Second harmonic generation of biotin and biotin ester microcrystals trapped in optical tweezers with a mode-locked Ti:Sapphire laser

Intense second harmonic generation (SHG) was observed from microcrystals of biotin and biotin ester trapped by optical tweezers, formed with a focused near-infrared pulsed laser beam. The intensity of SHG depends strongly on the states of the microcrystals and the excitation wavelength. Microscopic scanning images of biotin and biotin ester were obtained in high contrast with SHG. Simultaneous trapping and excitation of SHG and two-photon autofluorescence of biotin and biotin ester microcrystals allow us to investigate their structure and optical properties. These optically trapped particles (of submicron size) are useful as nonintrusive microscopic probes for high-resolution studies.