Faegheh Hajizadeh

Optimized optical trapping of gold nanoparticles

Metallic nanoparticles are of significant interest due to their particular optical and biological applications. Gold nanoparticles are proven to be excellent candidate for in vivo micro-manipulation using Optical Tweezers. This manuscript reports on stable 3-D trapping of 9.5-254nm gold nanospheres using substantially decreased laser power. The lower limit is approximately 2 times smaller than previous record. 5.4nm gold nanospheres were trapped for only 2-3 seconds. For the first time, our experimental data verify the volume corrected Rayleigh model for particles smaller than 100nm in diameter. Measuring the maximum applicable force for gold nanoparticles, we have shown that a few tens of milli-Watts of laser power can produce pico-Newton range forces.

Extended linear detection range for optical tweezers using image-plane detection scheme

Ability to measure pico- and femto-Newton range forces using optical tweezers (OT) strongly relies on the sensitivity of its detection system. We show that the commonly used back-focal-plane detection method provides a linear response range which is shorter than that of the restoring force of OT for large beads. This limits measurable force range of OT. We show, both theoretically and experimentally, that utilizing a second laser beam for tracking could solve the problem. We also propose a new detection scheme in which the quadrant photodiode is positioned at the plane optically conjugate to the object plane (image plane). This method solves the problem without need for a second laser beam for the bead sizes that are commonly used in force spectroscopy applications of OT, such as biopolymer stretching.

Extended linear detection range for optical tweezers using a stop at the back focal plane of the condenser

Optical tweezers are indispensable micro-manipulation tools. It is known that optical tweezers are force rather than position sensors due to the shorter linear range of their position detection system. In this paper, we have shown for the first time, that positioning an optical stop at the BFP of the condenser can overcome this problem by extending the linear detection range. This method would be valuable for the force spectroscopy applications of optical tweezers.