given to it. And then the energy
can be given back in the form of a quantum of energy or light of electromagnetic field.
This is very commonly used in lighting, literally. Some of you might have seen those beautiful
soft yellow lights that you see on street illumination or parks or parking lots. Those
are sodium lamps. Sodium is one of the elements of the periodic table is an atom and has some
specific energy levels such that when you excite a vapour of sodium atoms, then this
extra energy you provided to the sodium atom is given away in the form of a quantum of
light that is yellow. This is very nice, but what if you want to make a different colour?
Well sodium will only give you yellow light. What if you want red or blue or green? Here
is where modern technology comes to help us. We are now able to produce very small structures
that confine electrons in such a way that the electrons will have some very well defined
energies inside of these artificial atoms, that's why we call them that way, they behave
like atoms but their energy levels are designed by us as engineers and scientists. And so
we can make these artificial atoms to make green light, blue light, red light, whatever
we like. And in fact, if you go and buy the latest model flat screen TV you can find in
the shop, it will contain quantum dot technology. Quantum dot is another name that is given
to these artificial atoms. These man made structures have the ability to create light
with a very well defined colour that can be used for example in flat screen TVs.
If you have a confining potential like this, like a parabola, then all these energy levels
are equispaced but the lowest energy level in fact, does not have zero energy. It's not
just at the bottom of the parabola. This residual energy is called a zero point energy and is
again a purely quantum phenomenon that has a plethora of very interesting applications
but we're going to talk about that another time.