Ted Rappaport: Good morning everyone and thank you for coming to our landmark series: Circuits:
Terahertz and Beyond. My name is Ted
Rappaport, the director of NYU Wireless
and on behalf of NYU Wireless and the
Electrical and Computer Engineering
Department, it's a pleasure to welcome
you here live and also out on the web.
Thanks for joining. This seminar series,
Circuits: Terahertz and Beyond is the
brainchild of faculty and the NYU
Wireless Center which is supported by a
large industrial affiliate program
looking at the future of wireless and
where communications sensing and
computing is going. We have eight
remarkable speakers who are coming to
campus here at NYU at the Tandon School
of Engineering. During this fall semester, a lot of you
are taking this for your courses and the
range of speakers are just incredible.
We're going to learn a tremendous amount
and it's great to have these leaders as
allies, partners, to help us as we hire
faculty and build the resources to be a
leading player in the future of
terahertz. I'm delighted to introduce our
first seminar series speaker Professor Aydin Babahani. I've known Aydin since
his seminal PhD work where he was one of
the first people in the world to build
on ship antennas at millimeter wave
before people realized millimeter wave
was going to be a core part of the
wireless industry. Aydin's an
entrepreneur, he's a director of the
Integrated Sensors Lab and he's got some
great things to share with us about the
future of terahertz. Please welcome
Professor Aydin Babahani.
Aydin Babahani: Thank you Ted.
It's great to be here and thanks
for the invitation. I have to say that
I'm very impressed with all the
facilities and all the interesting
projects going on here; it was really
remarkable and it's great to be here so
what I'm going to do today, I'm going to
discuss the research that I've been
working on during the last 5-6
years mostly at Rice University and
recently at UCLA. So this is all about
really what we can do with with CMOS
silicon technologies from terahertz pulse sources to miniaturize our
spectrometers. So first I will discuss an
important paradigm shift that I believe
is happening in our field, in the field
of like integrating complex systems in
CMOS and then I will show you how we are
changing the way we need to do the
research to actually get the most out of
the silicon technologies and that's how
we were able to implement for the first
time ever terahertz
sources in a commercial standard silicon
process so we are able to produce
signals at 1.1 terahertz with 1 hertz, 2
hertz frequency line width without using
any lasers; all electronically you have
digital input, you have terahertz out and
then I will explain ok now we have all
these tiny chips around how can we use
them for wireless synchronization. How we can
synchronize a massive network with
soft picosecond timing accuracy and how
we use that for localization imaging and
all that and then I will change a topic
a little bit and discuss the first
electron spin resonance, a spectrometer
that we built that actually detects free
radicals and unpaired electrons in
various industry. So this is a technology
that's commercialized, not actually being
used to monitor the level of free
radicals in industrial processes. I will
show you some of the field experiments
on that and then I will also discuss
some other work in the last few years
that we are focusing on wirelessly
powering all these complex chips really
removing any wires from the chip and
then try to use medical implants to try
to use basically put the chips and
infrastructure in cements and everywhere.
You're going to see some of the examples
of that but let's see what's really
happening in the field of silicon. I mean
finally we are able to demonstrate
terahertz radiation from a standard
silicon but if you pause a second here
and think about terahertz and the
wavelength you see that the wavelength
in the air is about half a wavelength is
about 150 micrometer at at 1 terahertz
and in silicon that's even smaller so
that's your radiation component and
you're all familiar with the Moore's law
and integrating more and more complexity
and on a single chip. So if you for
example you look into a decent low-power
processor, this is an arm m0 processor in
40 nanometer process that's only 85
micron by 85 micron. So if someone gives
you a silicon area of 1 millimeter by 1
millimeter, what can you do there? You
already put the antenna there there's a
computational chip there that's your
processor suddenly there's 1 millimeter
by one millimeter seems a large area and
so you can actually add more antennas,
you can build phase rays, you can in
some application you can add
functionalization of surfaces, you can do
sensing so this is just really the
beginning of this field. You're cutting
every wire from the chip, you're
wirelessly powering these few sense chip
and you're doing anything from wireless
communication to sensing. So what are
the key changes? Obviously that
electromagnetic interface the chip
becomes very important you can't afford to
give design of the single circuit
component or amplifier to a circuit
design or ask an antenna person to
design a 50 ohm antenna. Everything has
to be done all together and as you're
gonna see that we are intending to use
no wires for clock you're gonna see some
of the chips that we are building, that
we are actually using wireless
synchronization down to picosecond
and then beyond that we are trying to
eliminate any wires for providing DC
power. So you're going to actually get
harvest electromagnetic energy produce a
stable DC power and use that for
sensing applications. So when you do this
you see that for the first time actually
traditional design techniques are not
necessarily optimum so and the reason we
are gonna see one example is that the
direct digital to impulse radiation for
example is the best approach so far for
producing these very broadband
picosecond pulses. So when we are looking
in there or working in this field there
are two sets of applications that are
enabled with this technology. The first
set is just because you're putting the
antennas on the chip you can move to
really high frequencies so some of the
applications are enabled by the fact
that we can produce and radiate high
frequency electromagnetic signals from
silicon so that enables terahertz and
terabit per second comlink. This is an
example of a basically true time domain
signal that they radiate from the
silicon, you're gonna see a zoomed
version of this. So it's about 1.9 pico
second decent nice signal and then we
use it in experiments to produce
imaging where each pixel tells you what
kind of material exists. So we're
actually working on hyper spectral
