Ted Rappaport: Good morning and welcome again to NYU from the campus of New York University.
I'm Ted Rappaport and welcome to the
sixth seminar series in our exciting
seminar Circuits Terahertz and Beyond. On
behalf of the NYU WIRELESS industrial
affiliates that sponsor our work at NYU
Wireless and who are also involved in
our research and recruiting and student activities, it's my pleasure to welcome you to a very exciting seminar
'Daniel Mittleman: Excited to come here. This is really
great. It's been a fantastic visit
already and I'm looking forward to the
rest of it. I'm here to talk to you today
about my group's contributions and
experiences in thinking about terahertz
wireless and this is something that, we
that as Ted said, I came to by way of
optics so I think I looked at the list
of people who are have participated in
this great seminar series over the
course of this semester and most of them
started at low frequencies and worked
their way up and I've come the other way.
So they all consider themselves high
frequency people and I consider myself a
low frequency person but we're talking
about the same frequencies so it's
really nice, it's really nice that the
field is finally coming to a place where
those two communities are now beginning
to shake hands right as Ted was saying
at breakfast this morning or at dinner
last night, I don't remember, you know. One
group started in California and started
moving east and the other group started
on the East Coast and started moving
west and we're meeting in Utah, at the
Golden Spike and shaking hands, right. So
this is this is fantastic. So what I'm
going to tell you about are just a few
ideas in devices for manipulating terror,
it's radiation and some measurements
we've been making on trying to
characterize terahertz channels and a
lot of this sort of the the overriding
theme here is the photonics approach, the
idea that we're thinking about these
systems in a photonic sort of way right.
For me, terahertz is not a broadcast. It's a beam,
it's more like a laser than it is like
an RF radio broadcast and that you know
changes the way you think about things
so that's kind of fun right it's fun and
interesting and gives us some new ideas.
So this is my one slide on
motivation, which I absolutely don't need
to show to this community because we all
know that wireless and also wired
systems, there's an increasing demand for
data and eventually we're going to need
more than we can do, right. 5G is coming
very soon, if not already here, and within
a few years it will also already be
saturated and we will need more
bandwidth than it can possibly provide,
so we will need to move to higher
frequencies, that's kind of I don't
really need to dwell on that I think for
this for this group right. This is one
slide which I like to show. I think many
of you might have already seen this but
I think it's important to emphasize, this
is a channelization standard that has
already been approved by the IEEE
and will be under consideration at WRC
2019 next year in Geneva and when I
first saw this, I had several reactions:
one of which was sort of the
overwhelming joy of knowing that
thinking about terahertz as a as a place
to do wireless communications is not
crazy because somebody else thinks about
it right and has thought about it enough
to put together a standards document
which is fantastic so thank you to
Thomas Kuerner and Iwao Hosako for
letting me use this slide. This is really
very motivating right because it tells
us that you know somewhere between
252 gigahertz and 321 gigahertz,
somebody might actually be doing
communications some day and if that
someday is you know not too far in the
future we better start thinking about it
now. So I will dwell for just a minute on
what do I mean by terahertz right and
you can see I circled down here is this
really terahertz it's it's less than 300
gigahertz which means the wavelength is
more than a millimeter and so is it
really terahertz? Should we call it
terahertz if it's below 300 gigahertz? I
hate this question right, who cares? In my
world, if it's above 100 gigahertz its
terahertz and you know that's just lingo
right it, doesn't really matter. You can
call it millimeter waves if you want to
but let me point out that the people who
say who are strict about this and say
you can't you have to say millimeter
waves if it's below 300 gigahertz, these
are the same people who refer to those
airport scanners made by L3 as
millimeter wave scanners even though
they're more than 10 millimeters. Ha, so there you go!
Okay so what's the plan for today? So a
little bit of background on the
terahertz field, this is sort of I know
many of you guys have heard a lot about
terahertz already but I'll spend a few
minutes just kind of giving my
perspective and from a true what I call
myself a tera-vangelis. I've been
evangelizing the terahertz field for
over 20 years now and so I guess I've
earned that title and then I'll give a
few examples from my research group over
the past few years of work we've been
doing on modulators for terahertz
radiation and these are external
modulators and I'll explain what I
mean by that.
The terahertz wave guides which are a
useful platform for doing all sorts of
signal processing operations like
filtering and switching and multiplexing
and I'll talk a little bit about how
that works
and then you know, what is the channel
like right. If we really want to send
terahertz signals from here to there
through the air, what's different? What's
the same at lower as at lower
frequencies and what's different and and
how do we measure how do we make those
measurements? And then I'll finish up
with our most recent work which is an
interesting question that we just
started thinking about very recently and
that is if you have a terahertz wireless
link and it's just a point-to-point link,
so not a network but a you know a single
transmitter and a single receiver and
they're pointing at each other, are you
still vulnerable to eavesdroppers right?
and now if you do this with RF, you are
and people think a lot about this and
it's a big issue at higher frequencies
presumably it gets harder but the
question is how much harder and so we
begun to think about how to quantify
that. So let me point out that the
terahertz field is not brand-new right
you know this is the very first paper
ever published in the journal Physical
Review right. The date on there is 1893
it's Volume 1, Issue 1, Page 1. It's the
very first paper ever published and it
was a terahertz spectroscopy paper.
They did not use the word terahertz, in
those days they called it obscure
radiation which it still is right but
they had a source of terahertz radiation,
they had a detector, they put things in
between and they measured absorption
coefficients right and so people have
been doing terahertz work for a long
time and in fact in my lab today we
still make measurements basically just
like that. We have a source, we have a
detector, we measure absorption. Ok so and
there's still a lot of materials that
haven't been characterized.
