Photoluminescence Procedures
This page summarizes the experimental set-up and procedures used to collect and analyze photoluminescence (PL) data.
Last updated by: SDM July 2016
Reading Up
Berkley Low Light Signal Measurements Lab (dead link)
System Overview and Slides
The last slide in the following presentation should be used when presenting our PL set-up at talks and conferences
Components
532nm DPSS pump laser
Since this laser does not have a built in power meter check the laser output power using the bolometer. While measuring power do not chop the beam.
532nm narrow bandpass filter to eliminate other wavelengths coming from the laser.
optical chopper: improves signal to noise ratio (SNR).
Detectors
Germanium detector, LN2 cooled to 77K, 500nm to 1500nm, set chopper to 100Hz
InGaAs detector, TE cooled upto -20C, 500nm to 1650nm, set chopper to 350Hz
InSb detector, LN2 cooled to 77K, 1500nm to 8000nm, set chopper to 350Hz
SR830 Lock-in amplifier
Acton SpectraPro 2500i spectrometer (0.5m optical path)
Sample holder
The sample holder is connected to house vacuum. A toggle valve located above the PL setup turns on/off vacuum. Place the sample on the sample holder and turn-on vacuum to hold the sample firmly in place.
It also has precision micrometer to move the sample forwards and backwards for precision alignment.
Data Sheets
Filter Wheels
Detectors
Safety and PPE
Wear long pants and close-toed shoes when handling liquid nitrogen (LN2)
Wear the yellow goggles to filter out the green laser light to prevent eye damage (i.e. have the correct goggles for the wavelength range of the laser)
Turn on the laser warning light indicator
In any setup make sure that the beam path is terminated at the beam dump and also make sure there are not stray reflections
Always place the beam block in front of the laser before making any changes to the optical beam path, this includes changing samples
Wear gloves to avoid contaminating samples
Set-up and Preparation
Construct the PL housing box using the black board pieces. The housing enclosure is a cavity that is filled with nitrogen to mitigate atmospheric absorption.
Close iris immediately in front of the laser
Turn ON the "LASER in use" by flipping the light switch located along the south side of the room. Check that the light immediately outside of the lab is ON.
Turn on laser driver and increase current slowly at a rate of 0.5 A/min to 3.3 A.
Turn on the chopper wheel and set the chopping frequency at 1000 Hz
Check that the chopper RG11 wire (telephone-looking wire) is connected to the chopper wheel next to the entry port of the PL housing box
Set up detector
InGaAs
Operation 0.8 - 1.7 um
Electrically cooled using temperature controller (ILK Lightwave LDT-5412 temperature controller). Keep at -20 deg C. Temperature controller should be pre-programmed to correct temperature, just turn it on and make sure that the output is "on".
Power supply required
No pre-amplifier needed -- output from detector feeds directly into lock-in amplifier
InSb
Operation 1.0 - 5.0 um
Cooled with LN2. Use funnel to fill with LN2 before using. Let sit for 10-20 minutes to cool to the proper levels.
No power supply required
Pre-amplifier needed -- output from detector becomes input to pre-amplifier. AC coupled output of pre-amplifier feeds into input of lock-in amplifier (Stanford Research Systems, Model SR830 DSP LIA)
Do NOT turn on Pre-amplifier until LN2 has been poured into the detector
When finished using for measurements
Unscrew and detach detector
Quickly cover spectrometer-facing side (use metal plate from where detectors are stored when not in use). The InSb is sensitive to high energy light (UV) and can become damaged if exposed while cool.
Pour out LN2
Set detector + cover in detector storage bin
Carefully handle detector when switch from one to the other. Gently pull from spectrometer port. Sometimes it is good to use some leverage if it is stuck (e.g. flathead screwdriver).
InSb detector properly connected and attached to spectrometer | Filling InSb detector with LN2 and funnel |
Pre-amplifier with input from a detector and output going to lock-in amplifier | Lock-in amplifier
|
Set spectrometer slits (both at the light entry port and exit port/detector)
If testing a bright emitter, close the slits down until the sample does not saturate the detector. After the detector stops saturating and you get sharp peaks, adjust the OD filters in front of the laser. NOTE: the OD numbers attenuate the signal by 10^(OD number)
For a crappy emitter, open to 0.5
For other stuff or super-crappy materials, open it up all the way
Set filter wheel (outside of housing box) to empty-empty. If attenuated laser intensity is needed (e.g. low-temperature measurements), adjust the filter accordingly.
Turn on the nitrogen gas lines. This can be done one of two ways.
OPTION 1: Open the line for labeled "GENERAL NITROGEN" from the gas drop along the north wall. The nitrogen from this line might be used to supply the nitrogen gun. Remove the hose from the gun and attach it to the cross joint.
OPTION 2: Open the "GENERAL NITROGEN" line on the gas drop located in-between the computer desk and the optics table.
Data collection
Place sample on vacuum chuck and load into the rails inside the housing box, using the door/flap on the south side of the housing box.
Allow the housing box to purge for several minutes. Use a hygrometer to monitor the humidity in the box. Measurements should be taken at or below 2% water content to prevent atmospheric absorption.
Open iris in front of laser
Open SimplePL software. Set up grating and filter wheel settings for the measurement. If unsure how to use software, ask another student for help.
Run the software to collect data
Measure the incident power by removing the sample holder and letting the pump beam fall on the bolometer
Repeat the same, but this time place the sample on the sample holder and the bolometer in the place of the beam dump to measure reflected power
The difference of the above two powers will give the effective pumping power
Close the iris when changing between samples. Reopen the iris before collecting data for a new sample.
The following are the default/optimal settings that should be used for the SimplePL software. Certain settings might need to change as the set-up is adjusted, e.g. diverters settings might change if a different entry port is used (such as during transmission and reflection measurements.
Software
The most recent software (as of July 2015) is a python GUI application called SimplePL, written by Scott Maddox. It is available on Github.
Finish Use
Close iris
Remove final sample
Turn off nitrogen to housing box
Slowly ramp down laser at a rate of ~0.5 A/min. At 0 A, Turn off laser.
Turn off chopper wheel
Turn off Output of temperature controller, then turn off
Turn the "sensitivity" on the lock-in amplifier all the way up
Turn off "LASER in use" light
PL for demonstrations: Pump-dependent PL
Set up the PL station using the InGaAs electronically cooled detector
Pick a standard quantum well sample
Measure and plot the PL data of the sample at different filter intensities
Range of measurement 800 to 1050 nm at 5 nm steps. Should see 2 peaks: (1) GaAs near 860 nm and (2) the quantum well peak near 1000 nm. Don't measure out to 1064 nm, because you'll see the second harmonic of the green laser.
Do not save data
Do not clear the plots
Plot on log plot. Recall the matrix element for absorption has an electric field squared term, so the output signal is roughly proportional to the intensity of the light. That is, changing the optical filter from 1.0 to 0.1 we would expect to see the output peaks reduce by a factor of 10, which is easier to see on a log plot.
After demonstration is over, clean up the PL set-up
Turn laser output to ~zero then shut off
Turn off the output (but not the power) to the temperature controller
Turn off filter wheel
Turn off light outside of the lab
Put away sample