imaging system this is an example of
image that we produce. We're working on
3D security imaging and also gaseous
spectroscopy. Turns out that when you
have this massive amount of spectrum you
can do so many other things like even a
single gas molecule vibrates with your
EM radiation so you can use it to
probe gas molecules and then obviously
precision time frequency transfer, how to
synchronize massive number of drones or
nano satellites; this is all going to
need high frequency and terahertz. Now
you have that, the question is okay
second set of applications are really
enabled by the fact that we have these
tiny chips that can be synchronized
wirelessly so why not just throwing them
around like dust particles and suddenly
you have this massive phased ray system
at terahertz that's not on the rigid
platform. You can stick elements of each
array to anywhere you want in the room
or on the car and then suddenly you have
a picosecond synchronization between
elements so you can build a
massive large arrays and now the second
set of applications are enabled because
we are putting everything on a single
CMOS chip no wires are
connected. What does that mean? That means
we can send these chips to do sensing in
places that were impossible to sense
anything before. For example we are
working on tracers for industrial
monitoring you're literally sending
these CMOS chips through pipelines in
oil and gas industry to reservoirs to do
mapping of temperature 10,000 feet
underground, wirelessly powering them, put
them in the cement to monitor
for corrosion for basically gas leaks
and everything. So I guess the vision
is that CMOS is gonna be in every
infrastructure, when you're building
these complex systems where each one is
few sense, you will use that. You will use
that in a lot of places and other
obviously a lot of applications are
medical implants so they're allowing
important applications where you cannot
afford to send a large instrument inside
the body so you have to shrink the size
to very small dimension and that's where
I'm gonna show you some of the results
of our wireless pacemaker where we are
intending to send these pacemakers that
are so small through a vein in the leg
to basically to multiple locations in
the heart and wirelessly synchronize
multiple pacing poising, do precision
timing of their electrical pulses on the
heart and we are showing the testing in
large animals and how successful this
one is and the last one is really
literally terahertz powered smart dust
sensors where it is going to be
potentially if these chips flying around
they are so light you can make
them fly and then now you can
synchronize them, you can do interesting
things with that. So let's now go over
some of the key examples of what we
have demonstrated so far and that brings
us to the first part of the discussion
on picosecond digital to impulse
radiators. So if you look into the
terahertz spectrum,
traditionally I mean terahertz is well
defined in frequencies about 100
gigahertz and down about basically 10
terahertz so in frequencies lower than
100 gigahertz it's easy to generate
signal with electrical methods and in
high frequencies in optical range, energy
of a single photon is large enough so
you can build room-temperature lasers
but when you come to
produce any electromagnetic energy in
terahertz these electrical systems are
not very efficient and optical methods
need to be cooled down, so optical lasers.
So this is one of their challenging
areas in our coming to you how to really
produce any signal and do the
measurements at terahertz frequencies
but if you do that you can do a lot of
interesting things you can for example
look into this different white powder in
an envelope it can be inside the
envelope and you can differentiate if
that's what kind of material that i,s is
that an explosive, is that poisonous, is
that not. You can detect a lot of
explosives just due to the vibration of
basically the rotation of the molecule and you can obviously build a
higher speed data links you can use it
for security imaging and all that. So now
let's try to look and see well why
terahertz is so powerful, why people
haven't really used it that often, why
it's not the mainstream technology and
this is slide tries to explain that. So
for the last 20-30 years the community
that actually produced picosecond pulse
and radiate that these are mostly optics
community that use a femtosecond laser.
This is a relatively expensive laser
that produces about few hundred
femtosecond pulses probably like 50
$100,000 big laser system and then you
have to actually build a photo
conductive antenna buy an antenna such
that when there's no optical
illumination there's no current but when
you're 100 femtosecond optical beam
arrives here you suddenly short the
antenna
and then you radiate a beam that's about
1 picosecond in duration so that's how
community in the past produce these
terahertz pulses. And then how they
detect it? They actually use a sub
sampling method so they delay the
optical beam mechanically or sometimes
electronically and then use a similar
PCA to detect the system and obviously
you see the problems there's a lot of
alignment issues you need to use a lot
of mirrors you cannot even a scale this
to large arrays. So when we talk about
this problem we said okay how can we
solve this in silicon? Can we use
similar methods to produce a picosecond
pulse in silicon and back then probably
the best state of the arts published
work on silicon pulse was around 40-50
picosecond. This is where we demonstrated
actually less than two picosecond so how
we did that was we tried to be very
inspired by this spark gap idea so you
build a huge voltage and suddenly
shorted and what happens and then there
was a lot of modification over that so
we ended up using a different idea that
actually uses a slot bowtie antenna on
the chip so this is the basically the
orange color here so we are passing a DC
current around this antenna so you can
think of this as an inductor in DC or
low frequencies. So when you pass this
you're building up magnetic energy
around it but now what happens if you
have an inductor but you disconnect the
current very fast inductor doesn't like
that it's gonna generate LDIDT which is
a large voltage but that's what we want
we want to generate a large voltage very
fast because at the same time this
aperture that we have here is not only
an inductor it's actually an antenna at
high frequency so when you produce a
large transient voltage it's going to
radiate so what you're doing is that
you're actually going to store magnetic
energy there, you're going to disconnect
it very fast and suddenly you have a
picosecond pulse radiated from the chip.
