• General Remarks
1. Wait 30 min for laser to stabilize after turning it on.
2. Sample should be placed at a right angle to the beam.
3. Samples are usually run as a function of temperature using the LabView programs
“temp control ver3.vi” to control the temperature and “Bire1,2,1on2,T3.vi”
to record the intensity and temperature data.
4. The sample temperature can be found by using the calibration charts on the
optical tables. (Sample temperature can be obtained by converting either the
computer recorded temperature or the set point temperature to the actual sample
temperature. However, recall that the sample takes ~10 minutes to its equilibrium
temperature.)
5. Experiments typically last at most ~24 hours depending on the temperature
range and number of cycles run; use time intervals of 30 sec for data collection.
Hold temperatures for around 30 min (until signal is fairly stable).
• Birefringence Equipment Setup
1. The initial setup is started with only the laser on the table.
Mark the position of the beam spot on the wall with crosshairs. Add the components
listed in the diagram below one by one starting with beam splitter, neutral
density, etc., making sure that the laser beam always hits the center of the
crosshairs.
2. With laser going through everything before the lens, check to see that the
beam hits the middle of the crosshairs on the wall. Add the lens. Move it until
the beam is focused on the crosshairs.
3. Add the detector (detector 1). The detector location for which the signal
is maximized is marked on the optical rail. Essentially it is the location that
gives the maximum voltage signal to the computer.
4. Open the program “Bire1,2,1on2,T3.vi”. Hit start. Hit cancel
so it doesn't save. Change the time interval to 1 second.
5. The detector signal is overloaded at a voltage reading of 10 (bire1 value
in the program).
6. Increase the neural density to reduce the beam intensity. Neutral density
calibration curves are located on the optical tables for their respective setups.
7. Individual components such as neutral density wheel, polarizer, analyzer
and half-wave plate can be calibrated at the point. To calibrate the neutral
density wheel remove it and insert a neutral density plate; record the intensity
value. Add the wheel and record the intensity at each wheel location. To calibrate
the half-wave plate, remove the polarizer and analyzer. Set the half-wave plate
to zero (horizontal orientation) and rotate the laser until the signal is maximized;
lock the laser in place and use the half-wave plate as the reference point for
polarizer calibration. To calibrate the polarizer, insert it into its holder
and set the holder to zero. Rotate the polarizer within its holder until the
signal is maximized. Lock it in its holder. Now with the half-wave plate and
polarizer calibrated, insert the analyzer and rotate it until the signal is
minimized. This is the crossed position. The uncrossed position is 90º
off from this position.
8. When not using the setup, cover all components with plastic bags.
9. Turn off the laser and put away the key.
Beam splitter: directs part of the beam to detector 2 and part to the polarizer.
Detector 2: Measures instantaneous fluctuations in the intensity of the laser
beam- can be used to determine if there is a problem with the beam itself.
½ wave plate: rotates the beam while maintaining its polarization spread
Polarizer: Filters the beam to pass one polarization of light.
Analyzer: is a polarizer that is crossed compared to the polarizer.
Lens: Focuses the scattered light down to one point.
Detector 1: monitors the forward scattered light focused by the lens.
• Birefringence Procedure
1. Turn on the
laser with the key, wait 30 minutes to warm up and stabilize.
2. Remove plastic bags from all components.
3. Start the program “Bire1,2,1on2,T3.vi” choosing 1 second interval.
Hit start and cancel to not save data.
4. Uncross the analyzer and set the neutral density such that the signal is
around 1. Record the signal and neutral density. This value is your Io.
5. Insert your sample and cross the polarizer. Set the neutral density such
that the signal is high (below 7 but above 1).
6. Stop the program. Change the collection interval to 30 seconds. Start the
temperature control program with your desired profile. The temperature step
must have the correct sign. For instance if you want to decrease the temperature
by 10C steps, the value must read -10. Also the Final minus Initial temperature
must be an integer multiple of the temperature step. For instance if you wanted
to heat the sample from 30 to 40 degrees by 2 degree steps, there will be no
problem. But if you used 3 degree steps the controller would never reach the
value 40C and would continue heating until the oven burned.
• Shut Down Procedure
1. Turn off the
laser and remove the sample.
2. Cover all of the optical components with the plastic bags to help keep off
dust.