Exploring the physics of efficient optical trapping of dielectric nanoparticles with ultrafast pulsed excitation

Abstract

Stable optical trapping of dielectric nanoparticles with low power high-repetition-rate ultrafast pulsed excitation has received considerable attention in recent years. However, the exact role of such excitation has been quite illusive so far since, for dielectric micron-sized particles, the trapping efficiency turns out to be similar to that of continuous-wave excitation and independent of pulse chirping. In order to provide a coherent explanation of this apparently puzzling phenomenon, we justify the superior role of high-repetition-rate pulsed excitation in dielectric nanoparticle trapping which is otherwise not possible with continuous-wave excitation at a similar average power level. We quantitatively estimate the optimal combination of pulse peak power and pulse repetition rate leading to a stable trap and discuss the role of inertial response on the dependence of trapping efficiency on pulse width. In addition, we report gradual trapping of individual quantum dots detected by a stepwise rise in a two-photon fluorescence signal from the trapped quantum dots which conclusively proves individual particle trapping.

On-the-Fly Calibrated Measure and Remote Control of Temperature and Viscosity at Nanoscale

“…46,47 Such technological developments in optical tweezers have resulted in exploring the three-dimensional world of micro- to nanoscale. 48,49 Here, for temperature sensing studies, we have used an IR780 dye solution that has high absorption at our trapping wavelength.…”

Structure and dynamics of optically directed self-assembly of nanoparticles

“…To obtain TPF from the trapped objects at low laser power, the laser was operated in the mode-locked condition (pulse-width ~120 fs at 76 MHz repetition rate having central wavelength ~780 nm). The tweezers microscope used for the experiment is a home-built inverted microscope that has been described in detail earlier [ 12 , 15 , 16 , 17 , 18 ]. Most importantly, the trapping laser beam was first expanded and then collimated using telescopic arrangements to overfill the objective back aperture so as to obtain nearly diffraction limited focal spot size.…”

“…2c,d , trapping of a nanosphere shows a sharp rise in the PMT detector signal with respect to background noise indicating TPF. Subsequent addition of each nanosphere into the trapping region causes an initial rise in the TPF signal by an amount corresponding to that of single nanosphere signal 12 15 16 17 18 . This initial rise of TPF signal on the PMT correlates well to the photodiode backscatter data and, therefore, independently confirms the backscatter step jump data.…”

Precise control and measurement of solid–liquid interfacial temperature and viscosity using dual-beam femtosecond optical tweezers in the condensed phase

“…Such a high peak-power Gaussian laser beam can easily trap and manipulate small microparticles, nanoparticles or even quantum dots. 6 This trapping wavelength is transparent through water, alcohol, most of the bio-molecule solutions, cells, and is confirmed to be safe and non-invasive due to its very low absorption coefficient. 7 Consequently, the temperature rise is negligible for the low laser powers used in such experiments.…”