So this is a little bit more explanation
on how we are doing this. So we have on
the chip we are passing a DC current
through a transmission line there's a
lot of little techniques here, how to
have a
distributed array of capacitors to deliver
charge very fast to the antenna on the
timescale of picosecond and then pass
it through the antenna and the only
thing you need to operate this is a
digital trigger so this is a standard
200 picosecond digital trigger you
sharpen that down to 30 picosecond and
further down to few picosecond you just
disconnect it. With every rise time of a
digital trigger you have a two
picosecond pulse that's radiated and how
good is the timing synchronization
between your digital and terahertz
that's on the order of 200 femtoseconds and I'm gonna share some of
the results that we have. So as you see
in CMOS they're all the practical issues
you need to put wire bond, there's an
inductor there basically that causes
issues so you need to you need to make
sure you have a stable chip supply
voltage to deliver the charge and then
what we did in the first version we use
a cascode basically bipolar transistors
in 130 nanometer C key and also another
version and 90 nanometer C key and then
the way it works is that you you send
your digital input, you delay it as much
as you want so you can delay the input
digital trigger with resolution or
literally 100 femtosecond with dynamic
range of beyond 100 picosecond and by
delaying that you're seeing that the
pulse that's radiated is delayed 
exactly same amount and then what we do
is that we sharpen this with digital
buffers down to 30 picosecond and using
an analog ,circuit it down to few picosecond and then we use that to turn on
and turn off this fast sewage and and do
the radiation. So because this is so
simple to build you can easily scale
this to an array. So we have an array of
2x4 picosecond radiators on a
single chip where you have independent
timing control on each radiator so you
can control the timing of each radiator
with about 200 femtosecond resolution
and everything works really nice and
radiates and I will show you a picture
of this picosecond terahertz pulse
radiator all on a single chip. This is
basically 2x4 array it only
needs DC bias here and digital input nothing
high-frequency no laser is used so you
need you now need to think okay when your
antenna radiates how is going to radiate.
The silicon substrate is usually has a
high dielectric constant so that behaves
like a low resistive or radiation it's
gonna absorb all the energy. So in most
cases we use a silicon lens to couple
that energy to basically to air in some
other cases we just don't use silicon
lens we optimize the thickness of
substrate and we radiate from top side.
So one of the very first problem we had
was okay we are producing these
picosecond pulses but we go by the best
oscilloscopes out there it can only
measure eight picosecond pulse what
would you do how do you characterize us
and everything else including like VDI
is down converters and all that they're
all banded there's nothing out there
that gives you a single chart like
measurements of the entire spectrum, so
we had to build that our self. So we ended
up using for characterization purposes
an asynchronous laser based system that
actually we only use a detector part of
the system this goes up to four
terahertz and these are two laser pulses
that are with they have a rep rate of 50
megahertz but they're one of the lasers
shifted by a few hertz compared to other
ones so what you do is that you actually
detect the pulses at a slightly
different shift at frequency than the
transmitted so you can do some sampling
and you can quickly capture the shape of
defaults. So what we do is that we take
the laser author of this we reduce
signal conditioning converter 250
megahertz basically divide that down to
10 megahertz reference signal we clean
that and now in the 10 megahertz
reference you can lock any microwave
source. So you can basically produce
signals at few gigahertz rep rate that's
locked your lasers and characterization
system and then you can feed that to
your silicon chip. So these are some of
the measurements we got from this array
chip. This is actually the real-time
measurement measure time domain signal
that so that the silicon chip radiates.
So if you look into the zoomed version
of that it's very clean
signal, it's about five picosecond is
this basically time duration here
there's some ringing effect so it gets
reflected from the lens and boundary but you see everything and then by playing
with the bias amplitude of the basically
bias of the transistor you can actually
do simple amplitude modulation so this
is how we can change the amplitude. We
are now hoping to produce two three bits
of amplitude modulation on each pulse
and everything is for you because yes.
