Jauffred, L.2; Kyrsting, A.2; Christensen, Eva Arnspang4; Reihani, S. N. S.2; Oddershede, L. B.2
1 MEMPHYS Center, Department of Physics, Chemistry and Pharmacy, Faculty of Science, SDU2 unknown3 Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, SDU4 Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, SDU
Colloidal quantum dots are luminescent long-lived probes that can be two-photon excited and manipulated by a single laser beam. Therefore, quantum dots can be used for simultaneous single molecule visualization and force manipulation using an infra-red laser. Here, we show that even a single optically trapped quantum dot, performing restricted Brownian motion within the focal volume, can be two-photon excited by the trapping laser beam and its luminescence can be detected by a camera. After two-photon excitation for a time long enough, the emitted light from the quantum dot is shown to blueshift. A quantum dot is much smaller than a diffraction limited laser focus and by mapping out the intensity of the focal volume and overlaying this with the positions visited by a quantum dot, a quantum dot is shown often to explore regions of the focal volume where the intensity is too low to render two-photon absorption likely. This is in accordance with the observation that a trapped quantum dot is only fluorescing 5-10 percent of the time. The results are important for realizing nano-scale quantum dot control and visualization and for correct interpretation of experiments using two-photon excited quantum dots as markers.