I mean a lot of stuff has been done but
there are many things that have not been
done and so there's still a lot of
interest in basic spectroscopy in this
range. So what has happened recently of
course as I mentioned at the beginning
is that there is this sort of growing
realization that the electronics people
in the photonics people are talking
about the same thing when they're
talking about these you know when
they're both talking about 300 gigahertz
right and so there's sort of a, you know
ongoing I would say, merger of the
electronics and photonics views of the
world which i think is great, it's what
the field needs and so you know I'm very
encouraged by this. It's been a long time
coming. There's been in recent years a
lot of progress in some key metrics and
this is this graph here shows one. This
is power produced by various different
sources and by the way I should point
out the the sources that are mentioned
on this graph that's by no means a
complete list right that's just a
representative list of some ways to
generate power in this spectral range
versus frequency and you can see, you
know this little blob down here is what
you could get from a silicon-based
terahertz sources in 2008 and here we
are in 2018 we've moved up by quite a
few orders of magnitude so and that's
silicon is one example. Other, other
material systems have improved in
similar ways and that gives you a lot of
hope that we really can eventually make
integrated systems where you generate
enough power to do something useful but
there are many important metrics right,
this is power. Power is not the only
metric of relevance here and this
cartoon is supposed to be and I should
credit Kaushik Senupta from Princeton who
made this graph and allowed me to use it,
thank you Kaushik. This is supposed to
illustrate you know some of the
different metrics that are relevant to
terahertz devices and where we are and
and and how what we have to think about
right it's not just power, its efficiency
right, or sensitivity of detection, it's
not just generating a terahertz signal.
But can you, can you change the, can you
tune it? Can you electronically control
the terahertz signal? Can you change the direction of the beam? Can you change the spectral content of
the beam? Can you change the polarization?
Can you do these things fast right. can
you do it in a programmable,
reconfigurable way. I think a lot of
systems that we can envision in the
future will need these sorts of
capabilities that we in
many cases don't yet have so there's a
lot of work still to do. There these days
there are many options for terahertz
sources and again this is only a
representative cartoon picture of a few
options, some of which may be familiar to
you people. They're certainly people in
this room who know about these sources
from Virginia diodes these Schottky
diodes and mixers that's the sort of the
electronics version but there are of
course terahertz lasers quantum cascade
lasers are just simply diode lasers that
operated at frequencies down below two
terahertz these days. You may not know
that one of the very first lasers ever
built was a terahertz gas laser from the
late 1960s. I don't know that you can buy
one anymore,
but up until a few years ago you could.
They're bulky and difficult to use and a
pain in the neck but they produce an
awful lot of power. If you really want a
watt right, add a watt, one watt, that's a
lot of power right. That's enough. When I
say a lot of power, what I mean is if you
put your hand in the beam it hurts.
Alright, that's my metric for
defining what a lot of power is and so
if you put your hand in front of a gas
left focused terahertz gas laser, it
hurts, you don't do that right. So on the
other hand you know, it's three feet long
and costs a lot of money and breaks off
all the time so you really don't you're
better off not using one I would say. If
you happen to have a spare hundred
million dollars, you can build a free
electron laser and get a lot, no hundreds
of watts probably but well, enough said
about that. My favorite source is this
one. This is where my career began or at
least my career in terahertz in the
mid-1990s the idea here is that you
start with a femtosecond laser which is
producing a very short pulse of optical
radiation in the near-infrared and you
down convert to terahertz using a
nonlinear optical process. I'm not going
to talk about the details too much this
is by well by now pretty well known. This
was a technology that was developed in
the late 1980s by groups at Bell Labs
and and at IBM and it is by now matured
to a fairly you know sophisticated level
where we can do an awful lot of
measurements with these systems and it's
very powerful because it's very
broadband right. So we really it's
continuous spectrum that spans the range
from well the low frequency end is
usually 100 gigahertz or so and that's limited
by the size of the optics actually and
the high frequency end can be several
terahertz two, three, four, terahertz is very
typical with these systems. So it's a
nice tool for making sort of
characterization measurements,
characterizing devices and things like
that. It's not really going to be easy to
imagine putting it in you know a form
factor that would fit in your pocket,
that's that's less likely but it's
nevertheless a useful tool which we make
use of extensively in my lab. So this is
the kind of thing that we measure, this
upper this picture in the upper right
here shows the terahertz electric field
versus time. That's a direct measurement
straight out of the instrument without
any processing, post-processing at all. So
it's a it's a photo current versus the
position of a mirror which corresponds
to a time delay and that's terahertz
electric field versus time you can see
it's a single cycle in the time domain
which means it's very broad in the
frequency domain and it's a powerful
tool I would say. It's also quite easy to
build one of these if you if you want or
you can buy one. So nowadays there's
quite a few companies that are selling
systems just like the one I showed you a
schematic of the whole system can be
packaged in a box about the size of well
this is about the size of a laptop
computer or thereabouts
including the laser it all fits in there
and you can make these terahertz time
domain measurements. They're not it's
still not cheap you know it's not the
cost of a cell phone, it's more the cost
of a extremely expensive automobile very
expensive automobile but ok you know
nevertheless it's a useful tool and as I
say you know there there's now a list of
I don't know maybe there's 10 or 12
companies in the world that are selling
these mostly for the research market but
there are also these are some of the
first terahertz systems that were sold
for a commercial market for actual
applications outside of you know
university based research so terahertz
imaging systems are used for quality
control in factories and these are the
systems this is this is the kind of
system that's used for that purpose. So,
here's you know so if you google
terahertz imaging you'll come up with
some sets of really interesting pictures
right some of these represent real world
applications that are in use today right.
This is this is from the news just a
couple weeks ago. This company through
vision is now going to be manufacturing
these passive terahertz imaging systems. It's
just a camera, it doesn't generate any
terahertz radiation, it's just looking at
thermal emission from people and this
will be for security screening and it's
already used for security screening in
European airports and it will be used in
US airports soon and then these are some
pictures that came from when I was a
postdoc at Bell Labs which I think are
funny but they're also interesting in
what they illustrate right. You really
actually can count the almonds in a
block of chocolate if you want to which
may seem funny but if you're the guy who
manufactures the chocolate and is
responsible for making sure there's
enough almonds in every bar well maybe
this is a good way to do it right. On the
other hand, there's a lot of hype right
so be very careful. There's also a lot of
stuff out there that is just absolute
nonsense right there you can very easily
find images like this on the internet
which suggests that terahertz imaging
could be used to see the bones in your hand
through living tissue which is complete
nonsense, don't get fooled by that right.