So our peak radiated power is about tens
of million what I'll show you the ERP
measurement over over over over time
yeah. It's actually better than the
conventional the expensive laser based
systems in terms of the average power
because they are limited by the physics
of their laser by the repetition rate so
they can only put pulse with 100 Meg rep
rate so that gives lowers their average
bar. No no our peak progress but our rep
rate duty cycle is like 10%, we
can go to about beyond 5 gigahertz rep
rate. So now by changing the digital
trigger you can actually delay these
pulses. I'm here focusing on the 0 0
crossing so that was a time domain
measurement now we are trying to build
communication systems where you have
literally a terabit terahertz bandwidth
but you also want to do beam-steering.
So in those problems none of those
conventional phase rays work if you
use like a low phase shifting every
frequency will be radiated one direction
if you use like RF domain phase shifting
you pass all that information to RF
delay you have those nonlinear delay
components that's gonna cause basically
a.m. to p.m. problems conversion issues
so we decided a completely separate our
timing control from the information
channel so we are sending always a
trigger signal that doesn't change its
shape based on the information you're
putting in and that sets the time of
radiation and everything else including
information just comes on the altitudes
of the signal so you can actually build
a system that has a terahertz bandwidth
and you can actually delay it with 100
femtosecond resolutions a true time
delay method. So when we build a system
we were also interested in
characterizing it in frequency domain so
if you think about a single pulse in
frequency domain you have a broader
spectrum but now you can program the
repetition rate of this pulse so that
means you can produce a pulse train so
that means a frequency domain you have a
frequency comb now the question is we're
interested in characterizing this we
want to know okay how how stable these
tones are what can we do with that in in
different experiments. So that's why we
build up a characterization system up to
1.1 terahertz using a series
of VDIs downconverter mixers with a
keysight spectrum analyzer so you can
now look into actually these pulses
frequency come directly on the spectrum
analyzer. Do these are some of the
results we got. We were actually
surprised but we're getting initially
but at 750 gigahertz over like 20
centimeter distance this is the raw data
includes all the last free free space
free power loss loss of the mixer and
everything. So we were getting roughly 30
DB SNR at 30 centimeter 750 Giga
Hertz. When you go down to one we for the
first time we show that beyond one
terahertz we can actually demonstrate a
tone using a silicon chip with basically
for you DSLR and you're gonna see that
we have another version that produces
much higher SNR up to 1.1
terahertz. But one of the remarkable
things here was that the line width so
we were able to produce these tones with
two Hertz frequency stability
so these tones are actually very very
very clean so you can produce anything
you want from 20-30 gigahertz to 1.1 terahertz with two Hertz alignment.
So these are the radiation pattern
measurement and each plane a plane at
different frequencies we characterize
all that and then this is the micrograph
of this array that's only like 1.4-5 millimeter
by 1.6. If you zoom into the
single element the majority of the area
is really the antenna you don't have any
amplifier you don't have anything else
this is only a very fast reaching
network that pushes the whole system
into extreme non-linearity. So we had
this one was done the array one what was
done in 99 meters CT process.
So let's see frozen little, maybe
keyboard does oh yeah it's frozen.
I guess I'll wait to see what happens.
It was thinking, I think it was listening.
There's some delay sorry just wait.
Now I think we have to wait it has a
little bit delay. So we are consuming 100
milliwatt DC power. That's a that's a
peak power yeah with duty cycle of few percent.
Alright, let's see.
I'm gonna be very careful when I
press. No okay okay so this is this is
right slide now, you're lucky. So this
is the basically it's another version of
the single chip radiator so when you put
in a single antenna on the chip
everything is a small 550 micron by 8 15
micrometer but we can actually push the
frequencies even further. If you look
into some of the numbers here we were
actually produced we can produce 1.9
picosecond pulse with this chip and if
you look into the ERP spectrum we have
everything from basically 30-40 gigahertz.
The ERP is flat from 100 gigahertz 200
gigahertz and then it drops basically
when you get to 1 terahertz that that's
how it looks. For this chip now
if you look into the spectrum look at
the frequency here at one point one
terahertz we are producing a decent SNR,
roughly 20 DB with two hertz alignment
and then when you go to lower frequency
600 gigahertz is now a low frequency for
the system. So you have basically a 40
gigahertz SNR so it's plenty for doing a
spectroscopy near field sensing and all
that. So the goal is really ok how to and
we have the receiver version of that I
will also show you some of that so how
to really convert a complex system
that's $100,000 to something that
potentially few dollars. It's bulky this
is much smaller millimeter this is a
slow has a rep rate of 100 megahertz we
can go up to 10 gigahertz this is not a
scalable you cannot build my mode or
phased arrays you can easily do that
and this one requires a laser but we
don't need the laser. So now we have this
source technology we are trying to see
ok can we build a spectroscopy system so
in one of the setups or the projects we
had we basically the goal was to build a
precision gaseous spectroscopy sensor so
here's our gas cell we are radiating
this frequency comb with
see a silicon chip and we are detecting
gears we're trying to see if we can
actually probe the rotational spectrum
of gas molecules and this was from two
years ago our old gas cell now we have a
better version of that but the whole
concept worked we actually demonstrated
that if you look into the theoretical
results the energy tree has an
absorption at 572 gigahertz. We clearly can
detect that and it's actually
interesting when you're putting more gas
so these gas motors hit each other
you're gonna expect pressure broadening
that means the absorption curve spectrum
gets broaden and we see that as well so
we demonstrate that as well. H2O has
another absorption it has an absorption
of 753 gigahertz. We were able to
demonstrate that and detect that. So now
we are also using this system for
hyperspectral imaging that's where we
have basically a couple of mirrors and
and the sample in the middle so this is
an image at 330 gigahertz the students
took from a piece of plastic and and
metal. This is another image they take
from a few basically capsules put the
powder and use a previous name of the
lab and this is the image one of my
favorite ones is this. This is a piece of
rock basically you don't see what is
inside the rock is taken from 10,000
feet underground and companies are
interested in see what what the hell's
inside. Is that oil, is that chemical, is
that basically water? Can be can we use
different complex EER methods to push
the oil and that's basically how this
hyperspectral image show this is a
transmission spectrum from 100 gigahertz
to close to 1 terahertz and the yellow
one shows basically less absorption blue
one shows higher absorption so these
blue patterns is probably we have more
water content here the and the yellow
one is more like oil and now we are
building a set up where you can inject
different chemicals and you see how how
they move inside inside the rock.
