I am using double beam Jasco V-670 spectrophotometer to measure the transmittance of some thin film samples . first I put to two bare glass substrates(one in the reference position and the another one in the sample position) and then I measure the baseline.
after that I remove the one on the sample position and put my sample (thin film deposited on a glass substrate) and remeasure the transmittance . but finally I found that the transmittance of the thin film higher than the transmittance of the bare glass substrate(attached).
Could you please suggest interpretation for this ?
It is probably due to the effect of thin-film interference. And the phenomenon of anti-reflective coating occurs.
If you create an anti-reflective coating, you could also think of that as a pro-transmissive coating. From your experiment, you can tell that the index of refraction of your thin film is somewhere between the index of glass and the index of air. The reflection at each interface is less than the reflection at the air-glass interface. Look up “index matching.”
I think this phenomenon is related to the thickness of the thin film, which led to the interference of the thin films. Transmittance improvement depends on the thickness of the thin film and its relationship to the incident wavelength. Sometimes the Transmittance increases and at other times it decreases.
In reference to the attached graph of T vs wavelength, the peaks are due to the interference effect at the interface between the coated film and air. If you use the envelope method to calculate the average T surely you will find it less than that for bare glass.
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