The absorption coefficient of water is
very large. Water is opaque you can't put
terahertz through it unless it's
extremely thin right kind of hundred
micron scale or less so the penetration
depth into you is very small in fact
with our terahertz systems it's easy to
just stick your hand in the beam and
your hand acts as a reasonably good
mirror for the terahertz beam so you can
you know you can build a system where
the beam makes a 90-degree turn, pull out
the mirror and put in your hand and you
know you get something like a fifty
percent reflection off your hand
something like that with penetration of
basically zero no less less than the
thickness of your skin in your palm. So
there's a lot of hype right so be
careful of that but there's also a lot
of real stuff and that's the fun stuff
right and so when you come to thinking
about the real stuff you have to say
well what are the tools we need to make
a system right and there are many many
challenges right and now I will focus
mostly on communication systems although
imaging sensing systems are also of
course very interesting in communication
systems. There are many challenges, there
are many challenges at many different
layers right at all layers right and
here I'm listing only a few right
there's many bigger challenges even than
the ones that I'm listing here the ones
I'm listing here happen to be the ones
that are of interest to me personally so
that's a rather biased choice but
there's lots of challenges right. What
are the what are the sources
we're gonna use, how we gonna modulate
the take the signals, how we gonna steer
the beam right, if the terahertz is a
laser beam, if it's really directed and
we want to do mobility right. Someone
walks across the room, the beam needs to
follow them and it needs to know where
to go and it needs to do that fast right
and that's that's a hard problem if you
don't have a phased array and we don't.
There's no phased arrays at these
frequencies that are at least not
commercially available I mean if you
have a lot of money and a big research
lab you can build one maybe maybe but
it's a challenge right. So let me just
give you an example right. Optical
modulators is or modulators is a nice
example right and here's where I want to
define what I mean by an external device.
I used that phrase earlier let me tell
you what I mean right. In in optical
systems you can modulate an optical
signal in two different ways, number one
you can turn the source on and off right
that's one way to do it but number two
is you can have leave the source on all
the time and then have a device in front
of it that just blocks it and unblocks
it right so that's an external device
and it turns out that in optical fiber
optic systems that's the way it's done.
For a variety of reasons that's better
and so the modulators are typically
external modulator and so it's this
here's a picture of something you can go
out and buy it's not even very expensive
this is a lithium niobate based device
you drive it with an RF signal and it
modulates the optical transmission and
you can you know run these things at up
to tens of gigabits per second right and
that's a fairly cheap device right it's
not expensive at all it's it's fiber
coupled it's easy to use you can
modulate both amplitude and phase or
both if you like the insertion loss
right when you put it in how much loss
do you get is typically a few DB, 6 DB
sort of a worst case. I guess the
modulation depth right on/off ratio is
20 DB or better in these systems right
and the speed is very high. Wouldn't it
be nice if we had something like that
for a terahertz beam something we could
put in our turrets beam and had all
those same characteristics we're nowhere
near that of course right so just to
give you an idea this was a paper from
colleagues of mine at Los Alamos which
is now about 10 years old where they
demonstrated that you could use a meta
surface which is just a tile a surface
tiling on a 3 5 semiconductor to do this
modulation
routine right you stick it in your
terahertz beam and when you put a
voltage bias on it you change the
density of electrons in this doped layer
and therefore you change the terahertz
transmission right and take the voltage
off the transmission recovers and
because of these little antenna patterns
that are tiled in this array
it's a resonant effect because that's
basically an antenna resonance. These
things are about lambda/10 in size
so they're sub-wavelength antennas and
you can tile the surface with them
that's called a meta surface right and
so then you get a resonant transmission
which you can turn on and off right so
to give you an idea of where the
terahertz field was or is right,this
paper got a huge amount of attention. It
was published in Nature, it's got you
know a thousand references or something
like that. It was such a big deal because
they had a modulation on/off ratio of 3
DB right factor of 2 between on and off
and the real and that was right that's
terrible compared to optical modulators
but compared to anything that had been
done in terahertz up to that point it
was better by a factors of a thousand
right I mean it was so much better and
it worked at room temperature which most
previous examples didn't so it's a big
step but it also shows you how far we
have still to go. So we my group got
interested in this technology and and
have done a number of kind of things
with it ok. So this is from that same
paper, this shows you sort of this is
you take your terahertz pulse this is
again one of those very short terahertz
pulses you shine it through the device
and you've got some voltage that you can
put on there to turn it on and off and
you can see that the transmission swings
on and off at this particular resonant
frequency in this case about 700
gigahertz by a 3 dB
right that's the ratio between on and
off. If you try to turn the voltage up
too much of course at saturation it
doesn't you don't get an each additional
change and because we're measuring the
electric field of the terahertz you get
not only the amplitude but also the
phase so you see this very nice resonant
response and that's a nice modulator
although 3 dB
you know we'd really like to do better
right. So my group got interested in this
and we realize that there's sort of a
good news/bad news story here for if you
want to use this as a modulator right.
The good news is by changing the size of
those little antennas I can tune that
resonance to any frequency I want so
it's very tunable right if I want a
modulator at 400 gigahertz I can make
one if I want a modulator at 600 it's
just scale the thing and make it a little
bigger that's all. It's reasonably fast,
it's room temperature, it's got a fairly
low insertion loss you know it's
controlled electrically not optically so
it's you know it has a lot of the
characteristics you would want. On the
other hand the modulation depth is not
great right three DB and the reason is
because there's sort of a small
interaction length and so the question
is is there a way you can improve that
and we've thought of it we've thought of
a way to do this which we've played with
a little bit and so one one step is to
instead of tiling the entire surface
with this meta surface array you can
break those pieces up into pixels and
imagine having one little pad for each
pixel so now instead of one meta surface
array I have sixteen of them a four by
four array each one of which can be
switched independently right so now
that's each pixel can be turned on and
off independently with a DC voltage so
that's now a spatial light modulator
right. Now the device that's broadcast
that's projecting this on the
screen right that's a spatial light
modulator, that that projector over there
has a spatial light modulator in it
right which has ten million pixels or
something like that Texas Instruments
has been manufacturing these for years
it's a great technology we got sixteen
pixels so you know we've got a little
ways to go but anyway it's you know this
was the first try and it works right so
here's a picture an image of that device
a terahertz image in which we're
wiggling two of the pixels on and off
and leaving the other ones grounded and
you can tell. So to make the device I
didn't mention how we made this this is
single step photo lithography on gallium
arsenide that's all it is it's it's it's
a put down a mask you do for the
lithography you do lift off that's it
right and the wafer is just semi
insulating gallium arsenide wafer with a
one micron thick doped EPI layer on top
so that's a wafer that you know if you
have a friend with an MBE machine he
will make it for you or maybe you can
even buy it right so it's a fairly
simple processing step and the feature
size the smallest feature size here is
you know a couple microns so it's sort
of 1980s era lithography right we don't
need a fancy stepper to make this work.