It's about five millimeter thick 25
minute dimension and the resolution of
image is roughly 500 micron.
Yes
This particular one is yes we move the sample we focus to me and we move the sample to
get these high-resolution. Yes, yes, yes,
yes. Now we are trying to look into other
interesting medical applications so
people have published using this select
era we use $100,000 system with the
medical probe to really look into it to
do dielectric imaging of a skin cancer
it turns out that in melanoma there are
some due to the change in the dielectric
constant of the cancer people can probe
basically a skin cancer we are working
on these problems. A lot of people used
it for basically for breast cancer when
you remove the tumor the question is can
you actually take in terahertz image to
to make sure nothing is left or or to
make sure you're not really removing too
much so there's a lot of things that's
that's terahertz signature some of that
is primarily due to the water content
but there's also other interesting
complex dielectric constant that you can
measure. So when you have the ability to
measure dielectric constant from 50 mega
Gggahertz to 1 terahertz things are
also frequency dependent so you can
classify different tissues you can
actually do a lot of interesting things
there. So one of the things we were
interested in we were trying to say ok
we have we show that we can actually
radiate signal from a single chip what
happens if we have two wide ear space
radiators can we if we synchronize that
with the low frequency digital trigger
what happens to the shape of the signal
that's being radiated? Can you combine
the radiated time domain signals from
two widely spaced radiators? So this
measurement shows that so the top one
shows basically a red one is a signal
radiated from the top silicon chip about
110 millimeters away we have another one
that radiates a blue one so if you do
the measurements separately you get
these two pulses if you add them in
MATLAB you will get this black curve but
if you concurrently radiate them
you get the pink one so it means
basically combining in air special
combining is almost ideal but we want to
quantify this we want to know okay what
is the timing to drop the combined
signal and that's this measurement that
shows we measured about 270 femtosecond
RMS jitter on on the combined signal. So
so we had previous ones were all built
in silicon germanium process we had
another one is 65 nanometers CMOS this
is a larger rate this is a 16 element
array and it has all the digital
components so it has basically Dax and
shift register for controlling the delay
and then terahertz radiator here. This the
CMOS chip produces a little longer
pulses about 14 picosecond pulses this
is when 14 channels are on and then then
four channels are on you see that
amplitude basically drops so you can
turn on turn off more channels to
quickly do change the amplitude and
basically this is just a measurement of
the time delay so I have the picture of
the CMOS chip about 2 2 millimeter by
3.3 millimeter in CMOS process we need
to comply with a lot of top metal
filling that's why you see these dots
here but you have to verify make sure
you don't have many of them around the
sensitive part of the antenna where the
electric field is a strong so there
that's why you have this block fills
that. So I will now that was
mostly the source technology now we have
that we wanted to use this in wirelessly
synchronization of different chips and
why we are motivated with that for four
different reasons one is when you put a
large array on the rigid platform you
can really put it everywhere let's look
at the corridor application the bumper
is curvy you cannot put a very flat
basically array there so ideally you
want to put these tiny chips on the
bumper everywhere you want and then use
wireless synchronization and then beyond
that you can also place these chips
widely spaced so you can increase the
angular resolution. So this is basically
shows that the effective angular beam
width of an array depends on the your
effective element spacing so we looked
into this problem from multiple
basically view points. In the first chip
we rebuild a continuous wave version of
the this I would say is the sensor node.
This is a node that receives a low power
signal at a hundred gigahertz it has an
on-chip patch antenna that actually
captures that amplifies that and it
locks this to an injection lock
oscillator applies phase shift you need
a phase shift for the array operation
and then it radiates back from an
on-chip dipole antenna so everything on
this single chip you amplify and radiate
back a signal at 50 gigahertz half of
the harmonic so when you do this and you
have roughly close to 80 to 100 DB gain
every little coupling from your output
to input will cause self-oscillation so
if you have to deal with that so
basically I will show you how we deal
with that. Let's assume exact dimensions
of the patch at 100 gigahertz capital to
injection lock basically oscillator we
have the phase shifter that covers 360
degrees and then the output differential
transmitter that puts the power on a
dipole antenna and radiates.
