Hello everybody,
Today I want to talk about my attempts to make logic gates based on optical interference. So, optical interference is probably best
known from when you place two semi-reflective surfaces close to each other and you observe
these bright and dark fringes under monochromatic lighting conditions. And these fringes are caused by variations
in the distance between the two surfaces, resulting in areas of constructive and destructive
interference of light. Now, a common explanation given for destructive
interference is that the light reflected of the two surfaces is sort of out of phase and
the electromagnetic waves sort of cancel each other out. But in terms of conservation of energy, this
explanation does not make sense at all. Because where would the electromagnetic energy
contained in the dark fringes go to? So, in my opinion, the term destructive interference
is kind of misleading, even though I myself use it all the time, for lack of a better
word. It suggests that the electromagnetic radiation
is somehow destroyed and that is definitely not the case. So where does the energy go when we observe
destructive interference? Well, the answer is simple: It is going somewhere
else. For example, in the case of the semi-reflective
surfaces, more light is transmitted in the areas that display destructive interference
in reflection. And in a more general sense, we can state
that electromagnetic radiation such as light cannot just disappear between its emission
and absorption. I mean, a lot can happen in between: the radiation
can be reflected, refracted, diffracted. But as long as it is not absorbed, the total
amount of energy in the electro-magnetic wave remains unaltered. And, whereas the processes involved in emission
and absorption always involve quantization, the transportation mechanism for the energy
between these two events always behaves as a pure and elastic wave, without any noticeable
quantization in space and time. Anyway, this is not the path I want to go
down on, at least not today. Because in this video, I want to discuss something
of a more practical nature, which is a way to make logic devices based on optical interference. But let us first take a look at what logic
devices, also called “logic gates” are exactly. Logic gates find wide-spread application in
modern electronics as all sort of logical switches. Here is an overview of some very common logic
gates. They all have in common that you send logic
values like true or false to the inputs, assigned as A and B in this figure. So, True is shown here as 1 and False as a
0. The gate consists of some hardware that takes
these input values and performs a logic operation on them and then sends the result an output. Each type of logic gate handles the input
values in a somewhat different way. Let's say we have a gate with 2 inputs. With 2 different states for each input, this
means that there are 4 distinct input states possible. So, let's take for example the OR-operation. The only state where the OR-gate results in
the output of the value 0 is when both inputs are 0. But if either of the inputs has a value of
1, or both, the output will be 1. Now, the logic gates used in electronics,
use voltage levels to represent the 0 and the 1, which are either higher or lower than
a specific threshold value. And in the case of optical logic, we can imagine
something similar, where the light intensity at the inputs or at the output are a measure
for the logic value. A purely optical OR-gate would actually not
be very difficult to construct. Basically, we would just lead the light of
both inputs to the output, and then determine if the light level at the output is above
a specific threshold value. If it is, then the output is considered a
1, otherwise it would be a 0. However, this is only true if there is no
destructive interference, because in that case we would get a 0 value result if both
inputs are on. So, if we look up what kind of logic gate
this represents in the overview, we arrive at the Exclusive OR gate. And similarly, we could also come up with
a gate that exploits local constructive interference and only outputs the value 1 if both inputs
are on. And this configuration would be an AND-gate. The idea of using optical interference to
perform logic operations has been on my mind for quite some time. And of course, I did what almost everyone
does when they have an idea: I quickly searched the Internet to see if someone had already
done this. And guess what, people did, but actually not
in the way I had imagined. So I quickly stopped looking, because I did
not want to get too distracted from the original idea. But, if you know of people who have done exactly
the same thing, please leave a comment. Let's go right to the core of concept I had
in mind: As a light source I wanted to use a coherent monochromatic beam such as the
one produced by a diode laser. The beam is divided into two paths A and B,
which can be switched on or off individually. So later in this video I will get back to
how to switch the beams, but let's for now focus on the logic operation performed by
the light. This is done by using two optically active
planes, and for reference I've named them the generator and the evaluator. The generator plane contains the actual diffractive
elements and generates an interferometric pattern. The optical interference pattern is then evaluated
in the second plane, which basically acts as a simple filter and produces the logic
output value. Let's have a look at how this could work in
practice. Say the 2 diffractive elements that serve
as the signal inputs both direct the light to a common focal point. In the case that the light from both elements
arrives in phase at the focal point, we will have constructive interference. So, if we place a pinhole in the evaluation
plane at the focal point, there will be light passing through the pinhole for 3 different
input states. And only if both inputs receive no light at
all, we will have no light at the output or in other words, a 0-value result. So the current configuration represents a
very simple OR-gate. But if we use this same configuration and
shift one of the two beams 180 degrees out of phase with respect to the other beam, we
will create a different kind of logic gate. In that case, light will be passing the evaluator,
when only one of the gate inputs is on. Because, if both inputs are on, interference
results in a zero intensity exactly in the center of the common focal point. And if we look at what kind of the logic scheme
this situation represents, it is equivalent to the exclusive OR gate. Let’s do another example: we could add an
additional light path that is always on, so it is actually not part of the logic evaluation. If we make this particular input such that
it is out of phase with the two switchable inputs, we can create an exclusive NOR gate. So, I guess by now you will probably get the
general idea. And by clever use of the principles of interference,