Is it the fact that it's gallium
arsenite important? No not necessarily.
Oh the thickness, the thickness of the
gallium arsenide the the substrate is
it's just a it's a semi insulating
substrate it's just a transparent backing
so it's just for mechanical support. The
one micron layer yeah that thickness
does matter if you make that right
because what you're doing is you're
creating a depletion zone by DC bias and
so if you make it too thick then you're
not going to deplete all the way through
and so then you'll still have a strong
attenuation and if you make it too thin
then you lose contrast so there's an
optimization there and one microns about
right. Okay so here's a device a
pixelated device which shows this sort
of modulation in you know and every
pixels the same of course so you know
this is sort of the spectrum each one's
about the same and once again about a
factor of two on/off ratio which is you
know for terahertz and not bad we'd
like to do better but what we can do is
modulate the terahertz wavefront right
and so wavefront engineering is
something that everyone knows is going
to be important for for wireless systems.
This is as far as I know the first
example of doing that other than
mechanically I mean you can put your
hand in the beam and block it and I call
that wavefront modulation right sure but
you know this is an electrical system
where and this is I love this example
because in this case you can see our 16
pixels what we're doing here is
grounding two columns the ones shown in
white and the other two columns were
flashing them on and off at a few
kilohertz and just detecting the
difference and so what you if you think
about what that is it's it's a Young's
double-slit experiment and we're just
turning this changing the widths of the
slits at three killers and detecting the
difference and so if you can calculate
you know and and I gave this problem to
a freshman in freshman physics who had
just learned about diffraction and the
Young's double-slit experiment right the
famous Young's double-slit said
calculate the difference between this
lid and this lid and that's the red
curves and that was so exciting for the
freshman that he decided to major in
physics which I consider a victory right
that's yay we win. okay so anyway you can
you can modulate terahertz wave fronts
right that's that's nice that's a good
thing to be able to do but what we
really want to do is improve our
modulation depth and so one way to do
that is to make your pixels instead of
making them square make them long and
thin all right so this is now as the
same sort of device but if the pixels
are now columns so each one of these
gray is a column and then the red is a
column and they're alternating right so
if you think about what that is
that's a bunch of columns each one with
its own bias if I bias them all the same
then this device is uniform across its
face and I'll get either transmission or
not depending on whether I've biased
them all zero or bias them all ten volts
right but if I bias them alternating now
it's a diffraction grading and it will
diffract light at some particular angle
so if I put my detector over there
I should see nothing if the device is
biased all the same and I should see
something if the device is biased
alternating and so something divided by
nothing is a big number and there's
there's your contrast instead of having
3 DB right you can do better that's
the idea. So this is just a zoom in on
the device the the split ring resonator
the little antennas are a little bit
they look a little different but it's
basically the same idea there just
LC resonators. In an array, the width
of the column is about one wavelength
this device was designed to operate it
for I think 400 gigahertz and so that's
about one wavelength for 400 gigahertz
and so it works. So you can shine your
terahertz beam through it and look at
the signal as a function of the angle of
the receiver and indeed what you see is
if you look straight ahead you see you
know this metamaterial resonance which
i've already showed you you see this
nice resonant behavior when the thing is
biased when we don't bias it we don't
have a depletion region and the
resonance goes away okay so that's the
resonance that's designed but now if you
look at a different angle what you see
is this big peak comes up if alternating
columns are biased which is not there if
all the columns are biased right. So in
other words you see the fraction peak
and you can calculate the angle at which
that should occur using again the
freshman level diffraction theory and it
works perfectly okay so it's just a
diffraction grating and so now what we
can do is we can park our detector over
here at the location of that diffracted
light and we can measure with alternate
the spectrum of what comes out with
alternating columns biased verses all
columns biased right and this is the
spectrum that what you see and indeed
what you observe is an on/off ratio
greater than 20 dB
so that's a big improvement in the
contrast for the modulator right. But on
the other hand what I'm not telling you
here is what's the efficiency you know
what fraction
the light is actually diffracted into
this direction versus just go straight
ahead. It turns out again you're working
with a thin film so the diffraction
efficiency is still fairly low so we
still have some work to do to turn this
into a really useful modulator but it's
got most of the characteristics other
than efficiency most of the
characteristics that you want for an
external modulator so it's still work in
progress but I think there's some
promise there. So now let me switch gears
a little bit and instead of talking
about these meta surfaces which I think
are very promising for a lot of
different things let me talk about
waveguides
so in this room people who are familiar
with Virginia diodes think of wave
guides as little holes in a piece of
metal right little circular pipes right
well that's a very nice waveguide if
you've got one frequency but if your
bandwidth is like mine you know three terahertz,
that's a not a very good waveguide
because it has a cut-off it's got
dispersion it's not a very useful device.
So in my world most people use something
which is open on the sides so then
you've got a pair of metal plates so
it's basically just again a metal
waveguide with air but in this case
instead of being closed on all four
sides it's open on two sides and what
that does is it removes the dispersion
so now the lowest order mode will
propagate without dispersion at all
which means if I send in a single cycle
pulse through one of these wave guides
what comes out looks exactly the same as
what went in. It's reduced a little bit
in amplitude from the loss but the shape
of the waveform has not changed at all
and that's the tool that most people
actually use in in my in the optics
community it's a nice tool for
spectroscopy because your waveform isn't
distorted by the wave guide itself. On
the other hand if I rotate the
polarization by 90 degrees now instead
of exciting the TEM mode of the
waveguide I'm exciting a transverse
electric mode which is dispersive and
has a cut off and you can see the
difference in the waveform it's spread
out now instead of being a single cycle.
I've taken all that energy and spread it
out over hundreds of cycles really it
right I'm only showing you the first 50
or so but it rings for a long time right
because the speed of light now depends
on frequency in this mode and so you
know that spreads out things a lot which
you would think would be a tremendous
disadvantage but it turns out you can
actually exploit that because that now
means that the space between these two
plates acts
like an effective dielectric material
it's just air but it acts like a
material with a refractive index less
than 1 in other words with a speed of a
phase velocity greater than C and you
can actually exploit that to make
devices so here's the basic idea rather
than just one pair of plates we're gonna
make a stack of plates so that we can do
things in the third dimension right and
so here's a picture of a device that we
made in my lab this is a stack of metal
plates with precise spacing all the same
right the you're looking at it in the
direction the beam would propagate the
beam would propagate into this picture
the width into the board is
you know a centimeter or less maybe and
this stack of waveguides acts like a
material with a refractive index less
than one in other words with a phase
velocity greater than one and it's
frequency dependent and you can actually
use this now it turns out this idea is
actually old this idea was proposed in
even in the 1940s to make lenses they
called them lens antennas I would just
call it a dielectric lens artificial
dielectric lens but you can see the size
makes it a little bit impractical right
so if you want to do this for 4
gigahertz that's an actual normal sized
human being right that's not a little
doll that's like a real person right so
you can see that's not a tremendously
practical device but now if you scale
the frequency up and the same idea
that's the device 470 gigahertz fits in
your hand all right it's this big so now
you have something that's really
practical that you can actually use to
make terahertz optics right terahertz
components that are passive but give you
capabilities that you don't have in any
other way now can we make terahertz
lenses out of plastic sure we can but
plastic even the best plastics absorb a
little bit right this thing absorbs
nothing because it's air right I mean
you have to worry about the water vapor
residences but other than that it
absorbs not at all so it's a lot almost
low lossless lens if you like. We can
also configure it in a variety of other
different ways. For example, if we take a
stack of those waveguides and now in
this case the stack is coming out of the
screen at you right so there's a stack
of plates coming out of the screen at
you and then here's another one exactly
the same but at a different angle and we
illuminate it from the side with the
terahertz beam. Now just because of
Snell's law refraction causes the beam
to change
direction and different frequencies will
change by different angles because their
phase velocity depends on frequency and
so now you have all of your frequencies
in your beam spread out where you can
access them individually which means
you've got a filter right an arbitrary
filter so I can just stick metal objects
in there and filter out pieces of my
beam so I can make a high-pass filter or
a low-pass filter with just metal and
air. You can't get cheaper than that
right it's passive but it's incredibly
robust
and it's completely reconfigurable; we
can do high pass, we can do low pass, we
can stick things in the middle of the
beam like this to make a band pass or a
band stop filter and this is arbitrarily
tunable and scalable to essentially any
frequency and not only that I mean I
don't know if it matters so much for you
know what for this kind of applications
but it's also can handle arbitrarily
high power it's never gonna damage
because it's just metal and air right so
it's a really a nice idea for a
reconfigurable filter which I think
could be really useful and you could
imagine packaging in this in a way I
mean we did it of course you know in
some bulk way and using a bulk optical
components so it doesn't look pretty but
you could easily imagine packaging this
in a way that would really make it very
robust. We've done other things with this
same idea so here's a demonstration of
the same sort of waveguide stack as a
polarization manipulation device so
there's a quarter wave plate, we can
rotate, we can turn linear polarization
to circular polarization, we can make an
isolator by combining a quarter wave
plate and a polarization filter and so
this is a terahertz isolator with
isolation of about 50 DB in a narrow
band I mean the bandwidth here's a few
gigahertz but it's 50 DB of isolation at
any frequency you want just by changing
the spacing between those plates you
tune the frequency so I don't know is
a 50 DB isolator useful? Could be, right?
And this is you know it's it but it
costs you know nothing to manufacture
right it's almost free because it's just
thin steel plates screwed together. This
is another interesting waveguide based
idea which is a little bit more
sophisticated a little bit harder to
make
but it gives you an interesting
advantage which is that it's an active
device you can electrically control it
and change his property. So this is again
a filter. In this case what we've
done is we've taken two waveguides and
coupled them together so here's a
waveguide on the top here's a waveguide
on the bottom and they're coupled
together with a gap in between and
actually that gap has in it a little
glass capillary which is filled with an
electrically conductive liquid and in
this case it's it's saltwater or
something equivalent and then a liquid
metal plug so it's an indium gallium
liquid plug that and it turns out if you
put a voltage on the liquid that plug
will move back and forth with just a few
volts so you can actually use this as a
channel add/drop filter because the
frequency that couples between the two
waveguides depends on the size of that
gap and so you can design it to a couple
of one particular frequency. So for
example here's one particular frequency
is coupled out of the upper waveguide
and into the bottom one. So it's
an add/drop filter with a single channel,
now we can make multiple channels as
well and then you've got a multi-channel
add/drop filter. Here's one with three
channels which and this graph here is
showing you the bit error rate of
digital data sent through on the upper
waveguide and measured on the either the
upper or the lower one depending on the
switching so this is an electrically
switchable channel add/drop for
terahertz operating at about hundred
this one's operating about a hundred
gigahertz but again it's scalable you
just scale the size by a factor of two
and all of a sudden you're at 200
gigahertz ,there's nothing in here that
won't scale. In this example the spacing
between the two waveguides is a little I
think a little bit over a millimeter so
it's you know single mode waveguide
that's all it is
millimeter and a half maybe I'm not sure
I don't remember exactly the numbers. So
we can also use these parallel plate
wave guides as a platform for
multiplexing so this is a leaky wave
device here. We have a pair of parallel
metal plates we cut a slot in one of
them and that lets some radiation leak
out and that turns out to be the basis
for frequency multiplexing in the
terahertz range so let me show you how
that works. So the device here's a
cartoon of what's going on you shine
light in over here it comes out over
here and the angle at which it emerges
is directly related to the frequency
so if you've put in multiple frequencies
then each one will come out of its own
angle and you can imagine distributing
different signals to different clients
according to the frequency at which
they're going to receive right so
there's a one-to-one relation there it's
not linear but it is at least one-to-one
between frequency and angle which is
shown there and so this is how you would
use this device as a transmitter right
and just reverse the arrows and it's a
receiver right. So this is I
figured I should show a picture of at
least one experiment this is sort of
what it looks like in my lab. Here's our
terahertz transmitter, this is you can't
see the optical fiber coming in on the
back here with the femtosecond laser but
it's there. These are plastic lenses
which focus the terahertz onto this
device which I can zoom in on over here
and that's our leaky wave device in the
receiver configuration. So we're shining
light into the device and the light will
only folk a couple into the device if it
satisfies that angle frequency relation.
