I have prepared tin and tin oxide nano particles at 60, 120, 180 and 240 mJ laser energy( 2n d harmonics of Nd : YAG laser system is used for 40 min) .
I have observed broad and high intensity spectrum in visible region at lower energy . However with the increase of laser energy the spectrum breaks in to two band one band appears in UV region and one in visible .The intensity also decreases .
It is important to mention here that my SEM result shows that particle size increases with the increase of laser energy.
I have read in one research paper that the intensity increases if the size of nanoparticles is small and they contribute more in emission
I think this can be explained by considering the light emission process. The Nd ions are at ground state （E0）before laser excitation. When excited by low energy laser, The Nd ions at ground state (most populated, E0) absorbs energy and are excited to excited state (E1). When excited by high energy laser, most of the Nd ions at ground state were excited to excited state （most populated, E1) , consecutive energy absorption will excite the ions at E1 to higher excited state E2. These ions excited to E2 will emit higher energy photons (UV region) by relaxation process. Given limited Nd ions, more ions relaxation from E2 means less ions relaxation from E1, ie, weak spectrum in visible region.
You didn't say what the spectrum relates to — is it absorption, extinction, emission spectrum, and if the emission then of what — the laser or the particles? What is the measurement setup — transmission or epi? Are those NPs separated well enough to excite single particles?
If you ask about the energy spectrum of laser, then I agree with the previous answers.
There may be plasmon resonance in Sn (article showing theoretical values is in the appendix) which enhances the absorption process and thus could possibly change the NPs geometry. In the SEM image you have one plane of view, so you can see it as increasing size.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.498.9334&rep=rep1&type=pdf
see this article for the reason that reduce the laser energy, increase the PL intensity.
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