I got a bit sceptical of the spare viewport measurement reported in entry 2580.

Indeed the absorption seems too uniform with really low value..

• Discussing with Matteo, a possibility could be that the spare viewport is tilted (indeed it is placed inside a tama size holder with the addition of 2 spacers to compensate the 1 cm thickness of the viewport). That would make the surface calibration not reliable.

So I reinstalled spare viewport and aligned the DC with Pin = 0.6248W.

First, I checked the tilt of the viewport by doing Z scan and checking the the Z center of the absorption (computed from the 2 positions where the phase is 0 deg) .

From these measurements, it seems that the difference of the Z center over the radius of the absorption map is ~0.35 mm. This is far smaller than the interaction length ~2 mm (see Manuel PhD).

So we can conclude that there is negligeable tilt with this holder.

• Another possibility could be that the lockin sensitivity was saturating at a low value, making a constant noise level fakes an absorption signal.

However, an easy check is to double the power and check if the AC signal is also doubled. That would confirm that the signal we are seeing is coming from absorption.

Therefore, I decided to do a new measurement with Pin = 1.2545 W (radius 30mm and step size 2 mm) which is roughly twice larger than the measurement reported in entry 2580.

The mean ac value went from 8.1 e-4 to 1.6e-3. This confirms that we are seeing absorption signal !

Finally I also took a new absorption map with this higher power as reported in figure 1.

In figure 2, you can see the same map but with the colorscale constraint between 15 and 20 ppm and we can see the (small)  non-uniformity of the viewport absorption.