So you can measure this in a receiver
configuration like I just showed you and
in fact of course it works beautifully
right and notice the range of
frequencies over which we're working
here, from about 50 gigahertz up to about
500. This is using the device in the
other configuration where you've got
light propagating in the waveguide and
looking at what comes out which again is
the exact same frequency angle
relationship and so now we can imagine
putting in two different frequencies
each of which has its own independent
data you know. On a terahertz carrier two
different terahertz carriers with two
different frequencies and each of them
will come out at a different angle and
therefore they will be D multiplexed
right and broadcast to some user 10
meters away or whatever who's expecting
a particular frequency and that
determines the data he gets. So that
works, turns out.
So here's coupling two different
frequencies in. This is the opposite idea
where you put two frequencies in from
two different angles and you see that
indeed if you shine light on this thing
from two different angles you get two
different frequencies and of course one
of them, oops one of them tunes as you
tune the angle you tune the frequency
again just as you would expect and this
works with data. So we then took two
different data streams at two different
frequencies, put them into one of these
devices let it propagate in the
waveguide for
centimetres and then another slot where
it can couple out and we actually can
get pretty good eye diagrams and
bit error rates of about 10^-5 or 10^-6 on
those two channels independently and of
course there's no crosstalk because the
frequencies are different right and this
is a passive device there's no way for
two frequencies to couple to each other
so it's a very robust multiplexer and we
did up to 50 gigabits per second
through this mock steam device.
There's an interesting effect of
detector aperture here which I think is
actually very general it's not I think
any multi frequency terahertz
communication system is going to suffer
from this problem. Well I don't know if I
should call it a problem, it was going to
experience this issue and that is that
if you have a very broad band of
frequencies and you've got anything in
your system that has diffraction or
refraction there'll be a frequency
dependence to the angle that's
inevitable. In this device, it's built-in.
But in any device, it's pretty much
inevitable and that means that your
detector is going to see different
frequencies across its face and that
changes your bit error rate. That
actually impacts your bit error rate and
if you have a MIMO system it's gonna be
even worse right. So this is something
that you really have to think about this
is some experimental measurements and a
simulation to show that and what the
effect that has in our system is as we
raise the data rate from one gigabit per
second to six gigabits per second the
angle over which we can receive the
signal decreases, because of this effect.
So that's a disadvantage but maybe it's
also an advantage if you have a MIMO
system you could actually use this to
improve your data rates with clever
signal processing which is something I
know nothing about but I'm sure it
worked because those guys are smart
right. Okay, so let me move on to other
measurements that we've made in my lab
recently since I moved to Brown a few
years ago we bought this system here
which is familiar to many of you this is
just Virginia diodes tools and we can
broadcast signals at 100 gigahertz 200
300 and 400 with reasonable power I mean
compared to the femtosecond laser based
systems way more power way way more
power but not tunable right single
frequency and we can modulate at a
gigabit per second so we can actually do
tests of sort of simulated real channels
right real wireless channels and we have
a license from the FCC to do outdoor
tests up to 400 gigahertz. So we can
actually do this legally which is kind
of nice let me show you this picture
here. This is a little bit,
that's a little distorted that picture's
a little weird. Okay so this is
an interesting this summary and I have
to apologize it's a bit out of date but
this shows a bunch of different
publications, we tried to capture them
all which have done different data rates
and different frequencies and here's a
distance and frequency and this one's
distorted in a way which I apologize is
you can't really see
that's strange why that happened but no
the bottom line here is that people are
doing measurements all the way up to 700
gigahertz at ranges from you know
centimeter to kilometer. Well of course
the higher frequency you go the shorter
the ranges but you can see even at the
at the low frequency ends even up to a
couple hundred gigahertz. People are
doing kind of hundreds of meters or
kilometer scale tests outdoors both in
we're doing you know indoor tests and
outdoor tests and I want to point out on
this graph over here which comes
from this paper so it's a couple years
old by now but all of the data points on
here are labeled according to the
country in which the experiment was done
and you will notice the very few of them
say U.S next to them. I'm hoping that that
situation will change because we need to
catch up. So we're trying to catch up in
my lab by doing some measurements and
here's one example where we just took
our DDI system and took it out into the
hallway outside the lab where there's a
cinderblock wall and just pointed it at
the wall and bounced it off the wal,l it
works. This is a the wall is acting like
a mirror so it's not scattering very
much. It's a specular reflection off the
wall and these are data curves at 100
gigahertz 200 300 and 400 and you can
see that in all four cases we can
achieve a bit error rate of 10^-9 which is pretty good right? We
can distinguish where the loss is coming
from by putting a conformal aluminum
foil plate or you know just aluminum
foil pressed onto the wall so that
preserves the surface roughness but
eliminates the penetration of the signal
into the cinderblock
and so therefore we can distinguish
between scattering losses and absorption
losses and what we find is that in this
particular case the absorption
accounts for like 10 out of the 15 10
out of the say 14 DB of total
something like that so the absorption is
really the biggest effect. The fact that
the surface is rough doesn't make that
much of a difference it's a few DB which
surprised me I thought it would be the
other way around
so that's it's nice you know it's nice
to learn things right. So this
really works I mean you really can
bounce the signal off a wall and in fact
you can do it twice if you want to this
is shows you the signal being bounced
off of the wall and then bounced off the