So let's look into the coupling
mechanism when you put everything on a
single chip you amplify you radiate how
do you resolve the self oscillation
problem. One issue is that okay let's
start with polarization orthogonality so
the patch the way it's design is only
sensitive to electric field horizontally
here and the dipole radius electric
field vertically so when they radiate
then this is how you can actually
improve their near feel isolation but
that that's not enough so we use
frequency separation obviously that's
one is 50 gigahertz the other one is at 100
gigahertz again you have harmonics and
their harmonics coupled to each other so
how do you deal with the harmonics like
at 200 key guards they will have
harmonics.
So to do that we used basically ground
shield to the patch and optimize the
patch so it's only efficient in terms of
radiation efficiency in our and even
harmonics of basically 50 years so
that's 100 years 200 years and optimize
the thickness of the substrate such that
the dipole is only efficient in all
harmonics of 50 grams so if you look
into the radiation efficiency this is
the blue one is the one with dipole and
the green one is the one with patch so
even the signal goes to the environment
packaging come back it won't cause
self-oscillation because your radiators
are not simply efficient at those
frequencies. So compared to two simple
dipole we you can increase isolation by
60 DB on the same single chip so we
build this we now trying to test it okay
we use a a basically horn antenna try to
lock this there's no wires connection
for connected for signal for
synchronization purposes so it gets the
signal at 100 gigahertz and radiates
locked signal at 50 gigahertz. Now when you
use the freerunning on the oscillator
you have this basically a noisy signal
that you're radiating at 51 gigahertz
when you turn on your transmit you lock
it suddenly you look really sharp
signals like laser looking like beam and
then you can obviously tune the
frequency and do all that. So we get
about 400 Hertz line width at 51 gigahertz 
wireless locking no wires used
and now if we measure the phase no
resolve the freerunning system that's
the blue one the phase noise of the
locked one which is the red one and also
the phase of the wired base system. So
now we want to push this one for a
further step we want to see okay what
happens if you have multiple of this can
you still lock them and beyond that can
you play with these phases to combine
them in air coherently and this is the
demonstration of that is our two chips
about 13 centimeter away. We lock them
wirelessly, we adjust their phase shift
to the power received at the horn
antennas maximum. So this one was
successful as well so we get about six
kilohertz line width at
51 gigahertz. So I have the
micrograph of this wirelessly
synchronize chip it's about 1.5-1.7
millimeter by 3.8 the patch antenna is
shown here on the left that received the
signal all the receiver transmitter
phase shifter and the on chip antenna on
the right side. So that was the first
chip, the second chip we tried see okay
now we are producing these picosecond
pulses can we build a receiver to
actually measure the energy of the pulse
and extract the repetition rate and
that's gonna give you your clock so if
you send these pulses with 10 gigahertz
you suddenly have a wireless
clock at the removed location with 10
gigahertz rep rate. So that's that was
the idea here so we do energy detection
the way it works is that basically we
use a nonlinear block everything is on
CMOS you amplify the signal and we get
the energy of the pulse and we do
basically filtering to just extract the
clock and I'm going to skip some of the
circuit details here these are all
publish work so I'm going to show
basically the results here the chip was
only 1.4 millimeters by 1.35 millimeter
this is the antenna and all the
circuitry now let's look at the
measurements. So here we use our own
pulse radiator that generates these
picosecond pulses and the receivership
that detects these pulses the question
is if you use a wired connection from
your keysight signal source to your
spectrum analyzer what does the
effective phase know is how good your
clock is and if you completely eliminate
wire you do it wirelessly how good the
signal is that was the ultimate goal of
this test. So in terms of the frequency
we demonstrated with wireless clock
transfer we can get 30 Hertz line width
at 3.1 gigahertz and if you look into the
phase noise measurement result this is
basically the wire complete. Wireless
testing gives you as a phase noise
that's almost very close to the your
source with wired connection. So the blue
one is the phase noise measurement when
you completely remove the transmitter
receiver just use a simple wire
and the in the wireless test this is
showing that bass demonstrate that true
air you can get almost same level of
accuracy and you're limited with now
with the instrumentation
basically sensitivity. Here is the
unfiltered clock at the output you can
easily filter the high-frequency signal
and you basically shows that you can get
decent priyad. This is your output clock.
So that was the second method. We did
try a few other methods, we also looked
into the optical ways of passing this
clock so in this experiment we use the
VCESL modulated this at basically 1-2
gigahertz and use it for longer
distance you want to probably use
optical frequencies and then what we do
is that we used optically modulated
very form to lock an RF oscillator. So
how do we do that is shown here. So
here's a cross couple RF oscillator that
operates at gigahertz.
