Hi. Is the phase-matching condition fulfilled? How? Do you have a delay line to make sure that the pump and Stokes pulses arrive at the same time on the sample? Sorry if these questions sound too obvious!

Hi Vincenzo, thank you very much for your answer. I am actually looking for an obvious mistake that I might have made, so no question is too obvious. Both the pump and the dye beam are co-aligned together with a dichroic mirror and travel together throughout the interrogation space. I think that this is indeed fulfills the phase-matching condition and expect the Anti Stokes beam to appear co-aligned with the pump and Stokes beam, until they all are separated by a prism.

I now think that I haven't checked that both beams arrive to the sample simultaneously, and I certainly do not have any delay lines. I haven't seen anything on that in the papers that describe these experiments. Is that something that is usually done in CARS experiments ?

Thank you for your help !

You should definetely check that your pump and Stokes beams are spatially and temporally overlapped. The Stokes beam, which is obtained from the dye laser is usually delayed compared to the pump beam and therefore you should have a delay line for the latter.

Dear Vincenzo and Ilana - thank you very much for your help, I'll definitely check that ASAP.

If the CARS experiment is running properly, the blue anti-Stokes signal (for atmospheric N2) can be seen on a white paper (in a dark room). Due to the prims dispersion, are you sure that the anti-Stokes beam alignment is correct to be detected at the output of the spectrometer.

Thanks a lot once again, guys. After building an appropriate delay line the AS beam occurred immediately.

Hi.

Check if your beam are simultaneous, otherwise the 4 photon interaction will not take

Place...

I've done a very similar CARS experiment to this. The common easy mistakes we would make were almost always alignment based.

1. How confident are you in your temporal overlap? I am guessing you are using nanosecond regime lasers? A photodiode and oscilloscopes are probably all you need to double check that all your beams are arriving at the focal point at the same time.

2. You say you're confident in your spatial overlap, but it never hurts to triple and quadruple check this. If you have a pinhole (100 micron is what I used with a 300mm focal length lens) that you can place on a translation stage, this makes your life a lot easier. Use an index card on the opposite side of the pinhole while adjusting one beam (block the others) til it goes through cleanly, then without moving the pinhole, align the other beams to the same spot. This can be tricky and iterative but it gave my colleagues and I the best alignments we had seen.

3. Can you see the blue-ish CARS beam after your separating prisms? Depending on the strength of the signal, this can be visible on a white reflective card with all the lights off. I've found it's really hard to see it the first time you try to spot it that way, but once you know what you are looking for it gets easier and easier. If you do see this, then it may simply not be aligned into your spectrometer.

I apologize if these are very simplistic responses that you've already tried. In my experience, it's the simple things that usually got me stuck when I was doing CARS. Good luck!