wall again. This transmitter and receiver
there's no direct line of sight between
them. They cannot see each other they're
around a corner but you can go bounce
off the wall twice and these are
specular reflections right angle of
incidence equals angle of refraction. The
wall is acting like a mirror and with
you know it's a loss there certainly is
some loss but even at 200 gigahertz
we can with a very modest output power
from the transmitter we can get a bit
error rate of 10^-9 going around a corner. So this really
suggests that these links are pretty
reasonably robust, that it's actually got
a good chance of actually working. We've
also done some outdoor tests this is
outdoors above you know just a sidewalk
and this is outdoors above a grass field
this is on campus at Brown University
and actually you see a difference
between propagating over a sidewalk
versus propagating over grass because if
you're propagating over grass then the
specular reflection off the ground is
eliminated because the grass is wet and
so you don't see interference from that
and so your signals are actually a
little better over the grass for that
reason, not very big difference because
it wasn't a very wet day but you know
you can imagine if we did it the day
after a rainstorm it would be different
right. We've also measured snow. This is
something that I could not do when I was
a professor in Houston because it never
snows but moving to Brown has its
advantages right we could we knew there
was a snowstorm coming, we looked at the
weather, we said there's snow coming
tonight and I said to my postdoc go put
the stuff outside and stand out there
all night and measured, well not all, he
wasn't out there all night but he's a
hero, he's an absolute hero because he
did these measurements. It was cold and
someone you know he had to stand there
with an umbrella over the equipment to
keep it dry right and everything oh what
a nightmare anyway he did it and what we
found is that at least for this
particular snowstorm, snow is bad because
the snowflakes scatter a lot, they absorb
but they also scatter a lot because
they're about a wavelength across and so
the scattering loss and the absorption
loss are very comparable for the
particular snowstorm on this particular
day I don't know if I could generalize
that but yeah three DB loss in 10 meters
or something which is a lot right so
snow is, snow is tough, worse than rain I
think. So now let me finish up by asking
this question right and and I think we
all know that wireless security is a big
deal right when you're broadcasting in a
120 degree sector and
your intended receiver is over there then
an eavesdropper can sit over there
and hear what you're saying right. So how
do you fix that problem? Well of course
there's a lot of ways to fix that
problem, you can do a front engineering,
you can encode you know encrypt your
data, I mean there's a lot of things
right. Everyone has said for a long time
that if you go to higher frequencies and
your beam gets narrower then your
security should improve right because
the eavesdropper can't sit over there
anymore right the eavesdropper has to
sit sort of in between you and the
transmitter which means that the
eavesdropper will block the transmission
which means that the eavesdropper will
be noticed right and that if the beam is
narrow enough that should mean that
eavesdropping is impossible right. That's
kind of been the conventional wisdom
which people have said for many years
right so if the beam is directional
enough this conventional attack will
fail right due to blockage. The
eavesdropper cannot avoid blocking the
intended receiver and thereby raising an
alarm right so we decided to test this
right and the bottom line here
is that you would never design a system
where the beam size is exactly equal to
the beam to the aperture of the receiver
because then you have no margin for
error in aiming so it's got to be bigger
and if it's bigger that means that an
eavesdropper can somehow get in there
right and so we did this test at three
different frequencies 100 gigahertz 200
and 400 and you can see our beam angular
width gets quite small right at the
highest frequencies it's less than 2
degrees angular width this data shows
the 200 gigahertz but all three of these
are very narrow. So you know if under
ordinary circumstances Eve would sit in
there and her shadow would fall on Bob
and then Bob would know she was there
and so the eavesdropping attack would
fail. The question we wanted to know is
can you do that? Can you instead of
putting your receiver in the beam can
you put a little piece of metal in the
beam something small that casts a tiny
shadow so that Bob doesn't get
and therefore Bob doesn't get blocked
but still large enough that it scatters
enough signal towards Eve so that she
can actually decode the signal right. Is
there a sweet spot where the object is
small enough to not cast the shadow but
big enough to scatter enough radiation
so that the eavesdropper can see it and
when we started out these measurements
the answer was not clear that would that
there would be such a sweet spot right
but it turns out there is at all
frequencies. You can make this work at
all frequencies, if you're smart. So
here's our testbed right it's a
transmitter and a receiver and a piece
of metal in between I mean that's that's
what we call a testbed at Brown
University right. The hardest part of
these experiments was finding a room big
enough to do these experiments which no
one was using for something else and we
had to wait till Christmas break to do
it which worked and so this shows you
some sample data of scattering signal
and this is this shows you the ratio of
the broadcast the scattered signal
received by Eve divided by the power
broadcast by Alice as a function of the
position of the receiver, in other words
the angular position and you can see
it's four these are four cylindrical
objects so cylindrical objects will
scatter in all directions so you can see
there's scattered signal at all angles,
alright. The dashed lines are predictions from
numerical ab initio numerical
calculation that assumes nothing at all
just you know the power and and the size
of the object so the physics is right so
here's this is the interesting result
right so this shows you how much of the
beam is blocked, that's the solid curves
and how secret is it in other words how
much what is Eve's signal-to-noise
compared to Bob's right. If Eve is
getting the same signal-to-noise as Bob
then the secrecy capacity is zero
meaning Eve decodes the signal 100% of
the time right so you can see that the
secrecy capacity does indeed go to zero
for larger objects but of course the
blockage is large too and so that would
be a fail for Eve right, Eve has failed.