We use one of the diodes in the cross
couple exposed to optical illumination
so the optical illumination generates
electron holes. The other diode is
actually covered with metal. When you do
that you're generating a current that's
basically differential at the load of
your oscillator and that frequency of
that current is frequency of your
optically modulated signal so that's how
you can actually lock many RF
oscillators that are widely spaced. So
this was really a small chip our antenna
was basically our diode which is really
small this is photo diode for optimal
illumination everything is really this
is the optically locked VCO that's what
we called it and now if you look into
the measurement results the freerunning
VCO puts out a spectrum like this. This
is the red one the optically locked
produces signal that's really nice and
clean so this just demonstrate that we
can actually use optical methods to do
that so for variety of application
depends on what your distance is you can
actually you can switch the optical ways
to to synchronize. So now what I'm gonna
do I have roughly 10 15 minutes so what
I'm going to do is I'm going to show
some of the other
sensor technologies that we have been
building by all by the power of CMOS we
are trying to miniaturize them and then
try to actually use them in the field. So
I'm going to completely change the topic
to electron spin resonance and how it
works and how we actually build a
miniaturized version of that. So this is
a very powerful spectroscopy method
the concept is similar to NMR or MRI but
the differences here you're probing
spins of electrons and the nice thing is
that electrons come in pair in most
materials so they actually don't have
any sr signal but well in some specific
materials like free radicals they
actually they have an unpaired electron
so you can actually see unpaired
electron and you're not gonna basically
see the interference by the water
molecules or other things in the
environment. So what are the applications
in physics? They actually use it for
basically detect studying the defects or
sample composition. In medical
application they use it for cancer
applications. We were when we did that we
were in Houston with all the oil
companies around us so we said okay what
are the applications of this in oil and
gas industry it turns out that there's
this interesting large molecules in oil
they're called asphaltenes and they have
this stable unpaired electron that lasts
for millions of this millions of years
so this is mostly because of the organic
content probably dinosaurs died many
years ago and there there's some
asphaltenes left there so I don't think
we feel exactly know what the origin of
that is but the issue is it's a huge
problem because what happens is that
then pressure temperature drops. It
actually blocks the pipeline and
intervention cost us half a million
dollar for on short a few million
dollars for offshore. So in medic, - yeah
yeah yes well, I'll show you how it works.
So you put the sample in the magnetic
field we energize the unpaired electron
you see if the magnetic field and you
probe it with microwave so one thing I
have to mention is that electron because
it's much lighter than basically portal
for the same magnetic field the
resonance frequency is 600 times larger
so you're actually dealing with
microwaves which is type of the problem
we are interested in. Yes so for a 5
gigahertz system that's 1500 grafts
roughly so one Tesla gives you 30
gigahertz. So this is a direct method for
probing partial pO2 in a cancerous tumor
so cancerous tumor has a higher level of
oxidant activity so you can actually use
this to directly propio to and people
have demonstrated that in basically
mouse and clinical work and in
pharmaceutical application and you can
actually use this to look into quality
of pills in engine oil when you use the
oil it generates free radical so there
are some applications there. So what are
the challenges? The challenges very
briefly like how the physics works so
you put an unpaired electron in a
magnetic field and it goes through
energy splitting and then you can
actually now probe that with the
microwave so you put a sample in a
magnetic field it goes through energy
splitting you sweep the magnetic field
you measure in a spectrum but because
the signal is extremely weak that's not
a now you cannot detect this way.  So you
have to modulate magnetic field at
certain frequency let's say 100
kilohertz and then what happens is that
the reflected signal that comes back is
actually at a slightly shifted frequency
so you were sending let's say something
at five gigahertz and you're getting back
a huge interference at five degrees but
100 kilohertz away from that you get
you're actually signals from your
electrons. So it's a huge self
interference problem it's similar to the
full duplex problem that people are
interested in wireless comm
communication so basically
but the concept is similar you send a
transmitter signal to the sample you get
you have a circulator but you get your
on the desired signal that's extremely
weak but your direct coupling from
transmitter to receive it completely
blinds your receiver in conventional
system you cannot use it you cannot
detect it. So we build a chip to
completely solve this full duplex issue
so there's a basically transmitted that
puts out the microwave signal at 5 gig
everything goes to the sample comes back
the desired signal is slightly shifted
from your five kickers about a hundred
kilos shifted so you cannot directly
inject you're canceling signal at the
beginning because you will completely
degrade the noise figure of your
receiver so you have to amplify it by a
little bit 10 DB here and then add the
phase and amplitude control to cancel
the interference and then further
amplify that so that was a complete chip
for this ESR transceiver it has the
active cancellation all again and this
one is by CMOS process and here shows
that we can get improve across all
frequency range from 4 to 4.6
gigahertz and across all the phase
numbers the isolation we are achieving
is more than 10 DB and when you do that
you actually your receiver conversion
gain at a high level of self
interference and signal can be still
very high so your it's not gonna blind
your receiver. So a very important plot
is noise figure how much degradation you
have on the noise figure. If we this is a
black one as a noise figure of receiver
and transmitter is completely off when
you start the active cancellation you
have a small degradation if you now turn
on your transmitter on the same chip
you're gonna huge interference you're
gonna huge degradation noise figure and
that's what's bad in full duplex systems.
