LIF after excitation with ultrafast laser irradiation, the response of a single cell and the effect of its scattering environment

摘要

New and fascinating field of highly scattering gain media, random lasers and the study of the behavior of such materials of biological significance is presented. Especially the study of the behaviour of various fluorophores embedded in highly scattering matrices. The observed fluorescence signal of the fluorophore quenches in both temporal and spectral domains due to the intense scattering that the photons undergo. This happens when the excitation energy reaches and exceeds a threshold value. Above that value the narrowed fluorescence exhibits the same features as the excitation laser pulse. Lifetime in the order of 50 psec and spectral FWHM in the order of few nanometers are observed. For energies bellow the threshold value the amplification is negligible and the samples behave exactly as those without scatterers. As the excitation energy increases the laser action requirements are fulfilled and the sudden narrowing takes place. Similar studies are underway when various fluorophores are embedded in biological tissues. The final goal is to take advantage of this effect towards a more spatially and spectrally confined agent in Photodynamic Therapy of target tissue lesions on skin or other types of superficial lesions. Very promising in the field of skin PDT would be thin polymer sheets with various dyes, which could be applied directly on the lesion and allow the selection of different irradiation wavelengths using the same laser as excitation source. Thus, improving by far the efficiency of the destruction of different types of cancerous cells. In the biomedical arena, the materials high conversion efficiency per unit volume (≥ 50 percent in a 250 μm thickness) allows for wavelength-shifted catheters and laser creams, potentially useful for the removal of tattoos and other skin discolorations. In addition, random lasers can be used for wavelength shifting the installed base of lasers for other applications such as photodynamic therapy where narrow-band excitations are required for drugs such as benzoporphyrin derivatives (BPD). As far as detection is concerned random lasers could be implemented in various applications such as medical imaging. The combined use of these materials with chromophores that are selectively absorbed by malignant tumors and the narrowband emission along with the high-emitted intensity could boost the detection efficiency.