The prelab (with unique parameters for each individual lab group section) is REQUIRED to be completed by each student and submitted at the beginning of the lab (like any other prelab). Failure to submit the prelab or submit it late (late prelab submissions are worth ZERO) will be penalized -30% on the whole lab. NO EXCEPTIONS!
Design a six layer QW stack using FilmStar with maximum reflectivity at the wavelength in the table below (dependent on lab group) as follows:
Calculations (done manually, and with all work shown explicitly) showing the predicted approximate maximum reflectivity of the coating at the design wavelength. Use the Fresnel equations to compute the reflection coefficient of all major reflection components - consider only the first term in the series for each component (i.e. you may neglect small sub-reflections). For example, if the reflection coefficient from the first air-to-film interface was 0.05, transmitted component from the first air-to-film interface would be 0.95, and hence the reflection from the next film-to-film interface would be 0.95 times the reflection, and so on. See lecture notes for details.
This design must consist of (i) a page of calculations for the Fresnel coefficients and (ii) a diagram showing the six layers and the absolute reflection coefficient for each relflection from each interface between materials. Include the reflection and a POLARITY ("+" or "-") to denote its phase relative to the top reflection (as we've done in the lecture many, many times).
An example of the diagram required as part of the prelab submission. Show each of the six layers (each with an n value), each reflection, and each transmitted component as coefficients (NOT percent ... don't square the values until the end). Text boxes work well for these values.
In this example, the first relection of 0.05 leaves a transmitted coefficient of 0.95 which then reflects from the next interface (film1 to film2) resulting in yet another reflection (out-of-phase in this example) and another transmitted coefficient which continues through the device.
Mirror wavelengths are chosen based on the fact that the reflectivity peak spans over a range of 0.9 to 1.1 times the target wavelength and so will cover a good portion of the gain spectrum of the target laser. Design the mirror based on your specific lab group as follows:
|Lab Group||Lab Date||Center Wavelength (nm)|
|Thursday 9:30 am Group A&B||2019/04/04||530|
|Thursday 11:30 am Group A&B||2019/04/04||550|
|Thursday 1:30 pm Group A&B||2019/04/04||570|
FAILURE TO USE THE CORRECT WAVELENGTH IN YOUR FILMSTAR DESIGNS WILL RESULT IN AUTOMATIC FAILURE OF THE PRELAB - If the wavelength in your designs does not match that of your assigned lab group it will be considered plagiarism and a mark of zero will result for the entire lab
Filmstar models MUST BE UNIQUE TO EACH STUDENT. Plagiarized models (including the prelab) will result in a mark of ZERO for the entire lab and a trip to the Chair's office for academic dishonesty resulting in possible expulsion.
Building the FilmStar Model
First, review the structure of a dielectric mirror and specifically the order of the layers from class notes. Now, leaving the basic units in OPTICAL thickness, begin by building the stack from quarter-wave layers (i.e. "0.25x" where "x" represents the symbol for the material used such as MgF2 or ZnS). Begin with a massive glass layer (5mm of glass for this OC) and build the stack from there. The order of the layers is very, very important! Be sure to set the design wavelength as mentioned in the lectures.
In the course of your prelab research you might have found two possible values for the index of refraction of ZnS: choose one for this model however during analysis of results try both values to see if one "fits" better than the other.
Each layer should be, nominally, of thickness λ/4n where λ is the wavelength of the design and n is the refractive index of the individual material used. Other combinations will work however ther may not be optimal and, regardless, will be much thicker so that the deposition may take longer than anticipated and may well consume more material than is available in a crucible!
When done, convert all layers to physical thickness for fabrication in the lab (remembering that these thicknesses are in nanometres and our XTM/2 monitor reads in Angstroms).
NOTE: This lab will be completed on Sparky, a high vacuum eBeam evaporator system in which layer thickness will be monitored using an Inficon XTM/2. Density, Z-Ratio, and Tooling Factor are pre-programmed into the unit for each material used: the target selection on the unit matches the film # on the monitor.