it should be possible to design every single type of logic gate shown in the overview. Having an idea is one thing, but in reality,
things are always different and more complicated than they are in your head. So, the next step was making and testing some
diffractive patterns. And in previous videos I’ve shown how I
do this, which is by using photolithography as a manufacturing method and a home build
digital wafer stepper as the main tool. And I will not repeat myself here but I will
just post links in the description for reference. For this experiment, the diffractive patterns
were etched in a thin non-transparent chromium layer on top of a quartz carrier. And with this process you can create these
microscopic transparent areas in the chromium which have diffractive properties, due to
their shape and size. And they have to be really tiny because you
can only effectively diffract light with features that are of the same order of magnitude in
size as the wavelength of light. Because the patterns are so very small, we
need a microscope to study the development of the optical wave front behind them. And for this, I also used a method which was
demonstrated in previous videos: basically, I used a property that most microscopes have
which is a very shallow depth of focus. This allows you to observe the light intensity
distribution present in the focal plane of the microscope. And to avoid confusion, the focal plane of
the microscope will be referred to as the “observation plane“ in the rest of this
video. Let me just show you what that looks like
when you move the observation plane through a beam of light that has been diffracted by
a pattern. Here we have such a diffractive pattern in
focus under the microscope and the pattern has a diameter of about 150 microns. So, this makes the smallest features in the
pattern only a few microns wide. The observation plane is now leveled with
the pattern etched in the chromium, so in focus and you see the light of a laser seeping
through the transparent parts of the pattern. As we move the observation plane away from
the diffractive pattern, we observe how the wave front of the light is concentrated towards
a focal point, which is in this case next to the area of the diffractive pattern. So, let’s make this type of pattern the
first input for the logic device and create a second input which is identical and at exactly
the same distance from the focal point. If we look at the common focal point, we see
that this results in nice constructive interference with the highest intensity right at the center
of the original focal point. But also notice these dark bands. Apparently, inside the common focal point
we have created areas of destructive interference. But if we were to add an evaluator plane at
this distance from the diffractive elements and create aperture right at the center of
the common focal point, we would have produced an optical OR-gate. Here I have replaced one of the input patterns
with another one, that has an identical focal distance but that shifts the light 180 degrees
out of phase. By the way this is done simply by inverting
the diffractive pattern. To see what is happening, we need to zoom
in on the focal point in a little more detail. As you can see, the two parts of the pattern
create a dark line right in the center of the overlapping focal points. So, now, if we were to use a slit-shaped aperture
in the evaluator plane, the output would be in the off state with both inputs on. However, if only one of the inputs is illuminated,
the result at the output will be the on state, because this same aperture will be in the
center for each of the two individual focal points. So by just inverting the phase at one of the
inputs, we have very easily converted an OR gate to an Exclusive OR gate. Now I must admit that I would have wanted
the effects and especially the areas of interference a bit larger. But I guess the main reason for the areas
to be so narrow is the large angle under which the light is refracted by the inputs. So, there is definitely room for improvement
here One of the things that I found while playing
around with different patterns is that indeed light always shows up somewhere else if you
create areas of destructive interference. During my futile quest to make a completely
focal-pointless lens, I designed a hand full of Fresnel zone plate lenses. As you can see, half of the lens surface diffracts
the light out of phase with respect to the other half of the surface. Only to find out that, instead of a single
focal point, I had just created a lens with 2, 4 or even 8 new focal points in return. Even this pattern, which I briefly considered
a demonstration of pure genius, turned out to be another hopeless attempt. So here, the out-of-phase elements are both
centered exactly around the common focal point. But even this attempt resulted in 2 focal
points, this time not next to each other like with the previous patterns, but now on top
of each other. They can be observed when I move the observation
plane through the focal point of the diffraction pattern. First, we observe a focal point right here
closer to the lens surface than for a normal, then we observe a ring with destructive interference
in the center. And when I move the observation plane further
away from the lens, we observe a second focal point, right on top of the other one. So far, I have not made full logic devices
and this video was basically intended to just to demonstrate the principle and maybe get
some feedback or ideas from you viewers before I attempt to do this. Also, I need my thermal evaporator to be up
and running to deposit the evaluator layer. And sure, I think one could come up with much
more effective diffractive patterns. But I think that in essence I have shown that
this could really work as a method to make all-optical logic gates. What about applications? Imagine creating a plate that contains several
millions of these logic elements, which is sandwiched between a small transmissive LCD
screen and a CCD sensor. The LCD would switch the inputs, the CCD sensor
would read out the results. This configuration can in principle perform
millions of logical operations in parallel. And by the way, they are not limited to the
simple true or false logic evaluations, they could also involve complex analog calculus. But whether this could compete with computer
chips in speed and versatility is of course very much the question, I guess it would be
very much dependent on the specific application. Another, and much more down-to earth application
would be to use the logic to make a truly unpickable optical lock. And that will put even the best lockpicking
criminal or lawyer out of business for good. So, I hope you found this interesting. And If you did, please subscribe to the channel,
because I'm definitely going to return to this subject in the future.
How about using polarized lenses (rotated 90 degrees where necessary) instead of Fresnel patterns?