If the photons interacting with material is not getting absorbed through any allowed transition that provide fluorescence. Then most of its energy will be dissipated through non radiative transition creating heating in the sample instead of fluorescence.

I think you sample is damaged.Have you tried second time with 532nm laser after exposed to 450nm?

Diar Muhammad,

Color centers (defects) in crystals, in addition to the luminescence spectrum, have other important characteristics: absorption spectra (AS) and excitation spectra (ES). The absorption spectrum is usually symmetrical to the luminescence spectrum relative to the ZPL. The excitation spectrum can contain one or more bands. With regards to the diamond I recommend to look J. Walker Optical absorption and luminescence in diamond.

best wishes

V. Mironov

Dear Mohammad,

The situation described is very common, NV- center usually exhibits a stronger signal when the laser wavelength (532 nm in this case) is closer to 637 nm , though other factors (such as presence of other centers) may influence too

I highly recommend you a useful article of Sally Eaton-Magaña and Christopher M. Breeding titled " An Introduction to Photoluminescence Spectroscopy for Diamond and Its Applications in Gemology" ( https://www.gia.edu/gems-gemology/spring-2016-photoluminescence-spectroscopy-diamond-applications-gemology )

Michael

Luminescense and excitation spectra of NV- are almos mirror. As luminescense of NV- ranges from 1.945 to (1.945-0.4)eV (800nm - approx 5% of intensity) so absorbtion and excitation ranges from 1.945 to (1.945+0.4) (530 nm). In fact, laser 532 is not the best for NV- excitation, but 450 nm is completely inappropriate. But due to energy transfer between centres it is not full truth :)Article Luminescence excitation spectra in diamond