Different types of in situ nanometric-sized sources have been recently developed to investigate the properties of surfaces at nanometer scale. In order to be used for experiments involving slow processes or for scanning probe tip functionalization, these nanosources have to be bright and photostable. A single molecule fulfills the first property but suffers from irreversible photobleaching at room temperature. One alternative system consists of photoluminescent quantum dot which is indeed very close to a bright point source. However, its blinking behavior will be a serious drawback for most applications. Absorption-based detection schemes also allow one to detect single nano-objects, like a nanometric gold nanosphere. Here we propose inorganic nonlinear KTiOPO{sub}4 (KTP) crystals of nanometric size with a strong second-harmonic coherent emission when illuminated by femtosecond infrared pulses. The non-resonant character of the nonlinear interaction prevents from any photobleaching or blinking. The signal therefore exhibits a perfect photostability. We show that analysis of coherently emitted non-linear field allows us to characterize isolated nanocrystal orientation.
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