Experimental analysis of recoil effects induced by fluorescence photons

The momentum transfer to a scatterer from fluorescence photons was detected using an optical system that permits one to simultaneously measure the radiation force exerted on and fluorescence emission from the scatterer. The core of this technique is a partially metal covered dielectric bead optically trapped in a liquid with dye molecules. Fluorescence emission from the volume that includes the bead is measured simultaneously with the Brownian motion of the bead. The perturbed motion of the bead is a result of photon momentum transfer from the fluorescence of the dye to the trapped scatterer. The bead position fluctuations indicate the presence of the fluorescence and its bleaching nature. The results demonstrate the capability of the photonic force microscopy technique to be a complement to spectroscopy in the study of optical processes.

Figure 1

(Color online) A schematic of the experimental setup with a drawing (inset) of the optically trapped metal covered bead illuminated by the trapping, pump, and position detection beams.

Figure 2

Position data ($z$ axis) of the SS trapped in water (no dye) over time while increasing pump power at certain intervals. The pump beam is blocked for some time before increasing the power to the next level. The positive value of $z$ corresponds to the shift of the probe position along the propagation direction of the trapping beam.

Figure 3

Force values ($z$ axis) acting on the SS and SCS trapped in water (no dye) under varying pump powers. Each point is the result of averaging of 20 probe position measurements during 1 s. Errors bars are about the size of the symbols.

Figure 4

(Color online) Force acting on the SCS trapped in water (no dye) without the pump (time interval $0<t<125\text{s}$) and at the pump 0.8 mW $\left(t>125\text{s}\right)$.

Figure 5

(Color online) Fluorescence difference spectrum obtained with an SCS trapped in dye solution in the confocal volume. The spectrum is the difference between the measured fluorescence in the presence (curve 1 inset) and absence (curve 2 inset) of the trapped SCS. The pump is 0.02 mW.

Figure 6

(Color online) Integrated fluorescence (right axis, red line) of dye solution with a trapped SS plotted with the measured force $z$ (left axis, blue line) on the bead over time. The pump beam is first off $\left(t<25\text{s}\right)$, then turned on $\left(50\mu \text{W}\right)$ $\left(25<t<75\text{s}\right)$ before the bead is expelled from the trap by blocking the trap beam $\left(t>75\text{s}\right)$.

Figure 7

(Color online) Low-pump power response. The pump power is $50\mu \text{W}$. Integrated fluorescence (right axis, red line) of dye solution with a trapped SCS plotted with the measured force (left axis, blue line) over time. The pump beam is first off ($0<t<60\text{s}$), no luminescence is observed, and the average bead position is assigned to 0. During $60<t<200\text{s}$ the pump beam is on. At $t>200\text{s}$ the trapping beam is off and the fluorescence signal corresponds to the emission of the dye molecules in the confocal volume.

Figure 8

(Color online) High-pump power response. From 0–17 s the pump power is 0.13 mW. The power is then gradually increased until it reaches 0.8 mW at 28 s. The power remains constant until at 95 s when it is increased to 1.3 mW and is held at this value until the end of the measurement. Force exerted on the bead (a) and integrated fluorescence (b, c, and d) of dye solution with a trapped SCS over time. In order to show only variations in the fluorescence intensity, we subtract the background signals observed for each interval of measurements with constant pump power. Rectangles denote the time intervals when the pump power was gradually changed.

Figure 9

(Color online) A flash of fluorescence intensity observed for the highest pump power (1.6 mW) used in the experiments. Measured integrated fluorescence (right axis) of dye solution with a trapped SCS plotted with the measured force $z$ (left axis) on the bead over time. In this case, the pump is turned on to a much higher power than Fig. 7. A flash of emission is observed at 190 s.