Abstract:
Measuring the energy of Terahertz generated fields through an electro-optic crystal
using pump-probe techniques. TOM GEORGE ( San Jose State University, San Jose, Ca
95112) ALAN FRY and Matthias Hoffmann(Linac Coherent Light Source, SLAC,
Menlo Park, Ca 94025)
The pump-probe method is an easy and common way to look into the state of a
material after it has been hit with a Terahertz pulse. An electro-optic sample is
“pumped” with the Terahertz pulse and soon afterwards is “probed” by another
laser pulse. This pumped induced probe goes through birefringence as it goes
through the electro-optic sample. Two photodiodes, which receive their sample
clock from the laser, capture the energy of each laser pulse and sends it to a data
acquisition card for easy processing. This method allows for detection of very small
pump-probe signals with relatively the same accuracy as a lock-in amplifier.
Intro:
Terahertz signals exist in an area called the THz gap which lies between the
infrared and microwaves in the electromagnetic spectrum. Discovered over 110
years ago [1] research was relatively stagnant due to the unavailability of sources to
produce powerful enough THz signals. However with the advent of femptosecond
lasers and sources such as the Free Electron Laser(FEL), production of powerful
enough THz signals have been possible. Research in Terahertz time domain
spectroscopy has revealed interesting phenomena in materials when used in
molecular to astronomic studies [1].
THz pulses have a wide variety of uses which range from medical, scientific,
and even security. What makes Terzhertz signals perfect for many applications is its
non-ionizing properties. It can pass through objects such as clothing, wood, paper,
plastics, etc but cannot pass through water or metal [5]. This could be an alternative
to screeners with it’s ability to not only detect illegal materials such as narcotics and
explosives such as C4 but it also has the ability to create images of the material like
an x-ray but without the harmful effects. Applications in the future can include
detection of explosives and medical imaging. This is all possible with Terahertz
spectroscopy since many materials give off distinct spectra when hit with Terahertz
signals [3].
Reliable Terahertz generating sources have just sprung up in the last few
decades. One way to use to generate Terahertz signals is to use ultra-short pulses
using a titanium-sapphire laser using a process called optical rectification [4]. In
optical rectification an ultrashort laser pulse goes through a crystal, such as gallium
selenide, which produces terahertz pulses[4]. When the ultrashort laser pulse is
sent through a specific crystal, it is quickly polarized which creates the terahertz
radiation [4].
To capture the electric field of a terahertz pulse as it goes through a certain
sample, electro-optic sampling is used. After the terhertz has been generated it will
go through an electro-optic crystal which becomes birefringent when a pulse
carrying an electric-field goes through it [4]. The birefringence of the crystal causes
the terahertz signal to break into two separate signals respective of their axis [4].
The two signals are then sent to two photodiodes for processing which can be used
to determine the strength of the terahertz signal.
The best way to measure the strength of a terahertz generated field as it goes
through an electro-optic sample is by using pump-probe techniques. The pumpprobe techniques essentially excites a sample of electro-optic crystal with a
Terahertz pulse which effects its phase and polarization [1]. Another pulse called
the probe captures information from the electro-optic crystal and then transmits
that information to the two photodiodes[1]. Normally this is done using a lock-in
amplifier but because of how a lock-in amplifier manipulates its data, this can take
an excessive amount of time. To get around this problem we have used a data
acquisition card(DAQ) which can easily measure the energy of every pulse and
through simple arithmetic plot the data.
Materials and Methods:
Using a DAQ systems provides advantages in terms of quantification speed
and versatility. The laser used is a 1kHz Ti: Sapphire with a wavelength of 800nm, a
pulse energy of 1.5mj and a pulse duration of 80fs [2]. This laser is split by a beam
splitter into a “probe” pule and another beam that goes into a Terahertz signal
generator. The pulse that was sent to Terahertz signal generator passes through an
optical modulator that runs at 500Hz which blocks every other pulse. The pulses
that do make it through, go through a gallium selenide crystal which produced the
Terahertz signals. This signal is then sent to an electro-optic sample which excites it
by putting it in a transient state. The “probe” signal on the other hand is sent to a
delay stage which delays the pulse by moving the stage back and forth, depending on
the desired time delay, before it goes through an electro-optic crystal that has been
excited by a Terahertz signal.
To capture the energy of the Terahertz through the electro-optic sample we use
two photodiodes. Two THOR 100A photodiodes and were used to capture the output as
the probe pulse entered the THz influenced electro-optic crystal. The sample clock of the
DAQ card was taken from the Ti: Sapphire laser, meaning every pulse from the laser is
measured. As the pulses exit the two photodiodes, it has a pulse decay of 80 ns which
decays before the next pulse arrives. From this point there are various ways to capture the
data. A few ways are to use a lock-in amplifier, a current preamplifier with a box-car
integrator, or use a sample and hold circuit/integrator circuit. Since we were trying to find
a better method than the lock-in amplifier the best choice would have been to use a
current preamplifier which would then go into a boxcar integrator to give us the energy of
each pulse. This would have been the easiest method to implement but also carries along
with it the highest cost at around $20, 000 for the setup. A cheaper alternative is to use a
sample and hold circuit which increases the time-base of the signal, making if easier for
the DAQ card to sample at its peak and capture the data. The DAQ card we used was a
National Instruments NI-6251, which has a maximum sampling rate of 1.25 ms/s at 16 bit
resolution.
Results and discussion:
The data acquired from the photodiodes were processed in LabVIEW. LabVIEW
allows you to create virtual instruments(VI) which can be used to control and obtain data
from various sources. Since we were using a DAQ card on a Windows machine we were
able to take advantage of the DAQmx function which makes data acquisition using an NI
DAQ extremely easy. The DAQmx function shown in fig.2 allows you to set where your
signal is coming from, triggering, and sampling all in one place. Two of theses functions
were used to capture the data from the DAQ card for further use.
My first VI iteration as shown in fig.3 was able to show produce data on an XY
graph and able to scroll through data points using the scrolling option implemented.
However, with my limited time and the features I still had to implement, I decided to try
and modify a VI that is currently being used to capture the energy of Terahertz signals
but was doing so using a lock-in amplifier. I was able to accomplish this by simply
removing parts of the code where information was being sent to the lock-in amplifier and
initializing it. After doing this I found where the data was being taken from the lock-in
amplifier and replaced with two DAQmx functions as shown in fig.4. The rest of the code
took care of data manipulation, graph output, saving, etc.
To capture the signals at the right time requires the use of a sample and hold
circuit. What the sample and hold circuit does is, when triggered, it holds the signal by
streatching its time base and having the signal decay over that amount of time. It is made
so that the signal decays right before the next pulse comes in. We created the circuit in
figure 5 with with only a resistor, a mosfet, and a capacitor. When the pulse from the
photodiode is sent to the sample and hold circuit, it first goes through the resistor which
slows the pulse before entering the capacitor. This is to prevent the capacitor from being
quickly charged with a large spike of voltage that could potentially damage and limit its
uses. Before the pulse enters the capacitor, however, it first goes through the mosfet
which is triggered by using the laser as a sample clock.
Conclusion:
Using a DAQ system to capture Terahertz signals is a versatile and viable option to
perform pump-probe measurements. By setting the DAQ card and photodiodes to sample
at the rate of the laser, we were able to measure every Terahertz pulse that was sent
through the electro-optic sample.
-Talk about benefits of using Matlab??
-Talk about LabVIEW problems??
Acknowledgements:
I would like to thank my Alan Fry and Matthias Hoffmann for opening my eyes to areas
of research I never new existed and for guiding me through my first research experience.
I also want to thank Juan Santiago and Salem Cherenet for being amazing RA’s and for
encouraging my research goals. Thanks especially to Jaun Santiago for showing me a
prime example of an Electrical Engineering and for spending countless hours helping me
with my project. Thanks you Stephen Rock and Maria Mastrokyriakos, and DOE for
giving us the opportunity to conduct research at one the world’s premier research
facilities.