I'd like to measure the defect contribution of 150 um in diameter MIM structure.
However our PL laser spot is about 1mm. Is it possible for converged light to be 150um? By using additional lens(es)? What kind of lens I can use? And how to set it up (determine the position of the lens)?
Hello, Mangasa! Your task requires a converging lens. 150 microns is quite affordable spot size for conventional optical lenses. The smallest spot size can be obtained at the focus of the converging lens, which can be determined experimentally by how spot size will vary with the distance from the lens to a screen.
I would advise to use a pinhole to create a virtual source followed by a converging lens to create an image of the pinhole on your sample. To improve the flux you may as well use a (cheap) microscope objective to focus the laser beam onto the pinhole.
The final setup would be: laser - microscope objective - pinhole - converging lens.
You may choose a pinhole of just 150 μm and a lens with suitable focal length (f) placed at 2f from the pinhole. This will create a 150 μm spot at a distance of 2f in front of the lens. The lens must have an aperture large enough to avoid a diffraction radius larger than 150 μm.
Theoretically, the spot size in the focus (waist) depends on the focal length of your lens and on the dimensions of the aperture from the laser. See for instance eeh book, LASERS IN NEUROSURGERY, 2001 by L Danaila and M.L.Pascu. You may find the book on the interne in electronic form. Knowing this, in order to make the beam circular and to focus it as you need you should use a combination cylindrical - spherical lens.
From what you mentionned and what is common in PL machines, I will suppose your setup currently measures spot by spot by moving the sample, and that you pump your sample with a collimated laser beam of 1mm size.
If not, I would need more details before answering the questions. For example, what is your optical setup / PL machine? Is the pump laser going through the collection optics?
If you pump by the side of your collection optics, you can only add a simple lens / microscope objective, depending on the working space you have. (see illustration). If your beam is well collimated, the lens should focus close to its design working distance / focal length. Any focal length should give you an acceptable spot size. A spot size below 100µm should be easily achievable under those conditions, if you keep your focal below ~75mm (as explained higher by Pierre Cheyssac).
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