Target Configuration (Current as of 2018/04)
#2: Magnesium Fluoride (ρ=3.150, z=0.637, Tf=220%)
#3: Zinc Sulphide (ρ=3.980, z=0.775, Tf=220%)
#6: Aluminum (ρ=2.700, z=1.080, Tf=220%)
The tooling factor was verified correct (within 7%) for substrates on the lower rung of the holder only.
The coating is to be applied using SPARKY, a six-target cryopumped eBeam system. When you enter the lab, the CTI cryopump will already be running (since it requires at least several hours to precool). Verify that the cryopump is ready (via the attached monitor) before proceeding.
Begin by venting the chamber then opening it. Load one substrate per student into the holders. Ensure the required target crucibles are filled (this is important since many layers will be deposited). Ensure that the crystal (XTM/2) is operating properly. Now lower the chamber, rough-pump to below 100mTorr, and open the gate valve. When the pressure reaches the low 10-5 torr range (< 2*10-5, usually about 20 minutes) deposition may begin.
Turn the CV-8 eBeam supply ON, ensuring all interlocks are closed. Before applying power, ensure both the operating voltage and the emission current controls are set to zero. Now proceed to deposit all layers in the proper order.
The deposition process is as follows:
The violet glow as seen through the window of the chamber. The filament is visible in the lower part of the photo and the crucible is seen in the upper part. The crucible is not visible directly, but through the periscope arrangement which ensures the window is always kept clear of the deposit. Of course, the deposited film is also deposited on the lower mirror and so the colour of the image may well change during deposition.
With MgF2 use a rate of 5 to 10 Angstroms/second and for ZnS a rate of 4 to 6 Angstroms/second. Since ZnS sublimes, be careful when controlling the rate of deposition - it may "run away" quickly. While exceedingly high rates are possible (especially since Sparky features a 13kW supply), splattering may occur resulting in poor surface quality of the coating and possible damage to the monitor crystal - be especially careful with a 'freshly' loaded crucible. The other concern is evolved gases (again, a problem with freshly loaded crucibles) which may cause a pressure increase sufficient to permit an electrical discharge (i.e. arcing) of the eBeam supply into the chamber - low rates, especially on the first few depositions, will prevent this! Remember, too, to degas each material the for the first use only by slowly heating it with the shutter closed (subsequent layers will not require this process but the shutter is still required to terminate the deposition).
Deposit six to ten alternating layers. Be prepared when you enter the lab as this is all you will have time for in the two-hour lab: each layer will take about 2 minutes to deposit plus a minute of cool-off between switching the hearth targets plus ramp-up time. Be sure to program the XTM/2 monitor for each film (simply select the new film number, all other parameters are preprogrammed including density, z-ratio, and tooling factor). Write-down the actual layer thickness deposited. More layers means higher reflectivity of the finished mirror. Be sure to also deposit the top protective layer.
When all layers are complete shut down the CV-8 power supply (remove the key for safety), close the gate valve, vent and open the chamber, and USE THE SAFETY GROUND TO DISCHARGE THE FILAMENT TERMINALS (Wherever a safety grounding probe is provided, USE IT!).
Check the XTM/2 monitor, as well, for the current tooling factor.
Quantitative analysis requires the use of a spectrophotometer. Using either the Lambda-3B or GeneSys-20 spectrophotometer in the spectroscopy lab, determine the reflection curve for the coating (all substrates should be the same - verify this by sampling at least three from different locations in the chamber) - the reflection curve may be determined using the reflection adapter.
If using the reflectance accessory ...
The UV lamp is not required, only the VIS. Before starting the unit ensure the alignment mirror is in place on the accessory. Once ready, carefully remove the alignment mirror and replace it with your dielectric mirror (coated side down). Now, run the spectrum from 900nm to 315nm. When all runs are complete, replace the alignment mirror.
Transmission only ...
If the reflection adapter is not installed, simply run the transmission spectrum as per the previous lab and take the inverse of the transmission curve (in Excel) by subtracting 100% minus transmission for each point. Be sure to use a glass blank the same as used for the deposition as a reference.
Compare the observed results to the theoretical prediction from FilmStar (using the actual layer thicknesses deposited) and try to "fit" the FilmStar model to the observed spectrum by modifying layer thicknesses as well as changing the n value of the ZnS layer (as outlined below).
Prepare a condensed lab report as follows: