A Comprehensive Study of A GaAs Spin Polarized Source
An overview of spin electron physics and the generation of spin polarized sources from GaAs shards. Experimental polarization results and description of the system used to measure them.
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- [00:00:00.960]Hi, I'm William Newman. And I will be
talking about gallium arsenide (GaAs),
- [00:00:04.740]and its pivotal role in the
production of spin polarized electrons.
- [00:00:08.490]This talk is meant
for the general public.
- [00:00:10.230]So I apologize if any of this
information isn't new too you.
- [00:00:15.660]First, we will begin
talking about electrons,
- [00:00:18.360]their property is spin and an
electron beam's polarization.
- [00:00:22.200]We will then move on to discussing the
history of GaAs and its
- [00:00:25.590]capabilities as a source of spin,
- [00:00:27.840]polarized electrons, our GaAs
source that we use for this experiment
- [00:00:32.370]and our results and
outlook for the future.
- [00:00:36.690]Electrons have a property called spin,
- [00:00:39.210]the spin of electron
points along a direction.
- [00:00:42.510]This is where it's
often best to visualize the
electron has a ball that is spinning.
- [00:00:47.460]Despite that analogy being far from
reality. Electrons have what we call one
- [00:00:52.440]half spin. This this is an important
property for any measurement involving
- [00:00:57.360]electron spin. Essentially
this one half quantity,
- [00:01:00.510]specifies that the electron can be
measured in one of two states with
- [00:01:05.160]spend parallel or anti-parallel
to our measurement direction.
- [00:01:10.380]This is commonly referred to
as up or down respectively.
- [00:01:14.160]To elaborate on this,
- [00:01:15.450]if you're going to make a measurement of
a quantity dependent on the spin
- [00:01:18.290]electron, you will only be able to
measure that quantity in two states up or down.
- [00:01:24.540]We then define the polarization of electrons
as the number of electrons with spin
- [00:01:28.770]up minus the number of electrons
of spin down divided by the sum.
- [00:01:33.150]The polarization for one electron
is always one or minus one.
- [00:01:37.620]For two electrons,
- [00:01:38.340]we can measure both spins as spin up
and thus our electron polarization
- [00:01:43.110]is one. Alternatively, we can measure
both electrons having opposite spin.
- [00:01:48.000]So our polarization for
the two electrons is zero.
- [00:01:51.930]This type of measurement is then scaled
up for measuring multiple electrons and
- [00:01:56.190]then percentage polarizations become
more applicable. For example,
- [00:01:59.940]if we had a beam of electrons with 30%
of them is being spin up and 70% being
- [00:02:04.830]spin down, we have a
net polarization of 40%.
- [00:02:10.950]Now that we've covered the basics of
spin and the measurement of polarization,
- [00:02:14.820]we move on to GaAs and how
we can generate a source of polarized electrons.
- [00:02:18.780]Briefly, before we
cover GaAs. First,
- [00:02:23.070]we need to discuss circularly
polarized light, like electrons,
- [00:02:26.940]photons have spin.
- [00:02:28.080]The light we measure can either be
left-handed or right-handed polarized light.
- [00:02:32.700]The two animated images you see show
how the electric field of light rotates,
- [00:02:36.900]depending on the helicity of light.
- [00:02:39.120]This helicity is important
when describing light's
- [00:02:41.670]interaction with materials and
electrons in that material.
- [00:02:45.750]We can use this light to control how
we interact with it and influence the
- [00:02:50.400]spin of electrons within
the material when they
- [00:02:53.100]when they interact with
light of a given handedness.
- [00:02:57.660]What makes GaAs especially interesting
in electron spin physics is that with
- [00:03:02.500]circularly polarized light,
- [00:03:03.880]you can create a source of electrons
where there are three times as many
- [00:03:08.320]electrons with one type
of spin than the other.
- [00:03:12.040]This results in a
theoretical maximum of 50%,
- [00:03:17.110]the polarization 50%. To achieve
this type of emission, however,
- [00:03:21.190]is difficult with GaAs. Typically,
- [00:03:24.010]what you would have to do is you have to
apply layers of cesium and oxygen onto
- [00:03:27.970]the surface of GaAs
in order for the electrons,
- [00:03:30.850]to be able to obtain enough
energy to escape out
- [00:03:33.430]into vacuum and away
from the bulk material.
- [00:03:37.390]This process is called activation and
it takes quite some time to accomplish.
- [00:03:42.370]It also requires a good vacuum
system and a clean sample.
- [00:03:46.150]The difficulty of this process is
what makes it not applicable to all
- [00:03:49.780]situations. Electron emission
can be achieved easily though,
- [00:03:54.520]using nanotips.
- [00:03:56.020]So our research group has looked towards
creating GaAs nanotips so
- [00:04:00.010]that we have a source that
can produce electrons easily.
- [00:04:03.460]This next section of
the talk we'll go over.
- [00:04:05.320]What is required to achieve
electronic emission from these tips,
- [00:04:09.820]images shown here are nano
tips made from tungsten,
- [00:04:13.180]and these can provide us with the
structure to easily emit electrons
- [00:04:17.800]sizes ranging from a few
hundred to a few thousand atoms.
- [00:04:21.880]Normally continuous wave light is
not strong enough to be able to emit
- [00:04:26.860]electrons. To do that, we
must employ pulsed light,
- [00:04:31.270]which is very strong, with electric field,
- [00:04:33.730]very strong for very
short periods of time.
- [00:04:37.270]This in combination with the nanometer
size structure works symbiotically to
- [00:04:41.860]emit electrons strongly.
- [00:04:44.830]We try to create similarly
small structures with
GaAs rather ineloquently.
- [00:04:50.200]We carefully and scientifically smash
gallium arsenide wafers to create these
- [00:04:55.000]which then when looked under an electron
microscope reveals that there are small
- [00:04:59.170]fine structures that are under
the size constraints needed for strong
- [00:05:02.890]electron emission.
- [00:05:05.860]Now that we have covered the details of
what we are trying to measure and the
- [00:05:09.730]source we want to achieve our goals with,
- [00:05:12.100]we can delve into the experimental
setup we have and the results of recent measurements.
- [00:05:18.940]With a pump laser,
we give enough laser power
- [00:05:20.926]to an oscillator which
generates pulses of light in
- [00:05:23.260]the infrared region.
- [00:05:24.790]We then send these pulses of
light through polarization optics,
- [00:05:28.000]which allows us to control the helicity
of light and make them circularly
- [00:05:31.030]polarize. We then send this,
- [00:05:33.370]this light into the chamber where it's
reflected off of a mirror inside that
- [00:05:37.690]focuses onto the GaAs.
- [00:05:39.940]Electrons emitted are then
accelerated towards our polarization
- [00:05:44.680]measurement device.
- [00:05:46.990]This device consists of a gold rod in
the center of which electrons scatter off
- [00:05:51.340]this scattering is spin dependent,
- [00:05:53.440]which is why we can use this to
measure the electron polarization.
- [00:05:58.840]We count electrons for both,
- [00:06:00.910]left-handed polarized light
onto the GaAs,
- [00:06:03.550]as well as right-handed polarized light.
- [00:06:05.680]This gives us a overall value that we can
then use to calculate the polarization
- [00:06:10.330]of our electron beam.
- [00:06:13.720]Here is a table,
- [00:06:14.850]that summarizes polarization data
for changes in two system parameters
- [00:06:19.690]and a color contour plot to compliment it.
- [00:06:22.690]We've been able to
achieve high polarization,
- [00:06:25.420]which is promising and exciting
for our group. However,
- [00:06:28.720]we can also achieve polarizations
that are larger than what we should
- [00:06:31.990]theoretically be able to achieve. Namely,
- [00:06:34.750]the one measurement of 63%
and the two of above 90%.
- [00:06:39.370]Right now, we are looking into these
inconsistencies and are resolving them.
- [00:06:46.000]For future work, we plan on
- [00:06:49.090]GaAs sources like
- [00:06:52.420]and a GaAs nano tip
array that we have created with
- [00:06:57.310]a highly polarized source of this type,
- [00:06:59.740]there are many experiments that can be
conducted from exploring fundamental
- [00:07:04.000]principles like Pauli exclusion
principle for free electrons.
- [00:07:08.440]Learning more about GaAs
as a source of electrons.
- [00:07:11.920]We can also use these to probe other
magnetic or spin dependent interactions
- [00:07:16.870]with molecules and bulk
materials. For example,
- [00:07:20.470]Jefferson laboratories uses a source
of spin polarized electrons to be able to
- [00:07:24.190]measure the size of a neutron.
- [00:07:28.240]I like to thank you for attending this
talk as well as I like to thank my
- [00:07:31.240]advisor, Dr. Tim Gay and our post-doc
Dr. Sam Kermati
- [00:07:34.632]as well as the rest of
our group members for providing their
- [00:07:38.260]valuable insight and
knowledge and overall support,
- [00:07:41.620]more information can be
found on our website.
- [00:07:43.960]And I also like to thank the national
science foundation for funding this
- [00:07:47.050]project. Thank you.
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