When you turn on your active
cancellation you reduce that down to
basically reasonable numbers but anyway
the summary is that we build a system we
miniaturize the magnet to something that
looks good looks a small like this all
the electronics are on a single chip and
the whole system is commercialized
actually being right now used invest
Texas on oil fields so all whatever they
produce we can go and see basically the
level of asphalting
that come to surface and a new cellphone
basically and this system is can detect
corrosion ions in the flow it can go to
5,000 PSI 175°C
and all that so there. So now what I'm
going to do is spend two three minutes
and our medical implants and wirelessly
power chips that we are using for
wireless pacemakers and here a
completely different topic like what are
the problems of a pacemaker and why we
are interested in this issue so if you
look into the heart electrical system of
the heart it's actually interesting
nonlinear electrical system. So the way
it works is that you have a central node
it's called an SA node that first the
heart electrical signal initiates from
that and it's distributed through
different part of the heart muscle in a
healthy person everything works
everything is nice but sometimes what
happens that the delay of the signal
that goes to different part of the heart
is not same so different parts of the
heart actually pulses a different time
delay so you're pumping efficiency is
not good enough. So companies like
Medtronic use this lead based system
this is a huge battery-powered a
pacemaker with a huge wire that has to
go through all this veins and blood go
to inside the heart to just deliver a
current to to synchronize a heart, so
that has a lot of infection problems.
They recently build a smaller one that
actually doesn't have lead but it still
has a battery if patch is depleted they
have to go through another surgery and
replace it. So what we were interested in
is can we build a battery less lead less
wirelessly powered microchips to get the
power wirelessly but produce enough
energy to actually control their the bit
rate of the heart and this was our first
pacemaker that was operating with an
tube antenna at higher frequencies but
what happens is that if you look into
the natural sinus rhythm of the heart
it's this one is I think in a pic at 102
beats per minute. If you pulse the signal
at a frequency that is less than
this heart doesn't respond but if you
pulse the signal frequency higher than
this you can actually lock
the heart so you can control the period
of the frequency of the heart. So this is
a complete this one we increase the
heart rate to exactly 120 beats per
minute that was program and again to 172
bits per minute so this one was
successful. The problem with the first
chip was when you it evade operated that
it harvests electromagnetic energy, puts
it on a cap and release it when it gets
to a certain level of energy to release
it. So that's not good because if you
bring your transmitters kind of put more
pulses so we fix that in the future
versions we demonstrated wireless pacing
from two different sites but in the
latest experiment we used actually six
different tiny chips on the heart and
demonstrated that we can synchronously
do the wireless pacemaking and we even
close the chest we demonstrated that yes
electromagnetic energies at those
frequency can penetrate through several
inches of the tissue and you can still
can build a wirelessly powered pacemaker.
So the course circuitry is only 200
micron by one millimeter we used it for
sciatic nerve stimulation to control the
movement of the leg we put that in deep
brain stimulation in in mice so this is
a freely happy moving mice Mahseer there
are two of them we use a single
transmitter to control the electrical
activities deep in the brain to look at
their behavior, how they move, how fast
they move, how we can control that and
now we have a very low power their
electrical recorder that only consumes
like two nano watt per channel and we're
trying to shrink it further to get to
the cell size where that's where we are
trying to use optical ways to basically
provide energy to the chip using this
surface grating apertures so the last
slide really here is showing that okay
what was the future what are the
directions we are taking the way we are
looking at yes this CMOS chip will be
almost everywhere in the in the
battlefield, in the environment you're
gonna have they're so light you can make
them basically with very little energy
to fly so in the room you're gonna have
tons of these CMOS chips. The question is
how to use them to do synchronization,
imaging, sensing and all
and eventually how to couple that with
self-assembly techniques to do precision
biomolecule detection or DNA DNA
detection so this is basically kind of
the trend and I'm just a speaker here so
the work was done by students and
postdocs and I'm gonna skip the
conclusions and hopefully I was
convinced to show that yes the CMOS
provides a really massive platform where
you have to think differently how to use
that to something beyond communication
and how to do this basically interesting
sensing and thank you very much for
listening to talk yeah okay.
Ted Rappaport: Professor Babahani, thank you for this
truly remarkable lecture. Professor Babahani: Thank you. Ted Rappaport: What you've done in inventing the future
of terahertz, creating measurement systems
to see what we've never been able to see,
to use wireless to synchronize to create
resolutions we've never seen in the
world. I'd say you're a true virtuoso of
circuit design wouldn't you say it
virtuoso making the future, let's give him a hand.
Professor Babahani: Thank you.
Ted Rappaport: So in conclusion with the
first seminar, we'd like to present you
with something. I'd like to ask Dr. Ivan
Selesnick, the ECE department chair to
give you a presentation.
Professor Babahani: Oh thank you very much!