But that's if you put the pipe right in
the middle of the beam where it's
guaranteed to cast a shadow on Bob. If
you move it over just by one beam
one pipe diameter now Eve wins right
you just move it over by one diameter
one pipe diameter and so all of a sudden
Eve wins right the blockage is still not
but it's much smaller and the secrecy
capacity is quite small so Eve can
actually win and this is with a
cylindrical object which means Eve's can
put herself at any angle she wants and
get the equal amount of signal
almost. If she uses flat plates it's even
worse because now Eve you know the signal
is all going in one direction now it's
just a mirror basically right a flat
metal plate is a mirror and so now all
the signals so Eve has lost her freedom
to be at any angle but her
signal-to-noise is much higher and the
blockage is basically zero right if the
plate is off axis so if you put a flat
metal plate in here you're not shadowing
Bob at all so the blockage is basically
zero and the secrecy capacity is also
very low so Eve basically has less
freedom but more success and this is
works at every frequency right every
frequency we tested. So it doesn't mean
that terahertz links are not secure they
are more secure than links at lower
frequencies that is still true but you
can't say they are 100%
secure because the beam is narrow which
people in the past have said and that's
kind of the lesson here is that you
still have to think about wireless
security. It gets even worse if Eve is
clever and has the freedom to engineer a
beam splitter that splits off a desired
fraction of the beam and she can choose
that fraction then she wins a 100%
of the time it turns out there's
a sweet spot here where she chooses just
the right transmission coefficient for
her beam splitter the blockage is too
low the back scatter is too low there's
no win mean Eve wins every time the bad
guy always wins. So if the bad guy is
sufficiently smart and agile and clever
then they will win no matter what you do
so you know unless you have other
mechanisms in place. Obviously you know
you're gonna encrypt your data right I
didn't mention anything about that right
so this is just considering whether or
not you know as the beam gets narrower
does it really get impossible for an
eavesdropper to grab a piece of it and
the answer is no even if the beam is
very narrow it's still possible. Even for
optical communications, even if you
communicate with lasers free space
lasers this is still true right you
could always still catch some of the
scattered light right. So this is sort of
a vulnerability note which is exists no
matter what frequency you're at
that people maybe previously haven't
considered so much. Okay so let me wrap
up. So my conclusions are sort of I guess
pretty obvious right. Number one I
believe that terahertz communications
will be inevitable, we will be using
these frequencies above 100 gigahertz
for wireless data links in the future so
we better get working on it. Now there
are many challenges I've only listed a
few of them right there are many many
many challenges that I have not
discussed so this is a big problem
you've heard about some of them from
some of the other speakers in this
series and so I'm hoping that my talk is
sort of a little complementary to theirs
in the sense that I'm focusing on other
aspects of the challenge than what
you've previously heard about. I think
these external devices are maybe not the
only answer but they're promising
platform for doing cool things with
signal processing which may be better
with respect to say energy efficiency or
speed or price or whatever. The channel
characteristics I think there's still a
lot that we don't know. There's a lot of
conventional wisdom that needs to be
overturned, there are still many people
in the world who believe that you can't
bounce the terahertz beam off the wall and
you really can no matter what your
frequency is you really can and you know
sort of my final answer here is that
when you come to talking about links
above 100 gigahertz this is not merely
microwaves with a few extra zeros right.
It's more subtle than that, the
differences there are significant both
in the physics and in the in the devices
and in the implementation it's different
in many ways
and that makes it interesting and fun of
course so thank you for your attention
Ted Rappaport: Thank you very much Professor Mittleman for that terrific talk, taking us from
optics to these devices you've made
waveguides, multiplexers, bouncing off
walls at 10 Hertz frequencies, that's
terrific. I'm sure we have some questions.
Questions for Professor Mittleman?
Prof Mittleman: The
question is would it be possible to
improve the link security from a
hardware standpoint and I assume you
mean from a physical layer standpoint. So
there's a variety of ideas that people
are exploring certainly you know at
lower frequencies people talk about
wavefront engineering right.
This idea of you broadcast a wavefront
that goes in many directions but the
data is only decodable in one direction
and in other directions it's noise. That
may that in principle oughta work at
terahertz 2 although it's a more subtle
problem because in this case you know
your beam is very narrow so you know you
the change has to be much more rapid as
a function of transverse position which
might get bit really hard so I don't
know how efficient how effective that's
gonna be that I think is still an open
question. Ted Rappaport: Other questions? So I've got one for you Dan. Great talk.
Really exciting. We're doing some of
these same kinds of measurements but
we're not at 300 and 400 gigahertz,
we're at 140 gigahertz.
Prof Mittleman: Yep. Ted Rappaport: And we've got this the wireless class actually looking at the scattering
models and we found the same thing that
the rough scattering reflection
coefficient model shows you'll get a lot
of energy bouncing off in the reflective
direction from rough surfaces. What have
you found to be in the literature the
best scattering models for up in these
terahertz range because the mobile
industry nevers really considered
scattering before because right really
such a low-level effect down at mobile
frequencies but as we go to
wavefront terahertz we need to know what
scattering is. What have you found in the
literature? Prof Mittleman: So the subject of
scattering off of rough surfaces is a
big deal in optics as you can imagine, people have developed extremely I
mean there are full text books on this
subject and Ted Rappaport: not a lot of work at how it
works at terahertz and sub terahertz.
What have you seen? Prof Mittleman: That's right so
there's been a little bit of work we
published a little bit of on that about
10 years ago,
one model which is basically a simple
you know estimate your surface roughness
with a single parameter that's sort of
the RMS height variation and then just
sort of exponentially attenuate your
beam according to that right and that
that works okay if your roughness isn't
too much scattering. Ted Rappaport: Direct scattering? Prof Mittleman: Yeah, but then if the surface gets
rougher and actually we're doing some measurements
on this right now if the surface gets
rougher we've we've engineered some
artificially rather rough surfaces and
made them out of metal so that we don't
have to worry about attenuation due to
prop you know penetration. Then you see
multiple scattering phenomena on the
surface so your beam will actually the
photons if I can use that word right
well actually scatter more than once on
the surface before they leave and that
leads to interesting correlations and
you can use that because actually what
we've done now is we've shown that even
in the non specular directions a few
angles a few degrees off you can still
get data transmission which is nice but
you need the surface to be pretty rough
so and there are models for this like I
say there are textbooks on this people
have done and what we're trying to do is
adapt models that have been developed
for optics and use them at terahertz. I
think it's an important problem because
among other things what we see on these
rough surfaces is polarization rotation
so the beam comes in polarized this way
some fraction of it comes out this way
and if you're trying to do polarization
multiplexing that's going to really
really ruin your life right so so we you
know one needs to characterize this because these sorts of rough
surfaces I mean it's not so rough it's
rougher than a typical indoor wall but
it's not for outdoor it's the kind of
roughness you're going to see all the
time. Ted: That's right. So other questions?
Well your students were out in snow
you're up at 500 gigahertz, you're down at 50
gigahertz you're sending 50 gigabit
per second. Let's give Professor Mittleman
a huge thank you for this great lecture
in the seminar series. Thank you Dan.
Prof Mittleman: Thanks for the invitation. I really
appreciate it.
